Effects of homogenization temperature on microstructure and mechanical properties of high-speed-extruded Mg–5Bi–3Al alloy

Je Won Ch,Sng-Cheol Jin,Je-Gil Jung,Sung Hyuk Prk,?

a School of Materials Science and Engineering,Kyungpook National University,80 Daehakro,Daegu 41566,Republic of Korea

b Advanced Metals Division,Korea Institute of Materials Science,Changwon 51508,Republic of Korea

Abstract This study investigates the effects of billet homogenization temperature on the dynamic recrystallization behavior during high-speed extrusion and resultant microstructure and tensile properties of the Mg–5Bi–3Al(BA53,wt%)alloy.Two billets homogenized at 350 and 450 °C(350H and 450H billets)are extruded at a high speed of 69 m/min.The 350H billet has a relatively smaller grain size and a higher abundance of fine Mg3Bi2 particles compared to the 450H billet.During extrusion of the 350H billet,enhanced dynamic recrystallization occurs as a result of its finer grains and abundance of particles,while the growth of recrystallized grains is suppressed by the grain-boundary pinning effect of particles.Ultimately,the extruded 350H material is characterized by smaller grains,relatively greater number of Mg3Bi2 particles,and a higher internal strain energy than the extruded 450H material.The tensile strength of the extruded 350H material is higher than that of the extruded 450H material owing to stronger grain-boundary hardening,particle hardening,and strain hardening effects.The extruded 350H material also exhibits a higher tensile elongation as its smaller grains inhibit the formation of crack-inducing undesirable twins during tension.The results from this study demonstrate that a decrease in the homogenization temperature from 450 to 350 °C leads to improved strength and ductility in the high-speed-extruded BA53 material.

Keywords:Mg–Bi–Al alloy;High-speed extrusion;Homogenization;Dynamic recrystallization;Tensile properties.?Corresponding author.

1.Introduction

Extrusion is an effective metal-forming process used to fabricate wrought magnesium(Mg)products.Unlike the rolling process,extrusion is able to yield wrought products with a variety of shapes,such as rods,tubes,beams,and channels.Moreover,the occurrence of dynamic recrystallization(DRX)during hot extrusion generates a uniform and fine grain structure;therefore,extruded Mg alloys exhibit much higher strength and elongation than their cast counterparts.Accordingly,extruded Mg alloys are particularly useful in the fabrication of body and chassis components,such as engine cradles,bumper beams,radiator supports,and subframes[1–3].Extrusion speed is very important in the mass production of extruded automobile profiles because it is directly related to productivity and cost of final products.Schumann and Friedrich[4]reported that the production cost of extruded Mg automotive profiles may be exceed three times that of extruded Al profiles;this considerably cost disparity has been largely attributed to the lower extrudability of Mg alloys compared to their Al counterparts.The maximum extrusion speed is defined as the speed at which a material can be extruded without any surface cracking.The AA6063 Al alloy has a high extrudability with the maximum extrusion speed of～60 m/min.Moreover,easily extrudable Al alloys such as 1350 Al alloy can be economically extruded at extremely high speeds up to 100 m/min.However,the maximum extrusion speed of commercial high-strength Mg alloys such as AZ80 and ZK60(～3–6 m/min)is approximately ten times lowerthan common Al alloys(＞40 m/min)[5,6].Thus,the increase in the extrusion speed of Mg alloys via the development of new Mg materials with excellent extrudability may significantly reduce the final cost of extruded products.Mg materials extruded at high speeds with moderate or high mechanical strength may have a considerably wider application range in the transportation industry,including automobiles,trains,and aircraft.The maximum equilibrium solubility limit of bismuth(Bi)in Mg is as high as 9.0%,and the Mg3Bi2phase generated through the addition of Bi has a considerably high melting point of 823 °C.Go et al.[7]reported that the addition of 6 wt% Bi to pure Mg significantly improves the tensile yield strength(TYS)of extruded material from 88 to 129 MPa;this is primarily because of the formation of fine Mg3Bi2particles.Accordingly,Mg–Bi-based alloys are potentially able to achieve high extrudability and strength,concurrently.Given this potential,a novel Mg–5.0Bi–3.0Al(wt%)alloy was recently developed(termed BA53).This alloy possessed extraordinary extrudability and it was successfully extruded at a die-exit speed of 67 m/min without cracking,despite its high alloy content(8.0 wt%)[8].Therefore,the BA53 alloy with excellent extrudability and high strength has the potential for widespread application in the transportation industry.

