Metastable Alloys

● The corrosion properties of selective laser melted (SLM) and of additionally heat-treated (T6) Al-3.5Cu-1.5 Mg–1Si samples were assessed in 0.001 M and 0.1 M NaCl solutions (pH 7) by means of electrochemical polarization measurements and SEM surface analysis. In both solutions these new alloy types exhibited improved corrosion behavior compared to wrought and heat-treated AA 2024 reference states. This is attributable to more refined microstructural states and to the formation of particular phases. The SLM samples comprise finely dispersed Q-phase particles which act cathodic and thus, trigger adjacent matrix phase dissolution. Additional T6 heat treatment was efficient for attaining the lowest corrosion current density of ~0.04 μA/cm² and a widening of the anodic passive range to ~400 mV in 0.1 M NaCl. Only this alloy state shows a repassivation ability upon reverse scanning. T6 treatment yields mainly the Mg₂Si phase acting as local anode and a widely passive Al-based matrix phase. Altogether, SLM processing with final heat treatment of the Al-3.5Cu-1.5 Mg–1Si alloy is suitable for achieving not only superior mechanical performance but also improved corrosion stability.

P. Wang, A. Gebert, …, K. Kosiba, U. Kühn, S. Scudino et al., Corrosion of Al-3.5Cu-1.5 Mg–1Si alloy prepared by selective laser melting and heat treatment, Intermetallics 124 (2020) 106871.
 

● Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glass specimen with nearly full density were fabricated by means of SLM. The SLM-BMG demonstrated wear and corrosion resistances which are very similar to those of as-cast BMG reference samples. From potentiodynamic polarization measurements, which were conducted in 0.01 M Na₂SO₄ and 0.1 M NaCl electrolyte, for the SLM samples a slightly reduced susceptibility to pitting corrosion and a better surface healing ability was derived. This may be attributed to an improved homogeneity of the additively manufactured glass.

L. Deng, A. Gebert, …, K.Kosiba, S. Pauly et al., Mechanical performance and corrosion behaviour of Zr-based bulk metallic glass produced by selective laser melting, Mater. Design 189 (2020) 108532.
 

● By means of selective laser melting (SLM) Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glass specimen with nearly full density were fabricated. The SLM-BMG demonstrated wear and corrosion resistances which are very similar to those of as-cast BMG reference samples. From potentiodynamic polarization measurements which were conducted in 0.01 M Na₂SO₄ and 0.1 M NaCl electrolyte for the SLM samples a slightly reduced susceptibility to pitting corrosion and a better surface healing ability was derived. This may be attributed to an improved homogeneity of the additively manufactured glass.

L. Deng, A. Gebert, …, K.Kosiba, S. Pauly et al., Mechanical performance and corrosion behaviour of Zr-based bulk metallic glass produced by selective laser melting, Mater. Design 189 (2020) 108532.
 

● The superposition of environmentally induced reactions and mechanical load gives rise to significant stress corrosion cracking (SCC) and corrosion fatigue (CF) phenomena, which are poorly understood at present for bulk metallic glasses. In early studies we analyzed basic interactions between shear bands and corrosion pits. The focus of a recent project was on the detailed analysis of SCC phenomena for cast Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ BMG samples in selected chloride-containing electrolytes. First, 3PB tests with un-notched bar specimen in solution under in situ control of the electrochemical response revealed significant effects of increasing applied elastic load levels and increasing anodic potentials towards a reduction of the total time to fracture. Further SCC tests with notched 3PB specimen conducted in 0.01 M NaCl solution demonstrated that due to corrosive attack the applied load to attain material failure load is much smaller than that determined when similar tests were performed in air. From fracture surface analysis and the in-situ monitored corrosion characteristics, it is concluded that pitting at surface defects is the typical initiation step followed by hydrogen embrittlement as the major failure cause for fracture under the harsh SCC test conditions. The chemical alloy composition with Zr and Cu as main constituents appears to be problematic as those facilitate the pitting process and enable the local generation of hydrogen. According to our present state of knowledge, the catastrophic stress corrosion failure evidenced for these BMG samples must be generalized to Zr–Cu-based BMG types with similar compositions.

A.Gebert, D. Geissler et al., Studies on Sress corrosion cracking of Vit105 Bulk metallic glass, Frontiers Mater. 7 (2020) 128.
D. Geissler et al., Catastrophic stress corrosion failure of Zr-base bulk metallic glass through hydrogen embrittlement, Corros. Sci. 159 (2019) 108057.
P.F. Gostin et al., Stress corrosion cracking of a Zr-based bulk metallic glass, Mater. Sci. Eng. A 639 (2015) 681.
A. Gebert et al., Interactions between mechanically generated defects and corrosion phenomena of Zr-based bulk metallic glasses, Acta Mater. 60 (2012) 2300.

● Ti-based metallic glass-forming alloys are mainly developed with view to biomedical applications. Appropriate design strategies comprise considerations regarding biological safety, i.e. the selection of non-toxic elements, and on the other hand, constituents which enhance the glass-forming ability. Typical examples are Ti₇₅Zr₁₀Si₁₅ and Ti₆₀Nb₁₅Zr₁₀Si₁₅. Via control of the rapid solidification conditions fully amorphous materials or bcc-Ti-phase reinforced metallic glass nano-composites can be obtained. Nb is beneficial for improving the glass-formation. Upon mechanical testing melt-spun ribbons showed an interesting combination of very high hardness values and moderate reduced elastic modulus values. This suggests these new materials for applications demanding high wear resistance. Those alloys are highly corrosion-resistant also in synthetic body fluids. Thermal and electrochemical oxidation treatments are feasible approaches to enhance the biocompatibility of the alloy surfaces.

