Co-reporter:Can Guo, Jincheng Wang, Junjie Li, Zhijun Wang, Sai Tang, and Yunhao Huang
The Journal of Physical Chemistry C April 13, 2017 Volume 121(Issue 14) pp:8204-8204
Publication Date(Web):March 28, 2017
DOI:10.1021/acs.jpcc.7b01311
Eutectic solidification under rapid solidification conditions has enormous applications, as it can produce microstructure-refined and interface-stable combined composite structures with low cost. However, investigations reported that the coupled interfaces will be destroyed under the condition of large undercoolings. For further understanding of the mechanisms of uncoupling growth, we investigated the uncoupling growth process of binary eutectics during rapid solidification using atomistic simulations. We find that both the nucleation rate and the crystallization velocity for the first phase of eutectics are very high; however, nucleation of the second phase is very inactive: its nucleation rate is low and nucleation incubation time is very long. As the nucleation of the eutectic second phase is severely suppressed during rapid solidifications, the crystallization of the second phase lags far behind, therefore we speculate that the uncoupling growth of eutectics during rapid solidification is nucleation-induced.
Co-reporter:Wenquan Zhou, Jincheng Wang, Zhijun Wang, Qi Zhang, Can Guo, Junjie Li, Yaolin Guo
Computational Materials Science 2017 Volume 127() pp:121-127
Publication Date(Web):1 February 2017
DOI:10.1016/j.commatsci.2016.10.031
Employing the modified phase-field-crystal (MPFC) model, we investigated the size effects of shear deformation response on nano-single crystals by examining the elastic strain distribution at atomistic scales. Here, three specimens with different aspect ratios are explored. Three types of shear deformation behavior have been found, namely, bending, simple shear and the mixed-mode based on the competition of bending and shearing. Further, a size scale analysis indicates that the slope of the shear strain response curve decreased dramatically as the aspect ratio changed from 1 to 0.125. However, when the ratio increased from 1 to 8, the slope increased slowly until saturated.
Co-reporter:Chunwen Guo, Junjie Li, Honglei Yu, Zhijun Wang, ... Jincheng Wang
Acta Materialia 2017 Volume 136(Volume 136) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.actamat.2017.07.002
We present an investigation of secondary and tertiary branching behavior in diverging grain boundaries (GBs) between two columnar dendritic grains with different crystallographic orientations, both by two-dimensional phase-field simulations and thin-sample experiments. The stochasticity of the GB trajectories and the statistically averaged GB orientations were analyzed in detail. The side-branching dynamics and subsequent branch competition behaviors found in the simulations agreed well with the experimental results. When the orientations of two grains are given, the experimental results indicated that the average GB orientation was independent of the pulling velocity in the dendritic growth regime. The simulation and experimental results, as well as the results reported in the literature exhibit a uniform relation between the percentage of the whole gap region occupied by the favorably oriented grain and the difference in the absolute values of the secondary arm growth directions of the two competitive grains. By describing such a uniform relation with a simple fitting equation, we proposed a simple analytical model for the GB orientation at diverging GBs, which gives a more accurate description of GB orientation selection than the existing models.Download high-res image (510KB)Download full-size image
Co-reporter:Feng He, Zhijun Wang, Qingfeng Wu, Junjie Li, ... C.T. Liu
Scripta Materialia 2017 Volume 126(Volume 126) pp:
Publication Date(Web):1 January 2017
DOI:10.1016/j.scriptamat.2016.08.008
CoCrFeNi alloy is an exemplary stable base for high entropy alloys (HEAs), but its phase stability is still suspicious. Here, the CoCrFeNi HEA was firstly identified to be thermally metastable at 750 °C. Composition decomposition occurred after annealed at 750 °C for 800 h. The minor addition of Al accelerated the composition decomposition and a second fcc phase with a different lattice constant occurred in the long time annealed CoCrFeNiAl0.1 HEA. The CoCrFeNi HEA cannot be seen as stable alloys anymore and researchers should be very careful when using the current phase selection models to predict stability of HEAs at intermediate temperatures.Composition decompositon occurred in the CoCrFeNi high entropy alloy after annealed at 750 °C for 800 h (see (a, b)). The addition of Al accelerated the decomposition and a second fcc phase with different lattice precipitated from the CoCrFeNi base, as shown in (c and d). These results indicated that the CoCrFeNi high entropy alloy is a metastable solid solution.Download high-res image (271KB)Download full-size image
