Effect of microstructure on impact toughness of TC21 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 degrees C for I 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 alpha grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature, the volume fraction of primary alpha phase decreases and the nucleation sites of microcrack at the primary alpha phase boundaries also decrease, the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on alpha/beta interface, or the boundary of colony, and/or grain boundary alpha phase. The crack propagates cross the colony, or along the colony boundary, and/or along beta 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 alpha/beta interface because of crisscross alpha lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.