<Original Papers>Effects of Carbon on the Ms Temperature and the Mechanical Properties in TiNi Alloys

In order to clarify effects of carbon on the Ms temperature and the mechanical properties in TiNi alloy, differential scanning calorimeter, tensile test, optical microscopic observation and EPMA were made for TiNi alloys and TiNiC alloys which were melted by arc furnace and high frequency induction furnace. The results are as follows : (1) When the quenching temperature was above 923K, the Ms temperature of TiNi alloys containing less than 49.60mol% Ni was 353K independent of X_, while that of TiNi alloys containing more than 49.60mol% Ni was represented by the experimental equation : Ms/K = 5010-93.9×(X_/mol%). (2) In the same heat treatment as mentioned above, the Ms temperature of TiNiC alloys (Xc=0.2～2mol%) decreased linearly with an increase of the corrected Ni concentration, (X_) over the range from 49.6 to 51.2mol% X_, assuming that all included carbon became stoichiometric TiC carbide. In this case, the Ms temperature was about 15K lower than that corresponding to same X_ in TiNi alloys. (3) When the quenching temperature was 673～773K, two-step transformations were observed in TiNi alloys and TiNiC alloys containing more than 50.0mol% X_. In this case, the Ms temperature was constant without X_ and the Ms^ temperature decreased linearly with an increase of X_. (4) Yield stress and yield strain obtained in tensile tests at room temperature and 418K were unaffected by including carbon, while fracture strain increased with including 0.2～0.5mol%C. (5) The dendritic or circular precipitates in arc melted TiNiC alloys were nonstoichiometric TiC carbide containing 1～2mol%Ni, 43～46mol%C. (6) In arc melted TiNiC alloys containing more than 51.0mol% X_, plate-like precipitates were observed which seems to be Ti_Ni_ as reported by Wasilewski et al