Homogenization heat treatment of billets is typically conducted prior to extrusion to reduce solute segregation and dissolve the second phases that are formed during solidification into the matrix.In high-alloyed Mg alloys such as AZ80 and ZK60,fine dynamic precipitates are generally formed from the supersaturated matrix during hot extrusion;these precipitates improve the strength of the extruded material via the precipitation hardening effect[9,10].For instance,when a homogenized AZ80 billet is extruded at a low temperature of 200 °C and a low exit speed of 0.84 m/min,the formation of abundant nanoscale Mg17Al12precipitates occurs during extrusion[11].This in turn induces the formation of ultrafine recrystallized grains through the grain-boundary pinning effect.This produces an extruded material with a significantly high strength(TYS)of 403 MPa and an ultimate tensile strength(UTS)of 437 MPa.However,extrusion at a higher speed generates a larger amount of deformation heat through plastic deformation and friction;accordingly,the temperature in the deformation zone increases with increasing extrusion speed[12,13].Kim et al.[14]reported that even when the extrusion of an AZ91 alloy begins at 350°C,the temperature in the deformation zone increases to 388 °C,397 °C,and 411 °C at exit speeds of 3.0 m/min,4.5 m/min,and 6.0 m/min,respectively.Therefore,when extrusion is conducted at an extremely high speed(＞60 m/min),it is likely that the temperature in the deformation zone will become considerably high owing to the large amount of deformation heat generated.Moreover,a tenfold increase in the extrusion speed is known to cause a～50% increase in extrusion pressure[15].An increase in the extrusion pressure is highly problematic as it requires larger-capacity extrusion equipment and reduces the service longevity of the die and tools,which eventually leads to an increase in extrusion cost.This means that extrusion at high speeds should be conducted at temperatures higher than those adopted under typical(i.e.,low-and moderate-speed)extrusion conditions to reduce extrusion load.Although highalloyed Mg alloys have been developed for high-speed extrusion(e.g.,BA53),dynamic precipitation hardly occurs under high-temperature,high-speed extrusion conditions because the solubility limit of alloying elements at high temperatures is high and the time available for precipitation is insufficient because of the high extrusion speed.The strength of a highalloyed Mg material extruded at a high temperature and high speed has been demonstrated to improve through the formation of static precipitates when additional aging treatment follows extrusion[8].However,the application of such postheat treatment increases both the processing time and energy consumption,ultimately elevating the cost of the final products.Techniques that utilize the advantages of high-alloyed Mg alloys(i.e.,possibility of formation of abundant secondphase particles)under extrusion conditions which restrain dynamic precipitation,without any additional processes(e.g.,post-aging treatment),are required to improve the strength of high-speed-extruded material while maintaining a low processing cost.This study attempts to improve the mechanical properties of Mg material extruded at high temperature and high speed by ensuring the uniform distribution of abundant fine second-phase particles throughout the starting material by varying the homogenization temperature of the billet.This objective was carried out using cast billets of the BA53 alloy which were homogenized at two different temperatures(350 and 450 °C).These billets were then extruded at a relatively high temperature of 400°C and a substantially high exit speed of 69 m/min;subsequently,the DRX behavior of the billets during extrusion and the microstructure and tensile properties of extruded materials were systematically analyzed.

2.Experimental procedure

A recently developed alloy,BA53(Mg–5Bi–3Al(wt%)),was used in this study.Two cast billets of the alloy were prepared in a CO2–SF6gas mixture using the conventional mold casting technique based on a previously reported procedure[16].One of the cast billets was homogenized at 350 °C for 10 h(denoted as the 350H billet)and the other one was homogenized at 450 °C for 10 h(denoted as the 450H billet);homogenization of both billets was performed in an inert gas atmosphere containing an Ar–SF6mixture in an electric furnace.Samples for extrusion were obtained by machining the homogenized billets into a cylindrical shape with a diameter and length of 68 mm and 120 mm,respectively.Hightemperature,high-speed extrusion was conducted according to the following procedure.The machined cylindrical samples and a flat-faced extrusion die were preheated at 400 °C for 30 min in a resistance furnace and then subjected to direct extrusion at 400 °C,an extrusion ratio of 76.5,and a ram speed of 15 mm/s using a 300 ton horizontal extrusion machine.The extruded bars exiting the extrusion die were naturally air-cooled to room temperature(RT,23 °C).The corresponding exit speed of the fabricated extruded bars was 69 m/min,and the diameter of these bars was 8 mm.

Fig.1.Optical micrographs of(a)as-cast,(b)350H,and(c)450H billets.d denotes the average grain size.

