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Plasticity in metals

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High Entropy Alloys

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Powder-in-tube processing

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High-temperature shape memory alloys

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Intermetallic phases

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Titanium-based alloys

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Accumulative deformation

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Highly strengthened conductors

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Plasticity in metals

Our reseach activities are driven by metal forming and mechanical testing. Consequently, plasticity and the investigation of the (dominant) deformation mechanism ist at the heart of our activities. This is completed by in-depth microstructural analyses to image the mechanisms. Furthermore, a basic understanding of the underlying mechanisms is deepend upon results whereof some are listed below.

D. Geissler et al. Acta Materialia 59 (2011) 7711–7723

By comparing the microstructural and texture evolution with tensile stress–strain response of an Fe–24Mn–7Ni–8Cr (mass%) alloy, a slip-dominated deformation process and, at a later stage of deformation, twinning-induced plasticity are observed. The occurrence of deformation twinning is texture sensitive and occurs only in the h1 1 1i fibre texture component. Based on these experimental observations, a model is presented, which reflects an orientational and configurational peculiarity of face-centred cubic stacking faults bordered by two Shockley partials. With this model, the onset point of stacking fault growth, i.e. movement of the leading partial and stopping of the trailing partial, is evaluated. This point reflects the formation of twins in the sense that a twin is regarded as an arrangement of stacking faults on every consecutive slip plane. Furthermore, based on the tensile test results, a model-compatible description of the mechanical behaviour is shown and a reasonable stacking fault energy of about 8 mJ/m2 is calculated for the onset of partial dislocation breakaway, i.e. the onset of deformation twinning.

 

D. Geissler et al., Philosophical Magazine 94 (2014) 2967-2981

Especially with respect to high Mn and other austenitic TRansformation and/or TWinning Induced Plasticity (TRIP/TWIP) steels, it is a current trend to model the stacking fault energy of a stacking fault that is formed by plastic deformation with an equilibrium thermodynamic formalism as proposed by Olson and Cohen in 1976. In the present paper, this formalism is critically discussed and its ambiguity is stressed. Suggestions are made, how the stacking fault energy and its relation to the formation of hexagonal -martensite might be treated appropriately. It is further emphasized that a thermodynamic treatment of deformation-induced stacking fault phenomena always faces some ambiguity. However, an alternative thermodynamic approach to stacking faults, twinning and the formation of -martensite in austenitic steels might rationalize the specific stacking fault arrangements encountered during deformation of TRIP/TWIP alloys.

 

A. Kauffmann et al., Acta Materialia 59 (2011) 7816–7823

The effect of low-temperature on the active deformation mechanism is studied in pure copper. For this purpose, cryogenic wire drawing at liquid nitrogen temperature (77 K) was performed using molybdenum disulfide lubrication. Microstructural investigation and texture analysis revealed severe twin formation in the cryogenically drawn copper, with a broad twin size distribution. The spacing of the observed deformation twins ranges from below 100 nm, as reported in previous investigations, up to several micrometers. The extent of twin formation, which is significantly higher when compared to other cryo-deformation techniques, is discussed with respect to the state of stress and the texture evolution during wire drawing.

 

A. Kauffmann et al., Materials Science & Engineering A 588 (2013) 132–141

We present the work hardening behaviour, mechanical and electrical properties of pure copper subjected to wire drawing at 77 K and 295 K, respectively. The deformation per pass is increased up to true strain of  0.4 by adopting pressure die/drawing die combinations in order to optimize lubricant residuals of MoS2 on the wire surface at 77 K. The onset of deformation twinning for wire drawing at 77 K was found to be 0.3 and 0.4 for a true strain of 0.1 and 0.4 per pass, respectively. Twinning activity, texture strength and homogeneity are enhanced by increasing deformation per pass while the number of processing steps required for a certain deformation are reduced significantly. A considerably altered electrical conductivity, medium strength increase accompanied with a loss of ductility and a limited thermal stability suggest the formation of non-coherent twin boundaries or destructed twin orientation relationship in cryo-drawn wires. Evidence was found for the latter possibility by local investigation of deformation twins in the final stage of deformation.

 

A. Kauffmann et al., Materials Science & Engineering A 624 (2015) 71–78

The microstructure of single phase copper alloys is altered by cold deformation. Depending on the processing parameters like temperature and intrinsic material parameters as stacking fault energy, the dominant deformation mechanism is different and the refinement of the microstructure bears other rates with respect to the deformation strain. The formation of deformation twins is activated at low homologous temperature or at low stacking fault energy. Both also lead to smaller grain sizes achieved at a certain deformation strain. Lowering the temperature only yields to a high efficiency in strain hardening with respect to room temperature deformation for intermediate stacking fault energies. The maximum efficiency is found to occur in the vicinity of the onset of deformation twinning at room temperature which was found for a stacking fault energy of 30 mJ/m2. The thermal stability of the microstructure is assessed by means of in situ resistivity measurements.

 

C.-G. Oertel et al. Scripta Materialia 65 (2011) 779–782

The tensile deformation behaviour of an extruded, polycrystalline YCu intermetallic compound was investigated from room temperature down to 77 K. A stress-induced martensitic transformation of the cubic B2 to the orthorhombic B27 phase was observed. The increasing amount of B27 phase with decreasing temperature leads to a decrease in ductility. A drastic decrease sets in at about 160 K when thermally induced martensite is formed, characterizing the brittle-to-ductile transition of YCu. Reasons for the relatively high ductility above 160 K are discussed.

