Universiẗat, Diss., 2001--Osnabrück.

Techn. Universiẗat, Diss., 2004--Berlin.

Effect of electrochemical absorption of hydrogen has been studied on the Zr-based amorphous alloys. The influence of hydrogen absorption on the stability of the amorphous phase and its crystallisation was investigated. Additionally, the cathodic hydrogen reaction mechanism on the surface of the alloy, the reversibility of the absorbed hydrogen and the hydrogen diffusion in the alloy were studied. These alloys are able to absorb large amounts of hydrogen (>1:1 hydrogen to metal ratio) but a rearrangement of the amorphous matrix takes place so that Cu rich areas are detected on the surface and Zr-hydride may precipitate. The thermal stability and crystallisation behaviour depends on the hydrogen concentration in the alloy. At low hydrogen concentration, the thermal stability deteriorates and primary crystallisation of Cu and/or Cu-rich phases is observed. At high hydrogen concentration, primary crystallisation of Zr-hydride takes place. The cathodic polarisation behaviour of amorphous Zr-based alloys as derived from Tafel plots reveals three characteristic potential regions reflecting the different mechanisms of hydrogen on the surface. In the Tafel region, hydrogen discharge and adsorption takes place on the alloy surface as fast steps reactions followed by the rate determining electrodic desorption reaction step in competition with hydrogen absorption as a fast step. In the further negative potential region, the current density is independent on the potential as both the Volmer and the Heyrowsky reactions take place at the same rate and the hydrogen mass transfer from the solution to the electrode surface is the rate-determining step. In the high polarisation region, all the partial hydrogen reactions take place intensively. The reversibility of the absorbed hydrogen tests reflects the possibility of hydrogen desorption from different energy sites in the amorphous alloy. The diffusion of hydrogen in the Zr-based alloys is comparable with that in the crystalline Pd and it is reduced in the pre-hydrogenated samples.

The ferromagnetic bulk metallic glasses (BMGs) started to be investigated only in the last 10 years.They are difficult to cast, but their properties are uniques. The work deals with casting, mechanical and soft magnetic properties of new Fe-based BMGs. Such alloys can be cast directly in samples with various geometries and they can be use as magnetic parts in different devices.

Amorphous and nanostructured Al-based alloys have attracted significant interest owing to their promising properties, including high strength combined with low density. Unfortunately, the production of these advanced materials is limited to powders or ribbons with thickness of less than 100 micrometers due to the reduced glass forming ability of the Al-based alloys. Powder metallurgy through pressure-assisted sintering is a good solution to overcome the size limitation of these materials. In this thesis, Al84Gd6Ni7Co3 glassy powders were consolidated into high-strength bulk materials by hot pressing. The sintering behavior and the microstructural evolution during hot pressing were analyzed as a function of temperature. The results reveal that, through the careful control of the sintering temperature, the combined devitrification and consolidation of the amorphous Al84Gd6Ni7Co3 powders can be achieved, leading to bulk samples with the desired hybrid microstructure and with excellent room temperature mechanical properties. Beside the sintering temperature, the microstructural state of the starting