The effect of fatigue deformation on microstructural evolution in a superplastic aluminum-zinc eutectoid alloy
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I.
INTRODUCTION
M A N Y alloys with a grain size under I0/~m are superplastic at temperatures above 0.4Tin (Tin is the absolute melting temperature). At these high temperatures, such processes as interface migration, grain growth, and precipitation are important. In fact, the rate of a particular process may be greater during deformation than during annealing. For example, in aluminum-rich grains in an AI-40 wt pet Zn alloy, the rate of dissolution of zincrich particles was faster during superplastic tensile deformation than during annealing, m In an aluminum-zinc eutectoid alloy, the rate of grain growth increased with tensile strain rate. t2] Aluminum-rich grains in the aluminum-zinc eutectoid alloy contain the sam~ precipitates that are found in aluminum-zinc alloys with smaller overall concentrations of zinc. Above 160 ~ the sequence of precipitation is as follows:t3] spherical Guinier-Preston (GP) t zones --~ ellipsoidal GP zones --* aRt --~ ~ P ---> (Zn) (OLR, a ' , and (Zn) denote rhombohedral a ' , cubic a ' , and the equilibrium zinc-rich terminal solid solution, respectively). Cubic a ' does not form at below 160 ~ t4] The fatigue deformation of a superplastic aluminumzinc eutectoid alloy was examined in a recent paper, tS] Interface sliding, interface migration, dislocation activity, and intergranular cracking were discussed. The purpose of the present paper was to examine three processes that occur during fatigue deformation: the decomposition of a~, the formation of precipitate-free zones (PFZs), and phase growth. II.
PROCEDURE
The preparation of fatigue test specimens, the fatigue testing procedure, and the preparation of thin foils were described in detail in another paper, tS] For the present work, only the thermal history of fatigue test specimens J.W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, is CSIRA Research Associate, Department of Mining and MetallurgicalEngineering,McGill University,Montreal, PQ H3A 2A7, Canada. B. RAMASWAMI, Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON M5S 1A4, Canada. Manuscript submitted February 3, 1989. METALLURGICALTRANSACTIONSA
is important. It will be briefly described. Extruded bars were solution annealed at 375 ~ and then quenched in ice water. They were subsequently annealed at 250 cC for grain growth. Specimens to be fatigue tested at 100 ~ and 200 ~ were annealed for 1 and 200 hours, respectively. After being annealed, all specimens were air cooled to room temperature. Thin foils were prepared from slices cut normal to the tensile axis from fatigue-tested specimens. They were examined in a Hitachi H-800 transmission electron microscope. Precipitates were identified using selected area electron diffraction. Fatigue-tested specimens and annealed specimens were examined to determine phase sizes and volume fractions. For each specimen, data from a minimum of eight fields were used for calculating average values. In each field,
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