Publication
Many commercially important alloys such as Fe-C, Fe-Ni, Cu-Zn, Cu-Sn and Ti-Al exhibit peritectic transitions on solidification. In the two-solid phase region of peritectic metallic systems, a wide spectrum of microstructures has been revealed during directional solidification experiments at low growth rates, where both α- and β-phases would grow independently as planar fronts [1, 2, 12–20]: discrete bands, islands, simultaneous growth of the α- and β-phases in the form of oriented lamellae or fibers and continuous oscillatory tree-like microstructures in highly convective peritectic systems [3, 21, 22]. Recently, Vandyoussefi et al. [27] and Dobler et al. [16] have done extensive solidification experiments on Fe-Ni alloys in the corresponding two-solid phase region. Depending on the growth conditions and local composition, a lamellar eutectic-like structure was observed. Despite recent works, the peritectic solidification at low growth rate is not fully understood. Additionally, coupled growth was observed only for peritectic systems for which the solidification interval of the primary phase ΔΤ 0α is fairly small and with negligible convection; but is it also possible for peritectic alloys with fairly large ΔΤ 0α such as Cu-Sn? If so, what would be the effect of convection? And finally, is the coupled growth front really isothermal? In the present contribution, the peritectic Cu-Sn system has been chosen because of its remarkable properties and technological importance. Indeed, two essential features distinguish Cu-Sn alloys from other peritectic systems: The equilibrium solidification interval of the a-phase decreases with the tin concentration in the hypoperitectic composition range. Additionally, the equilibrium solidification interval of the primary phase is about 25 times larger in the Cu-Sn system than in the Fe-Ni peritectic system. Since only few studies have been made on the detailed solidification of tin bronze alloys, the first goal of this study was to carry out thermal analyses in order to investigate precisely the solidification of Cu-Sn alloys and to measure temperatures of solid-state phase transformations. Therefore, an SPTA assembly has been built and successfully calibrated. Additionally, a heat flow model was developed in this work and coupled with a Cu-Sn thermodynamic database to treat solidification-dependent latent heat. Single pan thermal analyses on three Cu-Sn alloy compositions revealed that the corresponding phase diagram recently re-investigated in Liu et al. review [28] is the most reliable. Directional solidification experiments on Cu-Sn alloys of various compositions have been conducted at different velocities in a high gradient Bridgman furnace. In this context, the set-up already used by Dobler [14] has been modified in order to reduce the size of the samples and, accordingly, natural convection and the associated macrosegregation. During each solidification run, two different geometries were thus tested (3 mm