Morphology and Substructure of Ti–Cu Martensite and its Aged Martensite

Abstract

The morphology and substructure of Ti-Cu martensites containing 1, 2 and 5.3 wt% Cu were investigated by means of optical microscopy and transmission electron microscopy. Surface relief studies prove that the transformation of three Ti-Cu alloys during water-quenching is essentially martensitic. Detailed examinations exhibit gradual transitions in (1) the morphology, (2) the mode of the inhomogeneous shear and (3) the mode of stress accommodation, with an increase in copper content. A typical massive structure appears only in Ti-1%Cu. Above this concentration (2%Cu or more) the plate structure predominates. A transition in the operating mode of the inhomogeneous shear, from slip to twinning, occurs with increasing copper content. It occurs at a somewhat higher M, temperature than in high purity binary titanium or zirconium alloys. A parallel transition is first observed in the mode of stress accommodation, from accommodation by movement of dislocation boundaries or by formation of dislocation tangles at low Cu, to accommodation by {1011} deformation twinning at higher Cu. A mixture of the two modes is also observed, proving a possible interchangeability of dislocation tangles and twins during the accommodation process.Clear distinction between types of accommodation twins (stress induced twins and direct impingement twins) is given. These are compared to the transformation twins.The rate of nucleation and growth of precipitates on lattice defects during the aging process of the martensitic structure depends on the type of defects. The phenomena observed can all be explained on the grounds of surface energy, strain energy and diffusion rate considerations. The importance of lattice defects as preferred nucleation sites decreases in the order of martensitic plate boundaries,