Precipitation of α-Al(Fe,Mn)Si from the melt

The cubic and hexagonal α-Al(Fe,Mn)Si phases have been precipitated slowly and equilibrated at 630°C from a series of melts with approximately 4 wt% transition metal and 7.5 wt% Si in which the Fe : Mn ratio has been varied from 30 : 1 to 1 : 10. Chemical analyses of the phases, and of the microstructure that formed on solidification of the residual melt, show that the total transition metal concentration in the mixed phase is less than in the almost stoichiometric compositions found for the end-member ternary αc-AlMnSi produced previously under similar conditions. This is interpreted as evidence for a stabilising effect of vacancies which counters any destabilisation of the electronic structure arising from the second transition metal. The residual melt composition also exhibits a marked reduction in the total transition metal concentration that is further evidence of the enhanced stability of the mixed phase. The consequence for both liquidus and solvus precipitation reactions in dilute alloys is that the transition-metal enrichment necessary for precipitation is as low as 60% of that estimated from Phillips and Varley [1] who interpolate linearly between the end-member solubilities.