mobile meEffect of heat treatment on microstructure and mechanical properties of extruded ZM61 magnesium alloy
F.G.Qi1,2,D.F.Zhang*1,2,Z.T.Zhu1,2,X.X.Xu1,2and G.L.Shi3
The effects of T5and T6heat treatments on the microstructure and mechanical properties of extruded Mg–6Zn–1M n alloy were investigated in the prent study.The results showed that T5 and T6treatments could markedly improve the strengths of extruded ZM61alloy,and the precipitate strengthening effect of double aging was better than that of single aging.The precipitates formed in the T6treatment were much finer and more dispersive than the ones in the T5treatment,resulting in stronger precipitation strengthening effect.However,due to additional grain size refinement strengthening effect,the T5treatment could improve ductility without sacrificing strength over the T6treatment.Scanning electron microscopy obrvation and tensile test indicated that different cooling methods after extrusion,such as air cooling and water quenching,had no obvious influence on the microstructure and strengths of extruded and subquent heat treated Mg–6Zn–1Mn alloys.In addition,the Mn element mainly existed as fine Mn pha particles,which were well disperd in the matrix.Disperd Mn particles could be found in rod-like b’1precipitates,but not in the disc shaped b’2precipitates.
Keywords:Mg–Zn–Mn,Heat treatment,Precipitate,Microstructure,Mechanical properties
Introduction
As the lightest structure metal materials with high specific strength and stiffness,good damping capacity, excellent machinability and good castability,magnesium alloy is the most attractive material in aerospace, transportation and mobile electronics.1–4However,due to limited strength,poor formability and high cost of expensive composition elements ud,the application of magnesium alloy is still limited.5–8Therefore,it is pressing to develop some new wrought magnesium alloys with high strength and low cost.Mg–Zn alloys are the most widely ud wrought magnesium alloy.9,10 Owing to the problems with hot shortness and coar and uneven grain size in binary Mg–Zn alloy,commer-cially Mg–Zn alloys are always grain refined by the addition of Zr.11In addition,RE and Cu have been added to improve both casting characteristics and mechanical properties at elevated temperature.12–15 However,the additions of the elements also increa the alloy’s cost.
Mg–6Zn–1Mn(ZM61)alloy is a new promising alloy, which is developed to meet the above requirements. Zhang et al.16,17reported that the mechanical properties of ZM61alloy with solution and aging treatment can achieve the level of the commercial wrought ZK60A alloy.16Recently,Park e
211大概要多少分t al.18,19investigated the effect of the Al addition on the microstructure and tensile properties of ZM61alloy and found that ZM61–1Al alloy exhibited excellent tensile properties as a result of refined precipitates by Al addition.More recently,the microstructure and mechanical properties of the Mg–x Zn–1Mn alloy have been reported.20According to the reports,the Mg–6Zn–1Mn(ZM61)alloy had the best comprehensive mechanical properties.
It is well known that strengthening via grain size control is particularly effective in magnesium alloys becau of the higher Hall–Petch coefficient.21,22For the most part of Mg–Zn ries alloys,solution treatment after plastic deformation can significantly coarn the grain so that the mechanical properties of T6aged alloys are wor than T5aged alloys.23,24It was reported that the hardness and strength of extruded ZK60alloy under T5condition were higher than tho under T6condition becau the cond pha precipitated during the aging process wasfiner and more dispersive under T5 condition than that under T6condition.23The micro-structure and mechanical properties of forged ZK60-Y alloy under various heat treatments have been reported.24The results showed that T5treated alloy had superior tensile strength and plasticity compared with T4and T6treated alloys.
1College of Materials Science and Engineering,Chongqing University, Chongqing400045,China
2National Engineering Rearch Center for Magnesium Alloys,Chongqing University,Chongqing400044,China
3State Key Laboratory for Fabrication and Processing of Non-Ferrous Metals,General Rearch Institute for Non-Ferrous Metals,Beijing 100088,China
*Corresponding author,email zhangdingfei@cqu.edu
1426ß2012Institute of Materials,Minerals and Mining
Published by Maney on behalf of the Institute
Received15May2012;accepted27July2012
DOI10.1179/1743284712Y.0000000095Materials Science and Technology2012VOL28NO12
Although some rearches on the microstructure of ZM61alloy have been carried out,no systematical study was focud on heat treatment of extruded ZM61alloy.In the prent study,the effect of T5and T6heat treatment on the microstructure and mechanical proper-ties of extruded ZM61alloy were investigated.This study also aims to investigate the relationship between precipitations and mechanical properties and to opti-mi the heat treatment parameters.
