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

The coarse-grained pure Al was first pre-fatigued to different fatigue life fractions D(D=2%-75%) 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 investigated. The results show that with increasing D, the extrusion/intrusion phenomenon on the surfaces of the pre-fatigued coarse-grained pure Al 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 75%, 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 Al specimens were subjected to the uniaxial tension, the yield strength sigma(YS) obviously increases, but the change in sigma(YS) is not so obvious as D increases. Meanwhile, the ultimate tensile strength sigma(UTS) first decreases and then increases, and finally sharply re decreases. However, the pre-fatigued coarse-grained pure Al 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 Al has higher maximum uniform percent elongation.