Peritectic solidification of Cu–Sn alloys: Microstructural competition at low speed

Abstract

Directional solidification experiments on Cu-Sn peritectic alloys have been conducted at very low velocity in a high-thermal-gradient Bridgman furnace. The size of the samples has been reduced in order to decrease natural convection and the associated macrosegregation. At the lowest growth rates (0.5 and 0.58 lm s À1 ), eutectic-like a + b lamellar structures have been observed in near-peritectic composition alloys over several millimeters of growth. These structures resulted from a destabilization of a band structure in which a-and b-phases overlay each other. Electron backscattered diffraction measurements revealed that bands and lamellae of a solid phase are continuous and originate from a single nucleus.

Modeling of peritectic coupled growth in Cu–Sn alloys

“…As can be seen in Fig. 10, the lamellar front at steady state is not strictly isothermal, as previously noted by Kohler [29]. The temperature of the a-lamella is located below T per , whereas that of the b-lamella crosses the peritectic temperature; the temperature of the triple junction is always below the peritectic temperature (see Fig.…”

“…In contrast to the results reported in Ref. [11], the specimens used here were cylinders of 0.5 mm diameter. This diameter is small enough that solutal buoyancy-induced convection is considerably reduced.…”

“…In region (b), the first appearance of the b-phase seems to be highly unstable: we see a disordered pattern of a domains in a b matrix. The fact that most of these domains have the same orientation as the initial a phase in the EBSD analysis indicates that the a domain is in reality continuous in three dimensions, and that a and b undergo a competitive dynamics with successive overgrowth, as already observed in the early stages of eutectic coupled growth by Akamatsu et al [20] in eutectics and by Kohler et al [11] in peritectic coupled growth. The system then switches back to a before initiating coupled growth in region (c).…”

“…The experimental results reported herein were obtained with the same apparatus and experimental methods as described by Kohler et al [11] and Valloton et al [18]. We therefore limit the description here to the essentials, and refer the interested reader to the previous articles.…”

“…These alloys show a wide variety of microstructures in directional solidification at low speed [1][2][3][4][5][6][7][8][9][10][11][12][13], including regions of primary a, primary b, islands of one phase in the other and coupled growth. It is this latter microstructure that is of most interest here.…”