Analysis of the microstructural characteristics of the ascast billet,homogenized billets,extruded materials,and waterquenched extrusion butts was carried out using optical microscopy(OM),field-emission scanning electron microscopy(FE-SEM),X-ray diffraction(XRD),and electron backscatter diffraction(EBSD).All specimens for microstructural analyses were mechanically polished with progressively finer grades of emery paper up to #2000 grit and then polished with 1 μm diamond paste.Specimens for EBSD observations were additionally polished with a colloidal silica solution(0.04 μm)for 30 min to remove surface strains and obtain reliable crystallographic data.XRD measurements were conducted using Cu Kαradiation at a scan speed of 2°/min in the 2θrange of 20–80° All EBSD measurements were carried out at an accelerating voltage of 20 kV,with a working distance of 17 mm using an EBSD detector(Symmetry S2,Oxford Instruments)installed in a FE-SEM.Automated EBSD scans were conducted in stage-control mode with a step size of 2.0 μm using the Oxford Instruments AZtec 5.0 software.The resulting EBSD data were analyzed using the Oxford Instruments AZtecCrystal 2.0 software.Only reliable EBSD data with a mean angular deviation less than 1.0 were used to analyze the average grain size,texture,Schmid factor(SF),and the kernel average misorientation(KAM).

For tensile testing,dogbone-shaped specimens with gage dimensions of 5 mm(diameter)×25 mm(length)were machined from the extruded materials;the tensile loading direction corresponded to the extrusion direction(ED).Tensile tests were undertaken using a Shimadzu AGS-100kNX universal testing machine at RT at a strain rate of 1×10-3s-1.Tensile tests for each extruded material were carried out triplicate;the representative tensile curve and average tensile properties for each extruded material were displayed.To analyze the fracture modes of the extruded materials,microstructures on the longitudinal cross-sections of the tensile-fractured specimens were observed by OM,and the fracture surfaces of these specimens were observed by FE-SEM.

3.Results

3.1.Microstructures of as-cast and homogenized billets

Fig.1 presents the optical micrographs of the as-cast billet and homogenized billets,demonstrating that all billets have a relatively equiaxed grain structure.The average grain sizes of the as-cast,350H,and 450H billets are 120,118,and 149 μm,respectively.These values indicate that the grain size remains the same during homogenization at 350 °C while it increases by 24%during homogenization at 450°C due to grain growth by boundary migration.Although the 450H billet has larger grains than the 350H billet,its grain size is much smaller than those of other Mg–Al-and Mg–Sn-based Mg alloys.For instance,although the total alloy contents of the commercial high-strength AZ80 and the high-speed-extrudable Mg–7Sn–1Al–1Zn(TAZ711)alloys are similar to the BA53 alloy,the average grain sizes of the homogenized former alloys are as large as 410[10]and 420 μm[17],respectively.Grain boundaries in a homogenized billet become the primary nucleation sites for DRX during hot deformation;therefore,the fine grain structure of the BA53 billets is advantageous for the activation of DRX during extrusion.

SEM micrographs of the as-cast billet and homogenized billets are shown in Fig.2.In the as-cast billet,coarse rodlike particles(average length and width of 28.8 and 3.5 μm,respectively)are observed along grain boundaries(Fig.2a and b).Moreover,in the SEM image of the as-cast billet,the central area of the grains is dark whereas the area near the grain boundaries is relatively brighter(Fig.2a).This difference in brightness may be attributable to the fine lath-shaped(1–4 μm in length)particles that are less abundant in the grains than near the grain boundaries(Fig.2b).During the solidification of the molten metal,solute atoms shift from the initially formed solid phase to the remaining liquid phase.This enables the formation of greater number of second phases near the grain boundaries in the as-cast material because a larger amount of solute atoms are present in a later solidified region[18].In the 350H billet,coarse particles are still present along the grain boundaries,and their size,morphology,and amount are almost the same as those in the as-cast billet(Fig.2c and d).However,the 350H billet,unlike the as-cast billet,exhibits almost no difference in brightness between the central area and the near-boundary area(Fig.2c).This may be attributed to the formation of fine particles in the central area of the grains during homogenization(Fig.2d).As a result,a greater number of fine particles are more uniformly distributed throughout the 350H billet,in comparison to the as-cast billet.These observations indicate that homogenizationtreatment at 350 °C leads to the formation of fine particles,particularly in the central area of the grains,as opposed to the dissolution of the pre-existing particles(i.e.,those formed during solidification).In contrast,the size and amount of coarse particles along grain boundaries in the 450H billet are smaller than those in the as-cast and 350H billets.In the grains of the 450H billet,fine particles are larger in size while being significantly smaller in number compared to the other billets.Furthermore,most fine particles in the 450H billet are spherical,unlike the lath-shaped fine particles in the other billets.During homogenization at 450 °C,coarse particles along the grain boundaries are partially dissolved into the matrix,a considerable amount of fine particles disappears,while some of the fine particles become coarse via the Ostwald ripening phenomenon[19].

