Thomas J. Rathz

Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3

The crystallization kinetics of Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 were studied in an electrostatic levitation (ESL) apparatus. The measured critical cooling rate is 1.75 K/s. Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 is the first bulk-metallic-glass-forming liquid that does not contain beryllium to be vitrified by purely radiative cooling in the ESL. Furthermore, the sluggish crystallization kinetics enable the determination of the time-temperature-transformation (TTT) diagram between the liquidus and the glass transition temperatures. The shortest time to reach crystallization in an isothermal experiment; i.e., the nose of the TTT diagram is 32 s. The nose of the TTT diagram is at 900 K and positioned about 200 K below the liquidus temperature.

Beamline electrostatic levitator for in situ high energy x-ray diffraction studies of levitated solids and liquids

Determinations of the phase formation sequence, crystal structures and the thermo-physical properties of materials at high temperatures are hampered by contamination from the sample container and environment. Containerless processing techniques, such as electrostatic (ESL), electromagnetic, aerodynamic, and acoustic levitation, are most suitable for these studies. An adaptation of ESL for in situ structural studies of a wide range of materials using high energy (30–130keV) x rays at a synchrotron source is described here. This beamline ESL (BESL) allows the in situ determination of the atomic structures of equilibrium solid and liquid phases, undercooled liquids and time-resolved studies of solid-solid and liquid-solid phase transformations. The use of area detectors enables the rapid acquisition of complete diffraction patterns over a wide range (0.5–14A−1) of reciprocal space. The wide temperature range (300–2500K), containerless processing environment under high vacuum (10−7–10−8Torr), and fast data ac...

Undercooling, liquid separation and solidification of Cu-Co alloys

Large undercooling can induce not only various solidification pathways, but also a precursor reaction, or liquid separation. This paper deals with the latter effect of undercooling using examples of the Cu-Co system which has a flattened liquidus. Bulk Cu-Co alloys (about 7 mm in diameter) at compositions ranging from 10 to 100 wt % Co were highly undercooled using a fluxing technique. Except for compositions above 90 wt % Co, liquid separation was directly observed as undercooling exceeded a critical value depending on the composition. It was also confirmed by a microstructural transition from dendrites to droplets above the critical undercooling. Finally, calculations of the metastable miscibility boundary were made to analyze the experimental results.

Direct determination of the metastable liquid miscibility gap in undercooled Cu-Co alloys

Abstract Bulk Cu–Co alloys at compositions ranging from 10 to 80 wt.% Co were highly undercooled using a melt fluxing technique. The metastable liquid separation boundary has been directly determined from the measured temperature-time profiles. It was also calculated using a subregular solution model. The critical point of the miscibility gap was found to be about 80 K below the liquidus. A droplet-shaped microstructure was observed for all solidified specimens (Cu–10 to 80 wt.% Co), when the melts were undercooled into the metastable miscibility boundary.

The Marshall Space Flight Center Drop Tube Facility

A description of the structural and instrumentation hardware and the improved experimental capabilities of the 105‐m Marshall Space Flight Center Drop Tube Facility is described. The structure comprising the Drop Tube Facility is designed to provide a high‐vacuum facility in a vertical orientation. The low‐gravity environment obtained during free‐fall and the containerless materials processing capabilities are discussed. Two furnaces, an electron‐beam and an electromagnetic levitator, are used to melt metallic samples in a process environment ranging from 10−6 Torr to 1 atm. Specifics of the vacuum control and gas delivery systems are provided. The experimental instrumentation and data acquisition systems used to perform predrop and in‐flight measurements of the melting and solidification process are discussed. Typical experimental results are presented as an indicator of the performance of the Drop Tube Facility.

High Undercooling of Ni59Nb41 Alloy in a Containerless Electrostatic Levitation Facility

Utilizing the containerless electrostatic levitation facility at NASA/MSFC, we were able to undercool the Ni59Nb41 (atomic) alloy by 210 K which was 160° farther than the results of previous flight experiments. Undercoolings were clustered around 200 K during the repeated melting–freezing cycles on a single sample. Prior to this work, a metastable liquid separation had been presumed to limit the undercooling of this alloy. However, microstructural observations have revealed that undercooling was limited by crystal nucleation.

