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Compact specimens were produced by selective laser melting (SLM) from glass-forming Zr52.5Cu17.9Ni14.6Al10Ti5 powder. Below a critical energy density of Ecrit = 15 J/mm3 fully glassy samples were obtained. When the energy density is higher, the metastable big cube phase, stabilized by oxygen, precipitates next to other crystalline phases. We introduce density contour maps to identify the process window, which allows obtaining fully glassy samples with a high relative density of up to 98.5%. The systematic parameter variation illustrates the importance of laser power for obtaining dense SLM parts. Moreover, the distribution of pores in the present alloy is rather sensitive to the scanning strategy. The compressive strength of the SLM samples is comparable to values measured for as-cast glasses and even a distinct plastic strain is found. Finally, scaffolds were produced and high-energy X-ray diffraction proves that they are fully glassy irrespective of differences in the local cooling rates. Our experiments contribute to better understanding the behaviour of Zr52.5Cu17.9Ni14.6Al10Ti5 during SLM and for making glass-forming alloys progressively more accessible to laser-based additive manufacturing in order to overcome the intrinsic size limitation of bulk metallic glasses. Keywords: Selective laser melting, Metallic glass, Porosity, Mechanical properties, High-energy X-ray diffraction

In this work, the effect of Si additions on the glass-forming ability (GFA) and the mechanical behaviour of a Ti42.5Cu42.5Ni7.5Zr7.5 alloy is investigated. An optimum GFA occurs for addition of 0.5 at% Si and it is explained based on a balance between two factors: an increased thermal stability of the supercooled liquid and the detrimental effect of Ti5Si3 on vitrification. On rapid cooling, composites consisting mainly of the cubic B2 phase and glass can form. Their plasticity increases with increasing Si additions without sacrificing their high yield strength. A characteristic core–shell microstructure is formed in the rod with 2 at% Si, for which the combination of an amorphous outer surface and a refined dendritic structure in the centre gives rise to a composite with enhanced mechanical properties.

In the present work, the wetting behaviour of the Cu–GaX (X = 10–30 wt.%) liquid alloys on 304L steel was investigated in vacuum at temperatures between 1123 and 1373 K. Contact angle and surface tension measurements were performed using the sessile drop setup. In general, the Cu–GaX (X = 10–30 wt.%) alloys show good or even excellent wetting on 304L steel. However, the wetting mechanism strongly depends on the Ga content. While for Ga-poor alloys secondary wetting occurs, there is a partial dissolution of steel grains combined with an austenite-to-ferrite transformation for larger Ga contents. Cu penetrates along the grain boundaries of the substrate and provides a path for Ga diffusion into the steel. The dissolution of steel grains at the interface is suggested to be responsible for slowing down of the spreading at elevated temperatures. Understanding these interrelations between microscopic and macroscopic effects is important for the development of new Cu-based liquid alloys containing Ga, especially for applications involving steels. Keywords: Brazing, Reactive wetting, Gallium, Copper, Steel, Phase transformation

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu 50 Zr 50−x Ti x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu 10 Zr 7 and Cu(Ti, Zr) 2 , of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu 10 Zr 7 , Cu(Ti, Zr) 2 , and ZrTiCu 2 crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.

The glass formation in the Ti–Cu system is revisited by means of the predictive γ* and λ + Δh1/2 amorphization criteria. Rapidly solidified ribbons of six selected compositions were investigated and in order to improve the glass-forming ability (GFA) minor amounts of Si were added to the best glass former predicted, Cu54Ti46. A series of (Cu0.54Ti0.46)100−xSix (x = 0.5, 1, 2) ribbons and rods with a diameter of 1 mm were prepared and analysed regarding their phase formation and thermal stability. For the alloy with the maximum GFA, (Cu0.54Ti0.46)99Si1, the critical diameter for glass formation is below 1 mm. Nevertheless, a good correlation was found between the experimentally determined GFA of the alloys and the prediction given by the λ+(Δh)1/2 and γ* criteria. The addition of Si decreases the liquidus temperature and promotes the precipitation of the Cu4Ti3 phase instead of γ-TiCu.

