超轻双相镁锂合金的超塑性、显微组织演变与变形机理

采用熔铸、大变形轧制(加工率大于92%)和硝酸盐浴退火方法制备Mg-7.83%Li 合金与Mg-8.42%Li合金细晶板材,研究合金的超塑性、显微组织、空洞与断裂形貌和变形机制.计算α相(5.7%Li)和β相(11%Li)的扩散系数和Gibbs自由能,讨论573 K时超塑性晶粒长大的原因.结果表明:Mg-7.83Li和Mg-8.42Li合金分别获得850%和920%的最大超塑性;Mg-7.83Li合金在573 K时发生了显著的超塑性晶粒长大;在573 K和1.67×10~(-3) s~(-1)条件下制备的Mg-8.42Li合金中的空洞较少,且在变形区中随机而孤立地分布.断裂形貌观察发现Mg-8.42Li合金在573 K和5×10~(-4) s~(-1)条件下发生穿晶断裂;Mg-7.83Li合金在573 K和1.67×10~(-3) s~(-1)条件下发生沿晶界韧窝断裂.归一化实验数据与考虑位错数量的变形机制图对比表明合金超塑性变形机制为晶格扩散控制的位错调节的晶界滑移.

Superplasticity, microstructural evolution and deformation mechanism of super-light two-phase Mg-Li alloys

The fine-grained sheets of Mg-7.83%Li alloy and Mg-8.42%Li alloy were prepared by melting and casting, heavy rolling (reduction more than 92%) and nitrate bath annealing method, and their superplasticity, microstructure, cavitation, fracture morphology and deformation mechanism were investigated. The diffusivities and Gibbs free energy of α phase (5.7%Li) and β phase (11%Li) at 573 K were calculated to discuss the reason of superplastic grain growth. The results show that a few cavitations distribute randomly and isolatedly in the gauge length in Mg-8.42Li alloy at 573 K and 1.67×10~(-3) s~(-1). The transgranular fracture appears at 573 K and 5×10~(-4) s~(-1) in Mg-8.42Li alloy and dimple fracture along grain boundary appears at 573 K and 1.67×10~(-3) s~(-1) in Mg-7.83Li alloy. The maximum superplasticity of 850% and 920% are obtained in Mg-7.83%Li alloy and Mg-8.42%Li alloy. Obvious superplastic grain growth at 573 K appears in Mg-7.83Li alloy. The comparison of normalized experimental data with deformation mechanism map incorporating dislocation quantities inside grains reveals that the dominant deformation mechanisms in two alloys are grain boundary sliding accommodated by slip controlled by lattice diffusion.