Co-reporter:Feng He, Zhijun Wang, Peng Cheng, Qiang Wang, Junjie Li, Yingying Dang, Jincheng Wang, C.T. Liu
Journal of Alloys and Compounds 2016 Volume 656() pp:284-289
Publication Date(Web):25 January 2016
DOI:10.1016/j.jallcom.2015.09.153
•A strategy of designing a eutectic HEA was proposed in this paper.•The CoCrFeNiNbx eutectic HEA were designed and studied.•The CoCrFeNiNb0.5 alloy has a good balance of strength and ductility.Based on surveying the existing binary phase diagrams with eutectic points, a strategy of designing eutectic high entropy alloys (EHEAs) with desired high strength and ductility is proposed. Based on the computer-aided thermodynamic calculations, the pseudo eutectic binary alloy system of CoCrFeNiNbx (x = 0.1, 0.25, 0.5 and 0.8) was designed. The experimental results show that the eutectic alloys are composed of a ductile face centered cubic (FCC) phase and a hard Laves phase with fine laminar structures. The designed alloys show excellent integrated mechanical properties of ductility and strength. For the CoCrFeNiNb0.5 alloy, the compressive fracture strength and strain can reach above 2300 MPa and 23.6%, respectively.
Co-reporter:Feng He, Zhijun Wang, Sizhe Niu, Qingfeng Wu, Junjie Li, Jincheng Wang, C.T. Liu, Yingying Dang
Journal of Alloys and Compounds 2016 Volume 667() pp:53-57
Publication Date(Web):15 May 2016
DOI:10.1016/j.jallcom.2016.01.153
•A FCC precipitate was found in CoCrFeNiNb0.25 HEA for the first time.•The HEA was strengthened extensively without sacrificing compressive ductility.•It is the first time that a FCC phase precipitated from FCC matrix in HEAs.High entropy alloys (HEAs) are facing difficulties in balancing strength and ductility, and till now there is no satisfactory solution. In this paper, the CoCrFeNiNb0.25 HEA with good ductility was strengthened by a face centered cubic (FCC) precipitate after annealing heat treatment. When the CoCrFeNiNb0.25 HEA was annealed at 750 °C, a lath-shaped FCC precipitates with nano basket-weave microstructures appeared and distributed randomly in the proeutectic FCC phase. Due to the precipitation strengthening, the yield stress of the CoCrFeNiNb0.25 HEA almost doubled but its ductility was just slightly damaged.
Co-reporter:Yunhao Huang, Jincheng Wang, Zhijun Wang, Junjie Li, Can Guo, Yaolin Guo, Yujuan Yang
Computational Materials Science 2016 Volume 122() pp:167-176
Publication Date(Web):September 2016
DOI:10.1016/j.commatsci.2016.05.024
Understanding and controlling the selection of crystalline structure are of great significance in crystal growth, in which the preparation of metastable crystal structures is one of the most important issues to be understood. Although plenty of experimental and simulation investigations have been carried out to reveal the formation of metastable structures, the mechanism of metastable phase existence and forming still remains unclear. In the present work, by using the phase field crystal (PFC) model, we further explored the existence and formation mechanism of metastable structure by modeling the crystallization process with prestructured seeds in 2D. We found that the metastable structure can be survived in a certain region of the phase diagram. Our results show that the selection of final structures is determined not only by the thermodynamic driving force but also by competitions between the metastable and stable structures.fx1
Co-reporter:Can Guo, Jincheng WangJunjie Li, Zhijun Wang, Sai Tang
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 24) pp:5008-5014
Publication Date(Web):November 22, 2016
DOI:10.1021/acs.jpclett.6b02276
Crystallizations often pass through multiple intermediate structures before reaching the final state, such as amorphous precursors, polymorphs, or denser liquid droplets. However, the atomistic pathways from these metastable phases to final crystals still remain unclear. Here, we investigated the structure evolution process from liquid to final crystals of homogeneous nucleation by atomic-scale simulations and analyzed the intrinsic mechanisms that influence the nucleation pathways. Three different pathways of two-step nucleation were found by visualizing the precursors’ evolutions, and some new micromechanisms of two-step nucleation are revealed. We suggest that the solid bond fluctuations can trigger the formation of intermediate precursors, while the precursors’ packing density dominates the structural transformation pathways from intermediate phases to crystals. These findings not only shed light on the mechanisms of nucleation but also provide guidance for future refinements of two-step nucleation theory.
