Interaction of High Temperature Oxidation and Fatigue-Crack Growth of Ni-Based Directionally Solidified Super Alloy
Masaru SEKIHARA and Shigeo SAKURAI
Abstract:To evaluate the remaining lifetime of high-temperature parts of fossil-fuel power plants precisely, the influence of the solidified direction and the crack-surface oxidized zone on the crack growth rate of Ni-based directionally solidified super alloy was investigated by using the results of long-term fatigue tests. Three types of specimens, 0‹, 45‹ and 90‹ specimens, in which the load direction was parallel, diagonal and normal to the solidified direction, were prepared to evaluate the effect of solidified direction on the crack growth rate. The crack initiation lives were influenced by the solidified direction and the crack initiation life of the 0‹ specimen was longer than the 45‹ and 90‹ specimens. Also the crack growth rates were influenced by the solidified direction at the same cycle number. Under 600Ž, the fatigue crack growth rates of 45‹ and 90‹ specimens were almost the same. The fatigue crack growth rate 0‹ specimen was slower than the rates of crack growth rates of 45‹ and 90‹ specimens during the stress intensity factor range ƒ¢K was less than 30MPaãm. And under 860Ž, the fatigue crack growth rates of 0‹ ,45‹ and 90‹ specimens were almost the same. The long-term fatigue-crack growth rate under 860Ž once decreased with the growth of the crack. And this behavior seemed to arise from the residual-stress field produced by the volumetric expansion owing to the growth of the oxidized layer, the growth of which was evaluated by the experimentally obtained parabolic oxidation law. Key Words:Fatigue, Crack propagation, Elevated temperature, Oxidation, Ni-based super alloy, Directionally solidified