The microstructural evolution of a pre-alloyed AZ91magnesium alloy
powder through high-energy milling and subsequent isothermal annealing
M.A.Jabbari Taleghani a,b,n,J.M.Torralba a,b
a Department of Materials Science and Engineering,Universidad Carlos III de Madrid,Avenida de la Universidad30,28911Legane´s,Madrid,Spain
b IMDEA Materials Institute,C/Eri
c Kandel2,Tecnogetafe,28906Getafe,Madrid,Spain
a r t i c l e i n f o
Article history:
Received29October2012
Accepted9February2013
Available online17February2013
Keywords:
AZ91magnesium alloy
Pre-alloyed powder
High-energy milling
Isothermal annealing
Microstructural evolution
a b s t r a c t
This study analyzed the effect of high-energy milling(HEM)and subsequent isothermal annealing on
the microstructural characteristics of a pre-alloyed AZ91Mg alloy powder.To this end,the mentioned
powder was milled for14h using a horizontal attritor.Then,the mechanically milled powder was
isothermally annealed at temperatures ranging from2001C to5001C for1h up to4h in an
Ar atmosphere.HEM caused the b-Mg17Al12phase present in the microstructure of AZ91powder
particles to dissolve into their a-Mg solid solution matrix.In addition,the crystallite size of the a-Mg
phase decreased to25nm through HEM.In contrast to the annealing time,the temperature of
isothermal annealing had a significant effect on the microstructural features of the mechanically milled
AZ91powder.
&2013Elsevier B.V.All rights reserved.
1.Introduction
Mg alloys have received considerable attention as a structural
material in recent years due to their interesting properties,such
as low density,high strength-to-weight ratio,good damping
characteristics,superior machinability,and excellent castabil-
ity[1].Most of the research and development on Mg alloys has
been performed by the automotive industry;and die casting has
been the main manufacturing route for Mg products because of
the poor workability of Mg at room temperature,which is a result研讨学习环境
of its HCP crystal structure[2,3].
Although the market for Mg products continues to grow,many
opportunities remain untapped because of the low stiffness and
strength of Mg alloys[4].Due to the limited number of slip
systems of Mg and its correspondingly large Taylor factor,grain
refinement remarkably improves the mechanical properties of Mg
and its alloys.It is well known that thefine-grained Mg alloys
exhibit an interesting combination of high strength and high
八角楼上
ductility at room temperature and superplasticity at elevated
temperatures[5,6].
批判
理论
In the1960s,high-energy milling(HEM)wasfirst developed
by Benjamin and his co-workers to fabricate oxide-dispersion-
strengthened(ODS)nickel-based superalloys[7].In recent years,
this technique has been widely exploited for the production of
nanostructured materials.Grain sizes with nanometer dimen-
sions have been observed in almost all mechanically milled pure
metals,alloys,and intermetallics[8,9].
Although the processing of nanostructured light materials,
such as Al alloys and composites,by HEM has been the subject
of many studies[10–12],studies on the HEM of Mg and its alloys
are scarce.However,processing by HEM also has considerable
potential for Mg alloys becausefine-grained Mg alloys have a
unique combination of properties.Among all of the existing Mg
alloys,AZ91is the most widely used alloy in industry[13].黑龙江畜牧兽医网
Therefore,this study focused on the fabrication of a nanostruc-
tured AZ91Mg alloy by HEM.Furthermore,as the mechanically
milled powders are then consolidated by powder metallurgy(PM)
routes,such as cold pressing and sintering,powder extrusion,
powder forging,and powder rolling,the microstructural evolution
of the milled AZ91powder during subsequent consolidation
processes at high temperatures was also studied by isothermal
annealing at different temperatures.
2.Materials and methods
The raw material used for this study was a pre-alloyed Mg–Al–Zn
powder(Ecka Granules,Germany)with a chemical composition
equivalent to that of AZ91D alloy(8.8wt%Al,0.6wt%Zn,0.2wt%
Mn,0.03wt%Si,and the balance Mg).The above-mentioned powder
was milled in a horizontal attritor(CM01Simoloyer,ZOZ,Germany)
using the following milling parameters:ball-to-powder weight
ratio—20/1;ball diameter—5mm;ball material—AISI420stainless
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journal homepage:www.elsevier/locate/matlet
Materials Letters
0167-577X/$-see front matter&2013Elsevier B.V.All rights reserved.
