Oxygen Embrittlement and Time-Dependent Grain-Boundary Cracking of ALLVAC 718PLUS
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I.
INTRODUCTION
ALLVAC 718PLUS* is a newly developed Ni-base super*ALLVAC 718PLUS is a trademark of Allegany Technology, Inc., Cumberland, MD.
alloy that combines the processing attributes of INCONEL**
**INCONEL, WASAPALOY, and INCOLOY are trademarks of Special Metals Corporation, Huntington, WV.
718 with the elevated temperature capability of WASAPALOY. The main goal was to produce an alloy which had the workability of INCONEL 718 but with a 55 C increase in elevated temperature capability over alloy 718. The upper operating-temperature limit for INCONEL 718 is 650 C and is set primarily by the thermal stability of the gamma double prime (c¢¢) phase that strengthens this alloy. At temperatures exceeding 650 C, the c¢¢ phase becomes unstable and begins to coarsen. In addition to this, the c¢¢ phase begins to revert to the more stable equilibrium delta (d) Ni3Nb, therefore severely reducing the alloys resistance to deformation at these temperatures. There are currently a number of recent articles describing the development of ALLVAC 718PLUS,[1] R.W. HAYES, Director of Research and Development, is with the Metals Technology Inc., Northridge, CA 91324. Contact e-mail: [email protected] Manuscript submitted December 27, 2007. Article published online August 12, 2008 2596—VOLUME 39A, NOVEMBER 2008
therefore readers of this report are referred to the research for a detailed background on the development of this alloy. Here, we will briefly touch on some of the points which are pertinent to the present topic of timedependent GB failure. According to the recent literature,[2] the chemical composition of phases present within the structure which may potentially react with the embrittling species is quite important. The chemical compositions of ALLVAC 718PLUS, 718, and Waspaloy are given in Table I. There are several significant changes in the composition of 718PLUS relative to INCONEL 718, as shown in Table I. First, there is a significant reduction in Fe which has been replaced with 9 wt pct Co. Second, the Cr content in ALLVAC 718PLUS has decreased by 1 wt pct compared to 718. The last significant difference is in the Al content, which is increased by approximately 1 wt pct in 718PLUS. These compositional changes promote the presence of the more stable gamma prime (c¢) phase in 718PLUS, as opposed to the metastable c¢¢, which is the strengthening phase in alloy 718.[1] Very little c¢¢ has been reported to exist in the 718PLUS structure following aging heat treatments, and the strengthening phase is predominately c¢.[1] However, it has also been shown that significant levels of d can be brought out in 718PLUS structures depending on processing and heat treatment.[1] The d phase resides primarily at grain boundaries. Without the c¢¢ phase within the grain interiors, it seems that it would be more difficult to nucleate d phase within the grains of 718PLUS, compared to the nucleation of intragranular d in alloy 718.[3] In a previous study on the creep crack growth (CCG) of INCONEL 718, Valerio et al.[4] found a si
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