Annett Gebert

Structural bulk metallic glasses with different length-scale of constituent phases

Abstract Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.

Designing biocompatible Ti-based metallic glasses for implant applications

Ti-based metallic glasses show high potential for implant applications; they overcome in several crucial respects their well-established biocompatible crystalline counterparts, e.g. improved corrosion properties, higher fracture strength and wear resistance, increased elastic strain range and lower Youngs modulus. However, some of the elements required for glass formation (e.g. Cu, Ni) are harmful for the human body. We critically reviewed the biological safety and glass forming tendency in Ti of 27 elements. This can be used as a basis for the future designing of novel amorphous Ti-based implant alloys entirely free of harmful additions. In this paper, two first alloys were developed: Ti(75)Zr(10)Si(15) and Ti(60)Nb(15)Zr(10)Si(15). The overheating temperature of the melt before casting can be used as the controlling parameter to produce fully amorphous materials or bcc-Ti-phase reinforced metallic glass nano-composites. The beneficial effect of Nb addition on the glass-formation and amorphous phase stability was assessed by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. Crystallization and mechanical behavior of ribbons are influenced by the amount and distribution of the nano-scaled bcc phase existing in the as-cast state. Their electrochemical stability in Ringers solution at 310 K was found to be significantly better than that of commercial Ti-based biomaterials; no indication for pitting corrosion was recorded.

Investigations on the electrochemical behaviour of Zr-based bulk metallic glasses

Bulk amorphous ZrAlCuNi alloy samples can be prepared by slow cooling from the melt due to their low critical cooling rate for amorphization. The passivation behaviour of the amorphous Zr55Al10Cu30Ni5 alloy in a 0.1 M Na2SO4 (pH=8) electrolyte is characterized in comparison to its crystalline alloy counterpart and to zirconium. Potentiodynamic and potentiostatic polarization measurements reveal that the alloys form strong protective surface layers by anodization, which is, in general, quite similar to the behaviour of zirconium. The barrier effect of surface layers grown on the alloys is slightly lower than that of films on zirconium. From AES investigations, it is obvious that all alloying elements participate in the formation of anodic films, which also show a gradient of composition along the growth direction. In 0.001–0.1 M NaCl electrolytes, pitting occurs on bulk amorphous ZrAlCuNi samples due to the existence of micrometer-sized crystalline inclusions. The oxygen-induced formation of such crystalline phases during slow cooling is studied in detail by characterizing samples of different oxygen content with X-ray diffraction, optical and scanning electron microscopy and differential scanning calorimetry.

Stability of the bulk glass-forming Mg65Y10Cu25 alloy in aqueous electrolytes

Abstract The corrosion behaviour of the amorphous Mg 65 Y 10 Cu 25 alloy was studied in aqueous alkaline electrolytes and compared with that of the corresponding multiphase crystalline alloy and of magnesium. Alloy samples were prepared by melt-spinning and die-casting and characterised concerning their microstructure and thermal stability by X-ray diffraction, optical and scanning electron microscopy and differential scanning calorimetry. In 0.3 m H 3 BO 3 /Na 2 B 4 O 7 buffer solution with pH=8.4 and 0.1 m NaOH solution with pH=13 the samples were electrochemically investigated by recording Tafel plots and by performing potentiodynamic polarisation tests and current transient measurements at anodic potentials. Potentiostatically formed surface layers were characterised with Auger electron spectroscopy and scanning electron microscopy. For both, the amorphous and the multiphase crystalline Mg–Y–Cu alloy, differences in anodic surface layer growth mechanisms in the two investigated electrolytes were detected which are explained by the effect of the constituent copper. In the two electrolytes, the amorphous alloy showed the lowest corrosion rates and the highest passivation ability also in comparison with earlier investigated Mg–Y alloys. Differences between the corrosion behaviour of the amorphous and the multiphase crystalline alloy are mainly attributed to heterogeneity effects rather than to an effect of the amorphous structure.

Electrochemical hydrogenation of Mg65Cu25Y10 metallic glass

Rapidly quenched ribbons of a Mg65Y10Cu25 metallic glass were electrochemically charged up to a maximum hydrogen content of about 3.7 wt.%. The hydrogen content was determined by hot extraction. The microstructure of different hydrogen-charged samples was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal behaviour was studied by differential scanning calorimetry (DSC) and thermal desorption analysis (TDA). Samples heated to selected temperatures were characterised by XRD. With increasing hydrogen content a change from a single-phase amorphous to a very fine nanocrystalline microstructure was observed, which is a consequence of hydride-forming reactions at room temperature. This strongly affects the thermal behaviour. With increasing fraction of nanocrystalline phases in the hydrogenated samples, grain growth processes are more pronounced than crystallisation of the residual amorphous phase for temperatures up to 623 K. Correspondingly, the fraction of nanocrystalline products of hydride-forming reactions, i.e. YH3, MgH2 and Cu2Mg, increases. Significant hydrogen desorption occurs at temperatures above 623 K and is mainly related to the reverse of those hydriding reactions.

