The structure of eutectic Al-Fe composite IN SITU
Edward Guzik, Dariusz Kopyciński Akademia Górniczo-Hutnicza, Wydział Odlewnictwa, ul. Reymonta 23, 30-059 Kraków
Annals 2 No. 4, 2002 pages 139-142
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abstract The results of preliminary research on the directional solidification of eutectic Al-1.8%Fe alloy and Al-2.8%Fe alloy with addition of 0.1% V were described. Using a Bridgman apparatus for directional solidification, the structure of an oriented equilibrium α(Al)-Al3Fe eutectic was determined over a range of low growth rate v, i.e. from 9.03E-05 to 1.11E-03 cm/s. As a result of the directional solidification an Al-Fe composite in situ has been produced; its structure is shown in Figure 3. For the two chemical compositions, i.e. Al-1.8% Fe and Al-2.8% Fe (with 0.1% V added) two types of eutectic were obtained, i.e. an irregular lamellar eutectic (Fig. 3 a, c, d) with typical branching (Fig. 3b) and a regular rod-like eutectic (Fig. 3 eh). In composite of the first type an increase in the growth rate changed the irregular lamellar eutectic of the Eu1 type to a morphology characterized by the presence of parallel precipitates of Al3Fe in eutectic. The characteristic feature of the base structure was that, apart from Eu1 eutectic, it also contained precipitated dendrites of αAl phase (Figs. 1 and 2a). An increase in iron content by 1% changed the eutectic morphology from Eu1 to Eu2 of the rod-like type (Fig. 2b). Introducing vanadium hindered the transition from irregular eutectic to a regular one in the range of high growth velocities, which is visible in Fig. 3 c, d. In general terms it can be said that with increasing growth rate v the interphase spacing λ decreases in the eutectic structure of an Al-Fe composite in situ. The results of quantitative interpretation of the interphase spacing λ in eutectic referred to the growth rate v are shown in Figure 5. In this diagram, for the sake of comparison, the results of studies made by Adam and Hogan [7] and Wang and Jones [14-16] were also plotted. The results of the research described in [14-16] indicate that for the structure of Al-2.85%Fe composite in situ (containing 0.12% V) the exponent n assumes the value of 0.5 and the constant A is equal to 2.45 · 10-5 cm3/2 · s-0.5 for the irregular lamellar eutectic and to 1.38 · 10-5 cm3/2 · s-0.5 for the regular rod-like eutectic of the Eu4 type. Contrary to what has been reported by authors of [14-16], introducing vanadium to Al-2.8%Fe composite has not caused, in the examined range of growth rate v, the transition to a eutectic Eu4 structure, i.e. αAl-AlxFe; an irregular lamellar eutectic of the Eu1 type is still observed to exist. The extended range of occurrence of the irregular eutectic in an Al-Fe composite in situ with vanadium is expected to make easier future verification of the existing models of an irregular eutectic growth in the examined range of the eutectic growth velocities. The interphase spacing λ in the examined Al-1.8%Fe composite in a given range of the growth rate can be determined from relationship (1).