The size effect on the phase stability of nanograined Fe–12Ni powdersand the magnetic separation of face-centred-cubic–body-centred-cubicphases

Abstract

The phase stability of nanograined Fe–12Ni powders is studied bymeans of both experiment and theoretical calculations. By means of amagnetic field accompanied by an ultrasonic vibration dispersal, thenanograined Fe–12Ni powders synthesized by the gas condensationwere successfully separated into a face-centred-cubic (fcc, γ-) phasewith a grain size distribution ranging from 20 to 50 nm and a body-centred-cubic (bcc,α-)phase with a grain size less than 30 nm. The experimental results showthat the most significant factor affecting the phase stability of thenanograined powders is the grain size. The critical grain size (d∗)for the fcc phase to exist at 300 K was quantitatively calculated onthe basis of a dilated crystal model and the effect of magnetismenergy was considered in the calculations. The calculated d∗is 27 nm when the excess volume for nanograined Fe–Ni (ΔV) is taken as 0.1, sothe calculated d∗is comparable to the experimental result. The calculation results for thenucleation barrier to fcc-to-bcc phase transformation indicate that the grain sizeplays an important role in the fcc phase stability of nanograined Fe–12Ni powders.