material is critical in order to obtain bulk samples with the desired microstructure and related properties. Consequently, the variation of the initial structural state of the powders as well as of their thermal stability and phase evolution during heating may be used for further tuning the mechanical performance of the hot pressed Al84Gd6Ni7Co3 samples. In order to analyze this aspect, ball milling was used to vary the crystallization behavior of the gas-atomized Al84Gd6Ni7Co3 glassy powder. The influence of milling on microstructure and thermal stability was investigated as a function of the milling time. The results show that the traces of crystalline phases present in the as-atomized powder decrease gradually with increasing the milling time. The thermal stability of the fcc-Al primary phase increases while the thermal stability of the intermetallic phases decreases with increasing milling. Moreover, significant improvement in hardness occurs after milling, which is attributed to the amorphization of the residual crystalline phases present in the as-atomized powder. These finding demonstrate that milling is an effective way to change the initial structural state of the powders and to control the thermal stability of the material. The effect of the microstructural state of the starting material on the mechanical properties of the consolidated samples was investigated in detail. For this, the milled Al84Gd6Ni7Co3 glassy powders were consolidated into bulk specimens by hot pressing. These materials exhibit superior mechanical properties than the samples produced from the as-atomized powder: record high yield strength of 1.7 GPa and fracture strength exceeding 1.8 GPa. This is combined with a plastic strain of about 4 %, Young’s modulus of 120 GPa and density of 3.75 g/cm3. A bimodal microstructure consisting of coarse grained and fine grained regions was achieved in the hot pressed samples by properly controlling the milling process. The exceptionally high strength is attributed to the increased volume fraction of the fine regions, whereas the plastic deformation is favored by the coarse regions, which are able to hinder crack propagation during loading. In addition, the fracture toughness is also improved by the existence of the coarse regions. The tribological properties of the Al84Gd6Ni7Co3 bulk samples were also evaluated. The wear resistance of the bulk samples produced from the milled powder is enhanced with respect to the specimens fabricated from the as-atomized powder, and both alloys exhibit improved wear properties compared to pure aluminum and Al88Si12. Abrasive wear is the main mechanism for these alloys. Finally, the corrosion resistance of these alloys was studied. The results indicate that the Al84Gd6Ni7Co3 bulk material produced from the as-atomized powder has better corrosion resistance than the samples obtained from the milled powder. The main corrosion behavior for these alloys is pit corrosion, intermetallic particle etchout and the corrosion of the Al-rich inter-particle areas. These results clearly demonstrate that, by the proper selection of the sintering temperature and through the appropriate choice of the initial structural state of the powders, the combined devitrification and consolidation of amorphous precursors can be successfully used to produce bulk amorphous/nanostructured Al-based materials with tunable physical and mechanical properties. This expands the known boundaries of Al alloys and offers a new route for the development of novel and innovative high-performance Al-based materials capable to meet specific requirements.

