Hardening precipitation in a Mg–4Y–3RE alloy

Acta Materialia 51 (2003) 5335–5348www.actamat-journals.com

Hardening precipitation in a Mg–4Y–3RE alloy

C. Antion∗, P. Donnadieu, F. Perrard, A. Deschamps, C. Tassin, A. PischLaboratoire de Metallurgie et de Physico-Chimie Metallurgiques, Domaine Universitaire, 1130, rue de la Piscine, BP75 38402

Saint Martin d’Heres cedex, France

Received 25 April 2003; received in revised form 25 April 2003; accepted 13 July 2003

Abstract

Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150°C have been studied using TEM and smallangle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates,and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size andvolume fraction. Precipitation at 150°C involves formation of DO19 monoplanar precipitates, which further developinto theβ� andβ� phases having platelet and globular morphologies, respectively. TEM observations on samples agedat 150°C reveal the formation mechanism of the bco-β� structure by the ordering of monoplanar DO19-β� precipitates.Additional examinations at 250°C revealed the DO19-β�→bco-β� transformation, as well asβ1 precipitates. Estimationof the volume fraction deduced from SAXS is discussed on the basis of the TEM results. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Ageing; Magnesium alloys; Rare earth; TEM; SAXS

1. Introduction

The need for ever-lighter structures in transportindustries has indicated magnesium as a goodpotential element for many structural applications.When high mechanical properties are also required,magnesium–rare earth (Mg–RE) alloys appear asvery good candidates. They reach high specificstrength, creep resistance, good castability, andcorrosion resistance up to a temperature of 250°C.In these Mg alloys, strengthening results from adense fine scale precipitation, which remains quite

∗ Corresponding author. Tel.:+33-476-82-67-54; fax:+33-476-82-67-45.

E-mail address: [email protected] (C. Antion).

1359-6454/$30.00 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S1359-6454(03)00391-4

stable up to 5000 h in the 200–250°C range.Among the Mg–RE alloys, the Mg–Y–Nd systemprovides a combination of mechanical and cor-rosion properties, which allows the development ofcommercial alloys: WE43 (4wt.% Y, 3.3wt.% RE,0.5wt.% Zr) and WE54 (5wt.% Y, 3.3wt.% RE,0.5wt.% Zr), where RE consists of mischmetal(essentially Nd).

Mg–Y–Nd alloys precipitate from the solid sol-ution according to the sequence of phasesβ�→β�→β.

The β� phase, which is metastable and fullycoherent with the matrix, has been identified as anordered DO19 type structure[1–6]. The DO19 struc-ture results from a chemical ordering of magnes-ium and RE atoms. Chemical ordering of the atomsin the DO19 structure leads to a double hexagonal

5336 C. Antion et al. / Acta Materialia 51 (2003) 5335–5348

cell (a = 2aMg, c = cMg) [1]. It has been reported[1,4,7] that the DO19 phase in Mg–RE alloys hasa Mg3X type crystallographic structure (X = RE).In-situ energy dispersive X-ray analysis has beenused to determine the composition of the β� pre-cipitates, and yielded a Y:Nd ratio of 6:1 in WE54alloys [4,6]. Thus, the β� composition may beMg3(Y0.85Nd0.15) as reported in [4]. According to[1,4,7,8], the β� phase forms in the first stages ofprecipitation at low temperature (150–200 °C) andis expected to have a platelet shape. Most studiesreport that the β� precipitates in the Mg–Y–Nd ter-nary alloy lie only in the {1 �1 0 0} magnesiumplanes [4–7] in agreement with β� phases in otherMg–RE alloys. However, Polmear [1] mentionedtwo possibilities for habit planes, {1 �1 0 0} and{1 1 �2 0} planes. In addition, the formation ofquasi-DO19 and DO19 precipitates in binary Mg–Nd alloys lying respectively in {1 �1 0 0} and{1 1 �2 0} Mg planes has also been reported [8].

