Microstructural Evolution in β‐Metastable Ti–Mo–Sn–Al Alloy During Isothermal Aging

Abstract

The microstructure of β‐metastable Ti–3Mo–6Sn–6Al (mol%) alloy is systematically investigated before and after isothermal aging. Before aging, nanosized martensite‐type lattice modulations are observed that correspond to the nanodomains reported in oxygen‐added Ti‐Nb base alloys. All six nanodomain variants are homogeneously distributed in the β phase, which suppresses the formation of macroscopic α″ martensite. An isothermal α″ phase (α″iso) forms in the parent β phase during isothermal aging. The crystal structure of the α″iso phase changes continuously during isothermal aging, approaching that of the α phase. No composition change is detected on α″iso phase formation, demonstrating that its formation is not due to precipitation or β phase decomposition. The thermally activated process required for the α″iso formation is considered to be a short‐range atomic rearrangement, which weakens the suppressive effect of nanodomains on α″ formation. However, the microstructure of the α″iso phase is different from the self‐accommodating structure of α″ martensite that forms on quenching or cooling. The α″iso phase comprises a main (matrix) variant, with all five other variants observed inside this matrix, with a size of 10–100 nm.