The Effect of Initial Structure on Phase Transformation in Continuous Heating of a TA15 Titanium Alloy

The effect of initial structure on phase evolution in continuous heating of a near-alpha TA15 titanium alloy (Ti-6Al-2Zr-1Mo-1V) was experimentally investigated. To this end; three microstructures were obtained by multiple heat treatment: I-bimodal structure with 50% equaixed alpha, II-bimodal structure with 15% equiaxed alpha, III-trimodal structure with 18% equiaxed alpha and 25% lamellar alpha. Differential scanning calorimetry (DSC), dilatometry and quantitative metallography were carried out on specimens with the three initial structures at heating rates from 5 to 40 degrees C/min. The transformation kinetics was modeled with the Johnson-Mehl-Avrami (JMA) approach under non-isothermal condition. It was found that there exists a four-stage transformation for microstructures I and III. The secondary and third stages overlap for microstructure II. The four stages of phase transformation overlap with increasing heating rate. In the presence of alpha laths, the phase transformation kinetics is affected by the composition difference between lamellar alpha and primary equiaxed alpha. Phase transformation is controlled by the growth of existing large beta phase.