A.G. Pard

HELIUM EMBRITTLEMENT IN TYPE 304 STAINLESS STEEL.

Abstract Helium was uniformly injected into small tensile samples of type 304 (austenitic) stainless steel to concentrations of 1 × 10−7 and 3 × 10−5 atom fraction helium. Subsequent tensile testing above 540 °C revealed progressive ductility loss—as measured by total elongation at rupture—with increasing temperature. This effect was more severe in samples with the high helium content where elongations at 760 °C were a third or less of those of control samples. Yield and tensile strengths remained unaltered by the presence of helium. Above 650 °C, grain boundary sliding, which results in intergranular cracking, becomes important. The cracks begin as voids on carbide particles which act as obstacles to grain boundary sliding. In the presence of helium, bubbles attach themselves to these carbide particles and serve as void nuclei, thereby accelerating the process. Bubbles were also more prevalent on grain boundaries, dislocations and inclusion particles than isolated in the matrix.

Radiation damage in crystalline phases by (n, α) reactions

Abstract Crystalline materials containing 6 Li or 10 B were irradiated with thermal neutrons and the resultant (n, α) reactions produced structural damage. Unenriched cubic boron nitride was heavily damaged, with a density decrease of 34 ± 2%, by irradiation with 4 × 10 20 n / cm 2 (~16% B burnup). Fluences of 2 and 8 × 10 19 n / cm 2 produced density decreases of about 3 and 9%, respectively. This behavior, together with the effect of annealing, was generally similar to that previously observed in diamond irradiated with fast neutrons. However, the heavily damaged BN reverted to the hexagonal form at a lower annealing temperature than unirradiated material, whereas the opposite is true for heavily damaged diamond. Irradiation of 6 LiNbO 3 and 6 Li 3 NbO 4 , respectively, with a thermal neutron fluence of 4 × 10 18 n / cm 2 (~0.4% Li burnup) produced lattice parameter expansions of ~0.1%. Any lattice expansions in 6 Li 2 Ti 3 O 7 and 6 Li 4 Ti 5 O 12 irradiated to the same burnup were

Effect of high helium and hydrogen concentrations on the microstructure of SAP

Abstract Samples of SAP, 9.4 wt % Al 2 O 3 , were irradiated at temperatures between 350C and 500C with He + and then H 2 + ions to gas concentrations of about 1000 appm each. During the irradiation, the samples were continuously oscillated through an arc so as to spread out the end-of-range region and produce a uniform gas concentration in a section of the specimen about 3 μm thick. Following irradiation, foils extracted from the irradiated volume were examined by TEM and cavities were observed in all samples. Cavity size and number density varied with irradiation temperature. Calculations indicate that all of the helium can be accommodated in the cavities. Hydrogen analysis, with an Ion Microprobe Mass Analyzer, showed no accumulation of hydrogen in the cavities or elsewhere in samples irradiated at 400C and 500C.

Swelling and Tensile Property Changes in Neutron-Irradiated Type 316 Stainless Steel

Specimens of Type 316 stainless steel, given different thermomechanical treatments resulting in either a cold-worked or solution-annealed and aged structure, were irradiated in the Experimental Breeder Reactor-II (EBR-II)at 500 to 600 °C (932 to 1112 °F) to a fluence of 7.4 x 10 2 6 neutrons (n)/m 2 (E > 0.1 MeV). Three specimen configurations were used: small sheet tension specimens, small right-circular cylinders for immersion density, and thin foils for transmission electron microscopy (TEM). TEM revealed voids in all specimens. Immersion density indicated swelling in cold-rolled specimens only after irradiation at temperatures near 600°C (1112°F). Considerable recovery and precipitation were observed in the cold-rolled specimens. Results of tension tests revealed an increase in strength and decrease in ductility for specimens originally in a solution-annealed and aged condition. Cold-rolled specimens exhibited a decrease in strength and a slight increase in total elongation. True stress-true plastic strain was best described by the Ludwigson equation, o = K 3 e n 3 ′ exp (K 4 + n 4 e), in all cases. Irradiation causes a decrease in the work-hardening exponent, n 3 , and strength factor, K 3 . After irradiation, the values of n 3 and K 3 tended toward common values for both preirradiation treatments.

Studies of Void formation in proton-irradiated Type 316 and titanium-modified 316 stainless steels

Void formation was studied in annealed Type 316 stainless steel irradiated with 1-MeV protons at 400 C, 500 C, and 600 C (752, 932, and 1112 F) for amounts of damage up to 50 displacements per atom (dpa). Prior to irradiation, the samples were injected with about 5 atomic ppm helium to simulate the fast-reactor situation, where helium is generated by (n,α) reactions. The volume increase at each temperature, as determined by quantitative electron microscopy. varied exponentially with displacement damage and the exponent increased with temperature. Volume increases exceeding 20 percent were associated with 50 dpa at 500 C and 20 dpa at 600 C; no limits on volume increases were apparent at any temperature. Void number density decreased and average void size increased with increasing temperature for a constant damage. A titanium-modified 316 stainless steel developed a very inhomogeneous void distribution when proton irradiated at 600 C. However, the addition of titanium did not appear to diminish the volume increase at damages between 5 and 10 dpa, when compared with ordinary Type 316 stainless steel.

The Effects of Helium on the High-Temperature Ductility of Sandvik 12R72HV and Inco IN-744X

. . . . . . . . . . . . .