Post-irradiation fracture toughness of unalloyed molybdenum, ODS molybdenum, and TZM molybdenum following irradiation at 244 °C to 507 °C

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Abstract

Commercially available unalloyed molybdenum (Low Carbon Arc Cast (LCAC)), Oxide Dispersion Strengthened (ODS) molybdenum, and TZM molybdenum were subject to fracture toughness testing following neutron irradiation at temperatures of nominally 244 °C, 407 °C, and 509 °C to neutron fluences between 1.0 and 4.6 × 1025 n/m2 (E > 0.1 MeV). All alloys exhibited a Ductile to Brittle Transition Temperature that was defined to occur at 30 ± 4 MPa m. The highest post-irradiated fracture toughness values (26–107 MPa m) and lowest DBTT (100–150 °C) was observed for ODS molybdenum in the longitudinal orientation. The results for ODS molybdenum are anisotropic with lower post-irradiated toughness values (20–30 MPa m) and higher DBTT (450–600 °C) in the transverse (T-L) orientation. The results for ODS molybdenum are better than those for LCAC molybdenum (21–71 MPa m and 450–800 °C DBTT). The fracture toughness values measured for LCAC and T-L ODS molybdenum at temperatures below the DBTT were determined to be 8–18 MPa m. The role of microstructure and grain size on post-irradiated fracture toughness was evaluated.

Introduction

Irradiation embrittlement of molybdenum and its alloys is of concern due to the decrease in tensile ductility and fracture resistance [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. This irradiation embrittlement results from the formation of a high number density (>1013/cm3) of extended point defect clusters (loops and voids) that result in an increase in the flow stress to levels higher than the inherent fracture stress of the material so that brittle behavior is observed. Irradiation embrittlement of commercial molybdenum alloys generally occurs at irradiation temperatures less than 700–800 °C, and is characterized by an increase in the Ductile to Brittle Transition Temperature (DBTT) determined from a tensile test from below room-temperature to temperatures as high as 800 °C after irradiation [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Irradiation at temperatures greater than 700–800 °C results in rapid movement of point defect to sinks so that a low number density of coarse voids are formed that have little effect on mechanical properties. Thus, an understanding, quantification, and possible mitigation of embrittlement is desired for molybdenum at irradiation temperatures less than 700–800 °C.

Improved resistance to irradiation embrittlement has been observed for molybdenum alloys that have a refined grain size and with a fine oxide particle distribution, such as Oxide Dispersion Strengthened (ODS) molybdenum, where a DBTT of −50 °C to room-temperature was measured by tensile testing after irradiation at 600 °C, which is an improvement over the 600–700 °C DBTT values for unalloyed molybdenum [22], [23], [25]. Additionally a lower tensile DBTT of 450 °C, compared to DBTT values of 600–800 °C for unalloyed molybdenum, was also observed for ODS molybdenum following irradiations to low fluences of 2.43 × 1025 n/m2 (E > 0.1 MeV) at 300 °C [27]. The approach of increased purity has also resulted in improved resistance to irradiation embrittlement for irradiations at 300 °C and 600 °C where tensile DBTT values similar to those observed for ODS molybdenum were also measured [25]. These tensile DBTT values are an improvement over results that have been observed for commercially available unalloyed molybdenum and TZM molybdenum, where tensile DBTT values of 600–800 °C are generally observed following irradiations at 300–600 °C. Although improved resistance to irradiation embrittlement for ODS molybdenum and high-purity molybdenum alloys at low fluences has been quantified by lower tensile DBTT values, the tensile ductility is much less than observed prior to irradiation with total elongation values generally between 0.4% and 1.1% [23], [25]. No molybdenum alloy irradiated to date has demonstrated properties comparable to non-irradiated molybdenum such that they could be considered to have been completely relieved of irradiation embrittlement. One approach needed to quantify the resistance to irradiation embrittlement and brittle fracture is post-irradiated fracture toughness results. Non-irradiated fracture toughness testing has been performed on molybdenum alloys that can serve as a basis for comparison [32], [33], [34], [35], [36].

