Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys
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I.
INTRODUCTION
Table I.
A number of materials undergo cavitation during superplastic flow in tension. The cavities nucleate at grain boundaries and their growth and interlinkage can lead to premature fracture. Copper alloys are particularly prone to cavitation, and the factors which influence the level of cavitation in some systems have been identified.l-7 However, there is little information on cavity growth rates or on the mechanism of cavity growth during superplastic flow. In the present work, studies have been made on two commercially available microduplex a/B nickel-silvers. These alloys are processed primarily to give good ambient temperature mechanical properties, but because they have an a grain size of 2 to 3 p,m and B-phase size of - 1 /zm, they are also superplastic at elevated temperatures. 8,9,1~The alloys, which consist of approximately 80 to 85 pct of a-phase, are microstructurally very stable, but are prone to cavitation during superplastic deformation. ~ The level of cavitation resulting from tensile superplastic flow in these alloys has been examined as a function of strain, strain rate, and temperature using metallographic and densitometric techniques. Cavity growth rates have been measured and compared with values predicted by theories developed for cavity growth during creep, which assume that cavities grow either by a stress-directed vacancy diffusion mechanism, or that growth is deformation controlled. II.
EXPERIMENTAL
Two microduplex nickel-silver alloys, coded IN836 and IN629, were received in the form of sheets with thicknesses of 2.5 mm and 1.25 mm, respectively. The analyzed compositions of the materials, one of which was a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy, are given in Table I. Tensile specimens of 10 mm gauge length and 5 mm gauge width were milled from IN836 and stamped from IN629. Tensile straining was carried out in air in a furnace attached to an Instron machine. Specimens were strained to preselected elongations short of failure so that cavitation
Code IN836 IN629
Analyzed Compositions (Wt Pct) of the Nickel-Silver Alloys
Zn 38.4 28.1
Ni 15.7 15.0
Mn 0.13 13.3
Cu bal. bal.
could be examined using metallography and density measurements. Cavitation was studied as a function of strain, strain rate, and temperature and all superplastic deformation was carried out under constant strain rate conditions, unless specifically stated otherwise. Metallographic studies were made using optical and scanning electron microscopy. After polishing to 0.25/~m diamond finish, IN836 was electrolytically etched in citric acid (100 g 1-1, 5 to 15 V, 10 to 20 seconds), while IN629 was chemically etched in a solution of 20 ml K2Cr2OT, 10 ml saturated NaC1, 5 ml HC1, and 100 ml of water for five seconds, and then in acid ferric chloride for 10 seconds. A few specimens were mechanically polished and ion-beam etched using argon at a pressure of 10 -1 mm Hg and a discharge current of ~19 mA for 5 to 10 minutes. Quantitative metallographic measurements were made on mechanically po
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