Solidification of Nb-bearing superalloys: Part I. Reaction sequences
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INTRODUCTION
NICKEL-BASE Nb-strengthened alloys represent a significant portion of the superalloys currently in use. Commercial examples include alloys IN625, IN706, IN718, IN903, IN909, and Thermo-Span, to name a few. These alloys are often used for components that are fabricated by solidification processes such as casting and fusion welding. They also find use in dissimilar weld applications such as claddings on steel components subjected to highly aggressive wear and corrosion conditions.[1,2] In these applications, the fusion zone composition can become significantly enriched in Fe and C due to dilution from the carbon steel substrate, and this composition modification can alter microstructural development during solidification.[3,4] Previous studies conducted on commercial superalloys[5,6] have shown that minor variations in Nb, Si, and C strongly affect the type and amount of secondary phases that form during the terminal stages of solidification. In particular, two types of secondary phases, NbC and Laves, are known to form, and the wide range of possible solidification behavior has been shown to control the susceptibility for fusion zone solidification cracking.[3,7,8] Although detailed investigations have been conducted on specific commercial alloys and dissimilar alloy combinations, no general solidification model has been developed to provide more quantitative prediction capabilities of microstructural evolution during solidification. In the current article, reaction sequences are established for J.N. DuPONT, Research Scientist and Associate Director, Energy Liaison Program, and M.R. NOTIS and A.R. MARDER, Professors, Materials Science and Engineering Department, are with Lehigh University, Bethlehem, PA 18015. C.V. ROBINO and J.R. MICHAEL, Principal Members of the Technical Staff, are with Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted July 14, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
multicomponent (Fe, Ni, Cr, Nb, Si, and C) alloys. These alloys contain systematic variations in Fe, Nb, Si, and C, and simulate the wide range in solidification behavior observed in commercial alloys and Fe-enriched fusion zones of dissimilar welds made between Nb-bearing Ni-base alloys and steel substrates. In companion articles, this information is applied to develop pseudoternary solidification surfaces,[9] which have been included in a solute redistribution model for making semiquantitative predictions of microstructural evolution during solidification.[10] II.
EXPERIMENTAL PROCEDURE
A. Experimental Alloy Compositions A four-factor, two-level set of experimental alloys was designed to simulate the commercial compositions of interest in this study. The alloy compositions are summarized in Table I. The alloys contain factorial variations in Fe (in exchange for Ni), Nb, Si, and C at two levels. The high and low target levels of Nb, Si, and C are set as follows (all values in wt pct): 2 , Nb , 5, 0.02 , C , 0.15, and 0.10 , Si , 0.60. These limits were chosen to represent low and high
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