U. Kühn

ZrNbCuNiAl bulk metallic glass matrix composites containing dendritic bcc phase precipitates

We report on phase formation of a multicomponent Zr66.4Nb6.4Cu10.5Ni8.7Al8 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.

Structural bulk metallic glasses with different length-scale of constituent phases

Abstract Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.

Modeling deformation behavior of Cu–Zr–Al bulk metallic glass matrix composites

In the present work we prepared an in situ Cu47.5Zr47.5Al5 bulk metallic glass matrix composite derived from the shape memory alloy CuZr. We use a strength model, which considers percolation and a three-microstructural-element body approach, to understand the effect of the crystalline phase on the yield stress and the fracture strain under compressive loading, respectively. The intrinsic work-hardenability due to the martensitic transformation of the crystalline phase causes significant work hardening also of the composite material.

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 Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 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 μm. In contrast, the quasicrystals formed by annealing do not exceed a size of 5–10 nm.

Correlation between the microstructures and the deformation mechanisms of CuZr-based bulk metallic glass composites

The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-“double”-to-“triple”-double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.

Correlation between glass-forming ability, thermal stability, and crystallization kinetics of Cu-Zr-Ag metallic glasses

The glass-forming ability (GFA) of the Cu-Zr-Ag system is evaluated based on the large amount of literature data available and discussed in the frame of a predictive amorphization criterion which combines topological instability and electronic criteria. The correlation between GFA, thermal stability, and crystallization kinetics of (Cu0.5Zr0.5)100−xAgx (xu2009=u20090, 2, 6, and 10) metallic glasses is further investigated. The enhancement of the GFA of the alloys and the thermal stability/fragility of the supercooled liquid can be traced back to a large size effect/volume mismatch and electronic effects. However, the apparent activation energy of crystallization decreases with increasing Ag content in the alloys which may be due to a nanoscale microstructural heterogeneity induced by the Ag addition. At a certain Ag content, a small amount of AgZr crystals precipitate together with Cu10Zr7 and CuZr2 and the crystallization mechanism changes from interface-controlled one-dimensional growth to three-dimensional gro...

Nanostructured Zr- and Ti-based composite materials with high strength and enhanced plasticity

Multicomponent composite materials with the compositions Zr66Nb13Cu8Ni6.8Al6.2 and Ti66Nb13Cu8Ni6.8Al6.2 were produced by copper mold casting, and their microstructure and their room-temperature mechanical properties were investigated. The specific alloys were developed to circumvent the limited ductility of Zr- and Ti-based bulk metallic glasses by the formation of a heterogeneous microstructure consisting of a nanocrystalline matrix and ductile dendritic bcc precipitates. Comparing the microstructure of both alloys, two significant differences were observed. The volume fraction of the dendritic bcc phase is higher for the Ti-based alloy and the formed interdendritic matrix phase(s) have a different structure. The two alloys show an excellent combination of strength and plastic strain. Especially the Ti-based alloy exhibits exceptional mechanical properties, such as high fracture stress of more than 2000MPa and a plastic elongation to failure of almost 30%.

Stable fracture of a malleable Zr-based bulk metallic glass

We report a stable fracture phenomenon during the compression of a malleable Zr-based bulk metallic glass. In the process, the shear band along which the sample fails is constrained by the machine crosshead, thus causing a slow release of the stress and the elastic energy by small steps in the stress-strain curve. A novel and unique fishbone-like pattern was observed on the fracture surface after the final rupture instead of the typical vein-like pattern usually found upon catastrophic failure for metallic glasses. The formation of this pattern might be a result of the modest temperature rise during failure and the frustration of the meniscus instability in the crack tip due to stress redistribution in the constraint. This fracture behavior, where the crack propagation is at a much lower speed and the elastic energy is released in a stable way, might be suitable for studying the crack propagation process and the fracture mechanism in metallic glasses.

Microstructure evolution upon devitrification and crystallization kinetics of Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 melt-spun glassy ribbon

The crystallization behavior of glassy Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 produced by melt spinning was investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, and atom probe investigations. The devitrification of the as-spun ribbon occurs by primary crystallization of a metastable nanoscale quasicrystalline phase during the first stage of the crystallization process, followed by successive transformation into intermetallic compounds at higher temperatures. The kinetics investigation reveals that quasicrystal formation is characterized by two overlapping processes: the first step probably linked with the redistribution of one or more elements, most likely Al and Zr, between the quasicrystalline phase and the remaining amorphous matrix, and the second step corresponding to the crystallization itself. Furthermore, a higher value of the activation energy for quasicrystal formation compared to other quasicrystal-forming alloys suggests an increased complexity of the crystallizati...

Medium range ordering and its effect on plasticity of Fe–Mn–B–Y–Nb bulk metallic glass

Glass formation, thermal properties and mechanical behavior were investigated in a series of Fe65− x Mn13B17Y5Nb x (x = 0, 3, 5 and 7) alloys. Appropriate partial substitution of Fe by Nb in a Fe65Mn13B17Y5 bulk glassy alloy simultaneously enhances the glass-forming ability and the mechanical properties of the alloys. The Nb-containing glassy alloys exhibit an additional exothermic event in the supercooled liquid region. This exothermic reaction is correlated with the formation of medium range ordered (MRO) clusters originated from the large difference in mixing enthalpy between constituent elements such as Nb–Y (+30 kJ/mol) and Nb–B (−39 kJ/mol). The presence of MRO clusters in the as-cast state is dependent on the cooling condition from the liquid state, and plays a crucial role in providing plasticity in as-cast Fe–Mn–B–Y–Nb bulk metallic glass.