Effects of Sn addition on the microstructure and mechanical properties of extruded Mg-Bi binary alloy

Sng-Cheol Jin ,Jong Un Lee ,Jongin Go ,Hui Yu ,Sung Hyuk Prk,*

a School of Materials Science and Engineering,Kyungpook National University,Daegu 41566,South Korea

b Department of Materials Science and Engineering,Kyoto University,Yoshida-honmachi,Sakyo-ku,Kyoto,606-8501,Japan

cSchool of Materials Science and Engineering,Hebei University of Technology,Tianjin 300130,China

Abstract We investigated the effects of Sn addition on the microstructural characteristics and mechanical properties of an extruded Mg-Bi binary alloy by comparing Mg-5Bi (B5) and Mg-5Bi-4Sn (BT54).Both the extruded alloys exhibit a partially recrystallized grain structure with a strong extrusion fibe texture and numerous Mg3Bi2 precipitates.However,the addition of Sn significantl decreases the average grain size of the extruded alloy from 123.9 to 75.2 μm.The Sn solute atoms inhibit the activity of dislocation slip,which reduces the internal strain energy accumulated in the dynamically recrystallized (DRXed) grains during extrusion.Consequently,this reduced strain energy leads to the decrease in the DRXed grain size owing to weakened grain growth during natural air-cooling.The extruded BT54 alloy exhibits higher tensile strength and ductility than the extruded B5 alloy.The improvement in the strength by the Sn addition is attributed to the combined effects of grain refinement Sn solute atoms,and increased dislocation density.The formation of {10-11} and {10-11}-{10-12} twins during tension is suppressed by the grain refinement thereby improving the tensile elongation considerably.

Keywords: Mg-Bi alloy;Extrusion;Dynamic recrystallization;Microstructure;Mechanical properties.

1.Introduction

The application of lightweight metal materials for automotive components has received considerable attention in terms of the rapidly growing demand for higher fuel efficien y and reduction of carbon dioxide emissions.The use of extruded Al products in automobile components has become increasingly common;yet,recent efforts to further reduce automobile weight have focused more on extruded Mg-based products because its density (1.74 g/cm3) is considerably lower than that of Al (2.7 g/cm3).However,the extrudability and mechanical properties of commercial Mg alloys are generally lower than those of commercial Al alloys [1-3].The low extrudability of commercial Mg-Al-and Mg-Zn-based alloys is mainly due to the formation of Mg17Al12or Mg-Zn phases with low melting temperatures (＜440 °C) and localized melting during hot extrusion[3,4].Dilute Mg-Al or Mg-Zn alloys with small amounts (＜2 wt.%) of alloying elements added to pure Mg have been proposed to improve extrudability by suppressing the formation of Mg17Al12or Mg-Zn phases [5-7].However,the strength of the extruded dilute Mg alloys is quite low because of the lack of solid-solution and precipitation hardening effects caused by the alloying elements.Mg alloys with high Gd and Y concentrations have been developed to improve the strength of the extruded material [8-12].Although these Mg-Gd-Y-based alloys offer a very high tensile yield strength (TYS) (454-575 MPa) [13-15],the necessary addition of more than 10 wt.% Gd and Y inevitably leads to a large increase in the material price,which sharply reduces the price competitiveness of Mg alloys for replacing Al alloys.The development of new Mg alloys that consist of inexpensive alloying elements and have high extrudability and strength is therefore in high demand.

