Xiaojun Wang, Xiaoming Wang, Xiaoshi Hu, Kun Wu

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Abstract Mg matrix composites were often reinforced by non-deformable ceramic particles.In this paper, a novel Mg matrix composite reinforced with deformable TC4 (Ti-6Al-4V) particles was fabricated and then extruded.The evolutions of microstructure and mechanical properties of the composite during hot extrusion were investigated.Hot extrusion refine grains and eliminated the segregation of TC4 particles.TC4 particles, as deformable particles, stimulated the nucleation of dynamic recrystallization during extrusion.However, since the deformation of TC4 particles partly released the stress concentrations around them,the recrystallized grains are just slightly smaller around TC4 particles than that away from them, which is evidently different from the case in Mg matrix composites reinforced by non-deformable ceramic particles.Compared with AZ91 matrix composites reinforced by SiC particles, the present composite possesses the superior comprehensive mechanical properties, which are attributed to not only the strong interfacial bonds between TC4p and matrix but also the deformability of TC4 particles.

Keywords: Mg matrix composites; Ti particles; Hot extrusion; Mechanical properties.

1.Introduction

Mg matrix composites reinforced with hard ceramic particles, such as SiC, TiC and Al2O3[1-3], have been attracting more and more attention for critical structural applications due to their high specifi strength [4,5], high specifi stiffness [6,7], good wear resistance and excellent damping capacities [8].However, these ceramic particles at micrometer length scale significantl worsen the ductility of Mg matrix composites [9,10], which is primarily attributed to the considerable differences between Mg matrix and ceramic reinforcements in the crystal structure, the coefficien of thermal expansion and the elastic modulus [11].Recently, some researchers have utilized Ti particles instead of ceramic reinforcements [11-13].Ti particles have improved both the strengths and ductility of Mg matrix.Rashad et al.[14] used a semi-powder metallurgy method to fabricate the Mg-Ti and Mg-10Ti-1Al synthesized composites, and these composites exhibited improved elastic modulus, 0.2% yield strength and failure strain.Our previous studies also confirme that the as-cast TC4p/AZ91 composites exhibited better ductility than that of SiCp/AZ91 composites [15].The good ductility of the TC4p/AZ91composites is attributed to the following reasons.First of all, there are relatively small differences in physical and chemical properties between Ti and Mg, which would induce lower residual stresses in the composites.In addition,titanium has a better wettability with magnesium, which may result in strong interface bonding.Last but not the least, titanium is ductile and thus easy to be plastically deformed, so part of the applied stress might be transferred to the titanium reinforcements during plastic deformation.

Generally, the microstructures and mechanical properties of Mg matrix composites can be tailored through secondary processing.Hot extrusion is widely used to further improve the mechanical properties of as-cast Mg matrix composites.Deng et al.[16] found that the as-extruded composites showed relatively uniform reinforcement distribution and minimal porosity in comparison with the as-cast composites.Roy et al.[17] noticed that the matrix grains were significantl refine by extrusion, and the degree of grain refinemen was relatively higher in the composite than that in the unreinforced alloy.Rashad et al.[2] indicated that the as-extruded Mg-3Al-1Zn matrix composites reinforced by Al2O3and SiC hybrid particles exhibited excellent mechanical properties (including yield strength and elongation),regardless of the addition of ceramic particles.Our previous studies on SiCp/AZ91 composites confirme that ceramic particles stimulated dynamic recrystallization (DRX) of the matrix, so the DRX grains size were often much fine near particles than that far away from them [18].In addition, hot extrusion induced particle cracking in SiCp/AZ91 composites[18].All those results are based on the composites reinforced by the non-deformable particles.The ductile Ti particles may exhibit different effects on the microstructure evolution of matrix during hot deformation.However, up till now, there is very limited work focusing on the effects of the deformable particles on microstructures and mechanical properties of Mg matrix composites during secondary processing.

The aim of this article is therefore to investigate the evolutions of microstructures and mechanical properties of Mg matrix composites reinforced with TC4 particles during hot extrusion.The effects of the deformable TC4 particles on the microstructure and mechanical properties of the composite were discussed.