Fig.2.SEM micrographs of(a,b)as-cast,(c,d)350H,and(e,f)450H billets at(a,c,e)low and(b,d,f)high magnifications.

The XRD results demonstrate that both the coarse particles along the grain boundaries and the fine particles in the grains of the as-cast,350H,and 450H billets are of the Mg3Bi2phase(Fig.3a);no other second phases(e.g.,Mg17Al12)are present in any of the billets.The cause of the absence of other phases is shown in the equilibrium phase diagram of Mg–5Bi–xAl(x=0–12 wt%),constructed using FactSage software(Fig.3b).The phase diagram shows that during the solidification of a molten BA53 alloy in the casting process,only theα-Mg and Mg3Bi2solid phases,and not the Mg17Al12phase,are formed in the molten metal.The homogenization temperatures of 350 and 450 °C exist in anα-Mg+Mg3Bi2two-phase region,and the billets are water-quenched immediately after homogenization.Therefore,only Mg3Bi2-phase particles are present both in the as-cast billet and in the homogenized billets.As the extrusion temperature of 400 °C also lies in theα-Mg+Mg3Bi2two-phase region,Mg17Al12precipitates are not expected to be formed during extrusion.Based on the equilibrium phase diagram,the Mg17Al12phase can precipitate at temperatures below 208 °C in the BA53 alloy(see the blue point in Fig.3b).However,after the material exits the extrusion die,it experiences a rapid temperature decrease during natural aircooling because of the small diameter of the extruded bar(8 mm).Accordingly,Mg17Al12precipitates are not formed during this cooling because of insufficient time to form the equilibrium phase.A previous study also reported the absence of Mg17Al12precipitates in high-speed-extruded BA53 material[8].In this study,Mg17Al12phase peaks are not detected in the XRD spectra of extruded materials(Fig.3a).

Fig.3.(a)XRD spectra of as-cast billet,homogenized billets,and extruded materials.(b)Equilibrium phase diagram of Mg–5Bi–xAl(x=0–12 wt%),calculated using FactSage software.Text.denotes the extrusion temperature used in this study.

Fig.4.Optical micrographs of extruded(a)350H and(b)450H materials.d denotes the average grain size with standard deviation(in parenthesis)and ED denotes the extrusion direction.

3.2.Microstructural characteristics of extruded materials

Optical micrographs of the extruded materials are shown in Fig.4,which illustrates that both extruded 350H and 450H materials have a completely dynamically recrystallized(DRXed)grain structure without any unDRXed grains.A previous study demonstrated that Bi addition to pure Mg suppresses DRX during hot extrusion by the Zener pinning effect of fine Mg3Bi2particles[7].When a Mg–6Bi(wt%)alloy is extruded at 350 °C,an extrusion ratio of 20.3,and a ram speed of 1 mm/s,the extruded Mg–6Bi material has a partially DRXed microstructure where the area fraction of the DRXed grains(i.e.,DRX fraction)is 76.4%[7].However,in this study,the extruded BA53 materials exhibit a completely DRXed microstructure as the adopted extrusion conditions(temperature:400 °C;extrusion ratio:76.5;ram speed:15 mm/s)are favorable for DRX during hot extrusion.Generally,the average size of DRXed grains in extruded Mg materials increases with increasing temperature or extrusion speed due to the acceleration of grain growth behavior under these conditions[20,21].In this study,although extrusionexperiments are conducted at a high temperature of 400 °C and an extremely high exit speed of 69 m/min,the extruded BA53 materials have a relatively finer grain structure(average grain size:20.9–26.4 μm)than the AZ91 and TAZ711 materials extruded at the same temperature(400 °C)and a considerably lower speed(1.5 and 12 m/min,respectively)(the average grain sizes for the extruded AZ91 and TAZ711 materials are 41[22]and 75 μm[23],respectively).The average grain size of the extruded 350H material(20.9 μm)is smaller than that of the extruded 450H material(26.4 μm),which may be attributed to the suppression of grain coarsening by the abundance of particles in the former as a result of the grain-boundary pinning effect.The coarse particles along the grain boundaries in the 350H billet are rearranged along the metal flow direction during extrusion;consequently,these particles are distributed parallel to the ED in the extruded material(Fig.5a).Additionally,as is the case with the billet prior to extrusion,the extruded 350H material also contains a large amount of fine particles(Fig.5b).Although the extrusion temperature(400 °C)is higher than the homogenization temperature of 350 °C,very few or none of the fine particles in the grains of the 350H billet dissolve during extrusion because the time available for the decomposition of the particles is insufficient owing to the extremely high extrusion speed(the total time for which the billet is extruded is just 5 s).In contrast,a limited number of coarse particles are present in the extruded 450H material,and the amount of fine particles in this material is significantly smaller than that in the extruded 350H material(Fig.5c and d).Although a considerable amount of Bi is supersaturated in the matrix of the 450H billet,the dynamic precipitation of the Mg3Bi2phase during extrusion is restrained owing to the insufficient time available for solute atoms to diffuse and subsequently form the equilibrium phase.Both extruded materials contain larger particles along the grain boundaries compared to those in the grains,where their number density is lower along the grain boundaries(Fig.5b and d).The presence of coarser particles along the grain boundaries may be attributed to particle coarsening caused by pipe diffusion along the grain boundaries during natural air cooling after the materials exit the extrusion die.

