Microstructures and Magnetic Properties of Fe70−x Pd30Ni x High-Temperature Ferromagnetic Shape Memory Alloys

Abstract

This research paper mainly presents an investigation of the microstructures and magnetic properties of bulk ferromagnetic shape memory (FSM) Fe70−x Pd30Ni x (Ni x = 4, 8 at.%) alloys, by transmission electron microscopy (TEM), a magnetostrictive-meter setup, and a superconducting quantum interference device (SQUID) magnetometer. The FSM alloys were homogenized through hot and cold strain forging (SF) to a ∼38 % reduction in thickness, solution-treated (ST) with annealing recrystallization at 1100 °C or 8 h, quenched in ice brine, and then aged at 500 °C for 100 h (5 HTA). The investigation of the microstructures and magnetic properties indicated that the higher Ni content (N x = 8 at.%) in the Fe62Pd30Ni8 alloy SF and ST reduced the saturation magnetostriction at RT. However, with higher Ni content in the Fe62Pd30Ni8 alloy, the decomposition of L10 + L1m twin phases into stoichiometric L10 + L1m + α bct structures was suppressed after the alloy was ST and aged at 5 HTA, as confirmed by TEM investigation. The result was that the FSM Fe62Pd30Ni8 alloy maintained a high saturation magnetostriction and magnetization after the alloy was ST and aged at 5 HTA. This magnetic property of the Fe62Pd30Ni8 FSM alloy makes it suitable for application in high-temperature (T < 500 °C) and high-frequency environments. However, low Ni content FSM Fe66Pd30Ni4 (N x = 4 at.%) alloy SF, ST, and aged at 5 HTA underwent decomposition of the L10 + L1m twin phases into the stoichiometric L10 + L1m + α bct structures, as confirmed by TEM, leading to a decrease of saturation magnetostriction and magnetization. This magnetic property of the Fe66Pd30Ni4 FSM alloy is not suitable for application in high-temperature (T < 500 °C) environments.