2.Experimental procedures

2.1.Fabrication of the composite

Commercial AZ91D magnesium alloy (Mg-9.29Al-0.71Zn-0.23Mn, wt%) ingots were selected as the matrix material and TC4 particles (TC4p) with an average particle size of 15μm were used as the reinforcement.The TC4 particles investigated here (～1000MPa for strength and ～10% for elongation to fracture) were provided from Shaanxi Yuguang Metal Material Co.Ltd in China.TC4p/AZ91 composite with 10% volume fraction was fabricated by stir casting.The fabrication details were described in the reference [15].AZ91D alloy was firstl heated to 720°C and then cooled down to the semisolid condition at 575 °C, which is between the solidus and liquidus temperature (approximately 595 °C and 470 °C,respectively).Next, the melt was stirred in the semisolid condition.Meanwhile, TC4 particles preheated at 100°C were added into the melt during stirring.After semisolid stirring,the mixture melt was reheated to 700 °C and then kept at this temperature for 5min.During the reheating process, the melt was always stirred at the speed of approximately 250rpm to avoid settlement of TC4 particles due to their relatively higher density compared to matrix melt.Finally, the mixture melt was poured into a steel mould preheated at 400°C to be solidifie under a 100MPa pressure.It should be noted that we found no sedimentation of TC4 particles occurred in all the composites investigated here through the fabrication method mentioned above.

Fig.1.The extruded bar and die residue of the TC4p/AZ91 composite.

2.2.The extrusion of the composite

The billets of the composite and the unreinforced AZ91D alloy were solutionized at 415 °C for 24h (T4 treatment) before extrusion with the purpose of dissolving the Mg17Al12phases into the matrix.After that pretreatment, the assolutionized alloy and composite were machined to cylinders(60mm in diameter and 40mm in height) which well match the extrusion die.It should be noted that the material surface should be polished smooth in advance, which avoids the oxides and impurities on the surface from being incorporated into the materials during extrusion.Besides, the material surface and extrusion die were coated with graphite lubricant oil to reduce the frictional resistance between the billets and the mould, promoting the homogeneous fl w of the materials.The extrusion container,including the billets,pressure pad and dish-shaped die inside, was heated to 350 °C in a muff e furnace.The temperature of the container was monitored by a K-type thermocouple inserted into the container.The container was kept at 350 °C for one hour to allow the billets to reach a steady-state temperature.Subsequently, the extrusion was carried out using a 2000kN press machine.The billets were extruded with an extrusion ratio of 14:1 and constant ram speed of 15mm/s and then cooled in air.The extruded bar and die residue of the composite are shown in Fig.1.

2.3.Microstructure characterization

Optical microscopy (OM; Olympus DP11), scanning electron microscopy (SEM; Quanta 200FEG) and transmission electron microscopy (TEM; Tecnai F30) were used to investigate the microstructure of the composite before and after the extrusion.All the samples for microstructure observation were carried out in the central part of specimens parallel to the extrusion direction.The specimens for OM were ground, polished and etched in acetic picral(5ml acetic acid+5.5g picric acid+10ml H2O+90ml ethanol).The specimen preparation for SEM was identical with that for OM without etching.Specimens for TEM were prepared by grinding-polishing the sample to produce a foil of 50μm in thickness followed by punching it into 3mm diameter disks.The disks were ion beam thinned.

Fig.2.The specifi shape and size of specimen for the tensile testing.

2.4.Mechanical properties testing

All the samples for the tensile tests were machined parallel to the extrusion direction.The tensile tests of the composite and alloy were conducted on an Instron Series 5569 testing machine at room temperature (25 °C) in accordance with ASTM-B557-06 and the tensile rate was 0.5mm/min(5.6×10?4/s for the strain rate).The specifi shape and size of specimen for the tensile testing are shown in Fig.2.The average tensile data (yield strength/YS, ultimate tensile strength/UTS,elongation to fracture and elastic modulus/E)of each sample were obtained from 5 tests at the same condition to ensure the accuracy of the experimental data.

3.Results and discussion

3.1.Matrix microstructure of the as-extruded composites

Fig.3 shows the optical micrographs of the TC4p/AZ91 composite before and after extrusion.Before extrusion, the average grain size was about 54μm, as shown in Fig.3(a)and (b).After extrusion, the grain size was refine to 1.8μm,as shown in Fig.3(c) and (d).The coarse grains of the as-cast composite were completely translated into the fin equiaxed grains through hot extrusion, which indicated that DRX in the matrix has been accomplished during extrusion.Moreover,the grain sizes of the extruded composite were fairly homogenous.The grains near TC4 particles were slightly smaller than those away from the reinforcements, as shown in Fig.3(d).The TEM observations further confirme the minor discrepancy of grain sizes near and away from TC4 particles, as shown in Fig.4.This is different from the result observed in SiCp/AZ91 composites extruded at the same condition, in which the grains were much fine near SiC particles than those far away the reinforcements [18].This difference is caused by the deformable characteristics of TC4 particles.

