Grain boundary embrittlement of the iron-base superalloy IN903A
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1. I N T R O D U C T I O N L o w coefficient of thermal expansion alloys based on the system Fe-Ni-Co are attractive for certain aircraft engine applications, as they can reduce the spacing between stator and rotor. The first two such alloys developed were IN903 (Huntington Alloys, International Nickel Co.) and CTX 1 (Carpenter Technology), and have had their basic properties outlined in publications by Carpenter ~and Muzyka e t a l , 2 respectively. Their chemistries are very similar and based on an approximate composition of Fe-38Ni- 15Co with Nb, AI, and Ti additions to provide strengthening precipitates. An early indication of the extreme sensitivity of the mechanical properties of these alloys to the environment was given by the tensile tests of Carpenter.~ In these tests, a dramatic decrease in elongation was noted at elevated temperatures when testing was conducted in air as opposed to vacuum. It was subsequently discovered 3,4 that these alloys were also greatly sensitive to the environment on stress rupture, and particularly notched stress rupture testing, and that very limited lives were obtained on air testing in the range of operational interest, 540 to 650 ~ As the failures occurred intergranularly, and as the parted grain boundaries contained oxide wedges, it was concluded that these failures occurred by the penetration of oxide down grain boundaries, aided by the applied stress. Hence, this phenomenon was dubbed "Stress Accelerated Grain Boundary Oxidation" (SAGBO). Certain approaches had shown themselves to be capable of alleviating this problem. Firstly, additions of 1 pct Hf to CTX1 greatly improved the air notched R. H. BRICKNELLand D. A. WOODFORD are Staff Members, Metallurgy Laboratory, General Electric Corporate Research and DevelopmentCenter, Schenectady,NY 12301. Manuscript submitted December 30, 1980.
stress-rupture properties, 3 and secondly, removal of AI from IN903 produced the same effect. To compensate for the loss in strengthening precipitate, this new alloy (IN903A) has its N b + Ta level augmented to approximately 5 pct. 4 In addition, the thermomechanical history of the alloy may also play an important role. In the past decade, studies have been carried out on the embrittlement of nickel-base superalloys 5-~~and pure nickel t~ following high-temperature air exposure. This embrittlement has been shown 9,t2 to be due to oxygen diffusing down grain boundaries and is not connected with any continuous oxide formation. In addition, it has been noted in both tensile and creep rupture studies. This form of embrittlement and the poor stress rupture properties seen in the low CoE alloys have several points in common: 1) both require exposure in air; 2) both are manifested by a loss in mechanical properties in the mid-temperature range; 3) both are typified by a transition from transgranular to intergranular failure; and 4) both can be greatly ameliorated by Hf additions. These similarities suggest that the two embrittling mechanisms may be closely linked, and the current study seeks to expl
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