Abstract:  

The core-shell structure precipitatates of Fe-xCu-3.0Mn-1.5Ni-1.5Al alloys under internal and external strain was investigated by using a multicomponent continuous phase field model based on Gibbs free energy of sub regular solution. Results show that the early cluster nuclei are not pure Cu, and Mn/Ni/Al also gather in the same position of Cu rich nuclei, resulting in four core-shell structures in precipitation. In the absence of external strain, the morphology of precipitates is mainly determined by interfacial energy, intrinsic elastic anisotropy and lattice distortion between new phase and parent phase. Intrinsic elastic strain energy can inhibit precipitation, while has no obvious effect on particle morphology. In coarsening, the elastic energy decreases due to the combination of particles. The loading direction and magnitude of the applied elastic strain field can control the morphology of precipitates. The external strain and the interaction between Mn, Ni and Al promote the joining and merging of adjacent core-shell particles. This work has guiding significance for the design of Fe-xCu-3.0Mn-1.5Ni-1.5Al alloys and other core-shell precipitates materials.

Background Introduction:

Currently, phase-field method has become an important tool for exploring microstructure evolution and phase transition, numerous studies have elucidated the effect of strain or stress on the precipitation of second phase particles by phase-field method, including the evolution of γ′/γ microstructure in Ni based superalloys, nano α′ phase separation in Fe–Cr based alloys, deformation texture and martensitic transformation behavior in polycrystalline materials. However, the above research is mainly applied to binary system, ternary system and very few quaternary system, and the research on the five component system is just beginning , especially the five component alloy with core-shell precipitates under the condition of elastic strain field has not been reported.

The effect of elastic strain field on precipitation process and morphology evolution in Fe–Cu–Mn–Ni–Al alloy with coherent precipitates is still unclear. Thus, in order to predict and control the microstructure and properties of multicomponent steels reasonably, it is vital to clarify the influence of elastic strain field caused by internal and external strain on diffusion transformation. In this paper, a quinary continuous phase-field model incorporating elasticity is established, and the system free energy with elastic strain energy is calculated by using the sub regular solution model. The precipitation behavior of Cu rich core-shell particles in Fe-xCu-3.0Mn-1.5Ni-1.5Al alloys under internal and external strain, including the morphology evolution, solute distribution and coarsening kinetics during isothermal aging, is studied. The work in this paper is helpful to the design of multicomponent alloys and core-shell materials.

Article Highlights:

A continuous phase-field model for aging precipitation of quinary alloys has been established by introducing the elastic strain energy storaged in internal and external strain. The nucleation, growth, and coarsening process of   the core-shell structure precipitates induced by the synergistic effect of solute and strain were investigated, including the early random nucleation, nucleation-growth-coarsening accompanied process, revealing the formation mechanism of core-shell structure. This model can be extended to more alloy systems including HEAs and traditional systems.

Summary and Outlook:

A continuous phase field model of quinary alloy is established to study the morphology evolution and precipitation kinetics of Cu/NiAlMn core-shell precipitates in Fe–Cu–Mn–Ni–Al alloy with different Cu contents under elastic strain field, the main conclusions are as follows:

(1) Whether there is elastic interaction or not, whether the alloy is located in the metastable or unstable region, the Cu rich precipitates have complex core-shell structure, and the evolution sequence with aging time is: ① Cu/Mn/Ni/Al miscible aggregate; ② the (MnNiAl (Cu) AlNiMn) single Cu core + single shell (AlNiMn) structure; ③ the (Mn (MnNiAl (Cu) AlNiMn) Mn) single Cu core + double shell (Mn shell and AlNiMn shell) structure; ④ the Mn (NiAl (Mn (Cu) Mn) AlNi)Mn structure has a Cu core and three-layer shell: Mn shell, Al/Ni shell and Mn shell from inside to outside. The Cu rich core may be covered by one, two or three shells respectively and the shape of the shell variess with the shape of the core.

(2) In metastable region, the internal elastic strain energy can inhibit precipitation, but has no obvious effect on the particle morphology. With the external strain, because the shear modulus of the particles is less than that of the matrix, particles grow along the elastic “soft” direction and perpendicular to the direction of external strain.

(3) In instability region, under the internal elastic strain energy, the phase separation period is prolonged, and the concentration wave changes from random network structure to regular vertical cross distribution. At the same time, the morphology of particles under applied strain is similar to that in metastable region. The difference is that with the increase of particle coalescence, the ratio of particle length to diameter increases. The morphology of precipitates can be artificially controlled via the applied strain.

(4) As the core-shell particles just precipitate, the new phase leads to the increase of coherent elastic resistance and elastic energy. In coarsening, the combination of particles causes the elastic energy decrease slowly. In the absence of external stress, the morphology of precipitates is mainly determined by the elastic anisotropy itself and the lattice distortion between the new phase and parent phase; The external strain field can control the precipitation rate and growth direction of core-shell particles. The joint and coalescence of adjacent core-shell particles are promoted by the synergistic effect of applied strain and Mn, Ni and Al components.

Article Details:

Core-shell structure nanoprecipitates in Fe-xCu-3.0Mn-1.5Ni-1.5Al alloys: A phase field study

Yuhong Zhao*, Yuanyang Sun, Hua Hou

Article Link: https://doi.org/10.1016/j.pnsc.2022.04.001

Corresponding author: Yuhong Zhao

She is a professor of materials science & engineering at the University of Science and Technology Beijing/North University of China. Her main research interests are solid-liquid/solid-solid phase transition and multi-scale computational modeling.

She is the director of the Collaborative Innovation Center for Aluminum Magnesium materials research and application jointly established by the Ministry of Education (Al/Mg MECIC), an executive directior of the solidification branch of the Chinese Materials Research Society, the vice chairman of the Shanxi Meatl Society and Shanxi Casting Society, and founder of the Phase-field method and Integrated computational materials engineering (PFM-ICME) forum. She has been selected for the National High level Talent Program (“WanRen”), National Excellent Teacher, National Millions of Talents, and Representative of the 13th National Congress of Chinese Women, and enjoys the special government allowances from the State Council.

She has long been committed to the development and application of phase-field theory & modeling, high-performance light alloys, liquid/semi-solid squeeze casting, software EasyCast for casting process, and integrated phase-field software EasyPhase. She propose a unified phase-field theory perspective applied to the entire process of material design and forming. She as published over 300 articles in journals such as Sci Adv, Prog Mater Sci, npj Comput Mater, Acta Mater, IJP,etc. She has won 6 first prizes of science and technology awards in provincial/ ministerial level.