Fig.5.SEM micrographs of extruded(a,b)350H and(c,d)450H materials at(a,c)low and(b,d)high magnifications.

3.3.Mechanical properties of extruded materials

The engineering tensile stress–strain curves and corresponding tensile properties of the extruded materials are shown in Fig.6.The tensile strength of the extruded 350H material is higher than that of the extruded 450H material;the TYS and UTS are 210 and 268 MPa,respectively,for the former and 198 and 256 MPa,respectively,for the latter.Thismeans that when the homogenization temperature decreases from 450 to 350 °C,the TYS and UTS of the high-speedextruded BA53 material both increase by 12 MPa.This improvement in strength may be an outcome of strengthening of the grain-boundary hardening and particle hardening effects because the extruded 350H material has finer grains and a substantially larger amount of particles than the extruded 450H material.The presence of more Mg3Bi2particles in the extruded 350H material suggests that there is lower quantity of Bi solute atoms dissolved in theα-Mg matrix due to the consumption of the Bi content to form Mg3Bi2particles.The solid-solution hardening effect is observed to generally increase with increasing size difference between the solute and solvent atoms[15].As the atomic radius of Bi(143 pm)is nearly the same as that of Mg(145 pm),it is likely that the solid-solution hardening effect induced by Bi solute atoms in Mg is insignificant[7];therefore,the extruded 350H and 450H materials may have a similar solid-solution hardening effect.Texture hardening and strain hardening also known to be important metallurgical phenomena that influence the tensile strength of extruded Mg alloys.Fig.7 shows the(0001)pole figures,ED inverse pole figures,and maps and distributions of the SF for basal slip under deformation along the ED for the two extruded materials.Both extruded materials have a basal fiber texture with the basal planes of most grains aligned almost parallel to the ED;this texture is typical of hot-extruded Mg alloys devoid of calcium(Ca)or rare-earth elements.The extruded 450H material has a slightly stronger texture than the extruded 350H material;the maximum texture intensities in the pole figure and inverse pole figure are 7.3 and 5.0,respectively,in the former,and 6.2 and 4.3,respectively,in the latter.Although the distribution of SF for the basal slip of the grains slightly differs between the extruded 350H and 450H materials,both materials have the same average SF value of 0.12.These observations suggest that the texture hardening effect during tension along the ED is identical for both materials.Fig.8 presents the KAM maps of the extruded materials;the KAM is an indicator of the internal strain energy accumulated in a material[24,25].The KAM map of the extruded 450H material is overall blue,which corresponds to areas with low KAM values.In contrast,the KAM map of the extruded 350H material contains blue-and green-colored areas,the latter of which represents regions with higher KAM values than the former.A few red lines,which denote areas with the highest KAM value,are observed in both extruded materials;they are identified as low-angle grain boundaries from the grain-boundary maps.The extruded 350H and 450H materials are composed only of equiaxed DRXed grains without any unDRXed grains,whose internal strain energy is much higher than that of DRXed grains;however,the average KAM value of the former material(0.35)is 40% higher than that of the latter material(0.25).Therefore,in addition to the grain-boundary hardening and particle hardening effects,the enhanced strain hardening effect may also contribute to the higher tensile strength of the extruded 350H material.A study reported that the TYSs of AZ80 and TAZ711 alloys extruded at 400 °C and an exit speed of 1.5 m/min are 171 and 207 MPa,respectively[23].Although the extrusion speed of the BA53 alloy adopted in this study(69 m/min)is 46 times that of the AZ80 and TAZ711 alloys(1.5 m/min),the TYS of the high-speed-extruded BA53 material fabricated using the billet homogenized at 350 °C(210 MPa)is higher than those of the low-speed-extruded AZ80 and TAZ711 materials(171 and 207 MPa,respectively).

Fig.6.Engineering tensile stress–strain curves and tensile properties of extruded 350H and 450H materials.TYS,UTS,and EL denote the tensile yield strength,ultimate tensile strength,and tensile elongation,respectively.

