D.S. dos Santos

Hydrogen solubility in amorphous and crystalline materials

Abstract Electrochemical hydrogen permeation tests were performed in an electrolyte of 0.1 N NaOH at 313 K with the objective of studying the hydrogen solubility in metallic amorphous and crystalline materials. Samples were prepared from the amorphous metallic alloys Ni 81 P 19 and Fe 40 Ni 38 Mo 4 B 18 and crystalline pure nickel and palladium, as well as low carbon steel. It was shown that the hydrogen solubility in Ni 81 P 19 amorphous alloy is much bigger than in the Fe 40 Ni 38 Mo 4 B 18 glassy alloy and in the crystalline pure nickel, palladium and low carbon steel. Also that a terminal hydrogen solid solubility of 1232.0±334.1 mol H m - 3 is attained in the Fe 40 Ni 38 Mo 4 B 18 amorphous alloy by forming a hydride phase, while, under the same conditions, the Ni 81 P 19 glassy alloy dissolves 11645.0±341 mol H m - 3 as a partial hydrogen solid solubility without transforming into hydride. The cathodic hydrogen generation potential used in the hydrogen permeation tests was shown to influence the hydrogen permeation kinetics.)

Structural and thermodynamic properties of the pseudo-binary TiCr2-xVx compounds with 0.0≤x≤1.2

Abstract The TiCr 2− x V x compounds with 0.0≤ x ≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x >0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature ( P – C – T ) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P – C – T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.

Investigation of the vacancy-ordered phases in the Pd–H system

Abstract It has been shown that hydrogen–metal reactions operated at high pressures (3–5 GPa) may lead to hydrogen-induced lattice migration. The occurrence of fast diffusion processes that take place within the metal lattice has been established. Under these conditions, modifications of the diffusion kinetics and of the phases equilibria allow to produce vacancy-ordered phases with high vacancy concentrations (20%). An alternative route which leads to such phases that are stable at ambient pressure and temperature is presented. The structural properties of the Pd-(vacancy, H) system which have been studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy will be discussed. In the case of palladium, the vacancy-ordered state is characterized by the loss of superconductivity with respect to the Pd hydride. This spectacular modification of the physical properties will be presented and discussed in the light of band structure calculations that have been performed modeling different types of decorated vacancies with octahedral coordination.

Microstructural modifications induced by hydrogen in a heat resistant steel type HP-45 with Nb and Ti additions

Abstract Heat resistant stainless steel type HP45 modified by Nb and Ti additions, which is commonly used in chemical and petrochemical industrial plants, was submitted to hydrogen treatment at low and ultra-high pressures. In the low-pressure experiments, 0.1 Pa, hydrogen was supplied in a quartz tube and the material was heat-treated for 100 h at 1200 K. In the high-pressure experiments, 5 GPa, hydrogen was supplied in a NaCl container and the material heat-treated at 873 and 1073 K for 1 h. A coarsening of the M 23 C 6 (M=Fe, Cr, Ni) carbides, was observed in both cases, these being of a sharper shape for the low hydrogen pressure condition, compared with previously aged samples. In the high pressure experiments, where the atomic mobility is higher, a more intense coarsening of the carbides and the cracks associated with them was observed, which is deleterious for mechanical properties. The hydrogen-induced phase coarsening can be understood in terms of the vacancy formation that occurs under such conditions.

Hydrogen diffusivity and solubility in crystalline and amorphous alloys

The hydrogen permeation behaviour of iron- and nickel-based amorphous alloys was characterized using electrochemical methodology and compared with the properties of crystalline metals and alloys. The materials studied were amorphous Fe40Ni38Mo4B18, Fe74Ni4Mo3B17, Fe78B13Si9 and Ni81P19, as well as a crystalline low-carbon steel, pure nickel and pure palladium. The double potentiostatic electrochemical hydrogen permeation tests were performed at 40°C using a 0.1 N NaOH solution as electrolyte. It was found that the hydrogen diffusivity in the iron-based amorphous alloys is a few orders of magnitude lower than in carbon steel and iron, while a much smaller difference exists between Ni81P19 and pure nickel. Furthermore, the amorphous alloys showed a strikingly greater capacity to dissolve hydrogen in solid solution compared with their crystalline counterparts. In some cases, their apparent hydrogen solubility was even greater than that for the liquid metal of the main element present in their chemical composition.The hydrogen permeation behaviour of iron- and nickel-based amorphous alloys was characterized using electrochemical methodology and compared with the properties of crystalline metals and alloys. The materials studied were amorphous Fe40Ni38Mo4B18, Fe74Ni4Mo3B17, Fe78B13Si9 and Ni81P19, as well as a crystalline low-carbon steel, pure nickel and pure palladium. The double potentiostatic electrochemical hydrogen permeation tests were performed at 40°C using a 0.1 N NaOH solution as electrolyte. It was found that the hydrogen diffusivity in the iron-based amorphous alloys is a few orders of magnitude lower than in carbon steel and iron, while a much smaller difference exists between Ni81P19 and pure nickel. Furthermore, the amorphous alloys showed a strikingly greater capacity to dissolve hydrogen in solid solution compared with their crystalline counterparts. In some cases, their apparent hydrogen solubility was even greater than that for the liquid metal of the main element present in their chemical composition.

