The Effects of Hydrogen on the Physical Properties of Palladium
The Effects of Hydrogen on the Physical Properties of Palladium
Hydrogen-Phase “Naklep” Phenomenon During the Hydrogen Treatment of Palladium
Palladium is not polymorphic, which is why the physical metallurgy of palladium and its alloys is quite poor in comparison with those of polymorphic metals (such as iron, titanium, and others). In recent years advances in the physical metallurgy of palladium and its treatment have been taking place. The changes are based upon the physical effects that hydrogen has upon palladium when it is taken into the metal (1). In Ukraine and Russia this has been called “hydrogen-phase naklep” and was first noted in 1972. This observation has led to an improved concept in materials science and engineering (1–5).
When palladium is saturated with hydrogen, it becomes a polymorphic material and hydride transformations may be induced in it by processes of heating ↔ cooling and/or saturation ↔ desaturation. As there are differences between the specific volumes of the α-hydrogen solid solution and (β-hydride phases in the palladium, these hydrogen-induced phase transformations cause internal plastic deformation (relaxation) and controllable strengthening of the palladium. Some basic changes of structure also occur: the density of dislocations grows, the size of the ‘mosaic’ blocks decreases while their number increases, the angle of their disorientation increases, and so on.
Unlike the internal cold work which occurs during the martensitic transformations in iron alloys, the structure changes mentioned above in palladium-hydrogen alloys are not only caused by internal plastic deformation but also by the development of some very specific processes of interactions of the solute hydrogen in the palladium, generating hydrogen-containing phases and producing defects in the crystalline structure.
Annealed palladium has relatively poor mechanical properties, namely:
Ultimate strength σB = 180–200 MPa;
Yield limit σ0.2 =50–100 MPa;
Relative elongation δ = 20–25 per cent.
An early noted HPN-phenomenon (1) was achieved by thermocycling specimens of palladium wire in hydrogen (20 ↔ 250°C), after which the wire specimens (diameter 0.5 mm) were fully degassed. During the hydrogen treatment the strength properties of palladium increased by a factor of 2–4, but the plastic properties of palladium decreased. It was consequently surmised that the HPN-treatment could be effectively used for strengthening palladium articles, without noticeable changes to their dimensions and shape .
Superstrength Palladium Alloys: TRIP Alloys
Treating palladium alloys with hydrogen without immediate degassing can result in the formation of superstrength palladium-hydrogen alloys having high plasticities – the hydride TRIP-effect. (TRIP is transformation induced plasticity.)
The example shown in Figure 1 was achieved by a double (combined deformation-hydrogen) treatment (3). First, 90 per cent of the possible plastic deformation of a palladium wire was induced, until it reached a specimen diameter of 0.5 mm. After this initial mechanical treatment, the palladium specimen had very high strength properties (points a and b) and very low plastic properties (point c). Samples were then HPN-treated. Hydride transformations, α → β → α, were effected by a number (n) of pressure cycles (0.2 MPa → 1.33 Pa → 0.2 MPa) in a hydrogen atmosphere at 100°C. After this the specimens were not degassed and a new palladium hydrided material was obtained. From Figure 1 it is clear that this new hydrogen-containing material (n = 1–2) is much stronger and has been found to be more plastic than the pure, annealed palladium.
The nature of HPN-strengthened metals having hydride TRIP-plasticity has been reviewed and detailed analysis of the hydrogen treatment of metals and metallic materials, based on hydrogen phase naklep has been more fully discussed (3, 4).
HPN-strengthened palladium material has undergone recrystallisation annealing and its grain structure has been refined.
Samples of palladium wire were annealed in vacuum (about 10−4 Torr) for 6 hours at 900°C. A metallographic investigation of the samples was then undertaken. The samples were found to have extremely large grains (generally, only two or three grains in the 0.5 mm cross section of the wire). Their size was in the range 0.25 to 0.15 mm.
The samples were then HPN-treated and metallographic investigation showed them to have the same grain size as did the annealed samples. Annealings at 100 and/or 200°C did not significantly change the microstructure or the character of the high angle boundaries.
However, for samples annealed at 300°C, the development of first-stage recrystallisations may be seen. Small, newly crystallised grains could be seen near the boundaries of the initially large grains. The dimensionally smallest of these newly changed grains was of the order of 15 μm; but others may be 3–5 times larger. Annealing twins have also been observed. Annealing at 400°C led to further development of these recrystallisation effects, while above 500°C the grain sizes became significantly larger (by about 7–10 times).
HPN is the experimentally observed phenomena of the transitions of metals and alloys, in particular palladium, into controllable high strength states with special physical properties during courses of charging with hydrogen and transformation of α-phase solid solution ↔ β-phase hydride. Therefore, HPN-phenomena may form the basis for the hydrogen treatment of palladium materials aimed at improving their structure and properties without noticeably affecting their dimensions and shape. Based on this treatment, novel advanced palladium alloys have now been made (5).
V. A. Goltsov, Mater. Sci. Eng., 1981, 49, 109
V. A. Goltsov, “ The Phenomenon of Controllable Hydrogen Phase Naklep and the Prospects of Its Use in Metal Science and Engineering ”, Proc. Int. Symp. Metal-Hydrogen Systems, Miami, 13–15 Apr., 1981, Pergamon Press, Oxford, 1982, pp. 211 – 223
V. A. Goltsov, “ Phenomena Induced by Hydrogen and Hydrogen Induced Phase Transformations ”, in “ Interactions of Hydrogen with Metals ”, Science, Moscow, 1987, Chap. 9, pp. 264 – 292 (in Russian)
V. A. Goltsov, “ Hydrogen Barothermal Treatment of Metallic Materials – A New Field of Hydrogen Technology ”, Proc. 7th World Hydrogen Conf. Hydrogen Energy Progress VII, Moscow, 25–29 Sept., 1988, Vol. 3, pp. 1721 – 1740
Proc. Select. Papers of 1st Int. Conf . “ Hydrogen Treatment of Materials ”, Donetsk, Ukraine, 20–22 Sept., 1995, Guest Editor V. A. Goltsov, Int. J. Hydrogen Energy, 1997, 22, ( 2/3 )