Papers by Author: Reiner Kirchheim

Abstract: Nanocrystalline Fe-1.77at.%C and Fe-3.27at.%C alloys prepared by ball milling iron powders and graphite powders are annealed below 573K where the precipitation of Fe₃C does not occur. Upon annealing, a significant grain coarsening is observed in Fe-1.77at.%C alloy, whereas the grain coarsening is inhibited in Fe-3.27at.%C alloy. Within the framework of thermodynamic theories, the inhibition of grain coarsening in nanocrystalline Fe-C alloys is discussed. It is demonstrated that the inhibition of grain coarsening in the nanocrystalline Fe-C alloys can be ascribed to a vanished driving force for grain growth which is caused by the interaction between carbon and the grain boundaries of nanograins.

Abstract: Based on a novel defactants (defect acting agents) concept (R. Kirchheim, Acta Materialia 55 (2007) 5129 and 5139), a novel method of understanding and synthesizing NC material was proposed by introducing defactants (segregating solute atoms) into the materials to ease the formation of grain boundaries (GBs) and enhance the formation ability of nanocrystalline (NC) structures. The iron-carbon system was chosen as a model system where carbon acts as the so-called defactant. Iron powders mixed with different amount of graphite were ball milled to prepare NC iron-carbon alloys with different carbon concentrations (C0). After ball milling, the as-milled powder with relatively low carbon concentration was annealed at a certain temperature to achieve saturation of GBs by carbon atoms. The microstructures of the powders were investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. The mean grain sizes (D) of the powders were determined by analyzing TEM dark field images and X-ray line profiles. The results indicated that once the saturation of GBs is achieved, D of the NC iron-carbon powders will be strongly dependent on C0 and will follow a simple mass balance of carbon in a closed system, i.e. D=3ΓgbVm/(C0-Cg) with Cg the carbon concentration in grains, Γgb the grain boundary excess, and Vm the molar volume of iron. Based on the experimental results, the formation of NC iron-carbon alloys was treated in detail within the framework of the defactant concept. The increase of C0 significantly reduces the formation energy of GBs, leading to a substantial decrease of D.

Abstract: Titanium and its conventional alloys reveal a high affinity for hydrogen, being capable to absorb up to 60 at.% hydrogen at 600°C, and even higher contents can be alloyed with titanium at lower temperatures. Hydrogen exhibits a low solubility in the low-temperature hexagonal closed-packed (hcp) α phase and a very high solubility (up to 50 at.%) in the high temperature body-centered cubic (bcc) β phase. The presence of hydrogen in the amount exceeding 200 ppm leads to formation of hydrides in α and α + β titanium alloys. While the aforementioned hydrogen behavior within bulk titanium has been well-established and reviewed, this is not the case with titanium thin films. The interpretation of results in these nanosized systems is complicated because the exact determination of the hydrogen concentration is difficult. However, using electrochemical hydrogen loading technique under the proper conditions, the hydrogen concentration can be accurately determined via Faraday’s law. In this study the thermodynamics of the titanium films during hydrogen absorption were investigated by electromotive force (EMF) measurements. Titanium films of different thicknesses were prepared on sapphire substrates in an UHV chamber with a base pressure of 10-8 mbar, using ion beam sputter deposition under Ar-atmosphere at the pressure of 1,5ּ10-4 mbar. The crystal structure was investigated by means of X-Ray diffraction using a Co-Kα radiation. For electrochemical hydrogen loading, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step loaded with hydrogen by electrochemical charging, which was carried out in a mixed electrolyte of phosphoric acid and glycerin (1:2 in volume). An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the films microstructure. The role of varying thicknesses on the main characteristics of hydrogen's absorption behavior, as well as hydrogen-induced microstructural changes in titanium thin films, are discussed in detail.

Abstract: Titanium films were prepared on sapphire substrates in an UHV chamber, by means of ion beam sputter deposition under Ar-atmosphere at the pressure of 1.5ּ10-4 mbar, with a deposition rate of 2,1 nm/min. The crystal structure was investigated by means of X-Ray diffraction using a Phillips X-Pert diffractometer with a Co-Kα radiation. For electrochemical hydrogen loading, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step loaded with hydrogen by electrochemical charging, which was carried out in a mixed electrolyte of phosphoric acid and glycerine (1:2 in volume). An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the microstructure. The main characteristics of hydrogen's absorption behaviour, as well as the thermodynamics and phase boundaries of titanium-hydrogen thin films are discussed in detail with specific emphasis on the comparison to titanium-hydrogen bulk system.