Synthesis and characterization of amorphous Zr-based alloys with Ta and W additions

A reference Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ bulk metallic glass as well as a number of alloys obtained by addition of refractory elements Ta or W (combined with Sn) to the main Zr–Cu–Al–Ni system were elaborated by mould casting, twin roll casting and planar flow casting. The chemical compositions were chosen according to empirical rules, and as combinations of the binary eutectics for strongly interacting elements, taking into account the corresponding enthalpies of mixing. Optical microscopy, X-ray diffractometry, differential thermal analysis, differential scanning calorimetry, scanning electron microscopy and tensile mechanical testing were used to characterize the fully and partially obtained amorphous materials. Correlation of glass forming ability, thermodynamic parameters, crystallization behaviour and mechanical properties with chemical composition and production technology is discussed.     

Reliability of the plastic deformation behavior of a Zr-based bulk metallic glass

It is important to interpret the scattering of the plastic deformation behavior data for the structural-applications of bulk metallic glasses (BMGs), however, few studies have focused on statistical analysis of the variation and reliability of the plastic deformation behavior of BMGs. In this work, statistical analyses show unavoidable large variations in the maximum nominal strains of as-cast BMGs, although they exhibited greatly-enhanced average values of the maximum nominal strains with reduced sample sizes and in the presence of stress gradients. The large variations are attributed to the intrinsic variability of the atomic arrangements stemming from the solidification processes. Nevertheless, the investigations show enhanced cut-off nominal strains (safety threshold) in the specimens with stress gradients. The findings suggest that, despite large variations in the plastic deformation behavior, BMGs are still reliable in practical structural-applications where the materials always deform under more complex stress states.     

Effect of the volume fraction of the ex-situ reinforced Ta additions on the microstructure and properties of laser-welded Zr-based bulk metallic glass composites

To investigate the effect of the volume fraction of the ex-situ reinforced Ta additions on the weldability of Zr–Cu–Ag–Al bulk metallic glass composites (BMGCs), in this study, different Ta contents (0–6 vol%) of BMGCs are welded using the Nd:YAG pulsed laser technique with preselected welding parameters. After welding, the microstructure (including the parent material (PM), weld fusion zone (WFZ) and heat-affected zone (HAZ)), mechanical and thermal properties of the test samples are investigated.The test results show, for all BMGC welds, the micro-sized Ta particles in the PM, WFZ and HAZ to be covered by a crystallized interfacial layer (IL), ZrCu. For both un-welded and laser-welded BMGCs, as the Ta contents increase, the glass transformation temperature (T_g) increases, which in turn reduces the glass formation ability (GFA) indices, ΔT_x, γ and γ_m. However, when compared to that of un-welded BMGC, the GFA index, ΔT_x, of the laser-welded BMGCs is slightly improved. However, the γ, and γ_m of the BMGC welds seem not to be affected.In addition, due to the characteristics of the rapid thermal cycle of the laser welding process, two smaller sizes of Ta, nano-sized (mainly on the surface of WFZ) and sub micro-sized Ta, are found in the WFZ. These sub-micro-sized Ta particles normally locate near the micro-sized Ta, which tends to slightly reduce the hardness in this area.Furthermore, an increase in the volume fraction of Ta (0–6 vol%) in the BMGCs does not encourage the formation of the harmful crystalline phase in the amorphous matrix after the laser welding process. It is observed that, other than the IL (ZrCu) on the micro-sized Ta particles, no other type of crystalline is observed in the amorphous matrix of the laser-welded BMGCs.     

Enhancement of plasticity in Zr-based bulk metallic glasses electroplated with copper coatings

Electrodeposition was used to coat copper films on the surface of the BMG pillars (bulk metallic glasses) of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (Vit. 105) with the film thicknesses of 71.5 and 161.1 μm. The experimental results of the compression tests of the bare Vit. 105 pillars and the coated Vit. 105 pillars revealed that the copper costing increased the density of shear bands in the Vit. 105 pillars formed during the tests, resulting in the improvement of plasticity. The plastic strain was 6.1% for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7% of the bare Vit. 105 pillars. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the initiation and formation of multiple shear bands. The technique developed in this work provides an effective way to enhance the plasticity of BMGs at room temperature.     

