Arash Ilbagi

Microstructural analysis of rapidly solidified aluminium–nickel alloys

Abstract Powders of Al–50 wt-%Ni and Al–36 wt-%Ni were produced using the impulse atomisation technique, a rapid solidification technique. The molten droplets were cooled in flight by the stagnant helium or nitrogen in the atomising chamber. The resulting powders were sieved into different size ranges. X-ray diffraction and neutron diffraction were used in order to quantify the phase fractions in the samples. Profile refinement, using the computer software GSAS, was used to calculate the weight fraction of the existing phases, namely Al3Ni2, Al3Ni and Al, as a result of different processing parameters. The Scheil–Gulliver model was applied to investigate the extent to which it can predict phase fractions in the Al–Ni system. In Al–50 wt-%Ni, by increasing cooling rate, the ratio of Al3Ni to Al3Ni2 approaches that of Scheil–Gulliver’s prediction. Opposite behaviour was observed in Al–36 wt-%Ni. In addition, from the profile refinement, the effect of composition and cooling rate on the lattice parameter of Al3Ni2 was investigated. In Al–36 wt-%Ni, the c/a ratio is significantly smaller than the stoichiometric c/a ratio of Al3Ni2, and it decreases with increasing cooling rate. On the other hand, for Al–50 wt-%Ni, the c/a ratio is much closer to the stoichiometric value and it increases with increasing cooling rate. On a produit des poudres d’Al-50% en poids de Ni et d’Al-36% en poids de Ni en utilisant la technique d’atomisation par impulsion, une technique de solidification rapide. Les gouttelettes fondues étaient refroidies en vol par l’hélium ou l’azote stagnants dans la chambre d’atomisation. Les poudres résultantes étaient tamisées en différentes gammes de taille. On a utilisé la diffraction des rayons X et la diffraction des neutrons pour quantifier les fractions de phase dans les échantillons. Le raffinement du profil, obtenu grâce au logiciel d’ordinateur GSAS, était utilisé pour calculer la fraction de poids des phases existantes, soit Al3Ni2, Al3Ni et Al, comme résultat des différents paramètres de traitement. On a appliqué le modèle de Scheil-Gulliver pour vérifier à quel point il peut prédire les fractions de phase du système Al-Ni. Pour l’Al-50% en poids de Ni, en augmentant la vitesse de refroidissement, le rapport d’Al3Ni à Al3Ni2 s’approche de celui de la prédiction de Scheil-Gulliver. On a observé le comportement opposé pour l’Al-36% en poids de Ni. De plus, à partir du raffinement du profil, on a étudié l’effet de la composition et de la vitesse de refroidissement sur le paramètre de réseau de l’Al3Ni2. Pour l’Al-36% en poids de Ni, le rapport c/a est significativement plus petit que le rapport c/a stoechiométrique de l’Al3Ni2, et il diminue avec une augmentation de la vitesse de refroidissement. D’un autre côté, pour l’Al-50% en poids de Ni, le rapport c/a est beaucoup plus près de la valeur stoechiométrique et il augmente avec une augmentation de la vitesse de refroidissement.

Effect of cooling rate on solidification of Al-Ni alloys

Particles of Al-Ni alloys with different compositions (Al–50 wt-% Ni and Al–36 wt-% Ni) were produced using a drop tube-impulse system, known as Impulse Atomization. The microstructure of these rapidly solidified particles was compared with those solidified in a DSC at low cooling rates (0.083 and 0.33 K/sec). Also, the microstructure of the sample solidified in microgravity on-board of the TEXUS 44 sounding rocket was analyzed. Neutron diffraction was used to investigate the phases formed during different solidification processes. From SEM micrographs and neutron diffraction it was found that the inner parts of the TEXUS sample and the sample that was cooled at 0.083 K/sec contain almost no eutectic structure. The outer rim of the TEXUS sample showed the highest amount of Al3Ni and lowest amount of Al3Ni2 Increasing the cooling rate from 0.083 to 0.33 K/sec increased the Al3Ni/Al3Ni2 ratio. Opposite trend was observed in the impulse-atomized particles, where increasing the cooling rate decreased the Al3Ni/Al3Ni2 ratio.

