The crystallography of continuous precipitates with a newlyobserved orientation relationship in an Mg–Al-based alloy

O. Zheng,a,b J.P. Zhou,a,b D.S. Zhao,a,b,* J.B. Wang,a,b R.H. Wang,a,b J.N. Gui,a,b

D.X. Xionga,b and Z.F. Sunc

aDepartment of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education,

Wuhan University, Wuhan 430072, People’s Republic of ChinabCenter for Electron Microscopy, Wuhan University, Wuhan 430072, People’s Republic of China

cCollege of Material Science and Engineering, Chongqing Institute of Technology, Chongqing 400050, People’s Republic of China

Received 20 December 2008; revised 11 January 2009; accepted 11 January 2009Available online 20 January 2009

A new orientation relationship (OR) between  c-Mg17Al12  precipitate and magnesium matrix is identified by the selected-areaelectron diffraction technique, together with the convergent beam electron diffraction method. The stereogram for the new orienta-tion relationship has been calculated and discussed along with the stereograms for the Pitsch–Schrader OR, the Burgers OR, thePotter OR, the Crawley OR, the Porter OR and the Gjonnes–O stmoe OR in the Mg–Al-based system. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords:  Magnesium alloys; Precipitation; Ageing; Transmission electron microscopy (TEM)

Body-centered cubic (bcc) and hexagonalclosed-packed (hcp) structures are both important crys-tal structures for metals and simple compounds, and inmany alloys both the matrix and the precipitate willhave these two structures [1,2]. In most previous studiesof hcp/bcc systems, the precipitate was the element cor-responding to hcp  [3,4]. For these hcp/bcc couples, fourmain orientation relationships (ORs) had been identi-fied, i.e. the Burgers, Potter, Pitsch–Schrader andRong–Dunlop ORs  [1–4].

Recently, there has been increased interest in thelightweight Mg–Al-based alloy Mg–9 wt.% Al–1 wt.%Zn–0.2 wt.% Mn (AZ91), particularly in the automotiveindustry, because of its good combination of castability,mechanical strength and ductility  [5–8]. In AZ91 alloy,with its additions of a small amount of Zn and Mn tothe Mg–Al system, the volume fraction of secondaryprecipitates increases and the corrosion resistance im-proves  [5]. According to the Mg–Al binary alloy phasediagram, the precipitation phase in AZ91 is equilibrium

phase  c-Mg17Al12, which has a complex bcc structure,while the matrix a-Mg has an hcp structure [9]. The typ-ical lattice parameter for the   c-Mg17Al12   phase is1.05438 nm (space group  I 43m) [10]. The average latticeparameter of the AZ91 alloy after homogenization treat-ment is   aa = 0.31694 nm and   ca = 0.51582 nm, accord-ing to Vegard’s law   [11]. During the ageing process of the Mg–Al-based alloys, the  c-Mg17Al12  phase precipi-tates out in two ways: discontinuous and continuousprecipitation. Discontinuous precipitation takes placeon grain boundaries and reveals the lamellar eutectica/c   structure with Burgers OR, i.e. (00 01)a//(101)c,½12 1 0a==½1 11c; it ceases relatively early in the precipi-tation process  [5,12,13]. Continuous precipitation takesplace in the remaining areas of the matrix, and revealscomplicated morphologies and six orientation relation-ships, as shown in  Table 1   [5,11,14–23]. According tothe literature   [5], continuous precipitation plays animportant role in improving age-hardening effects inMg–Al-based alloys.

This paper will present the investigations of transmis-sion electron microscopy (TEM) on the continuous pre-cipitate particles with a different orientation relationshipin an Mg–Al-based alloy.

An AZ91 alloy ingot was cut into bars and heat-trea-ted at 688 K for 24 h in order to dissolve the  c-Mg17Al12phase and achieve homogeneous aluminum distribution,

1359-6462/$ - see front matter    2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.scriptamat.2009.01.016

* Corresponding author. Address: Department of Physics and KeyLaboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, People’sRepublic of China. Tel.: +86 27 87669170; fax: +86 27 68752569;E-mail: dszhao@whu.edu.cn

 Available online at www.sciencedirect.com

Scripta Materialia 60 (2009) 791–794

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and then the bars were quenched in water at room tem-perature. We heat-treated the solutionized AZ91 bars ata temperature of 473 K for 8 h in a muffle furnace. Thebars were then cut into slices and mechanically thinnedto foils. Disks 3 mm in diameter were punched from thefoils and polished electrochemically. Finally, the speci-mens were ion-milled by a Gatan precision ion polishingsystem PIPS691 under conditions of 3.0 kV and an inci-dent angle of 4  for about 10 min in order to clean thesurface of the specimens. Conventional TEM observa-tion of the continuous precipitates was conducted usinga JEM-2010 (HT) (HT: high angle tilt) transmissionelectron microscope operated at a voltage of 200 kV.High-resolution transmission electron microscopy(HRTEM) and energy-dispersive spectroscopy analysisof the same specimen was carried out in a JEM-2010FEF (UHR) (FEF: field emission gun and in-columnX-type energy filter; UHR pole-piece: ultra-high resolu-tion with a Scherzer resolution 0.19 nm) transmissionelectron microscope with an energy-dispersive X-rayanalysis system, operated at a voltage of 200 kV.

