Study on High Temperature Strength of Oxide-Dispersion-Strengthened Mechanical Alloy

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1. Crack propagation tests were conducted in creep-fatigue at 900 ﾟC and 1000 ﾟC in air using notched plate specimens of nickel base superalloy, Inconel MA754, which was an oxide (Y_2O_3) dispersion strengthened alloy made by mechanical alloying. The grains were elongated by directional recrystallization during the annealing process. As the crack tip approached a grain boundary perpendicular to the propagation direction on the specimen surface, the crack propagation drastically decelerated and temporarily halted. It implied that the grain boundaries were obstacles to the propagation. Despite of the microstructural inhomogeneity, the total area of crack increased smoothly because the randomness was averaged along the crack front through the specimen thickness of 5mm (10 grain diameters). The average crack propagation rate was correlated well with creep J-integral range which was a non-linear fracture mechanics parameter and represented the intensity of creep strain in the vicinity of crack tip.2. The crack propagation tests were conducted in creep-fatigue at 800ﾟC and 900ﾟC in air using notched plate specimens of Inconel MA754 without the directional recrystallization and base alloy without the oxide particle dispersion, Nimonic 75. The cracks propagated smoothly and the propagation rate was correlated well with the creep J-integral range. The relations coincided with that of directionally recrystallized Inconel MA754 was far superior to the others in terms of the high temperature strength.3. A stochastic model was developed for the random initiation and propagation of small cracks due to the microstructural effect. Three dimensional simulation was numerically carried out and the validity of model was examined by comparisons with experimental observation.