Effects of fatigue on the GP zones in Al-Zn alloys
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INTRODUCTION
PRECIPITATION-hardened alloys are known to be particularly vulnerable to the plastic strain localization which accompanies fatigue. Most theoretical attempts at explaining the poor fatigue resistance of these materials are based on the evolution, stability, and redistribution of the precipitate structure during fatigue. Mechanisms based on particle disordering, 1overaging,2 reversion,30stwald ripening,4 and aging inhomogeneities 5 have been suggested. These conflicting interpretations have been based primarily on evidence obtained with the transmission electron microscope, by imaging the local fatigue-induced structure within persistent slip bands (psb's). Precipitate-dislocation interactions can easily be obscured in such regions of high deformation, and destructive sample preparation techniques often prevent convincing "before and after" images throughout the various stages of fatigue. Small-angle X-ray scattering (SAXS) has been used in this investigation to obtain for the first time a quantitative estimate of the changes in GP zones in A1-Zn alloys due to fatigue. Bulk samples of AI-3.5 and 5,3 at. pct Zn have been cycled to failure in fully reversed loading at both room temperature and 77 K. The initial heat treatments have been varied to permit characterization of the damage at various stages of the age hardening process. At the same time as this study6 Steiner et al. 7 reported some interesting results on Cu-Co alloys, employing small-angle neutron scattering. They suggested that reversion occurred, primarily in the psb's as there was a decrease in the volume fraction of zones, but not their (average) size. However, these authors examined only two specimens and the changes were small. The zone sizes in this study were --27 A or greater. As will be shown below, our investigation indicates that the effects are much more pronounced for smaller zones.
II.
grain size to 1 to 3 mm. The specimen design was based on the need to facilitate the measurements of small-angle X-ray scattering. The coarse grain size was large enough to contain the 0.35 mm X-ray beam, and the specimen was made thin enough to allow penetration of this beam. The nominal dimensions are shown in Figure 1. The heat treatments given to the six specimens are summarized in Table I. B. Fatigue Testing Tension-compression cycling was carried out in laboratory air at room temperature and in dynamic displacement control (unless otherwise noted). An Instron model 1251 electrohydraulic fatigue machine was employed. Low temperature tests were carried out on a specially designed apparatus. 8 The sample was cycled in load control within a
T
7.95
76.2 2 _32
3.175
EXPERIMENTAL PROCEDURES
A. Specimens Dog-bone shaped fatigue samples were spark cut from alloy sheet which had been strain annealed to coarsen the R. G. PAHL, Jr., formerly Research Assistant, Department of Materials Science and Engineering, The Technological Institute, Northwestern University, Evanston, IL 60201, is now with Argonne National Laboratory, Idaho Falls, ID 83401.
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