Casting effect on softening of metallic glasses

Cooling rate-induced softening in Y-based metallic glasses were verified in this work. By varying the speed of the roller in the melt-spinning process, it is found the hardness of the ribbon decrease with the cooling rate. Nanoindentation test also indicated that the ribbon at a higher cooling rate shows higher deformation energy. Contrary to crystalline materials, whose hardness usually increases with the cooling rate, the softening of metallic glasses is attributed to high concentration of defects frozen by fast cooling, and thus more structural relaxation during deformation.     

Nanostructure controlling in Zr-based metallic glasses using icosahedral local structure

Glass-nano(quasi)crystal composite materials based on Zr–Al–Ni–Cu metallic glasses have been synthesized by controlling the nucleation and growth rates of the precipitated phase correlated with a unique icosahedral local structure. It is well known that the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass has a high glass-forming ability (GFA), which enables us to produce the glassy sample with a bulky shape. Controlling a substitution of QC-forming elements such as noble metals with Cu and annealing condition, the nanoscale icosahedral quasicrystalline phase (I-phase) with various grain sizes and nucleus densities can be formed. Moreover, we have succeeded to control the nano-QC phase nucleation by changing the atmosphere pressure during casting, which results in the formation of new bulk metallic glasses (BMGs). These nanoscale structure and nucleation controlling techniques in BMGs bring a significant improvement of mechanical properties such as high strength and good ductility.     

Room-temperature flow in a metallic glass - Strain-rate dependence of shear-band behavior

The rate dependence of serrated flow in amorphous Al_86.8Ni_3.7Y_9.5 has been investigated by nanoindentation. Three samples, containing different initial amounts and distributions of free volume, were used: as quenched, cold rolled and annealed below the crystallization temperature. When the cold-rolled sample is indented at low rates, no new shear bands form, and stable, time-dependent, flow takes place at pre-existing shear bands, well below the glass transition temperature. Aside from instrumental resolution, factors that likely affect serrated flow include the magnitude of the yield drop and the shear-band propagation velocity. The trends we observe are compared with calculations based on the free-volume theory.     

In-situ synthesis and mechanical properties of Zr-based bulk metallic glass matrix composites manipulated by nitrogen additions

Our study investigates in-situ synthesis and mechanical properties of Zr-based bulk metallic glass (BMG) matrix composites via arc plasma-induced accelerated displacement reaction (APADR) process. The aluminum nitride precursor under arc plasma-induced ultra-high temperature results in higher contents of dissolved nitrogen as well as precipitation of zirconium nitride (ZrN) particles in a Zr-based amorphous matrix. The nitrogen in the matrix results in a decrease of crystallization resistance (lower T_x and reduced glass-forming ability), but an increase of mechanical stability (a decrease of strain burst sizes). In particular, in-situ formed ZrN, which exhibits a homogeneous distribution and strong interfacial bonding with the matrix, causes an increase in compressive fracture strength and significant plastic deformation in the composite compared with the monolithic BMG. The formation of multiple shear bands and the enhancement of shear band interactions by the dissolved nitrogen as well as the in-situ formed ZrN particles were carefully confirmed by a statistical analysis on serrated flows. These results give us a guideline on how to manipulate nitrogen contents and fabricate in-situ BMG matrix composites with improved mechanical properties via APADR process.     

Mechanical heterogeneity and its relation with glass-forming ability in Zr-Cu and Zr-Cu-Al metallic glasses

Molecular dynamics simulations of nanoindentation using a spherical indenter were adopted to quantitatively probe the local mechanical heterogeneity (MH) in Zr-Cu and Zr-Cu-Al metallic glasses. Distinct MH at the nanometer scale has been revealed by statistically analyzing the first pop-in stress of different regions in the metallic glasses with an indenter of 4 nm in diameter. More interestingly, it is found that the degree of MH has a close relation to glass-forming ability of alloys, i.e., the smaller MH, the better glass-forming ability. Our findings not only shed light on the intrinsic feature of atomic structures, but also have important implications in understanding the structure-property relationship of MGs.     

Composition formulas of Ni-(Nb,Ta) bulk metallic glasses

It is known that bulk metallic glass compositions can be well interpreted by cluster formulas for stable liquids [cluster](glue atom)_{1 or 3}, where the clusters are derived from relevant devitrification phases. In the present work, the glass forming composition formulas in Ni-Nb and Ni-Ta systems are fully examined. The procedures include choosing the appropriate eutectic crystalline phases, selecting the principal clusters via the criteria such as spherical periodicity and cluster isolation degree, and determining the glue atoms. After strictly following these procedures, the best glass forming compositions Ni₆₂Nb₃₈ in Ni-Nb system and the range 59–62 at.% Ni in Ni-Ta system are explained by formulas [Ni-Nb₄Ni₈]Nb₂Ni and [M-Ni₆Ta₆]Ni₃ (M is a random mixture of Ni and Ta) respectively.     

