Journal of the Japan Institute of Metals and Materials

The effect of Laves phases, C36-(Mg,Al)₂Ca and C15-Al₂Ca, on high-temperature creep strength was quantitatively investigated for the Mg-5.0Al-1.5Ca alloy produced by die-casting. The homogenization treatment at 750 K for 1 h was carried out to divorce the interconnected skeleton of C36 phase, and the aging treatment at 523 K for 1-1000 h was performed to precipitate the C15 phase within the α-Mg grains. The creep tests to evaluate the creep strength were conducted at 447 K and 70 MPa. When the C36 skeleton is divorced, the minimum creep rate dramatically increases by a factor of 330. The coarsening of the C15 phase within the α-Mg grains increases the creep rate by a factor of 2.6. It was identified that the creep strength of the Mg-5.0Al-1.5Ca alloy is predominantly ascribed to the interconnected skeleton of C36 phase rather than the precipitation strengthening of C15 phase.

Fig. 2　Creep rate vs. time in a log-log diagram at 448 K under a stress of 70 MPa for the Mg-5Al-1.5Ca alloy: as die-cast (open) and after homogenization (solid). Fullsize Image

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In this study, the oxidative degradation of gamma-irradiated ultra-high-molecular-weight polyethylene (UHMWPE) was evaluated. Tensile yield stress, Young’s modulus, and Vickers hardness were measured to characterize oxidative deterioration. Furthermore, we evaluated the existence of residual radicals by the electron spin resonance (ESR) analysis, and discussed the relationship between ESR spectra change and mechanical property indexes. In the tensile test of small UHMWPE specimens, it is often difficult to measure the strain. Therefore, we tried to estimate the Young’s modulus simply by using Hertz’s theory of elastic contact, and confirmed its usefulness. It is possible to estimate the tensile Young’s modulus from the indentation test in this experimental range. This method is applicable to evaluate the relationship between mechanical properties such as Young’s modulus and ESR properties over a long term after irradiation.

Fig. 1　Test specimen and measurement points for indentation and hardness test: (a) Specimen dimensions and strain analysis area of tensile test, (b) traces of contact surface left on pressure-sensitive paper in indentation test, (c) typical indentation in Vickers hardness test. Fullsize Image

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