Hu Hung,Hongwei Mio,Gungyin Yun,Zhonghng Wng,Wenjing Ding,b

a National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite,Shanghai Jiao Tong University,Shanghai 200240,China

b Shanghai Innovation Institute for Materials,Shanghai 200444,China

c Department of Quantum Materials Science and Technology,International Iberian Nanotechnology Laboratory(INL),Av.Mestre JoséVeiga s/n,Braga,Portugal

Abstract We prepare a new type of patented biodegradable biomedical Mg–Nd–Zn–Zr(JDBM)alloy system and impose double continuously extrusion(DCE)processing.The lowest processing temperature is 250°C for JDBM-2.1Nd and 310°C for JDBM-2.8Nd,which increases with the Nd concentration.The highest yield strength of 541MPa is achieved in JDBM-2.1 Nd samples when extruded at 250°C and the elongation is about 3.7%.Moreover,the alloy with a lower alloying element content can reach a higher yield strength while that with a higher alloying element content can reach a larger elongation after DCE processing.However,when extruded under the same conditions,the alloy with a higher alloying contents exhibits better tensile properties.

Keywords:Magnesium alloys;Microstructure;Mechanical properties;Double continuously extrusion.

1.Introduction

As light-weight structural materials,wrought magnesium alloys hold substantial promise for a wide range of applications especially in automotive and 3C industry[1].However,they suffer from a couple of drawbacks,such as inferior formability at room temperature and low 0.2%proof strength[2].One effective approach to expand the application of Mg alloys is to improve their strength[3].However,most of the strengthening improvements by alloying are usually accompanied with the loss of ductility.Recently,Li and Lu[4]proposed to improve the properties of the metals by playing with defects instead of alloying.Alloying may have a negative impact on the long-term sustainability of high-performing metals and makes materials development more resource-dependent,exposing it to the risks associated with progressive resource exhaustion and unavailability of critical elements.Hence,microstructure adjustment is more important than alloying in strengthening metals.

Thermoplastic deformation such as hot extrusion[5–10],rolling[11–14]and forging[15–18]could refin microstructure of Mg alloys,and thus improve their strength and plasticity simultaneously.Usually,with the decrease of deformation temperature,the strength of the sample increases.Therefore,to obtain a high strength,a low-deformation temperature is generally preferred.In addition,double extrusion has also been found to be able to improve mechanical properties of Mg alloys.For instance,Zhang et al.[19]found that mechanical properties and corrosion resistance of biodegradable Mg–Nd–Zn–Zr alloys could be improved by double extrusion.Liu et al.[20]studied the microstructure and mechanical properties of Mg-12Gd-3Y-0.6Zr alloy after double extrusion and confirme that grain size of this alloy is refine and the strengthening effect is improved.To simplify such processing ways,we previously proposed a double continuous extrusion(DCE)method,and revealed that mechanical properties of double continuously extruded Mg–Zn–Gdbased Mg alloys are enhanced obviously[3].On the other hand,the patented Mg–Nd–Zn–Zr alloys(hereafter,denoted as JDBM)have been designed as biodegradable materials,and their mechanical properties[19],in vitro degradation behavior[21–26]and cytotoxicity[27,28]have been characterized systematically.Although it has been demonstrated that this alloying system is promising for potential biomedical applications,the strength of the JDBM alloys should be further improved to enhance its security as bone implant instruments.

In this study,to obtain high strength,the DCE processing was applied to two JDBM alloys with different compositions(Mg-2.1Nd-0.2Zn-0.4Zr and Mg-2.8Nd-0.2Zn-0.4Zr,wt%)by lowering deformation temperatures,and investigate how their microstructures and mechanical properties evolve under such processing.

