Characterization and modeling of the precipitation of the sigma phase in UDIMET 720 and UDIMET 720Li
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I.
INTRODUCTION
THE demand for aeroengines of greater thrust/weight ratio means that there exists a continuing requirement for materials that can tolerate ever more extreme conditions of temperature and stress, particularly for rotating components within the turbine. UDIMET 720Li (U720Li) is a high strength nickel-based superalloy that can potentially be used for high pressure (HP) turbine discs in the very latest high thrust engines.[1] It is derived from UDIMET 720 (U720), which was designed originally for use as steam turbine blading[2] and which exhibits an outstanding combination of strength and fatigue resistance; unfortunately, precipitation of detrimental phases such as sigma occurs at elevated temperatures, especially in the subsolvus heattreated condition, so that eventually the mechanical properties are degraded.[3,4] For this reason, the composition of U720Li differs from that of U720 with respect to the chromium content, which is reduced in an attempt to promote high-temperature microstructural stability, particularly with respect to the sigma phase. Furthermore, the carbon and boron contents are reduced in order to promote billet homogeneity and to infer better hot working properties. Sigma (s) is a topologically closed-packed (TCP) phase that can form in superalloys, generally in the g matrix next to large primary g' particles and carbides.[5] The phase has R.C. REED, Assistant Director of Research, and M.P. JACKSON, Postdoctoral Student, are with the Department of Materials Science and Metallurgy, University of Cambridge, Rolls-Royce University Technology Centre, Cambridge CB2 3Q2, United Kingdom. Y.S. NA, formerly Visiting Scientist, Department of Materials Science and Metallurgy, University of Cambridge, is Researcher, The Korea Institute of Machinery and Metals, Kyungnam 641-010, Korea. Manuscript submitted February 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
the tetragonal structure with a c/a ratio of 0.52.[6] It has two formula units and 30 atoms per unit cell and belongs to the [7,8] The composition space group D14 4h (P42/mnm, no. 136). is in the range AB7 to A7B, where A is a transition element from group IIIA to VIA and B is one from group VIIA or VIIIA.[7] In U720 and U720Li, the approximate composition of sigma is (Cr0.5, Mo0.1), (Ni0.2, Co0.2).[9] Platelet or needlelike sigma and sigma film along grain boundaries are nearly always detrimental to properties, but it is claimed that globular precipitation is not harmful.[10] However, sigma precipitation must inevitably deplete the g matrix of chromium, which is a strong solution strengthener, thus reducing the effective strength of the matrix. Traditionally, designers of new superalloys have attempted to avoid the formation of TCP phases (e.g., sigma, m, and Laves) using predictive procedures such as PHACOMP (‘‘phase composition’’),[7,11,12] in which an electron-vacancy number is assumed to correlate with the propensity for TCP phase formation. More recently, the use of thermodynamic software based upon sublattice models[13,14]
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