Experimental
The nominal composition (in wt-%)of the alloy ud in the prent study is Mg–6Zn–1Mn.The experimental alloy was prepared from commercial high purity Mg (.99?9%),Zn (.99?95%)and Mg–4?1%Mn master alloy by melting in an electrical resistance furnace under a SO 2z CO 2protective gas and then casting them into a steel mould.The actual composition of alloy was analyd by XRF-800CCDE X-ray fluorescence spec-trometer,and the result is Mg–5?9300Zn–1?0200Mn–0?0094Al–0?0049Fe–0?0058Si–0?0015Cu–0?0005Ni (wt-%).Experimental detail is schematically prented in Fig.1.First,cast ingots were homogenid at 330u C for 24h with air cooling.Before the ingots were extruded,the ingots and extrusion die were heated to 420u C for 90min.To study the effect of the preheating treatment on the microstructure,small samples for microstructure obrvation were also heat treated with the same heating regime and then quenched in water to retain the high temperature microstructures.Then,the homogenid ingots were hot extruded into bars 16mm in diameter at 420u C.The extrusion ratio was 25:1,and the ram speed was t at 3m min 21during extrusion.To investigate the effect of cooling methods after extrusion on the microstructure and mechanical proper-ties of extruded and subquent heat treated alloys,different cooling methods of air cooling and water quenching were ud.Following this,the samples were given T5or T6heat treat
ment.In the ca of T5treatment,the extruded bars were merely single aged (180u C for 16h)and double aged (90u C for 24h followed by 180u C for 16h)respectively.In the ca of T6treatment,the extruded bars were solution treated at 420u C for 2h followed by water quenching and then
immediately single aged (180u C for 16h)and double aged (90u C for 24h followed by 180u C for 16h)respectively.
Cylindrical tensile samples,50mm in gauge length and 5mm in gauge diameter,were machined from the extruded and aged bars along the extrusion direction.Tensile tests were conducted on a Sans CMT-5105electronic universal testing machine at room tempera-ture with a displacement rate of 3mm min 21.Each test condition was repeated at least three times for repeat-ability and accuracy.
Microstructure was obrved by an optical microscope (NEOPHOT30),a scanning electron microscope (SEM)(TESCAN VEGAII)equipped with an Oxford INCA Energy 350energy dispersive X-ray (EDS)spectrometer.Precipitates were examined using a transmission electrical microscope (Zeiss LIBRA 200FE)operating at 200kV.Pha constitutions were determined by a Rigaku D/max 2500PC X-ray diffractometer with the u of Cu K a radiation and a scanning rate of 4u min 21.
Results and discussion
comerMicrostructure of as cast and as homogenid alloys
Figure 2shows the microstructures of the as cast and as homogenid ZM61alloys.As shown in Fig.2a and c ,the as cast microstructure of the experiment alloy consists of a -Mg matrix and eutectic compounds.The eutectic com-pounds are Mg 7Zn 3pha by X-ray diffraction (XRD)analysis as shown in Fig.3a .Mn exists as pure a -Mn.The average grain size of as cast alloy is y 160m m.After homogenisation at 330u C for 24h,some of the eutectic compounds in the grain boundary dissolve into the matrix as shown in Fig.2b and d .Figure 3b shows the XRD pattern of the as homogenid ZM61alloy.It is clearly en that the peaks of the Mg 7Zn 3pha become weaker,and some peaks of the MgZn 2pha are detected,indicating that MgZn 2is precipitated during the Zn diffusion.
Microstructure of extruded and solution treated alloys
The preheating microstructures of ZM61alloy at 420u C for 90min and quenching in water is shown in Fig.4.
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1Extrusion and heat treatment schedule
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After homogenisation at 330u C for 2h (Fig.3),some of the Mg–Zn eutectic compounds in the grain boundary cannot dissolve completely into the matrix.The undissolved compounds,however,are found to dissolve into the matrix during the preheating of the ingots before extrusion,indicating a low thermal stability of the Mg–Zn compounds.
Microstructural changes after hot extrusion with air cooling and water quenching are shown in Fig.5a and b .
