Interfaces and precipitation

Abstract

An important class of alloy transformation involves the precipitation of a body-centred cubic phase in a close packed cubic or hexagonal matrix (or vice-versa). In the past ten years, numerous investigations have demonstrated that the precipitate grows as a lath or needle-shaped particle, with the growth direction of the axis of the particle parallel to an invariant line of the phase transformation. Although there are an infinite number of potential invariant lines in such a transformation, commonly one observes that the growth direction is close to the common close-packed directions e.g. <110>f and <111>b, with a rational or near-rational orientation relationship between the two phases. These observations can be rationalized by invoking the geometric principles embodied in Bollmann's O-lattice theory or by appealing to the minimization of strain energy principle associated with the set(s) of misfit dislocations lying parallel to the invariant line. Figure 1 shows an example of this characteristic morphology of a lath of γ (face—centred) precipitated in a matrix of α (body-centred) in a two-phase stainless steel. The lath is bounded by well-developed facet planes_(see Fig. 1) with a growth direction about 5° from the common close packed [111]α, [101]γ directions. Sets of misfit dislocations and steps are visible at the (416)α and (275)α facet planes.