Kinetics of transformations in the β solid solution of the magnesium-indium system

Abstract

Decomposition of the intermediate β solid solution of the magnesium-indium system has been examined in respect of the characteristics and kinetics of the reactions, using a resistometric technique. The β phase undergoes ordering of the type A1 → L1 2 (β′) in alloys containing up to 39% indium, and A1 → L10 (β″) between 39 and 55% indium. Both transitions occur with great rapidity, and the disordered state could not be retained at any temperature below T c . This behaviour is discussed with reference to the transformation kinetics expected of such first order reactions. Precipitation from the ordered solid solutions occurs within narrow homogeneity ranges at Mg 3In (β 1) and Mg 2In (β 2), although there are extensive two-phase regions about these compositions. The β 1 phase forms by continuous precipitation, and the formal kinetics of the reaction have been determined. For β 2, the precipitation changes from the continuous to the discontinuous mode as the indium concentration is increased through the stoichiometric composition.