The intermediate β� phase, which forms duringageing from 200 to 250 °C [1,4,7,8], is also meta-stable. Its structure is identified as orthorhombic(a = 0.640 nm, b = 2.223 nm, c = 0.521 nm) [2,4,6]but the Bravais lattice remains unclear: basecentred orthorhombic [2,3] as well as body centredorthorhombic [4–6] are reported. In the following,the β� phase structure will be referred as bco with-out specific mention of the Bravais mode. Accord-ing to [1–6], the β� phase forms plates parallel tothe {1 0 �1 0} planes. The composition of thismetastable phase is also cause of disagreement.According to [1,2,4], a Mg12NdY composition inWE54 alloys with equal proportion of yttrium andneodymium is found instead of a Y:Nd of 2:3, con-sistent with a Mg24Y2Nd3 composition [6]. Such acomposition dispersion is usually attributed to thedifference in nominal alloy composition since both[5] and [6] are in agreement with a Mg12NdY com-position of the β� phase in WE43 alloys.

The precipitation sequence ends with the equi-librium β phase, which develops in an fcc structure(a = 2.223 nm) after ageing for a long period oftime at 250 °C and higher temperatures [1–5]. Hereagain, the composition of the β phase depends onthe alloy composition. According to [2,3], the βphase has a Mg14Nd2Y composition in WE54alloys, whereas [4,6] propose an equal proportion

of neodymium and yttrium with a Mg12NdY com-position. In WE43 alloys, a Mg14Nd2Y compo-sition for the β phase is mentioned [5,6].

Regarding morphology, platelets are usuallyobserved for β� precipitates after 150 °C ageing,while heat treatment at higher temperature allowsthe formation of β� phase with different morpho-logies. For instance in WE54 alloys, Nie andMuddle [2,3] report the growth after a 250 °C age-ing of β� precipitates with a globular shape. Theyhave also pointed out the existence of anotherphase within the precipitation sequence: the β1

phase (fcc, a = 0.74 nm), which appears duringageing at 250 °C in WE54 alloys. The β1 precipi-tates form platelets in the {1 0 �1 0} magnesiumplanes connected to the globular β� phases [2].Under further heat treatment, the β1 phase trans-forms into the equilibrium β phase.

The present study aims at quantitatively study-ing the earlier stages of precipitation, as well asclarifying the structure and morphologies of thephases involved in the precipitation sequence. Thealloy of interest here is WE43 alloy (Mg–4wt.%Y–3wt.% Nd–0.4wt.% Zr). As the same phasesprecipitate at 150 and 250 °C, the 150 °C precipi-tation sequence will be studied in more details.Particular attention will also be given to the mor-phologies since, as suggested by the work of Nieand Muddle [2], the morphologies as well as thestructural relationships seem to play an importantrole in the formation of strongly connected net-work of precipitates. Transmission electronmicroscopy (TEM) will be carried out to study thestructure, morphology and relationships betweenthe β� and β� phases. Since TEM especially in itshigh-resolution mode is a very local technique, amore global approach by small angle X-ray scat-tering (SAXS) has been also used. SAXS is a veryefficient technique to characterize fine scale pre-cipitation in terms of average particle size (Guinierradius) and volume fractions [9]. Since the compo-sition of the precipitates is unknown, a full analysisof the integrated scattered intensity cannot be achi-eved though the complementary information givenby TEM allows establishment of the validitydomain of the composition hypothesis. However,it will be possible to follow the relative size evol-ution as a function of heat treatment.

5337C. Antion et al. / Acta Materialia 51 (2003) 5335–5348

2. Experimental procedures

WE43 alloys in as cast conditions were solutiontreated at 525 °C for 8 h, quenched in water, andaged at 150 °C and 250 °C for times ranging,respectively, from 864 to 1896 h and 1 to 16 h.

Specimens for TEM were ion milled using thePrecision Ion Polishing System (GATAN 691).

Microstructural examination was performed in aJEOL 3010 transmission electron microscopeoperating at 300 kV for high resolution electronmicroscopy (HREM) imaging; the point resolutionwas 0.25 nm. As precipitation occurs in the mag-nesium prismatic planes, the zone axes of interestwere [0 0 0 1] and [�2 1 1 0].

