R. Bormann

Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials

Abstract Nanocrystalline MgH2/MexOy- and Mg2NiH4/MexOy-powders were produced by high energy ball milling (MexOy=Sc2O3, TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, CuO, Al2O3, SiO2). The hydrogen absorption and desorption kinetics of the nanocomposite materials were determined with respect to a technical application. Some of the selected oxides lead to an enormous catalytic acceleration of hydrogen sorption compared to pure nanocrystalline materials. In absorption, the catalytic effect of TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, and CuO is comparable. Concerning desorption, the composite material containing Fe3O4 shows the fastest kinetics followed by V2O5, Mn2O3, Cr2O3 and TiO2. Only 0.2 mole% of the catalysts is sufficient to provide a fast sorption kinetics.

Comparison of the catalytic effects of V, V2O5, VN, and VC on the hydrogen sorption of nanocrystalline Mg

Abstract Nanocrystalline MgH 2 with additions of V 2 O 5 , VN, VC, or high-purity V was produced by high-energy ball milling. The hydrogen absorption and desorption kinetics of these novel materials were determined in order to compare the catalytic effects of the additions. The results show a significant enhancement of the hydrogen reaction kinetics under the chosen experimental conditions only for V 2 O 5 , VN, and VC, while the influence of high-purity V is negligible. This result is discussed with respect to previous reports.

Critical assessment and thermodynamic modeling of the Mg–H system

Abstract A comprehensive critical assessment has been made of the experimental data of the Mg–H system. Based on the selected original experimental phase diagram data from the literature, a set of thermodynamic functions for the Mg–H system was chosen and the parameters were optimized using the least squares method. Four different analytical descriptions were used to model the four different types of stable phases in the Mg–H system: gas, liquid, solid solution phase α -(Mg), and the stoichiometric compound β -MgH 2 . Most of the experimental information is in agreement with the modeling, especially the dissociation pressures of MgH 2 at different temperatures, the invariant equilibria, and the hydrogen solubilities in magnesium at one atmosphere. Calculation of the system has been used to thermodynamically analyze some experimental information.

Thermodynamic analysis of the hydriding process of Mg-Ni alloys

Abstract A consistent set of thermodynamic functions for the Mg–Ni–H system has been developed from experimental data, and the Mg–Ni–H phase diagram has been calculated thermodynamically. The extended ternary solubility of hydrogen in Mg 2 Ni, the ternary compound phase Mg 2 NiH 4 , and the effect of Ni addition on the hydrogen solubility in molten magnesium have been modeled thermodynamically. The ternary solid solution phases were extrapolated from the thermodynamic descriptions of the binary edge systems. The thermodynamic functions of the Mg–Ni–H system have been applied to study the hydriding process of Mg–Ni alloys.

Mechanical behavior of submicron-grained γ-TiAl-based alloys at elevated temperatures

Submicron-grained intermetallic compounds based on γ-TiAl were prepared by high-energy milling and hot isostatic pressing. At temperatures above 500°C, the flow stress is strongly reduced and work-hardening completely disappears. Compression tests performed at temperatures between 500 and 900°C reveal a marked strain rate sensitivity of the flow stress, suggesting superplasticity to occur. This could be confirmed by tensile straining of Ti–45Al–2.4Si samples, resulting in elongations of up to 175% at 800°C. Small silicide particles (d≈100 nm) of the type Ti5(Si,Al)3, embedded in the grain boundaries of the γ-TiAl matrix, impede a coarsening of the microstructure. However, at strain rates above , these dispersoids are suggested to promote the formation of voids and to reduce the overall deformability. At 800°C, an apparent activation energy of Qapp=351 kJ/mol can be derived. Superplastic behavior at 800°C is accomplished by grain boundary sliding accommodated by diffusional processes inside the γ-TiAl phase. Thus, the high temperature deformation mode is similar to the mechanisms found for more conventionally grained TiAl alloys at deformation temperatures ⩾1000°C.