The tensile elongation of the extruded 350H material(9.2%)is also higher than that of the extruded 450H material(7.9%)(Fig.6).When extruded Mg alloys with a basal fiber texture are subjected to tension along the ED,{10–11}contraction twins and{10–11}-{10–12}double twins are generally formed during the late stage of deformation as the applied loading conditions are unfavorable for both basal slip and{10–12}extension twinning[26,27].After the formation of contraction and double twins in grains during tension,deformation becomes highly localized in the twinned region;this eventually leads to the formation of microcracks along the twins[27,28].Similar to the critical resolved shear stress for dislocation slip,the critical stress required to activate twinning(i.e.,twinning stress)is also inversely dependent on grain size[29–31].Therefore,when tensile fracture is governed by this twinning,the elongation of an extruded Mg alloy is inversely proportional to its grain size because of the increase in twinning stress[32].Many narrow twins formed in the grains and sharp fracture lines generated along the twins are observed in the optical micrographs of the longitudinal crosssections of the tensile-fractured specimens in the extruded 350H and 450H materials(Fig.9a and b).A cleavage fracture surface comprised of layered cleavage planes is observed in the SEM fractographs of the tensile-fractured specimens of both extruded materials(Fig.9c and d),which is typical of a twin-induced fracture surface.Accordingly,in both extruded materials,tensile fracture occurs by deformation twinning and subsequent cracking along the twins.However,the finer grains of the extruded 350H material suppress undesirable contrac-tion and double twinning during tension,producing a higher tensile elongation of this material.This is supported by the lower number of formed twins and smaller size of the cleavage planes in the extruded 350H material compared to the extruded 450H material(Fig.9).

Fig.7.EBSD measurement results of extruded(a)350H and(b)450H materials:(0001)pole figure,ED inverse pole figure,and map and distribution chart of Schmid factor(SF)for basal slip under deformation along ED.

3.4.Dynamic recrystallization behavior during hot extrusion

After a material placed in a container exits the extrusion die,it is naturally air-cooled to ambient temperature.Therefore,the microstructure of the final extruded material is a result of the DRX and dynamic growth of grains during extrusion and the subsequent static growth of grains during air cooling.The microstructure of the extrusion butt,which was obtained by water-quenching of the remaining part of the billet immediately after extrusion,was observed by EBSD to compare the DRX and dynamic grain growth during extrusion and the static grain growth during air cooling between the 350H and 450H materials.EBSD observations were conducted at locations 9 and 1 mm away from the die exit along the centerline on a longitudinal cross-section of each extrusion butt;these locations correspond to positions A and B,respectively,marked in Fig.10a.The inverse pole figure maps of the remaining parts of the 350H and 450H billets at position A are shown in Fig.10b,and those at position B are presented in Fig.10c.At position A,both 350H and 450H materials have a partially DRXed grain structure consisting of fine,equiaxed DRXed grains and coarse,elongated unDRXed grains,because the strain imposed at this position is insufficient to cause complete DRX(Fig.10b).However,the DRX fraction at this position in the 350H material(74.4%)is higher than that in the 450H material(66.1%);this means that the DRX behavior during extrusion is more pronounced in the former material.The dominant DRX mechanism during extrusion at temperatures above 300 °C is known to be discontinuous DRX(DDRX),involving grain-boundary bulging and subgrain formation via the dislocation climb[33,34].The DDRX mechanism involves the initial formation of new DRXed grains along the original grain boundaries of a material owing to increased grain-boundary mobility at higher temperatures[35,36].In this study,the 350H billet has a greater number of DRX nucleation sites than the 450H billet because of its smaller grain size.Therefore,the predominant DDRX behavior during extrusion at 400 °C is stronger in the 350H billet,which results in a higher DRX fraction of this billet.In addition,particles larger than 1 μm in size may act as effective DRX nucleation sites during hot deformation because of the high strain energy accumulated around them[37,38].This is known as the particle-stimulated nucleation(PSN)phenomenon,which has been widely observed in various Mg alloys containing＞1 μm second-phase particles during hot extrusion[39–45].Because the amount of particlesthat are large enough to cause PSN is larger in the 350H billet than in the 450H billet(Fig.2),the enhancement of DRX via the PSN induced by abundant particles may also contribute to the higher DRX fraction of the former billet during extrusion.

Fig.8.Kernel average misorientation(KAM)maps of extruded(a)350H and(b)450H materials.KAMavg denotes the average KAM value.

Fig.9.(a,b)Optical micrographs and(c,d)SEM fractographs of tensile-fractured specimens of extruded(a,c)350H and(b,d)450H materials.

Fig.10.(a)Schematic illustration showing EBSD measurement positions(A and B)on longitudinal cross-section of extrusion butt.Inverse pole figure maps at positions(b)A and(c)B on extrusion butts of 350H and 450H materials.fDRX and dDRX denote the area fraction and average size with standard deviation(in parentheses),respectively,of the DRXed grains.

