Ｎｉ基超合金の微小疲労き裂進展に及ぼす温度の影響

Abstract

Based on the measurement of crack opening-closing behavior, the effect of temperature on small fatigue crack growth behavior was investigated in three kinds of Ni-base superalloys at the temperature range of 873K to 1123K, in comparison with the physically long crack properties; a polycrystalline alloy (CM247LC-CC), a directionally-solidified alloy (CM247LC-DS) and a single crystal alloy (CMSX-2) at temperatures of 873K, 1023K and 1123K. It was found that the propagation resistance and the fatigue threshold of long crack increased with temperature in all of the materials on an appearance. However, the long crack growth rates at three different temperatures were approximately represented by an unique curve, by taking account of temperature dependences not only of crack closure level but also of elastic modulus. On the other hand, the small fatigue crack growth resistance decreased with temperature even when the crack closure phenomenon was taken into account. In addition, the small fatigue crack exhibited considerably higher growth rate than the long crack at a given effective stress initensity factor range, and grew even at the lower effective stress intensity factor range than the long crack threshold. Based on the results thus obtained and the chemical analysis near crack propagation plane, the factors which lead to the lack of similitude in propagation law between small and long cracks were also discussed.