Destabilization Mechanisms Of Two-phase Lamellar Structure Fabricated By Nonequilibrium Solidification And Accumulative Roll Bonding

In recent years,with the rapid development of nonequilibrium processing techniques,a large number of nonequilibrium and metastable materials have been widely developed and applied.Among them,metallic materials with nonequilibrium two-phase lamellar structure have attracted great interests due to their high strength and radiation resistance arising from a large number of interfaces.The bulk nonequilibrium two-phase lamellar structures can be obtained by rapid solidification of undercooled eutectic alloy and accumulative roll bonding(ARB)the two-component heterogeneous metallic layers.The stability of the lamellar structures during above mentioned forming processes and high-temperature service is the key of adopting these materials with industrial scales.Different from grain boundaries in polycrystalline materials,the lamellar structure with a complete morphology and flat interface has no interface curvature.The capillary tension due to the curvature does not drive the destabilization of this kind of interface.Therefore,the nearequilibrium lamellar structure exhibits good stability.When the two phases in the lamellar structure are in nonequilibrium,they tend to evolve into equilibrium in thermodynamics.This will lead to the destabilization of the lamellar structure once the kinetic conditions for destabilization are satisfied.Compared to rapid development of the processing techniques,the destabilizing mechanisms of nonequilibrium lamellar structures still lack in-depth investigations.Understanding the destabilizing mechanism of the nonequilibrium two-phase lamellar structure is of great importance for obtaining stable nonequilibrium lamellar structure and designing new nanolayered composites.In this work,the destabilizing mechanisms of the nonequilibrium two-phase lamellar structures formed by the nonequilibrium solidification of undercooled eutectic alloy and the ARB technique are studied.Firstly,an undercooled Ni-18.7 at.%Sn eutectic alloy is selected as a model to observe the microstructure evolution process of primary lamellar eutectic structure(PLES)to anomalous eutectic structure(AES).On this basis,an analytical model is established to quantitatively describe the destabilization of PLES during the nonequilibrium solidification of undercooled eutectic alloy.Then,an undercooled Ag-39.9 at.%Cu eutectic alloy containing different coupled eutectic growth modes is selected as a model to discuss the influence of coupled eutectic growth modes on the destabilization of PLES.Finally,the destabilizing mechanism of the nonequilibrium Ag-Cu two-phase lamellar structure obtained by the ARB technique is investigated.The main conclusions are as follows:(1)The microstructure evolution of the formation of AES caused by the destabilization of PLES in the undercooled Ni-18.7 at.%Sn eutectic alloy constituted by Ni3Sn and α,-Ni phases is revealed by using an in situ observation in a high-temperature laser scanning confocal microscope.It is found that the rapid coupled eutectic growth leads to the formation of PLES during recalescence.When the temperature reaches the peak temperature of recalescence,the Ni3Sn phase with a lower melting point is subjected to pronounced remelting to the liquid,while the α-Ni lamellae rapidly transform into granules.The morphological transition of the α-Ni lamellae follows a sequence of lamella→rod→granule.As the temperature decreases,the liquid phase solidifies in the interregional area of the α-Ni phase,which eventually leads to the formation of AES.(2)An analytical model is established to quantitatively describe the destabilization of PLES during the nonequilibrium solidification of undercooled eutectic alloy.The results show that the PLES formed by rapid coupled eutectic growth will subject to a rapid destabilization during the post-recalescence period if the duration of post-recalescence period is longer than the time required for break-up of the lamellae.This process is caused by the unstable perturbation of interface driven by interfacial energy and solute concentration gradient.The solute concentration gradient is the necessary condition for the destabilization of PLES,which initiates the morphological transition of eutectic lamellae by forming high curvature regions in the trough of perturbed surface.Afterwards,the interfacial energy will dominate the subsequent morphological transition process.(3)Different from the undercooled Ni-18.7 at.%Sn eutectic alloy,which grows in an eutectic-dendrite mode within the undercooling range of coupled eutectic growth,the undercooled Ag-39.9 at.%Cu eutectic alloy exhibits two coupled eutectic growth modes,i.e.slow eutectic-cellular growth and rapid eutectic-dendritic growth.The destabilizations of PLESs formed by the eutectic-cellular growth and the eutectic-dendritic growth are ascribed to two different mechanisms.For the PLES formed by the slow eutectic-cellular growth,the destabilization of the PLES is caused by the mechanism of "termination migration" driven by interfacial energy.For the PLES formed by the rapid eutectic-dendritic growth,the destabilization of the PLES is ascribed to the mechanism of unstable interface perturbation driven by interfacial energy and solute concentration gradient.(4)The destabilizing mechanism of Ag-Cu nonequilibrium nanolamellar structure obtained by ARB exhibits both similar and different characteristics compared to the eutectic lamellar structure in the undercooled eutectic alloys.The similarities are that the destabilization of nanostructure Ag-Cu nonequilibrium two-phase lamellar structure is also caused by the unstable perturbation of interface driven by interfacial energy and solute concentration gradient.The differences are that compared to the eutectic lamellar structure in the undercooled eutectic alloy,the cross-sectional ratio of the nonequilibrium nanolamellar structure in the Ag-Cu layered composite is higher.Its destabilizing process significantly depends on the thicknesses of the Ag and Cu phases.That is,when the differences of the thicknesses of the adjacent lamellae are large,the destabilization of the thinner lamella will follow a sequence of perturbation→necking→pinch-off→termination migration.While when the thickness of adjacent lamellae is similar,the perturbed lamella will tend to form stable grooves.