The maximum solubility of Bi (an inexpensive alloying element) in Mg reaches 9.0% at 551 °C,and the Mg3Bi2phase that forms in the Mg-Bi alloys has a very high melting temperature (823 °C).Mg-Bi-based alloys therefore show great potential to achieve both high-speed extrudability and high mechanical properties through the formation of thermally stable Mg3Bi2particles.For this reason,Mg-Bi-based alloys with Bi as the main alloying element have been recently developed.For instance,Go et al.[16] reported that when 6 wt.% Bi is added to pure Mg,the TYS of the extruded material significantl improves from 88 to 129 MPa,which is mainly attributed to the formation of numerous fin Mg3Bi2precipitates.Meng et al.[17,18] developed extruded Mg-Bi-Ca alloys with exceptionally high ductility owing to the refine microstructure and tilted basal texture.In this alloy system,the Mg-1.3Bi-0.9Ca (wt.%)alloy exhibits a good balance of ductility and strength with a 40% elongation and TYS of 158 MPa.Our previous studies have demonstrated that the addition of Al to a Mg-Bi binary alloy significantl improves the strength and ductility of the extruded material[19,20].For example,the TYS of extruded Mg-5Bi alloy increases from 147 to 223 MPa upon the addition of 9 wt.% Al and the elongation significantl increases from 2.8%to 11.8%[19].A novel Mg-5Bi-3Al alloy with both high extrudability and strength was recently developed;this alloy can be extruded at an exit speed of 67 m/min without hot cracking and the high-speed-extruded alloy and subsequent peak-aged alloy have high TYS values of 188 and 214 MPa,respectively[20].These results demonstrate that the addition of Ca or Al to Mg-Bi binary alloys substantially affects their microstructural characteristics (e.g.,grain structure,texture,precipitates of the extruded materials),thereby improving their mechanical properties.The maximum solubility of Sn in Mg is considerably high (14.85 wt.%) and a thermally stable Mg2Sn phase with a high melting temperature (773 °C) forms in Mg-Sn alloys [21,22].Therefore,in addition to the previously reported Ca and Al,Sn can be also considered as an appropriate alloying element for improving the mechanical properties of extruded Mg-Bi alloy without degrading the extrudability.However,the effects of Sn in Mg-Bi binary alloys have not been reported.To address this question,we added 4 wt.% Sn as a third additive alloying element to Mg-5Bi and systematically investigated the variations of dynamic recrystallization (DRX) behavior during extrusion and resulting microstructure and mechanical properties of the extruded material.

2.Experimental procedure

In this study,4 wt.% Sn was added to a Mg-5Bi alloy for the purpose of developing a new high-alloying Mg-Bi-Sn alloy with a total alloying content similar to that of a commercial high-alloying AZ80 alloy.Cast billets of Mg-5Bi (B5) and Mg-5Bi-4Sn (BT54) alloys were fabricated via the conventional mold casting method by melting in an electric resistance furnace at 750 °C under an inert atmosphere containing a mixture of CO2and SF6,and then pouring into a steel mold pre-heated to 210 °C.The chemical compositions of the cast billets were confirme by inductively coupled plasma spectrometry as Mg-5.4Bi and Mg-5.5Bi-4.4Sn(wt.%),which are close to their respective nominal values.The billets were heat-treated at 500 °C for 48 h under an inert gas atmosphere containing a mixture of Ar and SF6.The homogenized billets were then machined to a cylindrical shape (68 mm diameter and 120 mm length) for direct extrusion.The machined billets were extruded at 400 °C using a 300-ton horizontal extrusion machine with an extrusion ratio of 25 at a ram speed of 1 mm/s.The extruded workpiece was naturally air-cooled after exiting a fla extrusion die with a circular hole.The cross-sectional diameter of the extruded rods was 14 mm.

The microstructural characteristics of the extrusion butts,extruded rods,and fractured tensile specimens were analyzed by field-emissio scanning electron microscopy (FESEM),X-ray diffraction (XRD),electron probe microanalysis(EPMA),and electron backscatter diffraction (EBSD) analysis.Detailed sample preparation procedures can be found elsewhere[16,23].The EBSD data were analyzed using Tex-SEM Laboratories orientation imaging microscopy (TSL OIM) 7.0 software,and data with a confidenc index greater than 0.1 were used for further analyses of grain size,DRX fraction,texture,grain orientation spread (GOS),and Schmid factor(SF).For tensile testing,dog-bone-shaped specimens with gage dimensions of ?6 mm × 25 mm (diameter × length)were machined from the extruded rods.Tensile tests were conducted using a Shimadzu AGS-100kNX universal testing machine at room temperature (RT) and a strain rate of 1 × 10-3s-1.The loading axis corresponded to the extrusion direction (ED).