Three different Cu–Zr–Co alloys, namely Cu 40 Zr 37.5 Co 22.5 , Cu 42.5 Zr 45 Co 12.5 and Cu 49 Zr 49 Co 2 , were obtained by rapid cooling. The microstructure and phase formation of as-cast rods with diameters of 2 mm are compared with those of the respective ingots. An increasing Co content stabilises the B2 CuZr phase and leads to the precipitation of a ternary Cu–Zr–Co phase. The variation of the cooling rate affects the size of the B2 dendrites as well as the volume fraction and the morphology of the interdendritic phases. The mechanical properties were determined in compression and all alloys show a certain plastic deformability despite the presence of several binary and ternary intermetallic phases. The deformation mechanisms are discussed on the basis of the microstructures and the constituent phases.

Starting from the brittle Cu46Zr46Al8 bulk metallic glass (BMG), the effect of substituting Al by Ti on the glass-forming ability (GFA) and the mechanical properties is systematically investigated. The supercooled liquid region (ΔTx), the reduced glass transition temperature (Trg=Tg/Tliq) and the γ parameter (γ=Tx/(Tg+Tliq)) tend to decrease with increasing Ti content in the present Cu46Zr46Al8−xTix (0≤x≤8) alloys. An increasing distance from the eutectic point might be the origin of the reduced GFA after the Ti addition. At the same time, the Ti additions significantly increase the plasticity of the respective alloys. The Cu46Zr46Ti3.2Al4.8 bulk glassy alloy is found to have the largest plastic strain in compression of around 12% and a fracture strength up to 1950 MPa while the Ti-free Cu46Zr46Al8 BMG exhibits values of 0% and 1754 MPa, respectively. The statistics of the serrated flow behavior, which is closely related with the dynamics of shear bands, are also investigated. The macroscopic and microscopic mechanisms, which contribute to the enhancement of malleability, are analyzed.

We report on the effect of Al and Co alloying in vitreous Cu50Zr50 on local deformation and serrated flow as a model for relating the size and localization of shear transformation zones (STZ) to Poisson ratio and strain-rate sensitivity of metallic glasses. Alloying with Al results in significant variations in mechanical performance, in particular, in Young's modulus, hardness and strain-rate sensitivity. Increasing strain-rate sensitivity with increasing degree of alloying indicates a reduced tendency for shear localization. In parallel, a gradual transition from inhomogeneous to homogeneous plastic flow is observed. Using a statistical analysis of the shear stress associated with the initiation of the first pop-in in the load-displacement curve during spherical indentation, the activation volume for plastic flow at the onset of yielding is reported. This analysis is employed for experimental evaluation of the compositional dependence of activation barrier, size and distribution of STZs. It is demonstrated that the STZ size does not change significantly upon Al alloying and encompasses a local volume of around 22–24 atoms. However, the barrier energy density for the initiation of a single STZ progressively increases. The broader distribution of STZs impedes their accumulation into larger-size flow units, leading to a lower number and reduced size of serrations in the load-displacement curve. On the contrary, lower barrier energy densities enable a larger quantity of STZs to be activated simultaneously. These STZs can easily percolate into large flow units, promoting plastic flow through their interaction. We employ Poisson's ratio as an indicator for plasticity to shown that this interpretation can be transferred to other types of metallic glasses. That is, larger flow units were found for metallic glasses with higher Poisson ratio and more pronounced plasticity, while the flow units in alloys with very low Poisson ratio and high brittleness are significantly reduced in size and more homogeneously distributed throughout the material.

Due to their inherent metastability, metallic glasses crystallize upon thermal activation, which involves nucleation and growth. It is debatable, however, whether steady-state kinetics of crystal nucleation and growth apply when glasses are heated quickly. We partially devitrified a CuZr-based bulk metallic glass at heating rates in the range of 0.08–180 K s −1 . Immediate quenching after heating preserves the partially transformed microstructure and allows direct observation of crystallite sizes and numbers. The crystallization reaction changes from eutectic to polymorphic at high heating rates and uniformly dispersed metastable shape-memory crystals precipitate in the glass. A beneficial side effect of this microstructure is a strongly enhanced room-temperature deformability. The analysis of crystal populations suggests that the nucleation rate on fast heating is effectively lower than in the steady state. At the later stage of crystallization by flash-annealing, the glass sample is traversed by a thermal front that migrates almost ten times faster than the crystal-liquid interface in levitation experiments. This phenomenon is related to the high population of crystals already present. Flash-annealing, thus, not only permits tailoring of BMG composites to overcome the intrinsic brittleness of monolithic BMGs, but also permits study of the fundamental crystallization processes in highly undercooled melts.