As we all know, the non-deformable particles restrain the matrix fl w during deformation, which leads to the localized stress concentration around particles.The stress concentration could generate the high-density dislocations and the large orientation gradients around the non-deformable particles,resulting in the formation of the particle deformation zones (PDZs)in the composites [19].Likewise, the residual stress around these particles caused by the quench during solution treatment due to the great differences of the coefficien of thermal expansion(CTE)between the particles and the matrix could further facilitate the formation of PDZs.In these ways, there is a larger driving force for subsequent DRX nucleation in PDZs.As such, PDZs are ideal sites for the recrystallization nucleation, and more favorable nucleation sites for recrystallization are formed in the matrix [19].That is to say, non-deformable particles could stimulate DRX nucleation during hot extrusion, which leads to the formation of the fine DRX grains near the non-deformable particles.In the present composite,TC4 particles possess much higher strength and elastic modulus than matrix, so the PDZs can be also formed around TC4 particles because of the resistant against the matrix fl w during deformation.Therefore, TC4 particles can also stimulate the DRX nucleation during extrusion, which is clearly shown in Fig.5.In addition, the grains between closed packed TC4 particles were very fin (1.2±0.2μm, calculated by measuring more than 200 grains), which is similar to the SiCp reinforced Mg matrix composites[18].The grain sizes away from TC4 particles were slightly larger(2.4±0.3μm,calculated by measuring more than 200 grains).Unlike non-deformable SiC particles, TC4 particles can be deformed during hot extrusion deformation.The deformation of TC4 particles can partly release the stress concentration around the particles, so the dislocation density and orientation gradient near TC4 particles are relatively lower than those around the non-deformable SiC particles under the same condition.As a result, the size difference of the grains between in the zones near the particles and far away from the particles was very significan in the extruded SiCp/AZ91 composite, but not evident in the present TC4/AZ91 composite.

It is a key issue to confir whether the TC4 particles were deformed during extrusion.If a TC4 particle is plastically deformed, it must be loaded to their yield strength during hot extrusion.One way this can be achieved, in principle, is by a shear stress at the interface [20].The strength of monolithic SiC is about 2000MPa,and the yield strength of TC4 is about 1000MPa.According to the shear lag theory, only a few TC4 particles can be loaded to their yield stress.However, cracks on TC4 particles were found after hot extrusion, as shown in Fig.6.This indicates the shear lag theory underestimates the stress on the TC4 particles.The shear lag theory ignores the internal stresses in the composite due to the differences of the thermal expansion coefficien between TC4 particles and the matrix [20].The residual stress in the particle (σ rp) is given by [20,21]:

in whichfis the volume fraction of particles,?is Poisson’s ratio,Eis the modulus;ε?is the eigen strain, which is given by:

and

Fig.3.Optical microstructures of TC4p/AZ91 composite before and after extrusion.(a) and (b) as-cast composite; (c) and (d) as-extruded composite.

Fig.4.The recrystallized grains in the extruded TC4p/AZ91 composite.(a) near TC4particles; (b) away from TC4particles.

whereΔαis the difference in coefficient of thermal expansion.The subscriptmandprefer to the matrix and particle, respectively.Using a temperature change of 400°C from the quench during solution treatment, these equations predict stresses ofσrp=?1280MPa for 10vol% TC4 particles.This calculation confirm that TC4 particles are easy to be loaded to their yield strength during hot extrusion.As shown in Fig.6, there were some TC4 particles fractured after extrusion, which indicates that they have been loaded to the high stresses enough to be deformed.Of course, these calculations assume the particles were not plastically deformed,which will not be the case considering the deformability of TC4 particles.The residual stresses in TC4 particles are significantl released due to their own plastic deformation.TC4 particles sizes were not evidently refine after extrusion, as shown in Fig.7.The observation indicates that most particles were loaded to be plastically deformed rather than fractured.Otherwise, the particle sizes would be evidently refined

Fig.5.The grains near and away from TC4p in the composite.

Fig.6.The fractured TC4 particles in extruded composite.