3.Results and discussion

3.1.Microstructural characteristics of the extruded alloys

Fig.1.Inverse pole figur (IPF) maps,(0001) pole figures and IPFs of the extruded (a) B5 and (b) BT54 alloys. davg denotes the average grain size.

Fig.1 shows inverse pole figur (IPF) maps,(0001) pole figures and ED IPFs of the extruded B5 and BT54 alloys.Both alloys exhibit a partially DRXed grain structure consisting of equiaxed DRXed grains and severely elongated un-DRXed grains.The basal poles of most grains are aligned nearly perpendicular to the ED in both alloys,as shown in the pole figure of Fig.1.They have an extrusion fibe texture with two intensifie 〈10-10〉 and 〈11-20〉 texture components,and the distribution of crystallographic orientations of the two alloys is nearly the same.The 〈10-10〉 texture component originates from the unDRXed grains (Fig.2a and d),whereas the 〈11-20〉 texture component mainly originates from the DRXed grains (Fig.2b and e).The crystallographic orientations of the unDRXed and DRXed grains are consistent with those previously reported for extruded Mg alloys[24,25].In Mg alloys,(0001)〈11-20〉 basal slip and {01-10}〈-2110〉 prismatic slip predominantly occur during deformation owing to their relatively low critical resolved shear stress (CRSS) [26].During hot extrusion,the activation of basal slip causes the basal plane to align parallel to the ED,and the activation of prismatic slip causes one prismatic plane to be perpendicular to the ED [27,28].The unDRXed grains that continuously deform during extrusion without recrystallizing therefore exhibit a strong 〈10-10〉 texture owing to lattice rotation associated with the basal and prismatic slip systems.The shape,size,and area fraction of the unDRXed grains are similar in the extruded B5 and BT54 alloys.However,the average size of the DRXed grains of the extruded B5 alloy (113.3 μm) is ?1.8 times larger than that of the extruded BT54 alloy (64.6 μm),and the size distribution range of the DRXed grains of the former (5-250 μm) is also significantly wider than that of the latter (5-150 μm) (Fig.2c and f).These results indicate that large DRXed grains with sizes of＞150 μm do not form in the presence of the Sn addition,which leads to the formation of a relatively fin DRXed grain structure in the extruded BT54 alloy.

SEM micrographs of the extruded alloys (Fig.3) reveal that numerous spherical fin particles (200-600 nm in size)are homogeneously distributed in both alloys with a similar size and number density.The XRD results show that all of the particles are the Mg3Bi2phase even in the BT54 alloy with 4 wt.% Sn (Fig.4a).The B5 and BT54 alloys both lie in the two-phase region consisting ofα-Mg and Mg3Bi2at an extrusion temperature of 400 °C according to the Mg-5BixSn alloy (x=0-10 wt.%) equilibrium phase diagram calculated using FactSage software (Fig.4b).On the basis of this phase diagram,only the Mg3Bi2phase forms during extrusion without precipitation of the Mg2Sn phase in the BT54 alloy.Additional SEM observations of the extrusion butts,which were obtained by water-quenching of the remaining part of the billet immediately after extrusion,confir the presence of fin particles having a similar size and number density with those in the extruded materials.This indicates that the fin particles are dynamic Mg3Bi2precipitates that formed during extrusion,not static precipitates that formed during natural air-cooling after extrusion.Fig.5 shows backscattered electron image and the corresponding EPMA scanning maps of Mg,Bi,and Sn elements in the extruded BT54 alloy.The EPMA result reveals that Sn added in the BT54 alloy is homogeneously distributed in the matrix of the extruded alloy as solute atoms without the formation of Mg2Sn phase.According to the equilibrium phase diagram,the Mg2Sn phase can precipitate at temperatures below 380 °C in the BT54 alloy (see the red point in Fig.4b).However,because the diffusion rate of Sn in Mg is quite low,the precipitation of Mg2Sn occurs considerably slowly during heat treatment.For example,Jung et al.[29] reported that the peak-aging time at 200 °C of a homogenized Mg-7Sn-1Al-1Zn alloy is as high as 128 h owing to the sluggish diffusion of Sn in the Mg matrix.Moreover,during natural air-cooling,the temperature of the extruded material rapidly decreases owing to the small size of the material (14 mm in diameter).Therefore,Mg2Sn precipitates are not formed in the BT54 alloy during air-cooling because of the low diffusion rate of Sn and insufficien time to form the equilibrium Mg2Sn phase.

