Plane strain fracture behavior of a new high strength Ti–5Al–3Mo–3V–2Zr–2Cr–1Nb–1Fe alloy during heat treatment

Plane strain fracture behaviors of a newly developed high strength Ti–5Al–3Mo–3V–2Zr–2Cr–1Nb–1Fe (Ti-5321) alloy with two typical microstructures were investigated in this study. The two microstructures were obtained by STA (solution treated & aged) and BASCA (β annealed with subsequent slow cooling & aged) heat treatments, respectively. The plane strain fracture toughness of STA and BASCA specimen were 73.1 MPa m^1/2 and 57.7 MPa m^1/2, respectively. Interaction between crack path and microstructure was investigated by SEM and EBSD to identify the microstructure characteristics that influence the plastic deformation in crack tip plastic zone (CTPZ) and crack propagation. The CTPZ in STA specimen exhibited a lot of uniform voids and microcracks located at α_p/β_t and α_p/α_p interfaces, and the tortuosity of crack path was 0.063 owing to the transgranular and intergranular propagation. The microcracks in CTPZ of BASCA specimen were focused on the partial α_GB/β_t interfaces, and the crack path tortuosity was 0.153 due to the intergranular propagation which are regarded as a low energy crack path. Both of the energy consumption during damage formation in CTPZ and crack growth in BASCA specimen were less than that in STA specimen owing to the presence of the continuous grain boundary α, leading to microcracks nucleation and fracture at grain boundaries. Besides, the crystal orientations of α phase around the crack path were investigated to further reveal the crack propagation mechanism. The results showed that the large misorientation angle (nearly 90°) between α/α phases could promote the crack diversion, and crack tended to be arrested prior to entering α plate with a c-axis nearly in alignment with the loading direction.