Cyclic hardening of an Ni-14.4 at. pct AI alloy containing coherent precipitates
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INTRODUCTION
STUDIES of the cyclic stress-strain response in alloys containing coherent precipitates have emphasized the cyclic softening which occurs when the alloys are in the underaged condition while the large cyclic hardening which precedes softening has received much less attention and has been discussed only in terms of monotonic hardening theories, xIt is well known that the work hardening rate of homogeneous metals under monotonic plastic straining is much greater than that for alloys with coherent precipitates. 2 In contrast, however, many studies on the cyclic behavior of the latter type of alloys have shown that very substantial cyclic hardening occurs. 3-1~The cause of this difference must originate from the back and forth motion of dislocations through the precipitates. Aside from the study by Stoltz and Pineau 3 on Waspalloy, studies relating the dislocation structures observed by TEM to the cyclic flow stress of alloys with coherent precipitates have dealt with precipitates limited to quite small sizes such as the G.P. zones in A1-Cu, Cu-Co, and A1-Ag 5-9 which are metastable. The present studies with an Ni-14.4 at. pet AI alloy have the advantage that the cuboidal coherent Ni3Al(y') precipitates may be 30 nm, t~ or larger if desired, allowing determination of the effect of coherent particle size on the cyclic deformation behavior over a wide range of sizes. The large particle size also facilitates direct TEM observation of the precipitate-dislocation structures. The results and discussion of this paper are divided into two sections. The first deals with the mechanical response of the material to cyclic deformation while the latter section encompasses the microstructural changes that occurred during cyclic straining. From discussion of the combined resuits, microstructural reasons were determined or offered for the phenomenological results obtained. D. L ANTON, formerly Graduate Assistant, Materials Science and Engineering Department, Northwestern University, Evanston, IL, is now a Research Scientist with United Technologies Research Center, East Hartford, CT 06108. M. E. FINE is W. P. Murphy Professor, Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, IL 60201. Manuscript submitted October 2, 1981. METALLURGICAL TRANSACTIONS A
EXPERIMENTAL PROCEDURES
The Ni-AI alloy investigated was received from INCO Research Laboratories as a 19.05 mm (0.75 inch) hot rolled plate. Chemical analyses furnished by INCO are given in Table I. Specimens of square cross section, 5 x 5 mm in the gage section which was 10 mm long, were initially solutioned in a vertical furnace at 1000 ~ for one hour in a dry argon atmosphere and quenched into ice water. Subsequent aging at 700 ~ in a vacuum of 3 x 10 -4 t o r t was performed for the various times given in Table II in order to obtain a range of precipitate sizes varying from quite small, underaged, through large, overaged, while maintaining a constant volume fraction of precipitates. Removing the
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