Keyou Mao

Deformation mechanism of a laser weld on neutron irradiated AISI 304L stainless steel was studied by in-situ microcompression test at room temperature. The deformation-induced austenite-to-martensite phase transformation occurs in {101}-oriented grains in the irradiated base metal, while deformation twinning prevails in {101}-oriented grains in the weld heat affected zone (HAZ). A high number density of irradiation-induced voids in the base metal provides sufficient nucleation sites for the austenite-to-martensite phase transformation under compression at room temperature. A deformation map is established to predict critical twinning stress for face-centered-cubic (fcc) metals and alloys. Our results show that irradiation-induced voids can tailor the deformation mechanisms of austenitic stainless steel.

The role of cavities on deformation-induced martensitic phase transformations is studied in a laser-welded and neutron irradiated austenitic stainless steel. Orientation dependent nanoindentation experiments are performed in the base metal and the weld heat affected zone (HAZ) at room temperature. Transmission electron microscopy study of deformed microstructures indicates indentation-induced α’-martensite forms in the base metal, whereas α’- and ε-martensite arise in the HAZ. The different pathway of martensite phase transformation is attributed to the laser weld-induced annealing of cavities. Our results suggest that deformation-induced martensitic phase transformation of austenitic stainless steel is correlated to neutron irradiated cavity structures.

Powder metallurgy with hot isostatic pressing (PM-HIP) is an advanced alloy processing method capable of fabricating complex nuclear reactor components near-net shape, reducing the need for machining and welding. For heat exchangers and steam generators, thermal aging of PM-HIP materials must be comparable or superior to conventional castings or forgings. This study compares thermal aging effects in PM-HIP and wrought alloy 625. Isothermal aging is carried out over 400–800°C for 100 h. Both PM-HIP and wrought materials have equiaxed grains with a uniform orientation distribution. The PM-HIP material has finer grains than the wrought material at all aging conditions. Both PM-HIP and wrought materials have a comparable hardness and modulus measured by nanoindentation. Hardness remains unchanged with aging except the wrought material aged at 800°C, which exhibits softening. Overall, PM-HIP alloy 625 responds comparably to wrought alloy 625 and is superior at 800°C. Results are used to calculate a Hall–Petch coefficient.