On oscillatory microstructure during cellular growth of directionally solidified Sn–36at.%Ni peritectic alloy

Abstract

An oscillatory microstructure has been observed during deep-cellular growth of directionally solidified Sn-36at.%Ni hyperperitectic alloy containing intermetallic compounds with narrow solubility range. This oscillatory microstructure with a dimension of tens of micrometers has been observed for the first time. The morphology of this wave-like oscillatory structure is similar to secondary dendrite arms, and can be observed only in some local positions of the sample. Through analysis such as successive sectioning of the sample, it can be concluded that this oscillatory microstructure is caused by oscillatory convection of the mushy zone during solidification. And the influence of convection on this oscillatory microstructure was characterized through comparison between experimental and calculations results on the wavelength. Besides, the change in morphology of this oscillatory microstructure has been proved to be caused by peritectic transformation during solidification. Furthermore, the melt concentration increases continuously during solidification of intermetallic compounds with narrow solubility range, which helps formation of this oscillatory microstructure.An important class of industrial alloys has been represented in peritectic systems, including Fe-Ni, Cu-Zn, Cu-Sn and Ti-Al which are the most widely known [1][2][3] . Solidification microstructures that form under steady state growth conditions have been reasonably well understood during peritectic solidification. However, under non-steady state growth conditions, the peritectic microstructures is also influenced by many other factors like Gibbs-Thomson effect 4 , and temperature gradient zone melting (TGZM) effect 5,6 etc. Besides, the selection of microstructures in peritectic systems is mostly governed by the coupling effects like convection, nucleation undercooling etc. 7,8 . Due to the above reasons, a wide variety of non-steady state microstructures have been revealed in directional solidification in both the hypoperitectic and hyperperitectic regions of peritectic systems [8][9][10][11][12][13][14] . Among them, the so-called banded 9 structures and the oscillatory structures 10,12 have been observed in several peritectic systems, including Sn-Cd, Pb-Bi, Fe-Ni alloys 1,3-11 . The former is featured by bands of both the primary and peritectic phases which form alternately normal to the growth direction 9 . And their formations have been attributed to nucleation followed by the growth of one phase (α or β ) ahead of a growing planar front of the other phase (β or α ). Besides, this banded structure is often related to the oscillatory structures 10,12 . In highly convective hyperperitectic alloys, tree-like structures which appeared as alternate bands and islands in longitudinal cross-sections have been revealed to be continuous structures in Pb-Bi and Sn-Cd alloys 13,14 . The oscillatory structures are, in fact, a continuous treelike morphology of the primary phase which is surrounded by the peritectic phase 12 . And these structures result from an...

Detachment of secondary dendrite arm in a directionally solidified Sn-Ni peritectic alloy under deceleration growth condition

“…In addition, the diffusion coefficient of atom in solid phases which is only about 1/1,000 of that in liquid phase decreases quickly with decreasing temperatures, which greatly restricts the growth of peritectic Ni 3 Sn 4 phase through peritectic transformation. From our previous work 33 , the growth thickness of the peritectic β (Ni 3 Sn 4 ) phase is this Sn-Ni peritectic alloy is only 5 μm in the vicinity of T P , which is much smaller than the experimental measurement of thickness of peritectic β (Ni 3 Sn 4 ) phase. In addition, the solid/liquid interface at the hot side of the liquid pool between secondary dendrite arms is much more zigzag than that at the cold edge of the liquid pool.…”

Solidification sequence and as-cast microstructure of a model Pb-Bi-Sn alloy with quasi-peritectic reaction

“…However, the above solidification sequence is based on the equilibrium thermodynamical analysis. Similar with the peritectic reaction of binary alloy [12,13] , the quasi-peritectic reaction must occur under the undercooling condition to achieve the driving force. Meanwhile, the third phase formed by the quasi-peritectic reaction will adhere to the existing phase, thus inhibiting the subsequent quasi-peritectic reaction.…”

Influence of a transverse static magnetic field on the orientation and peritectic reaction of Cu-10.5 at.% Sn peritectic alloy

“…Peritectic reaction is commonly observed in many binary alloys, such as Fe-based (Fe-C, Fe-Ni), Cu-based (Cu-Zn, Cu-Sn) and Al-based (Al-Ti) alloys 1 . In peritectic metallic system, various solidification structures have been experimentally observed during directional solidification 2 – 5 . A large number of numerical simulations and hypothesis models have been reported to explain the formation mechanism of the complex of solidification structures 6 – 9 .…”