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

The cubic and hexagonal alpha -Al(Fe,Mn)Si phases have been precipitated sl owly and equilibrated at 630 degreesC from a series of melts with approxima tely 4 wt% transition metal and 7.5 wt% Si in which the Fe : Mn ratio has b een varied from 30 : 1 to 1 : 10. Chemical analyses of the phases, and of t he microstructure that formed on solidification of the residual melt, show that the total transition metal concentration in the mixed phase is less th an in the almost stoichiometric compositions found for the end-member terna ry alpha (c)-AlMnSi produced previously under similar conditions. This is i nterpreted as evidence for a stabilising effect of vacancies which counters any destabilisation of the electronic structure arising from the second tr ansition metal. The residual melt composition also exhibits a marked reduct ion in the total transition metal concentration that is further evidence of the enhanced stability of the mixed phase. The consequence for both liquid us and solvus precipitation reactions in dilute alloys is that the transiti on-metal enrichment necessary for precipitation is as low as 60% of that es timated from Phillips and Varley [1] who interpolate linearly between the e nd-member solubilities. (C) 2001 Kluwer Academic Publishers.