Co-reporter:Yaolin Guo, Jincheng Wang, Zhijun Wang, Junjie Li, Yujuan Yang, Yaohe Zhou
Computational Materials Science 2015 Volume 109() pp:253-257
Publication Date(Web):November 2015
DOI:10.1016/j.commatsci.2015.07.037
Effects of a preexisting disconnection dipole on the migration of a Σ5(3¯10)/[001] grain boundary (GB) at low shear rates have been investigated by using the minimized bulk dissipation phase field crystal model. Simulation results show that the two disconnections play different roles during the process of shear-coupled GB migration via different behaviors regarding heterogeneous nucleation and propagation of disconnections. Specifically, the different roles can be attributed to their structural deviation from the equilibrated crystallographic structure caused by shear loading: one disconnection contributes to the onset of GB migration by heterogeneous nucleation of disconnections, while the other one slows down the corresponding GB migration dynamics through the impediment of homogeneous nucleation in the flat GB plane.
Co-reporter:Junjie Li, Chunwen Guo, Yuan Ma, Zhijun Wang, Jincheng Wang
Acta Materialia 2015 90() pp: 10-26
Publication Date(Web):
DOI:10.1016/j.actamat.2015.02.030
Co-reporter:Zhijun Wang, Junjie Li, Jincheng Wang, Yaohe Zhou
Acta Materialia 2012 Volume 60(Issue 5) pp:1957-1964
Publication Date(Web):March 2012
DOI:10.1016/j.actamat.2011.12.029
Abstract
Selection mechanisms of primary dendritic spacing in directional solidification are investigated by the phase field method. Results show that the lower and upper limits of primary spacing are determined by the interdendritic solutal interactions and the interdendritic undercooling respectively. The upper limit of primary spacing resulting from overgrowth of the tertiary arm could be about four times as large as the lower limit. The microstructural evolution from the onset of planar instability during directional solidification with a constant pulling velocity can be divided into three stages: an initial competition stage, a submerging stage and a lateral adjustment stage. Simulation results also demonstrate that the final primary spacing with a constant pulling velocity is very close to the lower limit due to the dendrite submerging mechanism.
Co-reporter:Junjie Li, Zhijun Wang, Yaqin Wang, Jincheng Wang
Acta Materialia 2012 Volume 60(Issue 4) pp:1478-1493
Publication Date(Web):February 2012
DOI:10.1016/j.actamat.2011.11.037
Abstract
The microstructure evolution of grains with different orientations during directional solidification is investigated by the phase-field method. For converging dendrites, in addition to the usually accepted overgrowth pattern wherein the favorably oriented dendrites block the unfavorably oriented ones, the opposite pattern of overgrowth observed in some recent experiments is also found in our simulations. The factors which may induce this unusual overgrowth are analyzed. It is found that in addition to the difference in tip undercooling, the solute interaction of converging dendrites, which has been ignored in the classical theoretical model, also has a significant effect on the nature of the overgrowth at low pulling velocities. Solute interaction can retard the growth of dendrites at the grain boundary (GB) and induce a lag of these dendrites relative to their immediate neighbors, which gives the unfavorably oriented dendrite the possibility to overgrow the favorably oriented one. However, this unusual overgrowth only occurs when the spacing between the favorably oriented GB dendrite and its immediate favorably oriented neighbor decreases to a certain level through lateral motion. These findings can broaden our understanding of the overgrowth mechanism of converging dendrites.