2.Experimental procedures

The ingots of Mg-2.1Nd-0.2Zn-0.4Zr(wt%,denoted as JDBM-2.1Nd)and Mg-2.8Nd-0.2Zn-0.4Zr(wt%,denoted as JDBM-2.8Nd)were fabricated and then solid solution treated at 540°C for 10h.The mold structure of the DCE was reported in our previous study[3].The billets were cut from the ingots with a diameter of 37mm and a height of 5mm,and then preheated under extrusion temperature for 1h in this extrusion mold,followed by DCE processing,i.e.they are extruded to 15mm firstl and then to 12mm in diameter under a total extrusion ratio of～9:1.Microstructures were analyzed by optical microscopy(OM).Specimens for OM observation were prepared by grinding and then etching in picric acid solution.The tensile test specimens were cut by electric spark machining along the extrusion direction.All test specimens were mechanically grounded using 600 grit silicon carbide impregnated emery paper in order to remove circumferential scratches and surface machining marks.At least three specimens were tested for every condition.Tensile tests were performed at an initial strain rate of 1×10?3s?1at room temperature by the Zwick/RoellZ100 testing machine.Texture analysis of the as-extruded bars was performed on the sections perpendicular or parallel to the extrusion direction,denoted as⊥ED and ‖ED,respectively,using X-ray diffraction(XRD,PW3040/60).For the XRD,the Cu–Kα radiation at 40kV and 100mA was used.The step was fi ed to 4°/min and the measurement angle ranged from 30°to 40°.

3.Results

3.1.Microstructure of the alloys before extrusion

Fig.1 shows microstructure of the JDBM-2.1Nd and JDBM-2.8Nd alloys before extrusion.As mentioned in the previous reports the secondary phase in the as-cast JDBM-2.1Nd and JDBM-2.8Nd alloys is Mg12Nd,which precipitated along the grain boundary[29,30].The average grain size of the two as-cast alloys is～ 38μm and 34μm,respectively.After solid solution treatment,the fraction of Mg12Nd decreased obviously.The average grain size of the two solid solution treated alloys is～ 41μm and 37μm,respectively.Therefore,with the addition of the Nd,the grain size decreased slightly,whereas the fraction of the secondary phase increased.

3.2.Microstructure of the alloys after extrusion

Fig.2 shows microstructure of the JDBM-2.1Nd and JDBM-2.8Nd alloys extruded under different temperatures.The equiaxed grains are found to be newly formed in the JDBM-2.1Nd alloy after extrusion,indicating that dynamical recrystallization occurs.The grains in this alloy are very fine With the increase of the extrusion temperature,the fraction of the recrystallization grains increase gradually,and complete recrystallization is fulfille when the extrusion temperature is 340°C.The JDBM-2.8Nd alloy shows a similar microstructural evolution with the extrusion temperature.However,non-recrystallized grains can still be identifie in this alloy until the extrusion temperature reaches 370°C,indicating that recrystallization is restrained with the addition of Nd.Comparing the microstructures of the two types of alloys,grains in the JDBM-2.8Nd alloy are fine than those in the JDBM-2.1Nd alloy when they are extruded under identical conditions,which is attributed to the fact that the equiaxed recrystallized grain is formed by nucleation and then grain growth.Since the Nd can restrain the recrystallization process,the grain growth is more difficul for the JDBM-2.8Nd alloy,giving rise to fine grains.

3.3.Tensile properties

Fig.3 shows representative engineering stress vs.strain curves for the two alloys.The JDBM-2.8Nd alloy exhibits a higher yield strength and elongation than the JDBM-2.1Nd alloy before extrusion(Fig.3(a)).During tensile test,the stress of the extruded JDBM-2.1Nd decreases gradually after yielding except for the sample extruded at 340°C(Fig.3(b)),which indicates weak strain hardening.Moreover,this also confirm that grains in the JDBM-2.1Nd alloys are very fine The yield strength of the JDBM-2.1Nd samples after extrusion under 250°C,280°C,310°C,340°C is ～541MPa,462MPa,337MPa and 232MPa,respectively,and their corresponding elongation is～3.7%,4.9%,9.5%and 13.8%,respectively.These results imply that with the increase of extrusion temperature,yield strength of the JDBM-2.1Nd samples decreases and their elongation increases gradually.Fig.3(c)shows the room-temperature tensile tests of the JDBM-2.8Nd alloy extruded under different temperatures revealing a similar variation trend for the yield strength and elongation.The yield strength of the JDBM-2.8Nd samples is～340MPa,370MPa,330MPa,251MPa,respectively,and their elongation is～1.8%,10.5%,18.4%,26.6%,respectively.The yield strength and elongation for the JDBM-2.8Nd sample extruded at 280°C are the lowest because the extrusion temperature is too low that many defects are formed in the samples.Likewise,if the JDBM-2.1Nd sample is extruded under temperaturelower than 250°C,the samples will be unableto form perfectly.Fig.3(d)shows relationship among yield strength,temperature and Nd concentration,uncovering clearly that high yield strength sample can be obtained by lowering Nd concentration and temperature.

