Dose Dependence of Strength After Low-Temperature Irradiation in Metallic Materials
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In high-strength refractory metals, irradiation softening is often profound after low-dose irradiation. In particular, some pure molybdenum showed a significant irradiation softening at room temperature and lower.[13– 15] In the single-crystal high-purity Fe, such irradiation softening or nonhardening behavior was quite significant in the low-temperature (£183 K [–90 C]) tensile tests, where the strength of the pure Fe is relatively high.[12] Some ferritic steels have a short constantstrength region before the hardening stress becomes measurable.[16,24] Lately, the low-carbon arc-cast pure molybdenum (LCAC Mo) has been tested after neutron irradiation to explore the temperature or strength dependence of irradiation softening.[14] The results have shown a significant decrease of yield stress with dose, along with prompt plastic instability or failure at relatively high doses.[14,15] In this paper, the DYS vs dpa data are displayed in a semilog plot (Figure 1) to S86—VOLUME 44A, JANUARY 2013
Fig. 1—Temperature dependence of irradiation hardening (DYS) in LCAC molybdenum.
reveal the change from radiation softening to hardening in the same material. It clearly shows that at lower temperatures, the irradiation softening is more profound and the initiation of hardening is delayed to higher doses: At a higher temperature, 373 K (100 C), or when strength is lower, the softening stress (~30 MPa) is much lower than the value at a lower temperature of 223 K (–50 C) (~220 MPa). At the highest temperature, however, the delay in irradiation hardening is minimal (~0.0007 dpa) or within error range. These results led to a speculation that the delay period in irradiation hardening is dependent on the strength of the material, and the irradiation softening or delay in hardening is a common phenomenon in metallic materials. Most of the strength data sets presented in the past do not have enough dose resolution in the low-dose region, and therefore, the phenomena in the early irradiation were largely ignored. This has been almost a common practice in the relatively soft materials where the irradiation softening or delay in hardening is negligible. In this study, such an early response of materials to irradiation is named as the nonhardening regime. To answer the question if irradiation softening is a common phenomenon in metallic materials, the data sets for commercial alloys with medium strength, including all bcc, fcc, and hcp structures, are plotted together in the semilog coordinate (Figure 2). It is evident that these materials have a delay in hardening, if not softening, before the irradiation hardening becomes measurable. The doses when the DYS values start to increase above the horizontal line are below 0.001 dpa for the six alloys. Since this softening or delayed hardening occurs at such a low dose in most of the commercial structural alloys, the phenomenon has been previously interpreted as a phenomenon unique to refractory materials or highly hardened materials only.[14,15] It is now postulated from the semilog plots
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