Correlation between Poisson ratio and Mohr–Coulomb coefficient in metallic glasses

Yielding in metallic glasses is often described in terms of the Mohr–Coulomb criterion, τ + μσ ≥ τ₀. It suggests that a material yields when a combination of shear (τ) and normal stresses (σ), linked through the friction coefficient (μ), reaches the critical shear stress (τ₀). In this paper, an increase of the friction coefficient for increasing Poisson ratio is foreseen, if the elastic limit observed under uniaxial (tensile or compressive) and shear stresses remain constant. Experimental values of Young and shear moduli and Poisson ratio of metallic glasses have been collected from the literature. Compressive and tensile yield stresses have been also collected and elastic limits have been calculated. The elastic limit observed under compressive stress decreases with increasing Poisson ratio and it appears similar for metallic glasses based on the same metals. The values of the friction coefficient for metallic glasses have been obtained from the fracture angle observed under uniaxial stress (compressive or tensile) and from the ratio between compressive and tensile strength. Experimental data of the friction coefficient appear rather scattered and a clear trend as a function of Poisson ratio cannot be outlined. The increase of toughness of metallic glasses with increasing Poisson ratio has been ascribed to the corresponding reduction of the compressive elastic limit.     

Tribological behavior of Zr-based bulk metallic glass sliding against polymer,ceramic, and metal materials

The tribological properties of zirconium-based bulk metallic glass (BMG) sliding against polymers, steels, and ceramics at different loads and speeds were investigated. Acoustic emission (AE) technology was used to analyze wear states. The frictional coefficients of the BMG slid against the steel and ceramic balls were high but decreased with increased applied normal load and sliding speed. As the steel balls were more ductile than the ceramic ones, the steel–BMG sliding pairs generated weaker AE signals and exhibited larger wear rates. The BMG tested against the polymer balls had much lower and more stable frictional coefficients than the ones against the steel or ceramic balls because of transferred polymer layers on the BMG surfaces. The BMG against the polymer balls also exhibited the highest AE signals among the three types of counter materials used, indicating that abrasive wear dominated in the polymer-BMG sliding pairs. These results demonstrated the potential application of the BMG as a new tribomaterial that could be an alternative to the traditional crystalline metals for various counter materials.     

Intrinsic versus extrinsic effects on serrated flow of bulk metallic glasses

In this work, we systematically investigate the serrated flow behavior in the compression of bulk metallic glasses by varying the intrinsic composition and various extrinsic material and experimental factors including the sample size, the strain rate, and the testing machine stiffness. We find that the serrated flow, characterized by the amplitude of load serrations, can be suppressed for the higher Young's modulus, larger sample size, higher strain rate, and larger testing machine stiffness, respectively, and that it could completely disappear at certain critical strain rates. Meanwhile, the shape of serrated flow, which tends to become more “blunt”, manifests as the increasing ratio of the duration time to the awaiting time of the serrated events. The dependence of the serrated flow on these various factors is interpreted from the stick-slip dynamics of a single dominant shear band in compression process and can be condensed into a unified theoretical parameter k/k_cr, where k is a parameter dependent on the Young's modulus, the sample size and the machine stiffness, and k_cr is expressed as a function of temperature and testing strain rate. The implication of the stick-slip shear band dynamics together with the tuning of these material factors and test parameters will lead to the design of ductile BMGs.     

