The effect of specimen dimension on martensitic transformation and phase stability in Ti-Ni shape memory alloys

The behavior of martensitic transformation and the phase stability of martensite have been found to be influenced by specimen dimension, as a result of DSC measurement of arc-melted Ti-Bi shape memory alloy specimens with different weight. With increasing specimen weight, Ms and Af have increased, while As and Mf have decreased, both parabolically, resulting in the increases of transformation temperature intervals(TTI), Ms-Mf and Af-As, and of equilibrium temperature T-0=(Ms+Af)/2. The TTI have been found to show linear relatioships against specimen volume per surface area ratio V/A. Stall TTI observed in thin fills deposited on Cu-substrates with magnetron sputtering have been found to be attributed to their stall size since their TTI are on the extrapolated lines of the arc-melted specimens. The increase of T-0 can be explained to be due to the increase of strain energy brought about by shape strain associated with the transformation. Not only chemical free energy but also nonchemical strain energy has been found to contribute considerably to the phase stability of martensites.