Publications of the department 'Metallic Glasses and Composites'

FeCo-based multiphase composites with high strength and large plastic deformation

A family of FeCo-based multiphase composites with a microstructure consisting of nano-lamellar phase strengthened α-(Fe,Co) dendritic cores surrounded by a network of reinforcement phases of ultrafine eutectics was produced by copper mold casting. The hypoeutectic composites exhibit a high yield stress, which is up to 7 times higher than the equiatomic FeCo alloy, and plastic deformation up to 18% during compressive test. Multiscale α-(Fe,Co)₃(B,C) reinforcement phases are responsible for the remarkable improvement of strength, and α-(Fe,Co) dendrites play a key role to inhibit the propagation of microcracks sourced from the eutectics. Furthermore, a fracture model for explaining the relationship between fracture strain and morphologic characteristics of the composites is presented.

R. Li, G. Liu, M. Stoica, J. Eckert, URL

DC- and RF-GD-OES measurements of adsorbed organic monolayers on copper

Our direct current (DC)- and radiofrequency glow discharge optical emission spectroscopy (RF-GDOES) measurements of adsorbed organic monolayers were inspired by the work of Shimizu et al., who presented the first example of depth profile analysis of an adsorbed monolayer by RF-GD-OES in 2004. The great potential of RF-GD-OES for analyses of layers with thicknesses in the subnanometer range was surprising.

Deformation-induced martensitic transformation in Cu-Zr-(Al,Ti) bulk metallic glass composites

Plastic deformation of Cu–Zr–(Al,Ti) bulk metallic glass (BMG) composites induces a martensitic phase transformation from the B2 to the B19' CuZr phase. Addition of Ti to binary Cu–Zr increases the temperature above which the B2 CuZr phase becomes stable. This affects the phase formation upon quenching in Cu–Zr–Ti BMG composites. The deformation-induced martensitic transformation is believed to cause the strong work hardening and to  ontribute to the large compressive deformability with plastic strains up to 15%.

Microstructural heterogeneities governing the deformation of Cu_47.5 Zr_47.5 Al₅ bulk metallic glass composites

Cu_47.5Zr_47.5Al₅ rods with different volume fractions of crystalline B2 CuZr phase were prepared by copper mould casting. Based on microstructural investigations a solidification mechanism is proposed for these bulk metallic glass (BMG) composites. The composite microstructure enhances the compressive plasticity (plastic strain up to 14%) and both plastic strain as well as yield strength scale with the crystalline volume fraction. Yield strength and fracture strain were successfully calculated using a strength model, which considers percolation and an empirical three microstructural element body approach, respectively. Furthermore, B2 CuZr was synthesized by means of a thermal cycling treatment and uniaxial compression tests were carried out at room temperature. The intrinsic work-hardenability caused by a martensitic transformation has strong implications on the deformation behaviour of the investigated Cu_47.5Zr_47.5Al₅ BMG composites.

Measurement of voltage and current in continuous and pulsed

rf and dc glow discharges

Electrical measurements are an important tool for the characterisation of glow discharges and have proved to be useful for a variety of needs in fundamental studies and as control parameter. Therefore, extensive hardware developments and studies of current-voltage (I-U) characteristics in continuous and pulsed, dc and rf modes have been made [1] and will be presented together with new results. In continuous dc mode, the I-U curves are non-linear and may be characterised by a threshold voltage U0 and saturation current Imax (both cathode material and pressure dependent). On the other hand P-U curves are to a large extent linear and very similar in the continuous rf mode [2].

V. Hoffmann, V.V. Efimova, M.V. Voronov, P. Šmid, E.B.M. Steers, J. Eckert, URL

Strain distribution in bulk metallic glasses

The macroscopic elastic strain can be determined relatively easily with the help of strain gauges during deformation tests. However, in order to get access to the atomic-scale strain tensor in situ tensile or compression tests in synchrotron radiation have to be performed.

Martensitic transformation in Cu-Zr-based alloys

The discovery of metallic glasses with casting dimensions up to several centimetres allows for the determination of the mechanical properties of these so-called bulk metallic glasses (BMGs). High yield stresses and relatively low Young’s moduli are inherent in BMGs. However, a major drawback is the limited plasticity in many alloy systems as they are prone to catastrophic failure. In order to improve the macroscopic deformability composite materials have been developed consisting of a crystalline phase embedded in an amorphous matrix. Particularly interesting under this aspect are Cu-Zr-based alloys as they can be quenched into a glassy structure. Furthermore, proper adjustment of the alloy composition as well as cooling rate permits the targeted precipitation of the B2 CuZr phase in a glassy matrix.>/p>
S. Pauly, U. Kühn, J. Freudenberger, N. Mattern, J. Eckert, PDF

Spinodal decomposition of Ni-Nb-Y metallic glasses

The ternary alloy system Ni-Nb-Y exhibits a miscibility gap in the liquid in accordance with the strong positive enthalpy of mixing between Nb and Y [1]. The liquid-liquid phase separation was explored to prepare new phase-separated Ni-Nb-Y glasses by rapid quenching of the melt. The temperature dependence of the critical temperature of liquid-liquid phase separation TC determines essentially the structure formation and consequently the obtained microstructures. For Ni contents < 60 at.% ( T_C > T_liquidus), coarsened hierarchical microstructures of two-phase glasses are obtained [2]. For higher Ni contents > 60 at.% ( T_C < T_liquidus), early stages of decomposition can be prepared with correlation lengths in the nanometer-scale. The observed fluctuation lengths range from 5 to 12 nm depending on the actual composition of the glass as determined by small angle X-ray scattering (SAXS) [3].

