A study of the fatigue crack propagation behavior of Haynes [Registered] alloy 242 [Trademark]

The fatigue crack propagation properties of HAYNES® 242™ ALLOY were investigated at ambient (24°C) and elevated (650°C) temperatures. These properties were tested for different material parameters, such as the heat treatment and the specimen orientation, and testing parameters including the R-ratio. The fracture surfaces obtained from the fatigue tests were examined under a scanning electron microscope for investigating fracture micromechanisms. Optical microscopy was done to evaluate the microstructure of the as-received and heat-treated material. Transmission electron microscopy was done to examine the second-phase particles, precipitated by a long-range- ordering reaction.

The results indicate that fatigue crack propagation resistance increased when the material was suitably heat treated. The crack propagation rates at both ambient and elevated temperatures were observed to be lower for the heat-treated samples, as compared to the as-received samples. The effect of orientation was observed to be minimal, both at ambient and elevated temperatures, and for the heat-treated and as received conditions. R-ratio had an interesting effect on the crack propagation properties at elevated temperatures. Whereas the room-temperature crack growth rates increased when the R-ratio was changed from 0 to 0.5 and to 0.8, the elevated-temperature crack propagation rates increased when R was changed from 0 to 0.5, and decreased when it was further increased to 0.8. This phenomenon was investigated using analytical techniques, such as optical microscopy and scanning electron microscopy.

Microstructural analyses showed that the fatigue cracks propagated in a mostly transgranular fashion. The standard fatigue fracture morphology of striations and tear ridges was observed for all the fatigued samples. Samples fatigued at R = 0.8 at elevated temperatures displayed more secondary cracking and interstriation cracking. Crack propagation for these samples was also associated with plasticity induced crack blunting.

The effect of temperature on the tensile and fatigue crack propagation behavior were investigated. At elevated temperatures, the material displayed dynamic strain aging during the tensile tests. The crack propagation rates were not sensitive to changes in testing temperatures, for both as-received and the heat-treated samples.