 

R. Schaarschuch et al. Acta Materialia 151 (2018) 149e158

The most ductile rare earth intermetallic compound, YAg, was subjected to an thermal activation analysis at low temperatures down to 4 K. Evaluation of the activation parameters and their dependence on stress and temperature yields strong indication for forest dislocation cutting as the rate-controlling deformation mechanism, similar to face-centered cubic metals. Surprisingly, nil temperature ductility was observed. Together with results of a detailed TEM analysis of the active slip systems it is concluded that, despite of violating the von Mises criterion for the plastic deformation of polycrystalline materials, a low elastic anisotropy and/or low Peierls stress is responsible for the appreciable ductility at low temperatures. This finding may help to search for other ductile systems in the broad class of intermetallic compounds.
                           

 

R. Schaarschuch, et al. Scripta Materialia 186 (2020) 95–98

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that the low yield stress and high work-hardening rate observed cause the moderate  ductility at low temperatures.


High Entropy Alloys

High-Entropy Alloys (HEAs) are of interest for gaining a fundamental understanding of composition determined materials properties, particularly solid solution hardening, or investigations of the deformation behaviour. One key question is how the conventional models for solid solution strengthening that consider matrix and solute elements, will have to be modified if no matrix element can be assigned. Furhtermore, investigations of the deformation mechanisms are perforemd to separate the material behaviour of the HEAs cleanly from that of single-phase conventional alloys. The research activities as given below aim at identifying which issues are special for HEAs leading to their peculiar properties.

J. Freudenberger et al.  Metals 7 (2017) 135

A single-phase solid solution is observed in quaternary and quinary alloys obtained from gold, copper, nickel, palladium and platinum. The lattice parameters of the alloys follow the linear rule of mixture when considering the lattice parameters of the elements and their concentration. The elements are a priori not homogeneously distributed within the respective alloys resulting in segregations. These segregations cause a large broadening of X-ray lines, which is accessed in the present article. This correlation is visualized by the help of local element mappings utilizing scanning electron microscopy including energy dispersive X-ray analysis and their quantitative analysis.

 

F. Thiel et al.  Acta Materialia 185 (2020) 400–411

The precious metal based High-Entropy Alloy (HEA) AuCuNiPdPt crystallises in a face-centred cubic structure and is single phase without chemical ordering after homogenisation. However, a decomposition is  observed after annealing at intermediate temperatures. This HEA shows extended malleability during cold work up to a logarithmic deformation degree of φ = 2.42. The yield strength ranges from 820 MPa in the recrystallised state to 1170 MPa when strain hardened by cold working with a logarithmic deformation degree of φ > 0.6. This work hardening behaviour is traced back to a steep increase in dislocation density as well as in deformation twinning occurring at low strain. The microstructure and the mechanical properties of AuCuNiPdPt are assessed in detail by various methods. EBSD and TEM analyses reveal mechanical twinning as an important deformation mechanism. The high strength in the recrystallised state is evaluated and found to originate predominantly upon solid solution strengthening.
 

 

F. Thiel et al. Scripta Materialia 18 (2020) 15–18

The compositional dependence of the yield strength σ y has been studied for a series of polycrystalline (AuNiPdPt )1−x Cux alloys by means of compression tests. σ y is found to decrease linearly with increasing Cu concentration. This behaviour is in contradiction to the generalised theory for solid solution strengthening in concentrated solid solutions provided by Varvenne et al. [1]. A breakdown of the scaling behaviour is found as σ y should be non-linear and slightly increasing when modifying the composition from AuNiPdPt to AuCuNiPdPt.
[1] C. Varvenne, A. Luque, W.A. Curtin, Acta Mater. 118 (2016) 164–176.

 

S. Haas et al., Entropy 20 (2020) 654

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp -measurements are carried out from −170°C to the materials’ solidus temperatures TS . From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical  composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy.

 

A.S. Tirunilai et al.  Journal of Materials Research 33(19) (2018) 3287-3300

This contribution presents a comprehensive analysis of the low temperature deformation behavior of CoCrFeMnNi on the basis of quasistatic tensile tests at temperatures ranging from room temperature down to 4.2 K. Different deformation phenomena occur in the high-entropy alloy in this temperature range. These include (i) serrated plastic flow at certain cryogenic temperatures (4.2 K/8 K), (ii) deformation twinning (4.2 K/8 and 77 K), and (iii) dislocation slip (active from 4.2 K up to room temperature). The importance of deformation twinning for a stable work-hardening rate over an extended stress range as well as strain range has been addressed through the use of comprehensive orientation imaging microscopy studies. The proposed appearance of e-martensite as well as a previously uninvestigated route of analysis, essentially a quantitative time-dependent, strain-dependent, and stress-dependent evaluation of the serrated plastic flow in CoCrFeMnNi is provided.

 

A.S. Tirunilai et al., Materials Science & Engineering A 783 (2020) 139290

The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the for­ mation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures.

 

K. Lu et al. Acta Materialia 215 (2021) 117089/1-12

Plastic deformation during low-cycle fatigue (LCF) in equiatomic face-centered cubic (FCC) CoCrFeMnNi high-entropy alloys (HEAs) is accumulated by dislocation substructure formation, which leads to crack initiation. Whilst these substructures have been reported before, little has been done to clarify their formation mechanisms and the effects of strain amplitude, cycle number and grain orientation. In this study, cyclic deformation behavior and microstructural evolution of CoCrFeMnNi were examined for two different grain sizes at room temperature. Microstructural investigations by transmission electron microscopy showed that, while the dislocation structures at low strain amplitude (0.3%) mainly consisted of planar slip bands, at higher strain amplitudes (0.5% and 0.7%), wavy-substructures including veins, walls, labyrinth and cells prevailed. Slip mode also changes from initially planar-slip to wavy-slip with cycle numbers. Dislocations in veins, walls, labyrinth and cells are found to have different Burgers vectors, suggesting that apart from wavy-slip, multiple-slip also contributes to their formation. Moreover, distinct dislocation substructure in grains is dictated more by the constraints from neighboring grains rather than by their orientation. Additionally, the formation of various dislocation structures in a single grain is also linked to the constraint effects from the neighboring grains.

