Dynamic recrystallization behavior and mechanical properties of bimodal scale Al2O3 reinforced AZ31 composites by soild state synthesis

Moling Hu,Shuihu Wei,Qin Shi,Zesheng Ji,Hongyu Xu,Ye Wng

aSchool of Materials Science and Engineering,Harbin University of Science and Technlolgy,Harbin 150040,China

b School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150000,China

Received 6 June 2019;received in revised form 22 February 2020;accepted 23 February 2020 Available online 30 May 2020

Abstract In this paper,(500nm 1%+5μm 3%)bimodal scale Al2O3p/AZ31 composites was fabricated by solid state synthesis and the effect of bimodal scale Al2O3 particulates on its dynamic recrystallization behavior and mechanical properties was investigated.The optical microscopy,scanning electron microscopy,transmission electron microscopy and electron universal strength tester composites were used to characterize the composites.The results indicate that the grains size of the composites are significantl refine and the mechanical properties are obviously improved.Due to the presence of the bimodal scale Al2O3 particulates,the high-density dislocation zone is formed around nano-Al2O3p and the particle deformation zone is formed near micron-Al2O3p.These zones are ideal sites for the formation of recrystallization nucleus.Meanwhile,the addition of the bimodal scale Al2O3 particulates may delay or hinder the growth of matrix grain through the pining effect on the grain boundaries,resulting in significantl improving the yield strength and tensile strength of Al2O3p/AZ31 composites.? 2020 Published by Elsevier B.V.on behalf of Chongqing University.This is an open access article under the CC BY-NC-ND license.(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:AZ31 magnesium alloy;Solid state synthesis;Dynamic recrystallization behavior;Mechanical property;Al2O3 particulate.

1.Introduction

As a light metal material with excellent properties such as low density,high specifi stiffness and specifi strength,magnesium alloy has broad application prospects in the field of automobile and aerospace[1-3].However,since magnesium and its alloy is hexagonal closed packed(HCP)structure with limited slip systems,resulting in its have relatively low strength and poor formability[4].Magnesium and its alloy is able to overcome their demerit and lead to a significan increase in strength and micro-hardness due to the addition of reinforcement phase,for instance ceramic particulates,whiskers or fibre[5-9].Furthermore,compared to fibe or whisker reinforced magnesium matrix composites,particulate reinforced magnesium matrix composites(PMMCs)show the unique advantages of lower cost and easier fabricability[10,11].

In fact,for particulate reinforced magnesium matrix composites,the particulate size of the reinforcement phase has a significan effect on the microstructure and mechanical properties of the composite.When the particulate size is micron size,the yield strength and tensile strength of the magnesium matrix composites are significantl improved,but the plasticity is lowered[12].Compared to micron particulates,nano particulate reinforcements efficientl enhance the mechanical properties of the magnesium matrix composites by promoting more effective particulate hardening mechanisms[13].In recent years,some researches have focused on the effect of the addition of micron and nano particulates reinforcements on the mechanical properties and the microstructure of PMMCs.Hu et al.[14]studied the effect of submicron size Al2O3particulates on the microstructure stability,it found that submicron size Al2O3particulates could increase the dislocation density by pinning the dislocation motion of the matrix and promoting the dynamic recrystallization nucleation.Wang et al.[15]have reported that the dynamic recrystallization(DRX)of 10μm SiC/AZ91 composite compressed at different strains and strain rates,revealing that the particle deformation zone(PDZ)was formed near 10μm SiC particle and the PDZ was considered as an ideal site for DRX.Hassan et al.[16]added 50 nm to 1μm size of Al2O3particulates to synthesize the Mg/Al2O3composite.Nano particulates were the most effective in increasing the combination of ductility and strength.Significan increase in ductility and work of fracture was attained by adding nano and submicron particulates simultaneously.However,few researchers have studied the bimodal scale Al2O3reinforced AZ31 composites using solid state synthesis,especially the effect of bimodal scale Al2O3particulate on dynamic recrystallization behavior and mechanical properties.

Accordingly,the primary aim of this paper is to prepare the bimodal scale Al2O3reinforced AZ31 composites using solid state synthesis,research the microstructure evolution of bimodal scale Al2O3reinforced AZ31 composites,and then reveal the effect of bimodal scale Al2O3particulates on the dynamic recrystallization behavior and mechanical properties.