The purpose of this work is to report the results of fracture toughness testing, fractography, and Transmission Electron Microscopy (TEM) examinations of microstructure following irradiations in the Advanced Test Reactor (ATR) at temperatures of nominally 244 °C, 407 °C, and 509 °C to neutron fluences between 1.0 and 4.6 × 1025 n/m2 (E > 0.1 MeV). Commercially prepared La-oxide Oxide Dispersion Strengthened (ODS) molybdenum, Low Carbon Arc Cast (LCAC) unalloyed molybdenum, and TZM molybdenum were used for this study. The LCAC serves as a baseline material for unalloyed molybdenum. ODS molybdenum has a finer grain size and fine oxide particles that has resulted in improved fracture resistance. TZM molybdenum has a finer grain size than LCAC molybdenum, but contains a distribution of carbide precipitates coarser than that of the La-oxide in ODS molybdenum. The results of fracture toughness testing, fractography, and TEM examinations of microstructure are used to understand the role of microstructure on the fracture resistance following irradiation.

Section snippets

Materials and experimental procedure

Wrought LCAC (9.53 mm and 6.35 mm thick), ODS (6.35 mm), and TZM plate (6.35 mm) were obtained from H.C. Starck, Inc., and are from the same heats used in previous work [32], [33], [34], [35], [36]. The processing, chemistry, microstructures, and tensile results have been described [32], [33], [34], [35], [36], and a summary of the certification chemistries, grain sizes, and DBTT values determined from toughness testing are provided in Table 1. LCAC and TZM were produced by arc casting ingots that

Non-irradiated fracture toughness and toughening mechanism

Non-irradiated fracture toughness testing was performed on additional spare specimens to provide results for comparison with the post-irradiated data. These additional non-irradiated fracture toughness results obtained for TZM, LCAC, and ODS molybdenum are compared with previously reported data [32], [33], [34] in Fig. 1, Fig. 2, Fig. 3, respectively. Results for ODS molybdenum in the stress-relieved and recrystallized condition are compared in Fig. 4. Only a limited amount of non-irradiated

Testing results

Post-irradiated fracture toughness results for ODS molybdenum in the T-L and L-T orientations for the stress-relieved condition, L-T orientation for the recrystallized condition, and stress-relieved LCAC molybdenum in the T-L and L-T orientations are provided in Table 3, Table 4, Table 5, Table 6, Table 7, respectively. Pre- and post-irradiated fracture toughness results are compared in Fig. 8, Fig. 9, Fig. 10. Representative load–displacement curves and fractography measured at temperatures

Examination of irradiated defect structure

Examination of the post-irradiated microstructure showed that dislocation loops and voids were present in ODS and LCAC molybdenum for the irradiations performed at nominally 244 °C, 407 °C, and 509 °C, see Fig. 22, Fig. 23. The size distribution of voids and loops are shown in Fig. 24 to generally exhibit a normal distribution that is skewed towards larger sizes particularly for irradiations at nominally 509 °C with almost the appearance of a bimodal size distribution. For molybdenum alloys

Summary and conclusions

The highest post-irradiated fracture toughness values and lowest post-irradiated DBTT was observed for LSR ODS molybdenum in the L-T orientation. The smaller grain size and fine oxide particle distribution of ODS molybdenum provides more grain boundary area and interface regions in the microstructure that serve as neutral sinks for point defect annihilation. However, the point defect mobility is relatively low for the irradiation of molybdenum at 244–509 °C so the spacing between boundaries is

Acknowledgements

This work was supported under USDOE Contract No. DE-AC11-98PN38206. The following ORNL personnel contributed to this work by specimen preparation and testing (M.M. Lee, J.P. Strizak, A.W. Williams, and J.L. Bailey). The authors acknowledge D. Ward at Bettis for void size/number analysis and R.W. Smith and J.E. Hack for numerous discussions on this work. Research supported in part by ORNL’s Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy

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