H. Thota et al. Journal of Alloys and Compounds 888 (2021) 161500/1-15

In the present study, strain-annealing based thermo-mechanical processing was employed to achieve a grain boundary engineered (GBE) microstructure in an equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) with a single phase, fcc structure. Cast, homogenized and recrystallized strips were cold rolled to 5%, 10% and 15% thickness reductions and annealed at temperatures from 1173 K to 1373 K for 1–6 h duration. Deformation twins were observed following cold rolling to 15%. From the deformed and annealed specimens, GBE microstructure was identified based on coincident site lattice (CSL) (Σ ≤ 29) boundary length fraction, number of twins per grain, triple junctions (TJs) character distribution and grain boundary plane orientations. Specimens rolled to 5% and annealed at 1223 K for 1 h exhibited GBE microstructure. The Σ3 fraction was enhanced from ~44% to ~62% in the GBE specimen with concurrent increments in TJs containing at least two CSL boundaries from ~20% to ~40% compared to as-recrystallized (AR) specimen. Potentiodynamic polarization studies revealed that the GBE specimen exhibited lower corrosion rate in both 0.1 M and 0.6 M NaCl solutions as compared to AR counterpart. The GBE specimen also displayed better passive film resistance due to higher polarization and charge transfer resistance, as evaluated from electrochemical impedance spectroscopy studies.

K. Lu et al. Scripta Materialia 194 (2021) 113667

We report on the low-cycle fatigue behavior of single-phase, face-centered cubic CoCrNi and CoCrFeMnNi at room temperature. Both alloys manifest cyclic hardening followed by softening and a near steady state until failure. CoCrNi exhibits higher strength, lower inelastic-strain, and longer lifetime than CoCrFeMnNi. For both alloys, microstructural investigations reveal no noticeable changes of texture, grain size and twin fraction. Nevertheless, CoCrNi exhibits planar dislocation structures, while CoCrFeMnNi shows well-defined wavy dislocation structures. This is due to CoCrNi lower stacking fault energy, which enhances planar slip and delays deformation localization leading to its superior fatigue resistance, compared to CoCrFeMnNi.

A. Srinivasan et al., Metals 12 (2022) 514

Serrated plastic deformation is an intense phenomenon in CoCrFeMnNi at and below 35 K with stress amplitudes in excess of 100 MPa. While previous publications have linked serrated deformation to dislocation pile ups at Lomer–Cottrell (LC) locks, there exist two alternate models on how the transition from continuous to serrated deformation occurs. One model correlates the transition to an exponential LC lock density–temperature variation. The second model attributes the transition to a decrease in cross-slip propensity based on temperature and dislocation density. In order to evaluate the validity of the models, a unique tensile deformation procedure with multiple temperature changes was carried out, analyzing stress amplitudes subsequent to temperature changes. The analysis provides evidence that the apparent density of LC locks does not massively change with temperature. Instead, the serrated plastic deformation is likely related to cross-slip propensity.


Powder-in-tube processing

Intrinsically brittle materials cannot be deformed on their own by cold work. The powder-in-tube (PIT) method is capable of deforming such materials in a composite made from a ductile metallic jacket containing the brittle matrix. This method is applied to functional materials  that require wire-shaped sample in application. 

A. Funk et al. Materials Today Energy 9 (2018) 223-228

The powder-in-tube (PIT) technology was applied to La(Fe, Co, Si)13 powder cladded by a thin seamless austenitic steel jacket. Wires appear to be promising in the search for alternative regenerator geometries, since they offer various possibilities of arrangements allowing to optimise heat transfer and pressure loss within the boundaries set by parallel plate and sphere beds. Here, pre-alloyed La(Fe, Co, Si)13 powder was  filled in a AISI 316L austenitic steel tube and swaged to wires with an outer diameter of 1 mm. By mechanical deformation, the steel jacket thickness was reduced to about 100 mm surrounding the magnetocaloric core. A post-annealing of only 10 min at 1050  C is sufficient to form the magnetocaloric NaZn13-type phase resulting in an entropy change close to the value of a pure reference sample. The presented technology is not limited to La(Fe, Co, Si)13/steel combination but can be extended to material pairs involving wire core materials with a first order transition, such as Fe2P-type or Heusler alloys.
 

 

M. Krautz et al. Materials and Design 193 (2020) 108832

Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ0ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔTad, and to determine the effective temperature change at the surrounding steel jacket, ΔTeff, during the field pulse. An inverse thermal hysteresis is observed for ΔTad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to hed light on their impact on ΔTeff at the outside of the jacket in comparison to ΔTad provided by the core.

 

L. Beyer et al. Journal of Magnetism and Magnetic Materials 536 (2021) 168115/1-6

Gd cladded in a seamless 316L austenitic steel tube has been swaged into wires by the powder-in-tube (PIT) technology, resulting in an outer diameter of 1 mm, a wall thickness of approx. 100 µm and a filling factor of around 62 vol%. Such wires provide an advantageous geometry for heat exchangers and have the benefit to protect the Gadolinium, i.e. from corrosion when being in contact with a heat transfer fluid. The magnetocaloric composite has been studied by static and pulsed magnetic-field measurements in order to evaluate the performance of Gd as a core material. By the analysis of magnetization and heat capacity data, the influences of deformation-induced defects on Gadolinium are presented. The subsequent heat treatment at 773 K for 1 h in Ar atmosphere allowed restoring the magnetic properties of the wire after deformation.
Data of the pulsed magnetic-field measurements on the Gd-filled PIT-wires and a Gd–core separated from the jacket are presented, with an achievable temperature change of 1.2 K for the wire and 5.2 K for the Gd in 2 T, respectively. A comparison to previously studied La(Fe, Co, Si)13-filled composite wires is included. It indicates that performance losses due to the passive matrix material cannot be overcome only by an increased adiabatic temperature change of the core material, but instead the wire components need to be chosen regarding an optimized heat capacity ratio, as well.

