Characteristics of shear bands formed in an austenitic stainless steel during hot deformation

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Abstract

A detailed investigation of the substructural characteristics of shear bands, formed in an austenitic stainless steel during deformation in torsion at 900°C, with particular emphasis on their nucleation stages, has been undertaken. Shear bands forming at large strains in a matrix of pre-existing microbands are composed of well-recovered, slightly elongated cells. These bands propagate along a similar macroscopic path and the cells, present within their substructure, are rotated relative to the surrounding matrix about axes close to a common macroscopic direction. The cell boundaries are frequently non-crystallographic, suggesting that the cells might often form through the operation of multiple slip. Shear bands appear to form through a cooperative nucleation of originally isolated cells that gradually interconnect with each other to form long, thin bands that subsequently thicken via the formation of new cells. When crossing a shear band, cumulative misorientations across consecutive cell boundaries display a typical sigmoidal profile, characterised by a gradual increase of misorientation angles to a peak value followed by a subsequent gradual decrease towards the matrix orientation. The formation of new cells thus appears to be assisted by the stress fields generated by lattice rotations of the previously formed cells.

Introduction

Shear bands have been observed to form as microstructural inhomogeneities at large strains during plastic deformation of a wide range of metallic materials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These bands are known to play an important role in the deformation process, as they are capable of accumulating large shear strains. Macroscopic characteristics of shear bands appear to be largely governed by the macroscopically imposed deformation geometry [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and these bands were frequently observed to propagate across several grain boundaries without any noticeable deviation. It has also been found that, in general, their macroscopic shear planes deviated from the crystallographic slip planes and thus appeared non-crystallographic [11]. In order to account for the formation of these bands, mechanisms of geometrical [9], [10], structural [3], or textural [12] softening have been suggested.

Although the macroscopic characteristics of shear bands appear to be rather well described; to date there has been only very limited information on their substructural characteristics available in the literature [2], [5], [10], in particular for early stages of their formation. Thus, the aim of the present work was to undertake a detailed investigation of the substructure of shear bands, formed in an austenitic stainless steel during hot deformation, with a particular emphasis on the study of the nucleation stages involved in the formation of these bands.

Section snippets

Experimental procedures

The material used was a 22Cr–19Ni–3Mo austenitic stainless steel with the chemical composition of 0.02 mass% C, 0.99% Mn, 0.55% Si, 0.023% P, 0.016% S, 21.54% Cr, 18.61% Ni, 2.93% Mo and the balance Fe. Specimens with a gauge length of 50 mm and a diameter of 6 mm were subjected to deformation in torsion at a strain rate of 0.7 s−1 at 900°C followed by rapid quenching. Torsional deformation was interrupted at von Mises strain levels of about 0.2, 0.5, 0.8 and 1.1. Transmission electron

Experimental results

At a strain of 0.2, one or two systems of parallel “microbands” [13], delineated by planar boundaries, started to develop on a background of pre-existing tangled dislocation cells. The planar walls delineating microbands were frequently aligned approximately parallel to one of the {1 1 1} crystallographic slip planes, although some non-crystallographic walls were also detected. The corresponding misorientation angles were found to be largely less than 2°.

When reaching a strain of 0.5, the

Discussion

It has been observed in the present study that shear bands propagated across grains along a similar macroscopic path. Moreover, the well-recovered cells, comprising these bands, were rotated with respect to the surrounding matrix of microbands, as well as relative to each other, about axes clustered around a common macroscopic direction. This suggests that characteristics of shear bands were largely governed by the macroscopically imposed deformation geometry, which is in correspondence with

Conclusions

A detailed investigation of the substructure of shear bands, formed in an austenitic stainless steel during deformation in torsion at 900°C, has been undertaken in the present work. It has been observed that shear bands formed at large strains in a matrix of pre-existing microbands and their characteristics were predominantly governed by the macroscopically imposed deformation geometry. These bands were composed of well-recovered, slightly elongated cells, largely formed by the operation of

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This paper is dedicated to Professor Pavel Lukáč on the occasion of his 65th birthday.

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