Peritectic solidification of Cu-Sn alloys: microstructure competition at low speed

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 diameter cylinder and 4/6 mm tube), allowing the observation of convection effects on solidifying microstructures. An essential outcome of this project is that α+β lamellar structures have been observed in both sample geometries, even when Vp is above the critical velocity for the α planar front growth, i.e., the α-phase grew with a cellular morphology. With the Cu-Sn phase diagram used in the present work and for positions close to the quenched interface, concentration profiles measured across the lamellar structures revealed that the corresponding operating temperature Τ*(x) seems to be non-isothermal, in contradiction with previous experimental observations in Fe-Ni peritectic alloys [14–16]. But, since the Cu-Sn phase diagram is not assessed accurately, Τ*(x) is still subject to large variations (±10°C). On the other hand, the peritectic transformation after solidification has been evidenced and modifies the compositions and fractions of phases. Additionally, local concentration showed that a partial banding mechanism and an unstable lateral propagation of both solid phases were responsible for the formation of lamellar structures. Various geometrical arrangements were revealed on transverse cuts of lamellar structures: straight α- and β-lamellae, labyrinth-like microstructures, disordered arrays of rods of one phase in the matrix of the other phase and uniform regions of β-phase. The dynamic evolution of lamellar structures was attributed to growth competition mechanisms and instabilities between phases originated from the same nuclei. Finally, it should be noted that the Kurdjumov-Sachs relationship was observed between the primary and the peritectic phases on both the straight lamellae and labyrinth-like microstructure. Finally, all lamellar structures ended by the termination of α-lamellae and the subsequent growth of a β planar front. In this context, the α-lamellae of lamellar structures observed in the 4/6 mm tube exhibited in all cases a "hammer-like" morphology before being overgrown by the peritectic phase. Careful investigations of this morphology revealed that it resulted effectively from growth competition mechanisms, which remain to be fully understood. In a simulated geometry corresponding to the 4/6 mm tube of Cu-Sn samples, 3D macrosegregation modeling revealed that a nearly helical convection pattern existed in the liquid phase ahead of a moving planar α-liquid interface. And between each plume, the fluid flow was mostly oriented parallel to the solid-liquid interface. As a consequence, the lateral propagation of the α- and β-phases during the 3D growth competition was surely affected by these convective motions. Finally, natural convection and the associated solute enrichment were assumed to be responsible for the destabilization of lamellar structures and the subsequent end of α-lamellae. The multi-phase field computations emphasized that the stability of the triple junction between α-, β- and liquid phases is function of the initial alloy concentration C0, of the lamellar spacing λ and of the thermal gradient Gl. In a near future, directional solidification experiments are planned to be conducted onboard of the International Space Station. Then, the obtained peritectic microstructures will have to be compared with the microstructures observed in the present work in order to assess more precisely the effect of natural convection on the formation of lamellar structures.


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