| The nucleation and growth mechanisms leading to the grain refinement of aluminium are examined. The literature shows that the grain refinement of aluminium is achieved commercially by the addition of Al-Ti or Al-Ti-B master alloys to the melt. The dissolution and dispersion of the components of the master alloys together with reactions occurring between the master alloy and the melt provides excellent nucleating particles. These particles are thought to be one or a combination of TiAl(,3), TiC, and TiB(,2).; The behaviour and the effectiveness of the different types of master alloy in the grain refining of aluminium was examined with respect to; grain refiner addition rate, time between addition of the master alloy and solidification of the melt, superheat, othe solutes in the melt, and cooling rate. The behaviour of the primary component of the mster alloys, TiAl(,3), in liquid aluminium was examined to determine its dissolution rate and stability upon precipitation from the melt. A number of the transition elements Ti, Hf, Zr, Cr, Ta, Nb, W, Mo, and V have peritectic reactions with aluminium at the aluminium-rich end of the phase diagram and have varying degrees of grain refining effectiveness.; The morphology of the primary trialuminide phases for Ti, Hf, Zr, Nb, and Ta was examined under varying conditions of cooling rate and composition. Ti, Zr and Hf were all found to have similar morphological responses to changes in cooling rate and composition. The morphology of these trialuminide phases is explained in terms of growth rate anisotropy and faceting tendencies. It was found that the trialuminide phases for Ti, Hf, and Zr form metastable cubic structures under conditions of high cooling rate. The results obtained often indicated large departures from equilibrium.; The grain refining effectiveness of the various transition elements examined is explained in terms of structure transformations at the nucleant surfaces and the effect of the growth-restricting solute field.; The assumption of local equilibrium at the solid/liquid interface is also examined and tested under conditions of rapid solidification for Zn-Cd and Bi-Sn alloys. Both these alloy systems have retrograde solidus curves and extensions of solid solubility beyond the maximum solidus compositions were obtained. This indicates a departure from local equilibrium at the solid/liquid interface. |
