Superplasticity in a bulk amorphous Pd-40Ni-20P alloy:a compression study

Compressive deformation behavior of a cast Pd₄₀Ni₄₀P₂₀ bulk metallic glass in the supercooled liquid region (589–670 K) was investigated at strain rates ranging from 10⁻⁴ to 10⁻² s⁻¹. The material exhibited excellent mechanical formability in the supercooled liquid region. However, in contrast to a Newtonian behavior generally observed in oxide glasses, the present alloy also showed a non-Newtonian behavior, depending upon the temperature and applied strain rate. Specifically, the alloy is like a Newtonian fluid at high temperatures, but becomes non-Newtonian at low temperatures and high strain rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution transmission electron microscopy. The non-Newtonian behavior is proposed to be associated with the glass instability during deformation.     

Plasticity and structural instability in a bulk metallic glass deformed in the supercooled liquid region

The deformation behavior of a bulk amorphous Zr–10Al–5Ti–17.9Cu–14.6Ni alloy was characterized in the supercooled liquid region. The alloy was observed to exhibit Newtonian behavior at low strain rates but to become non-Newtonian at high strain rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution electron microscopy. Experimental results showed the presence of nanocrystallites in the deformed samples, suggesting that the non-Newtonian behavior was associated with the concurrent crystallization of the amorphous structure during deformation; that is, a mixed crystalline-plus-amorphous structure was being tested. A mechanistic model based upon structural evolution has been developed to interpret the observed non-Newtonian behavior.     

Strain rate-dependent deformation in bulk metallic glasses

Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.     

High Temperature Deformation Behavior of High Strength and Toughness Ti-Ni Base Bulk Metallic Glass CompositesEI北大核心CSCD

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix in-creases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 degrees C and the intersection part with supercooled liquid region (SLR). When the temperature is 560 degrees C and the strain rate is 5x10(-4) s(-1), the stress sensitivity index m and the energy dissipation rate Psi are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.     

镁基大块非晶合金在过冷液相区流变行为本构关系

研究了Mg60Cu30Y10大块非晶合金在过冷液相区的流变行为.结果表明:随着温度升高和应变速率增加,平衡态的牛顿流转变为非平衡态的非牛顿流;其流变行为对于温度和应变速率非常敏感.由粘度与应变速率的关系,根据Arrhenius型VFT方程,确定了流动应力、应变速率和温度的关系.Mg60Cu30Y10大块非晶合金在过冷液相区的流变性能依赖于温度与变形速率,其微观机制可由自由体积模型解释,为大块非晶合金流变成形工艺的实现提供理论依据:温度高于玻璃转变温度以后,自由体积的增加使非晶合金变形过程中能够移动的原子数目随之增加,自由体积周围的原子沿外力的作用方向移动,即宏观上的塑性流变行为.应变速率增加,由热激活引起的自由体积增加不能满足更多原子流变所需的空间体积,导致牛顿流向非牛顿流转变.     

DEFORMATION BEHAVIOR AND CONSTITUTIVE EQUATION FOR Zr55Cu30Al10Ni5 BULK METALLIC GLASS IN SUPERCOOLED LIQUID REGION

Zr₅₅Cu₃₀Al₁₀Ni₅块体非晶合金(BMG)在过冷液态区内的单向压缩实验表明: 材料在过冷液态区内的形变行为强烈依赖于温度和变形速率. 随着应变速率的增加, 材料的流变特征由Newtonian流变转变为非Newtonian流变.利用扩展指数本构方程模型建立了非晶合金的流变应力、应变速率和温度的关系.     

Stabilization of metallic supercooled liquid and bulk amorphous alloys

Bulk metallic materials have ordinarily been produced by melting and solidification processes for the last several thousand years. However, metallic liquid is unstable at temperatures below the melting temperature and solidifies immediately into crystalline phases. Consequently, all bulk engineering alloys are composed of a crystalline structure. Recently, this common concept was exploded by the findings of the stabilization phenomenon of the supercooled liquid for a number of alloys in the Mg-, lanthanide-, Zr-, Ti-, Fe-, Co-, Pd–Cu- and Ni-based systems. The alloys with the stabilized supercooled liquid state have three features in their alloy components, i.e. multicomponent systems, significant atomic size ratios above 12%, and negative heats of mixing. The stabilization mechanism has also been investigated from experimental data of structure analyses and fundamental physical properties. The stabilization has enabled the production of bulk amorphous alloys in the thickness range of 1–100 mm by using various casting processes. Bulk amorphous Zr-based alloys exhibit high mechanical strength, high fracture toughness and good corrosion resistance and have been used for sporting goods materials. The stabilization also leads to the appearance of a large supercooled liquid region before crystallization and enables high-strain rate superplasticity through Newtonian flow. The new Fe- and Co-based amorphous alloys exhibit a large supercooled liquid region and good soft magnetic properties which are characterized by low coercive force and high permeability. Furthermore, homogeneous dispersion of nanoscale particles into Zr-based bulk amorphous alloys was found to cause an improvement of tensile strength without detriment to good ductility. The discovery of the stabilization phenomenon, followed by the clarification of the stabilization criteria of the supercooled liquid, will promise the future definite development of bulk amorphous alloys as new basic science and engineering materials.     

