High Temperature Fracture Toughness of Ni 3 Al Alloy IC-396M
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HIGH TEMPERATURE FRACTURE TOUGHNESS OF Ni3AI ALLOY IC-396M
DAVID J. ALEXANDER Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6151. ABSTRACT The fracture toughness of a nickel aluminide alloy IC-396M has been measured from room temperature to 8000C. The material was tested in the as-cast condition. Specimens were oriented for crack growth either parallel or perpendicular to the dendritic structure (C-R and R-C orientations, respectively). Both orientations had acceptable levels of toughness from room temperature to 6000C. At 6500C the toughness decreased markedly with some recovery at 8000C. Examination of the fracture surfaces revealed that the fracture mode changed from a ductile tearing mechanism at temperatures up to 6000C to interdendritic fracture at 650 0C and higher temperatures. At 6500C the crack grew along the dendrite axis for the C-R specimens, but for the R-C specimens the crack grew out-of-plane in attempting to follow the dendritic structure. It is believed that the presence of the sharp crack in the fracture toughness specimens allowed oxygen to penetrate to the crack tip, where straining exposed metal surfaces that were not protected by an oxide layer. The result was embrittlement of the interdendritic regions and low energy fracture, analogous to intergranular fracture in wrought materials. INTRODUCTION Nickel aluminide intermetallic alloys offer the potential for good mechanical properties and corrosion resistance at intermediate to high temperatures. One of the alloys developed at Oak Ridge National Laboratory for use as a casting alloy is designated IC-396M. The nominal composition of this alloy is 8AI-8Cr-3Mo-0.8Zr-0.005B (wt %). This work continues a previous investigation of the tensile and fracture properties of this alloy [1]. EXPERIMENTAL PROCEDURE The IC-396M material was vacuum induction melted and cast into a 75-mm-diam ingot, followed by electroslag remelting into a 100-mm-diam ingot. All processing was performed by Haynes International. Slices approximately 15 mm thick were taken from the ingot, and compact specimens 12.7 mm thick were machined, with the crack plane either in the radial or circumferential directions (C-R and R-C orientations, respectively). These specimens were precracked at room temperature with a final maximum stress intensity of approximately 25 MPaVm, and then side grooved 10% of their thickness on each side. The fracture toughness was determined from the J-integral-resistance (J-R) curves, which were obtained using the unloading compliance technique to monitor crack extension during the test, in general accordance with ASTM Standards E 813-89, Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
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Standard Test Method for J,., a Measure of Fracture Toughness, and E 1152-87, Standard Test Method for Determining J-R Curves. A rod-in-tube device was used to transfer the load-line displacement of the specimen outside the 0furnace where it was measured with a conventional clip gage. At temperatu
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