Dislocation network formation and coherency loss around gamma- prime precipitates in a nickel- base superalloy
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INTRODUCTION
NICKELbase superalloys are used extensively in gas turbine components. High volume fractions of ordered intermetallic precipitates of gamma-prime (Ni3(A1, Ti)) with LI2 structure essentially provide the strength for these alloys. Because of the small lattice misfit between the nickel-rich matrix, gamma, and the gamma-prime phases, the precipitates are coherent with (100) of the matrix parallel to (100) of the precipitates. In addition, they tend to form cuboidal precipitates with their faces aligned along cube directions. The degree of the lattice misfit, the difference in the elastic moduli of the matrix, and the precipitates and the anisotropy of these moduli are all important in determining the formation of this cuboidal configuration and the alignment of the precipitates. For the purpose of the present analysis, we consider the precipitates coherent because of the absence of any misfit dislocations. They become semicoherent with the reduction in the strain energy when misfit dislocations are formed. In addition to the size and distribution of the precipitates, the nature of the interface between the precipitates and the matrix plays an important role in determining the strength of the alloy. Although the initial interfaces are fully coherent, misfit dislocations form readily with plastic deformation at high temperatures or with prolonged aging. Because of the technological importance, several studies II "231 have been made of the role of the gamma-gamma prime interfaces and of the formation of semicoherent precipitates through the interactions of the mobile dislocations in the matrix with the precipitates. From these studies it is now clear that a network of prismatic dislocation (having Burgers vectors of a/2(110)) [4.51 ' is formed at the interface of each precipitate during deformation. While it is recognized that they are related to the glide *IN is a trademark of the INCO family of companies. A . K . SINGH and N. LOUAT, Senior Scientists and On-Site Contract Employees at Naval Research Laboratory, are with Geo. Centers, Inc., 10903 Indian Head Highway, Fort W a s h i n g t o n , MD 20744. K. SADANANDA is Head, Deformation and Failure Section, Physical Metallurgy Branch, Materials Science and Technology Division, Naval Research Laboratory, Washington, DC. Manuscript submitted September 8, 1987. METALLURGICAL TRANSACTIONS A
dislocations in the matrix, it is not clear how these glide dislocations form misfit dislocation networks of predominantly edge character around the gamma-prime precipitates. The objective of the present study is to examine the detailed process of the formation of this misfit dislocation network. Understanding of the misfit dislocation formation is important since strength of these alloys is intimately connected with the morphology of the precipitate and the nature of the precipitate-matrix interface, both being affected by the network formation. In particular, this network formation can reduce the strength of the alloy by decreasing the contribution from coherency stre
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