Neutron Diffraction Study of Strain/Stress States and Subgrain Defects in a Creep-Deformed, Single-Crystal Superalloy
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NTRODUCTION
THE strengths of single crystal (SC) Ni base superalloys at high temperatures are strongly dependent on their microstructures. The initial microstructure of the alloys has two phases composed of a fcc c-matrix and the coherently aligned L12-ordered c¢-precipitates. When the alloys are crept at high temperatures, the cubical microstructures change into a lamellar structure (so-called raftstructure). This microstructural change depends on internal strain/stress and lattice mismatch, and significantly affects the strengths and fatigue properties of the alloys.[1–3] The internal stresses are normally divided into three types according to their averaging length scales. Type I is the one averaged over the sample scale and often called long-range or macroscopic residual stress; type II is microstress averaged over the grain size for polycrystalline or precipitate size for two-phase materials; and type III is on atomic scale associated with defects or other imperfections. The effects of different types of internal strain/stress and lattice mismatch during creep deformation have been extensively studied by various methods.[4–10] Although some theoretical estimations have been made on creep associated strain/stress,[11–13]
ERDONG WU and JIAN ZHANG, Professors, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Shenyang 110016, P.R. China. Contact e-mail: [email protected] GUANGAI SUN, Research Scientist, and BO CHEN, Professor, are with the Institute of Nuclear Physics and Chemistry, Mianyang 621900, P.R. China. VINCENT JI, Professor, is with LEMHE/ICMMO, UMR 8182, Universite´ Paris-Sud 11, 91405 Orsay, France. VINCENT KLOSEK and MARIE-HELENE MATHON, Research Scientists, are with the Laboratoire Le´on Brillouin, CEA Saclay, 91191 Cedex, France. Manuscript submitted March 2, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
the measurements on the macroscopic strain/stress tensors in the SC superalloys by non-destructive diffraction method are still inadequate, as macroscopic strain/stress measurements on SC sample are much more complicated than that on polycrystalline samples.[14,15] Moreover, some sporadic strain/stress data in SC superalloys are mostly indirectly derived based on estimation from lattice mismatch.[16–18] The measurements of complete strain/stress tensors by X-ray diffraction have been described in a study of superalloy surface, but only normal stresses are derived.[14] Recently, we have used a method for determination of longrange strain/stress of SC crystal materials to treat the neutron diffraction data, and derived the triaxial stress tensors for the SC superalloys.[19–21] Due to the lack of stress-free sample, the derived stresses are still the relative values, rather than the absolute values. However, the data have provided some first-hand information on the effects of various treatments on internal stress states of the SC superalloys. Since the knowledge of creep deformation is essential for the studies of SC superalloys, we have therefore employed the method
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