Elsevier

Materials Letters

Volume 264, 1 April 2020, 127379
Materials Letters

An investigation by EXAFS of local atomic structure in an Mg-Nd alloy after processing by high-pressure torsion and ageing

https://doi.org/10.1016/j.matlet.2020.127379Get rights and content

Highlights

  • EXAFS was used to study the atomic environment of the HPT and aged Mg-1.44Nd alloy.

  • Two Nd-Mg bonds were found belonging to the α-Matrix and Mg12Nd second phase.

  • Interatomic distance and the coordination number are influenced by HPT and ageing.

Abstract

The local atomic structure of an Mg-1.44Nd (wt.%) alloy was investigated after solution annealing, high-pressure torsion (HPT) processing up to 1 and 10 turns and ageing at 250 °C for 5 h using X-ray absorption fine structure (XAFS) measurements at the Nd LIII-edge. The results show that HPT processing has no effect on the atomic structure around Nd atoms compared to the unprocessed state, whereas ageing at 250 °C for 5 h induces a significant modification in the coordination number and interatomic distances around the Nd atoms. These variations are analyzed based on the correlations between precipitation, defects and atomic mobility of the chemical species.

Introduction

Magnesium and its alloys are interesting materials for lightweight structural applications [1]. Nevertheless, these alloys exhibit in sufficient low temperature formability due to their limited slip systems and this serves to restrict their use for industrial applications [2]. In practice, small amounts of additions of rare-earth (RE) elements can improve the mechanical properties such as the ductility of Mg-based alloys [3]. In addition, the application of severe plastic deformation (SPD) processing, as in high-pressure torsion (HPT), has been used to achieve good mechanical properties through grain refinement and the introduction of high dislocation densities [4]. Thus, alloying elements and deformation processing may influence the sequence and kinetics of phase transformations such as recrystallization and precipitation [5].

The sequence and kinetics of precipitation were investigated in an Mg-1.44Nd (wt.%) alloy before and after HPT processing for up to 10 turns and ageing at 250 °C for 5 h using synchrotron X-ray diffraction and DSC measurements [5]. The results demonstrated that HPT processing modifies the kinetics of β1-Mg3Nd and β-Mg12Nd precipitation and decreases their activation energies relative to the non-deformed state [5].

There are no studies devoted to the local environment of solute species after SPD processing and ageing of Mg-RE alloys using X-ray absorption spectroscopy (XAS) techniques, for example. Thus, the aim of the present investigation was to examine the atomic structure environment around Nd atoms at the LIII-edge of the Mg-1.44Nd (wt.%) alloy before and after HPT processing and ageing using EXAFS measurementse.

Section snippets

Experimental

The Mg-1.44Nd (wt.%) alloy was supplied in an as-cast state. Disk samples were machined with diameters of 10 mm and solution annealed (SA) in sealed glass tubes at 535 °C for 6 h followed by water quenching. These disks were severely deformed by HPT at room temperature up to1 and 10 turns with a rotation speed of 1 rpm and an imposed pressure of 6.0 GPa.

The SA and HPT-processed samples were subjected to precipitation ageing at 250 °C up to 5 h and subsequently quenched in water. The samples

Results and discussion

Fig. 1 shows the X-ray absorption near edge spectroscopy (XANES) spectra collected at the Nd LIII-edge of the present alloy after different thermo-mechanical processing schemes. All spectra exhibit an intense peak (indicated by an arrow) which records the absorption at the LIII-edge of the Nd atom, followed by small peaks or oscillations due to constructive and destructive interferences which arise from the sum of the outgoing photoelectron wave from the Nd absorbing atoms and those

Summary and conclusions

  • 1.

    The local atomic structure around Nd atoms in the Mg-1.44Nd alloy was investigated using XAFS measurements after solution annealing, HPT processing and ageing at 250 °C up to 5 h.

  • 2.

    Excluding the β1-Mg3Nd phase, the EXAFS fitting for all samples showed that the first, third and fourth bonds were ascribed to the Nd-Mg (α-Matrix) bond while the second was associated with the Nd-Mg bond from the β-Mg12Nd second phase.

  • 3.

    The interatomic distance and the coordination number are strongly influenced by HPT

CRediT authorship contribution statement

Y.I. Bourezg: Writing - Original draft, Data curation. H. Azzeddine: Writing - Original draft, Data curation. M. Harfouche: Data curation. D. Thiaudiere: Investigation. C. Mocuta: Investigation. Y. Huang: Writing - Original draft. D. Bradai: Writing - Original draft, supervision. T. G. Langdon: Writing - Original draft, Format analysis, supervision.

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

Prof. D. Bradai wishes to deeply acknowledge the support of SOLEIL synchrotron, Gif-sur-Yvette Cedex, France, during the 20150973 proposal. Prof. T.G. Langdon and Dr. Yi Huang were supported by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS.

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    In addition, both conventional and severe plastic deformation (SPD) are known to improve the mechanicals properties, decrease the basal texture strength and enhance the material formability [7–10]. These deformation processing are also able to affect the sequence, kinetics and morphology of solid-state transformations such as recrystallization and precipitation [11–16]. Frequently, post-deformation annealing treatment (PDA) of severely deformed alloys is often carried out to reach a more or less stable state by recovery and recrystallization [17].

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