3.2.Particle distribution of the as-extruded composites

Fig.7 shows the particle distribution of TC4p/AZ91 composites before and after extrusion.For the as-cast composite,the TC4 particles distributed uniformly in the matrix and the particle clusters were not observed (in Fig.7(a)).There were some particles located in the interior of grains due to the good wettability between TC4 particles and Mg melts, although the majority of particles were prone to segregate along the grain boundaries, as shown in Fig.3(b) and Fig.7(b).The particle segregation, which was mainly caused by the “push” effect of the solid-liquid interface at the solidificatio front, leads to serious stress concentrations under the external load and deteriorates the mechanical properties of the composite.For the as-extruded TC4p/AZ91 composite, the distribution was evidently improved by hot extrusion, as shown in Fig.7(c) and(d).In addition, the particle segregation at the grain boundaries was almost eliminated and the particle major axes of TC4 particles were likewise parallel to the extrusion direction, because the matrix fl w facilitated the rotation of TC4 particles during extrusion.Some TC4 particles in the composite would be transformed into short fibers which further proves that TC4 particles were plastically deformed during extrusion.Xi et al.[11] have also found that TC4 particles were plastically elongated and the length-diameter ratio was obviously increased after extrusion.The deformation of TC4 particles may improve the ductility of the composite, because it can reduce the stress concentrations in the matrix around the particles.

3.3.Microstructure evolution during extrusion

The residues in the extrusion die have recorded the whole microstructure evolutions of the alloy and the composite during extrusion.The observed positions of the residues in this study are shown in Fig.8.The average equivalent strains for the different positions in the extrusion die residues can be calculated by the corresponding extrusion ratio (R) and the average equivalent strainwhich can be expressed as follows, respectively [22]:

whereA0is the cross-sectional area of the raw material billet before extrusion andAeis the cross-sectional area of the different positions.The cross-sectional areas,the extrusion ratios and the average equivalent strains of the different observed positions of the residue are shown in Table 1.

Table 1The average equivalent strains of the different observed positions of the residue.

Fig.9 shows the optical microstructure of the AZ91 alloy residue in the extrusion die.In the position I as shown in Fig.9(a), the grain sizes of the alloy were uneven.Most DRX grains were located at the grain boundaries of the initial grains, while only a small amount of DRX grains were located in the intragranular region.Thus, DRX non-uniformly and partly occurred at the beginning of the extrusion.During hot extrusion, the dislocation densities increased in the grains with the unfavorable orientations.The high-density dislocations glided towards grain boundaries during deformation,which causes the dislocation pile-ups in the vicinity of the grain boundaries.As a result, the nucleation of the recrystallized grains firstl occurred on the initial grain boundaries.In addition, there were some tangled dislocations or twins in the interior of the grains, which evolved into subgrains during the following deformation.Hence, some DRX grains were also observed inside the initial grains.

Fig.7.SEM microstructure for TC4p/AZ91 composites before and after extrusion.(a) and (b) as-cast composite; (c) and (d) as-extruded composite.

Fig.8.Observed positions for residue in the extrusion die.

As shown in Fig.9(b), the DRX areas were enlarged with the increase in equivalent strains, but the microstructure was still heterogeneously accompanied with some un-DRX regions.As the extrusion equivalent strain increased to the value corresponding to position Ⅲ, the amount of the recrystallized grains further increased, and only a small percentage of deformed initial grains(un-DRX regions) remained, as shown in Fig.9(c).When the AZ91 alloy was deformed to the positionⅣ, the initial grains were completely replaced by the fin equiaxed DRX grains, as shown in Fig.9(d).

Fig.10 shows the particle distribution of the composite residue in the extrusion die.When the composite was extruded to the position Ⅰ,TC4 particles did not exhibite evident orientations, as shown in Fig.10(a).As the equivalent strains increased, the particle distribution gradually became directional, as shown in Fig.10(b) and 10(c).Finally, the TC4 particles were aligned to the extrusion direction, as shown in Fig.10(d).In addition, the “necklace” particle distribution was gradually eliminated as the equivalent strains increased,which is mainly due to the radial stress perpendicular to the extrusion direction.The extrusion die constrained the composite fl w during hot extrusion.Simultaneously, the radial stress perpendicular to the extrusion direction was developed on the composite due to the constraint of the extrusion die, which accelerates the redistribution of the particles in the matrix.

Fig.11 shows the optical microstructure of the composite residue in the extrusion die.As shown in Fig.11(a),DRX has already taken place in position I, but DRX was incomplete with some large un-DRX regions located away from the TC4 particles.The DRX percentage was much larger in the composite than that in unreinforced alloy, which confirm that theTC4 particles stimulated the DRX of the matrix.With the increase of the average equivalent strains, DRX areas expanded rapidly in the composite with few un-DRX regions away from TC4 particles, as shown in Fig.11(b) and 11(c).The particle segregation at the grain boundaries significantl aggravated the stress concentrations near them, so the stored energy and the driving forces for recrystallization were evidently enhanced near grain boundaries in the composite.Therefore,DRX preferred to nucleate near the initial grain boundaries.The fin DRX grains progressively swallowed the coarse initial grains during extrusion, which leads to the expansion of DRX area from the grain boundaries to the interior of the grains.When at Position Ⅳ, the matrix of the composite was totally comprised of the fin equiaxed recrystallized grains,as shown in Fig.11(d).

