A Physics-Based Crystallographic Modeling Framework for Describing the Thermal Creep Behavior of Fe-Cr Alloys
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I.
INTRODUCTION
THE development and use of high-performance Cr-based steels, with superior high-temperature creep behavior, have been instrumental in improving the efficiency of thermal power plants.[1–8] Indeed, operation temperatures above 873 K (600 C) have been reached thanks, in particular, to the use of 9 to 12 pct Cr steels as boiler tubes and steam pipes. In parallel, other high Cr steel grades such as Fe-Cr-Al and modified Grade 91 (Fe-9Cr-1Mo) additionally exhibit low swelling during irradiation. Naturally, these alloys are candidate material systems for various nuclear energy applications (e.g., cladding). Their advanced high-temperature creep properties could prolong the service life and enhance the accident tolerance of both light water reactors (LWRs) and very-high-temperature reactors (VHTRs).[9–14] Under such high temperature, stress, and irradiation environments, the materials microstructure and part geometry will degrade over time. In particular, both thermal and irradiation creep largely contribute to the degradation process. Focus is placed here on thermal creep. Over the past two decades, a series of work has focused on the connections between the thermal creep behavior of high Cr steels and the specifics of their microstructures.[1–7,13,15,16] Following thermo-mechanical processing (e.g., tempering, tube extrusion), a polycrystalline sample will typically be textured, with most grains containing subgrain boundaries consisting of both geometrically necessary dislocations and M23C6
W. WEN, L. CAPOLUNGO, A. PATRA, and C.N. TOME´ are with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] Manuscript submitted October 31, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
carbide (M = Cr). The latter also decorates grain boundaries. M23C6 carbide can stabilize the subgrain structure by obstructing the dislocation annihilation in the cell walls, and hence decelerate the growth of subgrains.[7,17] Finally, the microstructure contains an additional level of complexity as subgrains also contain carbo-nitride precipitates MX (M = V or Nb; X = C or N). In consequence, precipitation hardening and precipitation-enhanced subgrain boundary hardening have been suggested to be the most important creep strengthening mechanisms in high Cr steels.[1] As a consequence of the complex microstructure, the creep rate is controlled by a broad spectrum of simultaneously active deformation mechanisms. Indeed, during thermal creep, plastic strain is likely to result from the activation of both diffusion creep and dislocation motion. The relative contribution of each depends on the imposed stress state, on the internal stress state, and on temperature. Vacancy-driven diffusion creep processes, such as the Nabarro–Herring creep and Coble creep, tend to play an important role in the high-temperature regime.[13,18] Shrestha et al.[13] show that diffusion creep is dominant in modified 9Cr-1Mo steel at 873 K (600 C) with a creep stress lower than 60 MPa.
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