Dynamic recrystallization, Laves phase evolution and mechanical performance of nuclear-grade Nb containing FeCrAl alloy joints fabricated by friction stir welding

Abstract

Nb containing FeCrAl alloys are a new class of promising candidates for accident-tolerant-fuel (ATF) materials used in nuclear power plants. The macrostructure, dynamic recrystallization, Laves phase evolution, tensile properties, and their correlations of the Nb containing FeCrAl alloy after friction stir welding (FSW) were systematically investigated at different welding parameters. FSW led to grain refinement and the stir zones (SZs) were mainly composed of fine equiaxed grains. The second-phase particles in the base materials (BM) were C14-type Fe2(Nb,Mo) with a diameter range of 500–900 nm. Under low heat input conditions, the majority of the Fe2(Nb,Mo) Laves phase particles in the SZs were dissolved into the matrix. With heat input increasing, very fine Fe2(Nb,Mo) Laves phase dispersions and Y2O3 in the nanoscale were re-precipitated. Microhardness distributions of Nb containing FeCrAl alloy joints obtained at different welding parameters exhibited identical “U-shaped” profiles. The average hardness of the base material (BM) was ∼352HV, while that in the stir zone (SZ) was reduced to 224-229HV. The joints obtained at 400rpm-80 mm/min held optimal yield strength (389 ± 11 MPa) and ultimate tensile strength (492±6 MPa). Micron-scale Fe2(Nb,Mo) Laves phase particles in the BM helped accumulate dislocation pile-ups. In contrast, the re-precipitated nanoscale Fe2(Nb,Mo) and Y2O3 precipitates could not pin dislocations as effectively as those larger particles in the BM. As a result, the mechanical properties of FSW joints were lower than those of the BM.