Zheng Chen

Microscopic phase-field simulation coupled with elastic strain energy for precipitation process of Ni-Cr-Al alloys with low Al content

Abstract The precipitation process of Ni-Cr-Al alloy with low Al content was studied at atomic scale based on the microscopic phase-field kinetic model coupled with elastic strain energy. The aim is to investigate the effect of elastic strain energy on precipitation mechanism and morphological evolution of the alloy. The simulation results show that in the early stage of precipitation, Do 22 phase and L 1 2 phase present irregular shape, and they randomly distribute in the matrix. With the progress of aging. L 1 2 phase and D 0 22 phase change into the quadrate shape and their orientations become more obvious. In the later stage, L 1 2 phase and D 0 22 phase present quadrate shape with round corner and align along the [100] and [010] directions, and highly preferential selected microstructure is formed. The mechanism of early precipitation of L 1 2 phase in Ni-17%Cr-7.5%Al (mole fraction) alloy is the mixed style of non-classical nucleation growth and spinodal decomposition and the D 0 22 phase is the spinodal decomposition. The mechanisms of early precipitation of L 1 2 phase and D 0 22 phase in Ni-12.5%Cr-7.5% Al alloy are both the non-classical nucleation and growth. The coarsening process follows the rule of preferential selected coarsening.

Microscopic phase-field simulation of atomic site occupation in ordering process of NiAl9Fe6 alloy

Abstract The process of γ(fcc)→γ(fcc)+γ′(L1 2 ) phase transformation was simulated by using microscopic phase-field method for the low supersaturation NiAl 9 Fe 6 alloy. It is found that in the γ′ phase, the ordering degree of Al atoms is obviously higher than that of Fe atoms, and the ordering of Al atoms precedes their clustering, while the case of Fe atoms is opposite. The α site is mainly occupied by Ni atoms, while the β site is occupied in common by Al, Fe and Ni atoms. At order-disorder interphase boundary, the ordering degree of Al atoms is higher than that of Fe atoms, and at the β site, the Fe atomic site occupation probabilities vary from high to low during ordering; the Al atomic site occupation probabilities are similar to those of Fe atoms, but their values are much higher than those of Fe atoms; Ni atoms are opposite to both of them. Meanwhile, during the ordering transformation, γ′ phase is always a complex Ni 3 (AlFeNi) single-phase, and it is precipitated by the non-classical nucleation and growth style. Finally, in the alloy system, the volume of γ′ ordered phase is less than that of γ phase, and the volume ratio of order to disorder is about 77%.

Microscopic phase-field study on aging behavior of Ni75Al17Zn8 alloy

Abstract The precipitating kinetics of Ni75Al17Zn8 alloy was studied at both 873K and 973K by microscopic phase-field model. The calculation results show that the order–disorder transformation experiences the matrix → lowly-ordered L10 phase → L12 phase at 973 K. And the nucleation of L12 particles belongs to the spinodal decomposition mechanism. As temperature increases, orderings of Al and Zn atoms are resisted, but coarsening of L12 particles is promoted. The value of coarsening kinetic exponents approaches to 1/2. In addition, the discussions about Ni-Al anti-site defect and Zn substitutions for Ni site and Al site exhibit that the higher the temperature, the more distinctive the Ni-Al anti-site defect, but the less the Zn substitution.

Atomistic modeling for interfacial properties of Ni-Al-V ternary system

Interatomic potentials for Ni-Al-V ternary systems have been developed based on the second-nearest-neighbor modified embedded-atom method potential formalism. The potentials can describe various fundamental physical properties of the relevant materials in good agreement with experimental information. The potential is utilized for an atomistic computation of interfacial properties of Ni-Al-V alloys. It is found that vanadium atoms segregate on the γ-fcc/L12 interface and this segregation affects the interfacial properties. The applicability of the atomistic approach to an elaborate alloy design of advanced Ni-based superalloys through the investigation of the effect of alloying elements on interfacial properties is discussed.

Modified embedded-atom interatomic potential for Co–W and Al–W systems

Abstract A semi-empirical interatomic potential formalism, the second-nearest-neighbor modified embedded-atom method (2NN MEAM), has been applied to obtaining interatomic potentials for the Co–W and Al–W binary system using previously developed MEAM potentials of Co, Al and W. The potential parameters were determined by fitting the experimental data on the enthalpy of formation, lattice parameter, melting point and elastic constants. The present potentials generally reproduce the fundamental physical properties of the Co–W and Al–W systems accurately. The lattice parameters, the enthalpy of formation, the thermal stability and the elastic constants match well with experiment and the first-principles results. The enthalpy of mixing and the enthalpy of formation and mixing of liquid are in good agreement with CALPHAD calculations. The potentials can be easily combined with already-developed MEAM potentials for binary cobalt systems and can be used to describe Co–Al–W-based multicomponent alloys, especially for interfacial properties.

