Viscoelastic Behavior of a Single-Crystal Nickel-Base Superalloy
Keywords:nickel-base single crystal superalloy, hard cyclic viscoplastic deformation, ratchetting, viscoelastic behavior, viscosity
Tensile specimens of an experimental single-crystal nickel-based superalloy with  orientation were subjected to cyclic tension-compression loading under strain control regime on materials testing system Instron-8516 at room temperature and ambient atmosphere. Recent research on the viscoelastic behavior of material has shown that their elastic deformation occurs in the initial portion of a stress-strain relationship, which is initially linear up to proportional limit with strain of ~0.55 %. Until then the material show’s fully elastic behavior at strain amplitudes of 0 % – 0.05 %, 0 % – 0.2 %, and 0 % – 0.5 % for 30 cycles and corresponding diagrams covers each-other. At the increased strain amplitude of 1 % the elastic limit occurs at first cycle and tensile strain of ~0.65 %. The specimen was plastically deformed from 0.65 % up to 1 % of tension with no load significantly increased. Next 29 cycles were conducted at strain amplitude of 1 %. The strain-stress curves on diagram covers each-other at 1 % of strain value. Material ordinarily possesses upscale elastic behavior stability due largely to improved resistance to deformation of a cuboidal intermetallic γ′-phase. However, under the cyclic straining the dendrite arms were decreased in dimensions about three times as compared to primary dendrite arm. The γ-phase tended to dissolve into the primary cuboidal intermetallic γ′-phase. The cast formed of niobium-tantalum intermetallic inclusions began to dissolve and supplies the γ-matrix during cycling. As a result, the raft microstructure was not formed, as it was usually formed during creep testing at high temperatures.
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