非比例载荷下Al-7Si-0.3Mg合金的循环特性及微观机理

FATIGUE FEATURES AND MECHANISM OF Al-7Si-0.3Mg CAST ALLOY UNDER NONPROPORTIONAL LOADINGS

在等效应变幅为0.22%时,研究了Al-7Si-0.3Mg铸造铝合金在比例、圆形、正方形、菱形、矩形与椭圆形路径下的循环变形行为,并用TEM观察了疲劳失效试样的位错结构.结果表明:在多轴加载条件下,材料均表现出循环硬化现象,循环硬化的速率和程度对加载路径有依赖性;非比例载荷下材料的疲劳寿命远小于比例加载时的寿命,且非比例加载下的疲劳寿命对各种非比例加载路径有依赖性,其中圆形路径下疲劳寿命最短;位错在不同的加载路径下形成不同的组态结构,位错与强化相、枝晶界及位错间的交互作用是铸造铝合金发生循环硬化的主要原因.

Widely application of cast aluminum alloy requires an understanding of its cyclic deformation behavior which is material dependent, and it is a complex function of strain amplitude, loading path, etc. In this study, multi-axial fatigue tests were conducted on cast Al-7Si-0.3Mg alloy with the same equivalent strain amplitude of 0.22% under six multi-axial path loadings, which were proportional, circular, square, diamond, rectangle and ellipse paths. TEM was employed to investigate the dislocation structures of the fatigue failure specimens. Cyclic hardening dominates the whole fatigue process under every loading path, but the rate and extent of cyclic hardening are quite dependent on particular loading paths. The fatigue life under nonproportional loading is much lower than that under proportional loading, and it also depends on the various nonproportional loading paths. The specimen with circular path loading has the shortest life and the most severe cyclic hardening among all the loading paths. The continuously changing of direction of maximum shear-stress plane is attributed to the complicated dislocation substructures and severe stress concentration during the cyclic process. The interaction among dislocation, particle and cell boundary is the main reason for cyclic hardening. The structure and density of dislocation in fatigue failure specimens under various loading paths exhibit quite different. From double dislocation bands, multiple dislocation bands, labyrinth structure to cell structure, the dislocation mobility decreases and stress concentration degree increases.