F. Jürries et al Scientific Reports 10 (2020) 7897/1-10

Alloys of the form (Mn54Al44C2)100-xCux (with x = 0, 1, 2, 4 and 6) were produced by induction melting. After homogenisation and quenching, most of the alloys consist entirely of the retained ε-phase, except for x = 6, in which the κ-phase was additionally present. After subsequent annealing, the alloys with x ≤ 2 consist entirely of a Cu-doped, ferromagnetic τ-phase, whereas the alloys with x > 2 additionally contain the κ-phase. The polarisation of the alloys at an applied field of 14 T decreases with increasing Cu-content, which is attributed i) to the dilution of the magnetic moment of the τ-phase unit cell by the Cu atoms, which do not carry a magnetic moment, and ii) at higher Cu-contents, to the formation of the κ-phase, which has a much lower polarisation than the τ-phase and therefore dilutes the net polarisation of the alloys. The Curie temperature was not affected by the Cu-additions. The stress needed to die-upset the alloys with x ≤ 2 was similar to that of the undoped alloy, whereas it was much lower for x = 4 and 6, due to the presence of intergranular layers of the κ-phase. The extrinsic magnetic properties of alloys with x ≤ 2 were improved by die-upsetting, whereas decomposition of the τ-phase during processing had a deleterious effect on the magnetic properties for higher Cu-additions.

L. Feng et al. Acta Materialia 199 (2020) 155-168

Rare-earth-free MnAl-C-Ni permanent magnets have been produced for the first time by extruding powders milled from bulk. The resulting materials, fabricated using different conditions, contained a large volume fraction (> 0.92) of the desired τ-phase. In terms of the maximum energy product, the best performance obtained for a whole, transverse section of the extruded material was (BH)max = 46 kJm−3, and was (BH)max = 49 kJm−3 for a sample taken from near the edge of this section. Analysis showed that this material was comparable to the long-established benchmark, comprising MnAl-C-based magnets extruded in industry from bulk or from gas-atomised powder. Such materials are no longer available. The microstructure of the materials produced here consisted of fine, recrystallised grains, which exhibited an <001> fibre texture with intermediate texture quality and of larger, non-recrystallised regions, which contained hierarchical twinning and a high density of defects. The volume fraction and size of the non-recrystallised regions was greatly reduced by decreasing the size of the initial powder particles. This led to a large increase in the squareness factor of the demagnetisation curve and consequently to the high (BH)max values observed.


High-temperature shape memory alloys

M. Vollmer et al. NATURE COMMUNICATIONS | (2019)10:2337

Iron-based shape memory alloys are promising candidates for large-scale structural applications due to their cost efficiency and the possibility of using conventional processing routes from the steel industry. However, recently developed alloy systems like Fe–Mn–Al–Ni suffer from low recoverability if the grains do not completely cover the sample cross-section. To overcome this issue, here we show that small amounts of titanium added to Fe–Mn–Al–Ni significantly enhance abnormal grain growth due to a considerable refinement of the subgrain sizes, whereas small amounts of chromium lead to a strong inhibition of abnormal grain growth. By tailoring and promoting abnormal grain growth it is possible to obtain very large single crystalline bars. We expect that the findings of the present study regarding the elementary mechanisms of abnormal grain growth and the role of chemical composition can be applied to tailor other alloy systems with similar microstructural features.


Intermetallic phases

A necessary step for materials development and optimization is the control Investigation of processing - microstructure – property – relationships (structural, chemical, mechanical, thermal, physical) and, in consequence, tailoring of composition and microstructures at different length scales for obtaining best possible performance.  The understanding of thermodynamic equilibria of intermetallic phases is highly important for materials development - even if these materials are obtained, processed, and applied in non-equlibrium conditions.

M. Kriegel et al. Intermetallics 83 (2017) 29e37

The binary system AleMo is a key system for the development of refractory aluminides. This development involves the thermodynamic modeling of multicomponent materials systems such as TiAl- and  NiAl-based systems. Due to inconsistencies within the literature data, the phase equilibria with the high-temperature b (AlMo) phase were re-investigated with alloys in the as-solidified state and after heat-treatment at 1773 K using complementary experimental methods such as metallography, electron-probe microanalysis (EMPA), X-ray diffraction (XRD) and thermal analysis. The obtained results show the congruent formation of b at nearly equiatomic composition and a good agreement with the phase equilibria at 1773 K, which were determined in an earlier work. In addition, the structural disorder in this high-temperature phase was confirmed by means of XRD and transmission electron microscopy (TEM) measurements. The diffuse scattering observed in the electron diffraction patterns taken at quenched b  (bcc) alloys indicate a soft phonon mode behavior of this phase, which differs substantially from the   vibrational characteristics of the bcc Mo-rich solid solution.
 

 

M. Kriegel et al. Journal of Alloys and Compounds 706 (2017) 616-628

The ternary AleMoeTi system is an important system for the development of refractory titanium alu- minides which are promising candidate materials for turbine blades. However, due to inconsistencies within the literature data, the phase equilibria close to the binary AleMo system need to be reinvestigated. Therefore, several alloys were analyzed in the as-solidified state and after heat-treatments at 1523 and 1673 K using complementary experimental methods such as metallography, electron-probe microanalysis (EMPA), powder X-ray diffraction (P-XRD) and thermal analysis. Based on these results, isothermal sections at 1523 and 1673 K, the liquidus projection and a partial Scheil reaction scheme were constructed. The phase equilibrium studies reveal the existence of a newly observed ternary t phase which crystallizes in a face-centered cubic g-brass related crystal structure. The crystal structure of t was investigated by means of P-XRD and transmission electron microscopy and also determined by single-crystal diffraction analysis from a crystal specimen obtained from an alloy with a composition 63Al-23Mo-14Ti (at.%).
 

