Porosity and tensile ductility in Al-Zn alloys
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I.
INTRODUCTION
IT is well known that porosity and defects, which act similarly, have a deleterious effect on material mechanical properties. Strength, toughness, and ductility are reduced in their presence and tensile ductility, in particular, is profoundly influenced by porosity even when the latter is present in amounts on the order of a percent or so. Certain A1-Zn alloys are useful model systems for studying the effect of porosity on ductility. This is so because these alloys are strong (O-y = 10-2 G, where G is the shear modulus) at low (77 K) temperatures,l yet are superplastic at moderate (approximately 570 K) ones. 2-6 Thus the effect of porosity on brittle, ductile, and quasi-uniform (i.e., superplastic) tensile fracture can be studied in one system over a fairly narrow temperature range. The results of such a survey are presented in this paper.
II.
EXPERIMENTAL PROCEDURE
A. Alloy Preparation A1-Zn alloys were cast and thermomechanically treated under different processing conditions (Table I). Alloys designated as 1 and 4 in this table were melted in air and, as will become apparent, this leads to porosity (or other kinds of defects) which affects their mechanical behavior even after subsequent hot rolling. As indicated in Table I, Alloy 1 has the monotectoid composition (22 wt pct A1), whereas Alloy 4 is aluminum rich relative to the monotectoid. A1B. HARRIPRASHAD, formerly Graduate Student at Michigan Technological University, is deceased. T. H. COURTNEY, formerly Professor, Department of Metallurgical Engineering, and Dean of the Graduate School at Michigan Technological University, is Professor and Chairman, Department of Materials Science, University of Virginia, Charlottesville, VA 22901. J.K. LEE is Professor, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49965. Manuscript submitted April 27, 1987. METALLURGICALTRANSACTIONSA
loys 2 and 3, both of the monotectoid composition, were melted and cast in vacuum, as described below. As reflected in their mechanical behavior, these alloys had a lower porosity/defect concentration than did Alloys 1 and 4. Preparation of Alloys 1 and 2 is described in detail as most of this study focuses on their behavior. Alloy 3 was prepared similarly to Alloy 2 except for an additional step (hot isostatic pressing) incorporated in its processing. However, no differences in mechanical behavior were found between the hot isostatically pressed material and that of Alloy 2. Alloy 4, as mentioned, was prepared in the same manner as Alloy 1. Although the composition of the alloy was different, the behavior of Alloys 1 and 4 was quite similar, indicating that processing affects properties to a degree as great as does compositional variation. Alloy 1 was prepared by first melting high purity AI and Zn (->99.9 at. pct) in a graphite container in air at 875 K. The molten alloy was vigorously stirred and then cast into a graphite mold (dimensions 1.27 cm • 5.08 cm • 15.25 cm). This alloy, as well as Alloy 4, could not be cold rolled o
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