Biocompatibility of Ni-free Zr-based bulk metallic glasses

Three Ni-free Zr-based BMGs with composition of Zr₆₀Nb₅Cu₂₀Fe₅Al₁₀, Zr₆₀Nb₅Cu_22.5Pd₅Al_7.5, Zr₆₀Ti₆Cu₁₉Fe₅Al₁₀ were fabricated by suck copper-mould casting. All the BMGs prepared exhibit good glassy forming ability and wide supercooled liquid region ranging from 38 to 99 K. These BMGs also show good mechanical properties under static compression with yield strength of over 1350 MPa, Young's modulus of 70–80 GPa, and plastic strain of 3.6–9.5%. Friction and wear tests revealed that the BMGs exhibit much better wear resistance than the medical alloy Ti6Al4V, although BMGs have a higher friction coefficient. In addition, the in vitro test indicated that the BMGs have a similar or even better cell viability and proliferation activity as compared with Ti6Al4V. Finally, the in vivo evaluation of the BMGs was carried out by the implantation of BMG samples into white rabbits. It is shown that the BMG implants performed as well as the Ti alloy, demonstrating that the Ni-free Zr-based BMGs developed in this work are promising in medical applications.     

Investigations of compressive strength on Cu-Hf-Al bulk metallic glasses: Compositional dependence of malleability and Weibull statistics

The study is focusing on how physical factors correlate with malleability for bulk metallic glasses (BMGs). Although great efforts have been done in these aspects, many problems still remain interesting. In this work, we investigated systematically the compositional dependence of the compressive malleability for three selected BMG samples of Cu₅₂Hf₃₉Al₉ (C1), Cu₄₉Hf₄₂Al₉ (C2) and Cu₄₅Hf₄₆Al₉ (C3) in the high glass-forming Cu-Hf-Al system. Our results demonstrated that (i) with increasing of Hf content the malleability of BMGs becomes stronger, (ii) shear stress has an important impact in controlling the compressive deformation behaviors and (iii) the malleability increases linearly as the glass transition temperature and shear modulus decrease but it does not directly correlate with Poisson’s ratio and glass forming ability. Finally, through two-parameter Weibull statistics we further suggest that the C3 BMG indeed exhibits a more uniform distribution on the failing strength in a more reliable manner with respect to the other C1 and C2 cases.     

Glass forming ability and crystallization of Zr-Cu-Ag-Al-Be bulk metallic glasses

The glass forming ability of Zr₄₆Cu_37.64−xAg_8.36Al₈Be_x (x = 0, 6 and 10 at.%) bulk metallic glasses (BMGs) were significantly improved by Be addition. The critical size of amorphous rods can be over 35 mm diameter. The high GFA achieved is mainly due to the decrease of melting point and liquidus temperature, and suppression of the formation of crystalline phases during solidification from liquid state. The high stabilization with supercooled liquid regime of 115 K was found for the BMG with x = 10 at.%. Two independent exothermic events happen in x = 0 and 6 at.% BMGs, corresponding to the formation of primary crystalline phases Cu₁₀Zr₇ and AgZr, then transforming to final stable crystalline phases Zr₂Cu and AlCu₂Zr. However, in the x = 10 at.% BMG, the precipitation of primary phases and transformation to final stable phases are within the first exothermic event and the AlCu₂Zr phase is totally suppressed.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

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.     

The oxidation behavior of Cu–Zr–Ti–base bulk metallic glasses in air at 350–500 °C

The oxidation behavior of two Cu-base bulk metallic glasses (BMGs), having compositions Cu–30Zr–10Ti and Cu–20Zr–10Ti–10Hf (in at.%), was studied over the temperature range of 350–500 °C in dry air. In general, the oxidation kinetics of both BMGs followed the parabolic rate law, with the oxidation rates increasing with increasing temperature. The addition of Hf slightly reduced the oxidation rates at 350–400 °C, while the opposite results observed at higher temperatures. It was found that the oxidation rates of both BMGs were significantly higher than those of polycrystalline pure-Cu. The scales formed on both BMG alloys were strongly composition dependent, consisting of mostly CuO/Cu₂O and minor amounts of cubic-ZrO₂ and ZrTiO₄ for the ternary BMG, and of CuO, cubic-ZrO₂, and Zr₅Ti₇O₂₄ for the quaternary BMG. The formation of ternary oxides (ZrTiO₄ and Zr₅Ti₇O₂₄) was inferred to be responsible for the fast oxidation rates of the BMGs.     

