稀有金属(英文版) 2020,39(08),909-917

Microstructure,mechanical properties and corrosion resistance of as-cast and as-extruded Mg-4Zn-1La magnesium alloy

Huseyin Zengin Yunus Turen Hayrettin Ahlatci Yavuz Sun

Department of Metallurgy and Materials Engineering,Karabuk University

Iron and Steel Institute,Karabuk University

摘 要：

Microstructure,mechanical properties and corrosion resistance of as-cast and as-extruded Mg-4 wt% Zn-1 wt% La magnesium alloys were investigated.The alloys were produced by low-pressure die casting method and extruded at 350℃ after homogenization at 400℃ for 24 h.The results show that the as-cast alloy mainly consists of primary α-Mg matrix and Mg-Zn-La ternary second phases(also called T-Phase) along grain boundaries and isolated spherical particles inside the grains.After extrusion at350℃,the average grain size decreases by 81% due to dynamic recrystallization mechanism and T-phase particles are distributed along the extrusion direction.The elongation,yield strength and tensile strength of the as-cast Mg-4Zn-1La alloy increase by 179%,90% and 40%,respectively,as a result of the extrusion process.The as-extruded Mg-4Zn-1La alloy shows better corrosion resistance than the as-cast alloy due to increased grain boundaries and decreased content of T-phase.

作者简介：*Yunus Turen,e-mail:yturen@karabuk.edu.tr;

收稿日期：20 May 2017

基金：financially supported by the Scientific Research Projects of Karabuk University (BAP) (No.KBUBAP-16/1-DR-075);

Microstructure,mechanical properties and corrosion resistance of as-cast and as-extruded Mg-4Zn-1La magnesium alloy

Huseyin Zengin Yunus Turen Hayrettin Ahlatci Yavuz Sun

Department of Metallurgy and Materials Engineering,Karabuk University

Iron and Steel Institute,Karabuk University

Abstract：

Microstructure,mechanical properties and corrosion resistance of as-cast and as-extruded Mg-4 wt% Zn-1 wt% La magnesium alloys were investigated.The alloys were produced by low-pressure die casting method and extruded at 350℃ after homogenization at 400℃ for 24 h.The results show that the as-cast alloy mainly consists of primary α-Mg matrix and Mg-Zn-La ternary second phases(also called T-Phase) along grain boundaries and isolated spherical particles inside the grains.After extrusion at 350℃,the average grain size decreases by 81% due to dynamic recrystallization mechanism and T-phase particles are distributed along the extrusion direction.The elongation,yield strength and tensile strength of the as-cast Mg-4Zn-1La alloy increase by 179%,90% and 40%,respectively,as a result of the extrusion process.The as-extruded Mg-4Zn-1La alloy shows better corrosion resistance than the as-cast alloy due to increased grain boundaries and decreased content of T-phase.

Keyword：

Magnesium alloys; La modification; Extrusion; Microstructure; Mechanical properties; Corrosion;

Received： 20 May 2017

1 Introduction

Owing to the growing demand for lightweight structural metals mainly in automotive and aerospace industries,magnesium alloys have recently received considerable interest since they have a unique high specific strength [ 1, 2] .However,improving several properties of magnesium alloys such as strength,ductility and corrosion resistance are still big challenges.Alloying and different processing conditions (heat treatment,hot forming,etc.)are considered as typical approaches to provide better properties [ 1] .Zn as one of the most common alloying elements in Mg has been widely employed to improve corrosion resistance and strength by grain refinement,solid solution and precipitation hardening [ 3, 4] .Zn has a high maximum solid solubility in Mg (6.2 wt%) and together with Mg can form binary coherent compounds [ 1, 5] .Therefore,Mg-Zn-based magnesium alloys are good candidates for being developed by further alloying additions and different processing routes.Rare-earth (RE) element addition to Mg-Zn-based alloys is an effective way to further improve mechanical properties at room and elevated temperatures.The improvement in mechanical properties has been attributed to the formation of thermally stable,fine and dense RE-containing precipitates [ 6, 7, 8, 9, 10, 11, 12, 13, 14] .However,high cost of RE limits the use in large amounts in Mg alloys.For this reason,La and Ce as similar and the most cost-effective rare-earth metals have recently attracted attention to be used in Mg-Zn alloys providing satisfactory properties [ 8, 12, 15] .The elongation of an extruded Mg-6 wt%Zn alloy was enhanced from 20%to35%with 0.2 wt%La addition,whereas no improvement in the strength was observed [ 15] .In another study,increasing La amount gradually improved the strength of the Mg-LaZr alloy due to the formation of Mg₁₂La phase [ 16] .La together with Ce formed Ce/La-containing Mg-Zn-Ca phase with high thermal stability,refined microstructure and increased strength of Mg-Zn-Ca alloys but decreased corrosion resistance due to the increased number of cathodic sites [ 17] .In Mg-Zn-RE alloy system,ZE41 is one of the most commercially used magnesium alloys for gearbox,housings,especially in helicopters which are exposed to corrosive environments in service [ 18, 19] .Neil et al. [ 19] reported that corrosion attacks at the Mg-Zn-RE ternary phase along the grain boundary and the Zr-rich regions in the centre of the grains in the Zr-containing ZE41 alloy.Zhao et al. [ 20] investigated the corrosion behaviour of Zr-free ZE41 alloy and found that corrosion initiated at junction betweenα-Mg matrix and the Mg₇Zn₃RE phase,also called T-phase with a c-cantered orthorhombic crystal structure [ 19, 20, 21, 22] .

