Local Atomic Structure And Rapid Solidification Of Undercooled Liquid Ni-Zr Alloys

Crystallization of a favored phase among many competing ones from the undercooled liquid alloys is one of the most fundamental nonequilibrium phenomena universal to a variety of materials.The inherent competition of different phases in undercooled liquid alloys is of principal interest but also of considerable importance for suppression of the undesired phases.In this thesis,three containerless processing techniques,i.e.,electromagnetic levitation,drop tube and electrostatic levitation,were used to systematically investigate the dendritic growth and phase selection of the undercooled liquid Ni-Zr alloys with the help of molecular dynamics simulations.The main conclusions are summarized as follows.1.Thermophysical properties of undercooled liquid Ni-Zr alloysThe thermophysical properties of the undercooled liquid Ni-Zr alloys were investigated by molecular dynamics simulations combined with a F-S potential.The calculated melting temperatures of Ni,Zr,Ni₇Zr₂ and Ni₅₀Zr₅₀ are in good agreement with the experimental values.The densities of the liquid Ni-Zr alloys were investigated at broad temperature ranges,which decrease with the enhancement of the temperature and agree well with the reported experimental values except for the Ni-rich composition alloys.We used the experimental density of liquid Ni₇Zr₂ alloy to re-gauge the current F-S potential and recalculated the densities of the Ni-rich composition alloys by the re-gauged potential.The liquid Ni-Zr alloy system shows a negative excess volume,which is ascribed to the strong attractive interactions between Ni and Zr atoms,which indicates that the liquid Ni-Zr alloys deviate significantly from the ideal solution,and the accuracy would be remarkably low if the thermophysical properties were estimated by the Neumann-Kopp rule.Besides,the thermal expansion coefficients were derived based on the density data,which rise with the increase of the temperature except for the liquid Ni₇Zr₂ alloy.The specific heat firstly increase and then decrease with the increase of the Zr content.2.Local atomic structure correlating to phase selection in undercooled liquid Ni-Zr peritectic alloyThe local atomic structure and its correlation with phase selection during rapid solidification of undercooled liquid Ni-16.75 at.%peritectic alloy has been investigated by combining electrostatic levitation technique and first-principles molecular dynamics simulations.The calculated density increase linearly with the decrease of the temperature in temperature range from 2000 to 1400 K,including a maximum undercooling of 212 K.The calculated density agrees well with the present experimental measurements.The partial pair distribution functions indicate that liquid Ni-16.75 at.%alloy exhibits a remarkable chemical short range order,which leads to a preferred association of Ni and Zr atoms.Atomic three-dimensional structure analyses reveal that a high fraction of atom pairs locates in perfect or distorted icosahedral environments（>36.9%）although the number of the fully developed icosahedra or distorted icosahedra is rather rare（<2.5%）,implying that this liquid contains a lot of fragmented clusters with perfect or distorted pentagonal faces.Moreover,we demonstrate that the short range orders of the undercooled liquid differ topologically from those in the ideal Ni₅Zr crystal but are similar to those in the ideal Ni₇Zr₂crystal in the temperature range from the liquidus temperature of 1612 K to 1400 K,which indicates that the nucleation of primary phase Ni₇Zr₂ from the undercooled liquid is structurally favored due to the low free energy barrier.However,the nucleation of primary Ni₇Zr₂ phase is presumed to be replaced by the direct formation of the peritectic Ni₅Zr phase if a sufficient undercooling is achieved.These results shed light on the relationship between the local atomic structure and phase selection during rapid solidification of the undercooled peritectic alloys.3.Phase selection in the undercooled liquid Ni-Zr peritectic alloysThe liquid Ni-16.75 at.