The Soret effect in bulk metallic glasses

Compositional inhomogeneity induced by the Soret effect was studied in two Zr-based bulk metallic glasses (BMGs): Zr₅₀Cu₅₀ and Zr₅₀Cu₄₀Al₁₀ (at%), and one Cu-based BMG: Cu₆₀Zr₃₀Ti₁₀ (at%), all of which were prepared by rapid solidification. The concentration of Cu increases from the surface to the interior, while the concentrations of Zr, Ti and Al decrease. The magnitude of the Soret effect is found to be highly dependent on the sample size and interactions between the diffusing atoms in bulk metallic glasses. For the Zr₅₀Cu₅₀ alloy, a large sample size favors the Soret effect, because of the longer diffusion time it affords compared to a small sample. Further, the additions of Al and Ti in the Zr–Cu BMGs reduce the magnitude of the Soret effect by the formation of short-range order and/or inter-atomic clusters.     

Elastic moduli and mechanical properties of bulk metallic glasses after quasi-static compression

The effect of quasi-static compressive stress on the elastic moduli and mechanical properties of a Cu₄₆Zr₄₆Al₈ bulk metallic glass (BMG) was investigated. When the applied quasi-static stress is below 2 GPa (equivalent to 1.4 times the yield strength of the BMG), the elastic moduli of the deformed BMGs are found to decrease with the applied stress, revealing the softening or dilatation of the bulk metallic glass. The Poisson ratio is relatively stable when the stress is below 1000 MPa, but it decreases significantly afterwards. Both the plasticity and strength of the BMG are found to increase at low applied stress, and achieve a maximum value before decreasing at higher applied stress. The applied stress is shown to enhance the mechanical properties of the BMG and the properties can be controlled by quasi-static compressive stress. The results demonstrate that an applied stress far below the macroscopic yield strength can still result in microscopic yielding and microstructure change in metallic glass systems.     

Understanding the mechanism for the embrittlement of a monolithic Zr-based bulk metallic glass by oxygen

Monolithic bulk metallic glasses (BMGs) with a nominal composition of Zr₆₅Cu_27.5Al_7.5 and the oxygen concentrations of 0.06 at.% and 0.68 at.% were prepared respectively. Oxygen effects on the deformation behavior and as-cast structural state were investigated. Although no crystalline phases were found in the BMG with higher oxygen concentration, the compressive plasticity was dramatically decreased. It was found that there is no direct correlation between the notable embrittlement and the initial free-volume content in the BMG. Geometrically-constrained compression tests for the BMGs shown that the processes of free-volume accumulation and shear band propagation during plastic deformation were obviously retarded by oxygen. It appears that the mechanism for the oxygen-induced embrittlement was closely related with the decreased atomic mobility by oxygen. As an indicator of atomic mobility, activation energy may be used as a parameter to evaluate the plasticity of monolithic Zr-based BMGs with different dissolved oxygen concentrations.     

Study of serrated flow and plastic deformation in metallic glasses through instrumented indentation

The plastic deformation behavior and serrated flow in seven bulk metallic glass (BMG) systems were investigated through instrumented indentation. These materials include Ce₆₅Al₁₀Ni₁₀Cu₁₀Nb₅, Mg₆₅Cu₂₅Gd₁₀, Pd₄₃Ni₁₀Cu₂₇P₂₀, Cu₆₀Zr₂₀Hf₁₀Ti₁₀, Pt_57.5Cu_14.7Ni_5.3P_22.5, Ni₆₀Nb₃₇Sn₃ and Fe₄₃Cr₁₆Mo₁₆C₁₅B₁₀ BMGs, which show a glass transition temperature (T_g) ranging from 360 to 908 K at a heating rate of 0.33 K/s. Remarkable difference in deformation behavior was found among these BMGs in the load–depth curves during nanoindentation. Prominent serrations are observed in Mg-, Pt- and Pd-based BMGs with medium T_g during the loading process, whereas no distinct serrated flow was found in Ce-, Ni- and Fe-based BMGs with quite low or high T_g. The subsurface plastic deformation regions after indentation were investigated using depth-sensing microindentation to characterize the shear band feature developed in various BMG systems. The size of the shear band upset is found to be larger in the alloys with lower T_g. The effect of T_g on the operation of shear bands and the serrated flow behavior in various BMG systems were discussed.     