2.Experimental

2.1.Raw material

Commercial AZ31 alloy was used as the matrix alloy and its chemical composition(wt%)was 0.28Al,1.00Zn,0.25Mn,0.06Si,0.01Cu,0.01Fe and balance Mg.Two kinds of Al2O3particulates with the average diameter of 5μm and 500nm were used as reinforcement.For simplification 1wt%(500nm)Al2O3/AZ31 composite is denoted as N-1,4wt%(5μm)Al2O3/AZ31 composite is denoted as M-4,and the(1wt%(500nm)+3wt%(5μm))Al2O3/AZ31 composite is denoted as N-1+M-3.

2.2.Solid state synthesis

The commercial AZ31 magnesium ingot was machined into chips,which had a length of 3.65-5.25mm,a width of 1.65-2.85mm,and a thickness of 0.15-0.85mm.The AZ31 alloy chips were placed in a mechanical mixer with the calculated Al2O3particulates at a speed of 200r/min for 240min,then a mixture was obtained.The mixture was placed in a 40-mm diameter cylindrical container and cold pressed into a cylinder,and then the cyliner was heated to 633K and held for 30 min.After that,it is hot extruded into a bar of 8mm in diameter,which is extrusion rate of 0.2mm/s and an extrusion ratio of 25:1.The experimental parameters of Al2O3/AZ31 composite are shown in Table 1.

2.3.Microstructural characterization

The microstructure of Al2O3/AZ31 composite was characterized by optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).After being etched with an acetic picric solution(5ml nitric acid+100ml ethanol(95%)),the samples were used for OM,SEM and TEM analysis.2D image-pro analysis software was used to analyze the grain size distribution.

Table 1The experimental parameters of Al2O3/AZ31 composite.

2.4.Tensile testing

Tensile testing is performed on a WDW-200 electronic universal tensile testing machine at room temperature.According to the GB/T228.1-2010 standard,the extruded bar was machined into tensile specimens of 5mm in gauge diameter and 25mm in gauge length.For each parameter,six samples were used for the tensile tests in order to ensure the accuracy of the experimental results.

3.Results

3.1.Microstructures

Fig.1 shows the optical microstructures of AZ31 alloy and Al2O3/AZ31 composites with different scales of Al2O3particulates parallel to the extrusion direction.The addition of Al2O3particulates has significan effect on refinin the grain size.A large number of the Al2O3particulates are scattered at the grain boundaries,and a few of Al2O3particulates are scattered inside the grains,as shown in Fig.1bd).Fig.1a)shows the optical microstructure of monolithic AZ31 alloy.The grains of monolithic AZ31 alloy are coarse,dynamic recrystallization(DRX)has occurred,but not completely.Compared with the grain size of monolithic AZ31 alloy,the grain size of the Al2O3p/AZ31/N-1 composite is significantl refined as shown in Fig.1b).Many fin recrystallized grains are observed around the large deformed grains and this phenomenon fully validates the dynamic recrystallization behavior.Due to the presence of nano-Al2O3particulates,the particulates are fin hard ones.The dislocations movement cannot be transferred through the particulates.The dislocations tend to pile-up around the particulates.The dislocation movement of the Al2O3p/AZ31/N-1 composite is easy to be pinned and the dislocation density near the particultes is obviously increased[17,18].The DRXed nucleuses preferentially occurred at these zones,and finall leads to grains refinemen of the Al2O3p/AZ31/N-1 composite.Previous studies by Humphrey's[19]have also confirme this phenomenon.When the particulate size is less than 1μm,it could promote recrystallization of Al alloy by the pining effect on grain boundaries.The average grain sizes of Al2O3p/AZ31/N-1 and Al2O3p/AZ31/M-4 were 28.37μm and 24.33μm.The grain of Al2O3p/AZ31/M-4 was fine than that of Al2O3p/AZ31/N-1,as shown in Fig.1c).Particle deformation zone(PDZ)can be formed in the vicinity of micron-Al2O3particulates during hot extrusion of AZ31 alloy containing micron-Al2O3particulates.The PDZ made for the recrystallization nucleation at the interface between Al2O3particulates and AZ31 alloy due to its high dislocation density,resulting in the refine grain of Al2O3p/AZ31/M-4 composite.Deng's[20,21]investigation also have shown that the micron-SiC particulates have a significan refinemen effect on the grain of the magnesium alloy by promoting DRX nucleation.Fig.1d)shows that the bimodal scale particulates are homogeneous distributed in AZ31 magnesium matrix.The average grain size of Al2O3p/AZ31/N-1+M-3 composite was 18.76μm.The grains of Al2O3p/AZ31/N-1+M-3 composite are further refined the necklace structure grains are observed around the grain boundaries and DRX is more sufficient The Al2O3particulate bonds are paralleling to the extrusion direction,as indicated by yellow dotted box in Fig.1d).In fact,micron-Al2O3particulates can effectively promote dynamic recrystallization nucleation,while nano-Al2O3particulates may have the pining effect.Two scale particulates work together,as a result,the Al2O3p/AZ31/N-1+M-3 composite grain were refine significantl.