Microscopic phase-field simulation of ordered domain interfaces formed between DO22 phases along [100] direction

Ordered domain interfaces formed between DO22 (Ni3V) phases along [100] direction during the precipitation process of Ni75Al(subscript x)V(subscript 25-x) alloys were simulated by using the microscopic phase-field model. The atomic structure, migration process, and compositions of interfaces were investigated. It is found that there are four kinds of stable ordered domain interfaces formed between DO22 phases along [100] direction and all of them can migrate. During the migration of interfaces, the jump of atoms shows site selectivity behaviors and each stable interface forms a distinctive transition interface. The atom jump selects the optimist way to induce the migration of interface, and the atomic structures of interfaces retain the same before and after the migration. The alloy elements have different preferences of segregation or depletion at different interfaces. At all the four kinds of interfaces, Ni and Al segregate but V depletes. The degrees of segregation and depletion are also different at different interfaces.

Microscopic phase-field simulation for precipitation behavior of Ni-Al-Cr alloy during two-step aging

Based on the microscopic phase-field model, the pre-aging temperature effects on the precipitation mechanism and microstructure evolution during two-step aging for Ni75Al9Cr16 alloy were simulated. The results show that the early precipitation mechanism of L12 phase is the mixed mechanism of spinodal decomposition and non-classical nucleation growth, whereas the early precipitation mechanism of DO22 phase is spinodal decomposition when the pre-aging temperature is 873 K. The early precipitation mechanism of L12 phase is non-classical nucleation growth, whereas the early precipitation mechanism of DO22 phase is spinodal decomposition when the pre-aging temperature is 973 K. Under the effects of elastic strain energy, the cubic particles exhibit directional alignment along [100] and [010] directions during the late precipitation, which is more obvious at lower pre-temperature. DO22 phases appear earlier than L12 phases under these two kinds of precipitation processes; and the nucleation incubation time becomes long with the increase of pre-temperature.

Simulation of interphase boundary of Ni75AlxV25-x alloys using microscopic phase-field method

Abstract The evolution of ordered interphase boundary (IPB) of Ni 75 Al x V 25- x alloys was simulated using the microscopic phase-field method. Based on the atomic occupation probability figure on 2D and order parameters, it was found that the IPB formed by different directions of θ phase has great effect on the precipitation of γ′ phase. The γ′ phase precipitated at the IPB that is formed by [100] θ direction where the (001) θ plane is opposite, and then grows up and the shape is strap at final. The IPB structure between γ′ phase and θ phase is the same. There is no γ′ phase precipitate at the IPB where the (002) θ and (001) θ planes are opposite, the ordered IPB is dissolved into disordered area. There is γ′ phase precipitation at the IPB formed by the [001] θ and [100] θ directions, and the IPB structure is different between γ′ phase and the different directions of θ phase. The IPB where (001) γ′ and (100) θ plane opposite does not migrate during the γ′ phase growth, and γ′ phase grows along [100] θ direction.

Calculation of free energy of Al-Cu-Li alloy under electric field

Based on Thomas-Fermi model, the interior potential boundary condition with the effect of electric field was defined, the calculation method of free energy for atom cluster under electric field was established. The change of free energy of Al-Cu-Li alloy under the effect of electric field was calculated quantitatively. It is shown that: near the zero electric field and the side of positive electric field, the free energy of Cu4LiAl7 compound at aging temperature 460 K is higher than that of free energy at solid solution temperature 725 K, but once the negative electric field increases to certain degree there will be opposite result. Under the effect of electric field, at 725 K the free energy of Cu4LiAl7 is higher than that of Al-1.0%Li-4.0%Cu, and at 460 K the free energy of compound is lower than that of solid solution. When the copper content in the Al-Li-Cu solid solution is below 5%, under the effect of electric field the free energy of solid solution increases gradually with the increasing of copper content, but the increasing amplitude reduces with the increasing of copper content. The free energy of binary solid solution increases with the addition of lithium, and with the increasing of electric field intensity the free energy margin of two kinds of solid solution becomes bigger.

Calculation of electric field effects on the Gibbs free energy of the Al-Li-Mg alloy

Based on the Thomas-Fermi model, the calculation methods of the monatomic Gibbs free energy were found. The interior potential boundary condition under electric field was defined. The calculation methods of Gibbs free energy for the monatomic, compound, and solid solution with electric field were set up. Under the influence of electric field, the Gibbs free energy of Al is the most sensitive, followed by those of Li and Mg. At the solution temperature the Gibbs free energies of Al3Li and its elements under electric field are not symmetrical about the zero point of electric field, whereas at the aging temperature their values are symmetrical about the zero point of electric field. At the solution temperature near the zero point of electric field, the Gibbs free energy of Al3Li is higher than that of Al-2.14%Li. And at 460 K the Gibbs free energy of Al3Li is lower than that of Al-2.14wt.%Li under electric field. The Gibbs free energy of 1420 alloy decreases from both sides of electric field to the zero point at the aging temperature.