Investigation on microstructure and mechanical properties of heat-treated Ti-47.5Al-3Nb-3.5Cr alloy

Three heat treatment processing routes (heat treatment at 1300 degrees C, 1350 degrees C and 1400 degrees C for 15min, 30min and 60min respectively, and following air cooling) were developed to investigate the dependence of mechanical properties on microstructure evolution in a new Ti-47.5Al-3Nb-3.5Cr (at.%) alloy. When the heat treatment temperature approaching the solution precipitation temperature of gamma phase (T gamma-sol) (T gamma-sol<1300 degrees C), the interlamellar spacing is found to increase obviously. Moreover, when the heat treatment temperature is between alpha phase transition temperature (T alpha) and T gamma-sol (1400 degrees C < T alpha), the interlamellar spacing and average grain size decreases, the distribution of the B2 phase is more uniform. The Vickers hardness test shows that the Vickers hardness of the heat-treated samples are improved compared with the as-cast sample (352.6Hv). The best Vickers hardness of lamellar microstructure after heat treatment is 465.2Hv. The tensile test at room temperature shows that the good tensile strength and elongation at failure of the heat-treated sample can be obtained after air cooling at 1400 degrees C for 60min, which are 359.54 MPa and 2.04%, respectively. Compared with 405.06 MPa and 0.66% of the as-cast sample, heat treatment can significantly increase the elongation of the sample. Meanwhile, the observation of the microstructure also exhibits the dependence of microstructure on the mechanical properties. The results shows that the improvement of Vickers hardness is mainly attributed to solid solution strengthening. In addition, the distribution of B2 phase, the refinement of the interlamellar spacing and the average grain size play essential roles in the improvement of the tensile strength and elongation at failure.