Fig.1.Optical microstructures:a)AZ31 alloy;b)Al2O3p/AZ31/N-1composite;c)Al2O3p/AZ31/M-4 composite;d)Al2O3p/AZ31/N-1+M-3 composite.

Fig.2.The statistical grain size distributions:a)AZ31 alloy;b)Al2O3p/AZ31/N-1 composite;c)Al2O3p/AZ31/M-4 composite;d)Al2O3p/AZ31/N-1+M-3 composite.

Fig.3.SEM micrographs and EDS results of Al2O3p/AZ31/N-1+M-3 composite:a)distribution of bimodal scale Al2O3 particulates;b)Al2O3 particulates;c)DRX grains;d)EDS results of point A in c);e)element distribution of Mg,Al,O,Zn in b).

Fig.2 shows the statistical grain size distributions of bimodal scale Al2O3p/AZ31 composite.It can be found that the average grain size decreased significantl with the addition of different scale Al2O3particulates.In the monolithic AZ31 alloy microstructure,the grain size is uniform and the peak of grain size distribution mainly concentrates at 25-45μm,as shown in Fig.2a).This peak is transferred to 20-40μm and 15-35μm as adding to 1% nano-Al2O3particulates and 4% micron-Al2O3particulates,respectively,as shown in Fig.2b)and c).Due to the presence of Al2O3particulates,the high-angle and low-angle grain boundaries of the magnesium matrix are pinned and the grain boundary movement is hindered,resulting in promoting to DRX nucleation and refinin the average grain of Al2O3p/AZ31 composite.By comparing with Fig.2b)and c),it can be found the grain size of Al2O3p/AZ31/M-4 composite are smaller than the grain size of Al2O3p/AZ31/N-1 composite.Compared to nano-Al2O3particulates,micron-Al2O3particulates have a more pronounced grain refinemen effect on the composite.Fig.2d)shows that the fin grains of 0-10μm increase significantl,the peak is mainly concentrated in 15-25μm,and the average grain size is decreased by 42.12% compared to monolithic AZ31 alloy.The phenomena illustrates that the existence of bimodal scale Al2O3particulates,significantl refine the grain size compared to the addition of single scale Al2O3particulates.

Fig.4.TEM microstructure of AZ31/Al2O3p/N-1+M-3 composite:a)microstructure near Al2O3 particulates;b)high-density dislocations near Al2O3 particulates;c)high magnificatio of microcrack near Al2O3 particulates;d)electron diffraction pattern of Al2O3 particulates.

Fig.3 shows SEM micrographs and EDS results of AZ31/Al2O3p/N-1+M-3 composite.The distribution of bimodal scale Al2O3particulates is nearly homogeneous in Al2O3p/AZ31 composites,as shown in Fig.3a).Some nano-Al2O3particulates were homogeneously distributed around micron-Al2O3particulates,and the interface between the Al2O3particulates and AZ31 magnesium matrix is intact,as shown in Fig.3b).The DRX grains are observed around the Al2O3particulates,as shown in the Fig.3c).Due to the bimodal scale Al2O3particulates scattered at the grain boundary,the movement of the grain boundary is validly restricted by micron-Al2O3particulates and the pining effect on the grain boundary is restricted by nano-Al2O3particulates,resulting in promoting to the DRX nucleation.The energy dispersive analysis(EDS)results of point A and element distribution further demonstrate that bimodal scale Al2O3particulates were homogeneously scattered at AZ31 magnesium matrix,as shown in Fig.3d)and e).

