ACCELERATED HIGH TEMPERATURE PERFORMANCE EVALUATION FOR ALLOY OPTIMIZATION, EMBRITTLEMENT, AND LIFE ASSESSMENT

Creep strength and fracture resistance are two properties that are critical in the selection and optimization of high temperature materials. These same properties may be progressively impaired by service exposure and are, therefore, key to the assessment of remaining life of high temperature components. In recent years, a methodology based on accelerated measurements of these properties has been developed. The two critical properties are decoupled in this approach, which allows a clearer separation of the effects of microstructural evolution, damage development and environmental attack. The evaluation of creep strength, based on a high-precision stress relaxation test (SRT), and fracture resistance, based on a constant displacement rate test (CDR), are illustrated using extensive data on a conventionally cast nickel based alloy (alloy 738) and also some recent work on a ferritic steel (T91). The well-known effect of section size on rupture life of conventionally cast alloys is shown to be an effect on fracture resistance rather than creep strength. By using miniature specimens the variation in creep strength in critical components can be quickly determined. The comparisons with long term creep rupture test data for both alloy classes show good agreement indicating that the accelerated testing is quite capable of yielding both comparative and design data. With the added advantage of a separate fracture resistance criterion the new methodology should be broadly useful