4.Discussion

4.1.Variation in grain coarsening behavior by different homogenization temperatures

Although the DRX fraction at position A of the extrusion butt is higher in the 350H material than in the 450H material,the average sizes of DRXed grains are similar in the two materials(6.4 and 6.7 μm for the 350H and 450H materials,respectively;Fig.10b).This insignificant difference in the DRXed grain size is consistent with the results from a previous study which found that the initial grain size of a billet has a negligible effect on the DRXed grain size of the extruded material[46,47].In both materials,complete DRX occurs at position B through the additional DRX caused by further deformation;this position is close to the last part of the material subjected to the extrusion load prior to exiting the extrusion die.Unlike position A,in position B the average sizes of the DRXed grains in the two materials are significantly different(11.8 and 21.4 μm for the 350H and 450H materials,respectively;see Fig.10c).This means as the extrusion proceeds from position A to position B,the coarsening of DRXed grains occurs in the two materials;however,the degree of grain coarsening is considerably smaller in the 350H material(from 6.4 to 11.8 μm)compared to the 450H material(from 6.7 to 21.4 μm).This less pronounced grain coarsening in the former is due to the abundant particles in the billet effectively inhibiting the dynamic growth of the DRXed grains under strain imposition on the material during extrusion.

Fig.11.(a)Variation in average size of DRXed grains during extrusion and subsequent air cooling.KAM maps at position B on extrusion butts of(b)350H and(b)450H materials.

In both materials,the grain size of the final extruded material is larger than that in the deformation zone immediately before the material exits the extrusion die(i.e.,position B);this implies that the static grain growth occurs during air cooling after the material exits the extrusion die.The increment in grain size during air cooling is 9.1 μm(from 11.8 to 20.9 μm)for the 350H material and 5.0 μm(from 21.4 to 26.4 μm)for the 450H material(Fig.11a).That is,the static grain coarsening during air cooling is more pronounced in the 350H material despite containing many more particles;this is in contrast to the above-stated result that dynamic grain coarsening during extrusion is more pronounced in the 450H material.When deformed materials are subjected to heat treatment,grain growth generally occurs through the movement of grain boundaries from regions with lower strain energy to surrounding regions with higher strain energy in order to reduce the overall strain energy of the material[38,48].This boundary migration caused by differences between the internal strain energies of adjacent regions is known as strain-induced boundary migration(SIBM)[49].Thus,an increase in the total internal strain energy of a material and the large difference between the internal strain energies of adjacent grains promote grain-boundary migration because the driving force for SIBM is higher under these conditions[49–52].In this study,complete DRX during extrusion occurs more rapidly in the 350H material than in the 450H material owing to enhanced DRX behavior in the former.Accordingly,higher strain energy is accumulated in the DRXed grains of the 350H material during further deformation after complete DRX.This difference in the internal strain energy can be confirmed from the KAM maps of the 350H and 450H materials at position B(Fig.11b and c,respectively).The average KAM value of the 350H material(0.97)is significantly higher than that of the 450H material(0.39),indicating that the higher driving force for grain growth via SIBM is generated in the former during extrusion.As a consequence,the growth of DRXed grains during air cooling is more pronounced in the 350H material.This enhanced static grain growth in the 350H material is also confirmed by the different degrees of reduction in the internal strain energies of the two materials after air cooling.Static grain growth via SIBM during air cooling decreases the overall internal strain energy of the material.Hence,after air cooling,the KAM value of the 350H material reduces significantly,by 64%(from 0.97 at position B to 0.35 for the extruded material),owing to vigorous static grain growth.However,the reduction in the KAM value of the 450H material after air cooling is relatively small(i.e.,36%;from 0.39 at position B to 0.25 for the extruded material),because the low internal strain energy at position B induces static grain growth to a smaller extent during air cooling.Although the degree of static grain growth during air cooling is larger in the 350H material,the degree of dynamic grain growth during extrusion is considerably larger in the 450H material(Fig.11a).Consequently,the extruded 350H material has a finer grain structure than the extruded 450H mate-rial because the abundance of particles in the former acts to effectively suppress the coarsening of DRXed grains during extrusion.

Fig.12.Relationship between TYS and extrusion speed for various extruded Mg alloys,including high-speed-extruded BA53 materials investigated in this study.

