Solid-State diffusion reaction and formation of intermetallic compounds in the nickel-zirconium system
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I.
INTRODUCTION
THE formation
of various intermetallics between two dissimilar metals has been a subject of great interest during the last two decades. The nickel-zirconium system finds its application in developing thermal barrier coatings on superalloys by a hot dipping process. [1] The latest nickel-zirconium phase diagram indicates eight intermetallic compounds (Figure 1), none of which exhibits extensive solubility. [2] The terminal solid solutions have very much restricted solid solubility. Three of the intermetallic compounds, namely, NiTZr2, NiZr, and NiZr2, are congruently melting alloys, while four others, namely, NisZr, NizlZrs, NimZrT, and NiuZrg, are formed by peritectic reaction. Ni3Zr is also formed by peritectoid reaction. Recently, Hammad and Shaaban have reported interdiffusion studies in ZIRCALOY*-4 and pure nickel. I31 *ZIRCALOY is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
Thin-film studies on the formation of intermetallic compounds in the zirconium-nickel system have been reported, t4] The present paper deals with solid-state diffusion studies on bulk samples. Results on the formation of intermetallic compounds and the kinetics of layer growth in the temperature range of 1046 to 1213 K have been discussed. Stability of various intermetallic phases has been rationalized based on their diffusion behavior. II. EXPERIMENTAL AND A N A L Y T I C A L P R O C E D U R E
A. Preparation of Diffusion Couples Rectangular pieces of pure zirconium (99.97 pct) and nickel (99.98 pct) approximately 10 • 5 x 3 mm in size
K. BHANUMURTHY, Scientific Officer, Metallurgy Division, G.B. KALE, Scientific Officer, Metallurgy Division, S.K. KHERA, Scientific Officer and Research Coordinator, Diffusion Research Group, Structural Metallurgy Section, Metallurgy Division, and M.K. ASUNDI, Head, Physical Metallurgy Division, are with the Bhabha Atomic Research Centre, Bombay 400085, India. Manuscript submitted September 15, 1989. METALLURGICALTRANSACTIONSA
were cut from the rolled sheets. These specimens were encapsulated in silica tubes in helium atmosphere and annealed at 1173 K for 7 days for grain coarsening. The average grain size of zirconium was 1 to 2 mm and that of nickel was 0.5 to 1.0 mm. These specimens were metallographically polished to a 1-/~m diamond finish. The polished faces of zirconium and nickel were kept in contact with each other and were loaded into a specially made jig under pressure in order to ensure intimate contact between two metal pieces. The entire assembl~( was placed in a vacuum furnace (vacuum better than 10 -~ torr) for diffusion bonding at 1053 K for 15 minutes. Metallographic and microanalytical examination of the specimens indicated that the width of the diffusion zone prior to annealing treatment was negligible as compared to that obtained after diffusion annealing. The diffusion couples thus prepared were sealed under helium atmosphere and subsequently annealed in a preheated furnace controlling the temperature to within ---1 K in the temperature range of 10
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