SAXS experiments have been carried out on thebeam line ID01 at ESRF (European SynchrotronRadiation Facility, Grenoble). The samples are 90µm thick; the X-ray wavelength was set at l =1.745 A; the position of the 2D-CCD detectorallowed to record the diffuse intensity for wavevectors q ranging from 0.001 to 0.13 A�1, whichallows the measurement of particle sizes rangingfrom 0.5 to 10 nm. Spectra were corrected forbackground, fluorescence, and Laue backgroundeffects [10].

3. Experimental results

3.1. Small angle X-ray scattering data analysis

SAXS measurements have been carried out onWE43 alloys in four different thermal states: sam-ple solution treated and quenched (ST), solutiontreated, quenched and aged for 864, 1324 and 1896h at 150 °C. The Mg–Y–Nd system provides a highenough electron density contrast between precipi-tates and the matrix to reach an efficient detectionof precipitation. The ST sample stands here as areference since it corresponds to the supersaturatedMg solid solution without any precipitation.

The data are presented as normalized intensity Ivs. the diffusion vector q. An estimate of the par-ticle size (Guinier radius) can be obtained if theGuinier approximation [I�exp(�q2R2

g /3)] is foundto be valid in a sufficient q-range [9,10]. This iseasily checked on a lnI vs. q2 or Guinier plot. In

the case of complicated precipitate morphologies,this approximation is often poorly satisfied. Then,it is best to estimate the particle size using themaximum in the Iq2 vs. q plot (see Fig. 1). In theGuinier approximation, the diffusion vector qmax

corresponding to this maximum is qmax = √3/Rg.Moreover, the area under the Iq2 curve, the so-called integrated intensity Qo, is independent of theparticle size and depends only on the particle vol-ume fraction and electron density contrast betweenthe precipitate and the matrix:

Qo � ��

0

Iq2dq � 2p2fv(1�fv)(�r)2

where

�r � rp�rm�Zp

�p�

Zm

�m

fv is the volume fraction, rp, Zp and �p (resp. rm,Zm and �m) are the average electron density,atomic number and atomic volume of the precipi-tate (resp. matrix).

In the case of the presence of multiple precipi-tate families, Qo will correspond to the total vol-ume fraction, and several maxima may be encoun-tered, corresponding to the different families.

As the composition of the precipitates isunknown, a calculation of the rs term has herebeen made assuming a DO19 type precipitationwith a composition of Mg3Y0.85Nd0.15, in agree-ment with the analysis given in [4]. The validityof this hypothesis will be discussed on the basis ofthe TEM results in Section 3.1. Under the DO19

composition hypothesis, it is then possible to fol-low the evolution of volume fraction between thedifferent samples with increasing ageing time, bycomparing their integrated intensity. The ST stateIq2(q) curve shows a small bump for high value ofq, revealing the presence of very fine objectswithin the magnesium matrix. Their average radiusis estimated from the curve to about 0.7 nm. Theprecipitate volume fraction is around 1.5%.According to the relative curves areas, the behav-iour of the three aged samples indicates clearly pre-cipitation with an important volume fraction ascompared with the ST state.

Table 1 shows the Guinier radius and volume


Fig. 1. Iq2 vs. q SAXS profiles (ID01 data) for several ageing time at 150 °C on WE43 samples; the maxima for each curve areindicated by the arrows. The reference ST state (solution treated and quenched) is also reported.

Table 1Guinier radius and volume fractions data obtained from assessment of Iq2 vs. q curves

ST state ST + 864 h at ST + 1324 h at ST + 1896 h at150 °C 150 °C 150 °C

Guinier radius from SAXS measurements (nm) 0.7 1.4 3 31 1.2

Volume fraction from SAXS measurements with the 1.5 5.4 6.7 10.7composition hypothesis Mg3Y0.85Nd0.15 (%)

fractions obtained by SAXS for the different ther-mal treatments. Estimation—with the samehypothesis of particle composition for each state—gives a volume fraction of 5.4% at 864 h, increas-ing to 6.7% at 1324 h and reaching 10.7% at 1896h, confirming continuous precipitation from themagnesium solid solution.

The Iq2 curve corresponding to the 864 h agedsample shows one well defined maximum, attestingone characteristic dimension for the diffusing par-ticles. Nevertheless, the peak is rather wide, whichindicates a rather large particle size distribution.The average size of particles (Guinier radius) canbe estimated to be 1.4 nm. However, it is possiblethat another characteristic dimension is hidden inthe spread of the curve.

