Flow localization accompanying the intergranular fracture of Ni 3 Al
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Flow Localization Accompanying the Intergranular Fracture of N%AI
solidification rate process (described elsewhere~); upon consolidation, dynamic recrystallization occurred, resulting in an aggregate of equiaxed grains = 15 p~m in diameter. A dumbbell-shaped tensile specimen was ground to specifications (2.7 mm diameter • 16.5 mm), annealed at 1000 ~ for 68 hours to yield grains 70/xm in diameter, and then electrochemically polished. Subsequently, the specimen was strained in tension at 10 -4 S-1 at room temperature, and then examined by scanning electron microscopy using a 30 Kv beam of -~5 • 10 -4 radians divergence, -~ 1 • 10 -9 amps, and -~5/xm diameter. Plastic strain was assessed by generating electron channeling patterns from selected grains using the rocking beam method,* and then by com*See References 2 and 3 for reviews of this technique.
paring the quality of patterns obtained from the fractured specimen with the quality of others obtained from Ni3A1 standards which had been similarly prepared but either compressed by 5 pct, by 3 pct, or not deformed. All patterns were generated during a single sitting, thereby ensuring that any variations in quality reflected variations in perfection of the scattering regions and not variations in the operating conditions. The specimen yielded at 157 MPa, and then after an elongation of 0.24 pct fractured via an intergranular mode (Figure !). Only very low quality patterns comprised essentially of a few diffuse channeling lines* could be generated *That the diffuse lines noted are electron channeling lines, and not distorted images of topographical features, is evident from the observations that they remained stationary while the specimen was slightly translated under the beam.
from any of the intergranular facets. In comparison, better patterns, although still poor, could be generated from individual grains located within the deformed gauge section but well away from the fracture surface. Good patterns could be generated from the standard which had not been deformed. Figure 2 illustrates these points. In addition, micrographs obtained from fracture facets using the electron probe in the channeling mode 2 (=2 • 10-3 radians, ~ 1 0 -9 amps,
E.M. SCHULSON, D. L. DAVIDSON, and D. VIENS The purpose of this note is to show that localized plastic flow accompanies intergranular fracture within the strongly ordered L12 aluminide Ni3A1. The work is part of a larger study on the brittle-to-ductile transition of this material, and was carried out to answer the question: Is the plastic strain which accompanies the propagation of cracks along grain boundaries greater than that which occurs throughout the bulk of the aggregate? The question is a qualitative one and is answered here by utilizing the qualitative aspects of the selected area channeling pattern (SACP) technique in scanning electron microscopy. The test material contained 25.0 at. pct A1 (nominal) and was prepared by extruding to rod at 1260 ~ rapidly solidified powders. The powders were made by Pratt and Whitney Airc
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