Andreas Masuhr

Equilibrium viscosity of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass-forming liquid and viscous flow during relaxation, phase separation, and primary crystallization

Abstract The flow behavior of the supercooled Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 bulk metallic glass-forming liquid is studied in isothermal three-point beam-bending experiments. The experiments lead to the determination of the equilibrium viscosity as a function of temperature. Comparison with other glass-forming liquids shows that the Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy is a strong liquid, similar to sodium silicate liquids. Flow measurements during phase separation and subsequent formation of crystals embedded in a non-crystalline matrix reveal a dramatic slowdown of the kinetics of the matrix that is expressed in an increase of the viscosity by several orders of magnitude.

Processing of carbon-fiber-reinforced Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass composites

Carbon-fiber-reinforced bulk metallic glass composites are produced by infiltrating liquid Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 into carbon fiber bundles with diameter of the individual fiber of 5 μm. Reactive wetting occurs by the formation of a ZrC layer around the fibers. This results in a composite with a homogeneous fiber distribution. The volume fraction of the fibers is about 50% and the density of the composite amounts to 4.0 g/cm3.

Thermodynamics and kinetics of Zr–Ti–Cu–Ni–Be bulk metallic glass forming liquids

Abstract The viscosity of the equilibrium melt as well as of the undercooled liquid of bulk metallic glass (BMG) forming Zr–Ti–Cu–Ni–Be alloys is several orders of magnitude higher than in pure metals and other alloys. The temperature dependence of the viscosity is consistent with that of a moderately strong liquid. Entropy, enthalpy and Gibbs free enthalpy of the supercooled liquids are closer to the crystalline mixture than in other alloys. Both, the sluggish kinetics and the relative thermodynamic stabilization are likely to have the same origin. The Zr–Ti–Cu–Ni–Be alloys are dense liquids with a small free volume and a pronounced tendency to develop short-range order. The viscosity can describe the crystallization kinetics at high temperatures, whereas at low temperatures it is controlled by solid like hopping of atoms.

Continuous refinement of the microstructure during crystallization of supercooled Zr41Ti14Cu12Ni10Be23 melts

Isothermal crystallization studies are performed on Zr41Ti14Cu12Ni10Be23 melts between the liquidus and the glass transition temperature. The resulting time–temperature-transformation diagram is in good agreement with the data obtained from containerlessly processed samples. The various solidification products are examined by electron microprobe. The investigations reveal the morphology and typical length scale of the microstructure as well as the primarily solidified phases after crystallizing at different degrees of undercooling. The typical length scale decreases continuously with increasing supercooling over five orders of magnitude. The number density of nuclei during primary crystallization is estimated from the microstructure. The results are discussed within several models, such as steady-state nucleation, spinodal decomposition, and a nonpolymorphic nucleation model.

Thermodynamics and kinetics of the Zr41.2Ti13.8Cu10.0Ni12.5Be22.5 bulk metallic glass forming liquid : glass formation from a strong liquid

Abstract The flow properties of Zr41.2Ti13.8Cu10.0Ni12.5Be22.5 have been measured both in the liquid and supercooled liquid state, spanning 15 orders of magnitude in viscosity. We used beam bending experiments in the vicinity of the glass transition regime and concentric cylinder rheometry near the liquidus temperature to find very large viscosities for a metallic system of 2 Pa s in the equilibrium liquid. The temperature dependence of the viscosity is discussed in the framework of the Adam–Gibbs entropy model as well as the free volume model for viscous flow. The thermodynamic functions on the one side and the specific volume on the other have been measured previously and allow for a detailed comparison of the two phenomenological descriptions.

Nucleation in undercooled Zr41Ti14Cu12Ni10Be23 melts

The undercooling and solidification of bulk glass forming Zr41Ti14Cu12Ni10Be23 liquids is investigated by processing in high purity graphite crucibles. Isothermal nucleation studies are performed between the liquidus and the glass transition temperature. The resulting time–temperature–transformation diagram is similar to the diagram obtained from containerlessly processed samples. Heterogeneous surface nucleation at the container walls does not effect the crystallization. The solidification products of these differently processed specimens are investigated by scanning electron microscopy as well as by electron microprobe. The investigations reveal the morphology and typical length scale of the microstructure after crystallization at different degrees of undercooling. The number density of nuclei during primary crystallization is estimated from the microstructure.

Crystallization of Supercooled Zr 41 Ti 14 Cu 12 Ni 10 B 23 Melts During Continuous Heating and Cooling

The crystallization behavior of the bulk glass forming Zr 41 Ti 14 Cu 12 Ni 10 Be 23 liquid was studied under different heating and cooling rates. Investigations were performed in high purity graphite crucibles since heterogeneous surface nucleation at the container walls does not effect the crystallization of the bulk sample. A rate of about 1 K/s is sufficient to circumvent crystallization of the melt while cooling from the equilibrium melt. In contrast, upon heating a rate of more than 150 K/s is necessary to avoid crystallization of Zr 41 Ti 14 Cu 12 Ni 10 Be 23 samples. The difference between the critical heating and cooling rate is discussed within classical nucleation theory and diffusion limited crystal growth. The calculated difference of the critical heating and cooling rate can be explained by the fact that nuclei formed during cooling and heating are expose to different growth rates.

Viscosity, Relaxation and Crystallization Kinetics In Zr-Ti-Cu-Ni-Be Strong Bulk Metallic Glass Forming Liquids

The high thermal stability of bulk metallic glass (BMG) forming liquids in the undercooled state allows for measurements of thermophysical properties in a large time and temperature window. In this contribution, results on viscous flow, relaxation and crystallization of Zr-Ti-Cu- Ni-Be BMG forming alloys are presented. The data are compared with the kinetics of other metallic and non-metallic liquids. BMG formers are relatively strong liquids with melt viscosities that are about three orders of magnitude larger than in pure metals and other alloys. The strong liquid behavior of these alloys is also reflected by a small entropy of fusion and a weak temperature dependence of the thermodynamic functions upon undercooling. The high viscosity and small driving force for crystallization are major contributing factors to the high glass forming ability and low critical cooling rate. The upper portions of experimental timetemperature- transformation diagrams down to the crystallization nose can be described well using the kinetics deduced from the viscosity data. For lower temperature the viscosity can not describe the crystallization kinetics. The time scale for structural relaxation becomes larger than for diffusive hopping processes. Diffusion stays relatively fast, whereas viscosity and structural relaxation time upon undercooling follow a Vogel-Fulcher-Tammann relation.