Fig.2.EBSD results of the extruded (a-c) B5 and (d-f) BT54 alloys:IPF maps,(0001) pole figures and IPFs of the (a,d) unDRXed region and (b,e)DRXed region and (c,f) size distribution and average size of the DRXed grains. funDRX denotes the area fraction of unDRXed grains.

Fig.3.SEM micrographs showing fin precipitates of the extruded (a,b) B5 and (c,d) BT54 alloys:(b,d) enlarged images of regions A and B marked in(a) and (c),respectively.

Fig.4.(a) X-ray diffraction spectra of the extruded B5 and BT54 alloys.(b) Equilibrium phase diagram for Mg-5Bi-xSn (x=0-10 wt.%) calculated using FactSage software. Text. denotes the extrusion temperature conducted in this study.

Fig.5.Backscattered electron image and corresponding EPMA scanning maps of Mg,Bi,and Sn elements in the extruded BT54 alloy.

Fig.6.EBSD results of the extrusion butts of the B5 and BT54 alloys:(a) longitudinal cross-sectioned extrusion butt and EBSD measurement positions A and B in the extrusion butt;(b-e) IPF maps at positions (b,d) A and (c,e) B of the extrusion butts of the (b,c) B5 and (d,e) BT54 alloys. fDRX and dDRX denote the area fraction and average size of the DRXed grains,respectively.

3.2.Suppression of DRXed grain growth with Sn addition

As mentioned in Section 3.1,the DRXed grains of the extruded BT54 alloy are more uniform and fine than those of the extruded B5 alloy.Fine particles in a material can greatly modify the DRXed grain size of hot-deformed material through grain-boundary pinning effects.In particular,dynamic precipitates that form during hot extrusion can inhibit the growth of DRXed grains during and after extrusion,which reduces the fina DRXed grain size of the extruded material[20,22,30].However,in this study,differences in the size and number density of the Mg3Bi2precipitates between the extruded B5 and BT54 alloys are insignifican (Fig.3),which suggests that the fine DRXed grain structure of the extruded BT54 alloy does not result from the enhanced grain-boundary pinning effect of the precipitates.To elucidate the reason for the fine DRXed grain structure of the extruded BT54 alloy,we investigated the DRX behavior during extrusion by analyzing the microstructural characteristics of the extrusion butts associated with the extruded B5 and BT54 alloys.EBSD measurements were conducted at positions 3 and 6 mm from the die exit along the center line on a longitudinal cross-section of the extrusion butts (positions A and B in Fig.6a).IPF maps measured at positions A and B are shown in Fig.6b-e.The driving force for DRX is the internal strain energy accumulated in a material [31].When the internal strain energy exceeds the critical value required for generating DRX,new DRXed grains are formed.In this study,while the material is deformed in the container,high strain energy enough to cause DRX is accumulated in the material because a high strain of 3.21 is imposed to the material during extrusion.Accordingly,in both alloys,the area fraction of the DRXed grains gradually increases as extrusion proceeds.For example,the DRX fraction in the BT54 alloy increases from 52% at position A to 71% at position B and to 92.6% after extrusion.However,the DRXed grain size at position A is similar to that at position B in each alloy,which indicates that the DRXed grain size remains almost unchanged during the extrusion process where deformation is imposed on the billet.The average sizes of the DRXed grains of the extrusion butts (11.2-14.9 μm) are substantially smaller than those of the DRXed grains of the extruded alloys(113.3 and 64.6 μm for the B5 and BT54 alloys,respectively).The uniform size of the DRXed grains in the extrusion butts differs from the large size distribution of the DRXed grains in the extruded alloys.These results show that during the extrusion process,DRX continues to occur without the growth of DRXed grains.After the material exits the extrusion die,it is difficul to form additional DRXed grains because the material is no longer subject to plastic deformation (i.e.,in a deformation-free state).However,the extruded material just exiting the extrusion die has considerable thermal energy owing to the high extrusion temperature (400 °C)and high strain (3.21) imposed to the material.Therefore,the DRXed grains inhomogeneously and rapidly grow during air cooling to RT through accelerated grain-boundary migration by high thermal energy.This grain growth behavior leads to the formation of a non-uniform coarse DRXed grain structure of the fina extruded alloys.

