Rüdiger Bormann

Fast hydrogen sorption kinetics of nanocrystalline Mg using Nb2O5 as catalyst

The efficiency of Nb2O5 as a catalyst for hydrogen sorption reaction of magnesium is investigated. Absorption and desorption kinetics are determined at 250 and 300 °C and are compared to other metal and oxide catalysts. The results demonstrate that the catalytic effect of Nb2O5 is superior in absorption as well as desorption.

Application of the CALPHAD method for the prediction of amorphous phase formation

Abstract The method of calculating the thermodynamic functions of an alloy system using the CALPHAD approach is described. In particular, the thermodynamics of the undercooled liquid and the amorphous phase are considered, taking into account the influence of the specific heat of the liquid phase. From the free energy functions metastable phase diagrams are predicted, assuming equilibria between the terminal solid solutions and the liquid or amorphous phase. In addition, T 0 lines are calculated for the solution phases. The results are compared with the experimentally observed concentration range of amorphous phase formation after melt spinning or mechanical alloying of the elemental powder mixtures.

Room temperature mechanical behavior of silicon-doped TiAl alloys with grain sizes in the nano- and submicron-range

Abstract Nano- and submicron-grained intermetallic compounds consisting of γ-TiAl and ξ-Ti 5 (Si,Al) 3 were produced by high energy milling and hot isostatic pressing. Owing to the pure and controlled processing conditions, the mechanical properties may be indubitably related to the microstructure. Both yield strength and hardness show a Hall–Petch-type dependence on grain size, resulting in extremely high flow and fracture stresses under compression of up to 3 GPa. With a reduction of grain size, the coefficient of strain hardening as well as the compressive fracture strain decrease and drop to zero for alloys with grain sizes of about 150 nm. Deformation at room temperature is accomplished by dislocation glide and mechanical twinning, with twinning attaining more and more importance as the grain size is further reduced. Diffusion-controlled deformation mechanisms can be ruled out even for intermetallics with crystallite sizes as small as 50 nm. A room temperature ductilization of intermetallic compounds by switching to a nanocrystalline microstructure seems to be rather unlikely.

Mechanical properties of submicron‐grained TiAl alloys prepared by mechanical alloying

Ti‐48 at. % Al powders of the metastable hexagonal‐closed‐packed solid solution with a grain size of 15 nm were prepared by mechanical alloying. The powders were consolidated to a density of greater than 99.5% by hot isostatic pressing (HIP) at 800 °C. After HIP the material exhibits a globular microstructure of the equilibrium phases α2 and γ with a mean grain size of 150 nm. Microhardness measurements show a Hall–Petch type [E. O. Hall, Proc. Phys. Soc. B 64, 747 (1951); N. J. Petch, J. Iron Steel Inst. 174, 25 (1953)] dependence on grain size. Room temperature compression tests reveal low ductility, but high fracture strengths ≥1800 MPa. On increasing the test temperature the yield strength drops sharply in the temperature range 600–800 °C to very low values. The results indicate that submicron‐grained TiAl alloys can be deformed at much lower temperatures than coarse‐grained material, making them suitable as precursors for net shaping, in particular if high deformation ratios are required.

Hydrogen Sorption of Nanocrystalline Mg at Reduced Temperatures by Metal‐Oxide Catalysts

Metal hydrides offer a safer alternative for storage of hydrogen for use as a zero-emission vehicle fuel than storage in compressed or liquid form. Mg hydride also has a high storage capacity by weight and is therefore favored for mobile applications. However, light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Recently, a breakthrough was achieved by preparing nanocrystalline hydrides using high-energy ball milling. The sorption kinetics can further be improved by adding catalysts to magnesium. In this communication results for MgH 2 with different oxide catalysts are compared. Special emphasis is put on the sorption kinetics at reduced temperatures, from 300 °C down to room temperature.

