Quantitative analysis of the effect of heat treatment on microstructural evolution and microhardness of an isothermally forged Ti-22Al-25Nb (at.%) orthorhombic alloy

The microstructural features of the 980 C isothermally forged Ti-22Al-25Nb (at.%) orthorhombic alloy during heat treatment were quantitatively investigated. The volume fraction of the O phase precipitates, the width and length of the lath O phase, and the diameter of equiaxed grains at different heat treatment temperatures were measured using an image analysis software. Quantitative relationships among heat treatment temperature, microstructure parameters, and microhardness were established. The relationship between microstructure parameters and microhardness was analyzed with a multiple regression analysis technique. The results indicate that the microstructure of this alloy is mainly depended on the heat treatment schedule. Only equiaxed O/α₂ grains and B2 matrix existed when the samples were solution-treated above 980 C, while equiaxed α₂ grains, rim O around α₂, and equiaxed/lath O could be obtained after the samples were solution treated below 980 C. The width of lath and acicular O phases, and volume fraction of total precipitates could be controlled in the range of 0.37-0.88 μm, 0.09-0.48 μm and 10.91-60.18%, respectively. Experimental and statistical analysis showed a linear relationship between the microstructure parameters and microhardness.