Molecular dynamics study on the effect of temperature on HCP→FCC phase transition of magnesium alloy

Chun Xue ,Shui Li ,Zhiing Chu,? ,Qinhu Yng ,Yugui Li ,Lifeng M ,Leifeng Tuo

aCollege of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China

b College of Mechanical Engineering, Taiyuan University of Science and Technology, Taiyuan, 030024, China

Abstract To explore the effect of temperature on the phase transformation of HCP→FCC during compression,the uniaxial compression process of AZ31 magnesium alloy was simulated by the molecular dynamics method,and the changes of crystal structure and dislocation evolution were observed.The effects of temperature on mechanical properties,crystal structure,and dislocation evolution of magnesium alloy during compression were analyzed.It is concluded that some of the Shockley partial dislocation is related to FCC stacking faults.With the help of TEM characterization,the correctness of the correlation between some of the dislocations and FCC stacking faults is verified.Through the combination of simulation and experiment,this paper provides an idea for the in-depth study of the solid-phase transformation of magnesium alloys and provides reference and guidance for the design of magnesium alloys with good plasticity and formability at room temperature.? 2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

Keywords: Temperature;AZ31 magnesium alloy;FCC stacking fault;Shockley partial dislocation;Phase transformation.

1.Introduction

As a new light weight metal material,magnesium alloy has been widely used in aerospace,transportation,electronic information,biomedical and other industries.A large number of researchers focus on strengthening the mechanical properties of magnesium alloys [1–8].The known strengthening mechanisms of crystal materials are "grain boundary strengthening","twin strengthening","solid solution strengthening" and"precipitation strengthening".The basis of these methods is to control defects and prevent dislocation movement [9–10].Because the number of slip systems that can be activated on the substrate of hexagonal close packed crystal structure is limited,and its ductility and formability are poor.[11].With the in-depth study of precipitation strengthening and fine grain strengthening of magnesium alloys,great breakthroughs have been made in the development of high plastic magnesium alloys[12–13].Among them,the grain refinement of the magnesium-aluminum alloy is considered to be one of the effective ways to improve the strength and plasticity of magnesium alloy at the same time [14–16].

Molecular dynamics methods can be widely used in the structural evolution and dislocation multiplication of materials at a nanometer scale [17–19].Many scholars have studied the effect of phase transition on improving metal strength by molecular dynamics method.Jianwei Xiao [20–21] studied the plastic deformation of nanocrystalline high entropy alloy model by molecular dynamics simulation.It is found that the plastic deformation of nanocrystalline high entropy alloy is mainly caused by partial reversible face-centered cubic transformation (FCC) to hexagonal compact packing phase transition (HCP) caused by stacking faults and partial dislocations,which is induced by stress and occurs through the nucleation and diffusion of partial dislocations.Hao Zhang [22] has studied the effects of uniaxial stretching directions [20],[010] and [0001]on the deformation mechanism of hexagonal dense packing(HCP) titanium crystals by molecular dynamics simulation.It is found that the phase transition behavior is different in different loading directions.N.Amadou [23] used molecular dynamics simulation to study the relationship between deformation,BCC-HCP phase transformation and orientation,initial defects,and temperature conditions of single-crystal iron under compression.Yuming Qi [24] analyzed the plastic deformation of single crystal and polycrystalline high entropy alloys under tensile and compression conditions by molecular dynamics simulation.It was found that FCC-HCP phase transition occurred in the process of plastic deformation.At the same time,under the action of compression load,the grain is refined,which leads to the inconsistency of crystal orientation,which effectively improves the strength of the material.Aleksandr V.Korchuganova [25] simulated the uniaxial tensile process of nano-copper crystals by molecular dynamics.It was found that the nucleation and slip of some dislocations on the free surface were due to the FCC→BCC→HCP phase transition.

