The Rate-Temperature Dependent Compressive Mechanical Behavior and Deformation Mechanism of CrCoNiSi0.3 Medium Entropy Alloy
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摘要: CrCoNiSi0.3中熵合金在准静态加载下展现出优异的强韧协同力学行为，温度和应变率对合金力学行为的影响亟待研究。通过霍普金森压杆装置，对CrCoNiSi0.3中熵合金进行了室温（20 ℃）动态压缩（
1776 ～5196 s−1）实验和高温（20～1000 ℃）准静态压缩实验，系统研究了CrCoNiSi0.3中熵合金在不同应变率和温度下的力学行为和变形机理。结果表明：在动态加载下，CrCoNiSi0.3中熵合金的屈服强度随着应变率的升高而增大，表现出较高的应变率敏感性，该现象源于随着应变率增大而增加的层错、变形孪晶和面心立方到密排六方相变；平均应变硬化率随着应变率的升高先略有上升，当应变率达到约5196 s−1时，由于剪切带形成，平均应变硬化率下降。随着温度的升高，压缩屈服应力和应变硬化能力下降，但直至1000 ℃仍未表现出明显的应变软化现象；在600 ℃下合金有高密度层错和位错锁，表现出与400 ℃时基本一致的屈服强度和流动应力。针对应变硬化随应变率升高而降低的力学行为，在模型中引入应变硬化函数，建立了修正的Johnson-Cook本构模型，该模型可以较好地预测CrCoNiSi0.3中熵合金的力学行为。Abstract: The CrCoNiSi0.3 medium entropy alloy exhibits excellent synergistic mechanical behavior of strength and toughness under quasi-static loading. However, the influences of temperature and strain rate on the mechanical behavior of the alloy urgently need to be studied. Through the split Hopkinson pressure bar (SHPB), dynamic compression experiments at room temperature (20 ℃) with strain rates ranging from1776 s−1 to5196 s−1 and quasi-static compression experiments at high temperatures (from 20 ℃ to1000 ℃) were carried out on the CrCoNiSi0.3 medium entropy alloy. The strain rate and temperature-dependent mechanical behavior and deformation mechanism of the CrCoNiSi0.3 medium entropy alloy were systematically investigated. The results show that under dynamic loading, the yield strength of the CrCoNiSi0.3 medium entropy alloy increases with the increase of the strain rate, exhibiting a high strain rate sensitivity. This is attributed to the comprehensive mechanism of stacking faults, deformation twins, and the phase transformation from face-centered cubic to hexagonal close packed, which increase as the strain rate increases. The average work hardening rate increases slightly at first with the increase of the strain rate. When the strain rate reaches about5196 s−1, the average work hardening rate decreases due to the formation of shear bands. As the temperature rises, the compressive yield stress and work hardening ability of the alloy gradually decrease. However, there is still no work softening phenomenon at a temperature of1000 ℃. Due to the high density of stacking faults and dislocation locks in the alloy at 600 ℃, it shows almost the same yield strength and flow stress as at 400 ℃. Aiming at the mechanical behavior that the strain hardening shows a decrease with the increase of the strain rate, a strain hardening function was introduced into the model, and a modified Johnson-Cook constitutive model was established, which can predict the mechanical behavior of the CrCoNiSi0.3 medium entropy alloy quite well.-
Key words:
- medium entropy alloy /
- mechanical behavior /
- strain rate /
- strain hardening /
- deformation mechanism
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图 1 (a) 压缩样品的理论尺寸示意图和实物，(b) 止动环示意图及其实物
Figure 1. (a) Schematic diagram of the theoretical dimensions and the actual object of the compressed sample; (b) schematic diagram and the actual object of the stop ring
图 2 CrCoNiSi0.3 MEA的XRD谱(a)、微观结构的反极图(b)以及晶粒尺寸分布(c)
Figure 2. XRD pattern (a), the inverse pole figure of microstructure (b), and grain size distribution (c) of CrCoNiSi0.3 MEA
图 3 室温不同应变率压缩下CrCoNiSi0.3的准静态工程应力-工程应变曲线(a)、动态工程应力-工程应变曲线(b)、真实应力-真实应变曲线(c)、屈服应力柱状图(d)、应力-应变率双对数曲线（应力和应变率的单位分别为MPa和s−1）(e)和应变硬化率-真实应变曲线(f)
Figure 3. Quasi-static engineering stress-strain curves (a), dynamic engineering stress-strain curves (b), true stress-strain curves (c), yield stress histogram (d), double-log stress-strain rate curves (The units of stress and strain rate are MPa and s−1, respectively.) (e), strain-hardening rate-true strain curves (f) of CrCoNiSi0.3 under compression at different strain rates at room temperature
图 4 不同温度、准静态（10−3 s−1）压缩下试样的工程应力-工程应变曲线(a)、真实应力-真实应变曲线(b)、不同应变下压缩屈服应力随温度的变化(c)和应变硬化率-真实应变曲线(d)