It is well known that hot forming processes of Mg alloys such as extrusion,rolling and forging,give rise to superior mechanical properties compared to their as-cast state.With regard to hot extrusion process,Mg alloys can be simply and cost-effectively formed to achieve homogenous and fine microstructures,resulting in high strength and ductility.Furthermore,extrusion temperature plays an important role in microstructure and mechanical properties,primarily by affecting the dynamic recrystallization behaviour.It was reported that larger dynamically recrystallized grains were observed in Mg-Zn-based alloys as the extrusion temperature increased,which influenced the strength adversely [ 23, 24, 25] .However,each Mg alloy has its own optimal extrusion parameters and it is still necessary for inpidual investigations of Mg alloys.Thus,the present study aims for understanding the relationship between extrusion process and the microstructure,mechanical and corrosion properties of Mg-4Zn-1La alloy.

2 Experimental

Mg-4 wt%Zn-1 wt%La alloy was fabricated by lowpressure die casting method (LPDC).High-purity Mg ingots were put into the stainless-steel crucible and heated to 750℃under a controlled Ar gas atmosphere.Then,pure Zn and Mg-30 wt%La master alloys were added to the molten Mg.The melt was held at this temperature for30 min and stirred for 15 min to ensure compositional homogeneity.Finally,a pressure of 2×10⁵ Pa was applied to the crucible and a steel mould (Φ34 mm×190 mm) preheated at 250℃was filled through the stainless-steel pipe,followed by air cooling.The chemical composition of the fabricated alloy was measured by wavelength dispersion X-ray fluorescence (XRF) using Rigaku's standardless semi-quantitative analysis programme and is listed in Table 1.The as-cast ingots were homogenized at400℃for 24 h followed by water quenching.The homogenized billets were machined into cylindrical bar(Φ32 mm×30 mm) and finally extruded at 350℃with an extrusion ratio of 16 and a ram speed of 1 mm·s^-1 by a vertical direct extrusion machine.

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Table 1 Chemical compositions of studied Mg-4Zn-1La alloy(wt%)

The as-cast,as-homogenized and as-extruded alloys were mechanically ground,polished and etched with a mixture of 6 g picric acid,5 ml acetic acid,10 ml distilled water and 100 ml ethanol.The microstructures were examined with a Nikon optical microscope (OM) and a Carl Zeiss Ultra Plus field emission scanning electron microscope (FESEM).Average grain size measurements were made on six microstructures for each sample by linear intercept method in accordance with the ASTM E112standard.The phases in the as-cast alloy were characterized by X-ray diffractometer (XRD,Rigaku UltimaⅣ) with Cu Kαradiation operating with a scanning rate of 3 (°)·min^-1and a scanning angle from 10°to 90°.The phase analyses were conducted according to the database of International Centre for Diffraction Data (ICDD).

According to EN ISO 6892-1,the dog-bone tensile specimens with a diameter of 5 mm and gauge length of25 mm were machined from the half radius of the as-cast alloy and parallel to the extrusion direction of the as-extruded alloys.Tensile tests were performed at a strain rate of 0.00167 s^-1 at room temperature,and each test condition was repeated three times.

