1991 Volume 55 Issue 11 Pages 1263-1268
A mixture of elemental powders of Ti(50 at%) and Ni(50 at%) was subjected to a mechanical alloying (MA) by a ball milling process. The processed powder was offered to the vacuum hot pressing at 1223 K for 3.6 ks under 150 MPa. The compact (relative density of approximately 99.8%) consisted of ultra-fine TiNi grains. The average grain size was 0.6 μm in diameter. The results of the examination of the temperature dependence of 0.2% proof stress showed that the flow stress of the compact rapidly deceased with increasing deformation temperature. During the compressive deformation at 1223 K and the initial strain rate of 6.7×10−4 s−1, the compact showed a steady state flow at relatively lower stress level such as about 16 MPa. The grain structure remained approximately equiaxed during the deformation. The value of strain rate sensitivity exponent of flow stress, m, was to be 0.32 at 1223 K. From these results, the deformation behavior of the compact at this temperature was presumed to be of superplasticity. The superplastic TiNi compact showed a large recovery stress of 300 MPa when the compressively 5% deformed (at room temperature, 294 K) specimen was heated at 373 K. Moveover, the strain by the deformation was fully recovered by shape memory effect at this temperature.
