Yongxin Wang

The effect of an electric field on the mechanical properties and microstructure of Al–Li alloy containing Ce

Abstract The influence of an electric field on the tensile properties and microstructure of Al–Li alloy containing Ce is investigated. The ductility of the alloy is improved, but the strength has no change under the effect of an electric field (electric field treatment). Scanning Electron Microscopy (SEM) and TEM results show that the fracture mode is changed by the applied electric field; the subgrains become larger with the increase of the strength of the electric field, the volume fraction of δ ′ phase increases and its distribution is more random with an electric filed. The relation between the mechanical properties, and the microstructure of those alloys is analyzed. On the basis of experiment results, the mechanism of the effect of an electric field is discussed.

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%.

Effect of atomic structure on migration characteristic and solute segregation of ordered domain interfaces formed in Ni75AlxV25-x

Based on the microscopic phase-field model, ordered domain interfaces formed between D022 (Ni3V) phases along [001] direction in Ni75AlxV25-x alloys were simulated, and the effects of atomic structure on the migration characteristic and solute segregation of interfaces were studied. It is found that the migration ability is related to the atomic structure of interfaces, and three kinds of interfaces can migrate except the interface (001)//(002) which has the characteristic of L12 (Ni3Al) structure. V atoms jump to the nearest neighbor site and substitute for Ni, and vice versa. Because of the site selectivity behaviors of jumping atoms, the number of jumping atoms during the migration is the least and the jumping distance of atoms is the shortest among all possible modes, and the atomic structures of interfaces are unchanged before and after the migration. The preferences and degree of segregation or depletion of alloy elements are also related to the atomic structure of interface.

Microscopic phase-field study on directional coarsening mechanism caused by interaction between precipitates in Ni–Al–V alloy

Abstract A microscopic phase-field model was used to investigate a directional coarsening mechanism caused by the anisotropic growth of long period stacking and different effects of phases on precipitation in Ni–Al–V alloy. The results show that DO 22 mainly coarsens along its short axis, which may press the neighboring L1 2 , leading to the interaction among atoms. Diffusion channels of Al are formed in the direction where the mismatch between γ′ and γ reduces; the occupation probabilities are anisotropic in space; and direction coarsening of L1 2 occurs finally. With a rise of ageing temperature, phases appear later and DO 22 is much later at a higher temperature, the average occupation probabilities of Al and V reduce, and Al changes more than V.

Ageing process of pre-precipitation phase in Ni0.75Al0.05Fe0.2 alloy based on phase field method

By utilizing phase field method combined with analysis on free energy and interatomic potentials, pre-precipitation phase formation and transformation process of Ni0.75Al0.05Fe0.2 alloy in early precipitation stage during the ageing process under 1 000 K were studied. And free energy, microstructures, compositions and volume fractions of pre-precipitation phase and equilibrium phase were analyzed. The simulation results indicate that nonstoichiometric L10 pre-precipitation phase formed first, and then would gradually transform into L12 equilibrium phase. It is discovered that the phase transformation process was closely related to free energy and interatomic potentials. Additionally, it is revealed that free energy of L10 pre-precipitation phase was higher and interatomic potential was smaller than that of L12 equilibrium phase. Therefore, it is concluded that L10 phase was unstable, and phase transformation would occur to L12 which was more stable.

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.

Unifying the crystallization behavior of hexagonal and square crystals with the phase-field-crystal model*

By employing the phase-field-crystal models, the atomic crystallization process of hexagonal and square crystals is investigated with the emphasis on the growth mechanism and morphological change. A unified regime describing the crystallization behavior of both crystals is obtained with the thermodynamic driving force varying. By increasing the driving force, both crystals (in the steady-state) transform from a faceted polygon to an apex-bulged polygon, and then into a symmetric dendrite. For the faceted polygon, the interface advances by a layer-by-layer (LL) mode while for the apex-bulged polygonal and the dendritic crystals, it first adopts the LL mode and then transits into the multi-layer (ML) mode in the later stage. In particular, a shift of the nucleation sites from the face center to the area around the crystal tips is detected in the early growth stage of both crystals and is rationalized in terms of the relation between the crystal size and the driving force distribution. Finally, a parameter characterizing the complex shape change of square crystal is introduced.

Quantitative determination of anti-structured defects applied to alloys of a wide chemical range*

Anti-structured defects bridge atom migration among heterogeneous sublattices facilitating diffusion but could also result in the collapse of ordered structure. Component distribution Ni75Al x V25−x alloys are investigated using a microscopic phase field model to illuminate relations between anti-structured defects and composition, precipitate order, precipitate type, and phase stability. The Ni75Al x V25−x alloys undergo single Ni3V (stage I), dual Ni3Al and Ni3V (stage II with Ni3V prior; and stage III with Ni3Al prior), and single Ni3Al (stage IV) with enhanced aluminum level. For Ni3V phase, anti-structured defects (VNi1, NiV, except VNi2) and substitution defects (AlNi1, AlNi2, AlV) exhibit a positive correlation to aluminum in stage I, the positive trend becomes to negative correlation or smooth during stage II. For Ni3Al phase, anti-structured defects (AlNi, NiAl) and substitution defects (VNi, VAl) have a positive correlation to aluminum in stage II, but NiAl goes down since stage III and lasts to stage IV. VNi and VAl fluctuate when Ni3Al precipitates prior, but go down drastically in stage IV. Precipitate type conversion of single Ni3V/dual (Ni3V+Ni3Al) affects Ni3V defects, while dual (Ni3V+Ni3Al)/single Ni3Al has little effect on Ni3Al defects. Precipitate order swap occurred in the dual phase region affects on Ni3Al defects but not on Ni3V.