The specimens with a diameter of 5 mm and length of15 mm for immersion corrosion test were cut from the ascast and as-extruded alloys.The specimens were immersed in 3.5 wt%NaCl solution at room temperature for 72 h after grinding and polishing.The immersion tests were conducted in a temperature-controlled laboratory,in which the temperature was kept at 20℃.After the immersion tests,the specimens were dipped into chromic acid solution for 10 min to remove corrosion products.The surface morphologies of the corrosion samples were also observed using SEM.The immersion tests were repeated three times.The electrochemical corrosion tests of the as-cast and asextruded alloys were performed in 3.5 wt%NaCl solution at room temperature by a Gamry model PC4/300 mA potentiostat/galvanostat with DC 105 corrosion analysis controlled by a computer.The polarization curves were generated at a scan rate of 1 mV·s^-1,starting from-0.25 V (vs.open circuit potential,E_oc) to+0.25 V (vs.E_oc).A classical three-electrode cell was used,consisting of a graphite rod as counter electrode,a saturated calomel electrode (SCE) as reference electrode and the sample with exposed area of 0.25 cm² as working electrode.The electrochemical corrosion tests were repeated five times.

3 Results and discussion

3.1 Microstructure

Figure 1 shows OM images of the as-cast and as-homogenized Mg-4Zn-1La alloys.The as-cast microstructure consists of primaryα-Mg matrix with an average grain size of about 50μm,a large number of coarse second phases along the grain boundaries and some isolated spherical particles inside the grains.After homogenization treatment,the content of the second phases decreases from～12vol%to～8 vol%and the precipitates are mainly distributed along grain boundaries.SEM images of the as-cast and as-homogenized Mg-4Zn-1La alloys with corresponding energy-dispersive spectroscopy (EDS) results of the second phases and the matrix indicated by arrows are presented in Fig.2 and Table 2.Owing to the limited solubility of La in Mg at the eutectic temperature(～0.79 wt%) [ 1] ,only 0.3 wt%La is detected inα-Mg matrix in the as-cast alloy.However,dissolution of second phases during homogenization results in an increase of 0.6wt%and 0.1 wt%in the Zn and La concentration inα-Mg matrix,respectively.The second phases are found to be rich in Zn and La,meaning that the second phases are MgZn-La ternary compounds also called T-phase.It is worth noting that the concentrations of Mg,Zn and La in the second phases show variations on each measurement.Du et al. [ 15] also reported similar variations in the compositions of the Mg-Zn-La ternary phases in Mg-6 wt%Zn alloy containing various amounts of La.They suggested that the compositions might depend on sizes and distributions of the ternary phases.EDS elemental mapping analysis of the as-cast alloy is illustrated in Fig.3.The matrix phase predominantly contains Mg,while there is no evidence of Mg in the second phases.Zn elements are in both matrix and second phases,and La elements are evidently present in the second phases although very few La elements are observed in the matrix phase since the matrix phase contains～0.3 wt%La (Table 2).

Fig.1 OM images of a as-cast and b as-homogenized Mg-4Zn-1La alloy

Fig.2 SEM analysis of a,b as-cast and c,d as-homogenized Mg-4Zn-1La alloy

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Table 2 EDS results of points indicated in Fig.2b,d

Fig.3 a SEM image and EDS elemental mappings (b Mg,c Zn and d La) of as-cast Mg-4Zn-1La alloy

XRD result of the as-cast alloy,as presented in Fig.4,reveals that the as-cast alloy consisted ofα-Mg,Mg-Zn-La ternary phase (T-phase) and a small amount of Mg-Zn binary compounds.Although the T-phase could not be distinguished according to standard XRD cards [ 17] ,the same peaks at～25°and～52°are also observed in previous studies [ 8, 17, 20] ,suggesting that these peaks corresponded to Mg-Zn-La or Mg-Zn-Ce ternary phases since Ce has similar characteristics with La [ 26] .

Figure 5 shows OM images of the as-extruded alloy taken from the longitudinal sections of the samples at different magnifications.The as-extruded alloy consists of fine grain structure with the second phase particles aligned along the extrusion direction.The extrusion process gives rise to a grain refinement compared to as-cast alloy (Figs.1,5) due to dynamic recrystallization during extrusion.The asextruded alloy exhibits an average grain size of about 9.3μm.That is to say,a grain refinement of 81%can be obtained by extrusion process.As it was stated earlier,only～4%of the second phases are dissolved during homogenization at 400℃for 24 h (Fig.2 and Table 2) due to the high thermal stability of the Mg-Zn-La ternary phases.In Fig.6 and Table 3,SEM images of the as-extruded alloy and EDS analysis of the second phases marked in SEM images are presented.The shape and distribution of the second phases in the as-extruded alloy are quite different from these in the as-cast alloy.The undissolved coarse second phases are broken and oriented towards the extrusion direction as a result of severe plastic deformation.In Table 3,EDS results show that the composition of the matrix phase does not change after extrusion and the second phases in the as-extruded alloy are T-phase with similar compositions with those in the as-cast and as-homogenized alloys.

