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

Abstract

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° - 800°C. Phase transformation in water quenched and furnace cooled samples is studied in the range from +140° to -170°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, A→R→M during cooling and M→A on heating, are observed in the as-received sample. For both the quenched and the furnace 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↔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, furnace cooling is found to promote the intermediate R-phase in Ni-rich Ni-Ti and Ni-Ti-Cr alloys annealed at higher temperature also.