Effect of temperature on deformation mechanisms of AZ31 Mg-alloy under tensile loading

https://doi.org/10.1016/j.msea.2020.138957Get rights and content

Abstract

Controlled in situ SEM tensile tests have been carried out between 200 and 300 °C at a constant strain rate of 5.10-5 s-1 to investigate the effect of temperature on deformation mechanisms operating in an Mg–3Al–1Zn (AZ31) Mg-alloy. Fiducial microgrids deposited using electron beam lithography are used to evidence grain boundary sliding as well as to determine the spatial strain heterogeneities as a function of temperature. Dislocation creep and grain boundary sliding coexist between 200 and 300 °C but their respective activity varies significantly as shown by the strain rate sensitivity value m which is about 0.2 at 200 °C but about 0.5 at both 250 and 300 °C. In addition, grain boundary sliding becomes predominant at 250 and 300 °C whereas its occurrence is relatively limited at 200 °C. Slip trace analysis shows that at 200 °C prism and pyramidal <c+a> slip already exhibit a great activity. Spatial strain heterogeneities determined by digital image correlation (DIC) based on microgrid displacements develop during the early stage of plastic deformation and persist at larger strains. It is shown that the strain in the vicinity of grain boundaries intensifies when the temperature rises from 200 to 300 °C while the core of grains accommodates less deformation in agreement with the fact that grain boundary sliding is predominant at 250 and 300 °C.

Introduction

Despite their low density, the use of conventional magnesium alloys in the industry remains limited, in particular, due to their poor workability at room temperature (RT) associated with the hexagonal close-packed structure. At RT, Mg alloys deform plastically mainly by dislocation glide on the basal plane and twinning, limiting their ductility. This lack of ductility can be accounted for by the fact that the aforementioned deformation mechanisms do not allow to accommodate strains along the c-axis. In addition, wrought Mg-alloys, typically the AZ-series, exhibit strong basal textures, leading to significant anisotropy, i.e. Lankford coefficient can reach 4 or more, see e.g. Refs. [1,2]. When the temperature increases beyond 150 °C, twinning activity vanishes and additional slip systems, namely prism and pyramidal <c+a> slips become easier to operate, see Refs. [[3], [4], [5], [6], [7]] among others. This results in a great improvement of the deformability that goes along with a reduced plastic anisotropy, see e.g. Refs. [2,8]. Finally, for alloys exhibiting relatively fine grains, typically less than 20 μm, grain boundary sliding can play a key role which results in an increase of the strain rate sensitivity parameter (m = ∂σ/∂ε) and, in some cases, in superplastic properties. Superplasticity of Mg-alloys has been widely reported and studied, see e.g. Refs. [[9], [10], [11], [12], [13]]. Cavitation, identified as the damage mechanism in superplastic conditions has also received attention from the community [14,15].

Nevertheless, the detailed effect of temperature on plastic deformation of Mg-alloys remains an open question and is still debated. In particular, the simultaneous activation of various deformation mechanisms as well as data related to microscale plastic strain distribution with temperature in Mg-alloys are not yet fully available and would deserve to be deeply investigated. A path to provide new insights regarding the effect of temperature on the plastic strain distribution in Mg-alloys can be to carry out highly-controlled mechanical tests including continuous strain mapping by Digital Image Correlation (DIC). At RT, recent work has contributed to determine local strain fields and the associated deformation mechanisms, see in particular [[16], [17], [18]]. However, running such mechanical tests at high temperatures (here >200 °C) is not an easy task. First Mg-alloys are prone to oxidation at high temperature, hence continuous imaging of the surface for further DIC analyses requires to work under vacuum. In situ mechanical tests within the SEM fulfills such a requirement but due to its low vapor pressure Mg might easily evaporate during high temperature (T > 250 °C) mechanical testing which can cause severe damage to the SEM. Finally, achieving a good control of the temperature when straining the sample during in situ tensile tests remains a challenge that needs to be overcome. For all these reasons, investigating high-temperature deformation of Mg-alloys using DIC during in situ SEM tensile tests is poorly documented, only some attempts to determine high-temperature strain fields can be found in Ref. [2]. Most of the studies employing DIC to investigate the deformation behavior of Mg-alloys have been conducted at RT [[16], [17], [18], [19]]. A recent study by Orozco-Caballero et al. [16] has shown how magnesium accommodates local deformation incompatibility at RT relying on HR-DIC. Note that in the present work, we did not use HR-DIC but we rather use fiducial microgrids allowing to reveal the occurrence of grain boundary sliding when this mechanism becomes predominant in superplastic conditions.

In the present work, highly controlled high temperature in situ tensile tests including DIC were performed in an SEM (controlled temperature within ±5 °C and controlled strain rate, see details in Ref. [20]). Particular attention was given (i) to the identification of deformation mechanisms as revealed by fiducial microgrids and slip trace analysis; and (ii) to the extent of strain localization through the microstructure as a function of temperature.