Fig.7.Grain orientation spread (GOS) maps of DRXed grains at position A of the extrusion butts of the (a) B5 and (b) BT54 alloys. GOSavg denotes the average GOS of the DRXed grains with the standard deviation listed in parentheses.

When deformed materials are subjected to heat treatment,grain growth generally occurs through grain boundary movement from regions with lower strain energy to the surrounding region with higher strain energy to reduce the material’s overall strain energy [32,33].This boundary migration caused by the difference between the internal strain energies in adjacent regions is known as strain-induced boundary migration(SIBM) [32,33].Therefore,an increase in the total internal strain energy of a material and a large difference in the internal strain energies of adjacent grains result in enhanced grain boundary migration owing to a higher driving force for SIBM[33-35].Fig.7 shows the GOS maps and average GOS values of the DRXed grains at position A of the extrusion butts.The GOS value is calculated as the average orientation deviation of each point in a grain from the average grain orientation and is known to represent the internal strain energy stored in individual grains [36].The average GOS of the DRXed grains that formed during extrusion is larger in the B5 alloy(1.64) than that in the BT54 alloy (1.46),and the standard deviation of the GOS is also larger in the former (Fig.7).These results indicate that the DRXed grains of the BT54 alloy have a lower driving force for grain growth than those of the B5 alloy.Accordingly,the growth of DRXed grains during natural air-cooling after exiting the extrusion die is less pronounced in the BT54 alloy and consequently leads to a fine DRXed grain structure.Fig.8 shows the GOS maps of the DRXed grains of the extruded alloys.The average GOS of the DRXed grains that are statically grown during air-cooling is smaller in the extruded B5 alloy (0.45) than in the extruded BT54 alloy (0.56).The smaller GOS in the extruded B5 alloy contrasts with its higher GOS value in the extrusion butt.Namely,during extrusion,the DRXed grains of the B5 alloy have a higher internal strain energy than those of the BT54 alloy,but after extrusion,the DRXed grains of the former have a lower internal strain energy.This is because the enhanced grain growth behavior in the B5 alloy rapidly reduces the material’s accumulated strain energy.

Fig.8.GOS maps of DRXed grains in the extruded (a) B5 and (b) BT54 alloys. GOSavg denotes the average GOS of the DRXed grains.

The addition of Sn to the B5 alloy therefore lowers the driving force for the growth of DRXed grains that form during hot extrusion,which consequently results in the formation of more uniform and fine DRXed grains in the fina extruded material.It is notable that the Sn added in the BT54 alloy is present as solute atoms throughout the entire extrusion process,which is supported by the absence of the Mg2Sn phase and the homogeneous distribution of Sn element in the extruded alloy (Fig.5).The extent of dislocation slip in a material during deformation depends on the specifi shear stress required to induce dislocations for movement (i.e.,CRSS)because dislocation slip begins when the shear stress on the slip plane in the slip direction applied by an external load reaches the CRSS [37].The magnitude of the CRSS of a material is determined by the interaction of dislocations with each other and with lattice defects(e.g.,vacancies,impurities,solute atoms).It is known that small amounts of impurities can significantl increase the CRSS of a material.For instance,as the purity of an Ag single crystal decreases from 99.99% to 99.97% and 99.93%,the CRSS considerably increases from 0.48 to 0.73 and 1.3 MPa [37].The addition of alloying elements more strongly enhances the CRSS owing to their higher concentration.Because the atomic radius of Sn (225 pm) is 1.3 times higher than that of Mg (173 pm),a relatively large quantity of Sn solute atoms (i.e.,4 wt.%Sn in the BT54 alloy) may cause considerable lattice distortion throughout the material,which will eventually increase the lattice resistance to dislocation motion,i.e.,the CRSS.Therefore,although the resolved shear stress imposed on the slip systems during extrusion is the same in the B5 and BT54 alloys,the extent of slip is less pronounced in the latter due to the higher CRSS.Consequently,the internal strain energy accumulated in the DRXed grains during extrusion is relatively small in the Sn-containing BT54 alloy compared to the Sn-free B5 alloy,and the growth of DRXed grains after exiting the die is suppressed by the Sn addition.In addition,the DRXed grain growth in the BT54 alloy can be suppressed by the solute drag effect of the Sn solute atoms.Unlike the B5 alloy,in the BT54 alloy,the Sn solute atoms are distributed along and around the boundaries of the DRXed grains.During grain growth,the DRXed grain boundaries have to drag the Sn solute atoms segregated along the grain boundaries to move together,consequently inhibiting the grain boundary movement [38].Therefore,the solute drag effect induced by the Sn solute atoms is likely to partially contribute to the suppression of the growth of DRXed grains in the BT54 alloy.

