Internal stress superplasticity in anisotropic polycrystalline zinc and uranium
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I.
INTRODUCTION
SUPERPLASTIC behavior can be observed either in materials that have ultrafine microstructures,l'Z i.e., fine-grained superplasticity, or in materials that have large internal stresses generated within them, 3'4's i.e., internal stress superplasticity (although in this case other names such as transformation and environmental superplasticity have also been used1). For the latter cases of internal stress superplasticity, there are several ways in which the necessary internal stresses can be generated. These methods include thermal cycling through a phase transformation, thermal cycling of polycrystalline pure metals or single phase alloys that have anisotropic thermal expansivity coefficients, and thermal cycling of composite materials in which the constituents have different thermal expansivity coefficients. Exampies of the generation of large internal stresses from each of these groups include white cast irons by cycling through a phase transformation, 6 polycrystalline pure Zn and polycrystalline pure a-U by taking advantage of anisotropic thermal expansivities, 7 and alloys of AI containing SiC whiskers by utilizing the different thermal expansivities of the SiC and the A1. 8 For cases where large internal stresses can be developed, especially under the application of a low externally applied stress, considerable plasticity can take place. It can be shown3 that under these conditions a high strain rate sensitivity exponent, m (m = d In tr/d In /0, is developed. High values of m (0.4 to 1.0) are responsible for gradual neck development and hence superplastic behavior. In the present paper, Zn manufactured by a powder metallurgy technique (hereafter called P/M Zn) has been selected to study the effect of internal stress on creep behavior. Zinc was chosen because it exhibits a large degree of anisotropy in its thermal expansivity coefficient; hence, in polycrystalline Zn, neighboring grains of dissimilar orientation change shape differently upon thermal cycling leading to the generation of large internal stresses at grain boundaries. In the paper, the experimental methods are described; MU YEH WU is Materials Scientist, Signetics Corporation, 811 East Arques Avenue, Sunnyvale, CA 94080; J. WADSWORTH is Manager, Metallurgy Department, Lockheed Palo Alto Research Laboratory, 3251 Hanover Street, Palo Alto, CA 94304; and OLEG D. SHERBY is Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted April 8, 1986. METALLURGICALTRANSAC'YIONSA
this includes a discussion of the techniques that are used for analyzing the thermal cycling creep data. Also, an internal stress creep model is proposed to describe the data and consideration is given to the physical mechanisms underlying the phenomenon. Finally, data from the literature on a-U and on polycrystalline wrought Zn are also examined using the creep model developed in this paper.
II.
EXPERIMENTAL MATERIALS
The pure Zn used in the study was originally obtained in the form of a single lot o
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