The effect of the thermal exposure on microstructure of MP159 alloy

Abstract

The effect of the thermal exposure at different temperatures on the microstructure is investigated for MP159 alloy by optical microscopy and transmission electron microscopy (TEM). Thermal exposure at 660°C is related to the aging induced hardening mechanism, thermal exposure at the neighborhood of 920°C is associated with static recrystallization behavior, and thermal exposure at a temperature range of 920 to 1070°C for a short time is investigated with a view toward obtaining the longest permissible thermal exposure time during which microstructural reconstitution of the cold deformed microstructure would not occur at the corresponding temperature, which would provide guidance for determining reasonable hot forging parameters of fasteners. The results indicate that both solution heat treated (ST) and cold worked (CW) MP159 alloy could be hardened by aging. This aging induced hardening is attributable to the precipitation of a very finely dispersed fcc ordered solid solution. The precipitating process is so rapid that the air cooling (AC) or water cooling (WC) process of the sample exposed to elevated temperature higher than normal aging temperature could produce an appreciable hardening effect. The recrystallization temperature of the alloy is as high as 920°C, such a high recrystallization temperature is attributed to deformation twins and a variety of alloying elements due to their inhibition to the nucleation and growth of recrystallization grains, other than to the hcp-phase which used to be believed to explain high recrystallization temperature in MP alloys in some literature.