3.3.Improved strength and ductility of Sn-containing extruded material

Fig.9.Tensile deformation characteristics of the extruded B5 and BT54 alloys:(a) engineering stress-strain curves,(b) work hardening rate curves,and (c)tensile properties.YS,UTS,and EL denote the tensile yield strength,ultimate tensile strength,and elongation,respectively.

Fig.9 shows the tensile stress-strain curves,work hardening rate curves,and corresponding tensile properties of the extruded alloys.The fl w stress and work hardening behaviors are considerably different in the two alloys.In the extruded B5 alloy,premature fracture occurs at a low strain level of 2.2% and the work hardening rate rapidly decreases during tensile deformation (Fig.9a and b).In contrast,the extruded BT54 alloy exhibits typical fl w stress curve and work hardening curve of ductile metals.Consequently,both tensile strength and elongation of the extruded BT54 alloy are higher than those of the extruded B5 alloy (Fig.9c),which implies that the addition of 4 wt.% Sn results in the simultaneous improvement of both material strength and ductility.The strength of extruded Mg alloys is determined by several hardening mechanisms,including texture hardening,particle hardening,grain-boundary hardening,solid-solution hardening,and strain hardening.In this study,the extruded B5 and BT54 alloys have a nearly identical texture distribution and intensity(Fig.1),which leads to nearly the same texture hardening effects during tension.Indeed,the SFs for basal slip under tension along the ED are identical in the two extruded alloys (0.096).The particle hardening effect of the extruded B5 and BT54 alloys also appears to be identical because of the similarity of the size and number density of precipitates between the two alloys (Fig.3).However,the average grain size of the extruded BT54 alloy (75.2 μm) is significantl smaller than that of the extruded B5 alloy (123.9 μm) due to the suppressed growth behavior of the DRXed grains in the former (Fig.1).Hence,grain-boundary hardening (i.e.,the Hall-Peth hardening) effect is more pronounced in the extruded BT54 alloy than in the extruded B5 alloy.In addition,as described in Section 3.2,the added 4 wt.% Sn does not form any secondary phases such as Mg2Sn and is present as solute atoms dissolved in theα-Mg matrix of the extruded material.The abundant Sn solute atoms produce compressive lattice strain owing to their larger atomic size,which leads to solid-solution hardening during deformation by interrupting dislocation movement.The residual strain energy of the DRXed grains is also higher in the extruded BT54 alloy compared to the extruded B5 alloy (Fig.8),which means that the former has a higher dislocation density.Accordingly,a higher strain hardening effect is induced in the extruded BT54 alloy during tension because the fl w stress is proportional to the square root of the dislocation density [39].The improved tensile strength of the Sn-containing extruded material is therefore attributed to the combined effects of enhanced grain-boundary,solid-solution,and strain hardening by grain refinement Sn solute atoms,and more dislocations.

