| In the practical engineering applications,the mechanical properties of materials will become weakened with the accumulation of fatigue damage.Therefore,it is of important practical significance to study the effect of cyclic pre-deformation on the mechanical behavior of materials.In recent years,the relevant research work has been gradually reported,but a deep understanding of the micro-mechanism still remains lacking.Therefore,in the present work,the Cu mono-and poly-crystals was well as the Cu-3at.%Al alloy are selected as the target material,and the dislocation structures are observed and the effect of cyclic deformation on uniaxial tensile mechanical behavior is studied systematically,in order for providing a useful experimental reference for the deformation micro-mechanism of typical FCC structure metals(such as Al,Cu,Cu-Al alloys)with different SFEs.For[011]multiple-slip-oriented Cu single crystals fatigued at different plastic strain amplitudes up to the occurrence of saturation stage,with increasing plastic strain amplitude from 1.3×10-4 to 5.0×10-3,the dislocation structures transform from veins and PSBs into labyrinth and loose cell-wall,and further into walls and cells;Compared to the unfatigued specimen,after[011]oriented Cu single crystal is pre-deformed,the tensile strength decreases slightly,whereas the yield strength and uniform elongation somewhat increase.For the specimen pre-deformed at high plastic strain amplitude,the surface damage is fairly slight after tensile deformation,and dense cell-wall structures formed provide greatly-enhanced tensile strength,while the pre-fatigue-induced preliminary deformation bands is the main reason for the increase of uniform elongation,so it shows comparatively good uniaxial tensile mechanical properties.For pure Cu polycrystals,with increasing total strain amplitude ??t/2 from 5.0 × 10-4 to 5.0 × 10-3,the dislocation structures transform from veins into walls,labyrinth and loose cells,and further into walls and cells;the cyclic stress increases and eventually all enters into a saturation stage.Compared to the unfatigued specimen,after pure Cu is pre-deformed to the same accumulated plastic strain at different ??t/2,the tensile strength decreases slightly,whereas the yield strength somewhat increases,but the uniform elongation decreases significantly at lower total strain amplitudes ??t/2 of 5.0 ×10-4 and 1.0 × 10-3;For the specimen pre-deformed at high total strain amplitude ??t/2 of 5.0 × 10-3,the surface damage is fairly slight after tensile deformation,and equiaxial cells formed provide greatly-enhanced tensile strength and uniform elongation,and cells and walls formed after pre-cyclic deformation increase the yield strength significantly,eventually resulting in better uniaxial tensile mechanical properties.For Cu-3at.%Al alloy,which is pre-deformed to the same accumulated plastic strain at different total strain amplitudes ??t/2(1.0 × 10-3-5.0 × 10-3),a typical planar slip dislocation structures of stacking faults occur at the low total strain amplitude ??t/2 of 1.0 × 10-3.With increasing ??t/2,the dislocation structures begin to transform into the wavy slip dislocation structures.The dislocation structures after cyclic pre-deformation are mainly composed of PSBs at the intermediate ??t/2 of 2.3 × 10-3,but of cells and walls at the high ??t/2 of 3.7 × 10-3 and 5.0 × 10-3.The cyclic stress of Cu-3at.%Al alloy increases with increasing of total strain amplitude,and eventually all enters into a saturation stage,just like the pure Cu polycrystals.Compared to the unfatigued specimen,after Cu-3at.%Al alloy is pre-deformed,the tensile strength increases slightly,whereas the yield strength increases obviously,although the uniform elongation decreases slightly.Especially for the specimen pre-deformed at the ??t/2 of 2.3 × 10-3,the surface damage is comparatively slight after tensile deformation,and the greatly-enhanced tensile strength and uniform elongation are attributed to dislocation cells and extended walls,respectively.Therefore,better tensile mechanical properties are achieved,showing a strengthening effect by low-cycle fatigue training. |
PDF Full Text Request