Microstructure evolution and fracture behavior of Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy during BASCA heat treatments

The microstructure evolution and fracture behavior of Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe (Ti-5321) alloy during BASCA (β annealed with subsequent slow cooling & aging) heat treatment were studied. Three different lamellar microstructures, termed as L1, L2 and L3, were obtained by solution treated at 880 °C for 2 h followed by 0.5 °C/min cooling to 800 °C, 750 °C and 710 °C, respectively and aging at 580 °C for 6 h. Microstructural investigation revealed that the three lamellar microstructures were composed of intergranular precipitation (α_GB, α_WGB), intragranular α_l and β_t. Both of the volume fraction of lamellae α and the size of α colonies increased with the increasing furnace cooling time. Both intergranular and intragranular precipitation in L1, L2 and L3 obeyed the Burgers orientation relationship (OR) with adjacent β grain. However, the texture intensity and variant selection degree of lamellar α increased with cooling process owing to the increased amount of α colonies. In addition, the L1 sample, with 15% α_l, displayed the lowest fracture toughness value (57.7 MPa·m^1/2), while the L3 sample with 61% α_l exhibited the highest (114 MPa·m^1/2). Although the crack path in all the cases were both translamellar and interlamellar, fracture mode varied from mixed mode (dimples and intergranular cracks) type fracture in L1 to predominantly dimple fracture in L2 and L3. Moreover, the fracture behavior of Ti-5321 alloy was also related to the crystallographic distribution and variation selection of α phase. Crack growth was much easier through continuous α_GB layer compared to the discontinuous α_GB, especially for the high angled α_GB/α_GB grain boundary. As a consequence, the higher probabilities of misorientation angles of 90° in L1 and L3 could significantly promote crack deflection compared to L2.