Doctoral Dissertations

Weiju Ren

Date of Award

5-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw, Thomas T. Meek

Committee Members

Charlie R. Brooks, Carl J. McHargue, John D. Landes

Abstract

The time-dependent fracture mechanics properties of HAYNES® HR160T™;at temperatures up to 950°C (1742°F) were investigated for the next generation of fossil energy systems. The material characterization with the time-dependent fracture mechanics parameters were studied under different testing conditions such as temperature, load level and specimen size; different material conditions such as heat, the as-received, coarse grain size, aged and welded conditions; different fatigue conditions such as with and without various hold times. The creep crack and creep-fatigue crack propagation mechanisms were studied through microstructural characterizations. Theoretical modeling was conducted on the dynamic process of creep crack propagation.

The results show that creep crack propagation rate can be uniquely correlated to the time-dependent fracture mechanics parameter, C*(t), in the form of da/dt = A[C*(t)]q;. This correlation is independent of temperature, load level, and specimen size. Desired data can be obtained by manipulating these parameters to save testing time and materials. Under the conditions of this study, the creep crack propagation rate of HR160 is not affected by heat difference, grain coarsening and aging, but increased approximately 15 times by welding, which causes a casting structure lack of creep ductility in the material.

The creep-fatigue crack propagation behavior was successfully correlated to the creep crack propagation behavior. The creep zone at the crack tip was found to accumulatively expand during the hold time, and the average value of C*(t) was proposed for characterizing the creep-fatigue crack propagation rate beyond the accumulative transition time. Decreasing hold time during creep-fatigue tests increases the average crack propagation rate. Therefore, increasing shut-downs or stress fluctuations in high temperature components decreases their lives.

Microstructural analyses show that creep cracks propagate mainly in an intergranular mode by the coalescence of creep cavities which occur along grain boundaries and twin boundaries. Load cycling causes a transgranular crack propagation mode within the creep zone, induces a mixed intergranular and transgranular crack propagation, and increases the total crack propagation rate.

A model has been established to show that the creep crack propagation rate depends on the creep rate at the crack tip, the creep zone geometry and the rupture strain of the material. The rate estimation number, Ren;, has been introduced and uniquely correlated to the minimum creep rate. The q' in this correlation is numerically close to the q in the da/dt versus C*(t) relationship for superalloy HR160 and can be used as a reference for q when long-term test results are predicted from short-term tests.

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