Containerless Solidification and Characterization of Industrial Alloys (NEQUISOL)

Containerless solidification using electromagnetic levitator (EML), gas atomization and an instrumented drop tube, known as impulse atomization is investigated for Al-Fe and Al-Ni alloys. The effects of primary phase and eutectic undercooling on the microstructure of Al-Fe alloys are investigated using the impulse drop tube and parabolic flight. The TEM characterization on the eutectic microstructure of impulse-atomized Al-Fe powders with two compositions showed that the metastable AlmFe formed in these alloys. Also, the growth undercooling that the dendritic front experiences during the solidification of the droplet resulted in variation of dendrite growth direction from to . For Al-4 at%Fe, it was found that in reduced-gravity and in the impulse-atomized droplets the primary intermetallic forms with a flower-like morphology, whereas in the terrestrial EML sample it has a needle like morphology. For Al-Ni, the effect of primary phase undercooling on dendrite growth velocity under terrestrial and reduced-gravity condition is discussed. It is shown that under terrestrial conditions, in the Ni-rich alloys with increasing undercooling the growth velocity increases, whereas in the Al-rich alloys the growth velocity decreases. However, the Al-rich alloy that was studied in reduced-gravity showed similar behavior to that of Ni-rich alloys. Furthermore, the effect of cooling rate on the phase fractions and metastable phase formation of impulse-atomized Al-Ni alloys is compared with EML. A microsegregation model for the solidification of Al-Ni alloys is applied to impulse atomized powders. The model accounts for the occurrence of several phase transformations, including one or several peritectic reactions and one eutectic reaction.

In-situ characterization of droplets during free fall in the drop tube-impulse system

Powders of copper were produced using a drop tube-impulse atomization technique. In this system, molten metal is pushed through orifices, forming ligaments, which eventually break down and spherodize into droplets. A 3-D translation stage was designed, constructed and installed in the drop tube to allow for measurements of velocity and droplet size in flight using a Shadowgraph and radiant energy using DPV-2000. A mathematical model of the evolution of droplet velocity and temperature for different sized copper droplets at various heights was developed. The experimental results from the Shadowgraph and the DPV-2000 are compared to the models results. In addition, the extent to which microgravity prevails during flight and droplet solidification was investigated by using the model and the Shadowgraph results. It was found that the acceleration of falling droplets near the melting point is close to gravitational acceleration and as a result the falling droplets do not reach their terminal velocity at their melting point. The results of in-situ measurements during the atomization of copper showed that the larger droplets have higher radiant energy than that of the smaller ones. Correlation between experimentally measured radiant energy and predicted temperature of falling droplets will be investigated. The current work is part of the NEQUISOL project supported by ESA within contract number 15236/02/NL/SH and CSA within contract number 9F007-08-0154 and SSEP Grant 2008.

Neutron diffraction analysis and solidification modeling of Impulse-Atomized Al-36 wt%Ni

Peritectic solidification reactions appear in many metallic systems such as Al-Ni alloys. However, microstructural evolution and effects of processing parameters on phase selection during peritectic reaction are not well understood. In this paper Impulse-Atomization experiments and simulations were used to study rapid solidification of Al-36 wt% Ni particles. Secondary dendrite arm spacing measurements were used to estimate the cooling rate achieved during the solidification of these particles. The weight fractions of the phases formed in different sizes of the particles were measured using neutron diffraction technique. Solidification paths were then simulated with a model previously validated for concurrent dendritic, peritectic and eutectic phase transformations in binary alloys. Model predictions are compared to the experimental results to understand the sequence of transformations that leads to the final metallurgical state of the particles.