A356合金低温拉伸力学性能变化机理研究

Variation Mechanisms of Tensile Mechanical Properties of A356 Alloy at Low Temperature

对A356合金在20℃、-20℃、-40℃、-60℃和-80℃低温环境下进行了拉伸力学性能测试,并利用扫描电子显微镜和金相显微镜对20℃、-20℃和-60℃三种环境的低温拉伸试样断口形貌和附近组织结构进行观察分析,并对断口深腐蚀后的Si相裂纹和铝基体中的位错滑移带进行分析。结果表明:当环境温度由20℃下降至-80℃时,A356合金的抗拉强度和屈服强度呈上升趋势,而塑性呈下降趋势;拉伸过程中,铝基体中位错在Si相边缘塞积,造成Si相破裂,形成大量微裂纹,在裂纹尖端产生的应力集中,使合金在低温下发生较小的塑性变形时就发生断裂,这是影响合金低温塑性下降的主要原因;随着温度的降低,铝基体内部的点阵阻力增加,塑性变形过程中位错运动困难,导致合金强度提高。同时合金的硬化指数升高,使合金的整体变形趋于均匀化。

The tensile mechanical properties of A356 alloy were tested at 20℃,-20℃,-40℃,-60℃ and -80℃.The fracture morphologies and the microstructures at 20℃,-20℃ and -60℃were researched by SEM and optical microscopy.The deep etching morphologies of damaged Si particles and dislocation slip band in the tensile specimens were investigated.The results showed that the tensile strength and yield strength increased,and the ductility decreased when the temperature dropped from 20℃ to -80℃.The dislocations piled up in ahead of Si phases during tensile test,which caused Si phase to crack.The stresses concentration in these cracks tip,which made the alloy to fracture before the large plastic deformation occurred.This is the main reason affecting the low temperature plasticity of the A356 alloy.With the temperature dropped,the Peirls-Nabarro force of aluminum increased,which caused the dislocations motions difficultly in the process of plastic deformation.So the tensile strength,yield strength and hardening exponent increased at low temperature,which caused the plastic deformation of the whole sample tended to homogenization.