G. T. Cashman, GE Transportation, Cincinnati, OH

Summary:

Two discrete and identifiable failure mechanisms have been observed in powder metal nickel-base Superalloys. These materials are extruded and isothermally forged René95 and René88DT. Low cycle fatigue testing in air at temperatures in the range of approximately 204°-649°C (400°-1200°F), failures initiating from surface features, such as isolated ceramic inclusions or an occasional large grain facet, have been shown to generate much shorter lives than from corresponding specimens demonstrating failure initiations from internal sites. Surface initiated failures tend to dominate at higher stresses, a mixture of failure initiation sites occurs at an intermediate stress level and a switch to predominantly internally initiated events occur at lower stresses. Figure 1 displays this response.

Vacuum LCF testing at stresses likely to demonstrate surface initiations were also conducted. Surface initiation resulted in all cases except one, and all lives were indicative of those for internally initiated failures. Figures 2 and 3 present these results. Fractographic evaluation in the scanning electron microscope identified the fracture initiation sites presented on these figures. The behavior of these results leads to the conclusion that an environmental effect at the initiation sites is reducing the lives of these specimens relative to those demonstrating an internal site. Since the latter can be considered to be operating in a vacuum during fatigue initiation and initial propagation, the role of environment is clearly indicated.