 

M. Kriegel et al. Journal of Alloys and Compounds 811 (2019) 152055

Titanium aluminides based on the ternary AleMoeTi system are promising candidates for the devel-opment of high-temperature materials. Close to the binary AleMo system, the stability of a ternary t phase was reported in a previous work. To investigate the relationship of this ternary phase with the binary high-temperature phase Al63Mo37 and to re-evaluate the heterogeneous reactions of Al63(Mo,Ti)37  a number of alloys were prepared and annealed at 1573, 1673 and 1773 K, respectively. The heat-treated alloys were analyzed by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), electron-probe microanalysis (EPMA) and differential thermal analysis/differential scanning calorimetry (DTA/DSC). Isothermal sections at 1673 and 1773 K as well as a consistent partial Scheil reaction scheme were constructed based on the obtained results. The phase equilibria investigations reveal that the ternary t phase extends up to the binary Al-Mo system. Hence, t and Al63Mo37 are the same phases. Inside the ternary Al-Mo-Ti system Al63(Mo,Ti)37 decomposes by the ternary eutectoid reaction Al63(Mo,Ti)37 <-> b + ε-Al3Ti + Al8Mo3 at approx. 1590 K.

 

T. Marr et al. Metals 2013, 3, 188-201

This study shows the possibility of processing titanium aluminide wires by cold deformation and annealing. An accumulative swaging and bundling technique is used to co-deform Ti and Al. Subsequently, a two step heat treatment is applied to form the desired intermetallics, which strongly depends on the ratio of Ti and Al in the final composite and therefore on the geometry of the starting composite. In a first step, the whole amount of Al is transformed to TiAl3 by Al diffusion into Ti. This involves the formation of 12% porosity. In a second step, the complete microstructure is transformed into the equilibrium state of γ-TiAl and TiAl3 . Using this approach, it is possible to obtain various kinds of gradient materials, since there is an intrinsic concentration gradient installed due to the swaging and bundling technique, but the processing of pure γ-TiAl wires is possible as well.

 

C. Wiesner et al. Journal of Electronic Materials 62 (2020) 8036-8051

A current issue in electrical engineering is the enhancement of the quality of solder joints. This is mainly associated with the ongoing electrification of transportation as well as the miniaturization of (power) electronics. For the reliability of solder joints, intermetallic phases in the microstructure of the solder are of great importance. The formation of the intermetallic phases in the Cu-Sn solder system was investigated for different annealing temperatures between 472 K and 623 K using pure Cu as well as Cu-1at.%Ni and Cu-3at.%Ni substrate materials. These are relevant for lead frame materials in electronic components. The Cu and Cu-Ni alloys were in contact to galvanic plated Sn. This work is focused on the unexpected formation of the hexagonal f-(Cu,Ni)10Sn3 phase at annealing temperatures of 523–623 K, which is far below the eutectoid decomposition temperature of binary f-Cu10Sn3 of about 855 K. By using scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction and X-ray diffraction the presence of the f phase was confirmed and its structural properties were analyzed.

 

R. Schaarschuch et al. Acta Materialia 223 (2022) 117489

The B2-type intermetallic compounds CoZr and Co39 Ni11 Zr50 were deformed in tension at low temperatures. While CoZr is ductile down to 4 K, Co39 Ni11 Zr50 becomes brittle below 125 K due to a martensitic phase transformation. Thermal activation analysis shows that CoZr follows the Cottrell-Stokes law indicating forest dislocation cutting as the dominant rate-controlling deformation mechanism, similar to face-centered cubic metals. The moderate ductility of both intermetallic compounds at low temperatures may qualitatively be related to a significant metallic character of bonding giving rise to a low Peierls stress estimated for primary {110}<100> slip and most likely also leads to an easier activation of secondary {110}<110> slip which was proven by transmission electron microscopy. Secondary slip is necessary for the fulfillment of the von Mises criterion for homogeneous plastic deformation of polycrystalline materials. The present results generalize the findings made on the ductile rare earth intermetallics YCu and, therefore, may help to search for other ductile systems in the broad class of intermetallic compounds.


Titanium-based alloys

Titanium-based alloys with hight strength are required for structural applications in transportation and mobility due to their comparably low density. Furthermore, ß-Ti alloys are potential candidates for the application as implant materials. The subsequently listed activities represent our contribution to the development of Ti-based alloys with a strong focus on metal forming and the related tailoring of the microstructure.

S. Pilz et al. Journal of the Mechanical Behavior of Biomedical Materials 79 (2018) 283–291

In this study, the effect of thermomechanical processing on microstructure evolution of the indium-containing β-type Ti alloys (Ti-40Nb)-3.5In and (Ti-36Nb)-3.5In was examined. Both alloys show an increased β-phase stability compared to binary alloys due to In additions. This leads to a reduced α’’-phase fraction in the solution treated and recrystallized state in the case of (Ti-36Nb)-3.5In and to the suppression of stress-induced α’’ formation and deformation twinning for (Ti-40Nb)-3.5In. The mechanical properties of the alloys were subsequently studied by quasistatic tensile tests in the recrystallized state, revealing reduced Young's modulus values of 58 GPa ((Ti-40Nb)-3.5In) and 56 GPa ((Ti-36Nb)-3.5In) compared to 60 GPa as determined for Ti-40Nb. For both In-containing alloys the ultimate tensile strength is in the range of 560 MPa. Due to the suppressed α’’ formation, (Ti-40Nb)-3.5In exhibits a linear elastic deformation behavior during tensile loading together with a low Young's modulus and is therefore promising for load-bearing implants.

