Microstructural heterogeneities governing the deformation of Cu47.5Zr47.5Al5 bulk metallic glass composites

Cu_47.5Zr_47.5Al₅ rods with different volume fractions of crystalline B2 CuZr phase were prepared by copper mould casting. Based on microstructural investigations a solidification mechanism is proposed for these bulk metallic glass (BMG) composites. The composite microstructure enhances the compressive plasticity (plastic strain up to 14%) and both plastic strain as well as yield strength scale with the crystalline volume fraction. Yield strength and fracture strain were successfully calculated using a strength model, which considers percolation and an empirical three microstructural element body approach, respectively. Furthermore, B2 CuZr was synthesized by means of a thermal cycling treatment and uniaxial compression tests were carried out at room temperature. The intrinsic work-hardenability caused by a martensitic transformation has strong implications on the deformation behaviour of the investigated Cu_47.5Zr_47.5Al₅ BMG composites.     

Triple yielding and deformation mechanisms in metastable Cu47.5Zr47.5Al5 composites

A triple yielding phenomenon resulting in high fracture strength (up to 2360 ± 10 MPa) and large ductility (up to 13.5 ± 0.5%) during compressive deformation is first discovered and systematically investigated in metastable Cu_47.5Zr_47.5Al₅ composites. Based on electron microscopy and X-ray diffraction studies, the major deformation mechanisms at the different stages of compression are revealed to be: (1) the constraint effect of the amorphous phase; (2) the pre-existence of a small number of microtwins and a small amount of martensitic phases; (3) the initiation and development of martensitic transformation; (4) the interplay between CuZr crystals and shear bands; and (5) the activation of partial detwinning, together with a high density of dislocations. The yield strengths of the composites are modeled as a function of the volume fraction of the constituent phases and a transition from the rule-of-mixtures to a load-bearing model is observed. The underlying ductile nature of the composites is further correlated with the unique electronic structure of the B2 CuZr intermetallics. The present results provide an understanding of the deformation mechanism of metastable CuZr-based composites and give guidance on how to improve the ductility/toughness of bulk metallic glasses.     

Improving glass forming ability of Zr50.5Cu36.5Ni4Al9 alloy by suppressing CuZr phase precipitation

CuZr phase is precipitated easily in the process of fabricating Zr_50.5Cu_34.5Ni₄Al₁₁ BMG. The critical diameter of amorphous rod is improved from 8 mm of Zr_50.5Cu_34.5Ni₄Al₁₁ to at least 20 mm of Zr_53.5Cu_26.5Ni₅Al₁₂Ag₃ by restraining the precipitation of CuZr phase. The great improvement of GFA is attributed to the declination of Cu content and the weakening of the drastic interaction between Cu and Zr atoms by Ag addition since Ag has similarly physical and chemical properties with Cu. Therefore, suppressing the precipitation of crystalline phase should be an effective method to enhance GFA.     

Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass

Significant tensile plasticity up to 0.7 ± 0.1% together with work-hardening and larger fracture strength was obtained in Cu_47.5Zr_47.5Al₅ bulk metallic glass (BMG) upon cold rolling with only 2.9 ± 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL).     

Ti–Cu–Ni shape memory bulk metallic glass composites

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti₂Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.     

Microstructure Evolution During Solidification of Cu–Zr–Ti Alloy Forming B2 Phase Particles Embedded in a Glassy Matrix

Microstructural evolution during injection casting Cu₅₀Zr_50?xTi_x (x?=?0–8) alloys has been investigated using X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. Cubic CuZr(Ti) B2 phase is competing against the glass transition during solidification for all the alloys and the primary B2 phase has transformed into the martensitic phase for x?<?6 alloys during cooling after solidification. The formation of spherical morphology and spatially inhomogeneous distribution of B2 phase in a glassy matrix can be rationalized in terms of reduced interface kinetics of solid/liquid interface and polymorphic nature of the primary solidification taking place without solute partition. The partial replacement of Zr with Ti improves not only glass forming ability but also suppresses the martensitic transformation of B2 phase, enabling the fabrication of BMG composites consisted of the B2 phase embedded in a CuZr(Ti) glass matrix. However, due to local cooling rate change during solidification, development of non-uniform microstructure in the BMG composites seems to be inevitable, which may be an obstacle in future application of the BMG composites.     

