Microscopic substructures of stainless steel 304 undergoing a uniaxial ratcheting deformation
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O R I G I NA L PA P E R
Yawei Dong · Zhiyong Zhang · Xu He
Microscopic substructures of stainless steel 304 undergoing a uniaxial ratcheting deformation
Received: 9 July 2020 / Revised: 1 August 2020 / Accepted: 15 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Under asymmetrical stress-controlled cyclic loading accompanied by ratcheting, the evolution of microscopic substructures of stainless steel 304 (SS304), a face-centered cubic metal, was detected using transmission electron microscopy (TEM). This observation demonstrates that dislocation slip is the main mechanism of uniaxial ratcheting in the non-steady stage (stage I). Dislocation proliferate rapidly duo to the high stress level of the cyclic tests, and dislocation density increases continuously with the the number of cycles. The ratcheting rate decreases continuously due to the interaction of moveable dislocations, and the planar dislocation substructures (dislocation pileups and tangles) are the dominant patterns in this stage. In the later stage I, distinct lath α-martensite was observed in the material due to the nucleation of martensite and the phase transformation zones increase gradually with the growth of axial ratcheting strain in the stage II of uniaxial ratcheting. The multiply and cross-slip were activated gradually in increasing numbers of grains, and the prevailing dislocation configurations evolve into more complicated and stable ones (dislocation walls and cells). In addition the dislocation configurations after various prescribed tensile strain of the monotonic tension and creep deformation with three different holding times were also observed by TEM. Comparing the evolution of microscopic substructures in various loading modes, the physical mechanism of uniaxial ratcheting of SS304 can be revealed as combination of the evolution of dislocation configurations and martensite transformation in the stage II of uniaxial ratcheting.
1 Introduction Due to its excellent corrosion resistance and mechanical properties, austenite stainless steel is extensively used in various engineering structures. Therefore, increasing concerns about the cyclic mechanical property, deformation mechanism, and corresponding constitutive model of austenite stainless steel have received widespread attention in the past four decades, especially for the asymmetrical stress-controlled loading case accompanied by ratcheting behavior. The ratcheting behavior and corresponding phenomenological constitutive models of various kinds of metallic material have been studied before the first decade of the twenty-first century [1–3]. Nevertheless, a large proportion of the experimental research focused on the influence factors on ratcheting behavior, such as cyclic hardening/softening, loading level, loading history, and environmental temperature. Little attention has been paid to the deformation mechanism. Therefore, most of the cyclic constitutive models were constructed directly from the macroscopic experimental data. The deformation mecha
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