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.     

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.     

Micromechanisms of serrated flow in a Ni50Pd30P20 bulk metallic glass with a large compression plasticity

Serrated flow is a characteristic feature of plastic deformation of bulk metallic glasses (BMGs) with a large compression strain. However, the underlying mechanisms of the discrete plasticity in the disordered solids have been debated for many years. Here, we report mechanical behavior and microstructural evolution of a Ni₅₀Pd₃₀P₂₀ BMG subjected to uniaxial compression testing. Extensive nanocrystallization within shear bands and in the vicinity of fracture surfaces was observed and various crystal defects, including dislocations, twins and kink bands, were detected in the resultant nanocrystals. These observations suggest a microscopic mechanism of the serrated flow of the BMG, i.e. the stress drop is caused by local strain-softening and the arrest of shear bands is associated with in situ nanocrystallization.     

表面粗糙度对Zr52.5Cu17.9Ni14.6Al10Ti5块体非晶合金塑性的影响（英文）

将Zr52.5Cu17.9Ni14.6Al10Ti5（Vit105）块体非晶合金棒用水砂纸和抛光膏打磨到不同粗糙度,研究表面粗糙度对试样压缩变形行为的影响。结果表明,随着试样表面粗糙度的降低,屈服强度并没有明显变化,但压缩塑性从2.3%提高到4.5%。在扫描电镜下观察断裂试样的侧面发现,塑性越大的试样,剪切带的密度越大。因此,对于非晶合金,要得到较大的塑性,降低表面粗糙度是必要的。     

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.     

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.     

Ductile to brittle transition in the Zr41.2Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass

The variation of impact toughness, Γ, of a Zr_41.2Ti_13.75Cu_12.5Ni₁₀Be_22.5 (Vitreloy-1) bulk metallic glass (BMG) within the temperature range of 123–423 K was evaluated by using an instrumented Charpy impact testing machine, in order to examine if the BMGs exhibit ductile-to-brittle transition (DBT) that is seen in rapidly quenched glasses. Results show an abrupt reduction in Γ when the testing temperature is lowered to below 150 K, implying that the BMGs are also prone to the DBT. Fractographic observations indicate a transition in the fracture mode; from ductile vein-like morphology above DBT to a cleavage-dominant fracture mode below it. Complimentary Vickers indentation measurements show no variation in hardness with temperature. However, the shear banded plastic regions that are typically seen around the indents were observed to be completely absent around the indents that were made at low temperatures, indicating that the inhomogeneous plasticity mediated by shear bands becomes inoperative below a critical temperature resulting in the DBT. This observation suggests that the minimum amount of free volume required for extensive plasticity (and hence high toughness) in metallic glasses is strongly dependent on the temperature. Testing of the structurally relaxed samples (through annealing at 530 K for 2.5 h that induces severe embrittlement at room temperature) at 423 K reveal almost complete recovery of Γ, supporting this hypothesis.     

A Dy-based bulk metallic glass with high thermal stability and excellent magnetocaloric properties

A new heavy rare-earth-based Dy₃₆Ho₂₀Al₂₄Co₂₀ bulk metallic glass (BMG) with high thermal stability and excellent magnetocaloric properties has been prepared by a copper mold casting. Compared with the other known rare-earth-based BMGs, this BMG alloy possess higher glass transition temperature and crystallization temperature, larger effective activation energy for glass transition and crystallization. Under a modest magnetic field, this BMG alloy exhibits a comparable or even larger magnetocaloric effect than that the previously reported RE-based BMGs and crystalline compounds. The high thermal stability and the excellent magnetocaloric effect together with other merits of the BMGs make this BMG alloy suitable candidate for use as magnetic refrigerant in a temperature range below 50 K.     

A new Cu–Hf–Al ternary bulk metallic glass with high glass forming ability and ductility

We have discovered a new Cu-based bulk metallic glass (BMG). Although of a simple Cu₄₉Hf₄₂Al₉ ternary composition, the as-cast alloy is a monolithic, uniform BMG with a critical diameter as large as 10 mm. The width of the supercooled liquid region ΔT_x and the reduced glass transition temperature T_rg for this glass are 85 K and 0.62, respectively. In addition to its high glass-forming ability and high density of 11 g/cc, this BMG exhibits high ductility with a compressive plastic strain of 11–13%, making it a good candidate for applications as well as for studies of deformation behavior of Cu-based BMGs.     

