Precipitation of an intermetallic phase with Pt 2 Mo-type structure in alloy 625
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INTRODUCTION
ALLOY 625 is a nickel-base superalloy that has found extensive use in the petrochemical and aerospace industries and in many marine engineering applications. The alloy possesses high strength and toughness over a wide temperature range and is endowed with excellent fatigue strength and oxidation and corrosion resistance. Although the alloy was originally designed as a solid solution alloy, it has been shown subsequently that the precipitation of intermetallic phases occurs in it during aging treatments or during service at temperatures in the range of 600 7C to 750 7C.[1] Unlike the majority of precipitation hardenable nickel-base alloys in which strengthening is brought about mainly by precipitates of the metastable g' or Ni3(Al,Ti) phase (ordered cubic L12 structure), in alloy 625, precipitation hardening is caused primarily by precipitates of the g" or Ni3(Nb, Al, Ti) phase, which has the ordered, body-centered tetragonal DO22 structure.[2] This metastable phase precipitates on aging at temperatures above 600 7C. The equilibrium intermetallic phase that forms in this alloy is the orthorhombic d or Ni3(Nb, Mo) phase (Doa structure).[2,3–7] This phase can form directly from the supersaturated solid solution on aging at relatively high temperatures.[8] Prolonged aging at relatively low temperatures also causes the precipitates of the metastable g" phase to be replaced by those of the equilibrium d phase.[6,9] The microstructure that develops in this alloy after aging or service for very long periods and the effect of such a microstructure on its properties constitute an aspect that needs to be examined in detail. This has been attempted by a few researchers in recent years. Radavich and Fort[9] have M. SUNDARARAMAN and LALIT KUMAR, Scientific Officers, G. ESWARA PRASAD, Head, Materials Characterisation Section, P. MUKHOPADHYAY, Head, Physical Metallurgy Section, and S. BANERJEE, Head, Materials Science Division, and Associate Director, Materials Group, are with the Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. Manuscript submitted March 10, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
studied the microstructure of alloy 625 after aging in the temperature range of 650 7C to 870 7C for up to 46,000 hours by scanning electron microscopy and X-ray diffraction (XRD), Thomas and Tait[10] have examined the microstructure and mechanical properties of the alloy after it had seen long-term service (about 50,000 hours) at temperatures of around 500 7C as the material of construction of feed stock superheaters in a petrochemical plant. They have found that, on being subjected to this treatment, the alloy hardens significantly, but its ductility and toughness at room temperature and weldability show a conspicuous deterioration. However, short aging treatments in the temperature range of 650 7C to 700 7C result in a considerable reduction in hardness and increase in toughness. While carrying out transmission electron microscopy (TEM) of specimens obtained from the postse
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