Microstructure and texture effect on the thermal expansion of a variously aged polycrystalline superalloy IN738LC
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I. INTRODUCTION
GAS turbines and aerospace engines, wherein the superalloy IN738LC is extensively utilized, operate most efficiently under close tolerance, which makes thermal expansion an important design factor. Similar thermalexpansion coefficients in mutually interacting components are oftentimes necessary. Usually, a low coefficient, acceptable to minimize thermal stresses, is preferred. Thermal expansion of materials is due to enhanced lattice vibration of the crystal lattice. It is generally very sensitive to composition, planar atomic density, electron configuration and its density in the crystals, as well as to near-neighbor interactions.[1,2,3] These are basic reasons why two phases in an alloy system with the same crystal structure would have different thermal-expansion coefficients. In polycrystalline aggregates, the grain-boundary type is also an important factor affecting the thermal-expansion coefficient of the material.[4] Since thermal expansion is reversible, similar to elastic strain, the two are interrelated. Inhomogeneous segregation of phases, local composition variations, and residual and interfacial stresses would also affect thermal expansion. Hence, processing methods and uniformity of the resultant microstructures are important in determining the accuracy of results.[5] Observations of directionally varying thermal expansion are also reported. Such variations are commonly due to structural anisotropy or to some directionally localized defects, such as microcracks and voids. Microcrack formation during processing of the materials may deteriorate the
ERCAN BALIKCI, Research Associate, and A. RAMAN, Professor, are with the Materials Group, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803. R.A. MIRSHAMS, Associate Professor, is with the Department of Mechanical Engineering, Southern University, Baton Rouge, LA 70813. Manuscript submitted March 2, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanical properties and lead to different thermal-expansion coefficients in different directions.[5,6,7] Also, in Reference 8, it is observed that the differences in lattice mismatch give rise to varying thermal-expansion coefficients due to misfit strains and constraints at the interface. Texture formation (i.e., preferred orientation (PO) of the lattice in the microstructure) would significantly affect the modulus of elasticity and the thermal-expansion coefficient of the material in the direction of the stressing or thermal expansion. Thus, properties of single crystals are commonly controlled by their orientations with respect to the axis of stressing or expansion.
II. EXPERIMENTAL The material used in this study is polycrystalline IN738LC, which is a precipitate-strengthened, Ni-based, investment-cast superalloy with a low carbon content. The alloy was provided by Howmet Corporation (Whitehall, MI) in the form of cylindrical rods, 15 mm in diameter and 110 mm in length. The cast rods were hot-isostatically pressed (“hipped”) at ,1200 8C for 2 hours to r
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