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

The effects of cooling rate following the β or α/β heat treatment on microstructure and phase transformation are investigated for BT25y alloy. For this purpose, BT25y alloy is soaked at 1000 °C, 980 °C, 960 °C, 940 °C or 920 °C for 10 min, and then control cooled at rate of either 0.15 °C/s, 1.5 °C/s, 15 °C/s, 45 °C/s, 90 °C/s or 150 °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 β phase field at the lower rate, the αGB and αWGB phases are precipitated, and this transformation process can be divided into the four stages: (a) the formation of αGB, (b) the connection of adjacent αGB, (c) the precipitation of αWGB, (d) the growth of αWGB. However, the increasing of the cooling rate will greatly restrain the precipitations of αGB and αWGB phases. In this case, the acicular martensite α′ is precipitated inside β grain. The primary equiaxed-α is retained when material is cooled down from the α/β phase field. The content and size of equiaxed-α decrease with the increasing of solution temperature, but is independent on the cooling rate. At the lower cooling rate, the lamellar α 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 α. Instead, the martensite α′ phase is precipitated and gradually takes the place of the lamellar α with the increase of cooling rate. In conclusion, whether material cools down from β single phase field or α/β two-phase field, a phase transformation law is summarized for BT25y alloy. The lamellar α is the only precipitated phase when the cooling rate ≤15 °C/s. The precipitated phase consists of the lamellar α and martensite α′ phase when the cooling rate is 15 °C/s ∼90 °C/s. The only martensite α′ phase is precipitated when the cooling rate ≥90 °C/s.