Fractal dimension of the grain boundary fracture in creep-fracture of cobalt-based heat resistant alloys

Abstract

The effects of grain boundary configuration and creep conditions on the fractal dimension of the grain boundary fracture (Df) were investigated using commercial cobalt-based heat resistant alloys, namely, HS-21 and L-605 alloys. Creep-rupture experiments were carried out under the initial creep stresses of 19.6–176 MPa in the temperature range from 1089–1422 K in air. The value of Df was larger in specimens with serrated grain boundaries than in those with straight grain boundaries in the HS-21 alloy under the same creep condition, and the difference in the value of Df between these specimens was large in the scale range of the analysis which was less than about one grain boundary length. However, there was almost no difference in the value of Df between the specimens with serrated grain boundaries and those with straight grain boundaries in the L-605 alloy, because there was no obvious difference in the microstructure between these specimens. The value of Df increased with decreasing creep stress in the scale range of the fractal analysis larger than about one grain boundary length in both HS-21 and L-605 alloys, while the stress dependence of Df was larger in the HS-21 alloy. The stress dependence of Df was explained by the stress dependence on the number of grain boundary microcracks linked to the fracture surface. The value of Df estimated in the scale range smaller than about one grain boundary length showed essentially no stress dependence in both L-605 and HS-21 alloys.