The Influence of the effect of solute on the thermodynamic driving force on grain refinement of Al alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.The Australian Research Council (ARC DP10955737)

Thermodynamic investigations of Al-Mg, Al-Li-Mg, Mg-Y, Mg-Si, Cu-Mg-Ni and Al-Mg-Si alloys Final report

The enthalpy of mixing #DELTA#H of liquid binary and ternary aluminium and magnesium alloys has been determined to understand the thermodynamics and the phase equilibria of these light alloys. The experiments were performed using high-temperature isoperibolic type of solution calorimeter. The obtained #DELTA#H-values of liquid Al-Mg alloys follow a regular solution behaviour with a minimum value of -2.2 kJ mol"-"1. The extrapolated #DELTA#H-values of Mg-Y and Mg-Si alloys show a maximum at about 23 at.% Y and 42 at.% Si respectively. The enthalpy of formation, the enthalpy of melting and the heat capacity of Mg_2Si, Mg_2Ni, MgNi_2, CuMg_2 and Cu_2Mg have been determined calometrically. The enthalpy of mixing of liquid Al-Li-Mg alloys was measured between 870 and 1030 K starting from all three bordering systems as bath material and adding the respective third component. The minimum value of #DELTA#H (i.e. -10 kJ mol"-"1) is to be observed in the binary Al-Li system at the stoichiometric composition AlLi. (orig.)SIGLEAvailable from TIB Hannover: F95B855+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany); Commission of the European Communities (CEC), Brussels (Belgium)DEGerman