/10.1016/j.matlet.2013.02.052
n Corresponding author at:IMDEA Materials Institute,C/Eric Kandel2,
林产化学与工业
Tecnogetafe,28906Getafe,Madrid,Spain.Tel.:þ34915493422; fax:þ34915503047.
E-mail addresses:mohammad.,
mo.jabbari@yahoo(M.A.J.Taleghani).
Materials Letters98(2013)182–185
steel;milling time—14h;milling speed—700rpm;and milling atmosphere—Ar.Stearic acid(2wt%)was also employed as the process control agent(PCA).Special precautions should be taken during unloading of the Mg-based powders after HEM.Immediately after milling,the milled powder is hot(or at least warm)and therefore the lid should not be immediately opened.Furthermore, after opening the
lid and prior to unloading,the milled powder must be exposed to atmosphere for a couple of hours.Otherwise,the powder may catchfire during unloading because of interaction with the oxygen.The isothermal annealing of the mechanically milled AZ91(MM AZ91)powder was then performed at annealing tem-peratures of2001C,3001C,4001C,and5001C for annealing times of up to4h in an Ar atmosphere.
The microstructural evolution and phase changes of AZ91 powder through HEM and subsequence isothermal annealing were studied by X-ray diffractometry(XRD).In addition,the crystallite size of the a-Mg solid solution matrix of MM AZ91 powder particles was determined from the broadening of XRD peaks using the Williamson–Hall method.
3.Results and discussion
Mechanical milling:The particles of AZ91powder possessed an irregular,flake-like morphology(Fig.1(a))with an average particle size of105m m and a very broad size distribution(D0.9–D0.1¼183m m).This morphology is typical of Mg-based powders produced by the mechanical grinding of casting ingots.
The MM AZ91powder(Fig.1(b))exhibited an equiaxed morphology,which implies that the employed m
illing 14h,was sufficient for the milling process to reach its steady state,in which there is a balance between the cold welding and fracturing of powder particles.The average particle size and size distribution(D0.9–D0.1)of MM AZ91powder were measured to be 37m m and100m m,respectively,suggesting that the milling process had a remarkable effect on the particle size characteristics of AZ91powder.
The XRD patterns of AZ91and MM AZ91powders are pre-sented in Fig.2.The microstructure of the AZ91powder particles was composed of the b-Mg17Al12precipitates and the a-Mg solid solution matrix.This structure is typical of AZ91castings[14]. HEM remarkably affected the intensities and widths of the diffraction peaks of the a-Mg phase,and the mentioned peaks became weaker and wider through HEM.This phenomenon can be attributed to the reduction in particle size,the refinement of crystallite size,and the enhancement of lattice strain,all pro-moted by the severe plastic deformation of the AZ91powder particles during HEM.Moreover,the diffraction peaks of the b-Mg17Al12phase are barely detectable in the XRD pattern of MM AZ91powder,which can be related to the dissolution of the b-Mg17Al12phase in the a-Mg matrix of the powder particles. Another possibility is that HEM caused the b-Mg17Al12phase to be refined into very small dispersoids distributed in the a-Mg phase,which are hardly detectable by XRD.The grain sizes of AZ91casting pr
oducts normally range between10m m and 150m m[14,15].The AZ91powder used for this study had been produced by the mechanical grinding of AZ91casting ingots. Considering the production method and the average particle size of the employed AZ91powder(105m m),it can be concluded that the grains of this powder should have had micrometer dimen-sions.The crystallite size and lattice strain of MM AZ91powder were calculated to be25nm and0.53%.A high lattice strain implies that the AZ91powder particles went through severe plastic deformation during the HEM process and,as a result, contained a high density of microstructural defects.The values obtained for crystallite size and lattice strain are in good agree-ment with those previously reported for high-energy milled Al-based powders[10,16].Various models have been proposed to describe the mechanism of formation of nanostructures by HEM[9].
Isothermal annealing:The XRD patterns of MM AZ91powder annealed for1h at2001C,3001C,4001C,and5001C are shown in Fig.3(a).The displacement of the a-Mg(101)diffraction peak through the annealing of MM AZ91powder at the above-mentioned temperatures is also illustrated in Fig.3(b).As shown in Fig.3,annealing at2001C for1h boosted the diffraction peaks of the b-Mg17Al12phase in the XRD pattern of MM AZ91
powder Fig.1.The morphologies of(a)AZ91and(b)MM AZ91
powders.