Fig.4 XRD pattern of as-cast Mg-4Zn-1La alloy

3.2 Mechanical properties

The tensile stress-strain curves of the as-cast and as-extruded alloys are shown in Fig.7.The tensile test results and grain sizes are presented in Table 4.After extrusion,the yield strength,tensile strength and ductility are significantly improved.The elongation,yield strength and tensile strength of as-cast Mg-4Zn-1La alloy can be increased by 179%,90%and 40%,respectively,by extrusion at 350℃.

Fig.5 OM images of as-extruded Mg-4Zn-1La alloy at a low and b high magnifications (ED:extrusion direction)

Fig.6 SEM images of as-extruded Mg-4Zn-1La alloy at a low and b high magnifications (ED:extrusion direction)

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Table 3 EDS results of points indicated in Fig.6b

Fig.7 Tensile stress-strain curves of as-cast and as-extruded alloys

A fully dynamically recrystallized microstructure is obtained in the as-extruded alloy.According to Hall-Petch theory [ 27, 28] ,the reduction in grain size results in higher yield strength due to increased amount of grain boundaries which create more opposition to dislocation motion.Therefore,the improvement in the mechanical properties is primarily attributed to the grain boundary strengthening.Furthermore,after extrusion,the coarse second phases distributing along grain boundaries in the as-cast alloy are broken and oriented towards the extrusion direction.Therefore,the second phases contribute to the improvement in the strength by providing more dispersive strengthening effect and acting as obstacles to dislocation motion [ 29] .In addition,Ce addition can result in a formation of fine precipitates from supersaturated solid solution during extrusion and accordingly an improvement of strength [ 8, 30] .Therefore,in the present study,similar dynamic precipitation of fine La-containing particles might have occurred as a result of the extrusion process and contributed to the overall strength of the studied alloy.

Figure 8 shows fractographs of the as-cast and as-extruded alloys.The fracture surface of the as-cast alloy consists of many cleavages and tearing edges which result in low ductility.Ductile fracture with a large number of uniform and shallow dimples with T-phase particles and tearing edges is seen in the fracture surface of the as-extruded alloy.The second phase particles can act as barriers to dislocations motion and contribute to strength.However,they can also cause crack propagation during deformation and accordingly decrease the elongation to failure.

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Table 4 Tensile properties of as-cast and as-extruded alloys

Fig.8 SEM images of fractured tensile samples:a as-cast and b as-extruded

Fig.9 Corrosion rates of as-cast and as-extruded alloys after immersion tests

3.3 Corrosion

The weight loss corrosion rates of the as-cast and as-extruded alloys after the immersion test in 3.5%NaCl solution for 72 h are presented in Fig.9.The as-extruded alloy shows higher corrosion resistance,exhibiting a lower weight loss rate (3.4 mg·cm^-2·day^-1) than the as-cast alloy(5.9 mg·cm^-2·day^-1).Zhao et al. [ 20] showed similar weight loss rate with that of as-cast ZE41 alloy,but the NaCl concentration of the working solution used for the immersion test is higher than that of the solution used in this study.Increased chloride ion concentration results in higher corrosion rates in ZE41 magnesium alloys [ 31] .Therefore,a lower weight loss rate can be expected in this study.It is thought that this behaviour is mainly due to the differences in Fe and Zn concentrations of the studied alloys.In Ref. [ 20] ,Zn and Fe concentrations of the ZE41alloy were 4.590 wt%and 0.006 wt%,whereas they were3.780 wt%and 0.018 wt%,respectively,in this study.Zn additions up to 5 wt%improved the corrosion resistance of magnesium [ 4] .Furthermore,Fe,Ni and Cu are the main elements which have detrimental effects on corrosion resistance of magnesium alloys since they act as active cathodic sites [ 32, 33] .It is commonly accepted that the implementation of tolerance limits for a specific alloy can be an effective way to minimise the deleterious effects of these elements on corrosion resistance of magnesium alloys.For example,pure Mg has a following tolerance limits:Fe<0.01 wt%,Ni<0.0005 wt%,and Cu<0.1wt%-0.3 wt% [ 33] .In this study,Fe content slightly exceeds 0.01 wt%,which is likely caused by the impurities in Mg ingots used in the casting (Table 1).It can also be expected that the stainless-steel crucible used in the casting process might lead to a release of Ni into the melt.XRF results (Table 1) reveal that the alloy contains trace amounts of Ni and Cu which are below the tolerance limits of pure Mg.Therefore,in this study,a possible inhomogeneous distribution of Ni and Cu impurities which can accelerate the galvanic corrosion is considered low and their effects on the corrosion resistance of Mg-4Zn-1La alloy are not taken into consideration.