Fe–C micro-alloying effect on properties of Zr53Al11.6Ni11.7Cu23.7 bulk metallic glass

(Zr₅₃Al_11.6Ni_11.7Cu_23.7)_1–x(Fe_77.1C_22.9)_x (x=0−2.2, at.%) bulk metallic glasses (BMGs) were prepared by copper mold suction casting method. Their glass forming ability and physical and chemical properties were systematically investigated. The glass forming ability is firstly improved with increasing x, and then decreased when x exceeds 0.44 at.%. Both glass transition temperature and crystallization temperature are increased, while the supercooled liquid region is narrowed, with Fe–C micro-alloying. The hardness, yielding and fracture strength, and plasticity firstly increase and then decrease when x reaches up to 1.32 at.%. The plasticity of the BMG (x=1.32 at.%) is six times that of the Fe-free and C-free BMG. In addition, by the Fe–C micro-alloying, the corrosion potential is slightly decreased, while the corrosion current density increases. The pitting corrosion becomes increasingly serious with the increase of Fe and C content.     

The characterization of plastic deformation in Ce-based bulk metallic glasses

The plastic deformation behavior of Ce₆₈Al₁₀Cu₂₀Nb₂ and Ce₇₀Al₁₀Cu₂₀ bulk metallic glasses (BMGs) at room temperature was studied by depth-sensing nanoindentation and microindentation. It is shown that the two BMGs exhibit a continuous plastic deformation without distinct serration at the all of the studied loading rates during nanoindentation. An obvious creep displacement was observed during the holding-load segment at the maximum load for the two alloys, and the magnitude of creep during holding-load increases with loading rate. The subsurface plastic deformation zone of the two BMGs after indentation at various loading rates was investigated through bonded interface technique using depth-sensing microindentation. A highly developed shear banding pattern can be observed in the plastic deformation region, though the global load–depth curves illuminate a “homogeneous flow”. The plastic deformation behavior of the Ce-based BMGs during indentation measurements is discussed in terms of localized viscous flow.     

New ternary Ni–Ta–Sn bulk metallic glasses

A new ternary Ni-base bulk metallic glass (BMG) system, Ni–Ta–Sn, was explored. There exists a wide BMG forming regime, in at.%, 33<Ta<38 and 2<Sn<9 with cast BMG rods of at least 1 mm diameter. The crystallization temperature being among the highest in Ni-based BMGs, 762–671 °C decreases with Sn content. These BMGs have wide super-cooled liquid region 57–61 K. They show high hardness, typically HV∼1000, and a compressive fracture strength 856–1192 MPa. The elastic strain limit is 0.0237–0.0266. The potentiodynamic studies in a solution of 12 M HCl show a passivation current density about 0.18 A m⁻² for Ni₅₈Ta₃₆Sn₆ BMG in the anodic region revealing a good corrosion resistance.     

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.     

Ternary La–Al–C bulk metallic glasses

Glass formation, thermal stability, elastic moduli and mechanical properties of La–Al–C bulk metallic glasses (BMGs) were investigated. The BMGs of La_57.5Al_32.5C₁₀, La₆₀Al₃₀C₁₀, and La_62.5Al_27.5C₁₀ with the maximal diameters up to 3 mm can be synthesized. Compared with other La-based glassy systems, like La–Al–(Co, Ni, Cu), the La–Al–C BMGs with the similar La concentration exhibit higher glass transition temperatures (507–577 K) and elastic constants. The La–Al–C BMGs with the unusual T_g and elastic moduli fill a gap in thermal and elastic properties between light rare-earth-based BMGs and heavy rare-earth-based ones. The compressive yield strength of La_57.5Al_32.5C₁₀ BMG reaches to ∼1.1 GPa, higher than that of other reported La-based BMGs. The ternary BMGs with simple element constituent, high glass-forming ability and superior mechanical properties are significant for fundamental research as model materials.     