In the course of this PhD thesis metastable Cu50Zr50-xTix (0≤ x ≤ 10) and (Cu0.5Zr0.5)100-xAlx (5 ≤ x ≤ 8) alloys were prepared and characterised in terms of phase formation, thermal behaviour, crystallisation kinetics and most importantly in terms of mechanical properties. The addition of Al clearly enhances the glass-forming ability although it does not affect the phase formation. This means that the Cu-Zr-Al system follows the characteristics of the binary Cu-Zr phase diagram, at least for Al additions up to 8 at.%. Conversely, the presence of at least 6 at.% Ti changes the crystallisation sequence of Cu50Zr50-xTix metallic glasses and a metastable C15 CuZrTi Laves phase (Fd-3m) precipitates prior to the equilibrium phases, Cu10Zr7 and CuZr2. A structurally related phase, i.e. the “big cube” phase (Cu4(Zr,Ti)2O, Fd-3m), crystallises in a first step when a significant amount of oxygen, on the order of several thousands of mass-ppm (parts per million), is added. Both phases, the C15 Laves as well as the big cube phase, contain pronounced icosahedral coordination and their formation might be related to an icosahedral-like short-range order of the as-cast glass. However, when the metallic glasses obey the phase formation as established in the binary Cu-Zr phase diagram, the short-range order seems to more closely resemble the coordination of the high-temperature equilibrium phase, B2 CuZr. During the tensile deformation of (Cu0.5Zr0.5)100-xAlx bulk metallic glasses where B2 CuZr nanocrystals precipitate polymorphically in the bulk and some of them undergo twinning, which is due to the shape memory effect inherent in B2 CuZr. Qualitatively, this unique deformation process can be understood in the framework of the potential energy landscape (PEL) model. The shear stress, applied by mechanically loading the material, softens the shear modulus, thus biasing structural rearrangements towards the more stable, crystalline state. One major prerequisite in this process is believed to be a B2-like short-range order of the glass in the as-cast state, which could account for the polymorphic precipitation of the B2 nanocrystals at a comparatively small amount of shear. Diffraction experiments using high-energy X-rays suggest that there might be a correlation between the B2 phase and the glass structure on a length-scale less than 4 Å. Additional corroboration for this finding comes from the fact that the interatomic distances of a Cu50Zr47.5Ti2.5 metallic glass are reduced by cold-rolling. Instead of experiencing shear-induced dilation, the atoms become more closely packed, indicating that the metallic glass is driven towards the more densely packed state associated with the more stable, crystalline state. It is noteworthy, that two Cu-Zr intermetallic compounds were identified to be plastically deformable. Cubic B2 CuZr undergoes a deformation-induced martensitic phase transformation to monoclinic B19’and B33 structures, resulting in transformation-induced plasticity (TRIP effect). On the other hand, tetragonal CuZr2 can also be deformed in compression up to a strain of 15%, yet, exhibiting a dislocation-borne deformation mechanism. The shear-induced nanocrystallisation and twinning seem to be competitive phenomena regarding shear band generation and propagation, which is why very few shear offsets, due to shear banding, can be observed at the surface of the bulk metallic glasses tested in quasistatic tension. The average distance between the crystalline precipitates is on the order of the typical shear band thickness (10 - 50 nm) meaning that an efficient interaction between nanocrystals and shear bands becomes feasible. Macroscopically, these microscopic processes reflect as an appreciable plastic strain combined with work hardening. When the same CuZr-based BMGs are tested in tension at room temperature and at high strain rate (10-2 s-1) there seems to be a “strain rate sensitivity”, which could be related to a crossover of the experimental time-scale and the time-scale of the intrinsic deformation processes (nanocrystallisation, twinning, shear band generation and propagation). However, further work is required to investigate the reasons for the varying slope in the elastic regime. As B2 CuZr is the phase, that competes with vitrification, it precipitates in a glassy matrix if the cooling rate is not sufficient to freeze the structure of the liquid completely. The pronounced work hardening and the plasticity of the B2 phase, which are a result of the deformation-induced martensitic transformation, leave their footprints in the stress-strain curves of these bulk metallic glass matrix composites. The behaviour of the yield strength as a function of the crystalline volume fraction can be captured by the rule of mixtures at low crystalline volume fractions and by the load bearing model at high crystalline volume fractions. In between both of these regions there is a transition caused by percolation (impingement) of the B2 crystals. Furthermore, the fracture strain can be modelled as a function of the crystalline volume fraction by a three-microstructural-element body and the results imply that the interface between B2 crystals and glassy matrix determines the plastic strain of the composites. The combination of shape memory crystals and a glassy matrix leads to a material with a markedly high yield strength and an enhanced plastic strain. In the CuZr-based metastable alloys investigated, there is an intimate relationship between the microstructure and the mechanical properties. The insights gained here should prove useful regarding the optimisation of the mechanical properties of bulk metallic glasses and bulk metallic glass composites.

