Bauschinger effect in haynes 230 alloy: Influence of strain rate and temperature
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INTRODUCTION
IT is difficult to quantify the role of internal stresses in the mechanical state of a material from the results of uniaxiaMoading experiments. In contrast, the reversibility of work hardening and the characteristics of the internal stresses can be ascertained by stress-reversal experiments. When certain materials are loaded uniaxially (e.g., in tension) into the plastic regime, unloaded, and then reloaded in the reverse direction (i.e., in compression), they may yield during the reloading, at a stress lower than if the reloading were carried out in the original direction (tension in this example). This direction-dependent yield behavior is known as the Bauschinger effect, after Bauschinger, who first reported this phenomenon in 1881.{~] Qualitative and quantitative information of hardening and softening mechanisms can be obtained from Bauschinger experiments. The drop in flow stress upon stress reversal has been attributed largely to the back stress developed in a dislocation pileup, which then assists the dislocation motion upon reversal of loading. To quantify the change in the flow stress upon stress reversal, a number of different parameters termed Bauschinger-effect parameters t2 6] have been used. These parameters reflect the mechanisms involved in lowering the yield stress. One of these parameters, the Bauschinger strain (/3), is defined to be the total reverse plastic strain obtained at the maximum forward stress (o)). In 1953, WoolleyI21 performed rigorous Bauschinger experiments on fcc and bcc metals. Using this "Bauschinger strain" as a measure, Woolley observed that the Bauschinger effect was more ANIRUDDHA THAKUR is Postgraduate Research Assistant, Department of Materials Science, University of Cincinnati, Cincinnati, OH 45221-0012. KENNETH S. VECCHIO, Associate Professor of Materials Science, and SIA NEMAT-NASSER, Professor of Solid Mechanics, are with the Department of Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, CA 92093-0411. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
pronounced in fcc metals compared to the bcc metals investigated. In addition, the Bauschinger effect in polycrystalline fcc metals which deform only by slip was determined principally by the stress level achieved and was independent of temperature and grain size. In the present study, the Bauschinger stress parameter,/3o, as defined by Moan and Embury,[7] is used to compare the internal stresses developed during the forward deformation in various experiments. The Bauschinger-stress parameter is the fraction of the total work hardening arising from the back stress and has the form /3~
--
o-~
o-s- Io-A
2o)
2o-s
where ~rb is the back stress, o-I is the maximum flow stress in th
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