Owing to the deformation and the occurrence of dynamic recrystallisation during the hot extrusion process,equiaxed grain microstructure is formed,and the average grain size is y 9m m.The effect of preheating treatment at 420u C on the microstructure has already been studied.As is stated above,almost all the eutectic compounds are solutionid into the matrix after
a ,
b optical micrographs;
c ,
d SEM images
2Microstructures of a ,c as cast and b ,d as homogenid ZM61alloys
3X-ray diffraction patterns of a as cast and b as homo-genid ZM61alloys 4Preheating microstructure of ZM61alloy at 420u C for
90min and quenching in water
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homogenisation and preheating treatment at 420u C.Therefore,little cond pha particles are retained,and the complete dynamic recrystallisation happens during extrusion at 420u C,resulting in equiaxed grain.In addition,it is found that there is no difference on the microstructure of extruded ZM61alloys with different cooling methods including air cooling and water quenching.Figure 5c prents an SEM image of ZM61alloy after solution treatment at 420u C for 2h.The average grain size of the solution treated is y 25m m.The dynamic recrystallid grains of the investigated alloy grew up sharply,and all the broken particles dissolved into the matrix,resulting in a high Zn solid solution concentration.The pha evolution was further determined by XRD analysis.Figure 6shows XRD patterns of the extruded and solution treated samples.It is obvious that the diffraction patterns of extruded specimens mainly contain a -Mg matrix,Mn and MgZn 2pha.However,the weak diffraction patterns of the MgZn 2precipitates in the extruded alloy significantly broadened.According to the Scherrer formula,25peak broadening qualitatively illustrates a decrea in grain
5a ,b images (SEM)of ZM61alloy after extrusion with a air cooling and b water cooling and c SEM and d TEM images
of ZM61alloy after solution treated at 420u C for 2h
6X-ray diffraction patterns of ZM61alloy after extrusion
with a air cooling and b water cooling and c X-ray dif-fraction patterns of ZM61alloy after solution treated at 420u C for 2h
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size in the corresponding pha,implying that some nanosized MgZn 2precipitates form during the cooling after extrusion.After solution treatment at 420u C for 2h,the diffraction patterns show that the MgZn 2pha disappears,which suggests that a uniform solid–solution structure is produced,as shown in Fig.5c .In addition,the detailed microstructure inside the a -Mg after solution treatment is shown in Fig.5d .From the TEM image,only one spherical pha can be obrved.No other phas are detected after solution treatment.Bad on the XRD result and previous studies,19,20we can preliminarily conclude that the spherical pha is pure Mn particle.
Microstructure of aged alloys
Figure 7shows the SEM images of ZM61alloy in the T5(single aging)and T6(single aging)state.Since the SEM images of single aged alloys are very similar to tho of double aged alloys,one of them is displayed here.By comparing Figs.5a and 7a ,the alloy in the T5(single aging)state shows the similar microstructure to the extruded alloy.The average grain size of the T5aged alloy is y 11m m.As shown in Figs.5c and 7c ,there is little difference on microstructure between T4treated and T6aged alloys under the SEM obrvation.In fact,many nanosized Mg–Zn precipitates that are formed during the aging treatment are obrved in Fig.8c .Figure 7a and b shows the SEM microstructures of ZM61alloy in the T5(single aging after extrusion with air cooling and water quenching respectively)state.It can be found that there is no obvious change on microstructures under SEM between the two.It is well known that magnesium metal and its alloys have high thermal diffusivity,high thermal conductivity and high efficiency of heat relea.26The diameter of extruded bars is only 16mm,so the extruded alloys with air cooling and water quenching have same macrostruc-tures.In addition,the average grain size of the T5treated alloy is much finer than that of the T6treated alloy due to high temperature solution treatment in the latter.
Figure 8shows TEM images of ZM61alloy in the T5and T6treatment states.It is obrved that two kin
ds of precipitates formed during aging treatments.Bad on previous studies,27–30we can conclude that the two precipitates are rod-like b ’1and disc shaped b ’2phas respectively.The interface between b ’1and the matrix is coherent,while micoherent between b ’2and the matrix.b ’1phas,which formed as rods with their long axis parallel to the [0001]a direction of the a -Mg matrix,can act as a more enormous impediment to the motion of dislocations than b ’2formed as plates on (0001)a ,as reported in previous studies.27–30In all samples,the precipitates after double aging (Fig.8b and d )are much finer and more disperd than tho after single aging (Fig.8a and c ).It is becau the nanosized s,which formed during the preaging at 90u C for 24h,could provide more effective nuclei for b ’1pha during the cond aging.On the other hand,b ’1and b ’2precipitates in T5treated alloys are relatively less than tho in T6states.This is becau the Zn solid solubility in T6states is slight higher than that in T5states,and a few broken particles formed after extrusion are grown and retained after T5treatment.
In addition,it is obrved that many spherical phas are well disperd in the matrix,which are found in the
a ,
b T5(single aging after extrusion with a air cooling and b water quenching);
c T6(single aging)
7Images (SEM)of ZM61alloy at different single aging
treatments conditions
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