Hello everyone:
During solidification process, a nucleus is usually very small, and the critical size may be as small as several nm that can be estimated from the interface energy and Gibbs free-energy difference. The nucleus then can grow up to a grain under a proper condition.
In another common case, if an alloy is solidified with a forced liquid metal flow, the already formed solid dendrites or particles with a large size (e.g. 300μm) drift into the bulk liquid and not remelt by the liquid metal. These solid dendrites or particles will finally grow to the grains, and their size will change (e.g. such as from 300 μm to 400μm). In this case, the large solid dendrites or particles can be considered as nucleation sites?
Dear Jie,
That's an interesting question to think of.
When a TiB2 type of particle (0.5 micron size) with limited coherency (than a drifted dendrite) with liquid Al can be called as a nucleating site, I dont think it is wrong to call a fine drifted dendrite as a nucleating site (which is has better coherency with the liquid), since the result is "grain-refinement" in either case.
The case of large dendrites may be looked at differently.
Anyway, I feel that the definitions are subject to a debate.
Cheers
Prasadd
Thanks for your reply. I also think that it may be questionable to call the large dendrite as nucleation sites.
However, since the size of drifted large solid dendrites will change finally, they also cannot be the same as their original state (i.e. we cannot simply consider that the floating grains enter the liquid metal, and survive and finally result in the grain refinement).
Above all, how can we express this phenomenon?
The survival of the drifted dendrites depends on the temperature and concentration gradients prevailing in the liquid surrounding the dendrites.
Although such dendrite fragments may become the origin of equiaxed grains, they are not nucleation sites in the classical sense. They are solid phase already and need not nucleate from the liquid phase in order to grow. They are one potential “source” for growth of equiaxed grains. In computational modelling, however, they could be treated like a nucleation site – but one which needs zero undercooling for activation.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Metals
- Alloys
- Steel