Chemnitz, Techn. Univ., Diplomarb., 2004.

Metallische Gläser erreichen aufgrund ihrer ungeordneten, amorphen Struktur eine Streckgrenze, welche andere metallische Materialien in den Schatten stellt (bis zu 5 GPa). Dabei wird aufgrund ihres geringen E-Moduls eine elastische Verformung von circa zwei Prozent erreicht und damit eine sonst unerreichte Menge elastischer Energie aufgenommen. Leider besitzen die metallischen Gläser auch einen enormen Nachteil: Aufgrund der Erweichung der Gläser bei plastischer Verformung kommt es zur Lokalisierung dieser in sogenannten Scherbändern (SBs). Dies führt nahezu ohne plastische Verformung zum Versagen des Materials. Es gibt bereits viele Methoden diesen Nachteil zu überwinden, indem man die Verformung durch das Einbringen von strukturellen Heterogenitäten auf eine Vielzahl von Scherbändern verteilt: Beispielsweise durch elastische Belastung, plastische Verformung mit eingeschränkten Freiheitsgraden, oder das Einbringen einer Fremdphase. Allerdings sind die dabei wirkenden Mechanismen und Wechselwirkungen zwischen der Struktur und der Verformung noch nicht vollständig verstanden. In dieser Arbeit wurde daher zum einen die Struktur des Glases beeinflusst und diese Änderungen charakterisiert und zum anderen die Wirkung dieser Veränderung auf das Verformungsverhalten untersucht. Als Ausgangszustand wurden dabei gegossene Stäbe und Platten vier verschiedener Legierungen (mit ansteigender Glasbildungsfähigkeit: Cu47,5Zr47,5Al5, Cu46Zr46Al8, Cu45Zr45Al5Ag5, Cu36Zr48Al8Ag8; von 1,5 mm bis 25 mm kritischem Gießdurchmesser) verwendet, welche mechanischen Verformungen unterzogen wurden, um strukturelle Heterogenitäten einzubringen. Zudem wurden B2-Glasmatrixkomposite mit der Formgedächtnisphase B2-CuZr beim Abschrecken der Schmelze (Cu47,5Zr46,5Al5-Sc1) bzw. beim Rascherhitzen über die Kristallisationstemperatur (Cu44Zr44Al8Hf2-Co2) hergestellt. Die angewandten Methoden decken ein breites Spektrum der in der Literatur diskutierten Vorbehandlungen mit einem Einfluss auf die Struktur und die mechanischen Eigenschaften ab. Die systematischen und umfangreichen Untersuchungen in dieser Arbeit beleuchten detailliert den Zusammenhang zwischen der Struktur und der Verformung metallischer Gläser. Die Struktur wird gezielt manipuliert um eine Veränderung ihrer Heterogenität zu induzieren. Dabei wird auf Veränderungen in atomaren Längenskalen ebenso eingegangen, wie auf die makroskopischen Unterschiede. Außerdem wird die Abhängigkeit der induzierten strukturellen Änderungen von der Stabilität des Glases betrachtet. Dies alles stellt einen wichtigen Beitrag zum Verständnis des Verformungsverhaltens metallischer Gläser dar. In letzter Konsequenz ermöglichen die Erkenntnisse, durch eine gezieltere Manipulation der Struktur, eine erhöhte plastische Verformung im metallischen Glas zu erreichen.

Co-based bulk metallic glasses (BMGs) are well known for their excellent mechanical properties with high fracture strength, hardness and elastic modulus. Since the first report of A. Inoue with co-workers in 2003 on Co43Fe20Ta5.5B31.5 BMG with fracture strength up to 5 GPa, a series of Co-based BMGs including Co-Fe-B-Si-Nb, Co-Fe-Cr-Mo-C-B-Er, Co-Ta-B systems have been developed. Co-Ta-B ternary BMGs, discovered recently, are characterized by even higher fracture strength of up to about 6 GPa. These BMGs with outstanding mechanical behavior are interesting for applications as advanced structural materials and coatings. Due to a relatively simple constitution (only three components), Co-Ta-B BMGs are very attractive for investigations of relationships between composition, structure, undercoolability, glass-forming ability, thermal and mechanical properties. However, there have been published just a few papers on Co-Ta-B BMGs focusing on the glass-forming ability in terms of the critical diameter and mechanical properties so far. In present work, a systematic study of the structure and properties of Co-Ta-B BMGs has been carried out on four intentionally chosen compositions  Co61Ta6B33, Co59Ta8B33, Co57Ta10B33 and Co53Ta10B37. Glass formation, thermal stability, crystallization kinetics upon isochronal and isothermal annealing, mechanical and magnetic properties were investigated. Co-Ta-B BMGs studied in this work are characterized by high thermal stability, ultrahigh fracture strength in compression, large Vickers hardness and high values of elastic constants. Increasing of B and Ta content is beneficial to the improvement of both thermal and mechanical properties. Based on the study of the short-range atomic order in Co57Ta10B33 BMG, Co-Ta, Co-B and B-B bonds are supposed to play an important role in the thermal and mechanical properties. A comprehensive picture on structure-composition-property relationship was established. In order to better understand the glass formation, non-equilibrium solidification of the undercooled alloys was investigated using electromagnetic levitation, high-energy X-ray diffraction and high-speed video observation. Three compositions with bulk glass-forming ability (Co61Ta8B31, Co59Ta8B33, Co55Ta8B37) were chosen to study the phase formation during non-equilibrium solidification. In addition, one ternary near-eutectic alloy Co64Ta5.5B30.5 and two binary alloys Co67B33 and Co63B37 with poor glass formation were comparably investigated using the same method. The phase formation, dendrite growth velocity and microstructure of the solidified samples were analyzed in detail as function of undercooling. The alloy composition, maximum undercooling and growth velocity were related closely with the glass-forming ability of the Co-Ta-B alloys studied.

