R. J. Bayuzick

Undercooling of pure metals in a containerless, microgravity environment

The 105‐m drop tube at NASA/Marshall Space Flight Center has been used in a series of undercooling experiments on pure metals. Ti, Zr, Nb, Mo, Rh, Hf, Ta, and Pt were undercooled 17–20% of the melting temperature in a containerless, microgravity environment. Ir and Ru were undercooled to 13% Tm. Sample sizes ranged from 175 to 880 mg.

Dual purpose pyrometer for temperature and solidification velocity measurement

A dual purpose pyrometer is described that allows both accurate radiance temperature measurement and fast temporal response. The system uses two silicon photodiodes with separate optical paths derived from a common spot on the sample. The optical bandwidths and response times of each detection circuit are tailored to the function of each radiometer. The radiance temperature of electromagnetically levitated metallic samples is measured over a narrow optical bandwidth with a high‐gain silicon detector. The velocity of solidification of undercooled melts can be deduced from the rise time of the second silicon detector which samples a broad optical bandwidth and has a fast response time. Results from experiments on the undercooling and solidification behavior of electromagnetically levitated pure nickel show that the solidification velocity approaches 17 m/s at high undercooling.

Solidification kinetics and metastable phase formation in binary Ti-Al

Near-equiatomic alloys of Ti-Al were solidified at various bulk undercoolings using electromagnetic levitation. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Re- calescence rise times were used to determine semiquantitative primary solidification kinetics for the different phases. Primary β solidification was observed at compositions well into the equi- librium α regime; this is presented as part of a near-equiatomic nucleation domain diagram mat shows the primary solidification phase (β, α, ordered γ, or disordered γ) that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary β (Vmax ≈ 15 to 18 m s-1) than they are for primary α (Vmax ≈ 10 to 12 m s-1). For undercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for γ than for α. However, at an undercooling of about 150 K, the solidi- fication kinetics for γ increase discontinuously. This discontinuity is associated with a change in the primary solidification phase from ordered γ (Vmax ≈ 0.5 m s-1) to disordered γ (Vmax ≈ 10 m s-1).

The velocity of solidification of highly undercooled nickel

At large undercoolings (τ;10 pctTM, present theories relating solidification velocity to degree of undercooling do not agree well with reported experimental data for the solidification velocity of nickel as a function of undercooling. The present work shows that this discrepancy is due to two factors. First, the majority of previously reported results overestimate the solidification velocity of nickel at large undercoolings. Second, the scatter in experimental data is so large that a functional relationship between undercooling and velocity is not evident. In this study, the solidification velocity of undercooled nickel was measured using a linear array of 38 photodiodes. The results indicate that the velocity of the thermal field generated by the solid/liquid interface approaches a maximum velocity of 20 m s−1 atΔT} ≈ 10 pctTM (173 K) and men remains constant with increasing undercooling. This suggests that the velocity of the solid/liquid interface, at undercoolings greater than 10 pctTM, could be limited by attachment kinetics at the interface.

Solidification velocity of undercooled Ni-Cu alloys

Abstract The solidification velocity of Ni–Cu alloys was measured as a function of bulk undercooling using high-speed thermal imaging of electromagnetically levitated samples. Two departures from power law growth (approximating plateaus) in the velocity versus undercooling data were observed: the first occurred at intermediate undercoolings and is attributed to copper solute, while the second occurred at high undercoolings and is hypothesized to be an effect of oxygen. The Ivantsov solution with marginal stability arguments (IMS model) is a widely used model that relates dendrite growth velocity to total undercooling for dilute alloy systems. However, the model does not predict a plateau at intermediate undercoolings for alloys with a large equilibrium partition coefficient, k E . Satisfactory agreement between the model and experimental results can be obtained by using a value of k E that is smaller than the alloy’s actual value, but this is physically unreasonable and causes disagreement with currently accepted kinetic models.

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetic ally levitated drops were melted and subsequently splat quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Micro-structures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the α-Nb + Nb3Si eutectic are also suggested.

Solidification of Nb-Ge alloys in long drop tubes

The 30 m and 100 m long drop tubes at the Marshall Space Flight Center have been used to obtain large undercooling in Nb-Ge alloys. Electron beam melting has been used to obtain drops approximately 2.5 mm in diameter. In the 30 m tube many specimens fell the length of the tube without solidifying, and were ultimately liquid quenched in oil. The amount of undercooling prior to the quench was usually around 0.13T;m. In the 100 m tube, freezing generally initiated during free fall, and the maximum undercooling was around 0.22Tm. Microstructures were characterized by a combination of X-ray diffraction, optical microscopy, and scanning electron microscopy with energy dispersive analysis by X-rays. A variety of interesting microstructures was observed.

A statistical approach to understanding nucleation phenomena

Abstract A series of undercooling experiments using pure metal samples of niobium and zirconium were performed using the Marshall Space Flight Center 105 meter drop tube, and for zirconium using both electromagnetic and electrostatic levitation in the laboratory. The results from a large number of experiments allowed nucleation frequencies to be measured, following the suggestion of Skripov. Using the nucleation frequencies, the nucleation probability distributions and the cumulative distributions, the critical free energy of nucleation and the magnitude of the pre-exponential in the classical nucleation equation were determined. The results for the pre-exponential and critical free energy for zirconium ranged from 10 8 to 10 13 and 15 kT to 24 kT respectively. Higher purity zirconium stock was observed to cause significant shifts in the undercoolings to higher values. For niobium the values were 10 31 and 73 kT for the pre-exponential and the critical free energy, respectively.

The solidification velocity of pure nickel

Abstract The solidification velocity of pure nickel processed in three different gas environments was measured using an ultra-high speed imaging technique (1 million frames per s). The results indicate that present solidification theories do not correctly predict the solidification behavior of undercooled metals. The solidification velocity as a function of undercooling was measured for pure nickel processed in 99.999% pure helium, processed in helium/20 wt.% hydrogen, and processed in a contaminated gas environment. The results indicate a transition in the solidification velocity/undercooling relationship in the first two data sets and a continually increasing, but significantly depressed, relationship for the last data set. A solidification model is proposed to explain the correlation and the deviation of the experimental results to solidification theory. The model asserts that at low undercoolings the solid–liquid interface is comprised of dendrites spaced widely enough apart that adjacent dendrites do not thermally interact. As the undercooling increases, the dendrites become more closely spaced and the thermal fields surrounding each dendrite begin to overlap. This occurrence causes the transition in the solidification velocity/undercooling relationship and the reduced dependence of the solidification velocity on undercooling at large undercoolings. Based on the experimental results, careful examination of the Boettinger, Corriell, and Trivedi (BCT) theory, and the proposed solidification model it is concluded that collision limited growth is an incorrect assumption for nickel solidifying at the undercoolings attained by electromagnetic levitation.

Microstructures of niobium-germanium alloys processed in inert gas in the 100 meter drop tube

The 100 meter drop tube at NASA’s Marshall Space Flight Center has been used for a series of experiments with niobium-germanium alloys. These experiments were conducted with electromagnetic levitation melting in a 200 torr helium environment. Liquid alloys experienced large degrees of undercooling prior to solidification in the drop tube. Several interesting metastable structures were observed. However, the recalescence event prevented extended solid solubility of germanium in the A-15 beta phase. Liquids of eutectic composition were found to undercool in the presence of solid alpha and solid Nb5Ge3.