Mechanical Behavior And Microstructure Analysis Of Nuclear Structural Materials

Austenitic stainless steel 316 LN is a favored structural material used in current reactors as a consequence of its good corrosion resistance,high temperature mechanical properties and adequate weld ability. It is also promising material for some Generation IV reactors. In the temperature where Generation IV reactors operate, type 316 LN stainless steel is susceptible to dynamic strain aging（DSA）, which may induce a significant change in material properties such as strength and plasticity. So it is necessary to investigate the serrated flow behavior of 316 LN steel. Tungsten-base alloy has many advantages like good thermal conductivity, the highest melting point in metals, a high energy threshold for physical sputtering and the lowest vapor pressure, and it is considered the most promising plasma-facing material for the future fusion reactors. Therefore, the research for W-La₂O₃ alloy is important and significative.The research in this paper includes two parts: in the first part, the serrated flow behavior of 316 LN austenitic stainless steel was investigated through tensile tests at initial strain rates from 2×10-5 to 10-4 s-1 at temperatures ranging from room temperature to 1048 K; in the second part, the microstructure of an as-forged commercial W-1%La₂O₃ alloy was investigated using transmission electron microscopes（TEM）.The investigation of serrated flow in 316 LN steel indicated that serrated flow occurred at 698–1048 K at a strain rate of 2×10-4 s-1, as well as at temperatures of 623–673 K at a strain rate of 2×10-5 s-1. Type A, A+B/A+C, C and E serrations appeared. The ultimate tensile strength and yield strength of the steel gradually decreased with increasing temperature, with a plateau being observed respectively, while the elongation initially decreased to a low level at about 573 K, then increased to a peak at around a temperature of 623 K, and thereafter it decreased to a minimum at about 723 K before an upward trend. Careful comparison suggested that serrations were observed in two temperature regions in 316 LN steel. The activation energy for the occurrence of serrated flow in middle and high temperature regions were about 228 kJ/mol and 327 kJ/mol respectively. The mechanism of serrated flow in middle temperature region in the steel is DSA involving diffusion of Cr atoms towards mobile dislocations. While in the high temperature region, the weak and ruleless serrated flows are attributed to a shearing of small-sized precipitates by moving dislocations.In as-forged W-1%La₂O₃ alloy, many kinds of phases were observed and identified. They are: micron-sized La₂O₃ particles with a long rod-like shape; submicron-sized particles consisting of spherical La6W2O15, ellipsoidal WO2, shuttle-like WO3, hexagonal W3O8; some other nano-sized phases, which have sizes ranging from about 20 nm to 100 nm, such as three kinds of La₂O₃ with different lattices, two kinds of WO3, W3O8, La0.14WO3 and（La₂O₃）5WO3. The possible formation processes of these phases are also discussed.