The influence of Co and Ga additions on the corrosion behavior of nanocrystalline NdFeB magnets

Abstract Isotropic nanocrystalline Nd14Fe80B6 and Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnets with different grain sizes in the range of 60–600 nm have been produced from melt-spun materials by hot pressing at 700 °C and subsequent annealing at 800 °C for 0.5–6 h. The microstructure has been characterized using XRD, SEM, energy dispersive X-ray analysis, and Kerr microscopy. The corrosion behavior of NdFeB magnets has been examined on 0.1 M H2SO4 by in situ inductively coupled plasma solution analysis, gravimetric and electrochemical techniques. The corrosion hydrogen absorption/desorption behavior has been investigated by thermal desorption analysis and hot extraction methods. Partial substitution of Fe with Co and Ga leads to an improvement in corrosion resistance and reduces the affinity and binding energy for hydrogen in these materials. Coarsening of the microstructure results in a better corrosion performance of these materials. The corrosion behavior of the magnets in relation to phase composition, phase distribution and grain size is discussed in terms of dissolution, hydrogenation and mechanical degradation.

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Abstract Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti–Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti–Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti–Nb alloys. In this work, the formation of martensites (α′ and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Plastic Deformation and Mechanical Softening of Pd 40 Cu 30 Ni 10 P 20 Bulk Metallic Glass During Nanoindentation

) under the standard loadcontrol mode. New results using the feedback displacement control mode are alsopresented. The dependence of the pop-in formation on the loading rate is investigated.Variations in hardness and reduced elastic modulus as a function of the indentationrate are observed. A softening effect occurs when increasing the loading rate. This isexplained by the differences in plastic deformation achieved at different indentationrates. The displacement control mode was used to avoid the shear localization of thefree volume, leading to the almost complete absence of pop-ins along the loadingcurve. The obtained results suggest that plastic flow in bulk metallic glasses isgoverned by the rate of creation of free volume, which depends on the strain rate andits localization into shear bands.

Hydrogenation and its effect on the crystallisation behaviour of Zr55Cu30Al10Ni5 metallic glass

Abstract Zr 55 Cu 30 Al 10 Ni 5 metallic glass exhibits high thermal stability, and as it contains early and late transition metal elements (ETM/LTM), it is of interest to study its hydrogenation properties. Charging melt-spun ribbons electrochemically to different hydrogen-to-metal (H/M) ratios and following the effusion of hydrogen by thermal desorption analysis (TDA) reveals hydrogen desorption from high interstitial-site energy levels at temperatures below 623 K. Zirconium hydrides are formed above 623 K. At higher temperatures partial desorption of hydrogen occurs. Simultaneously, transformation to different hydride phases takes place in the order tetragonal e-Zr-hydride, cubic δ-Zr-hydride and a mixture of (α+β)-Zr-hydride. Thermal stability investigations by differential scanning calorimetry (DSC) point out the exothermic peaks of formation/transformation to different Zr-hydride phases. The formation of zirconium hydride causes depletion in the number of free Zr atoms leading, in turn, to different crystalline phases upon crystallisation. X-ray diffraction (XRD) reveals the formation of different crystalline phases for different H/M ratios. For a H/M-ratio of 0.37 a hexagonal AlZr 2 phase forms at 753 K, whereas for high hydrogen contents of 0.7 2 Zr forms already at 713 K. In contrast, the uncharged ribbons crystallise at 771 K by formation of a mixture of metastable fcc NiZr 2 -type phase, orthorhombic NiZr and tetragonal CuZr 2 phases.

Surface treatment, corrosion behavior, and apatite-forming ability of Ti-45Nb implant alloy†

The low modulus β-type Ti-45Nb alloy is a promising new implant alloy due to its excellent mechanical biocompatibility and composition of non-toxic elements. The effect of surface treatments on the evolution of controlled topography and roughness was investigated by means of scanning electron microscopy and optical profilometry. Severe mechanical treatments, for example sand-blasting, or etching treatments in strongly oxidizing acidic solutions, like HF:HNO(3) (4:1) or H(2)SO(4):H(2)O(2) (1:1) piranha solution were found to be very effective. In particular, the latter generates a nanopatterned surface topography which is expected to be promising for the stimulation of bone tissue growth. Compared to Ti and Ti-6Al-4V, the β-type Ti-45Nb alloy requires significantly longer etching durations due to the high chemical stability of Nb. Severe surface treatments alter the passive film properties, but do not deteriorate the outstanding corrosion resistance of the Ti-45Nb alloy in synthetic body fluid environments. The Ti-45Nb appears to have a lower apatite-formation ability compared to Ti. Etching with H(2)SO(4):H(2)O(2) (1:1) piranha solution inhibits apatite formation on Ti, but not on Ti-45Nb.