 

A. Helth et al. Journal of the mechanical behavior of biomedical materials 65 (2017) 137–150

Different hardening strategies were evaluated regarding their potential to improve the mechanical biofunctionality of the cast and solution-treated low modulus β-Ti alloy Ti 40Nb. The strategies are based on thermomechanical treatments comprised of different hot- and cold-rolling steps, as well as annealing treatments aiming at the successive exploitation of different hardening mechanisms (grain boundary hardening, work hardening and precipitation hardening). Quasi-static tensile testing revealed that grain refinement by one order of magnitude has only a small impact on improving the mechanical biofunctionality of Ti-40Nb. However, work hardening effectively improves the tensile strength by 30% to a value of 650 MPa, while retaining Young׳s modulus at 60 GPa. The α-phase precipitation hardening was verified to have an increasing effect on both, strength and Young׳s modulus. Thereby, the change of Young׳s modulus dominates the change of the strength, even at low α-phase fractions. The pseudo-elastic behavior of Ti 40Nb is discussed under consideration of the microstructural changes due to the thermomechanical treatment. The texture changes evolving upon cold-rolling markedly influence the recrystallization behavior. However, the present results do not show a significant effect of the texture on the mechanical properties of Ti-40Nb.

A. Helth et al. Acta Materialia 61 (2013) 3324–3334

The influence of boron additions and different oxygen contamination levels on the microstructure and the mechanical properties in the Ti66ÀxNb13Cu8Ni6.8Al6.2Bx (0 6 x 6 1) system were investigated. The alloys were prepared by levitation copper mould casting as rods with a diameter of 5 mm using different grades of starting elements. The alloy without boron exhibits a maximum compressive stress of more than 2500 MPa, associated with a compressive strain of more than 30%. The ultimate tensile stress is $1075 MPa with a maximum elongation of 1.6%. Increased oxygen content leads to a rise of yield strength due to solid solution hardening. Boron additions promote grain refinement and reinforce the interdendritic phase compound by forming needle-like TiB precipitates. This change in microstructure increases the yield stress and the Young’s modulus and lowers the plastic strain. The microstructure was analysed in terms of the boron content by means of scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy. The presented mechanical properties are compared with the compression and tensile properties of the commercially available Ti6Al4V ELI (ELI = extra low interstitial) alloy.

 

I. Okulov et al. Applied Physics Letters 104 (2014) 071905

Tensile ductility of the Ti-based composites, which consist of a b-Ti phase surrounded by ultrafine structured intermetallics, is tunable through the control of intermetallics. The two Ti-based alloys studied exhibit similar compressive yield strength (about 1000 MPa) and strain (about 35%–40%) but show a distinct difference in their tensile plasticity. The alloy Ti71.8Nb14.1Ni7.4Al6.7 fractures at the yield stress while the alloy Ti71.8Nb14.1Co7.4Al6.7 exhibits about 4.5% of tensile plastic deformation. To clarify the effect of microstructure on the deformation behavior of these alloys, tensile tests were carried out in the scanning electron microscope. It is shown that the distribution as well as the type of intermetallics affects the tensile ductility of the alloys. 

 

I. Okulov et al. Materials Science & Engineering A 603 (2014) 76–83

New as-cast Ti–V–Cu–Ni–Al alloys with advantageous modulus of resilience and bioperformance were developed. Their microstructure is composed of a dendritic β-Ti phase and in-situ precipitated  interdendritic compounds. The tough and ductile β-Ti phase exhibits a relatively low Young's modulus.  Ultrafine intermetallics effectively strengthen the alloys. The effect of microstructure on tensile plasticity was studied on strained (in-situ) and fractured (ex-situ) samples in the scanning electron microscope. It was found that the ductility depends on the volume fraction/distribution of the intermetallic phases as well as local segregation. Already in the as-cast state Ti68.8V13.6Cu6Ni5.1Al6.5 exhibits a tensile strength of  about 1250 MPa and a ductility of about 4.5%.

 

A. Pukenas et al., Materials Today Communications 30 (2022) 103083

In this study multi-layered Ti/Al sheets prepared by accumulative roll bonding (ARB) underwent a two-step heat treatment (HT) to form intermetallic compounds. The microstructure and crystal structure of the samples were examined by scanning and transmission electron microscopy as well as energy-dispersive X-ray spectroscopy and synchrotron diffraction. In the first solid-state reaction annealing step, Ti-rich ARB samples containing 60 at% Ti and 40 at% Al were held at 600 ◦ C for 12 h under a uniaxial pressure between 0 MPa and 50 MPa applied along the normal direction of the sheets. At this stage, Al is completely consumed by forming mainly Al-rich inter­ metallic phases and to a lower extent other titanium aluminides such as Ti3Al and TiAl. In the second step, high-temperature annealing produces TiAl and Ti3Al as major phases during both pressureless annealing at 1100 ◦ C, 1200 ◦ C and 1300 ◦ C and annealing under an uniaxial pressure of about 100 MPa at 1200 ◦ C. Pore formation during the reaction annealing can be significantly reduced by the applied pressure. As a result, a TiAl-based semi-finished material was fabricated.


Accumulative deformation

Cold work is an effective means to strengthen metallic materials, which is even more effective when applied to composites as therein phase boundary strengthening also contributes to the strength. In order to obtain maximum strength accounting from phase-boundary hardening, the composites require severe plastic deformation. The research activities as listed in the following are focussed on accumulative deformation by means of swaging and bundling or roll bonding. Additionally they are focussed on light-weight materials with a high specific strength.

T. Marr et al. Metals 2011, 1, 79-97

An alternative deformation technique was applied to a composite made of titanium and an aluminium alloy in order to achieve severe plastic deformation. This involves accumulative swaging and bundling. Furthermore, it allows uniform deformation of a composite material while producing a wire which can be further used easily. Detailed analysis concerning the control of the deformation process, mesostructural and microstructural features and tensile testing was carried out on the as produced wires. A strong grain refinement to a grain size of 250–500 nm accompanied by a decrease in  111 fibre texture component and a change from low angle to high angle grain boundary characteristics is observed in the Al alloy. A strong increase in the mechanical properties in terms of ultimate tensile strength ranging from 600 to 930 MPa being equivalent to a specific strength of up to 223 MPa/(g/cm3) was achieved.