With increasing ageing times at 1324 and 1896h, curves are changing, revealing the presence oftwo peaks which cannot be separated, leading to a

difficult evaluation of their respective precipitateradius. However, in each case, the second peakcorresponds to a characteristic dimension, signifi-cantly smaller than the previous one.

The largest characteristic radius of particles isestimated to be 3 nm for both 1324 and 1896 hstates, whereas the smallest characteristic particledimension shifts from 1 nm at 1324 h to 1.2 nmat 1896 h. Two interpretations can be drawn fromthe two characteristic dimensions.

(1) First, the two dimensions correspond to onefamily of particles. This would suggest a mod-erate shape factor for particles. The fact thatonly the smallest of both dimensions evolveswith increasing ageing time shows that precipi-tates thicken slightly between the two differ-ent stages.

(2) Second, the two characteristic dimensions


come from two families of precipitates havingboth one characteristic dimension. In this case,the larger dimension may correspond to par-ticles still existing in the 864 h stage thatwould have coarsened; the second family, witha smaller characteristic size could be attributedto another family of particles, that may existat 864 h and that has slightly thickenedbetween the 1324 h stage and the 1896 h stage.

In both assumptions, no strong coarsening isdetected between 1324 and 1896 h, whereas vol-ume fraction strongly increases (with the hypoth-esis of a constant precipitate composition Mg3X).

So, the data extracted from the SAXS measure-ments on WE43 samples can be summed up as fol-lows:

� Very fine precipitation occurs in the WE43alloys solution treated and quenched; they maybe GP zones or solute clusters. Their volumefraction is small (1.5%).

� Precipitation of particles whose size is about 1.4nm occurs in sample aged 864 h at 150 °C.Their volume fraction is about 5.4%.

� Precipitation changes when ageing at 150 °C at1324 and 1896 h, with the presence of twocharacteristic sizes. They can be attributedeither to two characteristic dimensions of oneset of particles, revealing a moderate shape fac-tor, either to two distinct families of precipitates.

� The volume fraction of precipitates keepsincreasing from earliest ageing time (864 h) tothe 1896 h ageing state, reaching a high valueof 10%.

This average information on size and volumefraction obtained by SAXS will be complementedby TEM, which will provide information on crystalstructure, morphology and distribution of precipi-tates, and will help to estimate the validity of thehypothesis of composition made for SAXSinterpretation.

3.2. TEM results

3.2.1. Ageing for 864 h at 150 °CFigs. 2a,b shows typical electron diffraction pat-

terns of the sample aged for 864 h at 150 °C

Fig. 2. Diffraction patterns recorded from WE43 sample aged864 h at 150 °C. The beam direction is: (a) parallel to the[�2 1 1 0] zone axes and (b) parallel to the [0 0 0 1] zone axes.

recorded along the [1 1 �2 0] and [0 0 0 1]Mg zoneaxes. Both patterns present diffuse spots at half dis-tance of the (1 �1 0 0) Mg reflections. Such dif-fuse features are typical in Mg–RE alloys of theformation of a DO19 structure phase. Since the dif-fuse spots are narrow along the [0 0 0 1]Mg axis andquite diffuse in the perpendicular direction, thecorresponding microstructure is expected to be onthe nanometre scale or below in the basal Mg planebut well developed parallel to the [0 0 0 1]Mg axis.

HREM along the [0 0 0 1] zone axis (Fig. 3a)reveals numerous platelet precipitates with lengthsaround 1.5 nm. These platelets are distributed


Fig. 3. HREM images of WE43 sample aged 864 h at 150 °Crecorded with a beam parallel to [0 0 0 1]Mg. (a) Global viewshowing very fine platelets with the DO19 structure. (b) Detailof a (1 1 �2 0) precipitate showing the double hexagonal DO19

cell. (c) Detail of stacking of (1 �1 0 0) monoatomic layers;white arrows show the two ordered atomic planes.

homogeneously and are coherent with the Mgmatrix. Moreover, in many places, a hexagonalsupercell clearly appears, which is consistent withthe formation of the DO19 structure.

