Non-Isothermal Phase Transformation Behavior and Thermal Expansion Characteristics of TC21 Alloy

The dilatometric curve and its first derivative were utilized for investigating non-isothermal transformation behavior of α+β→β of TC21 alloy during the process of linear heating at four different heating rates. The quenched microstructure of TC21 alloy at different temperatures during the process of phase transformation and the microstructure after thermal expansion were observed by optical microscopy. Results showed that the starting and finishing β transformation temperatures of TC21 alloy increased as the heating rate increased. A high number of equiaxed α phase appeared in the microstructure of the edge area of TC21 alloy after thermal expansion, and the amount of equiaxed α phase decreased with the increase in heating rate. The transformed activation energy calculated by the kinetics equation was about 444.6 kJ mol. The transformation process of α+β→β of TC21 alloy under linear heating conditions could be divided into three sections based on the change law of local Avrami exponent with the increasing transformed volume fraction. These three sections could be explained by different nucleation and growth mechanisms.