In this work, the effect of cobalt on the phase formation and mechanical properties of rapidly solidified Cu50−xCoxZr50 (x = 2, 5, 10, and 20 at. pct) alloys was investigated. CuZr martensite forms in the case of low Co contents (x = 2 and 5 at. pct), while in the alloys with 10 and 20 at. pct Co, the B2 phase is stable even at room temperature. The deformation behavior of the rods under compressive loading depends strongly on the microstructure and, thus, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples, and deformation is accompanied by two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation.

Cu50−xCox Zr50 (x = 0, 0.5, 1, 2, 3, 4 and 5 at.%) alloys were cast in a cylindrical copper mold by a suction casting device. In order to investigate the thermal behavior of the as-cast rods, the samples were heated from 313 to 573 K and then cooled down to about 253 K. The structure of the samples was studied by X-ray diffraction and optical microscopy. Thermal cycling measurements were also done for alloys with 2, 3, 4 and 5 at.% cobalt. It was found that increasing the cobalt content decreases martensite (Ms) and austenite (As) start temperatures, while it increases the temperature region in which austenite is stable. Thermal cycling measurements revealed that by increasing the number of cycles, the austenite start temperature increases while martensite start temperature shifts to lower temperatures.

Ti-based bulk metallic glasses (BMGs) with large compressive plasticity were developed in the Ti-rich part of Vitreloy series BMGs (Ti65–x Zr x Cu9Ni8Be18 alloys with x = 0, 5, 10, 15, and 20). The current materials exhibit high fracture strength reaching ~2.3 GPa and plastic strains up to ~8.3 pct after partial substitution of Zr by Ti. The plasticity of the investigated alloys strongly depends on the Zr content, which affects the elastic constants, such as Poisson’s ratio and shear modulus. This, in turn, has an impact on the shear transformation zone (STZ) volume and, hence, on the shear banding of the glasses.

The correlation between the microstructure and mechanical behavior during tensile loading of Ti68.8Nb13.6Al6.5Cu6Ni5.1 and Ti71.8Nb14.1Al6.7Cu4Ni3.4 alloys was investigated. The present alloys were prepared by the non-equilibrium processing applying relatively high cooling rates. The microstructure consists of a dendritic bcc β-Ti solid solution and fine intermetallic precipitates in the interdendritic region. The volume fraction of the intermetallic phases decreases significantly with slightly decreasing the Cu and Ni content. Consequently, the fracture mechanism in tension changes from cleavage to shear. This in turn strongly enhances the ductility of the alloy and as a result Ti71.8Nb14.1Al6.7Cu4Ni3.4 demonstrates a significant tensile ductility of about 14% combined with the high yield strength of above 820 MPa already in the as-cast state. The results demonstrate that the control of precipitates can significantly enhance the ductility and yet maintaining the high strength and the low Young's modulus of these alloys. The achieved high bio performance (ratio of strength to Young's modulus) is comparable (or even superior) with that of the recently developed Ti-based biomedical alloys.

International audience; The structural evolution of Ti50Cu43Ni7 and Ti55Cu35Ni10 metallic glasses during heating was investigated by in-situ synchrotron X-ray diffraction. The width of the most intense diffraction maximum of the glassy phase decreases slightly during relaxation below the glass transition temperature. Significant structural changes only occur above the glass transition manifesting in a change in the respective peak positions. At even higher temperatures, nanocrystals of the shape memory B2-Ti(Cu, Ni) phase precipitate, and their small size hampers the occurrence of a martensitic transformation. (C) 2015 Author(s)

The phase formation and the glass-forming ability (GFA) of the Cu-Zr-Co system were studied. For the latter, predictions from a modified topological instability criterion were used to select six alloys. Five new glassy compositions were found and a good correlation was observed between the predicted GFA and characteristic thermal indicators. The addition of Co stabilizes a B2-type phase at room temperature in these alloys, which could be useful for the development of shape memory bulk metallic glass composites.

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.

We propose a strategy for the design of ductile bulk metallic glasses (BMGs) through minor substitution using relatively large atoms, which make the bonding nature become more metallic and with it less shear resistant. Such a locally modified structure results in topological heterogeneity, which appears to be crucial for achieving enhanced plasticity. This strategy is verified for Ti-Zr-Cu-Pd glassy alloys, in which Cu was replaced by In, and seems to be extendable to other BMG systems. The atomic-scale heterogeneity in BMGs is somewhat analog to defects in crystalline alloys and helps to improve the overall plasticity of BMGs.