At present,most of the researches on plastic deformation of magnesium alloys are at the micro-level.The room temperature deformation tests of AZ31 hot-rolled sheet and extruded bar were carried out by T.Ebeling [26],and the results were compared with the simulation results of the viscoplastic self-consistent model.Kenneth J.Tam [27] established a temperature-sensitive viscoplastic self-consistent(VPSC) model.The effects of temperature and texture on the deformation mechanism of hot-rolled AZ31 magnesium alloy were studied,and the deformation mode of magnesium alloy was predicted.Kai Zhang [28] compressed AZ31 magnesium alloy at room temperature and low temperature.The effects of twins and dislocations on flow stress,plastic strain,and strain hardening were studied.It was found that low-temperature deformation inhibited dislocation activity.Progress has also been made in the study of the atomic level of magnesium alloys.Xiangli Liu [29] has studied the dislocation structures of<0001>tilted and twisted grain boundaries in magnesium alloys at the atomic level.It is found that both symmetrically inclined and twisted grain boundaries have ordered atomic structures in the whole rotation angle range ofθ=0° -60°,which can be described by the grain boundary dislocation model.Ki-Hyun Kima [30] explained the role of alloying elements in improving the room temperature ductility of magnesium alloy by molecular dynamics simulation.It was found that as long as the alloy content was controlled,any solute element with a different particle size from magnesium could improve the room temperature ductility of magnesium alloy.A.M.Vlasova [31] has carried out an atomic simulation of the deformation of ideal magnesium crystal along the crystal axis and studied the evolution process of structural defects at T=300 -350 K.

Although there have been a lot of studies on molecular dynamics simulations in material deformation,few studies have focused on the role of stacking faults in the deformation mechanism of HCP metal [32–33].To explore the effect of temperature on the phase transformation of HCP→FCC during compression,the uniaxial compression process of AZ31 magnesium alloy was simulated by the molecular dynamics method,and the changes of crystal structure and dislocation evolution were observed.The effects of temperature on mechanical properties,crystal structure,and dislocation evolution of magnesium alloy during compression were analyzed.It is concluded that some of Shockley’s dislocations are related to FCC stacking faults.Through TEM characterization,the correctness of the correlation between Shockley dislocations and FCC stacking faults is verified.Through the combination of simulation and experiment,this paper provides an idea for the in-depth study of the solid-phase transformation of magnesium alloys and provides reference and guidance for the design of magnesium alloys with good plasticity and formability at room temperature.

2.Simulation and experimental scheme

2.1.Simulation scheme

In the study of molecular dynamics,the potential function is usually used to describe the relationship between the model particle total energyEand the particle coordinate (r)[34–37].The improved embedded atom method potential function (MEAM) is more suitable for the calculation of metals and alloys with FCC,BCC,HCP,and Diamond structures[38–44].Its function is as follows:

Fig.1.MD simulation diagram.

Fig.2.Experimental equipment and samples (a) Gleeble-3800 thermal simulator;(b) Hot compression process;(c) Compression pattern;(d) JEM-F200 transmission electron microscope.

Fig.3.MD simulated force and energy parameters (a) Stress-strain curves;(b) Potential energy curves;(c) Total dislocation density curves.

Fig.4.Mechanical properties (a) Stress-strain curves of compression test;(b) Yield stress curves.

In the function,Eis the total energy of atomi;Fis the embedded energy and is the Function of atomic electron densityρ;ρis the atomic electron density;γand?is a pair of potential interactions;αandβis the element type of atomsiandj.

The model of AZ31 magnesium alloy with the size of?200 ?A × 300 ?A is established,and the X-axis,Y-axis,and Z-axis are set to correspond to the crystal directions [20],[010] and [0001],respectively.The model is introduced into a large-scale atomic/molecular massively parallel simulator (LAMMPS) [45] for molecular dynamics simulation.X,Y,and Z are all periodic boundary conditions,NPT(constant-pressure,constant-temperature) ensemble is adopted,and the time step is 0.001ps.The NPT ensemble and the nose-hoover thermostat maintain the stability of the entire system at constant pressure and temperature.The calculation is carried out by using the potential function developed by Hyo-Sun Jang [46].This potential function can be used to study the deformation and recrystallization characteristics of Mg-Al-Zn alloy in the whole processing temperature range.The conjugate gradient (CG) algorithm is used to minimize the model energy to obtain the equilibrium structure at a given temperature in the NPT ensemble.First,relax the 10ps at a given temperature,and then compress the 40ps at the strain rate of 0.01ps-1,so that the deformation of the magnesium alloy reaches 40%,as shown in Fig.1.The mathematical formula of deformation is:

In the formula,εis strain;L1is the initial length of the model in [0001] crystal direction;L2is the length of the model compressed along the [0001] crystal direction.

The calculation results are imported into the visualization software OVITO,and the functions such as CNA (Common neighbor analysis),DXA (Dislocation analysis),GS (Grain segmentation) are used to analyze the structural change and dislocation evolution [47–53].

