CoCrFeNiAlx系高熵合金在室温高速动态冲击下的力学响应机理

The high-entropy alloys (HEAs) were recently developed based on the bulk metallic glass and the structural intermetallic compounds. Numerous research reports have revealed that the HEAs exhibit some exceptional properties compared with conventional alloys, such as high strength and good ductility, good wear, corrosion, and high-temperature softening resistances, excellent low- and high-temperature mechanical properties, and potential soft magnetic property, etc. These features lead to high-entropy alloys with a huge potential in the application of newly structural materials. It is known that structural materials often encounter with sudden occurrences during service, as an example for impact loading. However, most research directions in laboratory focus on quasi-static loading at ambient temperature, which yields a severe conflit between practical applications with experimental investigations on structural materials. Therefore, this project will firstly break through the limitation of quasi-static loading by adopting impact loading produced by the Split Hopkinson Pressure Bar. Based on this technique, the mechanical property and failure behavior of HEAs at high strain rates can be investigated in detail. Moreover, in combination with experimental data, to construct constitutive equations for HEAs undergoing high-strain-rate deformation will be done. This study aims on the elucidation of mechanical behavior and deformation mechanism of HEAs upon dynamic loading.

高熵合金是近年来基于大块非晶和金属间化合物结构材料的开发而提出的新型结构材料。大量的研究结果表明，高熵合金具有一些传统合金无法比拟的优异性能，如高强度大塑性，良好的耐磨、耐腐蚀和抗高温软化特性，优异的低温和高温力学性能，潜在的软磁学性能等，这些使得高熵合金非常具有成为下一代新型结构材料的潜质。结构材料在服役过程中会经常受到突发事件的影响，如冲击载荷。而实验室内的研究大部分基于室温准静态加载，这使得材料的实际应用与实验研究发生了严重的脱节。据此，本项目将突破准静态载荷的影响，采用分离式霍普金森杆进行高速动态冲击，研究高应变速率下高熵合金的力学性能和变形行为，并在实验基础上，构建起高应变速率下高熵合金的变形本构方程，通过实验和理论相结合，阐明高熵合金动态冲击载荷下的力学响应机理。

本项目突破高熵合金在准静态加载载荷的限制，采用分离式霍普金森压杆对其进行高速动态冲击，研究其在高应变速率载荷下的力学行为和变形机理。采用高真空电弧熔炼法成功制备出了CoCrFeNiAlx系高熵合金。研究表明，该高熵合金系在高速冲击下表现出显著的应变强化效应，更高的应变率敏感性，并且具有可观的压缩塑性，如具有面心立方（FCC）FCC+体心立方（BCC）双相结构的CoCrFeNiAl0.6高熵合金在应变率为4000s-1下其屈服强度接近1GPa，断裂强度超过1.5GPa，断裂应变约为40%，展现出了其在极端载荷下的应用潜力。透射电镜（TEM）和高分辨（HRTEM）分析认为，低Al含量的高熵合金，以其FCC结构和较低的层错能，在动态加载下往往会有大量的纳米孪晶产生，表现出显著的孪晶诱导塑性（TWIP）效应，而位错和孪晶强烈的交互作用，使得合金具有优异的强度和塑性的结合；而高Al含量的高熵合金，以其BCC结构和较高的层错能，在动态加载下只表现出单一的位错强化效应，这使得合金在强度提高的同时牺牲了部分塑性。最后，通过引入修正的John-Cook模型对CoCrFeNiAlx系高熵合金的塑性流变行为进行了动态本构描述，并建立了相应的动态本构方程。结果表明，这些方程可以很好的预测高熵合金在不同应变速率下的塑性流动行为。