We have identified two types of microstructureswhich can be schematically described as follows:(i) platelets lying approximately in the (1 1 �2 0)planes, the DO19 double hexagonal cell can berecognized within these platelets (Fig. 3b); (ii)

monoatomic layer lying in the (1 �1 0 0) planeshowing the contrast consistent with a DO19 typeordering (Fig. 3c). As illustrated in Fig. 3a, thesemonoatomic layers are frequently observed asbeing grouped by two layers spaced by three Mg(1 �1 0 0) planes. It should be noticed thatalthough the groups of two layers are quite easilyidentified, atomic organization within the mono-planar precipitates might be quite ill-defined insome places (Fig. 3c).

3.2.2. Ageing for 1324 h at 150 °CAfter ageing at 150 °C for 1324 h, the electron

diffraction patterns are similar to the ones obtainedafter 864 h ageing (see previous section). Though,according to HREM (Fig. 4a), noticeable particlegrowth has occurred, the microstructures can stillbe described in terms of two precipitate families.

(1) Precipitates lying in the (1 1 �2 0) planes (Fig.4b) are thicker than after the 864 h ageing (sizearound 2.5 nm). Fast Fourier transforms (FFT)carried out on these precipitates image (insertin Fig. 4b) are consistent with the DO19 struc-ture.

(2) The (1 �1 0 0) monolayer with a DO19 typeordering can still be observed forming largerstacking than at 864 h ageing ones (4.5 nm).A close examination of HREM image precipi-tates (Fig. 4c) indicates that in these stacking,the second layer is always translated withrespect to the first one by a Mg period alongthe (1 �1 0 0) direction. Besides, the DO19

type (1 1 0 0) monolayers are always separatedby three Mg planes. These features of theHREM image are summarized by the sche-matic representation given in Fig. 4d. Theresulting structure strongly reminds of the bcostructure of the β� phase (a = 2aMg = 0.64nm, b = 4aMg√3 = 2.22 nm, c = cMg = 0.52nm). According to the present observation, abody centred structure would be expected forthe orthorhombic β� phase, as it would gener-ate smaller elastic distortion than a basecentred structure.

In addition to the two families mentioned above,there are some monolayer precipitates with a DO19


Fig. 4. HREM images of WE43 sample aged for 1324 h at 150 °C recorded with a beam parallel to [0 0 0 1]Mg. (a) Global viewshowing platelets and stacking of monoplanar DO19 precipitates. (b) Detail of a (1 1 �2 0) precipitate having the DO19 structure asshown by its FFT pattern in insert. (c) Detail of ordered stacking of (1 �1 0 0) monoatomic layers. The resulting structure correspondsto the bco β� phase as shown by its FFT pattern in insert. (d) HREM schematic representation of monoplanar ordered stacking. Thedark spheres correspond to magnesium atoms whereas the light spheres correspond to solute atoms. Note that the magnesium compac-ity is not respected in order to clarify the position of the atoms.

type contrast lying in the (1 �1 0 0) plane as illus-trated by Fig. 5. However, such monolayers areless frequent than the groups of several layers.

Examination of the distribution of precipitatesreveals an equal proportion for both families. No

value of density and volume fraction could beextracted because the small precipitate volume andthe number of precipitates images were not con-sidered as significant.


Fig. 5. HREM image of a monoplanar precipitate obtained insample aged at 150 °C for 1324 h. Solute atom positions withinthe monoplanar precipitate are given in the schematic represen-tation of the insert: the monoatomic layer can be described bya chemical ordering of the solute atoms among the magnesiummatrix. The light spheres represent the solute atoms (yttrium,neodymium) after a DO19 type ordering.

3.2.3. Ageing for 1896 h at 150 °CAfter 1896 h ageing at 150 °C, the electron dif-

fraction pattern still shows the half distance streakstypical of DO19 type precipitates (Fig. 6). How-ever, a close examination reveals diffuse streaks at1/4 distance of the (1 �1 0 0)Mg reflection. Such afeature indicates the formation of the β� phase withthe bco structure.

