Carbide stability in nimonic 80a Alloy
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I.
INTRODUCTION
THE control of grain size during solution treatment is primarily dependent on the history of cold work and the stability of second phase particles. A greater degree of cold work will induce a finer recrystallized grain size. ~,2 Second phase particles act as pinning points, thus generally helping to restrict subsequent grain growth) The solution treatment temperature for NIMONIC 80A is 1080 ~ 4 In the range of temperature from 1040 ~ to 1090 ~ three carbides contribute to the second phase particle distribution. These have different stabilities and can therefore vary in proportion according to the heat treatment given: (1) Titanium carbonitride: Ti(C, N). A face-centered cubic B 1 phase which can exist in carbon- or nitrogen-rich forms 5 with lattice parameters of 4.33 ,~ and 4.24 A, respectively. The solvus temperature is above 1250 ~ (2) Mz3C6. A face-centered cubic phase where M is mainly chromium but where iron can substitute for chromium up to 30 pct. 6 The lattice parameter for Cr23C6 is 10.66 A. The phase forms on grain boundaries in a globular continuous morphology after slow cooling from solution treatment temperatures and aging at 800 ~ but it can precipitate in a discontinuous form in rapidly cooled and aged material, v The phase is also nucleated on slip planes and twins within the grains during aging, 5 The solvus temperature is between 1050 ~ and 1095 ~ (3) MTC3. This is a complex hexagonal phase where M can be chromium, iron, or both. 6 The lattice parameters are a = 14.01 A and c = 4.53 A for Cr7C3. The phase has a large atomic volume compared to the matrix, so that it nucleates heterogeneously on grain boundaries. It can precipitate on aging at temperatures above 930 ~ 8 However, after long aging times in the temperature range 930 ~ to 1050 ~ it is known to transform to M23C6.5 Above 1050 ~ it remains stable up to the solvus temperature range of 1095 ~ to t 150 ~ The stability of these phases in the solution treatment temperature regime (I000 ~ to 1150 ~ will be discussed in this paper. The implications of these discussions in relation to the effect of solution treatment temperature on grain size of NIMONIC 80A with a fixed cold working history wilt be highlighted. *NIMONIC is a registered trademark of Henry Wiggin and Co. Ltd. W.E. VOICE, Research Assistant, and R.G. FAULKNER, Senior Lecturer, are with the Department of Materials Engineering and Design, Loughborough University of Technology, Loughborough, LEII 3TU, United Kingdom. Manuscript submitted October 3, 1983. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL P R O C E D U R E
A. Alloy Samples of NIMONIC 80A (see T able I for composition) were cut from a single bar which had been previously extruded and hot rolled,
B. Analysis of Grain Size and Grain Boundary Precipitate Distribution The grain structure and grain boundary precipitates were studied by optical and scanning electron microscopy (SEM) techniques. The specimens were electrolytically etched prior to examination in 10 pct HC1 in methanol at room temperature for 5
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