Evolution of microstructure and tensile properties during the three-stage heat treatment of TA19 titanium alloy

This study reported the development of a three-stage heat treatment for TA19 titanium alloy to obtain a ternary microstructure consisting of equiaxed, lamellar and acicular alpha, which were successively acquired in the first stage (I-stage), second stage (II-stage) and third stage (III-stage) treatments. The content ratio among the equiaxed, lamellar and acicular alpha was tailored by controlling the I-stage and II-stage temperatures. Tensile test revealed that an increase of I-stage temperature improved both ultimate strength (UTS) and elongation (EL) due to the increased content of lamellar alpha. However, an excessively high I-stage temperature led to a very low content of equiaxed alpha, coarsening of beta grain and precipitation of grain boundary alpha, which reduced the EL. An increase of II-stage temperature resulted in an increase of UTS and a decrease of EL, because the increased content of acicular alpha produced a number of alpha/beta interfaces, which strengthened the alloy but was detrimental to plasticity. The fractography analysis indicated that majority of heat treated specimens exhibited a completely ductile fracture mode, while a mixed mode of brittle and ductile fracture was observed in specimens, which was subjected to an excessively I-stage temperature.