The age-hardening behavior of the Fe-Ni-based alloy HR6W without carbon was investigated at temperatures between 973 and 1173 K for a maximum of 3000 h to clearly identify the nose temperature of the Fe₂W Laves phase. The microstructure observation was carried out for the aged alloys using field-emission scanning electron microscopy. One-step increase of hardness was detected for the alloy, which corresponds to the precipitation of Fe₂W phase. It was found that the Fe₂W phase precipitates with a flake-like morphology within γ grain interior and with a granular morphology at grain boundaries. The TTP (time-temperature-precipitation) diagram for the alloy is established based on the results of hardness measurements and microstructure observations, and the C curve representing the time required to initiate precipitation of the Fe₂W Laves phase within γ grain interior was determined. The nose temperature of the C curve was identified as 1100 K, and the precipitation starts at 20 h at the nose temperature. The precipitation of the Fe₂W phase at grain boundaries occurs for one order of magnitude earlier than that within γ grain interior at temperatures below 1100 K.

Magnesium chips were coated with a high concentration of graphite using a binder, and then used as the raw material for thixomolding. The microstructure of the magnesium injection-molded product with the addition of graphite exhibited a dispersion of needle-like graphite. No significant voids were observed at interfaces between the graphite and matrix. The addition of more than 0.5 mass％ graphite decreased the proof stress and tensile strength of the injection-molded products. The Young’s modulus of graphite-added products showed a tendency to decrease with increasing addition of graphite, which was consistent with the lower limit of the rule of composites. Compared to AZ91D, the thermal conductivity of the 6.9 mass％ graphite-added product was increased, and the coefficient of linear thermal expansion was decreased. Both were within the ranges where the rules of mixtures were satisfied.

Al-10Si-Mg alloy have been used for die casting and additive manufacturing. In this study, solution-treated Al-10Si-Mg and Al-10Si alloys (mass％) were heat-treated at several temperatures and the linear dimensional changes arising from the heat-treatments were investigated. A theoretical model for calculating the linear dimensional change in Al-Si-Mg ternary alloys was proposed, and the theoretically and the experimentally determined linear dimensional changes were compared. It was found that the linear dimensional changes of Al-10Si-Mg and Al-10Si alloys were almost the same. In both alloys, the linear dimensional changes increased with the decrease of the heat-treatment temperature. The linear dimensional changes of Al-10Si-Mg and Al-10Si alloys agreed well with the values estimated by the proposed theoretical model.

Mater. Trans. 62 (2021) 1023-1029に掲載