Microstructural evolution and calorimetric evaluation of non-equilibrium states in rapidly solidified Al-Mn-Co ternary alloys

Evolutionary structures were observed in as-melt spun ribbons of Al--3.5Mn--0.8Co, Al--3.5Mn--1.3Co, Al--3.5Mn--2Co, Al--5Mn--0.8Co and Al--5Mn--2Co alloys. The evolutionary structures contained two zones, zone A and zone B, at the chilled and unchilled sides, respectively. A banded structure was observed between the two zones as a transitional region in the Al--3.5Mn--1.3Co alloy. The enthalpy differences, Delta H exp ne sub O , between a non-equilibrium state in as-melt spun ribbons and the equilibrium state in fully annealed ribbons of the Al--Mn--Co alloys were evaluated using data obtained by differential scanning calorimetry (DSC) measurements. The values of Delta H exp ne sub O for Al--Mn--Co alloys are lower than those for Al--Mn binary alloys. Delta H exp ne sub O corresponds to a decrease in Gibbs free energy, Delta G, which is stored in a solidified solid during rapid solidification. The formation of the banded structure is interpreted on the basis of an analysis of the distribution of Delta H exp ne sub O or the fluctuation in Delta G, which should oscillate in order to correspond with a gradual reduction of cooling ability for the constrained solidification by the substrate.