難燃性Mg-Al-Ca系合金押出材の疲労強度特性とその疲労強度に及ぼすAl添加量の影響の破壊力学的検討

抄録

Fatigue strength characteristics were investigated in two different non-combustible Mg-Al-Ca alloys, Mg-9%Al and Mg-4%Al controlled processability and strength by adjusting the amounts of added elements. Fatigue strength of Mg-9%Al with higher Al addition was higher than that of Mg-4%Al with smaller Al addition. According to fracture surface observation, a precipitate was observed at fracture origin in Mg-9%Al. However, precipitate was not observed at fracture origin in Mg-4%Al. Fatigue strength of Mg-9%Al was well evaluated based on fracture mechanics by precipitate size observed at the fracture origin and applied stress amplitude. In case of Mg-4%Al, the minimum size of a precipitate which can be the fatigue fracture origin estimated from the threshold stress intensity factor range obtained with fatigue crack growth test was larger than the maximum size of a precipitate estimated by statistics of extreme value. Therefore, it was able to confirm the validity of the experimental result based on fracture mechanics. Relationship between amount of Al addition and the fatigue strength at 10⁷cycles was estimated by using a crack closure model with assuming fatigue crack initiation from a precipitate. According to the calculation result with the crack closure model, degradation of the fatigue strength ratio (ratio for fatigue strength at 10⁷cycles to the tensile strength) was observed with increase in Al addition. In other words, it is considered that fatigue strength is significantly affected by increasing the precipitate size to play as a fracture origin rather than increasing of the tensile strength due to increase in Al addition. In additionally from the calculation result in the model, it was found that fatigue strength at 10⁷ cycles showed about 40% of the tensile strength when amount of Al addition is larger than 4%.