Low Temperature Superplasticity and Strain Induced Phase Transformation in Ti3Al Based Alloy

• Main Authors: Kai-Lin Yang, 楊凱琳
• Other Authors: J. Chih-Ching Huang
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/16795975209342398132

博士 === 國立中山大學 === 材料科學研究所 === 92 === Ti3Al based intermetallic alloys are attractive for aerospace and aircraft applications due to their superior high temperature properties. Excellent high temperature superplasticity in the Ti3Al-Nb based alloy has been widely published. However, the alloys become brittle and hard to deform at temperatures below 600oC so that low temperature superplasticity is difficult to develop. In the current super ��2 Ti3Al based alloy, the ordered BCC �� phase is the continuous matrix, with the DO19 hexagonal ��2 grains ~2.2 �慆 in grain size distributed uniformly in the �� matrix. The initial �� and ��2 volume fractions are around 60% and 40%, respectively, and strong textures are present in both phases. Although the alloy exhibits superior superplastic elongations over 1000% at 920-1000oC, the elongation drops appreciably to 600% at 900oC, 330% at 850oC and 140% at 750oC. Upon subsequent static annealing and superplastic loading at 700-960oC, the alloy tends to undergo �� to ��2’ phase evolution, approaching to the equilibrium phase partition at the respective temperature. The transformation seems to be enhanced during dynamic straining at temperatures lower than 900oC, suggesting the strain enhanced phase transformation. With the fine ��2’ laths inside the �� grains, the accommodation process across the BCC �� grains is impeded, leading to premature failure and lower tensile elongations at lower temperatures. Mechanical anisotropy is observed in this alloy and relatively higher tensile elongations are obtained in the 45o specimen as loaded at room temperature to 960oC. The texture characteristics appear to impose significant influence on the mechanical anisotropy at temperatures below 750oC (under the dislocation creep condition), as well as during the initial stage at a higher temperature of 920oC (under the superplastic flow condition). Systematic tracing of the texture evolution from the as-received to superplastically loaded specimens has been accomplished using electron backscattered diffraction. With the extensive dislocation motion plus a certain degree of grain boundary sliding and grain rotation during loading at 750oC, the ��2 grains gradually rotate to form the {0001} basal texture and some of the �� grains concentrate into the {111}< > orientation. At higher temperatures such as 920oC, extensive grain boundary sliding proceeds and results in grain orientation distributions for the ��2 and �� phases basically random in nature. Rationalizations for the mechanical anisotropy in terms of the Schimid factor calculations for the major and minor texture components in the ��2 and �� phases provide consistent explanations for the deformation behavior at lower temperatures as well as the initial straining stage at higher temperatures.