Transient regimes during high-temperature deformation of a bulk metallic glass: A free volume approach

The homogeneous deformation of a zirconium-based bulk metallic glass is investigated in the glass transition range. Compression and stress-relaxation tests have been conducted. The stress–strain curves are modeled in the framework of the free volume theory, including transient phenomena (overshoot and undershoot). This approach allows several physical parameters (activation volume, flow defect creation and relaxation coefficient) to be determined from a mechanical experiment. This model is able to rationalize the dependency of stress overshoot on relaxation time. It is shown that, due to the relationship between flow defect concentration and free volume model, it is impossible to determine the equilibrium flow defect concentration. However, the relative variation of flow defect is always the same, and all the model parameters depend on the equilibrium flow defect concentration. The methodology presented in this paper should, in the future, allow the consistency of the free volume model to be assessed.     

Deformation of metallic glasses with special emphasis in supercooled liquid region

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Isothermal nanocrystallization behavior of bulk metallic glass composites in supercooled liquid region

Bulk metallic glass composites(BMGCs)reinforced by micro-(spherical size of about 10 μm) and nano-sized(20-50 nm) Al_3Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu_(40)Zr_(44)Ag_8Al_8 BMGCs in the supercooled liquid region was studied by differential scanning calorimeter(DSC), X-ray diffractometer(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The mechanisms for nucleation and growth of the crystalline phases precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed.Results show that the BMGCs experience a primary crystallization process. Nano-sized Al_3Zr and Cu_(10)Zr_7 phases precipitate from the amorphous matrix successively. Furthermore, isothermal annealing has no effect on the preexisting particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately.     

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.     

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.     

Zr_(55)Al_(10)Ni_5Cu_(30)大块非晶合金在过冷液相区内虚拟应力模型的改进

针对描述非晶合金在过冷液相区流动行为的虚拟应力模型虽然能定性分析应力-应变关系,但定量计算误差很大的问题,以Zr55Al10Ni5Cu30为例,采用MATLAB遗传算法优化弹性模量、最大应力和最大松弛时间,提高了模型对应力峰值及稳态应力值的计算精度;提出时间调整因子概念,可提高对应力变化历史描述的准确度;改进后的虚拟应力模型与Zr55Al10Ni5Cu30大块非晶合金单轴压缩实验的结果吻合较好。     

Correlations between the relaxed excess free volume and the plasticity in Zr-based bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) with Nb minor alloying have been fabricated with different free volume (FV) trapped in. FV is evaluated by the relaxed excess free volume (REFV) after annealing just below T_g through loop thermal expansion tests. The results show that there is a qualitative correlation between the plasticity and REFV in Zr-based BMGs. The larger amount of excess FV the BMGs relax, the better plasticity they exhibit. With 1.5% Nb addition, the brittle Zr₆₅Cu₁₅Ni₁₀Al₁₀ BMGs possess REFV up to about 0.428% and exhibit the relatively good plasticity up to 25.6%. This provides a promising way to estimate the plasticity of BMGs and design new ductile BMGs through the minor alloying.     

Role of heterogeneity on deformation behavior of bulk metallic glasses

The isothermal section of the phase diagram of the Zr–Ni–Sb ternary system at 870 K in the whole concentration range has been constructed by means of X-ray and metallographic analyses. Eight ternary intermetallic compounds Zr₆NiSb₂ (Zr₆CoAl₂-type), Zr₅Ni_0.5Sb_2.5 (W₅Si₃-type), Zr₅Ni_0.9Sb₃ (Hf₅CuSn₃-type), ZrNi₂Sb (ZrPt₂Al-type), ZrNiSb (TiNiSi-type), Zr₃Ni₃Sb₄ (Y₃Au₃Sb₄-type), Zr₂Ni_0.7Sb_3.3 (Zr₂CuSb₃-type), and Zr₃NiSb₇ (own structure type) are formed in this system at 870 K.     

