CuZr-based Bulk Metallic Glass Research Articles

Developing centimeter-sized CuZr-based metallic glass composites via multi-element microalloying

A multi-element microalloying strategy has been implemented to manipulate the microstructure of CuZr-based bulk metallic glass composites (BMGCs) and, in turn, to improve their mechanical properties. It is found that microalloying with a combination of Ti, Hf, Co, Ni and/or Nb/Sn can effectively refine the austenitic B2–CuZr phase, resulting in finely dispersed B2 crystallites embedded in the BMG matrix while enlarging the sizes of BMGCs. Particularly, an optimum BMGC, i.e., Cu40.75Zr42Al4Ag5Sn0.75Co0.5Ti5Hf1Ni1, can be made by copper-mold casting in the range of diameter from 6.5 to 13.5 mm, much wider than those of BMGCs obtained from microalloying with merely one or two elements. Meanwhile, this centimeter-sized BMGC possesses good mechanical properties with a fracture strength as high as 1960 MPa and a compressive plastic strain up to 6.8 %. Our results demonstrate that multi-element microalloying would reduce the driving force for nucleation of B2 by lowering the melting entropy, and slow down the B2-growth kinetics by promoting faceted crystal growth while retarding multi-component diffusion, both accounting for the making of centimeter-sized BMGCs that contain finely dispersed B2 crystallites in the glass matrix.

Effect of cooling rate during solidification on structure and mechanical properties of Cu45Zr48Al7 glass-forming alloy

The CuZr-based bulk metallic glass matrix composites have attracted great attention in recent years owing to their unique mechanical properties compared to bulk glassy samples. The vital effect of B2 CuZr phase on mechanical properties was previously confirmed in CuZr-based alloys with equiatomic concentrations of copper and zirconium. In this paper, the structures and mechanical properties of the Cu45Zr48Al7 alloy, which has hypereutectoid composition with respect to stoichiometric B2 CuZr phase, were studied. This alloy exhibits high glass-forming ability, but low tendency to form the B2 CuZr phase during solidification. However, we demonstrated that depending on the cooling rate during solidification, governed by sample diameter, either bulk metallic glass or bulk metallic glass matrix composite can be produced. In order to allow partial crystallization of the B2 phase during solidification with the volume fraction allowing to observe the strain hardening effect, the minimum cooling rate should be lower than 40 K/s. The composite consisting of a glassy outer layer and a crystalline core composed of the B2 phase exhibits fracture strength of the same level as the bulk glassy sample (above 1800 MPa) obvious work-hardening with a plastic deformation of about 6%.

High temperature deformation behaviors of a CuZr-based bulk metallic glass composite

In this study, the deformation behaviors of a Cu47.5Zr48Al4Nb0.5 bulk metallic glass composite (BMGC) in the temperature ranging from 663 K to 763 K have been investigated using uniaxial tensile tests and high-energy X-ray diffraction. Within the supercooled liquid region (SLR), the studied BMGC exhibits superplasticity and the deformation of the studied BMGC basically involves two modes, i.e., homogeneous and inhomogeneous deformation. As the temperature increases in the SLR, the yield strength of the studied BMGC gradually decreases whereas the plastic strain increases, revealing that the amorphous phase plays a dominant role in the deformation. However, there is a significant increase in the yield strength above the crystallization temperature, which indicates that the deformation is mainly contributed by the crystalline phase. The results obtained in this manuscript provide a better understanding of the temperature dependence of deformation behaviors in BMGCs.

Strong dependency of the tribological behavior of CuZr-based bulk metallic glasses on relative humidity in ambient air

Thanks to their outstanding mechanical properties, Bulk Metallic Glasses (BMGs) are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission. However, the tribological properties of BMGs are still poorly understood, mostly because their amorphous nature induces counter intuitive responses to friction and wear. In the present study, four different BMGs (Cu47Zr46Al7, Zr46Cu45Al7Nb2, Zr60Cu28Al12, and Zr61Cu25Al12Ti2) underwent ball-on-disc friction tests against 100Cr6 steel balls (American Iron and Steel Institute (AISI) 52100) at different relative humidities (RHs) ranging from 20% to 80%. Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG. Interestingly, the friction coefficient decreased by a factor of 2 when the humidity was increased, and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface, composed of oxidized wear particles originating from the ball. The morphology of this tribolayer is highly correlated to humidity. The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.