Fig.1.Microstructure of the as-cast and solid solution treated JDBM-2.1Nd and JDBM-2.8Nd alloys.(a)as-cast JDBM-2.1Nd;(b)as-cast JDBM-2.8Nd;(c)solid solution treated JDBM-2.1Nd;(d)solid solution treated JDBM-2.8Nd.

3.4.Texture

Fig 4(a)shows XRD pattern for non-oriented polycrystalline Mg obtained by simulation software.The 2 theta value for the three highest diffraction peaks is 32.17°,34.39°and 36.60°,and their corresponding relative intensity is 24.7%,26.9%and 100.0%.Here,we apply the I(002)/I(101)to qualitatively measure the basal texture intensity[31–34].The I(002)/I(101)for the cross and longitudinal sections of the JDBM-2.1Nd after extrusion under 250°C is ～3.7 and 0.7(Fig.4(b)),respectively.On the other hand,the I(002)/I(101)for the cross and longitudinal sections of the JDBM-2.1Nd after extrusion under 340°C is ～0.74 and 0.08(Fig.4(c)),respectively.These results show that with the increase of the extrusion temperature,the intensity of basal texture decreases.Moreover,the I(002)/I(101)for the cross and longitudinal sections of the JDBM-2.8Nd after extrusion under 340°C is～1.46 and 0.20(Fig.4(d)),respectively,revealing that the JDBM-2.8Nd has a little higher intensity of basal texture than the JDBM-2.1Nd.Large amount of non-recrystallized grains can be found in JDBM-2.8Nd,whereas it basically cannot be found in JDBM-2.1Nd after extrusion under 340°C.Nonrecrystallized grains usually have stronger basal texture than that of dynamical recrystallized grains[35–37],which maybe is the reason for the JDBM-2.8Nd sample have slightly higher basal texture intensity than that of the JDBM-2.1Nd after extrusion under 340°C.

4.Discussion

4.1.Effect of alloying element concentration on extrusion

Fig.2.Microstructure of the two alloys after extrusion under different temperature:JDBM-2.1Nd,(a)250°C,(c)280°C,(e)310°C,(g)340°C;JDBM-2.8Nd,(b)280°C,(d)310°C,(f)340°C,(h)370°C.

Fig.3.Tensile properties of the two alloys after solid solution treatment and extrusion under different temperatures:(a)solid solution treatment,(b)JDBM-2.1Nd,and(c)JDBM-2.8Nd.(d)Relationships among yield strength,temperature and Nd concentration.

Comparing the tensile results of the two alloys,the lowest extrusion temperature is related to the concentration of the alloying elements in the sample(Fig.3).The alloy with a lower concentration of alloying element usually has a lower hardness and yield strength.Khomamizadeh et al.[38]investigated the effects of rare-earth(RE)element additions on high-temperature mechanical properties of AZ91 Mg alloy,and found that the addition of RE elements up to 2wt%improves both yield and tensile strengths at 140°C.Lü et al.[39]studied the effects of rare earths on microstructure,properties and fracture behavior of as-cast Mg–Al alloys,and found that microhardness of the matrix and yield strength increases with the RE addition.Moreover,RE shows little effect on ambient temperature tensile strength of AZ91 alloy but can greatly improve high-temperature tensile properties of the Mg alloys[39].Wang et al.[40]studied the effects of RE on microstructure and mechanical properties of Mg–8Zn–4Al magnesium alloy,and showed that the microstructure of ZA84 alloy can be refine and the room and high temperature tensile properties of as-cast and aged ZA84 alloys can be improved accordingly.In this sense,with the addition of RE elements,grains usually will be refine during solidifi cation which is beneficia for tensile properties.In addition,more secondary phases will form,improving strength of the alloy based on the Orowan strengthening theory.Since the only difference between the JDBM-2.1Nd and JDBM-2.8Nd alloys in our case rests with the Nd concentration.Before extrusion,the JDBM-2.8Nd has a higher yield strength and elongation than the JDBM-2.1Nd.When the two alloys are heated to extrusion temperature,strength decreases.Since the two alloys belong to the same alloying system,it is reasonable to believe that the yield strength of JDBM-2.1Nd alloy is still lower than that of JDBM-2.8Nd at elevated temperatures.Therefore,compared to JDBM-2.8Nd alloy,JDBM-2.1Nd alloy can easily be hot extruded because its yield strength is lower,which accounts for the observation that the JDBM-2.1Nd alloy can be extruded at lower temperature.Therefore,the lowest DCE processing temperature is found to increases with the increase of Nd concentration for the JDBM alloys.