Finite element simulation and experimental investigation of forming micro-gear with Zr–Cu–Ni–Al bulk metallic glass

Bulk metallic glasses exhibit some unique physical properties as compared to their corresponding crystalline alloys. Due to the superplasticity by behaving like a Newtonian fluid in their supercooled liquid region, the bulk metallic glasses can be used to make high strength microparts by net-shape forming. In this paper, the compressive tests of Zr–Cu–Ni–Al metallic glass are performed with different strain rates at a temperature of 683 K. According to the experimental results, the forming evolution of a metallic glass micro-gear is simulated using a finite element simulation software DEFORM 3D, and the forming load is predicted at different processing parameters. Meanwhile, the filling stages of bulk metallic glass in the micro-gear mold cavity are investigated by finite element simulation and experiment. The predicted workpiece geometry shows good agreement with experimental result. The forming experiments for micro-gear of Zr–Cu–Ni–Al metallic glass are carried out by hot embossing process, and the amorphous micro-gears are obtained successfully. It is found that the finite element simulation results are in reasonable agreement with the experimental observation.     

Fictive stress model based finite element analysis for bulk metallic glasses at an elevated temperature

It is important to manufacture micromachine parts and simulate the deformation behavior of bulk amorphous alloys in a superccoled liquid region. For these purposes, a correct constitutive model that can reproduce viscosity results is essential for good predictive capability. In this paper, finite element analyses of nonlinear behaviour in bulk metallic glasses during die compression in the supercooled liquid region have been carried out based on the fictive stress model in conjunction with the Maxwell viscoelastic model. The friction effect between the work piece and the die played an important role in inhomogeneous deformation.     

Ce基非晶合金的研究进展

Ce基块体非晶合金具有低弹性模量、低玻璃化转变温度、良好非晶形成能力等特点,与传统的Zr基、Ti基、Fe基、Pd基等体系的块体非晶合金相比,Ce基块体非晶合金在过冷液相区内进行均匀塑性变形加工时所需的温度条件更低,由于具有与塑料材料相似的低温热塑性行为特征,因此也被人们称为金属塑料。本文主要介绍了Ce-Al-M、Ce-Al-Cu-X、Ce-La系和Ce-Ga系等块体非晶合金体系,并结合Ce基块体非晶合金不同于传统非晶合金的独特微观结构特征和性能优势,对其在微器件制造、数据存储等领域的应用前景进行了讨论,同时针对Ce基块体非晶合金研究目前存在的问题以及未来发展的方向进行了分析。     

A thermoplastic forming map of a Zr-based bulk metallic glass

A thermoplastic forming (TPF) map of a Zr₃₅Ti₃₀Be_26.75Cu_8.25 bulk metallic glass was constructed through systematic hot-embossing experiments, spanning a wide range of strain rates and temperatures in the supercooled liquid region. By comparison with the corresponding deformation map, it is found that Newtonian flow, non-Newtonian flow and inhomogeneous flow regions correspond well to fully filled, partially filled and non-filled regions, respectively, in the hot-embossing TPF map. Furthermore, the spatio-temporally homogeneous flow facilitates the thermoplastic formability of the Zr-based bulk metallic glass, which is rationalized in terms of free volume theory as well as by finite element simulations. Finally, our results are corroborated by potential application tests.     

Temperature effects on mechanical properties,deformation behavior and formability of Zr−Ti−Cu−Ni−Be−Nb bulk metallic glass composite

The deformation behavior and macroscopic formability of a Zr-based bulk metallic glass composite (BMGC) has been investigated in this study by performing a series of compression and laboratory-scale extrusion tests under various deformation rates within the supercooled liquid temperature region. The morphology of Zr−Ti−Nb-rich dendrite precipitates after warm deformation was first examined by using optical microscopy (OM) and a field emission scanning electron microscope (FE-SEM). The extrusion of this BMGC alloy within the supercooled liquid temperature region was found more difficult than the extrusion of other Zr-based monolithic BMG alloys, possibly due to the existence of dendrite phases hindering the characteristic viscous flow generated in the amorphous phase. A FEM simulation has also been carried out by utilizing the stress-strain behaviors obtained from high temperature compression tests, and the results have been compared with the experimental extrusion test results. The FEM simulation results for the extrusion process as well as a processing map based on a dynamic materials model (DMM) were found to agree relatively well with the actual macroscopic extrusion formability.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

喷射成形大尺寸镧基块体非晶过冷液相区内的塑性变形行为

研究了喷射成形大尺寸La62Al15.7(Cu,Ni)22.3非晶合金在过冷液相区内的塑性变形行为.结果表明,随加热温度的增加和应变速率的减小,该非晶合金由非稳态变形向单一稳态变形行为转变.当应变速率为5×10-3S-1,温度为443 K和挤压比为6.25时,喷射成形La62Al15.7(Cu,Ni)22.3非晶合金样品的密度由挤压前的5.723增加到挤压后的5.924 g/cm3,达到了同成分吸铸态非晶合金密度(6.134 g/cm3)的96.6%.挤压后非晶合金样品依然保持完全非晶态.     