Analysis of the nanopores produced in nickel and palladium by high hydrogen pressure

Abstract Samples of pure nickel and palladium were submitted to a high hydrogen pressure (HHP) of 3.5 GPa at 800 °C for 5 h. Analysis of the resulting structural modification was performed using X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM) and small-angle X-ray scattering (SAXS), the latter specifically for Ni. The formation of superabundant vacancies (SAVs) was observed in the structure in both cases. For Pd, the pores, which formed by the coalescence of vacancies, had dimensions of 20–30 nm when present in the interior of the metal and 1–3 μm when condensed at the surface. The pores were seen to be dispersed homogeneously across the surface of Pd. For Ni, however, pores were created preferentially at the grain boundaries, which promoted significant decohesion in the metal. The distribution of pores induced by heat treatment of Ni subjected to HHP was determined by SAXS analysis and two populations of pores, with population mean diameters of 50 and 250 A, were observed.

Analysis of Grain Boundary Character in a Fine-Grained Nickel-Based Superalloy 718

Abstract In the current work, sheets of superalloy 718 were processed via thermomechanical route by hot and cold rolling, followed by annealing below the δ phase solvus temperature and precipitation hardening to optimum strength. Grain boundary character distribution throughout the processing was mapped via EBSD and its evolution discussed. The results show that it is possible to process the alloy to a fine grain size obtaining concomitantly a considerably high proportion of special boundaries Σ3, Σ9, and Σ27. The precipitation of δ phase presented a strong grain refining role, without significantly impairing the twinning mechanism and, consequently, the Σ3, Σ9, and Σ27 boundary formations.

Structural and hydrogenation properties of an 80wt%TiCr1.1V0.9–20wt%LaNi5 composite material

Abstract In order to monitor the hydrogenation/dehydrogenation characteristics of the bcc TiCr1.1V0.9 compound, two composites samples containing 20 wt % of LaNi5 were synthesized by melting and by mechanical alloying. The X-ray diffraction (XRD) analysis reveals that the melted material crystallizes with two bcc phases after a complete dissolution of the hexagonal LaNi5. Besides, the mechanical alloying (MA) procedure does not yield any modification of composition of the mixed phases. Therefore, a line broadening due to the particle size refinement is observed. For the melted alloy, the pressure-composition measurements carried out at 303 K reveal a decrease of both the maximum hydrogen capacity and the reversible hydrogen amount. Conversely after a MA treatment, hydrogenation/dehydrogenation properties are enhanced, even if a reduction of the maximum hydrogen capacity is pointed out. In both cases of alloying, the equilibrium plateau pressure is reduced reference to the starting bcc TiCr1.1V0.9 compound.

X-ray diffraction and scanning electron microscopic characterization of electrolytically hydrogenated nickel and palladium

The effects of hydrogen insertion by electrolytic charging in H2SO4 solution in pure nickel and palladium were investigated. The hydrogenated phases of Ni and Pd and their stability during aging were determined by X-ray diffraction. The NiH0,68 hydride and the α-Ni phase were distinguished by scanning electron microscopy (SEM) operating in the backscattered electrons mode. Interesting features about the intergranular cracks developed during and after charging are discussed.

Influence of cementite morphology on the hydrogen permeation parameters of low-carbon steel

In this investigation the effect of cementite morphology and distribution on the hydrogen permeation parameters of a low-carbon steel [chemical composition (in wt%): C 0.07, Mn 0.28, Si ~< 0.01, S 0.017, P 0.009 and A10.079] was studied utilizing an improved electrochemical technique. A two-compartment electrochemical permeation cell (based on the traditional apparatus described by Bees and Zfichner [1]) with the necessary dedicated hardware and software was developed to permit automated, high-precision isothermal permeation tests with real-time computer monitoring and data analysis via a programmable multichannel control unit. The results presented here were obtained using the above experimental system, selecting the double-potentiostatic option as the mode of operation, and a 2 s data-sampling rate. The specimens used for this investigation were of the order of 0.6 mm thick, having been prepared from 1 mm-thick sheet material by the usual metallographic polishing technique. Both compartments of the electrochemical cell were filled with deaerated 0.1 M NaOH, continuous nitrogen bubbling being maintained throughout the test. Hydrogen was generated cathodically at the sample surface in one of the compartments, utilizing a constant potential of 1 .35V versus saturated calomel electrode (SCE) and detected by anodic polarization at the corrosion potential in the adjacent compartment after permeating through the thin-sheet metallic specimen which separated the two. The progress of the hydrogen permeation process was followed by monitoring the evolution, over time, of the anodic current flow, measured between the specimen and the counterelectrode of the detection compartment. Thermostatic control of both cells was maintained throughout the test, the temperature (300 K) being monitored and controlled with an accuracy of +0.1 °C via a system of silicon-transistor sensors. Relatively small variations in temperature may significantly effect the precision and scatter of the data obtained. An analysis of the resultant data in terms of the unidirectional solid-state diffusion of hydrogen in the thin metallic sheet, as described by Ficks laws, permits the determination of the three interrelated parameters which quantify the permeation behaviour characteristics of the material: the permeability [P(t)], the apparent diffusivity (Dapp) and the apparent solubility (Sapp) [1, 2]. The term solubility is used here to signify the total hydrogen content,