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The development of refined microstructures for dispersion strengthening of Aluminum alloys by rapid solidification is related to the undercooling at nucleation and during competitive crystal growth. With controlled droplet samples the important processing parameters that govern undercooling in the powders have been identified and include powder size and coating melt cooling rate and superheat. The droplet method has yielded measured undercoolings of 175 C for AL and values near 300 C for some AL alloys. Droplet studies have been valuable in examining the solidification pathways including thermal history effects associated with rapid solidification and dispersoid formation in Aluminum-Nickel, Aluminum-Iron, and Aluminum-Beryllium alloys. Based on the analysis of solidification pathways, microstructure maps may be developed to identify the processing that can yield fine-scale eutectics, metastable structures or suppression of coarse primary intermetallic phases which are useful for the development of dispersoid structures. (jg).
The amount of liquid undercooling attained during rapid solidification processing (RSP) is an important factor in determining microstructural development. An effective experimental method that may be applied in assessing undercooling potential involves the examination of a dispersion of stabilized, fine (5 to 20-μm) liquid droplets. Substantial undercoolings (?T >= 160°C) have been measured by differential thermal analysis for slowly cooled (30°C/min) droplet emulsions of aluminum alloys. Results from Al-Si and Al-Fe alloys indicate that undercooling is a function of droplet size, melt super-heating, and material composition. Comparison studies have also been conducted relating the amount of undercooling to microstructural refinement in alloys prepared as droplet emulsions and by air atomization. The results indicate that at large undercooling (?T > 260°C) droplet solidification can yield a microstructure similar to that formed in atomized powder. Microstructural variations with powder size are related not only to a changing cooling rate but also to a modified undercooling behavior. These observations highlight the role of undercooling in RSP and indicate that the droplet approach can be effective in examining potential aluminum powder metal alloys and in optimizing processing procedures for rapid quenching treatments.
This portion of the program was conducted for the purpose of developing a predictive model of heat flow and solidification for aluminum alloys produced under the high cooling rate conditions achievable in atomization processes. It is a combined experimental and theoretical study of the relationship between the important solidification variables (e.g. cooling rate, temperature gradients, interface shape and velocity, supercooling and transformation kinetics) and the structure of rapidly solidified aluminum alloy powders. This report covers that portion of the investigations which addressed the characterization of the thermal history of rapid solidification in metal droplets, and its effects on powder microstructure. The main effort was focused on modelling the heat flow during solidification, and relationships were established between the atomization parameters, the growth kinetics, the interface velocity and undercooling, and other important variables. Numerical solutions based on the enthalpy model were developed, and their results compared to the trends predicted from the Newtonian model. The analysis covered situations of isothermal solidification at the melting temperature, as well as those where significant undercoolings are necessary for nucleation and growth. The implications of single vs. multiple nucleation were also discussed. The concepts developed from the heat flow analysis were coupled to microstructural observations in Aluminum alloy powders, mostly in the submicron size range. It was shown that reducing the particle size decreases the extent of segregation, promotes multiple nucleation and the formation of twins during solidification.
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