SUBSTRUCTURE IN TITANIUM-ALLOY MARTENSITE

Hexagonal plate martensite in titanium alloys contains a dense substru cture whose character and origins have not been completely addressed i n the literature. The substructure is shown to consist of columnar dom ains which extend along basal planes from the midrib of the martensite plates to the habit plane and are bound in the third dimension by sho rt segments of non-crystallographic surfaces roughly perpendicular to the habit plane. The domain structure gives rise to various diffractio n effects consisting of relrods of intensity along the c axis and relp lanes of intensity roughly perpendicular to the non-crystallo graphic interfaces. A mechanism for domain formation is proposed on the basis of the Burgers mechanism for the bcc-to-hcp martensite transformation. An atom shuffle which is an intrinsic part of this mechanism can occu r in either of two alternative directions and will result in domains r elated by the displacement vector 1/6[20<(23)over bar>]. The similiari ty between this substructure and the domain structure in martensites o f noble-metal alloys is pointed out, and orthorhombic martensite in a titanium alloy is shown to contain a similar domain structure which ca n form from an identical mechanism.