Tensile tests of rolled Mg-6.2mol%Li and Mg-11.7mol%Li alloys were carried out at room temperature to clarify effects of lithium addition on the relationship between mechanical properties and activities of slip systems. Ductility was found to increase with increasing lithium content. 0.2% proof stress increased when 6.2mol%Li was added. However, Mg-11.7mol%Li showed low 0.2% proof stress compared to pure magnesium. On the other hand, maximum stress decreased with increasing lithium content. Frequency of non-basal slips increased with increasing lithium content. Also, first order pyramidal <c＋a> slip showed the highest frequency among non-basal slips in Mg-Li alloys. Critical resolved shear stresses for non-basal slips, which were reduced by lithium addition, resulted in high ductility and low tensile strength of magnesium.
In the present study, tensile and creep properties of cast Mg-La-Zr and Mg-Ce-Zr alloys were examined and the microstructures of the alloys were observed by optical microscopy, TEM, and Cs-STEM to evaluate creep and work hardening characteristics. The room temperature tensile properties, plotted with lanthanum or cerium contents, did not reveal significant differences between the alloys except for slightly lower elongation for Mg-Ce-Zr alloys. However, at 150˚C, the values of tensile strength for Mg-Ce-Zr alloys were higher than those for Mg-La-Zr alloys, while the values of elongation for Mg-Ce-Zr alloys became significantly lower than those for Mg-La-Zr alloys. It was also found that Mg-Ce-Zr alloys tested at 150˚C exhibited a higher work hardening rate than Mg-La-Zr alloys. In creep curves, Mg-La-Zr alloys exhibited an initial strain after which the direction of curves abruptly changed to almost horizontal during creep testing, whereas Mg-Ce-Zr alloys did not exhibit the initial strain. It was found that Mg-Ce-Zr alloys contained numerous stacking faults on（0001）of the primary α-Mg grains, while Mg-La-Zr alloys did not, and these might have contributed to the differences in creep and work hardening characteristics.
The mechanical properties of aluminum alloys fabricated by selective laser melting (SLM) were examined with particular focus on their fatigue properties. The SLM aluminum alloys with various amounts of internal porosities were fabricated to investigate the effect of porosity on the fatigue properties. There were no remarkable differences in the microstructures of the SLM alloys. In terms of tensile properties, the tensile strength was significantly higher in the specimen with low porosity and decreased with increasing porosity. In terms of fatigue properties, crack initiations were observed at the internal pores in all specimens with the number of crack initiations increasing as a function of porosity. The fatigue strength decreased with increasing porosity, and porosity had a more significant effect on fatigue strength than tensile strength. The fatigue limit, which corresponds to the size and number of internal pores, changed remarkably in the low porosity regime because the size and number of internal pores changed drastically in this range. Considering the residual stress, these results demonstrate that by considerably suppressing the internal pores in the SLM aluminum alloys, a good fatigue performance can be expected, regardless of the occurrence of fatigue cracks at the internal pores.
Effects of heat treatment and additional zirconium on grain boundary (GB) fracture of Al-5%Mg alloys containing sodium were investigated. As a result of the tensile test, room temperature ductility decreased as the amount of Na increased from 4ppm to 200ppm in the homogenized specimen. GB fracture was observed at room temperature when the alloy contained 200ppm of sodium. On the other hand, the homogenized specimen containing 200ppm of sodium did not show GB fracture even at 300˚C. High temperature embrittlement (HTE) was observed in the Al-Mg-4ppm Na alloy with cold-swaging plus recrystallization heat treatment after the homogenization. HTE was partially suppressed by the addition of 0.2%Zr in an Al-Mg alloy with 3ppm of Na, while not fully be suppressed by the addition of 0.1% Zr in an Al-Mg alloy with 2ppm of Na. Furthermore, it was revealed that the HTE of the Al-Mg-0.1%Zr alloy with 2ppm of Na was fully suppressed by the additional annealing at 450˚C for 5h. It was presumed that Al3Zr precipitates is effective for the suppression of the HTE of Al-Mg alloys caused by sodium.