Study Of Surface State And Mechanical Properties Of Cast γ-TiAl Alloy By Machining And Exposure At High Temperature

TiAl intermetallics, with low density, high specific strength and good oxidationresistance, is considered as a potential material for structural application. In this paper, thesurface states by different machining conditions and exposed at high temperature of thecast Ti-47.5Al-2.5V-1.0Cr(at.%)alloy were analyzed. The influence of the surface statesfor the alloy on their mechanical properties were studied.The test specimens were prepared by turning, rough grinding and fine grinding forstudy on the surface state by different machining conditions, including surface roughness,work-hardening and residual stress. It was found by the study that the surface roughness ofthree kinds of specimens was decreased in turn, the surface residual compress stress onaxial direction was increased in turn, while the stress on radial direction had a little change,the work-hardening of surface appeared on all of three kinds of specimens. The results oftensile test showed that the elongation of three kinds of specimens were increased a little,but the variance of the tensile strength was minor. It was found by analysis that theductility of specimens by fine grinding was increased a certain extent.The surface conditions of castγ-TiAl alloy exposed at high temperature werestudied, including the micro-appearance, thickness, structures and compositiondistribution of the alloy's oxide layer. According to practical working temperature, thetemperature of exposure was designed at 700, 750 and 800℃, and the time was designedon 100, 200 and 300h. It was found by study that the oxide layer consisted mainly of TiO₂and Al₂O₃ on which surface there were oxide grains. The thickness of the oxide layer thatexposed at 700℃was from 0.5 to 5μm while the thickness exposed at 750 and 800℃wasfrom 14 to 37μm. The distribution of every element in oxide layer was similar under allconditions, which were rich Ti₃Al layer, rich Al₂O₃ layer, rich TiO₂ layer and mixture ofAl₂O₃ and TiO₂ in turn from their matrix to external layer. The results of tensile testshowed that the average tensile strength of the specimens exposed at 750℃for differenttime was 453. 439 and 442MPa respectively, the average elongation was 0.17%, 0.18%and 0.23% respectively. The exposure made the tensile strength and the elongation atroom temperature lower. There were saw-shaped closed angles on the junction layer of theoxide sub-layers and developed easily stress concentration under external force to result incracks.