Fig.9.Optical microstructures of the AZ91 alloy residue in the extrusion die.(a) PositionⅠ; (b) PositionⅡ; (c) Position Ⅲ; (d) Position Ⅳ.

3.4.Mechanical properties of the composites

Fig.12 shows the mechanical properties of the TC4p/AZ91 composites before and after extrusion,including YS,UTS and elongations to fracture.Hot extrusion drastically improved the YS,UTS and elongation of the composite.Especially,YS was enhanced more than twice after extrusion, which is mainly attributed to the following factors.Firstly, the DRX led to the grain refinemen during extrusion, which simultaneously improves the strengths and ductility of the composite.Secondly,fin Mg17Al12phase dispersedly precipitated in the matrix during extrusion, as shown in Fig.13.The precipitates improved its yield strength through the Orowan mechanism.In addition, particle distribution was improved after extrusion.The particle segregation at grain boundaries was eliminated by extrusion,which improves the reinforced efficien y of TC4 particles and reduces the stress concentrations in the composites.As a result, both the strength and ductility of the composite can be enhanced.Finally, as shown in Fig.13(b), the high density dislocations were generated around TC4 particles during extrusion, which also improves the strength of the composite.

Fig.14 shows the tensile fracture morphology of the TC4p/AZ91 composite before and after extrusion.For the as-cast composite, some big and shallow dimples were often observed in the fracture surfaces, as shown in Fig.14(a)and (b).TC4 particles were observed inside the dimples,which indicates that the fracture was caused by the interfacial debonding between TC4 particle and the matrix.For the as-extruded composite, the interfacial debonding was seldom observed although the fracture is characterized by the small dimples, as shown in Fig.14(c) and 14(d).This proves that extrusion improved the interfacial bonds between TC4 particles and the matrix and then enhanced the reinforced efficien y of the TC4 particles.It is worth mentioning that both the UTS and the elongation (namely 369MPa and 6.4%, respectively) of the extruded TC4p/AZ91 composite are better than those (less than 350MPa and 4%, respectively) of SiCp/AZ91 composite fabricated by the same process [23], even though the strengths of SiC are much larger than that of TC4.Thus, the TC4p/AZ91 composite realized the superior combination of the UTS and the elongation, which is attributed to the strong interfacial bonding and the deformability of TC4 particles which can coordinate deformation of the matrix to some extent.

Fig.10.SEM microstructures of the composite residue in the extrusion die.(a) PositionⅠ; (b) PositionⅡ; (c) Position Ⅲ; (d) Position Ⅳ.

Fig.11.Optical microstructures of the composite residue in the extrusion die.(a) PositionⅠ; (b) PositionⅡ; (c) Position Ⅲ; (d) Position Ⅳ.

Fig.12.The tensile properties of the TC4p/AZ91 composites at room temperature.(a) Stress-strain curves; (b) YS, UTS and Elongation.

Fig.13.TEM microstructures of the as-extruded TC4p/AZ91 composite.(a) Dispersed Mg17Al12 precipitates; (b) high-density dislocations around TC4 particle.

Fig.14.Tensile fractographs of the TC4p/AZ91composite before and after extrusion.(a) and (b) before extrusion; (c) and (d) after extrusion.

Conclusions

TC4 particles can facilitate the nucleation of DRX of Mg alloy matrix by the introduction of PDZs around them during hot extrusion.Thus, TC4 particles can promote the refine ment of the matrix grains during hot deformation.Besides,TC4 particles are able to be plastically deformed during extrusion, so the stress concentrations around the particles are released.As a result, the recrystallized grains near the TC4 particles are just slightly smaller than those away from them.For the as-extruded composite, the ductile TC4 particles can not only improve the strength of the Mg matrix composite but also maintain a good elongation.The superior comprehensive mechanical properties of the TC4p/AZ91 composites are mainly attributed to the strong interfacial bonding and the excellent deformability of the TC4 particles.

Acknowledgments

This work was supported by‘“National Key R&D Program of China’” 2017YFB0703100), “National Natural Science Foundation of China” (Grant Nos.51471059 and 51671066),Key Laboratory of Superlight Materials & Surface Technology (Harbin Engineering University), Ministry of Education and Key Laboratory of Lightweight and High Strength Structural Materials of Jiangxi Province.