Microstructure of a Rapidly Solidified Al-Fe-Co-Ni Alloy
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MICROSTRUCTURE OF A RAPIDLY SOLIDIFIED Al-Fe-Co-Ni
369 ALLOY
T. H. SANDERS, JR., Purdue University, West Lafayette, IN 47907 H. G. PARIS AND J. W. MULLINS, Alcoa Technical Center, Alcoa Center, PA 15069 ABSTRACT Observations made on different consolidated and wrought microstructures suggested that in addition to the simple variations in size and size distributions there were also differences in shape. Furthermore, clusters of oriented precipitates were often observed. The different particle morphologies This paper describes the changes were related to solidification rate effects. in microstructure which occur during the heating of rapidly solidified AI-3.3Fe-4.6Co-2.3Ni splat. INTRODUCTION Rapid solidification techniques applied to unique aluminum alloy compositions can result In significant Improvements in certain engineering properties over existing commercial aluminum alloys. Consequently, recent work (1-5) has centered on the utilization of the natural benefits of rapid solidification to produce metastable structures which decompose during processing into a dispersion of IntermatallIc phases. These phases Increase the elastic modulus and are more thermally stable than the metastable phases in 2XXX and 7XXX alloys. The results of a series of microstructure Investigations (5-7) in the Al-Fe-Co-Ni system have shown a clear correlation between the type of particulate (air atomized and splat), consolidation temperature, and composition with microstructure and tensile properties. The yield and tensile strengths were dependent upon the interparticle spacing. Thus those fabricating processes that resulted in a high volume fraction of small, closely spaced particles gave the highest strength. On the other hand, fracture toughness and ductility were affected not only by the particle size but also by the particle size distribution. The presence of coarse particles acted as stress concentrators, and the heterogeneity of the microstructure tended to Increasing the number of coarse particles and/or the localize deformation. heterogeneity decreased the fracture toughness. However, other basic differences in the microstructure of rapidly solidified material may also be important. The observations made on the different consolidated and wrought microstructures suggested that in addition to the simple variations In size and size distributions there were also differences In shape. Furthermore, clusters of oriented precipitates were often observed. The different particle morphologies most likely had their origins In the solidification process. Consequently, an experimental program was initiated to related the as-cast microstructure observed In the wrought products by observing in situ changes In appearance of the microstructure of splat particulate during heating. This paper will present the results of that investigation. MATERIALS AND METHODS An alloy having the nominal composition listed in Table 1 was obtained In splat particulate form by using argon with a dew point of
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