Zr基非晶合金在过冷液相区的高应变速率压缩变形行为

利用Gleeble1500热模拟机研究了Zr41.2Ti13.8Cu12.5Ni10Be22.5非晶合金在过冷液相区内633、653和673K,应变速率分别为2×10-2s-1和2×10-1s-1条件下的单向压缩变形行为.结果表明:在673 K时两种应变速率下,该合金都具有很好的塑性,尤其在2×10-2s-1时流变应力只有74 MPa,非常适于进行超塑性加工.对非晶合金的断口进行了观察,得到柱状非晶合金压缩变形时外观和断口形貌随着变形条件的变化规律.采用自由体积模型对非晶合金的形变和断裂的微观机制进行了分析.     

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.     

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.     

The effect of nanocrystallization and free volume on the room temperature plasticity of Zr-based bulk metallic glasses

To understand the mechanism of the room temperature plasticity of bulk metallic glasses (BMGs), microstructure observations, density measurements and positron annihilation studies were carried out for Zr-based BMGs cast at various temperatures and post-annealed under different conditions. We found that higher casting temperatures cause partial crystallization, which enhance the plasticity as long as the volume fraction of the crystalline phase is low. However, a similar nanocrystalline microstructure produced by post-annealing often leads to a large loss of plasticity, while certain conditions enhance the plasticity. Based on density measurements and positron annihilation lifetime spectroscopy, we conclude that the relative contribution of free volume and nanocrystallization is important for acquiring plasticity in metallic glasses.     

Shear transformation zone dynamics model for metallic glasses incorporating free volume as a state variable

A mesoscale model, shear transformation zone dynamics (STZ dynamics), is employed to investigate the connections between the structure and deformation of metallic glasses. The present STZ dynamics model is adapted to incorporate a structure-related state variable, and evolves via two competing processes: STZ activation, which creates free volume, vs. diffusive rearrangement, which annihilates it. The dynamical competition between these two processes gives rise to an equilibrium excess free volume that can be connected to flow viscosity via the phenomenological Vogel–Fulcher–Tammann relation in relaxed structures near the glass transition temperature. On the other hand, the excess free volume allows glasses to deform at low temperatures via shear localization into shear bands, even in the presence of internal stress distributions that arise upon cooling after processing.     

Diffusion bonding of a Zr-based metallic glass in its supercooled liquid region

Based on the analyses of the influences of temperature and pressure on diffusion behavior with time, a Zr-based bulk metallic glass is successfully bonded in its supercooled liquid region. The temperature is found to be a key factor for the success of the bonding process if sufficient pressure and time are adopted. Three-point bending tests show that the flexural performance of the samples bonded with various parameters is significantly different. The parameters in favor of diffusion bonding do not automatically benefit the performance of the bonded materials. Diffusion makes the nano-voids shrink to form metallurgical bonding at the interface and it causes structural relaxation leading to the embrittlement inside bulk. Balance between the two aspects is the key to the bonding success and the utility of the bonded materials.     

Homogeneous deformation of Ti41.5Cu37.5Ni7.5Zr2.5Hf5Sn5Si1 bulk metallic glass in the supercooled liquid region

The homogeneous deformation behavior of Ti_41.5Cu_37.5Ni_7.5Zr_2.5Hf₅Sn₅Si₁ BMG has been investigated by compression tests. The results show that its high-temperature deformation behavior is strongly dependent on strain rate and temperature, and there exists a transition from non-Newtonian flow to Newtonian flow with decrease in strain rate, which can be explained based on the transition state theory. In addition, this alloy can reach a large compressive strain than 0.8 at high strain rate; however the much higher flow stress and lower value of S parameter compared with typical BMGs indicates its worse formability in the SLR. A beneficial domain (temperature and strain rate) for optimum hot workability of this alloy has been roughly located by constructing the power dissipation efficiency map, where the power dissipation efficiency is larger than 0.8.     

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.