The aging characteristics including microstructures and properties were investigated for a new metastable beta titanium alloy named Ti-B20 subjected to different solution treatments. Microstructural examination shows that precipitate-free zones,which are present in the many metastable beta titanium alloys,are not produced in the aged new alloy. The decrease of yield stress with the increase of aging temperature is due to the coarsening of alpha platelet. Furthermore,the age hardening effect of the new alloy strengthens with the increase of solution treatment temperature. These aging characteristics are attributed to the relatively lower concentration of beta stabilizer in the new alloy.
The impact toughness of TC21 alloy after different types of forging and heat treatments was studied. The results show that heat treatment at 915 ℃ for 1 h followed by air-cooling can achieve the highest impact toughness. The crack propagation path of bimodal microstructure is different from that of lamellar microstructure. Boundaries of primary α grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature,the volume fraction of primary α phase decreases and the nucleation sites of microcrack at the primary α phase boundaries also decrease,the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on α/β interface,or the boundary of colony,and/or grain boundary α phase. The crack propagates cross the colony,or along the colony boundary,and/or along β grain boundary. The crack propagation path of lamellar microstructure is dependent on the size,direction of colony. The crack path deflects at grain boundaries,colony boundaries,or arrests and deviates at α/β interface because of crisscross α lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.
