The use of new PHACOMP in understanding the solidification microstructure of nickel base alloy weld metal
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I.
INTRODUCTION
SOLIDIFICATION during arc welding is an inherently nonequilibrium process. Microstructures generated during arc welding are often not those predicted by applying equilibrium considerations to existing phase diagrams. In some cases, such as the A1-Mg system, the formation of a relatively large volume fraction of a nonequilibrium eutectic constituent makes possible the fabrication of readily weldable alloys such as 5456 (AI-5.2 wt pct Mg). Usually, though, the formation of a low-melting nonequilibrium eutectic constituent is detrimental from a weld hot-cracking viewpoint. The sequence of solidification reactions in commercial alloys is of prime importance in diagnosing not only hotcracking propensity but also in understanding subsequent solid-state transformations and materials properties derived from these transformations. Unfortunately, phase diagrams are lacking for virtually every alloy system incorporating more than three components and are generally limited in scope even for ternary systems. In the case of nickel base alloys, much fundamental and empirical effort has gone into describing phase relationships in the solid state. Nickel base alloys are generally derived from at least ternary systems and more usually quaternary or higher order systems. The face-centered-cubic Ni matrix has a high solubility for many substitutional alloying elements, but commercial alloys often contain a large number of minor phases (carbides, nitrides, borides, intermetallic compounds, etc. ). The occurrence of certain topologicallyclose-packed (TCP) intermetallic phases, such as/x, ~r, and Laves, has been shown to influence the mechanical properties of nickel and cobalt base superalloys.l-4 As phase dia*HASTELLOY is a trademark of Cabot Corporation. INCONEL is a trademark of the INCO family of companies. M.J. CIESLAK and G. A. KNOROVSKY, Org. 1833, Process Metallurgy, T.J. HEADLEY, Org. 1822, Electron Optics and X-ray Analysis, and A. D. ROMIG, Jr., Org. 1832, Physical Metallurgy, are with Sandia National Laboratories, RO. Box 5800, Albuquerque, NM 87185. Manuscript submitted March 14, 1986. METALLURGICAL TRANSACTIONS A
grams were not available for complex systems, metallurgists compiled empirical data on the occurrence of these phases in commercial alloys. In the 1950's, several authors5-9 investigating binary and ternary alloy systems applied the electron vacancy concept of Pauling ~~and realized that cr phase was an electron compound, 11 the presence of which could be predicted in other alloy systems. Later, Woodyatt, Sims, and Beattie 12 advanced this predictive capability to complex, commercial alloys through a calculation scheme known by the acronym PHACOMP (for PHAse COMPutation). The average electron hole number, defined as Nv, for the austenitic matrix is calculated after accounting for the formation of all phases normally encountered in nickel base superalloys (carbides, y ' , borides, etc.). The algebraic definition of Nv is given as follows: Nv = ~ ( x i ) ( n ~ )
[1]
where xi is the atomic fractio
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