2.2.Experimental scheme

The cylindrical compression sample of ?10 mm × 15 mm in AZ31 magnesium alloy extruded bar was taken by WEDM,and the surface of the sample was polished with sandpaper to make the side of the cylindrical sample show metallic luster,remove the oxide scale on the end face and eliminate obvious scratches.The Gleeble-3800 thermal simulator was selected to carry out uniaxial compression along the axial direction of the sample,and the heating rate was 5 °C/s.To ensure the uniform distribution of the temperature field in the sample,the sample was kept at the experimental temperature for 100s before deformation.The setting temperature is 500 K,600 K,and 700 K respectively.Because of the difference between the molecular dynamics simulation and the macroscopic experiment,the strain rate of the experiment is not consistent with that of the molecular dynamics,so the experimental strain rate is chosen as 1 s-1.To reduce the friction between the sample and the compression fixture during deformation,the two ends of the sample are coated with graphite lubricant.

To further study the dislocation,atomic arrangement,crystal structure,and phase transformation,TEM experiments were carried out on 200 kV’s transmission electron microscope (model: JEM-F200).The atomic arrangement in AZ31 magnesium alloy after hot compression was observed,the diffraction pattern was obtained,and the dislocation and phase transformation mechanism in the process of hot compression were analyzed.

3.Results and analysis

3.1.MD analysis

Fig.3 shows the force and energy parameter curves in MD simulation.Fig.3(a)shows the stress-strain curves.The yield stress decreases with the increase of temperature.Fig.3 (b)shows the potential energy curves,which increases with the increase of temperature.Fig.3 (c) shows the curves of total dislocation density,which decreases with the increase of temperature.In the process of compression,with the increase of the amount of compression,the atomic position changes,the potential energy of the model increases,the total dislocation density decreases,and the stress increases.When the yield strain is reached,the total dislocation density begins to increase,and the model reaches the maximum yield stress.Enter the plastic stage.With the continuous increase of the amount of squeezing,the atomic position continues to change,and the potential energy still increases.With the increase of temperature,more potential energy is provided to the model,and the model is easier to deform,so the higher the temperature is,the smaller the yield stress is.

Fig.6.Crystal structure evolution.(a)～(c) CNA diagram: (a) 500 K;(b) 600 K;(c) 700 K.(d)～(f) Fraction curves of different structure: (d) 500 K;(e)600 K;(f) 700 K.

Fig.4 (a) shows the stress-strain curves of the compression test,which is consistent with the MD simulation,that is,the higher the temperature,the smaller the stress.Fig.4 (b)shows the yield stress curves.Due to the difference between MD simulation and experiment,the yield stress is not equal.However,no matter in the experiment or the MD simulation,the yield stress decreases with the increase of temperature,which proves the unity of the experiment and the MD simulation.

Fig.6.Continued

Fig.5 (a)～(c) is the diagram of grain evolution in MD simulation,and Fig.5 (d) is the curves of grain number.With the increase of the amount of compression,the grain refinement of magnesium alloy occurs in different degrees,but the degree of grain refinement decreases with the increase of temperature.Therefore,the high temperature can inhibit the grain refinement of magnesium alloy.

Fig.7.Dislocation evolution.(a)～(c) Dislocation diagram: (a) 500 K;(b) 600 K;(c) 700 K;(d)～(f) Dislocation density curves: (d) 500 K;(e) 600 K;(f) 700 K.

Fig.7.Continued

Fig.6 (a)～(c) is a CNA diagram.The red atom represents the HCP structure atom,the green atom represents the FCC stacking fault atom,and the white atom represents the OTHER atom including the grain boundary and the unrecognized structure.With the increase of the amount of compression,the grains are refined and the atomic structure changes.Whenε=0%,there are only HCP structure and OTHER structure in magnesium alloy.With the increase of compression amount,HCP structure decreases,OTHER structure increases,and FCC stacking fault occur.Because the number of atoms does not change,the phase transformation of magnesium alloy takes place in the process of compression.As the amount of compression continues to increase,so does the number of FCC stacking faults.Fig.6 (e)～(f) is a structural fractional curves.It can be found that when the strain is close to the yield strain,the FCC stacking fault occurs in the magnesium alloy,so the FCC stacking fault is related to the plasticity of the magnesium alloy.With the increase of temperature,the degree of transition from HCP structure to OTHER structure decreases,so the temperature can inhibit the phase transition.