Fig.4 shows the TEM microstructure of AZ31/Al2O3p/N-1+M-3 composite after solid state synthesis.The high-density dislocations appear near Al2O3particulates and microcracks can also be seen in Fig.4a).During the hot extrusion process,due to the difference in thermal expansion coefficien between the Al2O3particulates and the magnesium matrix,the particulates can both hinder the movement of dislocations and induce deformation mismatch between matrix and particulates.The dislocations tend to pile-up around the particulates[17].Therefore,the high-density dislocation zones are formed,as shown in Fig.4b).The high-density dislocation zones surrounding the Al2O3particulates can effectively promote its dynamic recrystallization nucleation,and finall the grain size of the Al2O3p/AZ31 composite is remarkably refined When the stress becomes greater than its interfacial bonding force,microcracks might take shape in the interface between AZ31 matrix and Al2O3particulates,as shown in Fig.4c).The electron diffraction pattern of Al2O3particulates is shown in Fig.4d).It demonstrates that the reinforcement is Al2O3particulates.

3.2.Mechanical properties

Fig.5.Mechanical properties of AZ31 alloy and Al2O3p/AZ31 composites.

Fig.5 shows the mechanical properties of AZ31 alloy and Al2O3p/AZ31 composites.Due to the addition of Al2O3particulates,the yield strength(YS),ultimate tensile strength(UTS)and elongation of AZ31 matrix are significantl improved.The bimodal scale Al2O3p/AZ31composite has higher tensile properties than single scale particulates reinforced composites.In comparison to single scale Al2O3p/AZ31/N-1 composite,bimodal scale Al2O3particulates improve the YS and UTS of AZ31 matrix as well as heighten the elongation of AZ31 matrix obviously.Compared with single scale Al2O3p/AZ31/M-4 composite,the YS and UTS of bimodal scale Al2O3p/AZ31 composite increased to 11.83%and 5.86%,respectively.Compared with nano-sized particulates,the elongation of Al2O3p/AZ31/M-4 composite slightly decreased.Due to the fact that the deformation mismatch between AZ31 matrix and micron-sized particulates in the tensile tests,it is prone to form the micropores and cracks,which ultimately leads to a decrease in elongation.

4.Discussion

4.1.Schematic diagram of DRX grains formation process

Fig.6 shows the schematic diagram of DRX grains formation process in Al2O3p/AZ31/N-1+M-3 composite during solid state synthesis.After mechanical mixing,the bimodal scale Al2O3particulates are decentralized at the surfaces and gaps of AZ31 alloy chips,as shown in Fig.6a).During hot extrusion,most of the micron-Al2O3p and nano-Al2O3p are dispersed at the grain boundaries and a few of nano-Al2O3p is dispersed in the grains due to difference in plastic deformation between Al2O3particulates and AZ31 alloy,as shown in Fig.6b).During the hot extrusion,PDZ is easily formed around the micron-Al2O3p,and high-density dislocation zones are generated in the vicinity of the nano-Al2O3p.Nano-Al2O3p can pin the movement of grains boundaries,while micron-Al2O3p not only impede the movement of dislocations but also induce the deformation mismatch between AZ31 matrix and Al2O3particulates.Hence,the dislocation density near nano-Al2O3particulates increases obviously and the PDZ is easy to be formed,as shown in Fig.6c).In general,the PDZ size rests with particulate size,which minishs with falloff in particulate size[22].As the particulate size is smaller than 1μm,the size of PDZ around them is too small to be considered[23].The DRX grains were observed in Fig.6d).Due to the presence of high-density dislocation zone and PDZ,the DRX nucleuses were easy to happen at these zones,resulting in forming the DRX grains.