Containerless measurements of thermophysical properties of Zr54Ti8Cu20Al10Ni8

Abstract:  High‐temperature measurement and study of reactive materials can be difficult with conventional processing methods because contamination from the measuring apparatus and container walls can adversely affect measurements. Containerless processing techniques can be employed to isolate samples from their environment, reducing contamination. Benefits of containerless processing include reduction in heterogeneous nucleation sites, which in turn delays the onset of solidification and allows the study of meta‐stable undercooled phases. However, property measurements must use noncontact methods as well. Fortunately, several optical‐based methods have been developed and successfully employed to measure thermophysical properties, including surface tension, viscosity, density, and thermal expansion. Combining these techniques with the electrostatic levitator (ESL) located at the NASA Marshall Space Flight Center (MSFC) has resulted in an excellent facility to perform containerless material studies which support microgravity flight projects. Currently, studies of the thermophysical properties of liquid quasi‐crystal forming and related alloys ranging from superheated to deeply undercooled states are being done with this facility in support of the NASA‐funded flight project Quasi‐crystalline Undercooled Alloys for Space Investigation (QUASI). While the primary purpose of these measurements is to support planned flight experiments, they are also a desirable resource for future manufacturing considerations and for fundamental insight in the physics of icosahedral ordering in liquids and solids. Presented here is an overview of the contactless measuring methods for surface tension, viscosity, density, and thermal expansion applied to Zr54Ti8Cu20Al10Ni8, for the superheated and meta‐stable undercooled liquid phases, in support of QUASI.

Study of the containerless undercooling of Ti-Ce immiscible alloys

Ti-Ce immiscible alloys of compositions across the miscibility gap were containerlessly processed in both a low-gravity and a unit-gravity environment. Although undercooling of the single-phase liquid into the miscibility gap could not be observed, undercooling did occur across the miscibility gap for the separated liquid Ti-rich phase. The low gravity, quiescent environment favored higher undercooling over the unit-gravity samples. Every undercooled sample had massive separation of the liquid phases. Metallurgical analysis of samples undercooled in unit-gravity showed signs of vigorous convective stirring and shearing of the L1 Ti-liquid by the applied levitation electromagnetic field. In low-gravity processed samples, the L1 liquid formed a near-concentric sphere within a Ce shell with some residual smaller spherical particles dispersed throughout the Ce. This configuration is predicted from wetting theory and from Marangoni separation. Plots of both the melting and solidification temperatures indicate that the monotectic temperature is 1831 ± 12°K rather than the 1723°K as reported in the literature. From chemical and diffraction analysis, the solubility of Ce in the Ti-rich phase was found to be extended; also, some cerium oxide precipitates formed but no perceptible dissolved oxygen within the Ce or Ti phases was found which indicates that the higher monotectic temperature reported here is probably not an oxygen effect.

Nonlinearities in the undercooled properties of Ti39.5Zr39.5Ni21

Using electrostatic-levitator (ESL)-based containerless processing combined with non-contact measuring techniques, surface tension and viscosity have been measured for Ti39.5Zr39.5Ni21, using the oscillating-drop method, in the superheated and metastable undercooled liquid states. Viscosity measurements show a non-Arrhenius behaviour that more closely matches a Vogel–Tamman–Fulcher trend, and a sudden divergence away from a high-temperature linear trend in surface tension has been observed for this composition. An overview of the measurement technique as well as the measurements are presented.

Undercooling of Cu-based binary alloys in a flux and long drop tube

Several Cu-based alloys such as Cu-Co, Cu-Fe and Cu-Nb, which are of considerable technological importance, possess a flattened liquidus implying a thermodynamic tendency to immiscibility upon undercooling. In this paper, both container and containerless processing were used to study the undercooling behavior, metastable liquid separation and microstructural development in the Cu-based systems. For undercooling experiments in an oxide flux, the melt separation temperature could be measured and the metastable liquid miscibility gap has been directly determined; while containerless processing in the 105 m drop tube permits larger undercooling to be achieved prior to solidification. All phase-separated samples were found to exhibit droplet- shaped morphologies, however there were various droplet size distributions, dependent upon composition, undercooling, and cooling rate.