Tribological behaviors of Zr-based bulk metallic glass versus Zr-based bulk metallic glass under relative heavy loads

As a novel engineering material, bulk metallic glass (BMG) has received much attention. However, the knowledge concerning the tribological behavior of BMG versus BMG under relatively heavy loads is still insufficient. In this study, Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 metallic glass pins and discs were prepared by copper-mold suction casting. The dry sliding friction and the wear characteristics of the as-cast Zr-based BMG versus Zr-based BMG were tested under loads of 100, 125 and 150 N, respectively, using a pin-on-disc tribological apparatus at room temperature. The worn surfaces were studied by scanning electron microscopy in order to identify the wear mechanisms. The results showed that both the coefficient of friction and the wear rate increased with both the normal load and the rotational sliding velocity. X-ray diffraction patterns recorded after the tribological experiments indicated that no sliding-induced crystallization occurred. Transmission electron microscopy was also used to confirm the amorphous of the BMGs after sliding tests. In addition, the wear mechanisms changed with the experimental conditions. For a normal load of 100 N, the main mechanisms were abrasive wear, slight grooves and micro-cracks. For higher loads, adhesive wear was predominant, accompanied by abrasive wear and deeper grooves and more micro-cracks. When the rotational sliding velocity was increased, the dominant wear changed from slight grooves to viscous flow and adhesive wear.     

Microstructure and mechanical properties of a spray-formed Ti-based metallic glass former alloy

A Ti_45.8Zr_6.2Cu_39.9Ni_5.1Sn₂Si₁ composite plate was spray deposited on a copper substrate. From the bottom substrate-contact surface to the upper free surface of the deposit, a layered microstructure evolution in the order of fully amorphous (for the region 1-2 mm perpendicularly away from the substrate), amorphous/nanocrystalline (3-4 mm from the substrate), ultrafine-grained crystalline (5-6 mm from the substrate) and micron-sized crystalline phases (7-8 mm from the substrate) was observed. The oversprayed powders bellow 50 μm exhibit fully amorphous structure, while the ones above 50 μm show certain crystallization behavior. The fracture strength of 1.58-1.85 GPa with obvious plastic strain can be achieved under compressive tests for the spray-formed deposit. Spray forming can therefore produce bulk-sized high strength Ti-based alloy which evolves gradually from certain non-equilibrium towards equilibrium during deposition, which were considered to be attributable to the chill effect at the bottom substrate-contact surface and the following heat entrapment from the successively deposited droplets or powders.     

Microstructure and mechanical properties of metallic glass/metallic glass composites

We report microstructure and mechanical properties of bulk metallic glass (BMG)/metallic glass composites fabricated by mechanical alloying with subsequent consolidation process. The microstructural investigations of a bulk composite reveal that a submicron-scale layered structure with irregular interfaces consists of three amorphous phases in tornado-like morphology. Based on these results, poor plasticity of the metallic glass composite can be understood possibly due to the irregular interfacial morphology of the submicron-scale heterogeneous amorphous phases throughout the materials.     

Local structure of metallic glasses as seen by mechanical hydrogen relaxation

As an application of hydrogen as a probe, the literature on the damping peak of hydrogen in about 30 amorphous alloys was examined with respect to the question of systematic structural differences between the different types of metallic glasses. A direct plot of measured peak temperatures, corrected to the same frequency, as a function of the peak height turned out to be most suitable for this purpose. Most of these data fall into a 40 K wide band, with no distinction between types like ‘metal–metalloid’, ‘metal–metal’, or ‘bulk’ glasses. This general result suggests, in particular, that the outstanding thermal stability of bulk metallic glasses does not come from a specific amorphous structure. Stronger deviations from the general behaviour, as observed very clearly for Pd-based glasses and tentatively also for phase-separating systems, still need a closer examination.     

Preparation and properties of quaternary CoMoPB bulk metallic glasses

A new series of Co_80−xMo_xP₁₄B₆ (x = 7, 9, and 11 at%) bulk glassy alloys were successfully prepared by a combination method of fluxing treatment and J-quenching technique. The glass-forming ability (GFA) of the obtained Co-based alloys is sensitive to the Mo content substituted for Co, and the maximum attainable diameter for a fully amorphous state can reach 4.5 mm at x = 9. The compressive tests show that the obtained Co-based BMGs exhibit a compressive strength of 3.3–3.9 GPa, but nearly zero compressive plasticity. The new Co-based BMGs possess good soft magnetic properties, and their saturated magnetization values decrease from 47 emu/g (0.45 T) to 14 emu/g (0.14 T) with increasing the content of the Co substitute from 7 at% to 11 at%, which may be attributed to the anti-ferromagnetic coupling between the Mo and Co atoms. Because of their good GFA, high Co content, few constituting elements, and relatively high strength, the obtained Co-based BMGs (especially Co₇₁Mo₉P₁₄B₆ BMG) can be considered promising as starting alloys to develop the new Co-based BMGs for the advanced structural and functional applications.