Segregation of B, P, and C in the Ni-Based Superalloy, Inconel 718
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TRODUCTION
INCONEL 718 (IN718) is a complex engineering material that can withstand the extreme environments experienced by jet engine components because of its excellent high temperature strength and superior corrosion and creep performance.[1,2] Trace amount additions of boron (B) and phosphorus (P) have been reported to be beneficial in increasing the creep resistance and fatigue life of IN718.[3–7] Because of the high cost associated with the material and production of IN718, welding is the preferred method to repair the damage suffered by the alloy while in service. Although B and P help to lengthen the service life of the alloy, they have an adverse effect on the weldability of IN718. Alloys with B and P additions suffer from microfissuring in the heataffected zone (HAZ) near the grain boundary region during welding. The microfissuring has been attributed to segregation of minor elements to the grain boundaries, locally lowering the melting point[8,9] or forming precipitates that may undergo constitutional liquation during welding.[10] Recently, additions of carbon (C) have been shown to counteract this negative effect,[11,12] decreasing the average fissure crack length significantly. The effect of each of these minor elements (B, P, and C) individually on the behavior of Ni-based superalloys TALUKDER ALAM and PETER J. FELFER, Ph.D. Students, LEIGH T. STEPHENSON, Research Associate, and JULIE M. CAIRNEY, Associate Professor, are with the Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, NSW 2006, Australia. Contact e-mail: [email protected] MAHESH CHATURVEDI, Professor, is with the Department of Mechanical and Industrial Engineering, The University of Manitoba, Winnipeg, MB R3T 5V6, Canada. MATTHEW R. KILBURN, Associate Professor, is with the Centre for Microscopy, Characterisation and Analysis (CMCA), The University of Western Australia, Crawley, WA 6009, Australia. Manuscript submitted May 15, 2011. Article published online March 16, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
has been investigated extensively and is well understood.[13] B is highly beneficial for improving creep properties; additions of ~0.01 wt pct have been shown to extend rupture life by an order of magnitude and improve rupture stress by a factor of 2 to 3.[14,15] However, a large number of studies have confirmed that microfissuring during welding increases significantly when B is present[11,13] and increases as the amount of B is increased.[16] P has also been shown to have a beneficial effect on the creep properties of 718 alloys.[17] Cao and Kennedy[18,19] reported that that optimal high-temperature properties were obtained with the addition of 0.012 wt pct B and 0.022 wt pct P, which is considerably higher than the normal concentration of B and P in Inconel 718 (30 to 50 ppm B and 3 to 5 ppm P). However, this higher B and P alloy was found to have a dramatically reduced weldability performance, with sharply increased microfissuring observed in the HAZ.[11] C is often added to Ni-based superalloys in
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