 

T. Marr et al. Metals 2014, 4, 37-54

An accumulative swaging and bundling technique is used to prepare composite wires made of Ti and an Al alloy. These wires show reasonable higher yield stresses than expected from the pure material flow curves. The additional strengthening in the composite is analyzed using nanoindentation measurements, tensile testings and investigations of the microstructure. In addition, these properties are analyzed in relation to the fracture surface of the mechanically tested wires. Additional strengthening due to the presence of phase boundaries could be verified. Indications for residual stresses are found that cause a global hardness gradient from the center to the wire rim. Finally, the yield stress of the wires are calculated based on local hardness measurements.

 

J. Romberg et al. Metals 2016, 6, 30

Co-deformation of Al and Ti by accumulative roll bonding (ARB) with intermediate heat treatments is utilized to prepare multi-layered Ti/Al sheets. These sheets show a high specific strength due to the activation of various hardening mechanisms imposed during deformation, such as: hardening by grain refinement, work hardening and phase boundary hardening. The latter is even enhanced by the confinement of the layers during deformation. The evolution of the microstructure with a special focus on grain refinement and structural integrity is traced, and the correlation to the mechanical properties is shown.

 

J. Romberg et al. Metals 2016, 6, 31

Differential speed rolling has been applied to multi-layered Ti/Al composite sheets, obtained from accumulative roll bonding with intermediate heat treatments being applied. In comparison to conventional rolling, differential speed rolling is more efficient in strengthening the composite due to the more pronounced grain refinement. Severe plastic deformation by means of rolling becomes feasible if the evolution of common rolling textures in the Ti layers is retarded. In this condition, a maximum strength level of the composites is achieved, i.e., an ultimate tensile strength of 464 MPa, while the strain to failure amounts to 6.8%. The deformation has been observed for multi-layered composites. In combination with the analysis of the microstructure, this has been correlated to the mechanical properties.

 

J. Scharnweber et al. Advanced Engineering Materials (2018) 1800210

The paper aims to summarize the research on Laminar Metal Composites produced by Accumulative Roll Bonding. After some notes on the general subject, frequent material combinations and the issue of bonding are addressed. Then, the evolution of microstructure, texture, and mechanical properties typically observed in such materials is briefly summarized.
Furthermore, the crucial aspect of layer continuity is discussed. In the main part, detailed experimental insight is provided for three representatives of different structural developments, namely Al2N/Al5N, Cu/Nb, and Ti/Al. The chosen systems represent different levels of component dissimilarity, as Al2N/Al5N is a combination of the same face centered cubic metal with varying purity while Cu/Nb and Ti/Al are combinations of face centered cubic with both body centered cubic and hexagonal close packed metals, respectively. As the layer thicknesses span different ranges, the composites also illustrate both the opportunities and experimental challenges of ARB Laminar Metal Composites.


Highly strengthened conductors

Strength and conductivity in metals are in principle conflictive materials properties as the same microstructural features that enhance the strength of metals also contribute to its resistance and, thus, lower the (electrical) conductivity. Consequently, metals show either a very high conductivity (as e.g. Ag, Cu) with low strength or they pocess very high strength values combined with a low conductivity (as e.g. high-Mn steels). Highly strengthened conductors are supposed to pocess both, high strength and high conductivity. The acivities as given below were performed to obtain the best possible combination of strength and conductivity in metals.

E. Bocharova et al. Journal of Alloys and Compounds 351 (2003) 119–125

Alloys with both high strength and high conductivity have been produced by mechanical alloying. In the present study, copper was mechanically alloyed with 5, 10 and 20 at.% Nb using a planetary ball mill. The Cu–Nb phase diagram shows a negligibly low mutual solubility in the solid state, but high energy ball milling can largely extend the region of solid state solution. Previously, it was observed that niobium partly dissolves in the copper lattice during milling. The present investigation demonstrates that this limit can be extended to a strongly supersaturated Cu solid solution of up to 10 at.% Nb provided the appropriate mechanical alloying method is applied. The change in the powder microstructure was followed by scanning and transmission electron microscopy (TEM) as well as by X-ray diffraction (XRD) analysis. In the case of Cu–5%Nb and Cu–10%Nb a homogeneous single-phase microstructure was obtained after 30 h of milling. Elemental Nb could no more be detected, indicating the formation of a metastable supersaturated Cu–Nb solid solution.
 

 

 

E. Bocharova et al. Acta Materialia 54 (2006) 3333–3341

Nanocrystalline Cu and Cu–Nb alloys were prepared by the consolidation of mechanically alloyed powder. The alloys show a microstructure with a grain size below 50 nm. The microstructure of the Cu matrix remains stable even at elevated temperatures of up to 900 °C, whereas the Nb precipitates coarsen during annealing. The mechanical strength as well as the electrical conductivity depend on the grain size of the Cu matrix, which can be influenced by the temperature of the heat treatment, i.e., a mechanical strength of about 1.6 GPa is measured for a Cu–10 at.% Nb alloy which shows an electrical conductivity of about 10% IACS (international annealing copper standard) at room temperature. The main contribution to the mechanical strength of the alloys is attributed to the grain boundary strengthening in Cu referring to the Hall–Petch relation, which is quantified. The grain boundaries are also found to influence considerably the electrical resistivity.

