| Research Abstract |
The compacts of Ti-Al intermetallic compounds were produced utilizing the powders prepared by mechanical alloying (MA) which has been recently developed as a new technique for combining metals. In the present study, the structure and mechanical properties were investigated, focusing on their high temperature workability.
First of all, as the bases of the present study, the small compacts of MA-processed powders having the several nominal compositions of Ti-50at%Al [TiAl(gamma) single phase], Ti-30at%Al [Ti_3Al(alpha_2) single phase], Ti-40at%Al and Ti-45at%Al (gamma+alpha_2) two-phase) were produced by vacuum hot pressing, and examined their structure and mechanical properties by compression tests. From these results, it was presumed that the compacts made from MA-processed powders exhibit superplastic deformation behaviors. Then, we intended to carry out the farther study by tensile deformation tests by making the larger compacts made by HIPing the similarly MA-processed powders.
The MA
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-HIPed compacts of a typical two-phase alloys, (1) Ti-45at%Al and so called Ti-rich TiAl alloy, (2) Ti-48at%Al, were prepared and offered to the X-ray diffraction and analytical TEM-observation using TEM accompanied by energy dispersive X-ray (EDX) analysis equipment. The compacts were consisted of the two-phase (gamma+alpha_2) structure, and composed of the fully ultra-fine equiaxed grain structure ( average grain size, d, of the compacts (1) and (2) were 1.2 and 0.9mum, respectively). The compacts (1) and (2) showed the maximum elongation to failure values of 405% at 1273K and the initial strain rate, epsilon_i=1.4x10^<-4>s^<-1>, and 549% at 1323K and epsilon^^・_i=5.6x10^<-3>s^<-1> which is comparatively higher strain rate, respectively. Namely, they showed typical superplastic behaviors. In addition to these facts, the activation energy of the creep, Q_c, at constant stress calculated from the results of the strain-rate-change tests of the compacts (2), in n=2 (n is the stress exponent, and strain rate sensitivity exponent, m=1/n) range of the present study, was 350kj/mole at high temperature (1173-1323K) and relatively higher epsilon range (between 5.6x10^<-4> to 2.0x10^<-2>s). Furthermore, the value of grain exponent, p, obtained from the grain size dependency of the steady creep rate of this compact was determined to be 2. Therefore, it concluded that the deformation mechanism of the superplastic Ti-rich TiAl HIPed compact (nominal chemical composition=Ti-48at%Al) made by the MA-processed powder), in the n=2 range, is grain boundary sliding accommodated by slip controlled by lattice diffusion of the slowest moving species in the Ti-Al phase. Thus, we proposed that the value of Q_c correspond to the activation energy for lattice diffusion, Q_L, of the slowest moving species is to be 350kj/mole. Less
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