Metastable phase formation in undercooled near-eutectic Nb-Al alloys

Abstract The non-equilibrium solidification behavior of undercooled hypoeutectic and eutectic melts (Nb-52 to 59.5 at.% Al) has been analyzed with respect to metastable phase and microstructure formation. A method combining controlled melt undercooling achieved by an electromagnetic levitation technique and rapid quenching of the undercooled drop on a chill substrate was established to study the primary solidification structure. In situ measurements of the recalescence process revealed thermal arrests characteristic for metastable phase formation and solidification rates typically for dendritic solidification. Dendrites of the metastable tetragonal σ*-phase were identified in quenched samples, and their composition was related to the calculated metastable phase diagram. The metastable dendrites gradually decompose into an anomalous eutectic microstructure on slow post-solidification cooling.

Inverse melting of metastable Nb–Cr solid solutions

Metastable body‐centered‐cubic (bcc) Nb–Cr solid solutions were produced by mechanical alloying over a wide concentration range, and investigated by x‐ray diffraction, differential thermal analysis, and transmission electron microscopy. For comparison, metastable bcc and amorphous Nb–Cr alloys were fabricated by sputtering. Upon annealing, the solid solutions prepared by mechanical alloying undergo a transformation to an amorphous phase. The vitrification is polymorphous for a composition Nb45Cr55 whereas two‐phase mixtures of amorphous and bcc phases are formed for other concentrations, indicating that metastable equilibria between these two phases develop upon heat treatment. These results, combined with a detailed analysis of the thermodynamic functions of the system, reveal that the amorphous phase has a lower enthalpy and entropy than the bcc phase near equiatomic composition. In consequence, the Gibbs‐energy curves of bcc and liquid/amorphous Nb45Cr55 phases versus temperature exhibit two points of ...

High resolution microstructure analysis of the decomposition of Cu90Co10 alloys

Abstract Homogeneous CuCo alloys with a Co concentration up to 10 at.% Co are prepared by rapid quenching. This allows for the first time the investigation of the decomposition process of the Cu90Co10 supersaturated solid solution on a nanometer scale using a combination of atom probe/field ion microscopy (AP/FIM) analyses and transmission electron microscopy. Annealing of a Cu90Co10 alloy at 440°C for various times leads to a compositional modulated microstructure. The composition profiles determined by AP/FIM analyses clearly exclude a classical nucleation and growth behavior and instead suggest a spinodal type decomposition whereby the compositions of the precipitates continuously increase. In contrast, at the grain boundaries of the Cu90Co10 alloy heterogeneous nucleation of pure Co particles is observed.

Microstructure development during rapid solidification of highly supersaturated Cu-Co alloys

Abstract We report a detailed microstructure analysis of rapidly quenched Cu 1− x Co x (0.1 ⩽ × ⩽ 0.5) alloys. The chemical homogeneity of the alloys was investigated on nanometer scale using a combination of AP/FIM- and TEM-studies. Cu-Co alloys with a Co concentration up to 10 at.% Co can be prepared homogeneously by rapid quenching. For larger Co contents, the microstructure is determined by a competition between polymorphic solidification, nucleation of the Co-rich solid solution from the melt, and spinodal decomposition of the unstable supersaturated solid solution. The microstructures are discussed in terms of the kinetics of the Cu-Co system, provided quantitatively by recent thermodynamic calculations.

Thermodynamics of stable and metastable phases in the NiTi system and its application to amorphous phase formation

The free energies of metastable and stable phases in the NiTi system are calculated by a Kaufman-type program taking into account different sources of thermodynamic data. However, direct measurements of thermodynamic values are required to determine the free energy of the amorphous alloy (treated as the undercooled liquid). Therefore, e.m.f. measurements were made on metallic glasses at T = 613 K. From the calculated free energies a phase diagram of metastable phases is determined, considering only the amorphous phase and the terminal solutions. The application of the so-called metastable phase diagram is demonstrated for the formation on amorphous co-sputtered and mechanically alloyed NiTi samples.