Interface sliding, migration, and cracking during fatigue deformation of a superplastic aluminum-zinc eutectoid alloy
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I.
INTRODUCTION
SUPERPLASTIC deformation has been the subject of many research papers, review articles, tz-4] and books, t5'61 While tensile deformation of superplastic materials has been extensively studied,~ cyclic deformation has received little attention, t7-~61 In particular, the microstructures of fatigue-tested superplastic materials have been investigated in only three papers. [13'15.16] Therefore, the first objective of the present work was to examine the fatigue deformation of a superplastic material, with emphasis on the changes in its microstructure. It is well known that during tensile deformation of superplastic materials, grain boundary sliding makes a significant contribution to the total strain, t17,18,19]Hence, the second objective was to investigate the fatigue deformation of a material in which grain boundary sliding makes a large contribution to the total strain. There is much information on tensile superplastic deformation of the aluminum-zinc eutectoid alloy. This material is superplastic at a relatively low temperature, and a fine grain size, which is necessary for superplastic deformation, can be obtained by heat treatment, t~71 Therefore, an aluminum-zinc eutectoid alloy was selected for the present investigation. II.
ice water. They were subsequently annealed at 250 ~ for grain growth. Bars annealed for 1 and 200 hours had respective phase sizes of 0.59 and 2.4/~m. Tensile and fatigue test specimens were machined from the heat-treated bars. After being ground and mechanically polished, specimens were electropolished. The composition of the electrolyte was as follows: 50 vol pct methanol, 35 pct butanol, 10 pct glycerol, and 5 pet perchloric acid. The electrolyte was cooled to - 4 0 ~ and a potential of 40 V was maintained between the specimen and a stainless steel cathode.
B. Mechanical Testing Constant plastic strain amplitude fatigue tests were done in air on an MTS model 810 materials test system. To measure strain at 200 ~ a resistance strain gage was bonded to the specimen. A strain gage extensometer was used for measuring strain at 100 ~ Strain rate change tests were done on an Instron model TFCML universal testing machine (Reference 20, method 3). In each strain rate change, the crosshead speed was increased by a factor of 2 or 2.5, and between strain rate changes, a true strain of 0.04 to 0.05 was accumulated.
PROCEDURE
A. Specimen Preparation
C. Metallography
Ingots of an aluminum-zinc eutectoid alloy (A1-78 wt pet Zn) were prepared from 99.999 wt pct aluminum and zinc. To produce an ultrafine grain size, extruded bars were solution annealed at 375 ~ and then quenched into
Before and after a fatigue test, two stage carbon replicas of selected surface areas were examined in a Hitachi model H-800 transmission electron microscope. The same areas were examined in a Hitachi model S-520 scanning electron microscope. After a fatigue test, slices with thicknesses between 0.5 and 1.0 mm were cut normal to the stress axis. They were ground to thicknesses between 0.25 and 0.3 mm us
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