Fe-based glassy alloys with high iron content and high saturation magnetization

Iron-based bulk metallic glasses (BMGs) are attractive due to their excellent magnetic properties. It is known that increasing the iron content in alloys would enhance the magnetic property of glassy alloys, but would reduce their glass forming ability. Despite researchers are keen to develop Fe-based BMGs with high Fe content, BMG with Fe content higher than 79 at.% has not been reported. Here we report that Fe-based BMGs with Fe content as high as 78 ～ 81 at.% have been successfully prepared. The as-prepared Fe₈₁Mo₁P_7.5C_5.5B₂Si₃ BMG possesses a saturation magnetization of 1.64 T, which is significantly higher than the reported value in BMGs.     

Zr67.8Cu24.7Al3.43Ni4.07非晶合金激光焊接晶化控制及组织性能分析

对Zr_67.8Cu_24.7Al_3.43Ni_4.07非晶合金进行激光焊接,研究激光功率和焊接速度变化对接头不同区域微观组织的影响,阐述非晶合金激光焊接接头晶化控制的工艺调控规律,并分析接头不同区域的微观结构特征与其硬度之间的关系.结果表明,采用高焊速及高能量密度的激光焊接有利于使Zr_67.8Cu_24.7Al_3.43Ni_4.07非晶合金接头的熔化区保持非晶态结构,同时伴随少量纳米晶产生.热影响区的晶化现象明显,激光功率对接头完全焊透具有较大影响,通过降低激光功率或提高焊接速度以减小热输入,热影响区的晶化程度得到有效控制.焊后接头的熔化区硬度略高于母材,而热影响区的硬度相比于母材显著降低.     

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.     

Effect of addition of Be on glass-forming ability,plasticity and structural change in Cu–Zr bulk metallic glasses

The present study reports the effect of the addition of Be in Cu–Zr bulk metallic glass (BMG) on glass-forming ability (GFA), plasticity and structural change. Although Be has a negative enthalpy of mixing with all the constituent elements of these glasses, Cu_47.5Zr₄₀Be_12.5 alloy exhibits apparent double glass transitions (T_g) and enhanced plasticity as well as improved GFA. Intensive structural analysis using extended X-ray absorption fine structure suggests that a large difference in the enthalpy of mixing between atom pairs in multi-component BMGs can cause atomic scale structural inhomogeneity and/or locally favored structures in the amorphous matrix, resulting in enhanced compressive strains, although the enthalpies of mixing for atom pairs are all negative. This concept may shed light on the development of BMGs with large plasticity as well as high GFA.     

Glass forming ability and mechanical properties characterization on Mg58Cu31Y11−xGdx bulk metallic glasses

Mg–Cu–Y–(Gd) alloy rods are made by arc-melting and injection casting methods in this research. The improvement of glass forming ability and mechanical properties by using Gd to substitute Y in Mg₅₈Cu₃₁Y₁₁ bulk metallic glasses (BMGs) is of interest. The results of thermal analysis present that the Mg–Cu–Y base alloys with the addition of 6 and 8 at% Gd are the best BMG former. The Vickers indentation tests and the compression tests are carried out in order to explore the mechanical properties of alloys. It reveals that there is no obvious change in Young's modulus (45 GPa) of the Gd-containing Mg-based BMG, in contrast with the base alloys. Vickers (micro-) indentation fracture toughness measurements are performed for comparison. Shear bands and the corner cracks around the inverted pyramind mark are showed. An average fracture toughness of Mg–Cu–Y–Gd alloy is calculated as 4 MPa m^1/2, which is a little higher than that of base alloys studied in the paper. Meanwhile, the fracture surface of Mg-based BMGs is dominant by featureless mirror-like and river-like pattern. Only nano-scaled shear bands and vein patterns are displayed, indicating that the plasticity of the Mg–Cu–Y–Gd BMGs are shown in nano-scale indeed.     

Cyclic compression behavior of a Cu–Zr–Al–Ag bulk metallic glass

The cyclic compression behavior of a Cu₄₅Zr₄₅Al₅Ag₅BMG was investigated in order to elucidate the damage initiation and growth mechanisms. The present Cu₄₅Zr₄₅Al₅Ag₅ BMG was found to have a fatigue-endurance limit of 1418 MPa and fatigue ratio of 0.77. Fracture under cyclic compression occurred in a pure shear mode. The fracture surface forms an angle of 41° with respect to the loading axis. This angle was similar to the monotonic compressive fracture angle for the present BMG. The cyclic compression fracture surface displays a morphology nearly identical to the monotonic compression fracture surface. In addition to many shear bands and cracks, areas of “chipping” were commonly found on the outside surfaces of the fatigue specimens. An attempt was made to measure crack growth rates, and two types of crack growth behavior were found. With the first type, the growth rate decreased with cycles due to the decrease in the driving force for crack propagation. With the second type, the crack growth rate increased with cycles after chipped areas developed. The fatigue deformation process for BMGs under cyclic compression was carefully studied and rationalized.     

