Mg-1.5%Gd-0.3%Zn-0.55%Zr alloy samples with and without neodymium（0.6%, mass fraction）were prepared by sand casting for the purpose of further improvements of various properties. Aged hardening behavior, solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM, XRD, FE-SEM, hardness tests and mechanical property tests. The main research results are as follows. 1）Compared with the alloy without the addition of neodymium, the alloys with the addition of neodymium shows the more remarkable age-hardening response. As neodymium concentration was increased, the peak hardness increased and the time to reach the peak hardness shifted to a long time side. 2）The crystallization phase increased with increasing neodymium concentration, and undissolved crystallized phase remained after solution treatment at 3.0 Nd and 6.0 Nd alloys. Precipitation hardening phase is Mg3Gd phase in 0 Nd without the addition of neodymium, and Mg3Nd in 1.5 to 6.0 Nd alloys with the addition of neodymium. 3）As neodymium concentration was increased, the 0.2% yield strength increased and the elongation decreased. In addition, the tensile strength showed the maximum tensile strength at 1.5 Nd alloy. From the surface fracture observation, the fracture mode changed from d uctile to brittle as the solute concentration became higher. Cracks propagating to the crystallized matter were also observed with neodymium concentration between 1.5～6.0Nd alloys, especially with 6.0Nd alloy.

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In this study, deterioration of adhesive joint strength was investigated by lap joint specimens and butt joint specimens which were aged under high temperature and high humidity conditions. As a result, lap shear strength of lap joint specimens decreased with increasing aging time. From the results of surface observation and FT-IR analysis, it was confirmed that decrease of joint strength results from deterioration of the adhesive/adherend interface and degradation of cohesion due to hydrolysis of urethane bond. From the results of XPS measurement for butt joint specimens fractured by tensile test, it was clarified that H2O is adsorbed at the adhesive/adherend interface after aging. It was suggested that adsorption of H2O causes decrease of hydrogen bonding force combining adhesive/adherend interface and thus interfacial strength of the joint decreases.

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