The Interdependence Theory: The relationship between grain formation and nucleant selection

The Interdependence Theory links grain formation and nucleant selection to improve the ability to reveal the mechanisms of grain refinement, predict as-cast grain size and account for observations that only a small proportion of added inoculant particles nucleate grains. The Interdependence Theory addresses these issues from a different fundamental perspective, which assumes that grain formation is the result of the interdependence between nucleation and growth acting in concert within an environment dictated by the alloy chemistry. The final grain size is determined by three components: (i) the distance that a previously nucleated grain must grow in order to establish sufficient constitutional supercooling (CS) ahead of a solid–liquid (S–L) interface to enable nucleation of the next grain; (ii) the distance from this S–L interface to the point where this critical amount of CS has been generated; and (iii) the additional distance to the nearest most potent nucleant particle. The first and second components together represent a nucleation-free zone where nucleation is completely suppressed. The relative significance of each component is assessed using experimental data on magnesium and aluminium alloys. To improve particle efficacy and promote nucleation and grain refinement it is critical to minimize the size of the nucleation-free zone by controlling alloy chemistry and/or growth rate. The Interdependence Theory clearly explains why only a small proportion of added inoculants particles are operative.