Projekt: EU ITN BIOTINET
 

● Various Ti alloys were studied regarding the impact of their composition-dependent microstructure on mechanical and chemical/ electrochemical properties.

A cast AlNbTiZr high entropy alloy (HEA) with solid solution bcc dendrites and inter-dendritic Zr₂Al intermetallic phase exhibits high specific yield strength and high fracture strength. Thermo-gravimetric analyses and long-term exposure tests in synthetic air up to 1273 K were conducted to analyze the oxidation behavior. Formed oxide layers were protective, intact, and spallation did not occur.
Cast ultrafine-structured Ti_70.5Fe_29.5 and Ti_67.79Fe_28.36Sn_3.85 eutectic alloys comprising beta-Ti and TiFe phases were electrochemically analyzed in halide-free aqueous media (pH 1–13). Both alloys exhibited excellent corrosion resistance due to spontaneous passivation. The passive film composition was found to vary with the pH value of the test solution. Pitting corrosion was investigated in strongly acidic media (pH≤1) containing 0.1–3M of chloride ions. The Sn-containing eutectic alloy revealed a higher pitting susceptibility.
A Ti–Y–Al–Co phase separated metallic glass was employed as starting material to generate nanoporous metallic glass structures by means selective phase etching. The alloy state with nanoporous surface showed in synthetic body fluid an improved corrosion resistance.
Zr₆₆Nb₁₃Cu₈Ni_6.8Al_6.2 and Ti₆₆Nb₁₃Cu₈Ni_6.8Al_6.2 alloys with in situ formed nanostructured matrix and dendritic bcc phase were obtained by Cu mould casting. Both alloys show excellent passivity chloride-free solutions due to the formation of very protective passive films. But the Zr-based alloy was more prone  to chloride-induced localized corrosion. For both alloys as a key feature preferential dissolution of the Cu-enriched and Nb-depleted nanostructured matrix was observed.

J. Jayaraj et al., Microstructure, mechanical and thermal oxidation behavior of AlNbTiZr high entropy alloy, Intermetallics 100 (2018) 9.
R. Sueptitz et al., Corrosion and pitting behaviour of ultrafine eutectic Ti–Fe–Sn alloys, J. Alloys Compd. 503 (2010) 19.
J. Jayaraj et al., Nano-porous surface states of Ti–Y–Al–Co phase separated metallic glass, Intermetallics 17 (2009) 1120.
P.F. Gostin et al., Comparing the corrosion behaviour of Zr₆₆/Ti₆₆–Nb₁₃Cu₈Ni_6.8Al_6.2 bulk nanostructure-dendrite composites, Intermetallics 16 (2008) 1179.
 

● A series of bulk amorphous steels (BAS) with castability of up to several centimeters thickness and extremely high strength and hardness has raised interest for structural applications. The corrosion behavior of a prominent example, (Fe_44.3Cr₅Co₅Mo_12.8Mn_11.2C_15.8B_5.9)_98.5Y_1.5, was studied in different aqueous electrolytes. In acidic solutions, due to its single-phase nature, the BAS exhibits a much higher stability than the multiphase crystalline counterpart and the conventional steel X210Cr12. The acid corrosion process based on multiple micro-pit formation was analyzed in detail. With increasing pH value, the overall elemental composition of the alloy plays a more important role than structural particularities. The reactive elements Mo, Mn, Co are considered to cause a diminished corrosion stability of the BAS.

P.F. Gostin et al., Acid corrosion process of Fe-based bulk metallic glass,  Corros. Sci. 62 (2012) 112.
P.F. Gostin et al., Comparison of the corrosion of bulk amorphous steel with conventional steel, Corros. Sci. 52 (2010) 273.
 

● Amorphous alloys based on the Mg-TM-RE system (TM= transition metal; RE= rare earth) show in many environments significantly reduced corrosion rates and improved anodic passivity as compared to Mg and conventional Mg alloys. However, they are very sensitive to local degradation phenomena like pitting and filiform corrosion. Moreover, those amorphous (or related nanostructured) alloys exhibit a high capability for hydrogen absorption which may cause materials embrittlement. On the other hand, some alloys are suitable for hydrogen storage. Fundamental aspects of their hydrogen reactivity upon electrochemical or gas phase charging were investigated.

A. Teresiak et al., Influence of Co and Pd on the formation of nanostructured LaMg₂Ni and its hydrogen reactivity, J. Alloys Compd. 582 (2014) 647.
A.Gebert, Corrosion behaviour of magnesium (Mg)-based bulk metallic glasses, In “Corrosion of magnesium alloys”, ed. G.L. Song, Woodhead Publishing Limited, (2011), p.207.
B. Khorkounov et al., Improving the performance of hydrogen storage electrodes based on mechanically alloyed Mg₆₁Ni₃₀Y₉, J. Alloys Compd. 458 (2008) 479.
A.Gebert et al., Stability of rapidly quenched and hydrogenated Mg–Ni–Y and Mg–Cu–Y alloys in extreme alkaline medium, J. Alloys Compd. 419 (2006) 319.

Project: DFG