The correlation between the temperature dependence of the CRSS and the formation of superlattice-lntrinsic stacking faul
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The Correlation between the Temperature Dependence of the CRSS and the Formation of Superlattice-lntrinsic Stacking Faults in the Nickel-Base Superalloy PWA 1480 WALTER W. MILLIGAN and STEPHEN D. ANTOLOVICH
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PWA 1480 is a single-crystal nickel-base superalloy, which contains about 60 vol pet of the strengthening 7' phase. Its structure and mechanical behavior have been well documented, tl,2] The low temperature mechanical behavior of this alloy is of interest because the alloy exhibits a unique minimum in the critical resolved shear stress (CRSS) at about 400 ~ This minimum is seen in Figure 1, which contains average (001) yield strength data from three studies, t2,3,4] The mechanism responsible for this CRSS minimum is the subject of this paper. As one possible explanation, Dollar and Bernstein LS] have proposed that the CRSS is controlled by the workhardening behavior of the matrix, and they have developed a model for the temperature dependence of the CRSS based on this hypothesis. (5] As an alternative explanation, we have observed a deformation mechanism whose temperature dependence correlates exactly with the reduction in the CRSS. After monotonic or cyclic deformation of PWA 1480 at 20 ~ the deformation substructures typically contain a high density o f superlattice-intrinsic stacking faults (S-ISF's) within the 7' precipitates. These faults are often present as a/3(112) faulted loops, as observed in single-phase Ni3A1 3~' deformed at 20 ~ A typical example is shown in Figure 2(a). The substructure contains S-ISF's, partial dislocation pairs, and partial dislocation loops within the 7'. Similar stacking faults are also visible in the micrographs presented by Dollar and Bernstein (after deformation at 20 ~ although the authors did not discuss this point in their paper. As the temperature of deformation is increased, the density of S-ISF's observed after deformation is reduced, until finally no faults are observed after deformation in the range from 400 ~ to 705 ~ These effects are illustrated in Figure 2. (The structure shown in Figure 2(c) is typical of that seen at 400 ~ and 705 ~ The reduction in fault density corresponds exactly to the reduction in the CRSS; the minimum in CRSS occurs at the temperature where the fault density has reached zero. This strongly implies that the material is strengthened at low temperature by a mechanism which is reWALTER W. MILLIGAN, formerly Graduate Student, Mechanical Properties Research Lab, School o f Materials Engineering, Georgia Institute of Technology, is now Assistant Professor of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. STEPHEN D. ANTOLOVICH, Director, Mechanical Properties Research Lab, and Professor and Director, School of Materials Engineering, is with the Georgia Institute of Technology, Atlanta, GA 30332-0245. Manuscript submitted February 10, 1989.
1888--VOLUME 20A, SEPTEMBER 1989
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