Nanoindentation of palladium β- hydride

The Pd-H system is a unique one. High hydrogen permeation in palladium allows to use palladium like a membrane material to extra pure hydrogen gas production, as catalyzer for hydrogen penetration promotion in different metals, etc. For aims of hydrogen energy development, in which different types of Me-H systems are applied, most important feature of Pd-H system is in the fact that it is a classic system for hydrogen-metal interaction modelling. Really, all the Me-H systems have rather complicated equilibrium diagrams, and just thermodynamically opened Pd-H has a simple one,1 which looks like binary state cupola in coordinates T,oC, P, MPa, n(H/Pd). Despite the fact that Pd-H is under investigations for almost 200 years, this system is still full of surprises for researchers.1 The simplicity of Pd-H diagram is as follows. Under conditions corresponding to the left of the binary state cupola region (Figure 1), there is a dilute solid solution of hydrogen in palladium which is named α-phase. To the right of the cupola there is a saturated solid solution of hydrogen in palladium, (denoted as βphase, rarely α’-phase). If the figurative point of the sample crosses the two-phase region, a hydride transformation develops in the sample. Hydride transformation products cannot be detected by etching, so their morphology is studied by investigating the development of surface relief on a pre-polished metallographic cross-section in an optical microscope in oblique lighting.2 It was discovered3 that during both α→β and βα hydride transformations, the previously polished metallographic section is irreversibly deformed. As a result of hydride phase transformations, metals and alloys strongly harden and all their physical properties change. This phenomenon was called “hydrogen phase hardening” (HPN).4 If, however, the palladium sample is hydrogenated up to β-hydride state by the way “out” of the two-phase cupola, i.e. by such a way that the figurative point of the sample does not intersect the twophase region, then the hydride transformation does not develop and the metallographic cross-section remains generally unchanged.2 As there were no phase transformations2 proceeding by the way “out” of the cupola, the sample saves its preliminary annealed structure, and has β-phase through all its volume. So samples hydrogenated by the technique ‘out’ of the binary state cupola we name β-hydrides of palladium (β-PdHx). Mechanical properties are one of main features characterizing materials in general and metal-hydrogen alloys particularly. Classic techniques on mechanical properties study used by specialists were always tensile tests, hardness measurements and so on. After the pioneer work of Oliver and Farr on nanoindentation was published,5 through decades the nanoindentation technique became a tool for the measurement of mechanical properties at small scales and even can have greater importance in science as a technique for experimental studies of materials physics fundamentals.6 Despite the fact that nowadays this method is widely used for a large variety of materials, we could find an only work on nanoindentation of palladium-hydrogen, by J.M. Wheeler and T.W.Clyne.7 In that work nanoindentation had been used to track the mechanical effects of hydrogen on palladium foils over a range of hydrogen concentrations. There were electrolytically fulfilled hydrogenation/dehydrogenation cycles in conditions through binary state cupola on the Pd-H diagram and was found that nanoindentation can measure the extent of hydrogen-induced phase transformations across the film thickness after hydrogen removal, with the α → β → α phase transformations yielding a ∼50% increase in local hardness. The aim of our work was to fulfill palladium hydrogenation from the gaseous phase by the way ‘out’ of the binary state cupola on the Pd-H diagram up to hydride of palladium β-PdHx state. Then we targeted identifying different mechanical properties of the produced by this way β-PdHx by classic tensile tests and by nanoindentation.