In the HREM image in Fig. 7, the β� phaseprecipitates are easily identified since they formprecipitates with a globular section in the basalMg-plane. FFTs made on these so-called globular

Fig. 6. Diffraction patterns of sample aged at 150 °C for 1896h (beam direction parallel to [0 0 0 1] zone axes).

Fig. 7. HREM image of sample aged for 1896 h at 150 °Crecorded with a beam direction parallel to �0 0 0 1�Mg as wellas FTT pattern performed on globular precipitates revealing abco structure and FTT pattern performed on a plate-shaped pre-cipitate revealing a DO19 structure.

precipitate are consistent with the bco-β� structure(see insert). The β� phase precipitates are approxi-mately 4 nm large. The globular precipitatescoexist with the platelets (platelet section around8 nm long, 3 nm large) lying in the (1 1 �2 0)planes. FFTs on these platelets indicate the DO19

structure associated to the β� phase. Here again,the two families are present in equal proportioninside the distribution of precipitates.

3.2.4. Precipitation sequence at 150 °Caccording to TEM

Regarding the GP zones, no particular trace wasdetected either on diffraction patterns or on theHREM image. Because of the oxide pollution,TEM is certainly not the most appropriate tech-nique to show evidence of GP zones. The TEMresults are rather limited in terms of earlier stagesof the β� and β� precipitations.

For summarizing, the present TEM study leadsto the following description for precipitation at150 °C:

1. Early stages of precipitation: A DO19 typeordering in the solid solution leads to the forma-tion of very fine precipitates in the (1 �1 0 0)and (1 1 �2 0) Mg planes. Monoplanar


(1 �1 0 0) ordered precipitates tend to formstackings of two DO19 ordered planes separatedby three Mg planes.

2. Formation of the b� and b� phases: During age-ing, the growth of DO19 type (1 �1 0 0) mono-planar layer appears limited, while the organiza-tion between planes rather proceeds. From thisordering, a long period structure results: a bcostructure consistent with the β� phase one. Thisorganization finally leads to the globular β� pre-cipitate. Simultaneously, with the formation ofthe β� phase, the DO19 (1 1 �2 0) almost mono-planar platelets grow, forming the DO19 β�plate-shaped precipitates in (1 1 �2 0) Mgplanes.

The first stage DO19 precipitates appear as pre-cursors for further precipitation. Depending on thehabit planes, these DO19 precursors give eitherDO19 platelets or bco globular precipitates. Finally,WE43 samples overaged at 150 °C contain twofamilies of precipitates of a similar proportion.Both phases β� and β� have grown with morpho-logies inherited from their mode of formation: theβ� phase with the DO19 structure forms plateletslying in a plane parallel to (1 1 �2 0), the β� phasewith the bco structure has a globular shaperesulting from a limited growth along (1 1 0 0) anda strong tendency to thicken by ordering of(1 �1 0 0) DO19 planes.

3.2.5. Transformation β�→β� at 250 °CAfter a 250 °C ageing for 2 h (resp. 4 h), a

strongly connected microstructure consisting ofglobular precipitates and platelets is observed (Fig.8a (resp. 8b)). Globular precipitates have coars-ened compared with 150 °C ageing stages; theirsize varies between 5 and 10 nm. Platelets are 3nm wide and their length varies from 8 to 15 nm.According to the FFTs (see inserts in Fig. 8b), theglobular precipitates are identified as the β� phasewhile the platelet structure depends on the age-ing time.

After ageing for less than 2 h, the platelet pre-cipitates are easily identified as DO19 β� phase.However, an interesting observation has been madeon the platelets at the 2 h ageing. Indeed, HREMexamination of the platelets after 2 h ageing time

Fig. 8. HREM images of samples aged at 250 °C (a) during2 h, (b) during 4 h. (c) Global diffraction pattern of WE43 alloyaged 4 h at 250 °C recorded with beam parallel to [0 0 0 1]Mg.

(Fig. 9) reveals that the DO19 platelets are turninginto the β� phase. As illustrated by the inserts inFig. 9, the continuous change can be evidenced byFFTs taken in different places along the platelet:the precipitate has the DO19 structure at one endand the bco structure at the other end. Note that inorder to rule out a possible HREM contrast arte-


Fig. 9. HREM image of a platelet precipitate in a sample agedfor 2 h at 250 °C and the corresponding FFT patterns madealong the precipitate. The high part corresponds to bco structurewhereas the down part corresponds to the DO19 structure.

fact, it was checked whether defocus or thicknessvariations could produce images similar to theDO19 structure ones on the bco β� phase.