Bendable bulk metallic glass: Effects of a thin,adhesive, strong,and ductile coating

We demonstrate, for the first time, that a thin, strong, ductile, and adhesive coating renders bulk metallic glasses (BMGs) bendable. The bending ductility of 3 mm thick BMGs, Zr₅₀Cu₃₀Al₁₀Ni₁₀ in this case, can be dramatically enhanced from ～0% to ～13.7% by the deposition of a thin bilayer film on the tensile side of the BMG sample. The bilayer, consisting of a 25 nm thick Ti adhesive layer with a 200 nm thick metallic glass (MG) overlayer, exhibits the required synergistic combination of good adhesion, high strength, and ductility compared with other single-layer films examined (Ti, TiN, and MG). Cross-sectional scanning and transmission electron microscopy, together with finite element modeling, reveal that the bilayer coating absorbs deformation while allowing more homogeneous formation of a high density of smaller shear bands at the bilayer/BMG interface. The bilayer coating, in turn, covers surface weak points and minimizes the formation of localized shear bands which lead to catastrophic failure under bending. As a result, the average shear-band spacing in bilayer-coated BMGs is small, 54 μm, and approximately equal to that found in bendable, 450 μm thick, MG ribbons. Thus, coated BMGs can accommodate large strains and overcome the MG size effect, without sacrificing their extraordinary mechanical properties. Our results for both coated and uncoated BMGs, as well as previously reported results for uncoated metallic glasses, with thicknesses ranging from ribbons to thin plates to bulk, are well described by a simple power law relationship between plastic strain to failure and shear band spacing. This scaling law may be useful in guiding future experiments toward producing more flexible BMGs.     

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.     

Forming of bulk metallic glass microcomponents

The present article considers forward extrusion, closed-die forging and backward extrusion processes for fabrication of individual microcomponents from two bulk metallic glass (BMG) compositions: Mg₆₀Cu₃₀Y₁₀ and Zr₄₄Cu₄₀Ag₈Al₈. Two types of tooling were used in the present work: relatively massive die sets characteristic of cold forming operations for crystalline metals and lightweight die sets adapted to the special characteristics of BMGs. In addition to demonstrating that microcomponents of several geometries can be readily fabricated from BMGs, rheological properties are combined with crystallization kinetics to formulate a generally applicable method that can guide selection of optimal forming parameters. Finally, the use of particulate-based lubricants for BMG forming is shown to result in individual lubricant particles becoming mechanically locked into the BMG surface.     

Formation,microstructure and properties of long-period order structure reinforced Mg-based bulk metallic glass composites

In situ Mg–Cu–Y–Zn bulk metallic glass (BMG) matrix composites, in which Mg solid solution flakes of 0.5–1 μm thickness and 2–10 μm length are dispersed, have been prepared by copper mold casting. The Mg flakes are characterized as a long-period order structure (LOS), i.e. periodic arrays of six close-packed planes distorted from the ideal hexagonal lattice of 6H-type. The formation mechanism of LOS is interpreted as the precipitation of the leading phase of the eutectic reaction above the glass transition temperature. In comparison with monolithic Mg-based BMG alloys, the composites with an LOS exhibit significant improvement in mechanical properties, e.g. a compressive plastic strain of ∼18% and ultimate strength of ∼1.2 GPa, have been measured in Mg₈₁Cu_9.3Y_4.7Zn₅ alloy. It is suggested that the enhancement of the mechanical properties of the composites can be attributed to the generation of multiple shear bands and the deformation of the LOS.     

Brazing method to join a novel Cu54Ni6Zr22Ti18 bulk metallic glass to carbon steel

This study joined a novel Cu-based bulk metallic glass (BMG) to carbon steel using a brazing process. Cu₅₄Ni₆Zr₂₂Ti₁₈ BMG was brazed to SS400 carbon steel using filler metal at 713?K under a compressive stress of 15?MPa for 3?min. Considering the relatively low glass-forming ability, pure Zn and Zn–Ag–Al filler metals were selected to suppress the crystallisation during a reheating thermal cycle. A sound bonding interface without notable chemical diffusion was obtained using the suggested brazing method, particularly with the Zn–Ag–Al filler. This gave rise to considerable improvement in shear bonding strength, from 9 to 45?MPa, when replacing the Zn filler with the Zn–Ag–Al filler. The present method also induced no marked change in phase composition of the BMG due to the low bonding temperature and short bonding time. Slight variations in glass transition temperature and crystallisation temperature would result from a thermal annealing effect.