Co-reporter:Sai Tang, Zhijun Wang, Yaolin Guo, Jincheng Wang, Yanmei Yu, Yaohe Zhou
Acta Materialia 2012 Volume 60(Issue 15) pp:5501-5507
Publication Date(Web):September 2012
DOI:10.1016/j.actamat.2012.07.012
Co-reporter:Sai Tang, Jincheng Wang, Gencang Yang, Yaohe Zhou
Intermetallics 2011 Volume 19(Issue 3) pp:229-233
Publication Date(Web):March 2011
DOI:10.1016/j.intermet.2010.09.012
The growth behavior of Ni3Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni3Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni3Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni3Al/γ and Ni3Al/β interfaces and the grain boundary migration of Ni3Al grains caused by fast grain boundary diffusion.
Co-reporter:J. Li, J. Wang, G. Yang
Acta Materialia 2009 Volume 57(Issue 7) pp:2108-2120
Publication Date(Web):April 2009
DOI:10.1016/j.actamat.2009.01.003
Abstract
The grain boundary (GB) motion in the presence of GB segregation is investigated by means of phase field simulations. It is found that the solute concentration at the moving GB may increase with increasing velocity and becomes larger than the equilibrium value, which is unexpected according to the solute drag theory proposed by Cahn, but has been observed in some experiments. A non-linear relation between the driving force (curvature) and the GB velocity is found in two cases: (1) the GB motion undergoes a transition from the low-velocity extreme to the high-velocity extreme; (2) the GB migrates slowly in a strongly segregating system. The first case is consistent with the solute drag theory of Cahn. As for the second case, which is unexpected according to solute drag theory, the non-linear relation between the GB velocity and curvature comes from two sources: the non-linear relation of the solute drag force with GB velocity, and the variation in GB energy with curvature. It is also found that, when the diffusivity is spatially inhomogeneous, the kinetics of GB motion is different from that with a constant diffusivity.
Co-reporter:Zhijun Wang;Jingcheng Wang ;Gencang Yang
Crystal Research and Technology 2009 Volume 44( Issue 1) pp:43-53
Publication Date(Web):
DOI:10.1002/crat.200800107
Abstract
Effects of surface tension anisotropy on the planar interfacial stability are studied with asymptotic analysis method in both the solidification from undercooled pure melts and the unidirectional solidification of binary alloys. The asymptotic approach developed by Xu is adopted to study the interfacial stability here, which is different from that used by other investigators previously in their works. A simple linear analysis result is obtained, i.e., the surface tension anisotropy may compete to determine interfacial stability near some critical conditions in unidirectional solidification of binary alloys. The exsitstence of the surface tension anisotropy enlarges the instability region of disturbed wave number. And the threshold of instability is strongly affected by surface tension anisotropy, especially at high pulling velocity or high temperature gradient. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Co-reporter:JinCheng Wang;YuXiang Zhang;YuJuan Yang
Science China Technological Sciences 2009 Volume 52( Issue 2) pp:344-351
Publication Date(Web):2009 February
DOI:10.1007/s11431-008-0155-0
By using the phase field model for the solidification of multi-component alloys and coupling with real thermodynamic data, the dendritic morphology transition and the dendritic micro-segregation of Ni-Al-Nb ternary alloys are simulated in two cases, i.e., varying the alloy composition at a fixed undercooling and varying the undercooling at a fixed alloy composition. The simulated results indicate that with the increase of the dimensionless undercooling U (U=ΔT/ΔT0, where ΔT is the undercooling and ΔT0 the temperature interval between the solidus and liquidus), the dendritic morphology transfers from dendritic to globular growth in both cases. As to the dendritic micro-segregation, both cases present a regularity of increasing at first and then decreasing.