In addition to the strength improvement,the tensile elongation also considerably increases by a factor of 2.63,from 2.2%to 5.8%.In extruded Mg alloys with a bimodal structure comprising fin DRXed grains and coarse unDRXed grains,the tensile elongation at RT is strongly dependent on the size and area fraction of unDRXed grains because {10-11} contraction twins and {10-11}-{10-12} double twins,which act as cracking sources,readily form in the coarse unDRXed grains during tensile deformation along the ED [16,19,40,41].Although the grain structure of extruded Mg alloys contains both DRXed and unDRXed grains,these contraction and double twins form in both grain types when their sizes are comparable [42].Fig.10a and b shows EBSD results on the longitudinal cross-sections of the fractured tensile specimens of the extruded B5 and BT54 alloys,respectively;the EBSD measurements were performed in the area near the fracture surface.The misorientation angles of twin boundaries and the specifi lattice rotations by twinning clearly demonstrate that the twins formed during tension are {10-11} contraction twins or {10-11}-{10-12} double twins in both materials.The formation of these contraction and double twins is consistent with the finding under tension along the ED of previous studies on extruded Mg alloys with an extrusion fibe texture [43,44].

Fig.10.EBSD results showing twins formed in fractured tensile specimens and their crystallographic orientations for the extruded (a) B5 and (b) BT54 alloys.The tensile loading axis is parallel to the horizontal direction in the EBSD maps.M,CT,and DT denote matrix,{10-11} contraction twin,and{10-11}-{10-12} double twin,respectively.

Fig.11 shows SEM fractographs of the fractured tensile specimens of the extruded alloys.Since the stress required for activating deformation twinning decrease with increasing grain size according to the Hall-Petch relation,deformation twins are more easily formed in larger grains than in smaller grains [45,46].The co-existence of small and relatively large DRXed grains in the extruded alloys produces a quasi-cleavage fracture surface consisting of small dimples and relatively large cleavage planes in both alloys (Fig.11a and b).However,the size of the cleavage planes in the extruded B5 alloy (80-180 μm) is much larger than that in the extruded BT54 alloy(60-100 μm)(Fig.11c and d).Similarly,the length of the twin band formed during tension in the extruded B5 alloy is larger than that in the extruded BT54 alloy because the DRXed grain size is larger in the former alloy(Fig.10).The size of the cleavage planes is roughly consistent with the size of the large DRXed grains in each extruded alloy,which indicates that the large DRXed grains act as initiation sites for cleavage fracture.These results demonstrate that the reduced DRXed grain size of the Sn-containing BT54 alloy inhibits cleavage fracture by suppressing the activation of {10-11} contraction and {10-11}-{10-12} double twinning during tensile deformation,which consequently results in a significan improvement in the ductility of the extruded material.

4.Summary

This study investigates the effects of Sn addition on the microstructure and mechanical properties of extruded Mg-5Bi alloy.The extruded B5 and BT54 alloys exhibit a partially DRXed grain structure consisting of severely elongated unDRXed grains and equiaxed DRXed grains with a wide size distribution.The influenc of Sn addition on the DRX fraction,developed texture,and amount of precipitates of the extruded material is negligible;however,the average size of the DRXed grains significantl decreases from 113.3 μm to 64.6 μm.The Sn solute atoms in the BT54 alloy reduce the extent of dislocation slip during extrusion due to the increased CRSS,thereby decreasing the internal strain energy accumulated in the DRXed grains.Accordingly,the growth of the DRXed grains during natural air-cooling after exiting the extrusion die is less pronounced in the Sn-containing BT54 alloy than in the Sn-free B5 alloy,which consequently results in the formation of a more uniform and fine DRXed grain structure in the former.The extruded BT54 alloy therefore has a higher tensile strength,which is attributed to the combined effects of refine DRXed grains,Sn solute atoms,and more dislocations.The tensile ductility is also substantially improved by the Sn addition because the reduced DRXed grain size suppresses the formation of the contraction and double twins that act as cracking sources during tension.These results contribute to the development of new Mg-Bi-Sn-based alloys that simultaneously possess excellent mechanical properties and outstanding extrudability.This high-performance alloy development could be accomplished through further studies on the optimization of alloying element contents,effects of additional alloying elements,control of extrusion process variables,and evaluation of high-speed extrudability.

Fig.11.SEM fractographs under (a,b) low and (c,d) high magnificatio of fractured tensile specimens of the extruded (a,c) B5 and (b,d) BT54 alloys.

Declaration of Competing Interest

The authors declare that they have no conflic of interest.

Acknowledgments

This research was supported by the 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).