Triple twins and martensitic transformation in Ti-5Al-2Mo-3Zr alloy

Abstract

In Ti-5Al-2Mo-3Zr alloy, TEM observations showed a special feature of “triple twins” in martensite plates after quenching, i.e. three sets of crystals in a martensite plate oriented with the {1¯101} 〈¯1 1 0 2〉 twin relationship between any two of them. This is possible because thec/a ratio of the lattice parameters equals 1.588 in this alloy, and thus the triangle consisting of spots 0 0 0 1, 1 ¯1 0 1 and the central spot in a selected-area diffraction pattern has the interior angles of approximately 30, 60 and 90°. The observations also showed that the boundary between twins II and III was the twin plane {1 ¯1 0 1}, but the boundary between twins I and II was close to {1 ¯1 0 3}, i.e. (1 2 1) of the b.c.c. parent phase. Formation of the triple twins could be explained by the phenomenological theory of crystallography. One pair of twin martensite plates may grow from a pair of twin nuclei which take (1 1 0) and (0 1 1) planes of the parent phase as their respective basal planes. However, the (1 1 0) and (0 1 1) planes are symmetrical with respect to (1 2 1) of the parent phase, which corresponds to a {1 ¯1 0 3} plane of the martensite. The third part of the triple twins forms to meet the need for simple shear according to the phenomenological theory. The formation of triple twins occurs to accommodate the shape change and reduce the elastic energy during the transformation.