Microstructural morphology effects on fracture toughness and crack growth behaviors in a high strength titanium alloy

Tensile properties and fracture toughness of Ti–5Al–3Mo–3V–2Zr–2Cr–1Nb–1Fe (Ti-5321) alloy with bi-modal (BM), basket-weave (BW) and heterogeneous lamellar microstructure (HL) were investigated. The results showed that HL specimen with hierarchical features (e.g., mixtures of elongated β_t matrix, discontinuous grain boundary α, intragranular α colonies and α plates) exhibited the best combination of strength, ductility and fracture toughness. The fracture toughness was up to 102 MPa m^1/2 with a yield strength of 1200 MPa and elongation of 14%. The toughening mechanisms and fracture behaviors in three microstructures were analyzed. The toughening mechanisms in HL primarily involved heterogeneous microstructure that acted to reduce local stress/strain distribution at crack tip and enlarged crack tip plastic zone (CTPZ), which lead to more energy consumption during plastic deformation. As for fracture behaviors, the fracture mode in three microstructures were predominantly dimple ductile fracture although the crack path were both transgranular and intergranular. Interactions between microstructure constituents and crack path were studied by EBSD to discuss the crystallographic crack propagation mechanisms in BW and HL.