Recovery stress and shape memory stability in Ni–Ti–Cu thin wires at high temperatures

Abstract

Abstract The recovery stress evolution during thermal cycling (heating, isothermal holding, cooling) to temperatures significantly above the austenite finish temperature in Ni – Ti – Cu thin wires was evaluated in a dynamic mechanical analyser DMA TQ800. Ni – Ti – Cu wires were annealed at four different temperatures (350 °C, 450 °C, 550 °C and 650 °C). It was observed that the maximum recovery stress significantly decreases with increasing annealing temperature. It was also observed that the recovery stress during four isothermal holdings shows similar behaviour for all annealing temperatures, i. e. stress relaxation occurs after initial stress increase during the first isothermal holding and no stress relaxation is observed during isothermal holdings in subsequent thermal cycles. The maximum recovery stress after each thermal cycle decreases and shows a tendency to saturate with increasing number of thermal cycles. It is found that even when the recoverable strain is significantly decreased during thermal cycling, Ni – Ti – Cu wires are still able to generate very high recovery stresses.