Hierarchical nanoporous metal/BMG composite rods with excellent mechanical properties

Hierarchical nanoporous structured NPC/BMG composite rods (NPC/BMG: nanoporous copper/bulk metallic glass) were facilely fabricated by one-pot chemical dealloying the Cu₅₀Zr₄₅Al₅ BMG rods in 0.05 M HF solution for 24 h. The cross-sectional SEM images illustrate that the NPC/BMG composite rods exhibit the perfect combination of inner rod-shaped Cu-Zr-Al amorphous phase core and outer tube-shaped NPC layer with a thickness of 85 μm. As compared to the reported NPC composites, the new composite rods demonstrate remarkable enhanced mechanical properties with an ultrahigh strength of 1500 MPa and a large compression strain of 2.9%. Additionally, the increase in the compression strain is attributed to the formation of the buffer deformation zone resulting from the existence of the outer nanoporous copper tube.     

过冷液态Zr48Cu36Ag8Al8块体非晶合金的相分离及组织演变

本文通过x射线衍射(XRD)、差示扫描量热法(DSC)和透射电子显微镜(TEM)研究了退火温度对Zr₄₈Cu₃₆Ag₈Al₈金属玻璃微观结构演化的影响。结果表明,快速凝固获得的样品为典型的非晶态结构。当样品在703K保温20分钟时,均一的非晶基体相分离成两种非晶合金,即,发生相分离。由于相分离结构与非晶基体在等温退火过程是竞争的关系,这个结构很容易向晶化态进行转变,形成AlZr₂ AlAg₃相。Zr₄₈Cu₃₆Ag₈Al₈金属玻璃的微观结构在过冷液相区等温退火过程中经历了的局部结构转变,相分离以及纳米晶转变,这个过程意味着Zr₄₈Cu₃₆Ag₈Al₈金属玻璃的微观结构对退火温度十分敏感。此外,相分离的形成可以加速纳米晶的形成。     

Bulk metallic glass composites ductilized by core–shell structured dual crystalline phases through controlled inoculation

Ductile crystalline phases of bcc Ta and CuZr(B2) were coupled in situ and assembled in a core–shell structure in bulk metallic glass (BMG) composites through controlled inoculation. These two phases were dispersed homogeneously within the amorphous matrix. The CuZr(B2) phase nucleated preferentially on the bcc Ta surface following well-defined crystallographic orientation relationships. A layer of intermediate transition zone containing interwoven amorphous structures formed at their interfaces to lower the interfacial energy. The composites exhibited significantly improved plasticity and simultaneous increase in strength. This study greatly aids in the microstructural design and tailoring for optimized mechanical properties of BMG composites.     

Plasticity improvement of (Cu43Zr48Al9)98Y2 bulk metallic glass composites by dispersed Ta particles

(Cu₄₃Zr₄₈Al₉)₉₈Y₂-based bulk metallic glass composites (BMGCs) with dispersed Ta particles (3vol.%, 6vol.%, 9vol.%) were successfully fabricated through suction casting. The thermal properties, microstructure, and mechanical properties of the BMGCs were systematically investigated. Ta particles are homogeneously dispersed in the amorphous matrix. Ta particle reinforced BMGCs exhibit similar thermal properties and glass-forming ability with the (Cu₄₃Zr₄₈Al₉)₉₈Y₂ base BMG. Compression test results show that the BMGC with 9vol.% Ta particles has superior mechanical performance with up to 15.7% compressive plastic strain, 2,216 MPa yield strength, and 2,260 MPa fracture strength at room temperature. These homogeneously distributed Ta particles act as discrete obstacles in the amorphous matrix, restricting the highly localized shear band. This results in the formation of multiple shear bands around the Ta-rich particles, which lowers the stress concentration, allowing the shear band to propagate further and improve plasticity.