Metal matrix composites reinforced with complex metallic alloys

Within recent years, complex metallic alloys (CMAs) have been attracting much attention ranging from scientific curiosity about their complex structure to technological aspects of preparation and potential applications. Complex metallic alloys display several interesting properties, such as good corrosion resistance and high-temperature strength, however, they are brittle at room temperature in the singlephase form, which limits their use in engineering applications. A way to improve the room temperature ductility is the development of a heterogeneous microstructure combining a soft metallic matrix with second-phase CMA particles. The CMA particles act as strength-bearing component, while the metallic matrix supplies ductility.

Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 BMGs: Sample preparation, thermal stability and mechanical properties

Bulk amorphous Fe-based alloys with the nominal composition Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 have been obtained by copper mold casting in different shapes: cylindrical rods with diameters up to 3 mm, rectangular bars of 2mm×2mm and discs of 10mm diameter and 1mm thickness. These alloys exhibit good soft magnetic properties, characterized by low coercivity and high saturation magnetization. Besides the magnetic properties, the Fe_65.5Cr₄Mo₄Ga₄P₁₂C₅B_5.5 bulk metallic glass (BMG) shows a high glass transition temperature Tg, as well as a high crystallization temperature Tx, with an extension of the supercooled liquid region of around 65 K. The mechanical behavior was investigated by compression and Vickers hardness tests. The fracture strength for the as-cast samples σf is 2.8 GPa and the fracture strain εf is 1.9%. Upon annealing at 715K for 10 min, i.e. at a temperature below the calorimetric glass transition, the fracture strain drops to 1.6% and no plastic deformation is observed.

M. Stoica, J. Eckert, S. Roth, A.R. Yavari, L. Schultz, URL

Superior mechanical properties of FeCrMoVC

This work presents results on the microstructure and mechanical properties of the steel composition Fe_84.3Cr_4.3Mo_4.6V_2.2C_4.6 subjected to preparation conditions typically used for manufacturing of bulk metallic glasses. Thermodynamical aspects and kinetic limitations on the specific solidification process of phase formation, particularly those, which are strongly dominated by diffusion controlled mechanisms, promote the formation of nonequilibrium phases, such as martensite and complex carbide structures already in the as-cast state. This combination of high strength phases yields material with highly desirable properties, such as an engineering compression strength of more than 4000 MPa surprisingly combined with a fracture strain of about 12%.

Multi-phase complex metallic alloys

Within recent years, complex metallic alloys (CMAs) have been attracting much attention ranging from scientific curiosity about their complex structure to technological aspects of preparation and potential applications. Thanks to their peculiar structural characteristics, CMAs may become attractive candidates for structural applications.However, one major drawback for their use in engineering applications is the limited plastic deformability at room temperature.

Superior mechanical properties of FeCrMoVC

The alloy Fe_84.3Cr_4.3Mo_4.6V_2.2C_4.6 was produced using preparation conditions typically applied for manufacturing of bulk metallic glasses (BMGs) in order to study the microstructure and the mechanical properties in comparison to similar composed BMGs. Thermodynamical aspects and kinetic limitations on the specific solidification process of phase formation, particularly those, which are strongly dominated by diffusion controlled mechanisms, promote the formation of nonequilibrium phases, such as martensite, bainite and complex carbide structures already in the as-cast state. This combination of dispersed high strength and ductile phases yields material with highly desirable mechanical properties, such as an ultimate engineering compression strength of more than 4000 MPa combined with a fracture strain of about 20 %.

Advanced analytical glow discharges

Instrumental improvements resulted in improved stability and reliability of GD-OES at depth profiling. Applying these improvements we were able to obtain reproducible information about concentration depth profiles in thin films. One example was the determination of impurities in 100 nm electrochemically deposited Copper layers. Even molecules, like monolayers of thiourea adsorbed on a copper surface were detected in dc mode, thus confirming rf measurements from Shimizu [1].

ZrNbCuNiAl bulk metallic glass matrix composites containing dendritic bcc phase precipitates

We report on phase formation of a multicomponent Zr_66.4Nb_6.4Cu_10.5Ni_8.7Al₈ glass-forming alloy upon copper mold casting. A bcc phase embedded in a glassy matrix forms for moldcast bulk samples yielding an in-situ bulk metallic glass matrix composite upon slow cooling from the melt. Upon annealing, the first exothermic transformation of the material is related to precipitation of an icosahedral phase from the glassy matrix. The formation of the bcc phase-containing metallic glass composite is strongly governed by the alloy composition and the actual cooling rate during solidification. Room-temperature compression tests reveal significant yielding and plastic deformation before failure.

As-cast quasicrystalline phase in a Zr-based multicomponent bulk alloy

An icosahedral quasicrystalline phase is obtained directly from the melt by copper mold casting of a Zr₅₇Ti₈Nb_2.5Cu_13.9Ni_11.1Al_7.5 alloy. On the other hand, rapid quenching of the alloy leads to an amorphous phase. Upon annealing, the amorphous structure precipitates quasicrystals in the first stage of crystallization. The microstructure of the quasicrystalline state is quite different for the two preparation routes, which is correlated with the asymmetry of the nucleation and growth rate upon cooling or heating. The quasicrystals formed upon slow cooling from the melt have a size of about 1 mum. In contrast, the quasicrystals formed by annealing do not exceed a size of 5-10 nm.