Some aspects of the precipitation of metastable intermetallic phases in INCONEL 718
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I.
INTRODUCTION
THE precipitation of intermetallic phases in the agehardenable, nickel-iron base, commercial superalloy INCONEL* 718 has been extensively examined, and it *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
has been found that the major intermetallic phases that precipitate in the austenite (7) matrix in this alloy are the metastable y' (cubic L12 structure) and 3'" (tetragonal D022 structure) phases and the equilibrium 6 (orthorhombic D0a structure) phase. I~,21All of these phases are of the N3M type. Age hardening in this alloy is brought about mainly by the homogeneously nucleated y" precipitates, which are coherent, disc-shaped particles with {100} habit and bear the orientation relationship {100}~,,//{100}r [001]~,,ff(001)~ with the host austenite. H'2'31 Some strengthening contribution also comes from the 3" precipitates which bear a cube-to-cube orientation relationship with the matrix and have a spheroidal/cuboidal morphology. [1,2,3] The "/-3'" matrix-precipitate misfit is concentrated in a direction parallel to the 3'" tetragonal axis (c-axis) and is quite small in magnitude. Studies on 3'" particles forming in Ni-Fe-Cr-Nb alloys have indicated that cr,, is about 2.5 pct greater than at,,. I4j However, it appears that the dependence of the extent of the tetragonal distortion on the particle size has not been examined in detail. When two or more types of metastable intermetallic phases precipitate in an alloy, it is generally important to know (1) the sequence of precipitation (whether these phases form simultaneously or in a sequential manner) and (2) whether the different types of precipitates nucleate independently of one another or whether the precipitation of particles of one type facilitates the precipitation of particles of the other type(s) and if so, what is the M. SUNDARARAMAN, Scientific Officer, Physical Metallurgy Section, Metallurgy Division, P. MUKHOPADHYAY, Head, Physical Metallurgy Section, Metallurgy Division, and S. BANERJEE, Head, Metallurgy Division, are with Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India. Manuscript submitted May 30, 1991. METALLURGICAL TRANSACTIONS A
mechanism that operates. In the context of INCONEL 718, it has been reported that y ' and y" precipitates generally form at random with respect to each other and that very rarely are y" particles found to be in contact with y ' particles, m However, in the course of their studies on the influence of changes in alloy composition on the stability and the morphology of y' and y" precipitates in INCONEL 718-type alloys, Cozar and Pineau ~51 have observed that a physical association between 3" and y" precipitates can occur quite frequently under certain conditions. It appears that as the total content of aluminum and titanium is increased with respect to the niobium content in these alloys, 3" precipitation is more and more favored compared to 3'" precipitation. I5,6,71 When the composition is so adjusted that the ratio of the combined aluminum and titanium content to
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