Meimei Li

Flow localization processes in austenitic alloys

Austenitic alloys are widely used for structural component applications in high irradiation environments. In general, they are more resistant to embrittlement than other classes of structural alloys, particularly at ambient and intermediate temperatures. Nevertheless, this class of materials suffers from highly localized flow when irradiated to moderate dose (∼ 1 to 5 dpa) at temperatures between 150 and 400°C. The loss of ductility is normally exhibited by very low values of uniform elongation in tensile tests. The processes that lead to plastic instability are examined here for several face centered cubic materials and alloys. It is found that there is a critical stress level at which necking initiates. This critical stress level is not influenced by irradiation exposure. However, irradiation exposure, which increases material yield strength, does result in proportional reductions in uniform elongation. Most of the materials examined here exhibit a bilinear strain hardening behavior. This leads to direct correlation between the material yield strength and the uniform elongation.

Dose Dependence of Strength After Low-Temperature Irradiation in Metallic Materials

This study intends to review and characterize the low-temperature (<473xa0K [200xa0°C]) irradiation-hardening behaviors in metallic materials and to propose new interpretations on the dose dependence of strength, particularly in the prehardening and saturation regimes. The analysis of results of yield stress-dose curves indicate that four dose-dependence regimes exist: the prehardening, main hardening, saturation, and embrittlement regimes. The semilog plots of yield stress vs dose data revealed that the prehardening regime displaying zero hardening or softening was common at least for the alloys with low-dose data available. It was observed that the dose range of the prehardening regime increased with the strength of material, which indicates that slower initiation in irradiation hardening is expected when strength is higher. For the majority of the metallic materials analyzed, it was reconfirmed that the exponent of the power-law hardening function was evaluated to be about 0.5 in the main hardening regime and about 0.1 in the saturation regime. In these positive hardening regimes, the low strength pure metals such as Fe, Ta, Cu, and Zr displayed lower hardening exponents. The minimum dose to the saturation of irradiation hardening was in the range of 0.003 to 0.08xa0dpa, depending on the category of materials. It was also reaffirmed that there exists a strong relationship between the saturation in irradiation hardening and the occurrence of plastic instability at yield.

Creep-Fatigue Behavior in High Strength Copper Alloys

Copper alloys are found to experience a stress relaxation during holds at maximum tension and maximum compression during cyclic loading patterns. This stress relaxation response has an effect on fatigue life by reducing the numbers of cycles to failure, particularly in the high cycle fatigue, long life regime. The stress relaxation behavior is found even for room temperature loading conditions, well below the temperature regime normally associated with creep and stress relaxation. It is suggested that the creep and stress relaxation deformation modes during hold periods is predominantly through primary creep process.