Mechanisms of the massive transformation

A massive transformation (MT) is here defined as a diffusional nucleation and growth process in which the product phase has a different crystal structure from but the same composition as the matrix phase. Particularly at low undercoolings below T-o, nucleation of a,MT at a grain boundary requires that the critical nuclei be as coherent as possible with both matrix grains. This is especially important when the MT takes place in a two-phase region; that this can occur is now well documented for Ti-Al and Fe-Ni alloys. Coherency with the matrix grain toward which the nucleus is irrationally oriented, and partial coherency during growth, may be achieved through the matching of plane edges (rather than plane faces), as recently pioneered by Zhang and Kelly; Nie and Muddle; and Howe, Reynolds and Vasudevan. Massive growth requires only that diffusional jumps across massive:matrix boundaries be readily feasible at some areas of these boundaries. Although MT products usually have a "massive" appearance, this no longer appears to be an essential characteristic of this transformation mode.