Effect Of Cyclic Pre-deformation On The Uniaxial Tensile Properties Of Cu-7at.%Al Alloy

In practical engineering applications, the mechanical properties of materials will decrease with the accumulation of fatigue damage. Therefore, great attention has been paid to studies about the cyclic pre-deformation effect on the mechanical properties of materials. However, the influencing micro-mechanism of pre-fatigue deformation is still lacking of understanding. Furthermore, the dislocation structures in cyclically deformed metal crystals with an intermediate stacking fault energy (SFE) like Cu-7at.% Al alloy has been rarely studied. These investigations are extremely significant for further understanding the fatigue micro-mechanism of face-centered cubic (FCC) metals with an intermediate SFE. In view of this, Cu-7at.% Al alloy mono-and poly-crystals were adopted as target materials in the present work, and their microstructures induced by cyclic deformation were systematically revealed by transmission electron microscope (TEM), focusing on the effect of cyclic pre-deformation on the static mechanical behavior of Cu-7at.% Al polycrystals.For [112]Cu-7at% Al alloy single crystals, a high slip planarity (i.e., only formation of planar slip bands) contributes to the occurrence of a gentle cyclic hardening with a much lower saturation stress at a low yp1 of 4.5×10-4, and a mixed planar/wavy slip mode (e.g., persistent Luder’s bands/wall-like microstructures) at an intermediate ypi of 2.2×10-3 causes an obvious cyclic hardening up to a comparable saturation stress to that for Cu single crystals, while the deformation mode is dominated by wavy slip (e.g., ill-defined dislocation cells and walls) at the highest ypi of 7.2×10-3, causing that its cyclic hardening curve is quite similar to that for [112]Cu single crystals, and a slightly higher saturation stress level than that for Cu single crystals is thus reached due to the additional solid solution strengthening.For Cu-7at% Al alloy polycrystals, at a low total strain amplitude Ast/2 of 1.0×10-3,the dislocation structures consist mainly of planar slip bands with a low dislocation density, and the cyclic stress response curve exhibits all along a slow hardening process. As Δεt/2=2.3× 10-3, planer-slip dislocation structures still dominate the microstructures, but a few loose cells form in some grains; and cyclic stress increases further. As Δεt/2 increases to 3.7×10-3, the dislocation structure is mainly manifested by wavy slip characteristics, such as walls, elongated dislocation cells and loose cells, and the cyclic stress increases significantly. When Δεt/2 increases to the highest value of 5.0×10-3, the dislocation structure consists of walls and cells, which are more remarkable wavy-slip type dislocation structures compared to those formed in the [112]Cu-7at% Al alloy single crystal at high plastic strain amplitudes. In this case, the cross slip of dislocations has been activated more markedly, leading to a dramatic increase in the cyclic stress.After the pre-fatigue deformation at different total strain amplitudes (1.0×10-3-3.7× 10-3), as the strain amplitude increases, the tensile strength of the Cu-7at.% Al alloy polycrystal sample increases first and then decreases and later rises again, while the yield strength increases all the time. After the pre-cyclic deformation at Δεt/2= 3.7 x 10-3 for different cycles (50,100 and 2900 cycles), both the tensile strength and yield strength increases to different extents, depending on the pre-cycles, while the elongation of all the samples decreases slightly, compared with the cases of unfatigued samples. In particular, it deserves to point out that, after the pre-cyclic deformation at Εεt/2= 3.7 x 10-3 for 100 cycles, the surface damage of sample is fairly slight, and some loose and well-developed dislocation cells have been induced in the microstructure; meanwhile, after subsequent tensile deformation, a large quantity of deformation twins with a nano-size thickness form. Therefore, such a pre-fatigue treated Cu-7at.% Al alloy sample shows optimal uniaxial tensile mechanical properties, i.e., greatly-enhanced yield strength and tensile strength as well as a minimum decrease in elongation.