Fig.2.The XRD patterns of AZ91(dotted)and MM AZ91(solid)powders. M.A.J.Taleghani,J.M.Torralba/Materials Letters98(2013)182–185183
and caused the a -Mg (101)peak to shift to a lower diffraction angle,which can be attributed to the rejection of Al atoms by the supersaturated a -Mg solid solution matrix of MM AZ91powder particles and the formation and growth of b -Mg 17Al 12precipi-tates.However,as the annealing temperature increased from 2001C to 3001C,the intensities of the diffraction peaks of the b -Mg 17Al 12phase decreased,and the a -Mg (101)peak shifted nearly to its initial diffraction angle,which can be related to an increase in the solubility of Al atoms in the a -Mg phase at this temperature.At annealing temperatures of 4001C and 5001C,the diffraction peaks of the b -Mg 17Al 12phase disappeared,and the a -Mg (101)peak shifted to higher diffraction angles,showing the complete dissolution of the b -Mg 17Al 12phase in the a -Mg matrix of the AZ91powder particles.To dissolve the b -Mg 17Al 12pre-cipitates in the a -Mg matrix,the AZ91cast products are normally annealed and homogenized at 4151C for 12–24h [14].Never-theless,annealing times as short as 1h were enough to comple-tely dissolve the b -Mg 17Al 12phase in the a -Mg matrix of MM AZ91powder particles,which suggests a very high diffusion rate for Al atoms in the a -Mg phase.Moreover,the accelerated dissolution of b -Mg 17Al 12phase for MM AZ91powder can also be attributed to the fact that the b -Mg 17Al 12precip
itates dis-persed in the microstructure of MM AZ91powder particles are significantly smaller than those normally found in the micro-structure of AZ91cast products.
The XRD patterns of MM AZ91powder annealed at 3001C for 1–4h are illustrated in Fig.4(a),showing that all of the micro-structural changes for MM AZ91powder occurred in the first hour of annealing,after which no significant microstructural change
was detectable.This can be attributed to the high diffusion rates of alloying elements in the a -Mg solid solution matrix of MM AZ91powder particles,promoted by the high density of structural defects and grain boundaries present in the microstructure of MM AZ91powder particles.
The crystallite size of MM AZ91powder annealed at 3001C for 4h was determined to be 80nm (Fig.4(b)),suggesting that the nanostructured AZ91alloy processed by HEM had a good thermal stability.This favorable thermal stability could result from the homogenous distribution of nanosized Mg oxides and carbides in the structure of MM AZ91powder particles.The surfaces of Mg-based powders are covered with a thin,stable oxide layer of 3–5nm [17],which can be fragmented and introduced into the structure of powder particles by HEM.Moreover,Mg may react with PCA and form nanosized Mg carbides during HEM.LECO measurements confirmed that the oxygen and carbon co
ntent of AZ91powder increased from 0.015wt%and 0.012wt%,respec-tively,to 0.045wt%and 1.52wt%through mechanical milling.The growth of MM AZ91grains at high temperatures can then be hindered by the Mg oxide and carbide dispersoids.
4.Conclusions
This study investigated the morphological and microstructural evolution of a pre-alloyed AZ91powder through HEM and subsequent isothermal annealing.The milling process signifi-cantly changed the morphology and size distribution of the AZ91powder.In addition,the microstructure of the AZ91
powder
Fig.3.(a)The XRD patterns of MM AZ91powder annealed at different temperatures for 1h and (b)the displacement of the a -Mg (101)diffraction
peak.
Fig.4.(a)The XRD patterns and (b)the crystallite sizes of MM AZ91powder annealed at 3001C for different times.
M.A.J.Taleghani,J.M.Torralba /Materials Letters 98(2013)182–185
184
was remarkably refined by HEM.The annealing of MM AZ91 powder at temperatures less than or equal to3001C boosted the intensities of the diffraction peaks of the b-Mg17Al12phase. However,annealing at4001C for1h was sufficient for the complete dissolution of b-Mg17Al12precipitates in the a-Mg matrix of MM AZ91powder particles.The nanostructured AZ91 alloy processed by HEM showed a good thermal stability,retain-ing its crystallite size of less than100nm after the annealing of MM AZ91powder at3001C for4h.
Acknowledgment
The authors would like to thank the Comunidad de Madrid for theirfinancial support of this work through the ESTRUMAT Grant #S2009/MAT-1585.
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