Quantitative analysis of alloy structures solidified under limited diffusion conditions

International audienceContents: The Need for an Instrumented Drop Tube. Description of IA. Powder Characteristics. Quantification of Microstructure. Secondary Dendrite Arm Spacing. X-Ray Microtomography. Neutron Diffraction. Modeling. Cooling Rate. Eutectic Undercooling. Peritectic System

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

International audiencePeritectic 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

Microstructural analysis of rapidly solidified particles 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 (5 and 20 ̊C/minutes). Also, the effects of cooling rate on the microstructure and the phase formation of the rapidly solidified droplets were investigated using scanning electron microscope and neutron diffraction. Rietveld analysis was performed to estimate the phase fractions of Al3Ni2, Al3Ni and eutectic Al. The results were compared to those achieved from electromagnetic levitation under terrestrial and microgravity conditions (TEXUS 44). Effect of cooling rate and microgravity condition on the crystal structure of Al3Ni2 was also studied. It was shown that increasing cooling rate as a result of decreasing particle size or using helium as a cooling gas, rather than nitrogen, would result in a refined microstructure. From Rietveld analysis on neutron diffraction data, it was shown that the increasing cooling rate increases the weight fraction Al3Ni in Al – 36 wt% Ni, while it has an opposite effect in Al – 50 wt% Ni. Also, from Rietveld analysis studies, a striking difference between the samples solidified in the drop tube-impulse system and those produced in microgravity was observed. The former always contain eutectic aluminum, while the latter showed no sign of this element. Crystal structure studies on Al3Ni2 showed that increasing cooling rate changes the c/a ratio in Al – 50 wt% Ni and Al – 36 wt% Ni. It was found that in the sample with higher nickel content, increasing cooling rate increased the c/a ratio, while in the sample with lower nickel content, it showed opposite effect. This work is part of NEQUISOL project supported by ESA within contract 15236/02/NL/SH and CSA within contract number 9F007-08-0154 and SSEP Grant 2008. The authors thank Stefan Schneider for assistance in conducting the TEXUS experiments

Solidification modeling: from electromagnetic levitation to atomization processing

International audienceContents: Introduction. Electromagnetic Levitation. Impulse Atomization. Modeling. General Assumptions. Mass Conservations. Specific Surfaces. Diffusion Lengths. Nucleation. Heat Balance. Thermodynamics Data. Growth Kinetics. Numerical Solution. EML Sample. IA Particles. Regime of Distinct Successive Growth. Regime of Shortcut of the Primary Growt

Solidification modelling: From electromagnetic levitation to atomization processes

International audienceContents: Introduction. Electromagnetic Levitation. Impulse Atomization. Modeling. General Assumptions. Mass Conservations. Specific Surfaces. Diffusion Lengths. Nucleation. Heat Balance. Thermodynamics Data. Growth Kinetics. Numerical Solution. EML Sample. IA Particles. Regime of Distinct Successive Growth. Regime of Shortcut of the Primary Growt

Containerless Solidification and Characterization of Industrial Alloys (NEQUISOL)

Containerless solidification using electromagnetic levitator, gas atomization and an instrumented drop tube, known as impulse atomization is investigated. Effect of primary phase undercooling on dendrite growth velocity of Al-Ni alloys under terrestrial and reduced-gravity condition is discussed. It is shown that with increasing undercooling in the Ni-rich alloys the growth velocity increases, whereas in the Al-rich alloys the growth velocity decreases. However, the Al-rich alloy in microgravity shows similar behavior to that of Ni-rich alloys. Furthermore, the effect of cooling rate on the phase fractions, metastable phase formation and Al3Ni2 lattice parameter of impulse-atomized Al-Ni alloys is discussed. In addition, the effects of primary phase and eutectic undercooling on the microstructure of Al-Fe alloys are investigated. The TEM characterization on the eutectic microstructure of impulse-atomized Al-Fe powders with two compositions shows that the metastable AlmFe forms in these alloys. Also, the growth undercooling that the dendritic front experiences in the solidification of the droplet results in variation of dendrite growth direction from to . For Al-4 at%Fe, it is found that in the sample solidified 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. A microsegregation model for the solidification of Al-Ni alloys is presented that accounts for the occurrence of several phase transformations, including one or several peritectic reactions and one eutectic reaction

Non-Equilibrium Solidification, Modeling for Microstructure Engineering of Industrial Alloys (NEQUISOL)

Within the ESA-MAP project NEQUISOL solidification and microstructure evolution of containerless undercooled drops and droplets of Al-based alloys are investigated. Individual drops in diameter of about 6mm are processed by electromagnetic levitation both on Earth and in Space. This technique allows to measuring the temperature time profile during melting and solidification yielding the undercooling prior to solidification. The dendrite growth velocities are measured by high-speed camera technique. Comparing experiments on Earth and in Space allows for determining the influence of convection on solidification kinetics and microstructure evolution. These results are used to extend multi-scale modeling to predict microstructure development as function of undercooling and cooling rate. The results are directly related to the production of Al-Ni powders in atomization facilities operated by some partners of the project. Atomization provides both containerless processing and reduced gravity conditions in solidification of small droplets during free fall on Earth. It is an industrial production route of advanced materials

Non-equilibrium solidification, modelling for microstructure engineering of industrial alloys (NEQUISOL)

International audienceWithin NEQUISOL project, crystallisation kinetics and microstructure evolution in undercooled melts of Al-based alloys is investigated. Different techniques are applied for containerless processing of the different alloys. These allow for undercooling a liquid far below its equilibrium melting temperature. An undercooled melt is in a metastable state giving access of different solidification pathways the system can take. Solidification starts with nucleation and is completed by subsequent growth of crystals. The negative temperature gradient in front of the solid-liquid interface and the concentration gradient in alloys destabilize a planar interface leading to dendrite growth. Dendrite growth dynamics and microstructure evolution in undercooled melts is investigated on drops undercooled by Electro-Magnetic Levitation (EML). The speed of the propagating solidification front is monitored by means of a high-speed camera with a maximum frequency of 120 000 pictures per second. Under Earth conditions strong alternating electromagnetic fields are needed to compensate the gravitational force. This, in turn, causes forced convection due to the strong stirring effects. Therefore, equivalent experiments are conducted under microgravity conditions using the TEMPUS facility for electromagnetic levitation in reduced gravity during parabolic flights and during TEXUS sounding rocket missions. Experiments on four selected alloys, Al 40Ni 60, Al 70Ni 30, Al 65Ni 35 and Al 89Cu 11 are in preparation to be performed on board the ISS using the Electro-Magnetic Levitator currently under development by DLR/ESA. In addition atomization facilities are operated that combines containerless processing with large cooling rates and reduced gravity on Earth. Atomization is an industrial processing route to produce metastable materials in large amount. We present a comparison of first experiments conducted in reduced gravity (parabolic flight, TEXUS) and reference experiments on Earth of measurements of the growth velocity as a function of undercooling of the congruently melt-ing Al 50Ni 50 alloy and the Raney type alloy Al 68.5Ni 31.5. The latter one is of special interest for industry because of its extraordinary potency as a catalyst. The experiments clearly demonstrate how important convection is in heat and mass transport processes which control dendrite growth dynamics and, hence, microstructure evolution. A sharp interface theory is presented that takes into account heat and mass transport by forced convection. This mesoscopic model is able to predict the dendrite growth kinetics obtained both on Earth as well as in reduced gravity. In addition, mesoscopic modelling is combined with macroscopic modelling to describe the entire solidification process involving several recalescences and the non-equilibrium solidification of several solid microstructures