4.2.Outstanding extrudability and high strength of BA53 alloy

It is notable that the BA53 alloy is successfully extruded at a high speed of 69 m/min and a high temperature of 400 °C even though the extrusion billets contain a considerable amount of second phase particles.The extrudability of a material is related to the solidus temperature of the material(i.e.,thermal stability of matrix)and the melting temperature of second-phase particles formed in the material(i.e.,thermal stability of second phases),which are essential characteristics for high-speed extrusion.When the local temperatures in the die land area during extrusion exceed the solidus temperature or the melting temperature of second phases,local melting and subsequent cracking occur on the extrudate surface[1].An increase in the extrusion speed causes an increase in both the velocity at the material/tool interface and the stress normal to the interface,thereby increasing the heat generation caused by plastic deformation and friction[15].Accordingly,in Mg alloys with a low solidus temperature or thermally unstable second phases,hot cracking can easily occur under high-speed extrusion conditions.Commercial high-Al-containing Mg–Al–Zn alloys,such as AZ80 and AZ91,have low solidus temperatures(＜470 °C)because both Al and Zn greatly lower the solidus temperature of Mg[53].In addition,the melting temperature of Mg17Al12phase formed in these alloys is low,437 °C.Therefore,commercial AZ80 and AZ91 alloys exhibit low extrudability owing to their vulnerability to local melting and subsequent hot cracking.Park et al.[54]confirmed that the incipient melting temperature of an AZ80 alloy is low as 427 °C through differential scanning calorimeter(DSC)measurements.Consequently,when extrusion is conducted at a relatively low speed of 6 m/min at 350 °C,severe hot cracking occurs in both AZ80[54]and AZ91[14]alloys.In contrast,the BA53 alloy has a high solidus temperature of 530 °C and the Mg3Bi2phase with a high melting temperature of 823 °C is formed in this alloy.This means that the BA53 alloy has a high thermal stability in terms of bothα-Mg matrix and second phase;this thermal stability is experimentally confirmed by its high incipient melting temperature of 533 °C,obtained from additional DSC tests.In this study,Mg17Al12phase peaks are not detected in the XRD diffraction patterns of all as-cast,homogenized,and extruded materials(Fig.3a),which indicates that no thermally unstable Mg17Al12particles are formed during the entire casting,heat treatment,and extrusion processes.Therefore,the BA53 alloy can be extruded without hot cracking at an extremely high speed of 69 m/min,irrespective of homogenization heat-treatment temperatures,because of the high solidus temperature and high melting temperature of Mg3Bi2phase.

Fig.12 shows the TYS of various extruded Mg alloys as a function of extrusion speed[54–60].The TYS of extruded Mg alloys decreases with increasing extrusion speed because an increased extrusion speed leads to the promotion of grain growth during extrusion.This trade-off relationship between strength and extrusion speed makes it difficult to achieve highstrength and high productivity concurrently,as required in the industrial fields.Ultimately,this hinders the widespread application of extruded Mg products.However,as shown in Fig.12,the high-speed-extruded BA53 materials investigated in this study overcome the typical trade-offs between strength and extrusion speed owing to their high thermal stability and high alloying content.Therefore,it is expected that the BA53 alloy will be widely applicable to extruded Mg products.Moreover,the homogenization treatment at a lower temperature is an effective means to improve the mechanical properties of high-speed-extruded BA53 materials.

5.Conclusions

This study investigates the effects of homogenization temperature of a billet on the microstructural characteristics and mechanical properties of the BA53 alloy extruded at a high temperature of 400 °C and an extremely high speed of 69 m/min.The following findings are obtained.

1.Homogenization treatment at 350°C leads to the formation of additional fine Mg3Bi2particles in the central area of the grains,while at 450 °C this leads to partial dissolution and coarsening of existing Mg3Bi2particles.Consequently,the 350H billet contains many more Mg3Bi2particles along grain boundaries and in grains than the 450H billet.

2.During extrusion,DRX occurs more vigorously in the 350H material because it contains a greater number of DRX nucleation sites.Dynamic grain growth during extrusion is significantly suppressed by the abundance of particles in the 350H material via the grain-boundary pinning effect.However,static grain growth during natural air cooling is more pronounced in the 350H material.

3.Both extruded 350H and 450H materials have a fully DRXed grain structure and a typical basal fiber texture,but the former has a smaller grain size and a substantially larger amount of particles than the latter.The extruded 350H material exhibits a higher tensile strength than the extruded 450H material,which is a combined outcome of stronger grain-boundary hardening,particle hardening,and strain hardening effects in the former.

4.In both extruded materials,tensile fracture is caused by twinning and subsequent cracking along the formed twins;however,the elongation of the extruded 350H material is higher than that of the extruded 450H material because the finer grains of the former inhibit the formation of undesirable deformation twins during tension.

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgments

This research was supported by a National Research Foundation of Korea(NRF)grant funded by the Ministry of Science,ICT and Future Planning(MSIP,South Korea)(No.2019R1A2C1085272)and by the Materials and Components Technology Development Program of the Ministry of Trade,Industry and Energy(MOTIE,South Korea)(No.20011091).