Shigeoki Saji

Microstructures and calorimetric evaluation of non-equilibrium states in rapidly solidified Al-Mn alloys containing less than 7 at % Mn

Morphology and enthalpy differences between a non-equilibrium state of as-melt spun ribbons and the equilibrium state of fully annealed ribbons were investigated by TEM observation, X-ray analysis and differential scanning calorimetry (DSC) measurements for Al-1.66, 2.56, 3.95, 5.37 and 6.98 at % Mn alloys. The icosahedral phase was observed with α-Al solid solution in as-melt spun 3.95, 5.37 and 6.98 at % Mn ribbons. The morphology of the icosahedral phase was complex; small particles, petal-like or coral-like branches and eutectic structure within the cell and small particles, films and networks along cell boundaries were observed depending on the manganese content. The differential enthalpy, d(ΔHone)/dT, versus T curve obtained by characteristic DSC measurements normally showed three peaks. The peak P1 at 520–560 K corresponds to the precipitation and growth of the icosahedral phase, the peak P2 at about 600–800 K to the transformation from the icosahedral phase to the equilibrium Al6Mn phase, and the peak P3 at about 700–880 K to the precipitation of Al6Mn from α-Al solid solution. The temperatures of the peaks P2 and P3 shift to the lower side and the height of the peaks becomes higher with increasing manganese content. The enthalpy difference due to the fine microstructure in as-melt spun ribbons was also estimated.

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, ΔHOne, 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 ΔHOnefor Al-Mn-Co alloys are lower than those for Al-Mn binary alloys. ΔHOnecorresponds to a decrease in Gibbs free energy, ΔG, 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 ΔHOneor the fluctuation in ΔG, which should oscillate in order to correspond with a gradual reduction of cooling ability for the constrained solidification by the substrate.