Influence of temperature on the precipitation phase transformation behavior of RR350 aluminum alloy during artificial aging and related mechanisms

Abstract

This study investigates the influence of artificial aging temperature on the evolution of precipitation phases and age hardening behavior of the RR350 aluminum alloy. The manner of nucleation and growth of different kinds of precipitates and phase transformation between them are analyzed, along with the corresponding mechanisms are discussed. The results reveal that the aging temperature affects little the precipitation process of precipitates which still follows the sequence of GP zones - θ″- θ′ in the RR350 aluminum alloy during artificial aging, but influences significantly the precipitation speed and phase composition, morphology, size and size distribution of the resultant precipitates in the alloy. An imperceptible relationship between the hardness of the sample and the composition of the precipitates were discovered. As the artificial aging temperature (160–180–200 °C) increases, the peak-aging hardness of the samples decreases and the time for reaching the peak-aging status shortens gradually. When the temperature is low, 160 °C for example, the precipitates in the peak-aging status are mainly composed of θ″ phase and some minor θ′ phase. As the temperature increases to 200 °C θ′ phase becomes prevalent. As a result, the alloy shows lower peak-aging hardness since the strengthening effect of the finer θ″ phase is much stronger than that of the θ′ phase. Increasing the aging temperature accelerates the competitive processes of nucleation of GP zones, growth of all precipitates, and the phase transformation. At higher temperatures, phase transformation from θ″ to θ′ overweighs the formation of GP zones, growth of both θ″ and θ′ phases, resulting in more amount of θ′phase in the peak-aging and over-aging statuses.