Structural evolution of a CuZr-based bulk metallic glass composite during cryogenic treatment observed by in-situ high-energy X-ray diffraction

In-situ high-energy X-ray diffraction and molecular dynamic simulation were employed to study the structural evolution of a B2 phase reinforced CuZr-based bulk metallic glass composite in different structural states during cryogenic treatment between 300 K and 100 K. No phase transformation occurs in the as-cast sample during cryogenic treatment, while the pre-strained sample undergoes martensitic transformation and reverse martensitic transformation. The ambient temperature, uniaxial stress, and hydrostatic pressure on the B2 CuZr phase are closely related to the phase transformation behavior of the CuZr phase in the pre-strained composite sample. Molecular dynamic simulation confirms the experimental results. The findings in this work will help to clarify the mechanism of the martensitic transformation in the B2 CuZr phase, and finely tune the structure and mechanical behaviors of CuZr-based bulk metallic glass composites.

Determining deformation behaviors in a CuZr-based bulk metallic glass composite

In this work, deformation behaviors of a CuZrAlNb bulk metallic glass composite (BMGC) with different volume fractions of the crystalline phase were characterized using in-situ tensile tests in an optical microscope. For the studied composite, it was found that the volume fraction of the crystalline phase plays a significant role in shear band types. Shear bands which are not restrained by the crystalline phase, disappeared with increasing the crystallization volume fraction. There exists a critical volume fraction (~ 29.63 %) of the crystalline phase determining deformation behaviors of the studied composite. Beyond the critical value, this composite exhibits the homogeneous plastic deformation. Otherwise, the deformation of the BMGC is affected by shear bands which are not restrained by the crystalline phase, resulting in a decrease in the plasticity of the BMGC. Besides, the crystalline phase with a large-size bears more strain than that with a small-size in BMGCs, thereby improving their mechanical properties.

CuZr-based bulk metallic glass and glass matrix composites fabricated by selective laser melting

Monolithic bulk metallic glass and glass matrix composites with a relative density above 98 % were produced by processing Cu46Zr46Al8 (at.%) via selective laser melting (SLM). Their microstructures and mechanical properties were systematically examined. B2 CuZr nanocrystals (30–100 nm in diameter) are uniformly dispersed in the glassy matrix when SLM is conducted at an intermediate energy input. These B2 CuZr nanocrystals nucleate the oxygen-stabilized big cube phase during a remelting step. The presence of these nanocrystals increases the structural heterogeneity as indirectly revealed by mircrohardness and nanoindentation measurements. The corresponding maps in combination with calorimetric data indicate that the glassy phase is altered by the processing conditions. Despite the formation of crystals and a high overall free volume content, all additively manufactured samples fail at lower stress than the as-cast glass and without any plastic strain. The inherent brittleness is attributed to the presence of relatively large pores and the increased oxygen content after selective laser melting.

Temperature-dependent deformation behavior of a CuZr-based bulk metallic glass composite

In this study, the effect of cryogenic testing temperatures on the deformation behavior of a CuZr-based bulk metallic glass composite (BMGC) has been investigated using in situ high energy X-ray diffraction. The yield strength and work-hardenability of the studied BMGC show an increasing trend from 298 K to 153 K. A higher yield strength at a lower temperature can be attributed to the larger difference in the volume shrinkage between the crystalline phase and the amorphous matrix. The increasing martensitic transformation degree of the crystalline phase from 298 K to 153 K results in the enhanced work-hardenability. These results lead to a better understanding of the temperature dependence of phase-transformation induced plasticity in CuZr-based BMGCs.

Shear transformation zone dependence of creep behaviors of amorphous phase in a CuZr-based bulk metallic glass composite

The creep behaviors of the amorphous phase in a CuZr-based bulk metallic glass composite (BMGC) are studied by nano-indentation. Samples fabricated via higher cooling rates are found to exhibit more prominent creep, but a smaller shear viscosity. The volume of the shear transformation zones (STZs) in the amorphous phase calculated based on a cooperative shear model increases with the cooling rate. The evolution of excess free volume created during creep deformation is clarified. A looser atomic arrangement leads to a larger STZ volume, thus facilitating creep deformation. This study gives a better understanding of the deformation behaviors of the amorphous phase in BMGCs.