Finite-element analysis for high-temperature deformation of bulk metallic glasses in a supercooled liquid region based on the free volume constitutive model

A constitutive relation based on the free volume model was developed to describe the strain-rate-dependent deformation behavior of bulk metallic glasses (BMG) at temperatures within the supercooled liquid region (SLR). Validity of the present approach has consequently been assessed by comparing the numerical results obtained from finite-element analyses with the experimental results previously obtained from compression tests of Vitreloy-1 BMG alloy. Finite-element-method simulations combined with free volume constitutive relations were found to reproduce well the plastic deformation behavior of Vitreloy-1 alloy, exhibiting a Newtonian viscous flow without stress overshoot and also a non-Newtonian viscous flow with stress overshoot at temperatures within the SLR. The present approach appears to provide a powerful means of understanding plastic deformation behavior in relation to localized and uniform deformation and also of making reliable formability estimations of BMG alloys.     

Arrhenius activation of Zr65Cu18Ni7Al10 bulk metallic glass in the supercooled liquid region

In the current research, the dynamic mechanical spectrum and compressive deformation of Zr₆₅Cu₁₈Ni₇Al₁₀ bulk metallic glass in the supercooled liquid region (SLR) are investigated. The experimental results prove the existence of transition from Newtonian flow to non-Newtonian flow in the metallic glasses. In addition, we found that the characteristic stress σ_tc, which is obtained by a stretched exponential function based on the normalized viscosity, can be regarded as a transition point from Newtonian to non-Newtonian flow. The correlation between strain rate sensitivity exponent and corresponding strain rate was obtained at a certain temperature. It is noted that the variation of transition strain rate from Newtonian to non-Newtonian flow with different absolute temperatures follows the Arrhenius equation. The activation energy is in good accordance with that using the mechanical spectroscopy method.     

Temperature,strain rate and reinforcement volume fraction dependence of plastic deformation in metallic glass matrix composites

A systematic study of mechanical properties is presented for Zr-based bulk metallic glass matrix composites, spanning a wide range of strain rates and temperatures, as well as various levels of reinforcement volume fraction. All of the experimental materials exhibit mechanical properties dominated by deformation of the amorphous matrix phase, including inhomogeneous flow and fracture at low temperatures, as well as homogeneous flow of both Newtonian and non-Newtonian character at high temperatures. In the homogeneous flow regime, the composites exhibit clear strengthening as the volume fraction of reinforcement increases. This strengthening effect is quantitatively explained in both the Newtonian and non-Newtonian regimes, and is found to arise from two contributions: (i) load transfer from the amorphous matrix to the reinforcements; and (ii) a shift in the glass structure and properties upon precipitation of the reinforcements. An additional source of apparent strengthening – in situ precipitation of reinforcement during deformation – is also discussed.     

镁基大块非晶合金在深过冷液相区的塑性变形

研究了Mg65Cu25Y7Nd3大块非晶合金在玻璃转化温度Tg附近及深过冷液相区的等温压缩变形行为。结果表明，Mg65Cu25Y7Nd3大块非晶合金的塑性变形与加热温度和加载时间紧密相关。在423K时该大块非晶合金具有一定的塑性，而在深过冷液相区则具有良好的塑性。通过系列试验，得出了Mg65Cu25Y7Nd3大块非晶合金的最佳加热温度为443～463K，加载时间约10min。对大块非晶合金在变形过程中的结构变化的分析表明，在本试验条件下，压缩变形对Mg65Cu25Y7Nd3大块非晶合金的晶化过程没有明显的影响。     

Mg-Cu-Y块体金属玻璃的塑性变形特性

通过对Mg-Cu-Y块体金属玻璃在深过冷液体区间塑性变形特性的研究，探讨了影响其塑性变形的因素及其影响规律。结果表明，加载温度和时间均对其塑性变形有明显的影响，在深过冷液体区间，要达到合适的变形量，加载温度和时间必须适中；Mg-Cu-Y块体金属玻璃在压缩条件下能够发生流变，较好地复制模具表面的显微形貌。同时，在加载条件下，Mg-Cu-Y块体金属玻璃更容易发生晶化。     

Deformation behavior of Zr-based bulk metallic glass in an undercooled liquid state under compressive loading

High temperature deformation behavior of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk amorphous alloy has been studied in a temperature range between 355 and 460°C under compressive loading after rapid heating. A transition of flow behavior, viz. from, a Newtonian to a non-Newtonian flow, has been reported by many researchers as the temperature is decreased at a given strain rate. In the present study, two different theoretical relations based on a viscous flow model and a transition state theory have been applied to analyze the transition behavior of deformation in terms of viscosity and flow stress. An experimental deformation map was then constructed to specify the boundaries between Newtonian and non-Newtonian flow, based on the relationship between the flow stress and strain rate in an undercooled liquid state. It has further been confirmed that the stress overshoot phenomena can be observed mostly in a non-Newtonian flow regime appearing in an intermediate temperature and strain rate region in this deformation map.