Effects Of Specimen Thickness And Pre-fatigue Deformation On The Uniaxial Tensile Behavior Of Coarse-Grained Pure Aluminum

With the development of Micro Electro Mechanical System (MEMS) and Micro System Technology (MST), studies of the size effect in mechanical properties of materials have attracted extensive research attention. In practical engineering applications, the mechanical properties of materials would often become weakened with the accumulation of fatigue damage during the process of their service. Therefore, studies on the general rule and mechanism for the influence of pre-fatigue damage on the mechanical properties of materials are of particular significance. In the present work, pure aluminium and copper, which possess typical face centered cubic (FCC) structure, are selected as the target materials to study the size effects in uniaxial tensile behavior and the effect of pre-fatigue on uniaxial tensile mechanical properties of coarse-grained pure aluminium.The plastic deformation and damage behavior of coarse-grained pure aluminium specimens with different thicknesses ranging from0.2mm to2.0mm were investigated at room temperature at a strain rate of10-3s-1under uniaxial tension. The corresponding results on the pure copper specimens with different thicknesses of0.1-2.0mm were also obtained for comparisons. As the ratio of specimen thickness t to grain size d, with increasing the Rt/d of the specimen, the ultimate tensile strength and uniform elongation increased. With decreasing the thickness of pure Al, the extrusion and intrusion phenomenon on the specimen surface near fracture area are not so serious, and necking phenomenon disappears, and meanwhile, the number of dimples on fracture reduces and their sizes decrease. As the thickness decreases to0.3mm, the dimples on fracture surface completely disappear. The shear lip area of the fracture of copper consists of dimples with different sizes, and the size of different dimples approaches and the number of dimples decreases with decreasing thickness, finally the dimples disappear as the thickness drops down to0.1mm. In these both types of materials, when Rt/d>1(i.e.,there are many grains in the thickness direction), because of the effects of grain boundaries, the dimples form on the tensile fracture surface. In contrast, when Rt/d≤(i.e., there is nearly only one grain in the thickness direction), the dimples are not found on the fracture surface. Since different levels of plastic deformation will be induced under the process of uniaxial tension of pure aluminium with different thicknesses, the dislocation structures change from irregular cell structures in thinner specimens into regular cell structures in specimen with intermediate thickness, and finally the dislocation structures are evolved into the subgrains in the thickest specimen (e.g.,2.0mm).The plastic deformation behavior of pure aluminium specimens, which were pre-fatigued at a stress amplitude of30MPa, were investigated at room temperature. With the fatigue damage grade D increasing, the extrusion and intrusion phenomenon on the fatigued specimen surface become more serious. As D>20%, the fatigue cracks start out to initiate on the surface of specimens. As D=5%, the fatigue dislocation structures consist mainly of loose dislocation tangles and dislocation cells, and with D increasing, the dislocation cell size and cell wall width decrease, and the dislocation density increases. As D is as high as50%, subgrains can be observed in this case. The plastic deformation of the pure Al specimens, which were subjected to pre-fatigue deformation to different levels of0%-75%, were investigated at room temperature with a strain rate of10-3s-1under uniaxial tension. It is found that, with D increasing, the ultimate tensile strength undergoes a process of decreasing, increasing, and sharp re-decreasing; this phenomenon can be attributed to the process of "hardening-softening-hardening" occurring in the cyclic pre-deformation of pure aluminium. The tensile deformation dislocation structures of the pure Al specimens, which were subjected to pre-fatigue deformation, were observed systematically by TEM, and it is found that, with increasing D, the cell structure tends to be observed in subgrains. Comparing to the remarkable strengthening phenomenon induced by the low-cycle fatigue training in other materials, there is only a slight effect of pre-fatigue deformation on the tensile properties of the present coarse-grained pure aluminium, resulting probably from the formation of such dislocation cells in subgrains.The plastic deformation and damage behavior of the coarse-grained pure aluminium specimens, which were subjected to a pre-fatigue damage of D=5%, were investigated at room temperature with a strain rate of10-3s-1under uniaxial tension. It is found that, with decreasing Rt/d of the specimen, the ultimate tensile strength and uniform elongation decrease. As Rt/d decreases to0.55, namely t=0.3mm, the ultimate tensile strength and uniform elongation decrease more sharply. With decreasing thickness, the extrusion and intrusion phenomenon on the specimen surface near fracture area-become much faint, and the plastic deformation degree decreases under uniaxial tension, and at the same time, dimples on fracture surface gradually disappear. As Rt/d≤1.82(i.e.,t≤1mm), the size and number of dimples drop more notably, and dimples on the fracture surface fully disappear, as Rt/d is reduced to0.36(i.e., t=0.2mm). Under the condition of Rt/d<0.9(t<0.5mm), the tensile deformation dislocation structures are mainly composed of cell structures, and meanwhile, ill-developed cell structures can be observed in the formed subgrains. As Rt/d is as high as3.64(i.e., t=2.0mm), the tensile deformation dislocation structures consist mainly of subgrains, in which ill-developed cell structures can still be observed.