Figure 4. Engineering stress-strain curves (a), true stress-strain curves (b), yield stress-temperature curves at different strains (c), and strain-hardening rate-true strain curves (d) of specimens under quasi-static compression (10−3 s−1) at different temperatures
图 5 室温压缩至应变为45.8%时试样的TEM形貌(a)、红色圆形区域（图5(a)）的衍射花样(b)和高分辨图像(c)、红色方框区域（图5(c)）的放大图(d)、剪切带形貌(e)以及红色圆形区域剪切带（图5(e)）的衍射斑点图像(f)
Figure 5. (a) TEM images of the specimen when compressed to a strain of 45.8% at room temperature; (b)−(c) diffraction pattern and high-resolution images in the circular region of Fig. 5(a), (d) enlarged image for the red rectangular area of Fig. 5(c), (e) shear band topography, and (f) diffraction spots in the circular region of Fig. 5(e), respectively
图 7 应变率为
5196 s−1的动态压缩后试样的TEM图像(a)～(b)、剪切带内部衍射斑点(c)、剪切带附近区域高分辨图像(d)及衍射斑点(e)～(f)Figure 7. TEM images of the specimen after dynamic compression with the strain rate of
5196 s−1 (a)−(b), diffraction spots inside the shear band (c), high-resolution images of areas near the shear band (d) and diffraction spots (e)−(f)
图 8 600 ℃准静态压缩后试样的(a) 形貌、(b)～(c) 图8(a)中红色圆形区域对应的衍射斑点和高分辨图像、(d) 剪切带形貌以及
1000 ℃高温准静态压缩后试样的(e) 形貌、(f)～(g) 高分辨图像、(h) 图8(g)中析出相的面扫图Figure 8. TEM images of specimens after high-temperature quasi-static compression experiment at 600 ℃ ((a)−(d)) and
1000 ℃ ((e)−(h)): (a) the topography; (b)−(c) diffraction spots and high-resolution images of circular region in Fig. 8(a); (d) the shear band; (e) topography images, (f)−(g) high-resolution images; (h) the surface map of the precipitated phase of Fig. 8(g)
图 9 (a)室温准静态（$ {T}_{\mathrm{r}}=20 $ ℃，$ \dot{\varepsilon }={10}^{-3} $s−1）压缩下试样的真实应力-真实应变曲线，(b) 高温准静态加载下$ \sigma /(A+B{\overline{\varepsilon }}_{\mathrm{p}}^{n}) $与T*的拟合曲线，(c) 高温准静态加载下真实应力-真实应变曲线，(d) $ {\sigma }_{i}/{\sigma }_{0} $-$ \mathrm{ln}\,{\dot{\varepsilon }}^{*} $拟合曲线，(e) 室温动态加载下试样的真实应力-真实应变曲线
Figure 9. (a) True stress-strain curve of the specimen under quasi-static compression at room temperature ($ {T}_{\mathrm{r}}=20 $ ℃, $ \dot{\varepsilon }={10}^{-3} $s−1); (b) $ \sigma /(A+B{\overline{\varepsilon }}_{\mathrm{p}}^{n}) $ - T* curve of the specimen under quasi-static compression at high temperature; (c) true stress-strain curves of the specimen under quasi-static compression at high temperature; (d) $ {\sigma }_{i}/{\sigma }_{0} $-$ \mathrm{ln}\,{\dot{\varepsilon }}^{*} $ fitting curves; (e) true stress-strain curves of specimens under dynamic compression at room temperature
图 10 第1次修正后的拟合结果：(a) 高温准静态压缩下不同应变试样的$ \sigma /(A+B{\overline{\varepsilon }}_{\mathrm{p}}^{n}) $- T*拟合曲线，(b) 高温准静态压缩下的真实应力-真实应变曲线，(c) $ {\sigma }_{i}/{\sigma }_{0} $-$ \mathrm{ln}\,{\dot{\varepsilon }}^{*} $拟合曲线，(d) 室温动态加载下试样的真实应力-真实应变曲线
Figure 10. Fitting results after the first revision: (a) $ \sigma /(A+B{\overline{\varepsilon }}_{\mathrm{p}}^{n}) $ - T* curve of the specimen under quasi-static compression at high temperature; (b) true stress-strain curves of specimens under quasi-static compression at high temperature; (c) $ {\sigma }_{i}/{\sigma }_{0} $-$ \mathrm{ln}\,{\dot{\varepsilon }}^{*} $ fitting curves; (d) true stress-strain curves of specimens under dynamic compression at room temperature
图 11 (a)
5196 s−1应变率下真实应力和温升随真实应变的变化，(b) 真实应力-真实应变实验数据与是否引入温度因素的J-C模型计算结果对比Figure 11. (a) Changes of true stress and temperature rise with true strain at a strain rate of
5196 s−1; (b) comparison of true stress-strain between the experimental data and the J-C model results with and without considering the temperature
图 12 (a) 不同应变率和参考应变率下真实应力-真实应变曲线屈服段斜率之比$ {k}_{\mathrm{J}}/{k}_{\mathrm{E}} $，(b) 第2次修正J-C模型得到的室温动态加载下试样的真实应力-真实应变拟合曲线
Figure 12. (a) Ratio of the slope in the yield stage of true stress-strain curves under different strain rates to that under the reference strain rate $ {k}_{\mathrm{J}}/{k}_{\mathrm{E}} $; (b) true stress-strain fitting curves of specimens under dynamic loading at room temperature obtained by the J-C model with second modification
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