Figure 10 shows the corrosion morphologies of the ascast and as-extruded alloys after immersion in 3.5%NaCl solution for 72 h and removal of the corrosion products.The surface of the as-cast alloy has deep corrosion pits which cover the whole working surface,while the surface of the as-extruded alloy consists of a low amount of deep pitting localized in the direction of extrusion and uncorroded regions.Previous studies revealed that corrosion pitting initiated adjacent to the T-phase and was followed by pitting withinα-Mg matrix phase,which was attributed to a galvanic interaction betweenα-Mg and T-phase in ascast ZE41 magnesium alloys [ 19, 20] .Therefore,the asextruded alloy shows some localized corrosion pits along the extrusion direction since uniformly distributed T-phases in the as-cast state are preferentially aligned along the extrusion direction,as shown in Figs.5 and 6.

Fig.10 SEM images of corroded surfaces with inset macroscopic images after immersion tests:a as-cast and b as-extruded

Fig.11 Potentiodynamic polarization curves of as-cast and as-extruded alloys in 3.5%NaCl solution

Figure 11 shows potentiodynamic polarization curves of the as-cast and as-extruded Mg-4Zn-1La alloys.The extrusion process shifts the corrosion potential (E_corr) to more negative potential by 0.04 V (from-1.590 to-1.630 V) and decreases the corrosion current density(i_corr) value by 0.045 mA·cm^-2 (from 0.084 to0.039 mA·cm^-2).After extrusion,the cathodic reaction kinetics considerably decreases,while the anodic reaction kinetics slightly increases.Therefore,it can be said that decreasedi_corr is dictated by cathodic activity.The decreased i_corr value indicates that the as-extruded alloy shows better corrosion resistance than the as-cast alloy,showing that the electrochemical corrosion test results agree with the immersion test results.

Both immersion and electrochemical corrosion tests indicate that the corrosion rate of the as-cast alloy is almost double that of the as-extruded alloy.Birbilis et al. [ 34] showed that decreased grain size resulted in better corrosion resistance in pure Mg,which was attributed to the formation of oxide layers with high coherency due to the increased content of grain boundaries.Argade et al. [ 35] also reported that grain refinement of Mg-4Y-3Nd alloy led to improvement in corrosion resistance.The improved corrosion resistance was attributed to the formation of more adherent passive layer by increased amount of high angle grain boundaries in the fine grained alloys.It was suggested that the decreased dislocation density and internal residual stress in dynamically recrystallized microstructure also contributed to the improvement in the corrosion resistance [ 36, 37] .Therefore,in this study,the improved corrosion resistance after extrusion of Mg-4Zn-1 La alloy is primarily attributed to the increased grain boundaries which can promote a more tenacious protective layer [ 35] .The decreased content of T-phase after homogenization treatment also contributes to the improvement of the corrosion resistance since they might act as cathodic sites.

4 Conclusion

Mg-4Zn-1La alloy was cast and extruded,and the micro structure,mechanical and corrosion properties were investigated.The micros true ture of the as-cast Mg-4Zn-1La alloy mainly consists of primaryα-Mg matrix and Mg-Zn-La ternary second phases (T-Phase) along grain boundaries and isolated spherical particles within grains.Extrusion process results in a grain refinement by 81%and distribution of T-phase along the extrusion direction.The mechanical properties of the as-cast Mg-4Zn-1La alloy are significantly improved by grain boundary strengthening effect after extrusion process.The extruded Mg-4Zn-1La alloy shows better corrosion resistance than the as-cast alloy mainly due to the increased grain boundaries and decreased content of T-phase.

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