Microscopic mechanisms of metastable phase formation during ball milling of intermetallic TiAl phases

Abstract Powders of the intermetallic equilibrium phases α 2 -Ti 3 Al, γ-TiAl and TiAl 3 were ball milled in order to investigate the microscopic origins of the energetic destabilization and the transformation into metastable phases during the milling process. It was found that the intermetallic phases were chemically partially disordered on milling followed by the transformation into solid solution phases after long milling. In detail, for the γ phase, the formation of numerous deformation twins, thin h.c.p. lamellae and lamellae of the 9R phase formed by an antitwin operation was observed by TEM. The disordering of the D0 22 -TiAl 3 phase occurred inhomogeneously in the material via the formation of antiphase boundaries on (001) planes, resulting in an f.c.c. solid solution in the final state. In summary, it can be concluded that the formation of the observed metastable phases results from chemical disordering, whereas the excess enthalpy of grain boundaries plays only a minor role for the energetical destabilization of the intermetallic compounds during milling in this case.

Thermodynamics of the Ni–H system

Abstract Thermodynamic properties of the Ni–H system have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases, and the model parameters have been optimized from the original experimental hydrogen solubility data. Magnetic ordering in solid nickel has also been considered. The nickel hydride that only forms at very high hydrogen pressure has not been included. No secondary values, such as the hydrogen solubility data at 1 bar derived from those values measured at lower pressures, were used in optimization. The heat of solution of hydrogen in nickel is calculated in dependence of temperature. The effect of the magnetic ordering in solid nickel on the heat of solution of hydrogen and, consequently, on the hydrogen solubility, is discussed. The CALPHAD method is demonstrated to be a powerful tool for determining the heat of solution of gaseous atoms in metals.

The effect of ultrafine grained microstructures on the hot-workability of intermetallic/ceramic composites based on γ-TiAl

Abstract Intermetallic/ceramic composites are prepared by mechanical alloying and subsequent powder consolidation. The bulk material of the composition Ti–45Al–2.4Si (at.%) exhibits a homogeneous microstructure of equiaxed γ-TiAl grains (dγ=196 nm) with a volume fraction of about 10% ξ-Ti5(Si,Al)3-silicides finely dispersed along the grain boundaries. Large fully dense specimens (m=500 g) allow for the demonstration of the easy hot-workability of ultrafine grained materials by industrial processing techniques like isothermal forging and extrusion. The temperatures for isothermal forging can be kept between 800 and 900°C for maximum strain rates in the range of 10−4 s−1. Deformations up to 75% at stresses below 160 MPa are achieved. Due to the stabilizing effect of the Ti5Si3-silicides, only slight grain coarsening is observed. Taking into account the high strain rate sensitivity of the material, extrusion tests ( e =1.5 s−1) are performed at temperatures between 1025 and 1250°C. While significant coarsening of both the γ-TiAl matrix and the silicide dispersion is observed at high deformation temperatures, extrusion at 1025°C retains the fine grained and homogeneous microstructure.

Production of nanocrystalline cermet thermal spray powders for wear resistant coatings by high-energy milling

Abstract TiC–Ni based nanocrystalline cermet powders for thermal spraying were produced by high-energy milling. Milling experiments were performed in an attrition mill and a vibration mill in kilogram scale, and powder morphologies and microstructures were characterized using scanning electron microscopy, X-ray diffraction, and laser scattering for particle size analysis. Milling time and powder input were optimized with respect to the desired microstructure and particle sizes, and the results using both types of mill were compared. Powders with homogeneously dispersed hard phase particles below 300 nm could be produced in both mills. Additional processes for the refinement of powder morphology and particle size distribution are discussed.

Tailoring nanocrystalline materials towards potential applications

Abstract The high interface area in nanocrystalline materials leads to advanced structural and functional properties that are interesting for a variety of applications and products. Three distinct examples for potential applications are given: Nanocrystalline and submicron-sized light-weight intermetallics based on γ-TiAl exhibiting favourable deformation behaviour at reduced temperatures, nanocrystalline cermet coatings produced by thermal spray process exhibiting improved hardness and wear resistance, and nanocrystalline Mg hydride-based composites for hydrogen storage in future mobile applications exhibiting extremely high, reversible storage capacity and fast kinetics.