Table 1 Bergson marks of common dislocations in closely packed hexagonal crystals[55]

Fig.7 (a)～(c) is a dislocation diagram.As the number of compression increases, the length and number of dislocation lines increase, and the dislocation density increases.Fig.7 (e)～(f) shows the dislocation density curves.It can be found that the100 dislocations and other dislocations have the highest dislocation density.Table 1 shows the Bergson markings of common dislocations in closely packed hexagonal crystals.Whereis a basal complete dislocation, -→T S([0001]) is a cylindrical full dislocation,is an incomplete dislocation,is a Shockley partial dislocation [54].According to Table 1, it is found that the strain of Shockley partial dislocation is similar to that of FCC stacking fault, so it is inferred that Shockley partial dislocation is closely related to the phase transformation of magnesium alloy.

To explore the correlation between Shockley partial dislocation and HCP alloy FCC phase transition, the HCP structure fraction curve, FCC stacking fault fraction curve, and Shockley partial dislocation density curves are combined into

Fig.9.TEM and SEAD images at different temperatures (a) 500 K;(b) 600 K;(c) 700 K.

To explore the correlation between Shockley partial dislocation and HCP alloy FCC phase transition,the HCP structure fraction curve,FCC stacking fault fraction curve,and Shockley partial dislocation density curves are combined into Fig.8 (a).It is found that with the increase of strain,whenε=2.6%,the HCP structure fraction first decreases.Whenε=4.6%,the density of Shockley partial dislocations begins to increase.Whenε=5.6%,the FCC stacking fault score begins to increase.However,this change occurs before yield,so the phase transformation of magnesium alloy occurs before the plastic stage.Fig.8 (b) shows the evolution of Shockley partial dislocations and the HCP→FCC phase transition at 600 K.There are atoms of HCP structure on one side of Shockley partial dislocations and atoms of FCC stacking faults on the other.With the increase of the amount of compression,the atomic position of the HCP structure changes,forming Shockley partial dislocations and forming grain boundaries.With the progress of compression,the atomic position continues to change,Shockley partial dislocations multiply and move,and the atoms of FCC stacking faults increase.Therefore,the specific law of phase transformation of magnesium alloy is HCP→GB→FCC,and Shockley partial dislocation is accompanied by the whole phase transformation process.

3.2.Experimental analysis

Fig.9 (a)～(c) shows the TEM and SEAD images under 500～700 K,respectively.It can be seen that the number of dislocations in the sample decreases with the increase in temperature.This is because in the hot compression process,with the increase of temperature,the deformation process begins to be accompanied by dynamic recrystallization,and the dislocation is no longer excited when the strain increases.

Fig.10 (a) is the HRTEM image of the sample.It is found that there are both FCC structure and HCP structure in the image,so there is a phase transition of HCP→FCC in the compression at 500 K.Fig.10 (b) for the lattice stripe image obtained by FFT and IFFT on the HRTEM image,it is found that there is a dislocation between the FCC structure and the HCP structure.Fig.10 (c) when the HRTEM image near the dislocation is enlarged,the atomic arrangement between the HCC structure and the HCP structure can be observed.Fig.10(d)and(f)calibrate the FCC structure and HCP structure,respectively.Fig.10 (e) shows the lattice stripes and atomic spacing of the FCC structure,and the atomic spacing in the two directions is 0.2403 and 0.2502 nm,respectively,which confirms the FCC structure.Fig.10 (g) shows the lattice stripes and atomic spacing of the HCP structure,and the atomic spacing in the two directions is 0.2447 and 0.4997 nm,respectively,which confirms the HCP structure.Therefore,this experiment verifies the conclusion obtained by MD simulation,that is,there is a phase transition of HCP→FCC in the hot compression deformation of magnesium alloy,and the FCC structure multiplies and moves with the progress of compression,and the atoms of FCC stacking faults increase.Therefore,the specific law of phase transformation of magnesium alloy can be summarized as HCP→Shockley partial dislocation→FCC.

Fig.11 (a) is the HRTEM image of the sample.It is found that there are both FCC structure and HCP structure in the image,so there is a phase transition of HCP→FCC in the compression at 500 K.Fig.11 (b) for the lattice fringe image obtained by FFT and IFFT on the HRTEM image,it is found that there is a dislocation between the FCC structure and the HCP structure.Fig.11 (c) the enlarged image of the lattice stripes near the dislocation,the Shockley partial dislocation between the HCC structure and the HCP structure can be observed.Fig.11 (d) and (f) calibrate the FCC structure and HCP structure respectively.Fig.11 (e) shows the lattice stripes and atomic spacing of the FCC structure,and the atomic spacing in the two directions is 0.2688 and 0.2857 nm,respectively,which confirms the FCC structure.Fig.11 (g) shows the lattice stripes and atomic spacing of the HCP structure,and the atomic spacing in the two directions is 0.5281 and 0.2840 nm,respectively,which confirms the HCP structure.

