Role of Coincident Site Lattice Boundaries in Creep and Stress Corrosion Cracking
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Role of Coincident Site Lattice Boundaries in Creep and Stress Corrosion Cracking G.S. Was1, B.Alexandreanu2, Peter Andresen3 and Mukul Kumar4 1
University of Michigan, Ann Arbor, MI Argonne National Laboratory, Argonne, IL 3 General Electric Global Research, Schenectady, NY 4 Lawrence Livermore National Laboratory, Livermore, CA 2
ABSTRACT Interfaces control many properties in engineering materials, several of which are critical to the integrity of the engineering structure. In single phase, solid solution, austenitic alloys, grain boundaries are often the weak link, displaying susceptibility to creep, corrosion and stress corrosion cracking. As such, grain boundary structure control affords the opportunity to improve the overall performance of alloys in a variety of applications. The role of coincident site lattice boundary (CSLB) enhancement and grain boundary connectivity is examined for how it affects the response of an alloy to stress and the environment. Specifically, the effect of grain boundary character on creep, grain boundary sliding, intergranular stress corrosion cracking, and irradiation assisted stress corrosion cracking in austenitic nickel-base (high purity Ni-Cr-Fe and alloy 600) and iron-base (high purity Fe-Cr-Ni and 304 stainless steel) alloys and for ferriticmartensitic alloy T91 is discussed. INTRODUCTION Owing to their highly ordered structures, the CSL-related boundaries are expected to possess special properties. Moreover, by exercising control over the population of boundary types it should be possible to tailor properties for specific applications. This concept, was first introduced by Watanabe [1] as “grain boundary design and control”, and later evolved into “grain boundary engineering” [2, 3]. In many instances low S (£29) boundaries have been found to display improved chemical and physical properties over the higher order CSL and high angle boundaries, and these observations have led to efforts to optimize structures by increasing the proportions of CSL boundaries. As an example, Lin et al. [4] presented the results of an intergranular corrosion test in which both non-sensitized and sensitized (1 h at 600°C) specimens of Alloy 600, containing different proportions of CSL boundaries, were tested in a 600 ml of boiling ferric sulfate and 50% sulfuric acid solution for 24 hours (ASTM standard G28). The authors found that an increase in the coincident site lattice boundary (CSLB) content resulted in a diminished the susceptibility to intergranular corrosion in both alloy conditions. The effect of grain boundary character distribution (GBCD) on intergranular corrosion of type 304 austenitic stainless steel was studied by Kokawa et al [5]. The base material (BM) and 5% strain-annealed (5%) specimens – identified by the processing parameters (strainannealing temperature-time) - were assessed by a electrochemical potentiokinetic reactivation (DL-EPR) test in ferric sulfate-sulfuric acid test following sensitization at 923 K for 2 hr. In such a test, the actual intergranular corrosi
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