%Zr peritectic alloy was substantially undercooled and containerlessly solidified by an electromagnetic levitator and a drop tube.Below a critical undercooling of 124 K,the primary Ni₇Zr₂ phase preferentially nucleates and grows from the undercooled liquid,which is followed by a peritectic reaction of Ni₇Zr₂+L→Ni₅Zr.Nevertheless,once the liquid undercooling exceeds the critical value,the peritectic Ni₅Zr phase directly precipitates from this undercooled liquid.However,a negligible amount of residual Ni₇Zr₂ phase still appears in the microstructure,indicating that nucleation and growth of the Ni₇Zr₂ phase are not completely suppressed.To completely suppress the formation of the primary phase,this alloy was also containerlessly solidified in free fall experiments.Typical peritectic solidified microstructure forms in large droplets,while only the peritectic Ni₅Zr phase appears in smaller droplets,which gives an indication that the peritectic Ni₅Zr phase directly precipitates from the undercooled liquid by completely suppressing the growth of the primary Ni₇Zr₂ phase and the peritectic reaction due to the combined effects of the large undercooling and high cooling rate.The undercooling-induced competitive growth between the primary Ni₇Zr₂ and peritectic Ni₅Zr phases in the liquid Ni-17 at.%Zr hyperitectic alloy was revealed by in situ observation of the recalescence process and further confirmed by analyzing the solidified microstructures,X-ray diffraction pattern as well as dendritic growth velocity.When the liquid undercooling is less than a critical value of 106 K,the primary Ni₇Zr₂ phase precipitates initially from the parent liquid,which is subsequently followed by the nucleation and growth of the peritectic Ni₅Zr phase around it.The orientation relationship and interface characteristics of the Ni₇Zr₂ and Ni₅Zr phases were investigated by electron backscatter diffraction（EBSD）and transmission electron microscopy（TEM）.The EBSD results clearly demonstrate that Ni₇Zr₂ and Ni₅Zr phases have the orientation relationship of{111}Ni₇Zr₂//{111}Ni₅Zr.TEM analysis reveals that a large-scale flat interface exists between the Ni₇Zr₂ and Ni₅Zr phases,indicating a good lattice matching of the two phases along the phase boundary.Once the critical undercooling is exceeded,the peritectic Ni₅Zr phase preferentially nucleates and grows from the undercooled melt by completely suppressing the formation of primary Ni₇Zr₂ phase.The EBSD analysis shows that the peritectic Ni₅Zr phase is highly orientated and its growth mode is almost parallel to<110>directions.The liquid Ni-20 at.%Zr hyperperitectic alloy was substantially undercooled by electromagnetic levitation technique up to 225 K.A sudden drop for the measured growth velocity is observed around the critical undercooling of 155 K.This is confirmed as a transition of growth mode from primary Ni₇Zr₂ dendritic growth to coupled growth of Ni₇Zr₂ and Ni₅Zr phases.Similar interface attachment kinetics and the planar front growth of Ni7Zr2 and Ni₅Zr phases are responsible for the formation of peritectic coupled growth.The eutectic-like lamellar structure also forms in a free fall experiment,which further clearly demonstrates the existence of peritectic coupled growth induced by undercooling.4.Dendritic and eutectic solidification in the undercooled Ni-Zr eutectic alloysThe liquid Ni-13 at.%Zr hypereutectic alloy was substantially undercooled by electromagnetic levitation technique up to 270 K.With the enhancement of the liquid undercooling,the transition of the morphologies of the primary Ni₅Zr phase from faceted to columnar dendrites takes place.The micromechanical property of the priamry Ni₅Zr phase in terms of the Vickers microhardness decreases firstly and then rise with the enhancement of the liquid undercooling,which is ascribed to the transition of the growth manner.The rapid solidification of the undercooled Ni-5 at.%hypoeutectic alloy was investigated by electromagnetic levitation method.The maximum undercooling obtained in the experiments is 262 K,wherein a maximum dendritic growth velocity of 78 mm/s,which is extremely slow.By comparing with the reported results,we found that the dendritic growth velocity of the（Ni）phase in the undercooled Ni-Zr alloys decreases with the increase of the Zr content.The competitive growth between the primary（Ni）phase and eutectics was investigated by electromagnetic levitation technique.The microstructure is composed of only eutectics（Ni+Ni₅Zr）in the upper zone of the master alloy.However,the primary（Ni）phase appears in the samples solidified in the undercooling region from 12 to 180 K.This gives an indication that eutectic coupled zone is remarkably small of this alloy and the（Ni）preferentially nucleates and grows from the undercooled liquid during the competitive growth between the（Ni）phase and eutectics.