 

A. Gaganov et al. Z. Metallkd. 95 (2004) 6

The microstructure and the mechanical properties of Cu - Ag alloys with 7 and 24 wt.% Ag are investigated. The microstructure of the alloys is mostly determined by the silver content. That of Cu-24 wt.% Ag alloys consists of a Cu-rich solid solution and the eutectic. Otherwise, the microstructure of Cu-7 wt.% Ag alloys consists of primarily solidified dendrites of a Cu-rich solid solution and small Ag-rich particles. The composition strongly influences the work hardening rate. In order to achieve an ultimate tensile strength of 1 GPa a logarithmic cold deformation strain, g, of about 3.7 is required (g = ln A0/A) for the 7 wt.% Ag alloy, whereas for Cu-24 wt.% Ag alloys g 1#4 3.1 is sufficient. In as-cast alloys with 7 wt.% Ag a strong segregation is observed, which consequently leads to a strong decrease of the age hardening effect. Therefore, the Cu-7 wt.% Ag alloy has to be homogenised before aging. The application of Cu - Ag alloys with a Ag-content below 8 wt.%, i. e. the maximum solubility at the eutectic temperature, bears mainly two advantages: (i) less addiction to shear band formation, and (ii) a higher electrical conductivity in comparison to equivalently treated Ag-rich alloys due to the small Ag content.      

 

A. Gaganov et al. Materials Science and Engineering A 437 (2006) 313–322

Adding up to 0.05 wt.%Zr to Cu–7 wt.%Ag alloys suppresses the discontinuous precipitation mode in these alloys. As a consequence, the continuous precipitation mode, which is also commonly observed in these alloys, is enhanced. The addition of Zr in the mentioned range guarantees a minimum of secondary phases that contain Zr. The subjection of Cu–7 wt.%Ag–0.05 wt.%Zr alloys to a certain deformation strain followed by an intermediate heat treatment gives rise to a logarithmic drawing strain of up to φ = 5.82 in these alloys. In this cold worked condition, an ultimate tensile strength of 1.4 GPa and a strain to failure of 1.6% are observed. Furthermore, an electrical conductivity of 60%IACS (IACS, International Annealed Copper Standard, i.e. the conductivity of copper) is measured. The influence of the Zr content, the thermal treatments, and the  deformation process on the microstructure and thereby on the mechanical and electrical properties is discussed.

 

F. Bittner et al. Materials Science & Engineering A 597 (2014) 139–147

Dynamic recrystallisation of CuAgZr alloys within a composition range of (3–7) wt% Ag and (0.05–0.3) wt% Zr is studied as a function of alloy composition, temperature and strain. Dynamic recrystallisation was  investigated using hot-compression and hot-rolling experiments at temperatures between 500 1C and  850 1C. For CuAgZr with 7 wt% Ag and 0.05 wt% Zr, an optimised hot-rolling temperature of 750 1C was found and a mean grain size of 25 μm was established at a true strain of 2.2. Similar grain size distributions were found for the extended range of alloy compositions while the active mechanism for changes from necklace towards a particle stimulated nucleation mechanism. This change is driven by the volume fraction of the ternary phase Cu4 AgZr as these particles are identified to stimulate nucleation of dynamic recrystallisation in the samples with increased Zr content. The final tapes exhibit an outstanding combination of ultimate tensile strength of 1 GPa and an electrical conductivity of 70%IACS at a true strain of 4.8 of cold work being applied.
 

 

J. Freudenberger et al. Materials Science and Engineering A 527 (2010) 2004–2013

Ultra strong CuAg-based conductor materials have been developed and tested in pulsed high-field magnets. The yield strength of a cold deformed CuAgZr conductor material has been assessed on the basis of different hardening mechanisms: solid solution, grain boundary, precipitation and dislocation hardening. The experimental value for the yield strength when transferred to the shear strength by Schmid’s law is between a linear and a quadratic superposition of the individual critical shear stresses and hence found to be in good agreement with the theoretical predictions. The conductor material shows an ultimate tensile strength of more than 1.1 GPa at room temperature (yield strength about 1 GPa, plastic strain: 0.7%). Based on the properties of the CuAgZr material a new coil has been designed and tested. The coil features additional internal reinforcement layers, which are optimised using computer simulations. In combination with refined computer simulation techniques, such as finite element modelling, significant progress was made concerning the use of these materials for pulsed magnet applications. The coil generated a field of 66 T without being destroyed, which constitutes a new high-field record of the Clarendon Laboratory in Oxford, UK. Performance and measurements are in good agreement with simulations.

 

J. Freudenberger in Copper Alloys: Preparation, Properties and Applications

Materials with apparently conflictive combination of materials properties, such as high conductivity and strength are facing growing interest. Their availability is claimed in a wide scope of application. On the one hand there are micro-electromechanical units in which the energy absorption, which is caused by mechanical loading, has to be raised beyond the present limits. Thus, the efficiency can be increased, while the power durability is warranted even at high energy. On the other (macroscopic) side highly strengthened conductors with a good formability are required as e.g. for the windings in non-destructive high field magnets which are operated in a pulsed mode. The mechanical and physical properties of the conductors is strongly related to their microstructure. Thus, the control of the microstructure is a crucial item when adjusting an optimised combination of materials properties. Due to its high thermal and electrical conductivity Copper is the ideal candidate to form alloys and materials that serve as conductor. Alloying bears the potential to apply several hardening mechanisms to enhance the strength. A special interest is paid to the increase of the strength due to the formation of precipitates. In contrast to dispersion strengthened materials they face a higher strengthening potential. However, they are less thermal stable and their potential can be used at low temperatures, only. Nevertheless, if the temperature of the conductor can be kept at or below room temperature these conductors are highly recommended to be used. The possibility to adjust the microstructure by thermal and mechanical treatments builds the basis to develop highly strengthened conductor materials, which materials properties have to be fitted to the case of applications within a broad range of application. This article reviews the properties of age hardenable, highly strengthened Copper based conductor materials that are developed for applications at room temperature and below.