The evolution of microcrystalline structures in supercooled metal powders

Abstract

A numerical model has been developed to study the relative effects of nucleation and growth kinetics on the evolution of ultrafine grain structures observed in some supercooled metal powders. The thermal history during solidification is analyzed using a Newtonian heat transfer formulation coupled to classical models for homogeneous nucleation and continuous growth with a diffuse interface. The results indicate that decreasing particle size increases both the supercooling prior to solidification and the thermal excursion beyond the nucleation temperature. After the first nucleus appears, a compctition is established between the formation of new nuclei and the growth of the existing one(s). There is a range of particle sizes in which the achievable supercoolings are high enough to produce massive nucleation before any significant growth—and the ensuing recalescence—can take place. The probability of multiple nucleation may be evaluated from a dimensionless parameter combining the characteristic frequencies of the nucleation, growth, and heat transfer processes at the moment of nucleation. Calculations for Al and Ni model systems confirm the experimental observation that the latter has a stronger tendency to supercool and develop microcrystalline structures.