4.2.Effect of alloying element concentration on tensile properties

Usually,the alloys with the increased alloying element concentration exhibits better tensile properties due to grain refinemen and Orowan strengthening of precipitates.The alloy with a lower concentration of alloying elements can reach a higher yield strength after DCE(Fig.3)since it can be extruded at a lower temperature.

Fig.4.XRD analysis of JDBM-2.1Nd and JDBM-2.8Nd alloys after extrusion at different temperatures:(a)simulation results;(b)JDBM-2.1Nd,250°C;(c)JDBM-2.1Nd,340°C;and(d)JDBM-2.8Nd,340°C.The ED represents the extrusion direction.The cross section,which is denoted as ⊥ED,is the plane perpendicular to the ED;and the longitudinal section,which is denoted as//ED,is the plane parallel to the ED.

Generally,grain size of as-extruded specimens strongly depends on extrusion temperatures[41].With the decrease of the extrusion temperature,the average grain size turns small due to dynamical recrystallization.For example,the grain size of the pure Mg extruded at 453,367 and 363K is 55,5 and 1μm,respectively[42].The grain size of the AZ91 alloy ingots extruded at 573,673 and 753K is 7.6,15.4 and 66.1μm,respectively[43].The Mg-1.50Zn-0.25Gd alloy after extrusion at 373K exhibits excellent tensile properties at ambient temperature with a ultimate tensile strength of 417MPa,0.2%proof stress of 395MPa and elongation to failure of 8.3%[44].Moreover,it has also been reported that high strength of over 400MPa is achieved in the Mg–8Al–0.5Zn alloy extruded at a relatively low temperature of 473K.These superior mechanical properties are attributed to the combined effects of ultra-fin recrystallized grains,numerous nano-scale Mg17Al12precipitates,and strong basal texture[21].In this study,a combination study of optical microstructures(Fig.2),tensile curves(Fig.3)and XRD patterns(Fig.4)for the two alloys also confirme that lower temperature results in fine recrystallized grains and stronger basal texture,which contribute to a higher tensile strength.This findin offers an explanation to the observation that the alloy with a lower concentration of alloying elements can reach a higher yield strength after extrusion.

On the other hand,when the two alloys are extruded under the same conditions,e.g.at 310°C and 340°C,the alloy with a higher concentration of alloying elements shows a higher yield strength and elongation(Fig.3).The addition of Nd in the alloys induces the formation of increased amount of Mg12Nd precipitates.Apart from the Orowan strengthening of these precipitates,strong basal texture(Fig.4)and grain size refinemen(Fig.2)strengthening,leading to improved tensile properties for the JDBM-2.8Nd alloy.

5.Conclusions

We have investigated microstructure,texture and mechanical properties of the JDBM-2.1Nd and JDBM-2.8Nd alloys and found that the lowest DCE processing temperature is 250°C for JDBM-2.1Nd and 310°C for JDBM-2.8Nd,which increases with the increase of the concentration of Nd.The highest yield strength of 541MPa is achieved in JDBM-2.1 Nd samples when extruded at 250°C and the elongation is about 3.7%.Moreover,we also fin that the alloy with a lower alloying content could reach a higher yield strength,which is quite unusual because alloying is conventionally believed to be able to strengthen metallic materials.We also fin that when extruded under the same conditions,the alloys with a higher alloying content exhibit improved tensile properties.

Acknowledgments

The authors gratefully acknowledge the support by the national key research and development plan (No.2016YFC1102100),the National Natural Science Foundation of China(Nos.51501110,51728202,11332013 and 51501115),the Natural Science Foundation of Shanghai(15ZR1422600),the Shanghai Jiao Tong University Medical-engineering Cross Fund(No.YG2015MS66 and No.YG2014MS62).