Experimental investigation of the Al-Mn-C system. Part II: Liquidus and solidus projections

Abstract

Phase equilibria upon crystallization in the Al-Mn-C system were studied using DTA, X-ray diffraction, SEM and electron probe microanalysis. The liquidus and solidus projections and the melting diagram (liquidus + solidus) for this system were constructed covering the whole concentration range. The ternary compound Mn3AlC (κ) (antiperovskite structure CaTiO3-type, ≿Р5-Pm-3m) melts congruently at 1320 °C. The liquidus surface consists of the binary compounds, the ternary compound Mn3AlC (κ) and solid solutions primary crystallization fields. The carbide Mn5C2 in the ternary system participates in equilibrium with the liquid phase in contrary to the binary system where it is formed from the solid state equilibrium. The solidus surface of the Al-Mn-C system in the region up to 50 at.% Al is defined by the co-existence of the κ-carbide with almost all phases of the binary subsystems: (С), (γMn), (εAlMn), (Al4C3), (Mn7C3) and (Mn5C2). The solidus surface in the region >45 at.% Al is defined by the co-existence of the carbide Al4C3 with all phases from Al-rich region.The DTA curves of the alloys with compositions close to the homogeneity region of phase (εAlMn), exhibit (on heating) an exothermic effect at ∼570 °C. This exothermic effect corresponds to the formation of the ordered metastable τ-phase, which is formed by mechanism: ε → ε' (B19) → τ. τ-phase (AuCu, tP2-P4/mmm, a = 2.760, c = 3.600 Å) and which was observed in the alloys after heating up to 700 °C and slow cooling (5 °C/min).