Effect of Sample-Size on the Compression Deformation Behavior of a CuZr-Based Bulk Metallic Glass Composite

The compressive deformation behaviors and microstructures of Cu45Zr48Al4Nb3 bulk metallic glass composites with diameters of 3, 2, and 1 mm were investigated systematically. It was found that the smallest sample showed the highest yield strength and compressive plasticity. The yield strength of the samples was found to depend on the fraction of their crystalline phases in the glassy matrix. The smaller samples showed larger free volumes, which is favorable for plastic deformation. The deformation behavior of the samples was found to depend on their size. The results obtained in this study will be helpful for investigating the mechanical behavior of metallic glass composites.

In situ high-energy X-ray diffraction study of thermally-activated martensitic transformation far below room temperature in CuZr-based bulk metallic glass composites

Abstract The martensitic transformation within B2 phase plays a critical role for the ductility of CuZr-based bulk metallic glass (BMG) composites, yet the underlying mechanism for the martensitic transformation process needs further investigations. Here, we found that the onset temperature of the thermally-activated martensitic transformation for CuZr-based BMG composites was reduced to 162.7 ± 2.5 K, which is far lower than those of polycrystalline samples, due to the stabilization of the austenitic phase induced by crystalline-amorphous interfaces. In situ high-energy X-ray diffraction measurements (HEXRD) further demonstrate that the thermally-activated martensitic transformation is governed by an extremely incomplete martensitic transformation from B2 to B19’ and B33 phases without the decomposition of B2 phase which generally appears in polycrystalline samples. Due to unconfined stress states during the thermally-activated martensitic transformation, B33 phase forms more early during thermal process than deformation process. The present study provides a further understanding on the martensitic transformation behaviors of shape memory type CuZr-based BMG composites.

Effect of mechanically induced structural rejuvenation on the deformation behaviour of CuZr based bulk metallic glass

Severe plastic deformation of bulk metallic glasses leads to rejuvenation of the amorphous structure. Here we present results demonstrating an improved plasticity for a Cu45Zr45Al5Ag5 bulk metallic glass alloy rejuvenated by severe plastic deformation via high-pressure torsion (HPT). Nanoindentation as well as atomic force microscopy measurements performed on the cross-section of HPT samples provide evidence for the enhancement in plasticity. A robust algorithm developed to reliably detect and quantify the discontinuities (pop-ins) in the nanoindentation load–displacement curves is applied to a large dataset. The results related to the rejuvenated structure demonstrate (i) a reduction of the number of detectable pop-ins as well as (ii) a reduction of their displacement offset. Furthermore, (iii) a pronounced increase in the total creep value is observed and correlates with a plasticity criterion as well as a reduced hardness and Young’s modulus that are heterogeneously distributed across the samples due to localized strain softening during HPT. Finally, (iv) atomic force microscopy topographies obtained from selected indents support the findings, showing less pronounced steps in the plastic pile-up. The experimental results indicate that rejuvenation by HPT processing leads to structural and dynamical heterogeneities, increased atomic mobility and an improved plastic deformation response.

Long-term room-temperature aging treatment of a bulk metallic glass composite

In this work, a long-term room-temperature aging treatment of the order of years is applied to a B2 phase-reinforced CuZr-based bulk metallic glass composite (BMGC). Presumably governed by residual stress, B19’ CuZr martensitic phase gradually precipitates in the crystals after such long-term aging, while structural relaxation takes place in the glassy matrix, which finally induces precipitation of Cu10Zr7 nanocrystals. Tensile tests prove that these time-dependent structural changes have a measurable effect on deformation behaviors of BMGC. The findings are helpful for understanding the thermodynamic stability and the kinetics of structural evolution of metastable alloys and have useful implications for tailoring the structure and mechanical behavior of BMGCs.

Cryogenic-temperature-induced phase transformation in a CuZr-based bulk metallic glass composite under tensile stress

In this work, the cryogenic-temperature-induced structural evolution of B2 CuZr phase-reinforced Cu48Zr47.2Al4Nb0.8 bulk metallic glass composite (BMGC) is in-situ studied under tensile stress. During the cooling process from 280 K down to 80 K, the B2 CuZr phase in the nearly yielding BMGC undergoes the martensitic transformation, whereas the composite sample retains its structure at lower stress. Molecular dynamics results confirm the structural changes of the B2 CuZr phase during the cooling process. The findings will be helpful for fine tailoring the structure of CuZr-based BMGCs and thus widening their applications.

