Some structural effects related to the abnormal grain growth in FeMnAlNi shape memory alloys

Abstract Oligocrystalline Fe43.5Mn34Al15Ni7.5 alloys, with an average grain size larger than their thickness/ diameter, experience superelasticity between 223 … 423K which recommends them for large scale potential applications in seismic-protection industries. Besides oligocrystallinity which is necessary in order to avoid the presence of triple junctions between grains, the body centered cubic (bcc) austenite matrix has to be reinforced by the coherent precipitation of ordered β-NiAl bcc, in such a way as to enable the reversible stress induced formation of γ’ face centered cubic (fcc) martensite. The present paper describes the processing procedure of three alloys with nominal chemical composition Fe43.5Mn34Al15±1.5Ni7.5∓1.5 After hot rolling, annealing, cold rolling, cyclic heat treatment around solvus temperature (enhancing γ-fcc phase decomposition and the occurrence of a discontinuous phenomenon called abnormal grain growth) and solution treatment with final water quenching (meant to avoid γ-fcc formation and to enable coherent precipitation), chemical composition effects have been analyzed by optical microscopy (OM). In each of the processing steps, the evolution of average grain size was monitored and correlated with chemical composition effects. A limited oligocrystalline structure was obtained at Fe43.5Mn34Al16.5Ni6 after cyclic heat treatment.