Hydrogenation effect on microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys

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Abstract

This work explores the ways of manipulating the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys of various compositions using hydrogen treatment. Changes to phase composition, microstructure, and mechanical properties of the alloys upon hydrogenation were studied. Prior to hydrogenation, the alloys were extruded at different temperatures with or without subsequent aging. Hydrogen treatment was performed on bulk rods after thermo-mechanical processing. X-ray diffraction and scanning electron microscopy studies showed that a single rare-earth (RE) hydride phase, Gd0.5Y0.5H2, was formed in all samples. As a result, the 14H long period stacking ordered (LPSO) structure, detected before hydrogenation, is completely destroyed due to the clustering of RE atoms into large hydride crystals and annihilation of the specific spatial order within the superlattice. Prolonged hydrogen treatment at high temperature (≈ 0.74·Tm) causes recrystallization and grain growth in the magnesium matrix, defect annealing and reduction of preferred orientation, which together with the complete destruction of the LPSO phase lead to the substantial decrease of alloys’ strength and concomitant increase of their ductility to the record-high value of above 20%.

Introduction

The Magnesium-Rare Earth (Mg-RE) alloys are a subject of intensive study [1], [2], [3] because of their enhanced mechanical and creep resistance properties. Specifically, the binary and ternary alloys of different systems, e.g. MgYZn and MgGdZn, have been investigated in detail [2], [3]. It has been shown that several strengthening mechanisms contribute to the improvement of mechanical properties, such as solution strengthening, formation of long period stacking ordered (LPSO) structures, precipitation of β′ -phase during solid solution decomposition, grain refinement under hot deformation and precipitation of secondary phases during ageing [4], [5]. Particular attention was paid to formation mechanisms of the LPSO phase.

The optimization of mechanical properties can be achieved by using two different RE-metals for Mg alloying. The synergic strengthening effect of two RE-metals was observed in Mg-Gd-Y alloys, where mechanical properties were significantly improved after hot deformation and ageing [6], [7], [8], [9]. Moreover, it was shown [10] that small variations in the concentration of alloying elements not only cause the strength increase, but completely eliminate asymmetry in yield stress due to changes in the degree of preferred orientation, as well as the LPSO type and its volume fraction. The enhancement of mechanical properties of Mg-RE alloys was also observed by adding non-RE elements, such as Zr, Zn Al, Ni or Ca [11], [12], [13], [14], [15].

With few exceptions, hydrogen atoms dissolve in metals and alloys causing a catastrophic decrease of their ductility, a phenomenon known as hydrogen embrittlement [16]. Studies on the effect of dissolved hydrogen on mechanical properties in pure Mg revealed that dissolved hydrogen causes some increase of the brittle-to-ductile transition temperature, though the quantitative change in the stress-strain curves is negligible [17]. The intergranular fracture in embrittled hydrogen-rich Mg was attributed to hydrogen segregation at the triple junctions of the grain boundaries [17]. It should be noted that the equilibrium solubility of hydrogen in Mg is very low, at the level of several tens of ppm, though it can be significantly increased in rapidly solidified Mg alloys [18].

As RE metals have a high affinity for hydrogen and a high stability of the respective hydrides, the microstructure and properties of bulk structural Mg-RE alloys could be manipulated by annealing them in hydrogen atmosphere, without significant conversion of the Mg matrix into MgH2 hydride. To avoid the formation of the MgH2, the alloy should be heat-treated under hydrogen pressure that is lower than the plateau pressure corresponding to the Mg-MgH2 equilibrium at a given temperature, yet higher than the respective equilibrium hydrogen pressure of the RE hydride. Such hydrogenation treatment will change the structure of RE-rich phases and precipitates and modify their interface with the Mg matrix, resulting in the formation of new stable RE hydrides precipitates and alteration the mechanical properties of the alloy. It is expected that the change of mechanical properties of the treated alloys due to hydrogen-induced changes of their microstructure will be much larger than the intrinsic changes of the mechanical properties of Mg matrix due to hydrogen embrittlement [16]. To the best of our knowledge, such an approach to modifying the microstructure and mechanical properties of structural Mg-RE alloys has not been reported in literature.

It is important to note that the interaction of Mg-RE alloys with hydrogen has been studied in several works in the context of hydrogen storage through the formation of the MgH2 phase. It was hypothesized that abundance of interfaces in the LPSO structure may provide additional fast paths for hydrogen diffusion and accelerate the hydrogenation reaction of Mg matrix. The kinetics of hydrogen adsorption/desorption in magnesium alloys with LPSO structures, namely Mg-Zn-Y system [19], [20] and Mg-Ni-Y system [21], [22], [23] have been investigated. It was shown that LPSO phase decomposes into Mg and Y-hydrides for the MgZnY system and Mg and MgNi-hydrides for the MgNiY system, which accelerates the kinetics of formation of the MgH2 hydride. These studies, as well as the majority of other studies on the formation of MgH2 in Mg-based alloys, were performed with dispersed powders (with particle sizes up to a few tens of micrometers), thin tapes or powders prepared by rapid solidification technique, or pelletized porous compacts [24]. This is preconditioned by low diffusivity of hydrogen through the MgH2 hydride phase, which prevents the full hydrogenation of the bulk specimens. A hydride layer only a few micrometers thick formes on the surface of bulk samples [25]. For this reason why, to the best of our knowledge, no studies of the hydrogenation effect on LPSO in bulk Mg-alloys are documented in literature.

In this work, the hydrogenation behaviour, structural changes and mechanical properties after hydrogenation are investigated for two Mg-Gd-Y-Zn-Zr alloys with different contents of Gd, Y, and Zn. The bulk rod samples were hydrogenated under conditions preventing formation of MgH2.

Section snippets

Experimental

Two ingots with compositions, Mg-8Gd-4Y-1Zn-0.4Zr (wt%) (herein referred to as M1) and Mg-10Gd-5Y-1.8Zn-0.4Zr (wt%) (herein referred to as M2), were prepared by induction melting of pure Mg (99.99%) and Zn with three master alloys (compositions in wt%): Mg-25Y, Mg-25Gd, and, Mg-25Zr. Melting was performed in a titanium crucible under argon atmosphere at temperature of 760 °C after preheating the mould to 130 °C. The molten alloys were casted into the water-cooled brass moulds. Thus fabricated

Results and discussion

The gravimetric hydrogen absorption curves of the M1 and M2 alloys in Fig. 1 show typical saturation behaviour occurred after 10–15 h of absorption. The hydrogen amount absorbed by the samples is between approximately 0.18 wt% in M1 samples and 0.24 wt% in M2 samples. Similar results were obtained for M1a and M2a alloys. The difference in absorbed hydrogen between the M1 and M2 samples is seemingly related to the higher concentration of Gd and Y in the M2, M2a alloys. In order to estimate the

Conclusions

Hydrogenation of bulk Mg-Gd-Y-Zn-Zr alloys with different amounts of RE elements and different volume fractions of 14H LPSO results in the formation of the cube-shaped and plate-like Gd0.5Y0.5H2-hydride crystals within the samples’ volume. XRD and SEM data provide solid evidence that no other hydrides, including Mg-hydride (MgH2), were formed in any of the compositions and for all heat treatments used.

The hydrogen absorption capacity increased with RE content from 4.9 at% (for M1 and M1a alloys

Acknowledgements

Prof. R. Lapovok acknowledges financial support through the Lady Davis Fellowship (2014-15) of her four month working visit to Technion. The authors express their gratitude to technical and engineering staff of Technion. The authors are thankful to Mr. Kyle Stephan Nicholson at Deakin University for useful discussion.

References (35)

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