Effect of heat treatment and silicon addition on the microstructure development of Ti–6Al–2Cr–2Mo–2Sn–2Zr alloy

Microstructural development of Ti–6–22–22–0.22Si and Ti–6–22–22–0.02Si after beta, alpha/beta, and aging heat treatment was investigated. Primary alpha morphology was determined following beta heat treatment. Cooling rate from beta treatment was found significantly influence the HCP transformation products, ranging from principally alpha-prime martensite and acicular alpha at rates exceeding 5.5 °C s−1, to mixtures of Widmanstatten and colony alpha at intermediate cooling rate, to colony alpha at the slowest rates of 0.27 and 0.055 °C s−1. Varying cooling rates from the alpha/beta heat treatment significantly influenced the volume fraction of retained beta phase and secondary alpha in the microstructure. Higher cooling rate resulted in greater retention of the beta phase and transformation of the phase into secondary alpha upon aging. This higher amount of fine, secondary alpha phase promoted higher strength and lower toughness. Alpha/beta heat treatment temperature significantly influenced the heat treatment response of the microstructure. High heat treatment temperatures promoted greater amounts of retained beta with fine, transformed alpha in the final microstructures. Lower alpha/beta heat treatment temperatures promoted retained beta microstructures which were less responsive to aging treatment. Aging treatment promoted decomposition of retained beta phase, particularly in larger retained-beta regions that exhibited lower stability. Microstructural characteristics of heat treated high and low Si alloys appears identical in most cases. However, very fine silicides were observed in the high Si alloy after aging at 593 °C, which may have a significant effect on fracture toughness.