Variant selection of orthorhombic phase precipitation and its effect on colony side plates formation in Ti-22Al-25Nb alloy

Heat treatments with controlled cooling rates have been used to investigate orthorhombic (O) phase precipitation and its variant selection (VS) in Ti-22Al-25Nb alloy, a latest Ti2AlNb-based intermetallic alloy with excellent comprehensive properties. This study uncovers the misorientation types of different O variants mainly belong to two types with a sharing angle of 90(degrees) and distinct rotated axis of <012> or <100>. A notable lamellar structure, colony side plates (CSPs), frequently develop on both sides of adjoining B2 grains, which always have a detrimental effect on the final properties. This structure significantly influenced by VS, variants having (001) planes that are paralleled to the common {110} plane of adjacent B2 grains are selected in colony plates on both sides. If the <111> axis of the two B2 grains coincidentally close as well, the colony plates may appear to penetrate the GB. Moreover, the CSPs structure could form on condition of much slow cooling rate after holding in rather high temperature. These findings could provide insights on how to control processing conditions and tailor the lamellar structure to obtain the desired properties.