Thermally-Activated Martensitic Transformation Behaviour Below Room Temperature in CuZr-Based Bulk Metallic Glass Composites

During the thermally-activated martensitic transformation (MT) of CuZr-based bulk metallic glass (BMG) composites containing B2 CuZr crystals, the MT temperatures were reduced to 162.7 ± 2.5 K due to the constraint effect from glassy matrix, which is far lower than those of crystalline counterparts. In situ high-energy X-ray diffraction measurements demonstrate that the thermally-activated MT is governed by an extremely incomplete MT from B2 to B19' and B33 phases without the decomposition of B2 phase which appears in crystalline counterparts. Due to free stress states during the thermally-activated MT, B33 phase forms more easily during thermal process than deformation process.

Enhanced tensile plasticity of a CuZr-based bulk metallic glass composite induced by ion irradiation

N+ ion irradiation is utilized to tune the structure and mechanical properties of a Cu48Zr47.2Al4Nb0.8 bulk metallic glass composite (BMGC). Ion irradiation increases the disorder near the surface, as probed by neutron diffraction, and, moreover, causes the phase transformation from B2 CuZr to B19’ CuZr martensitic phase in the studied BMGC. The tensile plasticity of the BMGC is dramatically improved after ion irradiation, which results from multiple shear banding on the surface and the martensitic transformation of the B2 to B19’ CuZr martensitic phase. The experimental results are strongly corroborated by complementary molecular dynamic simulations.

In situ study of the shear band features of a CuZr-based bulk metallic glass composite

To understand the shear banding evolution and the effect of individual shear band on deformation mechanisms of bulk metallic glass composites (BMGCs), tensile tests of CuZrAlNb BMGCs have been in situ studied. It is found that the type and sequence of shear bands are independent of volume fraction of crystalline phase. All the types of shear bands can be considered as shear bands controlled by stress affected zones (SAZs) at crystal/matrix interface and those far away from SAZs. Based on the analysis of elastic energy dissipation, the shear bands far away from SAZs play a dominant role in the deformation stability. For current BMGCs, the propagation of shear bands far away from SAZs can be delayed by reducing the average elastic energy dissipation. High Young's modulus and volume fraction of crystalline phase are effective for decreasing the average elastic energy stored in shear bands. It is expected that the obtained results can offer better understanding of deformation mechanism in bulk metallic glass composites, which should be beneficial to design bulk metallic glass composites with improved mechanical performances.

Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding

Structural heterogeneities in monolithic metallic glasses, i.e. free volume variations or stress/strain fields, are known to be pivotal for their plasticity but the mechanisms how they affect irreversible deformation remain poorly understood. We show that flash-annealing is a capable tool for modifying the free volume content and its distribution, which creates non-affine stress fields in a monolithic CuZr-based bulk metallic glass. Rather than the overall free volume, μm-scale regions of relaxed and rejuvenated glass govern plastic deformation. Complementary molecular dynamics simulations underpin the importance of the interfaces between relaxed and rejuvenated volumes for shear band proliferation. Not only can we reach unmatched states of metastability in metallic glasses with the present approach but also better understand the mechanisms underlying plasticity in monolithic but structurally heterogeneous metallic glasses.

Quantitatively determining the martensitic transformation in a CuZr-based bulk metallic glass composite

Here, the compressive deformation behaviors of a CuZr-based bulk metallic glass composite (BMGC) has been studied under both unconfined and confined conditions. The threshold strain for the martensitic transformation (MT) is experimentally determined to be 0.8%, and 1.9% for the unconfined and confined compression, respectively. The complicated stress state in the confined compression postpones MT of the B2 phase, which has been verified by both experiments and molecular dynamic simulation. The yielding of the B2 phase is dominated by the uniaxial compressive stress. It is expected that the obtained results can provide more insights into the deformation mechanism of BMGCs.

Serration dynamics in the presence of chemical heterogeneities for a Cu-Zr based bulk metallic glass

The serration flow behavior for Cu47Zr47Al6 and (Cu47Zr47Al6)99Sn1 (at. %) bulk metallic glasses (BMGs) was investigated through statistical analysis. The statistical analysis reveals that the (Cu47Zr47Al6)99Sn1 BMG has more stable self-organized critical (SOC) dynamics. The chemical heterogeneities formation in the microstructure has a triple effect on the deformation dynamics, which leads to high ductility for the (Cu47Zr47Al6)99Sn1 BMG.