The Effect of Specimen Dimension on Martensitic Transformation and Phase Stability in Ti-Ni Shape Memory Alloys

¹ Teikyo University, School of Science and Engineering, Toyosatodai 1-1, Utsunomiya 320, Japan
² Teikyo University, now at Toho Electronics Inc., Tana 10313-2, Sagamihara 229, Japan
³ Metal Economics Research Institute, Japan, Toranomon 4-chome Mori Bldg., Toranomon 4-2-12, Minato-ward, Tokyo 105, Japan
⁴ Showa Wire and Cable Corp., Odasakae 2-1-1, Kawasaki-ward, Kawasaki 210, Japan

Abstract

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-Ni 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 T₀ = ( M s + A f ) / 2 . The TTI have been found to show linear relatioships against specimen volume per surface area ratio V/A. Small TTI observed In thin films deposited on Cu-substrates with magnetron sputtering have been found to be attributed to their small size since their TTI are on the extrapolated lines of the arc-melted specimens. The Increase of T₀ 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.