Xianglin Wu

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.

EBSD Study of the Role and Activation of Mechanic Twinning in the Plastic Deformation of 316L Stainless Steel

Type 316L stainless steel has been widely used as a structural material for current Light Water Reactors and continues to be selected as a candidate material for various advanced nuclear applications due to its superior strength and corrosion resistance properties. Examples are ITER and the conceptual design of Generation IV reactors. However, certain irradiation conditions lead to the development of a damaged microstructure where plastic flow is confined to very small volumes or regions of material, as opposed to the general plastic flow observed in unirradiated materials. This process is referred as flow localization. Previous study [1] shows that the true stress at the onset of necking, defined as the “critical stress,” is a constant regardless of the irradiation levels. This critical stress appears to be insensitive to the irradiation dose but exhibits a strong dependence on the test temperature. The role of mechanical twinning in plastic deformation and its influence on the temperature dependence of critical stress are investigated in this study with electron backscattered diffraction (EBSD) orientation mapping of 316L stainless steel under tensile loading conditions from room temperature to 400C.

Notch Strengthening and Its Impact on the Deformation and Fracture of 316L Stainless Steel

316L stainless steel has been widely used for structural component applications in irradiation environments. The impact of manufacturing or service-induced defects on material tensile response and fracture behavior has been a longstanding concern. In this study, electron backscattering diffraction and finite element methods have been utilized to examine and model the tensile response of 316L stainless steel with different notch geometries. The tensile response is influenced by twinning at room temperature, while it is completely absent at temperatures higher than 200°C. In addition, irradiation exposure increases the yield point of the material and reduces the ductility. It is found that the larger plastic deformation area and severe tip blunting in ductile, unirradiated material promote uniformly distributed post-yield strain hardening processes. In this case, fracture initiates internally for tensile and C-notched specimen geometries, but from the notch tip for sharp V-notched specimen geometries. When irradiation-induced hardening is considered, similar fracture modes are found, however, strains to fracture are significantly reduced.

Grain size effect on room temperature fatigue and creep-fatigue behavior of OFHC copper

Abstract The fatigue and creep-fatigue response of OFHC copper with three different grain sizes has been studied. Tests were carried out at room temperature and hold times were applied at maximum tensile and compressive strain to simulate the creep effect. The results show that fatigue life decreases with increasing grain size for a fixed applied strain range. Hold times resulted in a major reduction in the number of cycles to failure. This reduction was largest at the lowest strain amplitudes and the longest fatigue lives, the region of most interest for component design. The large reduction in fatigue life is apparently due to a change in the crack initiation mode from transgranular in continuous cycle fatigue to intergranular in creep-fatigue conditions.