After ageing for longer than 4 h at 250 °C, FFT(insert in Fig. 8b) shows that the platelets now havethe bco β� structure. This is validated by diffractionpatterns such as seen in Fig. 8c. This complex dif-fraction pattern can be fully interpreted by theoccurrence of bco-β� phase precipitates, which arepresent under the three variants allowed by thesymmetry.

3.2.6. Overageing at 250 °CIn a final step, the 16 h ageing at 250 °C has

been examined because of the associated high levelmechanical properties observed for this so-calledT6 conditions and to check the possibility of thepresence of β1 precipitates found by Nie andMuddle [2,3] in WE54 alloys. The TEM image(Fig. 10) also reveals the presence of β1 precipi-tates with platelet morphology. Detailed examin-ation shows that they lie in the (1 �1 0 0) planesand always end by facets parallel to (1 �1 0 0)plane, which are at both extremities in contact withglobular β� particles. Such a connected microstruc-ture has been reported in WE54 alloys [3]. Smallerprecipitates, both plate-shaped and globular, arealso bco-β� phase, formed previously at 250 °C.

4. Discussion

4.1. Complementary SAXS and TEM study ofearly precipitation stages

4.1.1. Solution-treated sampleAccording to the SAXS data analysis, the ST

state is characterized by precipitates having a

Fig. 10. Microstructure of sampleT6 WE43 with a beam par-allel to [0 0 0 1]Mg.


Guinier radius of 0.7 nm and a 1.5% volume frac-tion with the DO19 hypothesis. No precipitation hasever been reported for WE43 alloys in the solutiontreated and quenched state, even in terms of GPzones. However, the size we measured is actuallyconsistent with the GP zones detected in a TEMstudy by Pike and Noble [7] in Mg–Nd alloys.Besides, this size is reminiscent of the cell para-meter of the DO19 structure (a = 0.642 nm, c =0.521 nm), which suggests Guinier zones con-sisting of a DO19 ordered cluster. In such a case,the DO19 hypothesis was quite relevant, the volumefraction estimated by SAXS is then reliable.Though GP zones are the typical first stage of hard-ening precipitation in numerous alloys, their for-mation in Mg alloys is not well documented. InMg–Nd alloys, the exact initial exposure con-ditions for GP zone formation are not established;a reasonable assumption would be that they formwhere there is a local supersaturation of neodym-ium [7]. Indeed, Mg–3wt.% Nd alloys are reportedto form GP zones whereas their existence in Mg–Yand Mg–Y–Nd alloys remains unclear. However,Lorimer et al. [4] detected their presence after verylong ageing in a WE43 alloy. These GP zonesshould be disk-shaped, lying in the Mg basalplanes and giving rise to specific streaks in[-2 1 1 0]Mg diffraction pattern. The present SAXSresults are consistent with the description given byPike and Noble, i.e. needle-like GP zones parallelto the c axis and with a diameter of 0.7 nm thoughthe Lorimer et al. description cannot be discarded.

4.1.2. Ageing for 864 h at 150 °CThere is good agreement between SAXS and

TEM study for WE43 sample aged at 150 °C for864 h. Both reveal evidence of precipitation withan average size of 1.5 nm. The TEM study indi-cates that the precipitates are made of ordering ofatoms and belong to the DO19 structure, whichvalidates the SAXS analysis presented in Table 1for the 864 h ageing. Therefore, the value of 5.4%for the volume fraction obtained by SAXS analysisis reliable. According to the nominal compositionof the alloy and still considering the relevant Mg3Xstructure for potential clusters, the maximal vol-ume fraction of precipitates available from solutecontent is 6.5%. A comparison of both values

shows that some quantity of solute elements is stilldissolved in the magnesium solid solution and isavailable for further precipitation. The volumefraction cannot be discussed with TEM imagesbecause of the distribution of precipitate and alsoof the large errors due to the small size of precipi-tates.

