STRESS RELAXATION DYNAMICS FOR AMORPHOUS ALLOYS BASED ON THE EVOLUTION OF MICROSTRUCTURAL HETEROGENEITY
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摘要: 本文研究了Zr48(Cu5/6Ag1/6)44Al8 非晶合金应力松弛行为与固有微观结构非均匀性之间的关联. 非晶合金是典型非平衡固体, 其应力松弛过程伴随着老化效应, 本文首次考虑了经典KWW方程最可几特征时间
$\tau $ 和扩展指数$\beta $ 在应力松弛过程中的耦合演化, 这表明探究应力松弛过程中应力松弛响应必须考虑结构状态的时间依赖性. 基于所研究非平衡状态非晶合金结构状态的演化, 厘清了非晶合金应力松弛行为中老化效应. 研究结果表明, 非晶合金应力松弛行为具有典型非指数特征, 单一特征时间的指数弛豫形式与有限特征时间的有限谱方法均无法合理描述非晶合金应力松弛实验, 这是由于非晶合金微观结构非均匀性所导致的特征时间谱连续分布. 此外, 非晶合金的名义弹性区域应力松弛行为与初始应变无关, 这主要是因为非晶合金应力松弛行为的流变本质, 即弹性和滞弹性可逆性变形随时间推移逐渐转化为黏塑性不可逆变形. 最后, 考虑了老化效应引起的结构参量演化以及进一步导致的变形行为改变, 应力松弛特征时间随时间逐渐增加, 扩展指数随时间逐渐减小.Abstract: The correlation between stress relaxation behavior and the evolution of microstructural heterogeneity of Zr-based amorphous alloys was probed. Amorphous alloy is the typical non-equilibrium solid system. Stress relaxation process is accompanied by aging effect of the amorphous alloy. In the current work, the coupling evolution of the most probably characteristic time$\tau $ and stretched exponent$\;\beta $ in the classical stretched function is considered for the first time, indicating the time dependence of structural state in the stress relaxation process. Based on the evolution of the non-equilibrium structure state of the amorphous alloys, the aging effect in the stress relaxation behavior was clarified. The results demonstrated that the stress relaxation behavior of amorphous alloys has non exponential characteristics. The exponential relaxation form of single characteristic time and the finite spectrum method of finite characteristic time cannot describe accurately the experimental results. This phenomenon is ascribed to the continuous distribution of the characteristic time spectrum caused by the microstructural heterogeneity of amorphous alloys. In addition, the stress relaxation behavior in the apparent elastic region is independent of the initial strain, which is due to the macroscopic flow nature of the stress relaxation behavior. Elastic and anelastic reversible deformation is transformed into viscoplastic (irreversible) deformation. Finally, Evolution of structural parameters caused by aging during stress relaxation is considered, which is reflected in the increase of characteristic time$\tau $ and the decrease of stretched exponent$\;\beta $ .-
Key words:
- amorphous alloy /
- stress relaxation /
- high temperature deformation /
- nonelastic deformation /
- aging
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图 1 Zr48(Cu5/6Ag1/6)44Al8 非晶合金DSC曲线, 升温速率为20 K/min. 插图为X射线衍射图谱
Figure 1. DSC curve of Zr48(Cu5/6Ag1/6)44Al8 metallic glass with a heating rate of 20 K/min. Inset shows the XRD pattern of the model alloy
图 2 Zr48(Cu5/6Ag1/6)44Al8 非晶合金在给定应变0.6%条件下的应力响应. 测试温度为620 K, 加载前保温120 min以使结构状态相对稳定. (a) Debye弛豫模型(式(1))拟合曲线; (b)和(c)分别为n = 2和n = 4情况下有限谱模型拟合曲线
Figure 2. Stress response of Zr48(Cu5/6Ag1/6)44Al8 metallic glass at a given strain of 0.6%. The test temperature is 620 K, and the temperature is kept for 120 min before loading to make the structural state relatively stable. (a) Debye model fitting curve (Eq. (1)); (b) and (c) are the fitting curves of the finite spectrum approach when n = 2 and n = 4, respectively
图 3 (a) 非晶合金微观结构非均匀性示意图; (b) 动力学特征时间分布; (c) Zr48(Cu5/6Ag1/6)44Al8 非晶合金在给定应变0.6%条件下的应力响应. 红色曲线为式(3)计算曲线, 绿色曲线为特征时间Gauss形式连续谱; (d) 应力松弛实验曲线及KWW方程最小二乘拟合结果.
Figure 3. (a) The schematic illustration of microstructural heterogeneity of MG. (b) Distribution of dynamic characteristic time. (c) The stress relaxation data at 620 K was calculated by Eq. (3), and the corresponding continuous
$ \tau $ spectrum was obtained. (d) Comparison between theoretical calculations (lines) and experiments (symbols) for stress relaxation behavior
图 4 (a) Zr48(Cu5/6Ag1/6)44Al8 非晶合金不同应变(0.2% ~ 0.4% ~ 0.6% ~ 0.8%)典型应力松弛曲线. 测试温度为620 K, 加载前保温5 min; (b) 归一化应力松弛曲线
${\sigma \mathord{\left/ {\vphantom {\sigma {{\sigma _0}}}} \right. } {{\sigma _0}}}$ Figure 4. (a) Typical stress relaxation curves at different given strain (0.2% ~ 0.4% ~ 0.6% ~ 0.8%). The test temperature is 620 K, and the temperature is kept for 5 min before loading. (b) Normalized stress relaxation curves
${\sigma \mathord{\left/ {\vphantom {\sigma {{\sigma _0}}}} \right. } {{\sigma _0}}}$ 
图 5 老化效应. (a)老化导致非晶态固体体积(V)、焓(H)、熵(S)等参量降低和缺陷位点湮灭; (b)归一化储能模量
$E'/{E_u}$ 和损耗模量$E''/{E_u}$ 随退火时间演化.${E_u}$ 为室温未弛豫储能模量; (c)不同退火时间后应力松弛曲线, 测试温度为620 KFigure 5. Aging effect. (a) Evolution of volume (enthalpy, entropy) and annihilation of defect sites caused by aging. (b) Evolution of normalized storage modulus
$E'/{E_u}$ and loss modulus$E''/{E_u}$ during annealing;${E_u}$ is unrelaxed storage modulus at room temperature. (c) Stress relaxation curves after annealing for different time
图 6 Zr48(Cu5/6Ag1/6)44Al8 非晶合金在给定应变0.6%条件下应力松弛曲线, 测试温度为620 K. 红色曲线为采用扩展指数方程的最小二乘拟合
Figure 6. Stress relaxation curve at given strain of 0.6% (experimental temperature is 620 K). The red solid curve is the least square fitting using the KWW equation
图 7 Zr48(Cu5/6Ag1/6)44Al8 非晶合金在给定应变0.6%条件下应力松弛演化曲线, 测试温度为620 K, 加载前保温 5 min. 红色曲线为修正KWW方程计算结果. 绿色曲线为根据初始态参量计算曲线
Figure 7. Stress relaxation curve at given strain of 0.6 %. The test temperature is 620 K, and the temperature is kept for 5 min before loading. The red solid curve is calculated by the modified KWW equation. The green solid curve is the calculated curve based on the initial state
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