Plastic Deformation of a Rapidly Solidified Al-Li-Zr Alloy
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PLASTIC DEFORMATION OF A RAPIDLY SOLIDIFIED Al-Li-Zr ALLOY N. J. Kim and S. K. Das Allied Corporation, Metals and Ceramics Laboratory, P.O. Box 1021R, Morristown, New Jersey 07960
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ABSTRACT The deformation behavior of a rapidly solidified Al-3.7%Li-O.5%Zr alloy has been investigated using transmission electron microscopy. Aging of the alloy in a conventional manner produces a homogeneous precipitation of 6' with small amount of heterogeneously distributed composite precipitates. The deformation behavior of the alloy in such aging conditions is dominated by the readily shearable 6', resulting in a pronounced planar slip. A homogeneous distrubution of composite precipitates promoted by double-aging treatment, however, alters the deformation mode to wavy slip by inducing the by-passing of dislocations around the shear resistant composite precipitates. INTRODUCTION There is a growing need for structural alloys with improved specific strength to achieve substantial weight savings in aerospace applications. Aluminum-lithium alloys offer the potential of meeting the weight savings due to the pronounced effects of lithium on increasing the elastic modulus and decreasing the density of aluminum alloys. Moreover, heat treatment of alloys results in the precipitation of a coherent, metastable phase, 6' (Al 3 Li) which offers considerable strengthening. Nevertheless, development and wide spread application of the Al-Li alloy system has been impeded mainly due to its inherent brittleness. The poor toughness of Al-Li alloys is mainly due to the pronounced planar slip. Additions of Cu and Mg to Al-Li alloys have been shown to promote slip dispersion through the precipitation of Al-Cu-Li, Al-Cu-Mg and/or Al-Li-Mg intermetallics (See e.g., Ref. 1-3). Recently, it has been suggested that the development of Zr modified 6' precipitates might modify the deformation behavior of Al-Li alloys (4). It has been shown that, in Al-2.34%Li-1.07%Zr alloy, solution treatment produced a fine distribution of A13 Zr precipitates, which acted as nucleation sites for 6' during subsequent aging, forming composite A13 (Li,Zr) precipitates. In the present paper, a new approach to promote the precipitation of composite phase is presented, emphasizing the effect of resultant composite precipitates on the deformation behavior of a rapidly solidified Al-Li-Zr alloy. EXPERIMENTAL PROCEDURE The material used in this investigation was a ternary Al-3.7%Li-O.5%Zr alloy. This alloy was rapidly quenched from the melt into continuous ribbon using the jet casting process. The ribbons were mechanically comminuted to -60 mesh powder and then consolidated into bulk compacts by vacuum hot pressing followed by hot extrusion to a final rectangular shape of 63.5 mmby 3.8 mm (18:1 extrusion ratio, 6:1 aspect ratio). The extruded bar was solutionized at 540 'C, quenched in cold water, and was given various aging treatments. Slices 0.3 mm thick were taken from the broken tensile specimens, and thin foils for transmission eletron microscopy were made by jet poli
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