Nature of massive transformation
- PDF / 58,770 Bytes
- 2 Pages / 684 x 858 pts Page_size
- 54 Downloads / 260 Views
LERT, Professor Emeritus, is with the Department of Materials Science and Engineering, KTH, SE-10044, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted January 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
When the OR to a parent grain cannot yield particularly low-energy interfaces, the migrating interface may be fairly smooth and show low tendency of faceting. The resulting particles are described as grain-boundary allotriomorphs. At sufficiently low undercoolings, there may only be grainboundary allotriomorphs. The absence of acicular particles may be explained by the buildup of stresses during acicular growth. At higher undercoolings, that problem will be less due to the larger driving force. Acicular growth will then be favored by the so-called point effect of diffusion and can dominate the reaction. If the parent phase by cooling is brought into the onephase field of the new phase, no long-range diffusion will be required and the transformation could turn compositioninvariant. The thermodynamic conditions for a compositioninvariant transformation may even be fulfilled already when the parent phase is cooled below T0, the temperature of equal Gibbs energy for the two phases. This will not have a strong effect on nucleation, which will still be concentrated to the grain boundaries, but the growth rate will increase dramatically. It would be expected that the resulting shapes of the allotriomorphs should be similar but they should develop to larger, more blocky shapes because their growth would not be retarded by the buildup of a composition gradient in front of the migrating interface. It is now proposed that the massive grains are actually formed in this way. If so, one could simply define the massive transformation as the composition-invariant counterpart of diffusional growth of the allotriomorphic form of precipitation. Some characteristics of the massive transformation would follow as direct consequences of this definition since they are shared with the allotriomorphic precipitation. When the driving force for composition-invariant growth is low, the growth of nuclei with an OR resulting in composition-invariant acicular growth will be retarded and the massive particles will predominate. From rapid quenching experiments with Fe-base alloys, it is known that the composition-invariant transformation will first be dominated by massive grains but then by an acicular structure when the driving force is higher at larger undercoolings.[5,6] This is similar to the competition between allotriomorphic and acicular precipitation. At even lower temperatures, a new phenomenon appears when the driving force is sufficient for the creation of glissile interfaces, which results in martensite. The concentration of nucleation to grain boundaries in the parent phase is another feature that the massive transformation has in common with allotriomorphic precipitation for small composition differences. The massive transformation is sometimes described as diffusional. However, it should be emphasized th
Data Loading...
