Elsevier

Acta Materialia

Volume 84, 1 February 2015, Pages 80-94
Acta Materialia

Effect of crystallographic texture on mechanical properties in porous magnesium with oriented cylindrical pores

https://doi.org/10.1016/j.actamat.2014.10.024Get rights and content

Abstract

The tensile and compressive deformation in porous Mg with unidirectionally oriented cylindrical pores and a unique fiber texture in which the normal direction of the {1 0 1¯ 3} plane was preferentially oriented was studied. Porous Mg specimens with unidirectional pores and texture were prepared by unidirectional solidification in a hydrogen atmosphere using a continuous-casting technique and their quasi-static tensile deformation and quasi-static and dynamic compressions were investigated. In tensile loading parallel to the orientation direction of the pores (the “pore direction”), the porous Mg exhibited a large tensile elongation of ∼60% strain despite the presence of ∼42% porosity, whereas it showed high energy absorption of ∼30 kJ kg−1 along the same direction. To clarify these superior mechanical properties, the underlying operative deformation modes and rotation of crystallographic orientation during loadings were analyzed by X-ray pole figures, optical microscopy and crystal plasticity finite-element modeling. The analyses revealed that in the initial stage of both the compression and tensile loadings along the pore direction, basal slip mainly operated. Importantly, the activity of basal slip was enhanced during the tensile loading by rotation of the crystallographic orientation, which resulted in high tensile elongation. On the other hand, the activation of basal slip was initially suppressed by the crystal rotation during compression. However, the localization of basal slip originating from the elongated grains with the unique texture subsequently enhanced the activity of basal slip, which suppressed the steep increase in the flow stress. This unique localized deformation gave rise to the superior impact energy absorption.

Introduction

Porous metals or metallic foams are increasingly being used in a variety of engineering applications because of their unique properties, such as energy absorption, sound absorption, thermal insulation and fluid permeability [1], [2], [3]. These unique properties stem from the porosity of the material; however, the porosity also causes simultaneous degradation of the mechanical properties [4], [5], [6], [7]. For open-cell and closed-cell metallic foams with high porosity of more than ∼80% the compressive yield stress and ultimate tensile strength are significantly lower than those of the matrix material, and the tensile elongation is usually limited to <10% [4], [5], [6], [7], [8], [9], [10], [11]. To expand the applications of porous metals further, there is significant demand to improve their mechanical properties. In the present study, therefore, we focused on the simultaneous control of the pore morphology and crystallographic texture in the matrix metal.

We first focused on enhancing the mechanical properties of porous Mg by controlling the matrix texture. In most porous metals, Al and its alloys are frequently employed as the matrix materials [4], [8], [11], [12], [13] because of their light weight and superior mechanical properties. However, research on porous Mg and its alloys is minimal compared with that on porous Al, even though porous Mg exhibits ultra-lightweight properties [14], [15], [16], [17], [18], [19]. The reasons may be related to the flammability, low-corrosion resistance and brittleness of the Mg matrix, which originates from its hexagonal close-packed structure with strong crystallographic plastic anisotropy. Recently, improving the ductility in Mg and its alloys by controlling the material texture has attracted much interest [20], [21]; in AZ31Mg alloy, the uniform tensile elongation was increased from ∼20 to 45% due to the formation of texture [20]. Thus, it may also be possible to enhance the ductility in porous Mg by controlling the texture.

Next, we focused on the simultaneous control of pore morphology and matrix texture in the porous Mg. Recently, porous metals with oriented, elongated pores have attracted much interest [22], [23], [24], [25], [26]. Such porous metals exhibit superior mechanical properties in the orientation direction of the pores (hereinafter the “pore direction”) because the stress concentration around pores caused by the loading is minimal. Therefore, for porous Mg, the combination of unidirectional pores and a controlled texture could possibly give rise to significant improvement of the mechanical properties, a topic that has not been studied yet.

In the present study, the tensile and compressive deformation of porous Mg with oriented cylindrical pores and a controlled crystallographic texture was studied, with particular focus on the effect of crystallographic texture. The operative deformation modes and changes in texture during deformation were analyzed using scanning electron microscopy (SEM), optical microscopy, X-ray pole figures, and crystal plasticity finite-element modeling (FEM), and their effects on the deformation were discussed.

Section snippets

Experimental procedure

Porous Mg ingots with oriented cylindrical pores were prepared using a continuous-casting method [27] in a mixed atmosphere of hydrogen and argon. Raw-material Mg ingots were melted at 973 K and the melt was unidirectionally solidified; the cross-section of the mold was 18 × 50 mm2. Mg ingots of 99.9% purity were used as the raw material. As a reference, a nonporous Mg ingot was prepared using unidirectional solidification in a mixed atmosphere of hydrogen and argon by the continuous casting of the

Microstructure and porous structure

Fig. 1 shows the pore morphologies of the prepared porous Mg specimens in cross-sections (a) perpendicular and (b) parallel to the solidification direction. In Fig. 1a and b, the pores are spherical and elongated along the solidification direction, respectively. This indicates that cylindrical pores elongated along the solidification direction were formed during the unidirectional solidification, as reported previously [31], [32]. The average porosity and pore diameter of the prepared specimens

Analysis of underlying operative deformation modes by crystal plasticity finite-element modeling

For a detailed understanding of the high tensile elongation and superior energy absorption in the porous Mg, the underlying operative deformation modes were analyzed using crystal plasticity-based FEM. In this modeling, the effects of anisotropic pore shape, texture, grain shape, elastic properties, rotation of crystallographic orientation, and the operative deformation modes of basal, prismatic and pyramidal slips, and deformation twinning were taken into account.

Conclusions

Tensile and compressive deformations in porous Mg with unidirectionally oriented cylindrical pores and fiber texture were studied. Porous Mg specimens with unidirectional pores and texture were prepared by unidirectional solidification in a hydrogen atmosphere using a CC technique. Quasi-static tensile deformation and quasi-static and dynamic compressions in these samples were investigated using a universal testing machine and the SHPB method. In addition, the underlying operative deformation

Acknowledgements

This work was supported by JSPS KAKENHI Grant Nos. 24109505, 26109713 and 26709053 and a research grant from the Light Metal Educational Foundation. This work was also supported by the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”). Analyses by XRD and X-ray pole figure were performed at the Comprehensive Analysis Center, ISIR, Osaka University. The authors would like to thank Dr. T. Ide of Osaka University for the preparation of nonporous and porous

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    Present address: The Wakasa Wan Energy Research Center, 64-52-1 Nagatani, Tsuruga, Japan.

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