Influence of cooling rate following heat treatment on microstructure and phase transformation for a two-phase alloy

The effects of cooling rate following the beta or alpha/beta heat treatment on microstructure and phase transformation are investigated for BT25y alloy. For this purpose, BT25y alloy is soaked at 1000 degrees C, 980 degrees C, 960 degrees C, 940 degrees C or 920 degrees C for 10 min, and then control cooled at rate of either 0.15 degrees C/s, 1.5 degrees C/s, 15 degrees C/s, 45 degrees C/s, 90 degrees C/s or 150 degrees C/s to room temperature. Microstructure observations indicate that the microstructure of BT25y alloy is significantly influenced by the cooling rate. When material is cooled from the beta phase field at the lower rate, the alpha(GB) and alpha(WGB) phases are precipitated, and this transformation process can be divided into the four stages: (a) the formation of alpha(GB), (b) the connection of adjacent alpha(GB), (c) the precipitation of alpha(WGB), (d) the growth of alpha(WGB). However, the increasing of the cooling rate will greatly restrain the precipitations of alpha(GB) and alpha(WGB) phases. In this case, the acicular martensite alpha' is precipitated inside beta grain. The primary equiaxed-alpha is retained when material is cooled down from the alpha/beta phase field. The content and size of equiaxed-alpha decrease with the increasing of solution temperature, but is independent on the cooling rate. At the lower cooling rate, the lamellar alpha is precipitated and its thickness increases with the increasing of solution temperature. But the increasing cooling rate will weaken the precipitation capacity of the lamellar alpha. Instead, the martensite alpha' phase is precipitated and gradually takes the place of the lamellar alpha with the increase of cooling rate. In conclusion, whether material cools down from beta single phase field or alpha/beta two-phase field, alpha phase transformation law is summarized for BT25y alloy. The lamellar alpha is the only precipitated phase when the cooling rate <= 15 degrees C/s. The precipitated phase consists of the lamellar alpha and martensite alpha' phase when the cooling rate is 15 degrees C/s similar to 90 degrees C/s. The only martensite alpha' phase is precipitated when the cooling rate >= 90 degrees C/s. (C) 2016 Elsevier B.V. All rights reserved.