Fracture characteristics and microstructural factors in single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe

The influence of microstructure on fracture toughness of single and duplex annealed Ti–4.5Al–3V–2Mo–2Fe alloys was investigated. For the single-annealing treatment between 1103 and 1173 K for 3.6 ks, fracture toughness, JIC, decreases first with the increase of annealing temperature from 1103 to 1123 K and then increases with increasing annealing temperature upto 1173 K. As a result, there is a minimum of fracture toughness at 1123 K. This anomalous relation between fracture toughness and annealing temperature also exists in the duplex-annealed specimens at 993 K for 3.6 ks followed by the above single-annealing treatment. Examining the change of JIC and microstructure with annealing temperature, it can be found that JIC is related to volume fraction, aspect ratio and grain size of primary α phase, width of acicular α phase and prior β grain size. The decrease of fracture toughness in the temperature range from 1103 to 1123 K is mainly due to the decrease of volume fraction of primary α phase, while the increase of fracture toughness above 1123 K is mainly caused by the increase of prior β grain size.