Fig.12 (a) is the HRTEM image of the sample.It is found that there are both FCC structure and HCP structure in the image,so there is a phase transition of HCP→FCC in the compression at 500 K.Fig.12 (b) for the lattice stripe image obtained by FFT and IFFT on the HRTEM image,it is found that there is a dislocation between the FCC structure and the HCP structure.Fig.12 (c) an enlarged image of the lattice stripes near the dislocation,the Shockley partial dislocation between the HCC structure and the HCP structure can be observed.Fig.12 (d) and (f) calibrate the FCC structure and the HCP structure,respectively.Fig.12 (e) shows the lattice stripes and atomic spacing of the FCC structure,and the atomic spacing in the two directions is 0.2655 and 0.2529 nm,respectively,which confirms the FCC structure.Fig.12 (g) shows the lattice stripes and atomic spacing of the HCP structure,and the atomic spacing in the two directions is 0.5278 and 0.2515 nm,respectively,which confirms the HCP structure.

Fig.10.Transmission experimental image at 500 K.(a) HRTEM image;(b) Lattice fringe image;(c) HRTEM magnified image near dislocation;(d) FFT image of FCC structure;(e) Lattice fringe and atomic spacing of FCC structure;(f) FFT image of HCP structure;(g) Lattice fringe and atomic spacing of HCP structure.

Fig.11.Transmission experimental images at 600 K.(a) HRTEM image;(b) Lattice fringe image;(c) HRTEM magnified image near dislocation;(d) FFT image of FCC structure;(e) Lattice fringe and atomic spacing of FCC structure;(f) FFT image of HCP structure;(g) Lattice fringe and atomic spacing of HCP structure.

Fig.12.Transmission experimental images at 700 K (a) HRTEM image;(b) Lattice fringe image;(c) HRTEM magnified image near dislocation;(d) FFT image of FCC structure;(e) Lattice fringe and atomic spacing of FCC structure;(f) FFT image of HCP structure;(g) Lattice fringe and atomic spacing of HCP structure.

4.Conclusion

In this paper,to explore the effect of temperature on the phase transformation of HCP→FCC during compression,the uniaxial compression process of AZ31 magnesium alloy was simulated by the molecular dynamics method,and the changes of crystal structure and dislocation evolution were observed.The effects of temperature on mechanical properties,crystal structure,and dislocation evolution of magnesium alloy during compression were analyzed.It is concluded that some of the Shockley dislocations are related to FCC stacking faults.Through hot compression experiments,with the help of TEM characterization means.The correctness of the correlation between some dislocations of Shockley partial dislocation and FCC stacking faults is verified.In this paper,through the combination of simulation and experiment,the following conclusions are obtained:

(A) In both experiments and MD simulations,the yield stress decreases with the increase of temperature,and temperature can restrain the grain refinement and phase transformation of magnesium alloy.

(B) The phase transition of HCP→GB→FCC occurs in magnesium alloy,which occurs before the plastic stage.At the same time,the Shockley partial dislocation is accompanied by the whole phase transformation process,with the atoms of the HCP structure on one side and the atoms of FCC stacking fault on the other.

(C) The experiment verifies the conclusion obtained by MD simulation,that is,there is a phase transition of HCP→FCC in the hot compression deformation of magnesium alloy,and the FCC structure multiplies and moves with the progress of compression,and the atoms of FCC stacking faults increase.Therefore,the specific law of phase transformation of magnesium alloy can be summarized as HCP→Shockley partial dislocation→FCC.(Fig.2)

Declaration of competing interest

The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria;educational grants;participation in speakers’ bureaus;membership,employment,consultancies,stock ownership,or other equity interest;and expert testimony or patent-licensing arrangements),or non-financial interest (such as personal or professional relationships,affiliations,knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Acknowledgments

This work was supported by the National Key Research and Development Project (2018YFB1307902),Shanxi Province Joint Student Training Base Talent Training Project(No.2018JD33),Shanxi young top talent project,Shanxi Province Science Foundation for Youths(201901D211312),Excellent young academic leaders in Shanxi colleges and universities(No.2019045),Excellent Achievements Cultivation Project of Shanxi Higher Education Institutions(No.2019KJ028),Shanxi Province emerging industry leader talent project,Shanxi Graduate Education Innovation Project(No.2019SY482).