预疲劳变形对粗晶纯Al单向拉伸行为的影响

EFFECTS OF PRE-FATIGUE DEFORMATION ON THE UNIAXIAL TENSILE BEHAVIOR OF COARSEGRAINED PURE Al

在恒应力幅下对粗晶纯Al进行不同疲劳寿命分数D(D=2%-75%)的预疲劳变形,研究预疲劳变形对其单向拉伸行为、断口表面变形特征和位错组态的影响.结果表明,随着D的提高,粗晶纯A1的表面挤出和侵入现象加重,晶内的不均匀变形程度增大,导致沿滑移带和晶界萌生微裂纹和微孔洞以及随后的连接,当D高达75%时,三叉晶界节点处产生较长的沿晶裂纹.疲劳位错组态由退火时松散的胞状结构演变成规整的胞状结构和亚晶,但亚晶尺寸并未发生明显变化.预疲劳的粗晶纯Al单向拉伸后,其屈服强度明显提高,但随D的提高,屈服强度变化不大,抗拉强度呈现先降低再升高和最终急剧下降的趋势,但其加工硬化能力较弱.单向拉伸断口由纤维区和剪切唇区组成,纤维区韧窝的数量随D的提高而增多,尺寸减小,当D=50%时,韧窝数量下降,尺寸增大,断口呈现撕裂特征.单向拉伸后的亚结构主要为亚晶和亚晶内的位错胞状结构,随着D的提高,亚晶先减小后增大,亚晶的细化和亚晶内位错胞状结构的形成使预疲劳后的粗晶纯Al具有较高的最大均匀延伸率.

The coarse-grained pure A1 was first pre fatigued to different fatigue life fractions D（D=2%-75 70） at a constant stress amplitude, and then the effect of pre-fatigue deformation on its uniaxial tensile behavior, fracture surface deformation features and dislocation structures were investi- gated. The results show that with increasing D, the extrusion/intrusion phenomenon on the surfaces of the pre-fatigued coarse-grained pure A1 becomes more serious, and the non-uniform deformation in grain interiors is also enhanced, leading to the nucleation of micro-cracks and micro voids along slip bands （SBs） or at grain boundaries （GBs） as well as their subsequent propagation. As D is as high as 7570, the longer intergranular cracks are produced at triple grain boundary nodes. With increasing D,the fatigue dislocation structures transform from loose cellular structures under annealing state into regular cellular structures and sub grains, but the size of sub grains nearly does not change. After the pre fatigued coarse grained pure A1 specimens were subjected to the uniaxial tension, the yield strength σYS obviously increases, but the change in σYS is not so obvious as D increases. Meanwhile, the ultimate tensile strength σUTS first decreases and then increases, and finally sharply re decreases. However, the pre fatigued coarse-grained pure A1 has poor ability to work hardening. The tensile fracture surface consists of fibrous and shear lip zones, and the number of dimples in fibrous zones increases and the size reduces with increasing D; as D reaches 50%, the number of dimples re reduces and the size raises, and the fracture surface exhibits tearing characteristics. The sub-structures after the uniaxial tension are mainly composed of sub-grains and cellular dislocation structures inside sub grains, and with increasing D, the size of sub grains first reduces and then increases. The formation of fine sub grains and cellular dislocation structures inside sub grains results in the fact that the pre- fatigued coarse grained pure A1 has higher maximum uniform percent elongation.