On the evolution of cyclic deformation microstructure during relaxation test in austenitic stainless steel at 823 K

doi:10.1016/j.msea.2006.12.167

Materials Science and Engineering: A

Volumes 483–484, 15 June 2008, Pages 422-425

https://doi.org/10.1016/j.msea.2006.12.167 Get rights and content

Abstract

The evolution of cyclic patterns during relaxation time has been studied in terms of long-range internal stresses and effective stress evolution as a function of time at 823 K. The main features are the decrease of intra-granular back stress in relation to dipole annihilation promoted by self-diffusion and a decrease of inter-granular back stress which results in a homogenization of dislocation distribution in all the grains. Moreover, the static aging leads to an increase of the athermal component of the effective stress in association with the hardening effect of solution segregation on dislocation junctions.

Introduction

In the past, the creep–fatigue interaction has been extensively studied in austenitic stainless steel at high temperatures [1], [2], [3], [4], [5]. Generally, a decrease of stress amplitude Σ_a is observed as a function of the hold time, t_m [1], [2], [3], [5]. However, some time a previous hardening was reported before the softening stage [5]. The origin of this hardening stays unclear. However, the “time softening” has been correlated with recovery of dislocation structures. It was suggested that phenomena arise in association with dipole annihilation in dislocation walls [4], [5]. The competition between dipole annihilation during the hold time and the formation of this one during cyclic loop conduces to an impossibility to identify the kinetic parameters associated with recovery process. Consequently, in present study we propose an alternative experimental procedure to evaluate recovery of dislocation structure arising to cyclic loading. Moreover, this experimental approach allows discussing the origin of creep–fatigue behaviour at high temperature (823 K).

Section snippets

Experimental procedures

Experiments were performed on polycrystalline austenitic stainless steel AISI 316L. This alloy presents an average grain size of 53 μm and a dislocation density below 10¹⁰ m⁻² [6], [7]. Cylindrical specimens with a 8 mm diameter and a 10 mm gauge length were machined for tension-compression tests. All tests were performed at 823 K with a constant strain rate of 3 × 10⁻³ s⁻¹ and a true plastic strain control. In a first step, a cyclic loading is conduced to stress saturated stage with a constant plastic

Experimental results

After cyclic loading sequence the saturation stress reaches a value equal to 370 MPa. This stress is partially reduced during relaxation sequence to reach a value near 260 MPa after 4 days. The evolution of the different components (X, Σ_μ, Σ^*) of stress during relaxation stage is presented in Fig. 1b. Back stress X and thermal part of effective stress Σ^* decrease as a function of time. However, athermal part of effective stress Σ_μ increases as a function of time. These results can be correlated

Discussion

According with a large number of works on cyclic behaviour of fcc metals, there exist not only forest dislocations but also debris such as long dipole loops and point defects in fatigue deformation. Point defects like vacancies intersected by mobile dislocations give rise to much larger temperature and strain rate sensitivity than for tensile deformation. Under a relaxation sequence vacancies can be also contributed to dipole annihilation. Recently, TEM investigations of dipole height in

Conclusion

The physical origin of the time relaxation of cyclic saturation stress has been identified in austenitic stainless steel at 823 K. Two mechanisms occur during relaxation sequence: the dipole annihilation promoted by self-diffusion and static aging in relation with chrome segregation on dislocation junctions.

Acknowledgments

The authors thank the Centre Commun d’Analyse, Université de La Rochelle for electron microscopy facilities and the Roberval Laboratory, Université de Technologie de Compiègne for fatigue testing facilities. Financial support from Commissariat à l’Energie Atomique is gratefully acknowledged.

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On the evolution of cyclic deformation microstructure during relaxation test in austenitic stainless steel at 823 K

Abstract

Introduction

Section snippets

Experimental procedures

Experimental results

Discussion

Conclusion

Acknowledgments

Fatigue Eng. Mater. Struct.

Metall. Trans. A

Nucl. Eng. Des.

Philos. Mag. A

Nucl. Eng. Des.

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Rec. Res. Dev. Mater. Sci.