Anisotropic tensile properties of a lamellar Al-CuAl 2 eutectic composite
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The influence of the stress direction on the ambient temperature tensile properties of a lamellar Al-CuAl 2 eutectic composite is investigated using metallographic observation and tridirectional extensometry. Noncoincidence between stress and strain principal directions is pointed out and studied. Deformation leading to failure may occur by three different modes according to the value of the angle between the tension direction and the plane of the lamellae. The model of failure proposed by Kelly for fibrous composites is adapted to the case of lamellar composites. It accounts for the variation of the ultimate tensile strength with the tension direction.
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OME lamellar eutectics are presently developed as high temperature resistant alloys. In order to study the influence of the applied stress direction on the behavior of a lamellar composite, Al-CuAl 2 eutectic was chosen as a model system.' By a series of bend tests Hertzberg et al. z already showed the influence of lamellae orientation on the plastic behavior of this eutectic. In the present work, the laws governing this influence are investigated more precisely by tensile tests at room temperature and are related to micro-
some risks of explosion, and the samples needed an immediate cleaning with ammonia. The second electrolyte, a mixture of 10 volumes orthophosphoric acid, 4 volumes acetic acid, 4 volumes butylcellosolve, 0.2 volume of water, gave good results too. The midplane of all the specimens contained the ingot axis and the normal to the mean plane of the lamel-
structural observations. EXPERIMENTAL PROCEDURE Specimen Preparation: Our specimens were made out of cylindrical eutectic ingots 12 mm in diam, grown by Bridgman solidification process i and contained a single eutectic grain over a 5 cm to 10 cm length. Within such specimens the microstructure of the alloy consists of alternating lamellae of the phases a and 0, nearly parallel to eachother all along the cylinder. The a phase is a primary solid solution based on aluminum, face centered cubic, with a very low copper concentration at ambient temperature. No precipitation could be detected in this phase in our samples. The 0 phase is the CuAl2 intermetallic compound. Its crystal structure is body-centered tetragonal. 4 In our samples, the mutual orientation of the two lattices may be characterized by the following relationships (see Fig. 1(a): (iii'
(211) °
(a)
[101] `1 // [135] B
It is the ß5 mutual orientation according to Bonnet's rationalized classification.' From these single grains, slices were machined with a cut-off wheel, and from these slices flat tensile specimens were cut by spark-machining. The tensile specimens were mechanically, then electrolytically polished. Two electrolytes were used. The first was a mixture of 1 volume perchloric acid, and 2 volumes acetic anhydride. It gave the best results, but with MICHEL DUPEUX and FRANCIS DURAND are Assistant and Professor respectively, at Laboratoire de Thermodynamique et Physicochimie Métallurgiques, Associé an C.N.R.S. (L.A. 29), E.N
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