Co-reporter:Jincheng Wang, Gencang Yang
Acta Materialia 2008 Volume 56(Issue 17) pp:4585-4592
Publication Date(Web):October 2008
DOI:10.1016/j.actamat.2008.05.016
Abstract
The process of isothermal dendritic coarsening in a Ni–Al–Nb ternary system is simulated by using the phase-field method. The coarsening behaviors and coarsening mechanisms with different solid fractions fS are investigated in detail. Simulated results show that, as fS increases from 64% to 88%, the coarsened morphology of the liquid phase varies from connected plate to rod-like shape, which is similar to the lamellar to rod transition in eutectics due to the interface energy. Simulated results also indicate that the dendritic isothermal coarsening mechanisms are dominated by remelting of the third arms, coalescence and smoothing in the case of low solid fraction, while in the case of high solid fraction, coalescence, smoothing, Rayleigh instability, rounding and shrinking away of small liquid droplets are the main mechanisms of dendritic coarsening. The increase of liquid diffusion coefficient DL will also trigger the Rayleigh instability to accelerate the morphology transition from plate to cylindrical for the liquid phase.
Co-reporter:Tomonori Kitashima, Jincheng Wang, Hiroshi Harada
Intermetallics 2008 Volume 16(Issue 2) pp:239-245
Publication Date(Web):February 2008
DOI:10.1016/j.intermet.2007.10.001
In order to simulate multi-component diffusion in a Ni-base superalloy using the phase-field method, the Gibbs free energy of the γγ and γ′γ′ phases in the quaternary Ni–Al–Re–Ta superalloy system was linked to the CALPHAD method using a four-sublattice model. The free energy curve and phase equilibrium compositions in this work were in good agreement with the results of the Thermo-calc calculation, indicating that the model was successfully linked to the CALPHAD method. A one-dimensional phase-field simulation showed a pileup of Re at the γ/γ′γ/γ′ interface during the growth of precipitates as well as a spike of the Re concentration in a γ′γ′ phase, accompanied by precipitate coalescence.
Co-reporter:JinCheng Wang;JunJie Li;YuJuan Yang
Science China Technological Sciences 2008 Volume 51( Issue 4) pp:362-370
Publication Date(Web):2008 April
DOI:10.1007/s11431-008-0041-9
The influences of pulling speed V and temperature gradient G on morphology evolution, concentration distribution, solute trapping and interface stability during directional solidification of binary alloys have been studied with the B-S phase field model. Simulated results reproduced the morphology transitions of deep cell to shallow cell and shallow cell to plane front. The primary cellular spacing, depth of groove and effective solute redistribution coefficient for different V and G are compared. The absolute stability under high pulling speed and high temperature gradient has also been predicted, which is in agreement with the Mullins-Sekerka (M-S) stability theory.
Co-reporter:J.J. Li, J.C. Wang, Q. Xu, G.C. Yang
Acta Materialia 2007 Volume 55(Issue 3) pp:825-832
Publication Date(Web):February 2007
DOI:10.1016/j.actamat.2006.07.044
Abstract
Polycrystalline solidification in binary alloys proceeding by nucleation and subsequent anisotropic growth is studied by a newly developed phase field model. Simulation results show that, although the growth rates at different locations of the dendrite arm are diverse and cannot be correlated through a multiplicative factor, the time dependence of the dendritic growth area also satisfies a certain simple power function as in the case of linear growth or parabolic growth of a convex particle, but the growth exponent is different from those cases. Through phase field simulation it can be obtained that the growth exponent in two dimensions approximately equals 2 for the dendrite with well-developed side branches and 1.5 for one without side branches. The transformation kinetics of polycrystalline solidification obtained by phase field simulation is compared with the Johnson–Mehl–Avrami–Kologoromov (JMAK) theory based on analyzing the growth kinetics of a single dendrite. For the dendrites grow without side branches, it is found that the JMAK model will overestimate the transformed fraction because of the neglecting of the blocking effect. For the impingement of dendrites with fully developed side branches, the blocking effect is negligible because of the small anisotropy of the dendrictic envelope shape.
