Kinetics of nanoscale precipitation in Ni-Fe-Al alloys: A magnetic monitoring approach

In this study, time-temperature dependence and kinetic aspects of nanoscale precipitation responsible for strengthening of Ni(50)Fe(x)Al(50-x) alloys with x = 20, 25 and 30 were investigated by temperature-scan and isothermal magnetic measurements in a vibrating sample magnetometer. Temperature-scan magnetization curves contained a magnetization rise for all studied alloys at temperatures above the Curie transition of the primary phase. These transient rises at relatively higher temperatures were associated with the formation of secondary ferromagnetic precipitates, identified as nano-sized body centered cubic alpha crystallites by microstructural observations under the transmission electron microscope. The isothermal kinetics of ferromagnetic alpha-phase precipitation was analyzed with the Johnson-Mehl-Avrami model. The Avrami exponent was determined to be close to unity and independent of the extent of precipitation, annealing temperature and alloy composition. It was concluded that alpha-phase precipitation was governed by a diffusion controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. The activation energies, ranging between 75 and 83 kJ/mol, were utilized to construct magnetically assessed isothermal transformation diagrams of precipitation. Conforming to the kinetic analysis, annealing at an intermediate temperature ensures precipitation of fine second phase particles and brings about a significant hardening effect, whereas a higher annealing temperature yields coarser precipitates and a smaller extent of precipitation hardening. (C) 2011 Elsevier B.V. All rights reserved.