Section snippets

Materials and specimen preparation

The as-received material was a commercial AZ31 Mg-alloy (3 wt%Al-1wt%Zn-0.4 wt%Mn) hot-rolled sheet with an initial thickness of 2 mm supplied by Satzgitter Magnesium Technology GmbH. The hot-rolled sheet was annealed at 345 °C for 1 h to achieve the “O” metallurgical state.

Samples were mechanically ground and polished using diamond (3 and 1 μm) and alumina (0.3 μm) suspensions. A final electro-polishing stage in a solution consisting of phosphoric acid (60%) and ethanol (40%) at 20 °C under 3V

Rheology

To characterize the rheology of the fine grained AZ31 alloy at high temperatures, both sets of mechanical testing were carried out.

First, strain-rate controlled in situ high-temperature tensile tests were conducted at 200, 250 and 300 °C at 5.10-5 s-1. Recording the true-stress/strain response during in situ tensile tests within the SEM is not straightforward, in particular, because no local extensometer attached to gauge length could be used to accurately measure the displacement, hence strain

Effect of macroscopic tensile strain

We first looked at the effect of strain on the development of deformation heterogeneities by plotting strain maps after different strain increments. An example is given in Fig. 8 where the spatial strain heterogeneity at 300 °C within the region containing the microgrid is plotted for various macroscopic tensile strain. The strains calculated by DIC are represented as color maps corresponding to the magnitude of the normalized equivalent strain: εeq/<εeq>. The normalized equivalent strain is

Conclusions

The main conclusions emerging from the present work can be drawn as follows:

  • A specific environment has been developed to conduct in situ strain-rate controlled high-temperature tensile tests within the SEM of Mg-alloys. This set-up was coupled with EBSD and DIC measurements allowing to provide new insights into the deformation mechanisms and the development of microscale strain heterogeneities as a function of temperature.

  • Dislocation creep and grain boundary sliding (GBS) coexist between 200

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

CRediT authorship contribution statement

Thibaut Dessolier: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Visualization. Pierre Lhuissier: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Resources, Writing - review & editing, Supervision. Francine Roussel-Dherbey: Validation, Investigation, Resources. Frédéric Charlot: Validation, Investigation, Resources. Charles Josserond: Software, Validation, Resources. Jean-Jacques Blandin: Conceptualization, Methodology,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was performed within the framework of the Center of Excellence of Multifunctional Architectured Materials ‘‘CEMAM’’ n°AN-10-LABX-44-01 funded by the ‘‘Investments for the Future Program’’. Doctoral school I-MEP2 is gratefully acknowledged for funding.

References (34)

Cited by (9)

  • Comparison on crack propagation under tension at 150 °C of Mg-2Zn-1.5Mn alloy sheets with and without crack notch

    2023, Journal of Magnesium and Alloys
    Citation Excerpt :

    As the temperature increases, the activation of non-basal slips (prismatic and pyramidal slips) increases due to the decreased CRSS [34]. Dessolier et al. [35] reported that prismatic and pyramidal 〈c + a〉 slips exhibited a great activity under tensile loading for AZ31 sheet at 200 °C through slip trace analysis. Therefore, the operation of prismatic 〈a〉 slip during tension at 150 °C in ZM21 alloy sheet is mainly ascribed to the decreased CRSS at elevated temperature in this work.

  • Uncovering the unexpected changes of creep properties in AZ-series Mg alloys

    2022, Materials Science and Engineering: A
    Citation Excerpt :

    But if the shape and the volume fraction of precipitates are considered, the dislocation mobility can turn out to be another reason for the different creep properties. As is well-known, AZ31 alloy hardly has any obvious precipitation under creep loading [47]. Thus, the dislocations have abundant spaces for movement once they are activated.

  • Elementary growth mechanisms of creep cavities in AZ31 alloy revealed by in situ X-ray nano-tomography

    2022, Acta Materialia
    Citation Excerpt :

    The average grain size measured was 12 µm with a standard deviation of 4 µm. More information about the material, microstructure and its high temperature deformation behavior can be found in [15–18]. For the experiment, a constant tensile load was applied to the sample at high temperature, while it was simultaneously imaged using X-Ray single-distance, phase contrast nano-tomography.

  • Overcoming the trade-off between stretch formability and heat resistance in magnesium via alloying dilute neodymium

    2022, Journal of Alloys and Compounds
    Citation Excerpt :

    But on the other hand, fine grain size and weak texture are always against the creep resistance. At elevated temperatures, grain boundary has a low strength and grain boundary sliding becomes easier to activate compared to the case at room temperature [24]. Meanwhile, the number of active slip systems increases, leading to frequent dislocation motions [25,26].

View all citing articles on Scopus
View full text