On the mechanisms of growth of faceted precipitates

Journal Article · Thu Dec 01 00:00:00 EST 1988 · Scr. Metall.; (United States)

DOI:https://doi.org/10.1016/S0036-9748(88)80234-5· OSTI ID:6166674

Stephens, D E; Brechet, Y J.M.; Purdy, G R

For some time it has been recognized that, depending on the magnitude of the driving force, an interface between a precipitate and its parent phase can progress by normal motion or by a lateral mode, involving the propagation of steps. In the case of lateral motion, the steps may appear via a homogeneous nucleation process; alternatively, they may be provided by some heterogeneous process; e.g. at points of intersection of screw dislocations with the interface. When the driving force is large enough to permit an element of the interface to ''ride over'' the energy barriers, normal motion is expected; for smaller driving forces the interface occupies energy minimum positions during growth, except at the ledge. For sharper interfaces, perhaps more typical of precipitation systems in which a major structural transformation occurs (e.g. O' in Al-Cu), it seems that lateral migration is the rule. In the case of normal growth, a precipitate starting with a polyhederal shape should remain polyhedral during its further growth. The case of lateral growth is more complicated. On the facets, the steps propagate without hindrance, but when they reach the edges, the steps must annihilate in order for the edges to move, i.e. a slow volume diffusional structural relaxation process is required. If steps are nucleated easily, the rate of annihilation at the corners may be insufficient, and the steps will pile up and create a curvature, and a singularity at the edge. The rate of annihilation of steps now will determine and be governed by the solid angle at the cusp. The authors' main purpose here is to investigate the various shapes, which include such singularities, of precipitates developed by lateral growth.

• Research Organization: Dept. of Materials Science and Engineering, McMaster Univ., Hamilton, Ontario (CA)
• OSTI ID: 6166674
• Journal Information: Scr. Metall.; (United States), Vol. 22:12
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM ALLOYS
ORDER-DISORDER TRANSFORMATIONS
COPPER ALLOYS
ANNIHILATION
CRYSTAL GROWTH METHODS
DIFFUSION
NUCLEATION
PHASE STUDIES
PRECIPITATION
SHAPE
ALLOYS
BASIC INTERACTIONS
ELECTROMAGNETIC INTERACTIONS
INTERACTIONS
PHASE TRANSFORMATIONS
SEPARATION PROCESSES
360102* - Metals & Alloys- Structure & Phase Studies