The nature of radiation damage occurring in solid materials at cryogenic temperatures is of both theoretical and practical interest. The basic mechanisms of radiation damage can be further elucidated by conducting irradiations at cryogenic temperatures because under these conditions diffusion can be greatly reduced, thereby allowing observed radiation effects to be studied and correlated with fewer fundamental variables. In addition, the problem of radiation damage occurring in structural materials at 20 K is of major importance in design studies of nuclear powered upper stage rockets. In this type of vehicle, large pressurized propellant tanks containing liquid hydrogen at 20 K are subjected to large doses of both neutron and gamma fluxes. Thus the structural materials experience a combined environment of cryogenic temperature and nuclear radiation.
To study the nature of these effects, specimens were exposed to an integrated fast neutron flux of about 2 × 1017 (E > 0.33 Mev) nvt while being soaked in liquid hydrogen. After radiation exposure, the specimens were subjected to tension tests while being held at 20 K without intervening warmup. The alloys studied were types 301 and 310 stainless steels cold rolled 60 and 75 per cent respectively, 5Al-2.5Sn titanium (A110AT), and 2014-T6 aluminum. The tension tests performed gave yield strength, tensile strength, elongation, notched tensile strength (Kt = 6.3), and the tensile strength of simple heliarc butt-welded joints. Specimens were tested in the unirradiated and irradiated conditions at both 294 and 20 K.
The largest and most consistent effect of cryogenic irradiation was a decrease in tensile strength and elongation after irradiation at 20 K. This effect varied from a 27 per cent decrease in tensile strength for the 301 stainless steel to a 3 per cent decrease for the 5Al-2.5Sn titanium. Elongations decreased in excess of 30 per cent for all of the alloys except the aluminum, but only minor changes in yield strength accompanied these decreases in tensile strength and elongation. Cryogenic irradiation severely restricts the amount of plastic deformation the specimens can sustain after yield and prior to fracture, in accordance with the theory that neutron radiation acts to introduce vacancies and interstitials which restrict the amount of plastic strain the specimens can withstand before fracture. This effect is believed to be more severe at cryogenic temperatures because the neutron generated vacancies and interstitials can diffuse out at the higher temperatures, but are “frozen in” at cryogenic temperatures. Notched tension tests showed small amounts of embrittlement in the 301 stainless steel after cryogenic irradiation, while the 310 stainless steel showed no embrittlement. It is believed that this condition resulted from the radiation induced embrittlement of the body-centered martensite phase which constituted about 45 per cent of the 301 stainless steel specimens. The 5Al-2.5Sn titanium and the 2014-T6 aluminum alloys showed no embrittlement as a result of cryogenic irradiation.