Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy
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I.
INTRODUCTION
A L T H O U G H superplastic forming is becoming an increasingly important method of shaping metals, there is little published information on the effect of superplastic deformation on ambient temperature mechanical properties. Property changes can arise from strain-enhanced grain growth at the relatively high temperatures required for superplastic deformation or, more importantly, may occur if the material undergoes cavitation during superplastic flow. In the present work a superplastic a/~ Cu-Zn-Ni alloy (nickel-silver), which is particularly prone to cavitation during superplastic flow in tension, has been used to evaluate the effect of cavitation on room temperature tensile properties. The alloy has good microstructural stability so the effect of strain-enhanced grain growth during superplastic flow on subsequent room temperature properties should be relatively small. The Cu-Zn-Ni alloys have previously been studied with respect to their superplastic deformation and cavitation behavior, l 5 After superplastic tensile deformation at 570 ~ (843 K) to introduce different levels of cavitation, using strains well below those associated with fracture, specimens were pulled to failure at room temperature. To determine whether heat treatment could significantly reduce the level of cavitation and thereby improve post-deformation properties, superplastically deformed specimens were annealed at 527 ~ (800 K), a temperature selected in an attempt to minimize grain growth, and then tested at room temperature.
II.
EXPERIMENTAL
A. Material An a/[3 Cu-Zn-Ni alloy was prepared in the laboratory in the form of round bar of diameter 5.4 ram, using a fabrication route described previously. 4 Swaging, followed by an annealing treatment at 480 ~ (753 K) produced a microN. RIDLEY, Senior Lecturer, and D. W. LIVESEY, Postdoctoral Research Assistant, are with the Department of Metallurgy and Materials Science, University of Manchester/UMIST, Grosvenor Street, Manchester M1 7HS, England. A. K. MUKHERJEE is Professor, Division of Materials Science and Engineering, Department of Mechanical Engineering, University of California, Davis, CA 95616. Manuscript submitted July 5, 1983. METALLURGICAL TRANSACTIONS A
duplex structure with an a grain size of ---2.8 pm (quoted as 1.74L, where L is the mean linear intercept). The analyzed composition of the alloy is given in Table I. Tensile specimens of 10 mm gauge length and 2.7 mm gauge diameter were machined from the bar.
B. Superplastic Deformation Specimens were superplastically deformed in tension to preselected strains, at various initial strain rates at 570 ~ (843 K) using a Mand machine. A thermocouple was attached to the specimen to monitor temperature. Straining was performed in air inside a furnace for which a temperature variation of +2 ~ was measured over the central 200 ram. Specimens were equilibrated at the test temperature for approximately 40 minutes prior to testing.
C. Densitometry Hydrostatic weighings in ethyl iodide were performed on the gauge heads and
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