Influence of the palladium coating on the hydrogen embrittlement of Ni61Nb33Zr6 amorphous tapes obtained by melt spinning

Abstract

The current work is focused towards the properties of Ni61Nb33Zr6 amorphous alloy for use in hydrogen-related energy applications. The master alloys were prepared by arc melting using high purity metals in a Ti-gettered argon atmosphere. The alloys were melted several times to improve homogeneity. The ingots were induction-melted under a argon atmosphere in a quartz tube and a graphite crucible, injected through a nozzle onto a Cu wheel to produce rapidly solidified amorphous ribbons. The characterization of the amorphous ribbons was done by X-ray diffraction, DSC analysis and hardness tests. The hydrogen charging was done electrochemically for low temperature tests and by heating in a hydrogen atmosphere at different temperatures in the case of the high temperature tests. It was found that the palladium plating reduces the hydrogen embrittlement limit by 50 °C. Introduction The amorphous alloys have been proposed for hydrogen separation membranes, because amorphous alloys absorb generally hydrogen without forming metallic hydride and show good mechanical properties. However, since amorphous alloys are thermally unstable, using them as dense, hydrogen permeation membrane at elevated temperatures is very hard. Maintaining an amorphous alloy close to its glass transition temperature will trigger crystallization, decrease of the hydrogen permeability and ultimately its mechanical failure. From this point of view it at utmost importance to have a Tg as high as possible. Generally, Ni-Nb amorphous alloys have high Tx [1] and according to Inoue [2] it could be further improved by adding more elements to the alloy. Zirconium on the other hand has excellent hydrogen permeability and in general improves the glass forming ability of the alloys [3]. On the other hand, increasing the zirconium content will lead to the reduction of the Tg, so, an optimal balance of these two issues must be found. Different nickel niobium alloys are studied [4, 5] which could be used as a separation membrane. The studied alloy has a supercooled liquid region of ~ 50K, which would allow it to be shaped by hot-pressing in this temperature range. The purpose of this paper is to evaluate hydrogel embrittlement behavior of the amorphous Ni61Nb33Zr6 alloy and identifying a temperature range in which the alloy could be used as the hydrogen separation membrane from this point of view. Experimental The master alloy (Ni61Nb33Zr6 ) was prepared by arc melting using high purity materials in a Tigettered argon atmosphere. The alloys were melted several times in order to improve homogeneity. The alloy ingot was induction-melted under a high-purity argon atmosphere in a quartz crucible and injected through a nozzle onto a rotating Cu wheel to produce amorphous Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 89-94 doi: http://dx.doi.org/10.21741/9781945291999-10 90 tapes. The obtained tapes were 4 mm wide and approximately 50 μm thick. The rotation speed used during the present experiments was 32 m/s. The amorphous nature of the ribbons was investigated by X-ray diffraction using a Shimadzu XRD – 6000 diffractometer and CuKα1 radiation. The samples behavior on heating was investigated by differential scanning calorimetry (SETARAM Labsys system) at the heating rate of 40 K/min. The ultimate tensile strength of the tapes was estimated from the Vickers micro-hardness measurements (40 gf. applied for 15 seconds) as UTS = HV*10/3 [MPa]. The palladium layer was deposited by thermal evaporation in a base pressure of 5*10 torr. The hydrogen embrittlement behavior was studied by heating the palladium coated and uncoated samples in flowing hydrogen to different temperatures (250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 540°C and 580°C). Heating to higher temperatures would result in the crystallization of the tapes. The critical bending strain was determined by measuring the radius of curvature at which fracture occurs in a bending test between two parallel plates. The strain is then calculated using the following equation: = t 2r−t ∙ 100 [%] , where r is the bending radius and t is the sample thickness. Results and discussions The amorphous structure of the sample is confined by XRD measurement. The X-ray diffraction pattern shown in Fig. 1a presents a broad maximum (FWHM = 6.3°) characteristic for glassy structures. Fig. 1. X-ray diffraction pattern (a) and DSC curve (b) of the as cast tapes. DSC measurements were performed to determine the thermal transformations that took place in the material and to approximate the thermal stability. The DSC heating curve of an amorphous material presents certain critical temperatures such as: glass transition temperature (Tg), crystallization temperatures (TX and TP) and melting temperature (TS and T1). The amorphous material remains in vitreous state until the TX temperature is reached. The crystallization of the amorphous material is indicated by the presence of exothermic peaks, their number depending on the number of crystallization steps through which the material undergoes. The DSC curve presented in Fig. 1b, shows at 420 °C a structural relaxation followed by a glass transition (Tg at 601 ° C and two crystallization steps (Tx1= 638 °C and Tx2= 702 °C). From the combined analysis we can conclude that these tapes are x-ray amorphous structures. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 89-94 doi: http://dx.doi.org/10.21741/9781945291999-10 91 Another advantage of the amorphous structure is the outstanding mechanical properties. Although not as precise, the ultimate tensile strength evaluation from the hardness measurements is a simple and strait forward way to go since even if the samples are prepared by grinding and polishing, there will still remain edges on the margins that act as tension concentrators, leading to an erroneous measurement. In table 1 the microhardness measured using the Vickers method is summarized. Table 1. Microhardness and estimated UTS of the selected tape. HV0.04/15 [daN/mm] Rm