AN INTERNAL FRICTION STUDY OF PHASE TRANSFORMATIONS IN MAGNESIUM-INDIUM ALLOYS

Order-disorder reactions in magnesium-indium alloys have been studied by internal friction techniques. Damping measurements during continuous slow heating and cooling of alloys between 21 at% In and 62 at% In have shown that the FCC β' and FCT β'' superlattices form by solid-state reactions which are thermodynamically first-order. A characteristic feature is the occurrence of discontinuous changes in damping, at two distinct temperatures, the ordus and disordus temperatures, To and Td. Between these temperatures, the damping changes rapidly and is directly dependent on the relative volume fractions of the phases present. The overall decrease in damping accompanying the development of the β'-phase, near the composition Mg3In, is explained by an inherently low background damping in structures exhibiting long-range order. The overall increase in damping associated with the formation of the ordered β''-phase, near the composition MgIn, results from an appreciable damping contribution from transformation twins in that phase. A necessary condition for the occurrence of discontinuities in damping is that the microstructure should be essentially at equilibrium at the heating and cooling rates employed