PALS Investigation of Structural Vacancies During Phase Transitions in Fe-27Ga and Fe-27Ga-0.1Tb Alloys

Good mechanical properties and relatively low switching magnetic field make Fe-Ga alloys very useful magnetostrictive material for wide range of practical applications. Doping with rare-earth elements (such as Tb) can significantly increase magnetostriction, because Tb rearranges vacancy structure during phase transitions in Fe-27Ga alloys. It was found that A2 and D0(3) structures in Fe-27Ga have high density of Fe-monovacancies with the positron lifetime approximate to 180 ps regardless of the presence of Tb. Transition into L12 structure is accompanied by formation of a significant number of larger vacancy defects, which increases the e+ lifetime by 5 /0. Addition of 0.2% Tb suppress formation of these large defects at annealing temperatures of 400-550 degrees C. Presence of Tb also decreases concentration of vacancies in D0(19) structure. After annealing at 650 degrees C D0(19) volume fraction in Fe-27Ga-0.2Tb decreases to 60 %, (according to the positron measurements) against 95 % in Fe-27Ga. In both alloys at 700 degrees C number of monovacancies increases dramatically during formation of the bcc B2 phase.