J. Phys. IV France
Volume 112, October 2003
Page(s) 747 - 749

J. Phys. IV France
112 (2003) 747
DOI: 10.1051/jp4:2003990

process during heat treatment on martensitic transformation in Ni-Ti and Ni-Ti-Cr alloys

J. Uchil, K.G. Kumara and K.K. Mahesh

Department of Materials Science, Mangalore University, Mangalagangotri, Mangalore 574199, India

This paper presents the effect of cooling process during heat treatment on martensitic transformation in 45% cold worked, Ti-rich, Ni-rich Ni-Ti and Ni-Ti-Cr shape memory alloys, annealed in the temperature range of $100^\circ-800^\circ$C. Phase transformation in water quenched and fumace cooled samples is studied in the range from $+140^\circ$ to $-170^\circ$C, employing four-probe electrical resistivity measurement technique. The heat treatment process has a distinct effect on the martensitic transformation in these alloy systems. In the case of Ti-rich alloy two-stage martensitic transformation, $\rm A \to R
\to M$ during cooling and M $\to$A on heating, are observed in the as-received sample. For both the quenched and the fumace cooled samples this behavior persists even after annealing at 420°C. The process spreads over a temperature interval of 60°, above 0°C, whereas, after annealing above 420°C only A $\leftrightarrow$M transformation is observed. Ni-rich quenched alloy annealed below 500°C exhibits a two-stage martensitic transformation, spreading over a wide temperature interval of 150°, below +50°C. For that alloy annealed above 500°C, R-phase gradually decays narrowing the transformation region. In the furnace cooled sample, R-phase region steadily narrows, for annealing at temperatures up to 580°C, suddenly broadens for annealing above 580°C and persists prominently thereafter. In the case of Ni-Ti-Cr, though for both the processes two-stage martensitic transformation is exhibited, for quenched one, R-phase disappears for annealing temperatures above 600°C. Thus, fumace cooling is found to promote the intennediate R-phase in Ni-rich Ni-Ti and Ni-Ti-Cr alloys annealed at higher temperature also.

© EDP Sciences 2003