Surface relief effects of α precipitates in a Ti-Mo alloy

The characteristics of the formation of surface relief associated with h.c.p. alpha precipitates in a Ti-Mo alloy were studied by atomic force microscopy coupled with electron back scattering diffraction and three-dimensional morphological observation. Tent- or inverse tent-shaped relief was formed with normal a plates (thick plates formed often with a ragged interface at smaller undercoolings) progressively to a lesser extent with decreasing temperature. It was replaced entirely by invariant plane strain (IPS) type relief as the plate morphology evolved to black plates (thin straight plates formed at larger undercoolings). In contrast to IPS relief the tilt angles of tent- and inverse tent-shaped relief were often considerably greater than expected from the motion of biatomic structural ledges. There seems to be a marked correlation of relief type with the surface orientation of matrix grains. A phenomenological explanation is presented for unduly large relief angles on the basis of previous theories. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.