Formation mechanisms of nanostructures in stainless steel during high-strain-rate severe plastic deformation

doi:10.1016/j.msea.2005.08.022

Materials Science and Engineering: A

Volumes 410–411, 25 November 2005, Pages 252-256

https://doi.org/10.1016/j.msea.2005.08.022 Get rights and content

Abstract

We have investigated the formation mechanisms of nanostructures within adiabatic shear bands formed in stainless steel samples deformed by high-strain-rate forced shear. Twinning is shown to play a critical role in the initiation of nanostructures. Secondary twins directly led to the formation of elongated subgrains. Microtwins inside shear bands promoted division and break-down of the subgrains, which further refined the microstructures.

Introduction

Adiabatic shear localization can be considered as a unique consequence of severe plastic deformation (SPD) at high-strain-rates. The process is controlled by a thermally assisted mechanism. Once thermal and strain softening leads to rapid deformation localization, a band-like region (shear band) forms via a nearly adiabatic process. The total shear strain within shear bands may reach 5–20, which qualifies it as severe plastic deformation. Experiments show that significant grain refinement can occur within shear bands. Although the formation and microstructural characteristics of shear bands have been extensively reported in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], little systematic work has been conducted on the formation mechanisms of nanostructures within shear bands. These nanostructures are formed under very high-strain-rate, a condition very different from those in conventional severe plastic deformations [10].

Various fine substructures within shear bands with sizes ranging from a few nanometers to submicrons have been reported [11], [12], [13], [14]. Several mechanisms have been proposed to explain the formation of these substructures. They include phase transformation [2], recrystallization [12], dynamic recovery [13] and mixed modes. There is no consensus concerning which mechanism is the controlling one. Large plastic deformation, high-strain-rate and high local temperature are expected to affect the structural evolution within a shear band. Yamaguchi et al. [10] reported that higher strain rate did not display a significant effect on the microstructure formed during equal channel angular pressing (ECAP). However, the strain rate in their work is much lower than that experienced inside a shear band. Murr et al. [15] discussed the grain refinement in shear bands as the result of the SPD process but did not clarify the formation mechanism of the substructures. To our knowledge, no systematic experimental study has been reported on the formation mechanisms of nanostructures in materials subjected to high-strain-rate severe plastic deformation.

This paper reports a detailed study on the nanostructures formed within shear bands in cold-rolled AISI 316L stainless steel deformed by a Hopkinson bar test at a shear strain rate of 10⁵ s⁻¹. The formation mechanisms are discussed based on our experimental observations.

Section snippets

Experimental procedures

A cold-rolled AISI 316L stainless steel (SS 316L) plate was used in the current study to investigate nanoscale deformation within shear bands. The cold-rolled SS 316L plate was obtained from an annealed plate subjected to multiple passes of cold rolling to an accumulated strain of 32%. The composition of SS 316L is 0.019 C, 16.82 Cr, 1.72 Mn, 2.07 Mo, 10.04 Ni, 0.028 P, 0.01 S, 0.4 Si and 68.1 Fe, all in weight percentages. Forced shear experiments were carried out on a compressive split

Results and discussions

The microstructure of the cold-rolled SS 316L prior to forced shear is shown in Fig. 2. The grains were seen to be elongated slightly along the rolling direction and have an average grain size of 36 μm. A high-density of deformation twins was observed in all grains. Extensively intersected twin boundaries constructed a network pattern in some of the heavily deformed grains. We call these twins the primary twins.

An adiabatic shear band (ASB) was observed in the sample sheared at high-strain-rate.

Conclusions

Grain refinement within adiabatic shear localization, an extreme case of severe plastic deformation at high-strain-rates was studied in a 316L stainless steel using TEM. Multiplication of deformation twins was found to play an important role in the formation of substructures within shear bands. The secondary twins were aligned to the shear direction and have fine spacing close to the width of the elongated substructure inside the shear band and became the dominant initial substructure. A

Acknowledgement

This work is supported by the US DOE/DoD MOU Program and DOE IPP and BES Programs.

References (21)

T.G. Shawki et al.
Mech. Mater.
(1989)
A. Marchand et al.
J. Mech. Phys. Solids
(1988)
M. Zhou et al.
J. Mech. Phys. Solids
(1996)
S.P. Timothy
Acta Metall.
(1987)
D. Yamaguchi et al.
Scr. Mater.
(1999)
M.A. Meyers et al.
Acta Metall.
(1986)
K. Cho et al.
Acta Metall. Mater.
(1993)
Y.B. Xu et al.
Acta Mater.
(1996)
M.E. Backman et al.
H.C. Rogers
Ann. Rev. Mater. Sci.
(1979)

There are more references available in the full text version of this article.

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Formation mechanisms of nanostructures in stainless steel during high-strain-rate severe plastic deformation

Abstract

Introduction

Section snippets

Experimental procedures

Results and discussions

Conclusions

Acknowledgement

Mech. Mater.

J. Mech. Phys. Solids

J. Mech. Phys. Solids

Acta Metall.

Scr. Mater.

Acta Metall.

Acta Metall. Mater.

Acta Mater.

Ann. Rev. Mater. Sci.