Tensile behavior of inconel alloy X-750 in air and vacuum at elevated temperatures
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INTRODUCTION
A variety of high temperature materials has been investigated at elevated temperatures in vacuum and in air under constant load creep conditions.l-~~ These tests have not provided any systematic trend on the creep behavior, and the results are full of contradictions. For example, the rupture life of nickeP '2 and Ni-19 Cr alloy ~~increased in air at low stresses and high temperatures while vacuum gave marginally better creep performance at low temperatures and high stresses. The material Udimet 500 was found slightly stronger in vacuum than in air 7 whereas opposite results 8 to this were obtained in Udimet 700. At low stresses, air increased the rupture life of Inconel alloy X-750, but a modification to the heat-treatment process displayed a reversal effect in the atmospheric effect. In several 80 Ni-20 Cr alloys, 6 creep tests conducted in argon and air have shown that the latter had a strengthening effect in all cases. The mechanism of this effect was found to depend on the melting history, silicon content, and Mn/S ratio. In a recent paper, Pandey and Taplin" have reanalyzed the results of the Ni-19 Cr alloy, ~~ tested in air and vacuum, in terms of cavity nucleation by gas bubble formation of CO2. ~2It is interesting to note that vacuum obtained during creep testing of the Ni-19 Cr alloy at high temperatures (815,927, and 1038 ~ was nearly 1.3 • 10 -2 Pa which is approximately the same as used in the investigation of Inconel alloy X-750. ~2.~3The flux of oxygen along grain boundaries in the alloy Inconel X-750 at elevated temperature was found much faster in the vacuum than in air. Evidence suggested that in the vacuum ( ~ 2 . 6 0 • 10 -2 Pa), oxygen diffused along grain bound~ aries without forming any oxides of chromium and nickel and instead formed gas bubbles of CO2 which acted as cavity nuclei. The cavity nuclei weakened the material, and during creep testing accounted for the enhanced overall creep rates and reduced fracture ductilities and lifetimes. On the other hand, exposure to air at elevated temperature, ~3 prior to testing, resulted in the formation of oxide scales at the
M.C. PANDEY, Postdoctoral Fellow, and A.K. MUKHERJEE, Professor, are with the Division of Materials Science, Department of Mechanical Engineering, University of California, Davis, CA 95616. D. M. R. TAPLIN is Professor and Chairman, Department of Mechanical and Manufacturing Engineering, Trinity College, University of Dublin, Dublin-2, Ireland. Manuscript submitted November 7, 1983. METALLURGICALTRANSACTIONS A
specimen's surface, and the flux of oxygen was the same in the grains as well as along the grain boundaries. Further, the material did not exhibit any embrittlement during creep testing. Thus, based on the work of the alloy Inconel X-750, it can be argued that, in the Ni-19 Cr alloy at low stresses and high temperatures, cavities nucleated due to CO2 gas bubble formation in the vacuum, very early during creep, giving lower creep life than in air. It is now becoming clear that while carbon provides improved c
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