Abstract
Magnesium and hafnium, two hydride-forming and biocompatible metals with hexagonal close-packed crystal structures, are thermodynamically immiscible even in the liquid form. In this study, these two elements were mechanically mixed by high-pressure torsion straining, and a new FCC (face-centered cubic) phase was formed although these two elements do not form the FCC phase even under high pressure or at high temperature. Microstructural examination by scanning-transmission electron microscopy combined with an ASTAR automatic crystal orientation and phase mapping technique confirmed that the FCC phase was stabilized mainly in the Hf-rich nanograins with localized supersaturation. Attempts to control the phase transformations under a hydrogen atmosphere to produce ternary magnesium–hafnium hydrides for hydrogen storage applications were unsuccessful; however, the material exhibited enhanced hardness to an acceptable level for some biomedical applications.
Highlights
Mg-based binary systems containing an element from the group 4 of the periodic table (Ti, Zr, and Hf) are of high scientific interest for hydrogen storage.[1]
x-ray diffraction (XRD) profiles illustrate that the powder mixture contains HCP-Mg and HCP-Hf, but new peaks appear after highpressure torsion (HPT) processing, and the intensity of these peaks increases with an increase in the number of turns from 10 to 100
Detailed examination of the XRD profiles using the Rietveld analysis, as summarized in Table I, confirms that the new peaks correspond to an FCC phase with a lattice parameter of a = 0.4670–0.4671 nm and a volume fraction of ∼7.5% after 100 turns
Summary
Mg-based binary systems containing an element from the group 4 of the periodic table (Ti, Zr, and Hf) are of high scientific interest for hydrogen storage.[1] First-principles calculations suggested that some particular ternary hydrides in these systems can have low hydrogen binding energy compared with MgH2, which makes them potential candidates for low-temperature hydrogen storage.[2] the main drawback of these systems is their full thermodynamic immiscibility in the solid and liquid forms.[3,4] There have been a few attempts to mix these elements using high-energy ball milling[5,6,7,8,9] and severe plastic deformation (SPD) via the highpressure torsion (HPT) method.[10,11,12,13,14] These studies reported that mechanical alloying can result in the formation of HCP (hexagonal close-packed), BCC (body-centered cubic), and FCC (face-centered cubic) metallic phases and ternary cubic hydrides in Mg–Ti5–8,10,11 and Mg–Zr9,12 systems Despite these studies on Mg–Ti and Mg–Zr, there have been no attempts to examine phase transformations in the Mg–Hf system. One main advantage of the HPT method, compared with the ball milling technique, is the absence of processing contaminations, which makes the interpretation of results more reliable.[23,24] the HPT method induces shear strain under high pressure, which is quite effective in controlling phase transformations[25,26] and in synthesizing new phases in immiscible Mg-based systems.[27,28] this article reports the first results of the application of the HPT method to the binary Mg–Hf system, there have been earlier studies on the application of HPT to pure Mg29–32 and pure Hf.[33,34] These studies reported no phase transformations in the HPT-processed Mg and Hf systems within the detection limits of x-ray diffraction (XRD)[29–33] a study using transmission electron microscopy (TEM) suggested the formation of a monoclinic phase in the HPT-processed Hf system.[34]
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