Precipitate Rafting in a Polycrystalline Superalloy During Compression Creep
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IN nickel-base superalloys, gamma prime (c¢) precipitates, with an ordered-FCC (Face centered cubic) (L12) crystal structure embedded in an FCC gamma (c) matrix, have a lattice parameter which is close to that of the matrix. This similarity in the crystal structure gives rise to elastic coherency strains between c¢ and c with a small value of the lattice misfit. Coherency strains are the main factors of strengthening in superalloys.[1] When the lattice misfit of the precipitates is larger than that of the matrix, the alloy has a positive misfit, otherwise it has a negative misfit. The precipitates evolve in size and morphology during the utilization of superalloys. During the growth and coarsening of the precipitates, the instantaneous morphology of the precipitates is dictated by the minimization of sum of the elastic strain energy and the interfacial energy.[2] The morphology that is spherical in the beginning of the growth evolves into a cubical one due to lattice matching along {100} planes of the matrix (c) and the precipitates (c¢). The coherency strains are
ARUN ALTINCEKIC, Ph.D. Candidate, and ERCAN BALIKCI, Associate Professor, are with the Department of Mechanical Engineering, Bogazici University, South Campus, 34342 Bebek, Istanbul, Turkey. Contact e-mail: [email protected] Manuscript submitted October 31, 2013. Article published online September 26, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
maximized as precipitates coarsen; however, the coherency is lost after a critical size.[3,4] An alignment of precipitates along elastically soft h001i directions is observed during isothermal aging and stressed aging (creep). This directional coarsening is called ‘‘rafting.’’ Plates or rods of precipitates may form after rafting.[1] During an isothermal aging, rafts may form because of concentration gradients in a dendritic structure.[5] On the other hand, dislocations generated during early stages of creep are the main reasons for rafting.[6,7] The precipitates align either parallel or perpendicular to the loading direction depending on the sign of the lattice misfit and the applied stress and the difference between the elastic constants of precipitates and matrix.[1,8] Although few exceptions have been reported in the literature,[9,10] when the signs of the applied stress and the misfit are same, P-type rafting occurs whereas when the signs are different, N-type rafting occurs. In P type, rafts are parallel to the applied load, while in N type, rafts are normal (perpendicular) to the applied load. In fact, N-type rafting yields plate-shaped rafts rather than rod-shaped rafts that result from P-type rafting. Plate-shape rafting is desirable since it better stops dislocations and decreases the surface energy of the system more.[11] In positive misfit alloys, the matrix channels are in tension and the precipitates are in compression. When a compressive load is applied, the stresses in precipitates parallel to the loading direction increase, whereas the ones perpendicular to the loading direction decrease.
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