Jan R. Rogers

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...

Machine vision for high-precision volume measurement applied to levitated containerless material processing

By combining the best practices in optical dilatometry with numerical methods, a high-speed and high-precision technique has been developed to measure the volume of levitated, containerlessly processed samples with subpixel resolution. Containerless processing provides the ability to study highly reactive materials without the possibility of contamination affecting thermophysical properties. Levitation is a common technique used to isolate a sample as it is being processed. Noncontact optical measurement of thermophysical properties is very important as traditional measuring methods cannot be used. Modern, digitally recorded images require advanced numerical routines to recover the subpixel locations of sample edges and, in turn, produce high-precision measurements.

Convection in Containerless Processing

Abstract: Different containerless processing techniques have different strengths and weaknesses. Applying more than one technique allows various parts of a problem to be solved separately. For two research projects, one on phase selection in steels and the other on nucleation and growth of quasicrystals, a combination of experiments using electrostatic levitation (ESL) and electromagnetic levitation (EML) is appropriate. In both experiments, convection is an important variable. The convective conditions achievable with each method are compared for two very different materials: a low‐viscosity, high‐temperature stainless steel, and a high‐viscosity, low‐temperature quasicrystal‐forming alloy. It is clear that the techniques are complementary when convection is a parameter to be explored in the experiments. For a number of reasons, including the sample size, temperature, and reactivity, direct measurement of the convective velocity is not feasible. Therefore, we must rely on computation techniques to estimate convection in these experiments. These models are an essential part of almost any microgravity investigation. The methods employed and results obtained for the projects levitation observation of dendrite evolution in steel ternary alloy rapid solidification (LODESTARS) and quasicrystalline undercooled alloys for space investigation (QUASI) are explained.

X-ray and electrostatic levitation undercooling studies in Ti–Zr–Ni quasicrystal forming alloys

The first undercooling measurements on electrostatic-levitated droplets of TiZrNi alloys that form the icosahedral quasicrystal phase are presented. The reduced undercooling for crystallization decreases with an increasing polytetrahedral order of the primary solidifying phase, suggesting the development of icosahedral short-range order in the undercooled liquid. X-ray diffraction measurements made at the advanced photon source on liquid droplets of these alloys, aerodynamically levitated and heated to near their liquidus temperature, however, show only weak evidence for increased icosahedral order. This suggests that significant ordering occurs below the melting temperature.

Contrasting electrostatic and electromagnetic levitation experimental results for transformation kinetics of steel alloys.

Abstract: The delay between conversion of metastable ferrite to stable austenite during ternary Fe‐Cr‐Ni alloy double recalescence is seen to differ by over an order of magnitude for tests conducted using electrostatic and electromagnetic levitation. Several possible reasons for this deviation are proposed. Thermodynamic calculations on evaporation rates indicate that potential composition shifts during testing are minimized by limiting test time and thermal history. Simulation indicates that deviation would be limited to a factor of 1.5 under worst‐case conditions. Possible effects due to differences in sample size are also eliminated since the metastable array, where stable phase nucleation must occur, is significantly smaller than the sample. Differences in internal convection are seen to be the most probable reason for the observed deviation.

Role of Ti in the formation of Zr-Ti-Cu-Ni-Al glasses

It has been widely reported that glass formation improves in Zr62Cu20Ni8Al10 alloys when small amounts of Ti are substituted for Zr. Glasses containing greater than 3 at. % Ti crystallize to a metastable icosahedral phase, suggesting that Ti enhances icosahedral short-range order in the liquid/glass, making crystallization more difficult during cooling. However, based on containerless solidification and in situ high-energy synchrotron diffraction studies of electrostatically levitated supercooled liquids of these alloys, we demonstrate that Ti inhibits surface crystallization but neither increases the icosahedral short-range order nor improves glass formation.

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.

A Review of Electrostatic Levitation for Materials Research

Electrostatic levitation (ESL) has been applied to research on bulk high-temperature materials for over 15 years. ESL is a non-contact method performed in vacuum or high-pressure gas, making it especially applicable in studying undercooled and/or hightemperature materials. ESL has been applied to metals, ceramics, glasses, and semiconductors from room temperature to over 3800 K. Experiments conducted using ESL span the range from measurement of thermophysical properties, phase diagrams, and rates of nucleation and solidification to the structure of undercooled liquid metals. Through national user facilities and individual laboratories, ESL is now widely available. We review the range of measurements being performed using ESL, with special emphasis on several recent innovations in measurements important to materials research. K e y w o r d s : electrostatic levitation, review, thermophysical properties, mechanical properties, solidification, containerless processing.

Structural studies of a Ti-Zr-Ni quasicrystal-forming liquid

Employing the technique of electrostatic levitation coupled with high-energy x-ray diffraction, Ti39.5Zr39.5Ni21 liquids were shown previously to develop significant short-range icosahedral order with supercooling. However, that conclusion was based on the assumption of a single dominant cluster type in the liquid and the observed evolution of the high-q shoulder on the second peak in the structure factor, S(q). Here, new diffraction data that were obtained using more rapid data acquisition methods are presented. These allow structural studies to be made down to and through recalescence to the icosahedral quasicrystal. The liquid structures obtained from a Reverse Monte Carlo analysis of these data are characterized by their bond-angle distributions, Honeycutt and Andersen indices and bond orientational order parameters. These analyses indicate that while there are several different types of local order, the icosahedral short-range order is dominant and increases gradually with supercooling.