Tuning the morphology of Ti–5Al–5Mo–5V–3Cr–1Zr alloy: From brittle to ductile fracture

The tensile properties and crack initiation of Ti-55531 alloy were studied by tuning the microstructure from lamellar microstructure (LM) to bimodal microstructure (BM). The results indicate that the mechanical properties change from ultimate tensile strength of 1287 MPa with zero plasticity to ultimate tensile strength of 1310 MPa and ductility of ∼10%. Further transmission electron microscopy and scanning electron microscopy characterization show that the bimodal microstructure have shown a coarsen primary α surrounded by ultra-fine α precipitation, which could be the origin of combination of strength and ductility. TEM images after loading indicate that multiple slip systems have been activated in primary α of BM. Plastic strain localized within primary α due to the low strength, which results about 83.2% voids initiation within or at boundary of primary α. The cracks propagate not only along the interface between primary α and matrix, but also along slip bands in primary α in BM, which results in transgranular fracture. The results show that the micron-scale α phase is necessary to accommodate plastic strain and bring about ductility for β alloys with ultra-fine α precipitation.