     

Microstructural tailoring and improvement of mechanical properties in CuZr-based bulk metallic glass composites

A strategy for homogenizing the B2 CuZr phase in CuZr-based bulk metallic glass (BMG) composites based on inoculation is presented. The sizes and distribution of B2 CuZr particles can be effectively homogenized by Ta additions in rapidly solidified Cu₄₇Zr_48?xAl₅Ta_x (0 ≤ x ≤ 1, at.%) alloys. Mechanisms of the homogenizing effect were investigated by analyzing the microstructures of the composites as well as the nucleation and growth processes of the B2 CuZr phase. Mechanical properties of the alloys were significantly improved by the uniform B2 CuZr particles under both compression and tension. The inhibition on the propagation of shear bands and cracks by the ductile crystals and deformation-induced martensitic transformation of the B2 phase was proved to account for the superior tensile properties. Fracture mechanisms were proposed to correlate the tensile fracture behaviors to microstructural features of the alloys. Furthermore, the tensile plastic strain was quantitatively modeled by using the empirical microstructural element body approach as well as percolation theory. This study has important implications for the development and structural applications of high-performance BMG composites.     

Effect of Ta on glass formation,thermal stability and mechanical properties of a Zr52.25Cu28.5Ni4.75Al9.5Ta5 bulk metallic glass

The effect of Ta on glass-forming ability, crystallization behavior and mechanical properties of Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ bulk metallic glass (BMG) is investigated. The solubility of Ta in the Zr-base BMG alloy depends on the arc melting conditions. 3.2 at.% Ta dissolve in the alloy inducing an increase of about 20 K in both glass transition temperature and crystallization temperature of the BMG. However, Ta does not significantly change the extension of the supercooled liquid region. The remaining Ta particles in the master alloy may induce a composition-segregation layer around the particles upon subsequent casting. This further induces the crystallization of Zr₂Cu that deteriorates the ductility of the samples. The compressive strength and ductility of the as-cast 3 mm diameter Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ samples are improved in comparison with the Zr₅₅Cu₃₀Ni₅Al₁₀ BMG alloy. The fracture plane of the present alloy has an angle of 31–33° with respect to the stress axis, which remarkably deviates from the maximum shear stress plane. The improvement of the mechanical properties and the peculiar fracture feature for the Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ BMG alloy can be attributed to the effect of dispersed Ta particles.     

Thermomechanical characterization of Cu47.5Zr47.5Al5 bulk metallic glass within the homogeneous flow regime

We present a systematic study of the high temperature deformation behavior of a Cu_47.5Zr_47.5Al₅ ternary bulk metallic glass over a wide range of strain rates within the homogeneous flow regime. The apparent viscosity and the effective strain rate determined by thermomechanical analysis in the low stress regime strongly depend on the isothermal annealing temperature and the applied compressive force. Three distinct flow modes, viz. inhomogeneous, non-Newtonian and Newtonian flow, can be distinguished from compression tests. The strain rate–stress data, deduced from both thermomechanical analysis and quasi-static compression tests, were used to construct a Norton-type plot indicating a transition from Newtonian to non-Newtonian flow. The significance of these findings for the expected macroscopic shaping capability based on the dynamic materials model as well as the change of the amount of atomic-scale flow defects such as free volume is also investigated.     

Effect of minor Nb addition on mechanical properties of in-situ Cu-based bulk metallic glass composite

In-situ formed (Cu_0.6Zr_0.3Ti_0.1)₉₅Nb₅ bulk metallic glass (BMG) composite with Nb-rich dendrite randomly dispersed in hard glassy matrix was prepared by casting into a water-cooled copper mold. The dendrite has much smaller hardness and elastic modulus than glassy matrix, and the stress concentration at interface provides a channel for the initiating and branching of shear bands upon loading, thus leading to a high compressive fracture strain of 6.08% and fracture strength about 2200 MPa. Comparing with other Cu-based BMG composite, the fracture strength of present (Cu_0.6Zr_0.3Ti_0.1)₉₅Nb₅ composite is not significantly reduced, indicating that the addition of Nb in the current work is an effective and effortless way to fabricate new practical BMG composites with enhanced strength and good plasticity.     

On the transformation-induced work-hardening behavior of Zr47.5Co47.5Al5 ultrafine-grained alloy

The work-hardening behavior of Zr_47.5Co_47.5Al₅ ultrafine-grained alloy, obtained by water-cooled copper mold casting, has been investigated. The Cu- and Ni-free alloy shows high compressive strength combined with large plasticity. The work-hardening behavior and the remarkable enhancement of the ductility and strength origin from the B2 to B33 deformation-induced martensitic transformation associated with the high strength ultra-fine grains. The fracture mode of the alloy is both intergranular and transgranular fracture. Our current finding may offer useful guidelines to design materials with improved strength and ductility as practical engineering and biomedical applications.     