Formation and interesting thermal expansion behavior of novel Sm-based bulk metallic glasses

Two kinds of Sm-based alloys, Sm₅₅Al₂₅Co₂₀ and Sm₅₅Al₂₅Cu₁₀Co₁₀, have been cast into full glassy rods up to 4 and 3 mm in diameter, respectively, by copper mold casting. The substitution of 10 at% Co for 10 at% Cu leads to the increase in glass forming ability and thermal stability of the Sm₅₅Al₂₅Co₂₀ bulk metallic glass (BMG). An interesting phenomenon is observed that Sm-based BMGs exhibit different thermal expansion behaviors compared to previously studied Cu-based BMGs above the glass transition temperature. The phenomenon correlates to the degree of contraction for the sample in the supercooled liquid region.     

Crack-resistance curve of a Zr–Ti–Cu–Al bulk metallic glass with extraordinary fracture toughness

For the emerging bulk metallic glasses (BMGs), damage tolerance is a key mechanical property needed for their practical applications. To reach a fracture toughness on a par with, or even better than, conventional engineering alloys, the only route reported so far is to compositionally base the BMG on high-cost palladium (Pd), which has a very high Poisson’s ratio (～0.42). Here we report the discovery of a Zr₆₁Ti₂Cu₂₅Al₁₂ (ZT1) BMG that has a toughness as high as the Pd-based BMG, but at the same time consists of common engineering metals and has robust glass-forming ability. The new BMG, while having an unimpressive Poisson’s ratio of 0.367, derives its high toughness from its high propensity for crack deflection and local loading-mode change at the crack tip due to extensive shear band interactions. The crack-resistance curve (R-curve) of this BMG has been obtained from fatigue pre-crack samples, employing standard “single-specimen” and “multiple-specimen” techniques.     

Formation of centimeter Fe-based bulk metallic glasses in low vacuum environment

The formation of a Fe_43.7Co_7.3Cr_14.7Mo_12.6C_15.5B_4.3Y_1.9 bulk metallic glass (BMG) was attempted in low vacuum environment and in air using commercial raw materials. The glass forming ability of the Fe-based alloys was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analyzer (DTA). It was found that cylindric rods with diameters ranging from 10 mm to 5 mm could be successfully fabricated by copper-mold casting in the pressures from 1.5 Pa to 10⁵ Pa (10⁵ Pa = 1 atm). All BMGs exhibit a distinct glass transition and wide supercooled liquid region. The preparation condition seems not significantly affected by the thermodynamic parameters of BMG, such as supercooled liquid region, glass transition temperature and melting process. The oxygen content of the alloys prepared in different vacuum conditions was measured by a LECO oxygen analyzer, which revealed that the oxygen content was less than 100 ppm for all BMGs prepared, even in air. The good glass forming ability and excellent oxidation resistance for the present Fe-based alloy are discussed.     

Influence of Superheat on Microstructure and Mechanical Properties of Ductile Cu47.5Zr47.5Al5 Bulk Metallic Glass-Matrix Composite

Generally bulk metallic glasses (BMGs) posses very less ductility and toughness at room temperature. Over the recent past years to improve up on these properties in many alloy system BMG composites have been developed. It was also reported that Cu_47.5Zr_47.5Al₅ BMG composite shows a very high strength together with an extensive work hardening-like behavior of large ductility around 18%. In this study, the influence of superheat on microstructure and the resulting mechanical properties in Cu_47.5Zr_47.5Al₅ bulk metallic glass-matrix composite alloy has been studied. The Cu_47.5Zr_47.5Al₅ melt solidifies into a composite microstructure consisting of crystalline precipitates embedded in an amorphous matrix. The crystalline phase consists of B2 CuZr (cubic primitive with CsCl structure) with a small amount of monoclinic CuZr martensitic structure embedded in an amorphous matrix. The volume fraction of crystalline phases varies with melting current as well as position along the length of the as-cast rod, depending on the local cooling condition. The volume fraction and the distribution of the crystalline precipitates are heterogeneous in the amorphous matrix. Room temperature uniaxial compression tests revealed high yield strength ranging from 796 to 1900 MPa depending upon the volume fraction of the crystalline phases present. The presence of the dendritic B2 CuZr significantly improved the ductility. The BMG composites show a pronounced plastic strain up to 14% for the higher volume fraction of crystalline phase.