Amorphous and nanostructured Al-based alloys have attracted significant interest owing to their promising properties, including high strength combined with low density. Unfortunately, the production of these advanced materials is limited to powders or ribbons with thickness of less than 100 micrometers due to the reduced glass forming ability of the Al-based alloys. Powder metallurgy through pressure-assisted sintering is a good solution to overcome the size limitation of these materials. In this thesis, Al84Gd6Ni7Co3 glassy powders were consolidated into high-strength bulk materials by hot pressing. The sintering behavior and the microstructural evolution during hot pressing were analyzed as a function of temperature. The results reveal that, through the careful control of the sintering temperature, the combined devitrification and consolidation of the amorphous Al84Gd6Ni7Co3 powders can be achieved, leading to bulk samples with the desired hybrid microstructure and with excellent room temperature mechanical properties. Beside the sintering temperature, the microstructural state of the starting material is critical in order to obtain bulk samples with the desired microstructure and related properties. Consequently, the variation of the initial structural state of the powders as well as of their thermal stability and phase evolution during heating may be used for further tuning the mechanical performance of the hot pressed Al84Gd6Ni7Co3 samples. In order to analyze this aspect, ball milling was used to vary the crystallization behavior of the gas-atomized Al84Gd6Ni7Co3 glassy powder. The influence of milling on microstructure and thermal stability was investigated as a function of the milling time. The results show that the traces of crystalline phases present in the as-atomized powder decrease gradually with increasing the milling time. The thermal stability of the fcc-Al primary phase increases while the thermal stability of the intermetallic phases decreases with increasing milling. Moreover, significant improvement in hardness occurs after milling, which is attributed to the amorphization of the residual crystalline phases present in the as-atomized powder. These finding demonstrate that milling is an effective way to change the initial structural state of the powders and to control the thermal stability of the material. The effect of the microstructural state of the starting material on the mechanical properties of the consolidated samples was investigated in detail. For this, the milled Al84Gd6Ni7Co3 glassy powders were consolidated into bulk specimens by hot pressing. These materials exhibit superior mechanical properties than the samples produced from the as-atomized powder: record high yield strength of 1.7 GPa and fracture strength exceeding 1.8 GPa. This is combined with a plastic strain of about 4 %, Young’s modulus of 120 GPa and density of 3.75 g/cm3. A bimodal microstructure consisting of coarse grained and fine grained regions was achieved in the hot pressed samples by properly controlling the milling process. The exceptionally high strength is attributed to the increased volume fraction of the fine regions, whereas the plastic deformation is favored by the coarse regions, which are able to hinder crack propagation during loading. In addition, the fracture toughness is also improved by the existence of the coarse regions. The tribological properties of the Al84Gd6Ni7Co3 bulk samples were also evaluated. The wear resistance of the bulk samples produced from the milled powder is enhanced with respect to the specimens fabricated from the as-atomized powder, and both alloys exhibit improved wear properties compared to pure aluminum and Al88Si12. Abrasive wear is the main mechanism for these alloys. Finally, the corrosion resistance of these alloys was studied. The results indicate that the Al84Gd6Ni7Co3 bulk material produced from the as-atomized powder has better corrosion resistance than the samples obtained from the milled powder. The main corrosion behavior for these alloys is pit corrosion, intermetallic particle etchout and the corrosion of the Al-rich inter-particle areas. These results clearly demonstrate that, by the proper selection of the sintering temperature and through the appropriate choice of the initial structural state of the powders, the combined devitrification and consolidation of amorphous precursors can be successfully used to produce bulk amorphous/nanostructured Al-based materials with tunable physical and mechanical properties. This expands the known boundaries of Al alloys and offers a new route for the development of novel and innovative high-performance Al-based materials capable to meet specific requirements.