Microstructure Evolution Of Triple Junction Region Of Peritectic Reaction And Its Mechanism During Directional Solidification Of Cu-Ge Alloys

Peritectic reaction widely exists in many structural and functional materials. Peritectic reaction under different conditions will influence the morphology and volume fraction of peritectic two phases, and thus influences mechanical and physical properties of the materials. However, there is limited understanding of the peritectic reaction process during directional solidification. For example, at present, there is still controversy of solute and thermal diffusion mechanism. Hillert was the first to predict that both dissolution and some resolidification of the primary phase during peritectic reaction are required. But till now, the resolidification phenomenon has not been well substantiated and the clear morphology of triple-phase junction is rarely observed. It is generally known that the peritectic reaction occurs below the equilibrium temperature. As a metastable phase, the resolidification phenomenon of the primary phase is incomprehensible. Therefore, it is necessary to have a deep study on the peritectic reaction mechanism.In this study, we selected different composition of Cu-Ge peritectic alloys to investigate the microstructure evolution and morphological characteristics of triple junction region of peritectic reaction by directional solidification technology. Usually, the acquired triple-phase region of the peritectic reaction is too small to clearly present above phenomenon. To reveal the morphological characteristics of the triple-junction region, a macro-separation structure of the two phases is used to magnify the triple-phase region. In addition, a novel lateral remelting phenomenon occurring on the primary dendrite is observed during directionally solidified Cu–Ge alloys. And a model is set up to explain the phenomenon.Because the Ge atom is lighter than the Cu atom, the combination of temperature and concentration gradients in front of the primary interface give rise to double-diffusive convection. When the lateral confinement (the ratio of the crucible diameter to the untable wavelength at the threshold for an infinite medium) is smaller than unity and the distance between the tip and the root of the primary cells is bigger than the distance between the α tip and ζ front, double-diffusive convection will induce the formation of the macro-separation structure of the two phases. In the two-phase separated structure, a large trijunction region of peritectic reaction forms around the cylindrical α-Cu phase, which provides a convincing experimental evidence for studying the morphological characteristics of peritectic reaction trijunction region. This two-phase separated growth process creates new opportunities for the fabrication of functionally layered materials. In the paper, the re-solidification of the primary phase after melting, which is predicted by Hillert, is confirmed at lager trijunction region of peritectic reaction. As a metastable phase, the resolidification of α is also attributed to the diffusion coupling between the groove of the α phase and the interface of the ζ phase under the constraint of mechanical equilibrium at the triple junction. A mechanism is proposed to explain the phenomenon. A groove structure of the primary phase near the triple-phase junction must be required for the mechanical equilibrium. Then the resolidification and remelting of the primary phase occurs in the groove. And a near couple growth of the resolidified α and the ζ phase forms in the vicinity of the triple junction. The migration of the primary groove will leads to the concentration decrease of the liquid. Namely, the groove of α is an absorber of Ge atoms. The growth of the peritectic phase will provide Ge atoms. Thus, at the temperature below the equilibrium temperature of peritectic reaction (TP), liquid diffusion of Ge atoms towards the groove of the α phase and Cu towards the interface of the ζ phase occurs in the vicinity of the triple junction. The couple growth is different with the classic theory of peritectic couple growth, which needs a negative undercooling (growth temperature above TP). Thus the resolidification depth is controlled by the new diffusion couple, which is closely related to the the pulling velocity, the temperature gradient and the thickness of the peritectic phase. However, at the lowest point (bottom) of the groove, the melting and solidification of the α phase can not occur concurrently at one point because the two process are contradictory and asymmetrical in kinetics. Thus, the local motion of the trijunction region can be unstable.It is found that, there is not a one-to-one correspondence between the pulling velocity and the thickness of the peritectic phase, which cannot be explained by the Fredriksson-Nylén model. This means that the peritectic phases grow close to the limit of stability rather than at the maximum velocity. Under lower velocity, the peritectic reaction is controlled by the direct solidification of the peritectic phase. According to the geometrical characteristics of the trijunction region, it is found that the central temperature of the ζ-planar front is higher than that of the triple junction. This means that the peritectic growth undercooling is different along the ζ interface and has a maximum value in the triple junction. And the remelting regions have a big temperature interval, about5K, which suggests that the peritectic reaction during directional solidification occurs in a changeable composition, which is different from the classic reaction model.The measurement of the concentration distributions of the trijunction regions support the solute diffusion mechanism. The peritectic reaction mainly is controlled by the supersaturated zone of the primary phase, which depends on the coupling of the solute and temperature fields. The remelting degree of the primary phase is related with the pulling velocity, the temperature gradient and the thickness of the peritectic phase. During the peritectic reaction, as G/V increases, the supersaturated zone ahead of the peritectic interface increases. Thus, the size of the remelting region increases. The thicker peritectic phase can provide more Ge atoms to the remelting region, and thus increase the size of the remelting region. Under higher pulling velocity, the interfacial instability of the peritectic phase forms at the larger triple junction region because the liquid at and ahead of the solidifying interface is constitutionally undercooled with respect to the peritectic phase. It is found that the morphological stability of ζ is irrelevant to initial alloy composition, which is mainly dependent on the growth conditions. Constitutionally undercooling in the solid primary phase increases the interfacial instability of the remelting primary phase. When the remelting phase is thicker and the pulling velocity is higher, the loss of Ge at the trijunction region can be not timely compensated and nonplanar remelting α interface can form in the vicinity of the trijunction.During peritectic solidification, besides the re-meting of the primary phase above the kinetic temperature of peritectic reaction (TPK), a lateral re-melting phenomenon of the primary phase below TPK is observed under high velocity in directionally solidified Cu-Ge alloys. The lateral re-melting occurs continuously along a liquid channel as temperature decreases, whose reaction velocity is far greater than that of peritectic transformation. The lateral re-melting leads to the morphological change of the primary dendrites. Even when the composition is up to Cu-18.0wt.%Ge and the pulling velocity is up to150μm/s, the dendrite arms is completely fragmented. During the growth of the ζ phase around the boundary of one α dendrite, the constitutional undercooling at the ζ tip induces that the ζ phase presents cellular morphology. The lateral remelting phenomenon is caused by the solute enrichment between two neighboring ζ cells.The longitudinal remelting of the secondary dendrite arm is obervered, which is caused by TGZM effect (Temperature Gradient Zone Melting). The longitudinal remelting of the second dendrite arm is related to the coarsening of the primary phase. When the coarsening of the primary phase is weaker and the secondary dendrite is very developed, the longitudinal remelting of the secondary dendrite arm is very obvious. When the growth conditions meet the instability of the primary and peritectic phase, the lateral and longitudinal remelting of the primary phase occurs concurrently at the the secondary dendrite arm. The intersection interface of the lateral and longitudinal remelting has a maximum remelting velocity. The velocity direction is vector sum between the lateral and longitudinal remelting velocity, which deviated from the pulling direction of the sample. In addition, the longitudinal diffusion of the Ge atoms from the peritectic interface to the groove also raises the lateral remelting velocity.