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