4.2.Strengthening mechanisms

For Al2O3p reinforced magnesium matrix composites,the Al2O3particulates belong to ceramic particulates and have high strength and hardness,which can effectively transfer loads and have a certain effect on improving the yield strength(YS)of composite.The increment of YS caused by the effect of load transfer(Δσload)can by estimated by the following equation[24]:Whereσmis the YS of monolithic AZ31 alloy andVpis the volume fraction of Al2O3particulate in AZ31 matrix given by Table 2[25,26].

Owing to the mismatch of the coefficien of thermal expansion(CTE)between Al2O3p and AZ31 matrix,highdislocation density can be generated around Al2O3p,which has a positive effect on the improvement of YS.The increment in YS can be estimated by following equation[27]:

where,ωis regarded as constant 1,andρis the dislocation density given as[28]:

where,Δβis the coefficien of thermal expansion(CTE)different of the AZ31 matrix and Al2O3particulates,ΔTis the temperature change from solid state synthesis temperature to room temperature.Gis the shear modulus of AZ31 alloy;bis the burgers vector of Mg(HCP),andcVpare the average diameter and volume fraction of Al2O3particulates,respectively.The parameter values are given in Table 2.As mentioned above,the addition of Al2O3particulates leads to the decrease of grain size,which causes the increase of YS according to Hall-Patch relationship[29].The Grain refine ment improves the YS of composites can be calculated by the equation:

wheredCompositeanddAZ31are the average grain size of composite and monolithic AZ31 alloy,respectively.Qis the Hall-Patch coefficien which is given as 0.13Mpa·m1/2[30].

For the particulates fine than 1μm,Orowan strengthening mechanism exists in the composite due to the interaction of dislocations with nano-Al2O3p[31],thus,it can be computed by equation[32]:

Fig.6.Schematic diagram of DRX grains formation process in Al2O3p/AZ31/N-1+M-3 composite during solid state synthesis:a)Al2O3p distribution before extrusion;b)Al2O3p distribution after extrusion;c)high-density dislocations and PDZ around Al2O3p;d)DRX grains at the grain boundaries and near Al2O3p.

Table 2The parameter values for calculating the increased yield strength[25,26].

Table 3Respective contributions from different mechanisms for bimodal scale Al2O3p/AZ31 composites.

whereλindicates the average inter-particle spacing,andcNindicates the average diameter of nano particulates.The interparticle spacingλis estimated by following equation[33]:

Based on the above analysis,the contribution of bimodal scale Al2O3p to improve the YS can be calculated by Eq(7).[34]:

The theoretical yield strength of AZ31/Al2O3p/N-1+M-3 composite(σP)is calculated and compared with the experimental results(σexp),as shown in Table 3.The experimental value of yield strength is significantl higher than the theoretical value.The PDZ existed around micron particulates,which promoted to the DRX nucleation,and it might strengthen the effect ofΔσCT E,ΔσLoad,ΔσHall?PetchandΔσOrwan.So the difference between the experimental and theoretic value of yield strength appears in Table 3.Table 3 also shows that theΔσCT Eplays an important role in improving the YS of the composite.The effect of load transfer(ΔσLoad)on the YS of the composite is almost negligible compared to other mechanisms.

5.Conclusions

1)The Al2O3p/AZ31 composites with different scale Al2O3particulates was successfully fabricate by solid state synthesis.Micron-Al2O3particulates were mainly dispersed at the grain boundaries,while nano-Al2O3particulates were distributed on the grain boundaries and inside the grains.

2)Due to the deformation mismatch between AZ31 matrix and Al2O3p,the dislocation density near nano-Al2O3p obviously increased and the PDZ around micron-Al2O3p was formed.It promoted to the DRX nucleation,resulting in the significan grain refinemen Al2O3p/AZ31 composites.

3)Compared to single scale Al2O3p reinforced composites,the yield strength,ultimate tensile strength and elongation of bimodal scale Al2O3p/AZ31 composite is higher.On the one hand,bimodal scale Al2O3particulates have the synergistic mechanism effect on the dynamic recrystallization behavior of the Al2O3p/AZ31 composite.On the other hand,the mechanical properties of the Al2O3p/AZ31composite are improved by different strengthening mechanisms.

Acknowledgment

The authors gratefully acknowledge the financia support by the National Key Research and Development Program(2019YFB2006500)and National Natural Science Foundation of China(51404082).