340 zyxwvutsrqpon N. zyxwvutsrqponm A. El-Mahallawy and M. zyxwvutsrq A. Taha zyxwvutsrqp 2. Werkstofftech. zyxw 8, 340-343 (1977) Overlag Chemie, GmbH, D-6940 Weinheim, 1977 Structure and Mechanical Properties of Al-40 70 Cu Directionally Solidified Alloy" Gefiige und mechanische Eigenschaften einer gerichtet erstarrten A1-40% Cu-Legierung zyxwv N. A. El-Mahallawy and M. A. Taha Controlled dendritic growth has been used to produce fibre-reinforced composites. The principle of the technique is that the primary phase is controlled to grow directionally in such a way that the dendrites have only one branch, i. e. a fibre. If during solidification the fibre-like dendrites are aligned before the metal matrix-phase solidifies, then an axially fibre-reinforced alloy will be obtained. One advantage of this approach over that of purely eutectic growth is that higher volume fractions of fibres can be obtained by in situ growth. However, Davies etal. (1) found a decrease in ultimate tensile strength of directionally solidified dendritic hyper- eutectic A1-Ni alloys with increasing intermetallic fibre volume fraction which was attributed to the presence of different types of fibres (or dendrites) with low strength. This paper describes results obtained with commercial purity A1-40 wt% Cu alloys. The microstructures obtained by directional solidification under a wide range of growth rates were studied and correlated with the solidification mechanism. The mechanical properties of the directionally solidified alioys were examined and used to calculate those of primary dendrites observed in the microstructures. Experimental Procedure The alloy used in the present investigation was prepared from 99,7% commercial purity aluminium and high purity electrolytic copper. The alloy had a composition (wt pct) of 40% Cu, 0.1 zyxwvutsr 5 Fe, 0.10 Si, 0.015 Mn, 0.022 Ti, balace Al. Machined rods of 8 mm- diameter and 80 mm length were directionally solidified vertically under steady state con- ditions using graphite crucibles under dynamic argon atmo- sphere. The directional solidification apparatus used was based on withdrawing an electrical resistance heating furnace from the heated fixed crucibles. The furnace withdrawal speeds were found to correspond to the growth rates after calibration using a rippling technique similar to that described by Chad- wick (2) Growth rates werevaried from 8 to 305 pm/sec and temperature gradient was kept constant at about 9 K/mm. Longitudinal and transverse sections were prepared from the directionally solidified rods for microscopic examination. The line intersect method was used in all microscopic measurements where each measurement was the average of about 20 individual observations covering the entire specimen area. Tensile testing was carried out using standard Houns- field specimens of 14 mm gauge length and 3,5 mm diameter. zyxwvut * This work had been done at the Dept. of Production Engineering, Ain Shams Univcrsitv. Cairo; the authors are now with Max-Planck- All tensile tests were carried out at a speed of about 0,465 mm/sec. The results presented here are the average of at least three tests. Results and Discussion Microstructure and Solidification At all growth rates employed a cellular structure was observed in directionally solidified commercial purity A1-40% Cu. Representative optical micrographs of transverse sections through the directionally solidified alloys at growth rates of 33 and 142 pmlsec are illustrated in Fig. (I). As shown in the figure, each cell can be divided into three zones. The first zone is in the cell centre and has a single primary CuAl2 dendrite. The second zone surrounds the first one and has a fanned-out lamellar structure of A1-CuA12 eutectic. The third zone, at the cell boundary is an A1-CuAl2 eutectic with CuAl2 phase taking the form of a coarser microstructure. Examinations of the longitudinal sections revealed that the cells were always elongated in the growth directions, and that their length became shorter with increasing growth rate. The primary CuA12 dendrites in the cell centres were also always elongated in the direction of the cell axes. Comparison between these cells and the eutectic cells, previously observed by directional solidification of commercial purity AI-CuA12 eutectic (3) shows that the aligned lamellar eutectic structure observed in the eutectic cell centre was replaced by the single primary dendrite in the A1-40% Cu. Other features of the second and third zones were similar in both cases. Quantitative measurements of the average intercellular spacing, Z, were carried out on transverse specimens and plotted vs growth rate, R, on a log-log scale as shown in Fig. 2. These values were found to coincide with those of the average spacing between the primary CuAl2 dendrites obtained by another set of measurements. The variation of Z with R was found to obey the relationship z3 R = const. This is in agreement with previous results of intercellular spacing in directionally solidified Al-CuA12 and Al-Al3Ni commercial purity eutectics (3, 4) and in directionally so- lidified commercial A1-5, 7% Cu (5). Comparison with A1-CuAl2 eutectic in Fig. 2 shows that the present values of (Z) are about 40% higher. However, other investigations on some single phase and eutectic alloys (6, 7) have reported different exponents for the relationship between Z and R. The volume fraction of the primary CuA12 dendrites did not varv with the growth rate and had almost constant value I Institut fur iiisenforschung, Dusseldorf of 0,35 similar to that expected from the phase diagram. The