Journal of Japan Institute of Light Metals

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The hardness and tensile properties of Mg–La–Zr alloys with various lanthanum contents were investigated, and the microstructures of the alloys were examined. The microstructure was composed of fine globular primary αMg grains and eutectic areas. The values of yield stress and modulus of the eutectic were, respectively, about 3 times and 1.7 times higher than those of the primary αMg. The increase in the yield stress of the alloys with less than 1.2%La is due to a rapid increase in coverage of the αMg grain boundary by the eutectic. As the increment in the grain boundary coverage decreases with lanthanum content, the composite strengthening (composite materials effect) starts to play a more significant role in increasing yield stress. As the grain boundary coverage reaches a plateau at about 2%La, further increase in the yield stress is mainly due to the composite strengthening, the effect of which is dependent on the volume fraction of eutectic. During tensile-testing at 150°C, the alloys with higher lanthanum content exhibited dual yield points, the first from yielding of the primary αMg, followed by the higher-yield point associated with the eutectic. This suggests that composite strengthening is in effect.

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The effects of homogenization and brazing conditions on material properties of brazed Al–Mn base alloys were investigated. Low-temperature homogenization enhanced the alloy strength in comparison with high-temperature homogenization. In contrast, high-temperature homogenization induced higher electrical conductivity. Both the strength and electrical conductivity were affected by brazing. The strength increased with a high heating rate, short holding time, and high cooling rate. On the other hand, electrical conductivity increased with a slow cooling rate. Therefore, combining low-temperature homogenization with low cooling-rate brazing should allow maintaining a balance between high strength and good electrical conductivity. The variation of strength after brazing with different homogenization and brazing conditions can be explained by the contribution of solid solution strengthening due to Mn and dispersion strengthening caused by Al–Mn–Si dispersoids.

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