The interrelationship of fracture toughness and microstructure in a new near β titanium alloy Ti–7Mo–3Nb–3Cr–3Al

A new near β titanium alloy Ti–7Mo–3Nb–3Cr–3Al (Ti-7333) with ultrahigh tensile strength and reasonable ductility after subtransus solution followed by aging treatment has been developed in China. The fracture toughness tests were carried out on the material at room temperature in order to understand the fracture mechanism. The results show that Ti-7333 alloy has a high fracture toughness value (> 140 MPa·m1/2) under the β solution followed by slow cooling, and then aging heat treatment process. Meanwhile, the elongation and reduction of area remain at about 19% and 45% respectively, and its corresponding tensile strength maintains at about 1000 MPa. The microstructure observation and analyses show that the intragranular α phase laths and grain boundary α become coarser roughly with the increasing of aging temperature, whereas, the continuity of grain boundary α is subjected to change and turned to zigzag. The coarser and longer intragranular α and more multidirectional arrangement of the colony of α lamellae, as well as discontinuous α precipitates on the grain boundary lead to a more tortuous crack propagation path and higher fracture toughness. In addition, the bigger the size of the plastic zone of the crack tip is, the higher fracture toughness it has. Compare with other similar alloys (Ti-5553, Ti-1023, VT22, etc.), Ti-7333 exhibits higher fracture toughness and close tensile strength. That indicates that Ti-7333 alloy has excellent combinations of mechanical properties and giant competitiveness.