By using the selected-area electron diffraction (SAED)method with the tilting technique, the reported orienta-tion relationships and morphologies of the continuousprecipitates have been reconfirmed as follows. The pre-dominant fraction of continuous precipitates is thin lath,with the primary habit plane parallel to the basal plane of the matrix (00 0 1)a, and possesses the Burgers OR withthe matrix   [23], i.e. (0001)a//(101)c,   ½12 1 0a==½1 11c.We have also observed the Pitsch–Schrader OR((0001)a//(101)c,   ½0 11 0a==½

1 0 1c;   ½211 0a==½0 1 0c)

among these thin lath-shaped precipitates  [23]. Anothertype, comprising only a small fraction, has the primarygrowth direction perpendicular to the basal plane(0001)a and has the Crawley OR, i.e. (0001)a//(111)c,½1 12 0a==½

2 1 1c  or  ½0 11 0a==½1 0 1c, as reported by

Crawley et al.  [15,16]. The morphology of the CrawleyOR precipitates was observed to be in the form of a hex-agonal prism-shaped rod [18–20]. The fourth type of con-tinuous precipitate possesses an orientation relationshipin the form of   ½0 0 0 1a==½1

5 1c;   ½0 11 0a==½1 0 1c, de-

noted as the Porter OR, and is even less common[5,17,18,20,21]. The Porter OR precipitates have the pri-mary growth direction lying at an angle of about 16 tothe normal of the (0001)a [5,17,18,20,21]. Due to smalldifferences between the Burgers OR and the Potter OR,the Potter OR has seldom been mentioned in Mg–Al-based alloy. Recently, however, we had observed contin-uous precipitate particles with the Potter OR in an AZ91alloy using HRTEM [23]. However, precipitate particleswith the Gjonnes–O  stmoe OR were not found in ourexperiment.

With the lattice parameters ac = 1.05438 nm for the  c-Mg17Al12phaseand aa = 0.31694 nm and ca = 0.51582 nmfor the matrix, we have calculated six stereographic pro- jections for the reported orientation relationships listedin Table 1, and these are shown in  Figure 1. In  Figure 1,the thin lines are the plane traces of the  a-Mg and thedirections marked by h1, h2, h3, h4, h5, h6, h7 and h8represent the   ½0 33 1;   ½0 33 2;   ½0 11 1;   ½0 22 3;   ½0 11 2;½1 12 1;   ½2 24 3  and  ½1 12 3  directions, respectively. InFigure 1, the thick lines are the plane traces ofthec phase,and the directions marked by B1, B2 and B3 correspondto ½1 0 0;   ½11 0  and  ½11 1, respectively.

According to the literature  [11,18,20,22], when the  clattice is rotated anticlockwise by about 5.8 around the[0001]a  direction from the exact Pitsch–Schrader OR(Fig. 1(a)) while retaining the [0001]a//[101]c condition,a quasi-invariant line will be produced and the near Bur-gers OR is then obtained (Fig. 1(b)). The existence of aquasi-invariant line represents low strain energy whenthe c phase particle precipitates from the matrix with thisOR [4]. The near Burgers OR is about 0.54 away from theexact Burgers OR [11,18,20,22]. According to the litera-ture [21], the previouslydetermined Burgers OR was actu-ally within a small angle range from the exact BurgersOR. According to Duly  [4], when the  c  lattice is furtherrotated clockwise for about 2  around the direction of ½12 1 0a  from the Burgers OR while retaining the condi-tion of ½12 1 0a==½1

1 1c, a quasi-invariant line will be pro-duced and the Potter OR is then obtained (Fig. 1(e)). Inaddition, when the c lattice is rotated clockwise for about35.8around the direction of  ½0 11 0a  while retaining thecondition of  ½0 11 0a==½

1 0 1c, a quasi-invariant line willbe produced. (The detailed model about a quasi-invariantline in the near Crawley OR will be discussed in anotherpaper.) The near Crawley OR is then obtained from theexact Pitsch–Schrader OR, and is about 0.4 away fromthe exact Crawley OR (Fig. 1(c)). According to Luoet al. [20], the Porter OR (Fig. 1(d)) has a  h1 1 2ic  pseu-do-twin relationship with the Crawley OR. Also, whenthe  c  lattice is rotated clockwise for about 35.3aroundthe direction of [0001]a while retaining the condition of ½0 0 0 1a==½1 0 1c   in   Figure 1(a), the Gjonnes–O stmoeOR is obtained, as shown in Figure 1(f).