Co-reporter:J.C. Wang, M. Osawa, T. Yokokawa, H. Harada, M. Enomoto
Computational Materials Science 2007 Volume 39(Issue 4) pp:871-879
Publication Date(Web):June 2007
DOI:10.1016/j.commatsci.2006.10.014
A quantitative simulation with phase field method (PFM) in a real alloy system was performed with a new strategy of modeling the microstructure evolution of Ni-base superalloys, in which, the calculation of phase diagrams (CALPHAD) method and cluster variation method (CVM) are combined with the PFM. In this strategy, the four sub-lattice model is used to evaluate the chemical free energy density while CVM to calculate some parameters, such as the lattice misfit and gradient coefficients in phase field equations. With this strategy, the microstructure evolution of a Ni–Al binary alloy at the temperature of 1000 K has been simulated; the elastic energy due to the lattice misfit and elastic inhomogeneity between γ and γ′ phases has also been taken into account. Moreover, the directional coarsening phenomenon of Ni–Al alloy has been reoccurred when the various external stress conditions are applied, which shows good agreement with Pineau’s directional coarsening map.
Co-reporter:Junjie Li, Jincheng Wang, Gencang Yang
Journal of Crystal Growth (1 February 2009) Volume 311(Issue 4) pp:1217-1222
Publication Date(Web):1 February 2009
DOI:10.1016/j.jcrysgro.2008.11.078
Microsegregation within the columnar dendritic array is analyzed by performing a two-dimensional phase-field simulation. The influence of microstructure morphology on microsegregation is studied for various back diffusion conditions. Under the condition of no back diffusion, it is found that for the region without second dendrite arms the simulation result agrees well with the Scheil equation, but for the region with well developed second dendrite arms there is a severe deviation. This deviation is attributed to the dendrtic coarsening and the inhomogeneity of interdendritic liquid concentration caused by various interface curvatures. Under the condition of moderate back diffusion, it is found that the effect of dendritic morphology on microsegregation can be accounted by enhancing the Fourier number which characterizes the solid-state diffusion. However, this effect decreases with enhancing the back diffusion of the system.
Co-reporter:Zhijun Wang, Junjie Li, Jincheng Wang
Journal of Crystal Growth (1 August 2011) Volume 328(Issue 1) pp:108-113
Publication Date(Web):1 August 2011
DOI:10.1016/j.jcrysgro.2011.06.021
Determinant factors for tilted growth dynamics of dendritic arrays during directional solidification are analyzed. A vector model of solutal gradient is proposed to predict the growth direction of tilted dendritic arrays based on the competition between the preferred crystalline dynamics and the solutal diffusion interaction of dendritic neighbors. Effects of the spacing Péclet number and thermal gradient on the tilted dynamics are clarified. The predicted growth direction from this vector model is in good agreement with previous results.Highlights► Tilted growth dynamics of dendritic arrays origins from the competition between the preferred crystalline dynamics and the solutal diffusion interaction of dendritic neighbors. ► A simple model is presented to predict the tilted growth direction. ► The spacing Péclet number and preferred orientation are the main factors in determining the tilted growth direction. ► An analytical expression is also proposed to describe the tilted growth direction. ► The model's predictions are in good agreement with previous results.
Co-reporter:Feng He, Zhijun Wang, Qingfeng Wu, Sizhe Niu, Junjie Li, Jincheng Wang, C.T. Liu
Scripta Materialia (1 April 2017) Volume 131() pp:42-46
Publication Date(Web):1 April 2017
DOI:10.1016/j.scriptamat.2016.12.033
Co-reporter:Yujuan Yang, Jincheng Wang, Yuxiang Zhang, Yaochan Zhu, Junjie Li, Gencang Yang
Journal of Crystal Growth (1 April 2009) Volume 311(Issue 8) pp:2496-2500
Publication Date(Web):1 April 2009
DOI:10.1016/j.jcrysgro.2009.02.011
Three-dimensional (3D) multi-phase field simulations are performed to study the lamellar stability of a directionally solidified hypereutectic CBr4–C2Cl6 alloy in both the longitudinal and the transverse directions. The 3D simulation results show that the morphology evolution in the longitudinal direction is consistent with that in two dimensions, but the morphology evolution in the transverse direction is much complex and some new patterns form. The discussion on the mechanism of lamellar spacing adjustment shows that the lamellar spacing adjustment in 3D is through lamellar termination and lamellar creation.
