Effect of heat treatment on fracture toughness of a Ti-15Mo alloy

Fracture-toughness and tensile tests have been carried out on a Ti-15Mo alloy bar, solution treated in the β− and α+β-fields and aged to contain a high volume fraction of small ω particles. The purpose of the investigation was to determine whether or not the alloy is tough as well as ductile when containing these small ω particles. Comparative tests were also made on bars solution treated in a similar way and aged to contain larger ω particles, a mixture of the ω- and α-phases, and the α-phase alone. Microstructures were examined by transmission electron microscopy and X-ray diffraction. Fracture-toughness K_IC values ranged from 86 MPam^1/2 for a β+α-microstructure to 22 M Pam^1/2 for material aged to contain large ω particles. The K_IC values for the alloy containing small ω particles were in the middle of this range, and this indicates that high ductility, in the absence of any uniform elongation (i.e. in a situation in which strain hardening does not occur) can give a misleadingly favourable impression of toughness. The best combination of toughness and strength was achieved for the alloy heat treated to contain the β-, ω-, and α-phases.