Glass formation and mechanical properties of (Cu50Zr50)100−xAlx (x = 0, 4, 5, 7) bulk metallic glasses

The effect of Al content on the glass formation and mechanical properties was studied for (Cu₅₀Zr₅₀)_100−xAl_x (x = 0, 4, 5, 7) bulk metallic glasses. The crystallization temperatures of fully amorphous Cu₅₀Zr₅₀, Cu₄₈Zr₄₈Al₄, Cu_47.5Zr_47.5Al₅ and Cu_46.5Zr_46.5Al₇ as-cast rods are 724 K, 753 K, 758 K and 782 K, respectively. The mechanical properties were investigated under compression at room temperature. As-cast Cu_46.5Zr_46.5Al₇ shows the highest yield strength (1867 MPa), whereas Cu_47.5Zr_47.5Al₅ shows the largest fracture strain of 11.2%. The fracture surfaces of the compressed samples were investigated by scanning electron microscopy and their morphology has been correlated with the compressive plasticity.     

Crystallisation by laser for Zr based bulk metallic glass

Abstract

Bulk metallic glasses (BMGs) show high amorphous phase stability. The solidification process of BMG occurs with the cessation of epitaxial crystal growth. For a laser heated and melted surface of BMG, crystallisation occurs during cooling, along with crystalline growth from the heat affected zone. Such crystallisation stops at a certain location where the residual melt solidifies as amorphous. The amorphous formation can occur after crystal nucleation. Thermal treatment using a diode laser was used for surface modification of 2 mm thick Zr₅₅Al₁₀Ni₅Cu₃₀ (at.-%) BMG plates. When laser conditions (scanning speed, defocused irradiated spot size and output power) were changed, the maximum temperature and heating cooling rate were changed, and microstructures were changed. Cessation of crystallisation was observed.     

Microstructure and mechanical properties of dissimilar spot friction stir welded Zr55Cu30Al10Ni5 bulk metallic glass to pure copper

A Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass (BMG) plate with thickness of 2 mm was successfully spot friction stir welded to a commercial pure Cu plate. Microstructural characterization revealed that some serrations formed at the edge of the BMG plate and some nano-sized nucleuses were dispersed in the BMG side along the Cu/BMG joint interface. In the upper Cu side close to the keyhole, some blocky and strip-like BMG fragments were embedded in the Cu matrix. As a result, significantly elongated Cu grains with width of about 200 nm were formed between the strip-like BMG fragments. Shear tensile tests showed that the Cu/BMG spot joints fractured at a maximum load of about 2300 N through the stir zone pullout fracture mode.     

Enhanced wear resistivity of a Zr-based bulk metallic glass processed by high-pressure torsion under reciprocating dry conditions

Wear properties of bulk metallic glasses (BMGs) are important for industrial applications as much as strength and ductility. Free volume of BMGs is a significant factor which decides wear mechanism and resistance. Increased free volume of a Zr₅₅Al₁₀Ni₅Cu₃₀ BMG processed by high-pressure torsion (HPT) affected wear resistance under dry reciprocating conditions. Two- and three-body abrasive wear as well as the delamination of oxide layers simultaneously operated during the wear tests of both as-cast and HPT-processed BMG (HPT-BMG). However, the HPT- BMG had a larger area of the oxide layers on a worn surface compared to the as-cast BMG at the early stage of the wear tests. The increased free volume by the HPT process resulted in ductile plastic deformation, prohibited crack propagation, and delayed delamination of the oxide layers. Therefore, the HPT-BMG had thicker oxide layers, which acted as an adequate protection and increased wear properties of the Zr-based BMG.     

Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites

The Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 bulk metallic glass forming liquid is reinforced with WC, SiC, W, or Ta particles. Structure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remains amorphous after adding up to 20 vol.% of particles. The reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy and electron microprobe. The mechanical properties of the composites are studied in compression and tension. The influence of the introduced particles on the thermal stability of the matrix as well as on the mechanical properties is discussed.