In this study, a new orientation relationship of contin-uous precipitates (Fig. 2(a)) was observed in the AZ91 al-loy. By using the SAED method withthe tilting technique,it was confirmed that the new orientation relationship is inthe form of  ½0 0 0 1a==½

13 1c;  ð1 12 0Þa==ð3 0 3Þc.  Figure2(b) shows the SAED pattern along the direction of ½0 0 0 1a, including the matrix and particle A shown inFigure 2(a).Figure 2(c) is the simulatedelectrondiffractionpattern of  Figure 2(b). By comparing the stereogram for

Table 1.  The reported crystallographic orientation relationships and morphologies of the c-Mg17Al12 phase (bcc) with the matrix of  a-Mg (hcp) inthe Mg–Al system.

Name Pitsch–Schrader OR Burgers OR* Crawley OR Porter OR Potter OR Gjonnes–O stmoe OR

Crystallographyorientation relationship

½0 0 0 1a==½1 0 1c   ½0 0 0 1a==½1 0 1c   ½0 0 0 1a==½1 1 1c   ½0 0 0 1a==½15 1c   ½0 0 0 1a1:97 ½0 0 0 1a==½1 0 1c

½0 110a==½1 0 1c   ½12 1 0a==½1 11c   ½1 12 0a==½

2 1 1c   ½0 11 0a==½1 0 1c   from ½1 0 1a   ½211 0a==½1 21c

½211 0a==½0 1 0c   ð1 01 0Þa==ð1 2 1Þc   ½0 11 0a==½

1 0 1c   ½12 1 0a==½1 11cMorphology Lath-shaped Lath-shaped Rod/prism

shapedShort rodshaped

Platelets,rod shaped

 — 

References   [23]   [2,5,11,14–23] [5,15,16,18,20] [5,15,16,18,20]   [21,23] [21]

* Including the near Burgers OR. The near Burgers OR is about 0.54  away from the exact Burgers OR.

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this new orientation relationship (Fig. 2(e)) with the stere-ograms inFigure 1, it is deduced thatit is different from thereported orientation relationships. This new orientationrelationship can be obtained from the Pitsch–SchraderOR by rotating the c lattice in two steps: thec lattice isfirstrotated clockwise for 90 around the direction of  ½01 1 0awhile retaining the condition of   ½01 1 0a==½

1 0 1c   inFigure 1(a), and then the c lattice is rotated anticlockwisefor 25.3around the direction of  ½211 0a.

It was further determined from this new orientationrelationship of continuous precipitates in the AZ91 alloythat [0001]a  deviates by about 1.0   from   ½13 1c   andð1 12 0Þa  deviates by about 1.0   from   ð3 0 3Þc  by usingthe convergent beam Kikuchi line diffraction method.

According to the literature [1,2,5,21], the diversity of the crystallographic orientation relationships existing inMg–Al-based alloys is because the bcc/hcp couple in theMg–Al system is a high-mismatch system and does nothave coherent boundaries. In addition, for the develop-ment of an orientation relationship, a subsequent mod-

ification of the precipitation crystallography may occurduring growth, such as the Burgers OR and the new ori-entation relationship of continuous precipitates existingwithin a small angle range in the Mg–Al system  [24,25].

In summary, to fully understand the mechanical proper-ties and the age-hardening mechanism of the AZ91 magne-sium alloy, the morphologies and orientation relationshipsof the continuous precipitates have been analyzed byTEM. A new orientation relationship of continuous precip-itates in the AZ91 alloy heat-treatedat 473 K for 8 h was ob-served in the form of  ½0 0 0 1a==½

13 1c; ð1 12 0Þa==ð3 0 3Þc,with a small angle range.

This project was supported by National Nat-ural Science Foundation of China (Grant Nos.50771074 and 50571075).

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Figure 2.   (a) TEM bright-field image including a newly observedorientation relationship of a continuous precipitate particle (markedby A) in AZ91 magnesium alloy aged at 473 K for 8 h along the½0 0 0 1a  direction. (b) The SAED pattern for the particle A and thematrix along the  ½0 0 0 1a==½

13 1c  direction in (a). (c) The simulatedelectron diffraction pattern for (b). (d) Stereogram showing the newobserved orientation relationship in the Mg–Al system.

Figure 1.  Stereographic projections of six orientation relationships which have been observed in Mg–Al-based alloy. (a) Pitsch–Schrader OR, (b)Burgers OR, (c) Crawley OR, (d) Porter OR, (e) Potter OR, and (f) Gjonnes–O stmoe OR.

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