4.1.3. Samples aged for 1324 and 1896 h at 150°C

According to TEM, the precipitation at 1324 and1896 h cannot be described by a unique compo-sition since the bco phase, which appears at 1324h, has a composition that strongly departs from theDO19 one (Mg3X against Mg12NdY). Since theDO19 and bco phases coexist, the SAXS data can-not be analysed as presented in Table 1 where cal-culations of the volume fractions are derived,assuming a single composition corresponding toMg3X. If the hypothesis of the Mg12NdY compo-sition is made, then calculations of the volumefractions from the SAXS measurements lead tohigher values of fv. This explains why the volumefractions are larger than the maximal volume frac-tion corresponding to solubility of rare earths inmagnesium with the assumption of one compo-sition.

Although precipitate distribution is more com-plex than one family of particles with a single crys-tal structure, the data extracted from SAXSmeasurements give useful information. Evaluationsof Guinier radius (3 nm) fit quite well with whathas been determined by TEM analysis (2.5 and 4.5nm). As far as volume fractions are concerned, theSAXS results are also relevant if we comparedthem with theoretical values of volume fractionsthat can be expected from the nominal compositionof the alloy and several hypotheses of crystal struc-tures. Indeed, we have estimated the maximal vol-ume fraction under two extreme assumptions ofphases: a bco one and an equal proportion of DO19

and bco phases. In the first case, the volume frac-tion cannot exceed 11.7%, whereas in the case ofmixing precipitation of DO19 and bco phases, thevolume fraction cannot exceed 9%. These figuresare in good agreement with SAXS-determinedvalues. Actually, a remarkable aspect of precipi-tation here is that the precipitates which grow after


long ageing (bco-β� phase) have a lower solutecontent than the first ones. So, the bco phase pre-cipitation is able to lead to higher volume fractionthan the DO19 one.

4.2. Chemical ordering and precipitationsequence at 150 °C

Regarding knowledge of precipitation in Mg–RE alloys, our work reconciles different points ofview. The formation of β� phase with a DO19 struc-ture is expected to occur in the (1 �1 0 0) and(1 1 �2 0) planes [1] though only precipitation inthe (1 �1 0 0) plane is reported. Nevertheless, inthe earliest stages, precipitates of unknown struc-ture in the (1 1 �2 0) plane was mentioned [2,3].Regarding the β� phase, though apparentlyresulting from homogeneous precipitation, it dis-plays several morphologies: globular precipitatesas well platelets [2,3]. Actually, the present workprovides an explanation for these features. Wehave shown: (i) a DO19-ordering occurring in boththe (1 �1 0 0) and (1 1 �2 0) planes, (ii) theordering of the (1 �1 0 0) DO19 planes, whichleads to the formation of the bco-β� structure witha globular morphology. Thus, our observationsprovide an origin for globular morphology of β�phase formerly observed by Nie and Muddle [3]in the WE54 alloy, (iii) the formation of the β�phase by thickening of monoplanar precipitates inthe (1 1 �2 0) magnesium planes, (iv) the trans-formation of β� phase forming the (1 1 �2 0) plate-lets in the β� phase with the same morphology. Fol-lowing this precipitation scheme, the bco β� phasewill display two different morphologies: globularor platelets depending on the formation mode. Theunknown (1 1 �2 0) platelet precipitates men-tioned by [3] are likely to be the bco phaseresulting from the transformation of the (1 1 �2 0)DO19 platelets.

The present TEM study points out a strongstructural relationship between the β� and β�phases: they both derive from the chemicalordering on the hexagonal Mg lattice. Chemicalordering, which was already mentioned as a driv-ing force for the formation of precipitation in Mg–RE alloys [11], seems strongly involved in the pre-cipitation sequence. Indeed, some authors interpret

very small structures as the result of complexordering [11]. In the present work, we limit ourinterpretation to ordering leading to the bco-β�phase; nevertheless, some HREM images of corru-gated layer such as the DO19 plate (Fig. 4b) mightbe interpreted by a complex ordering. In such acase, we have chosen to emphasize on the meandirection of platelets instead of the atomic detailof the corrugation. From our point of view, themajor point is that, as a consequence of ordering,the complex network microstructure inherited fromthe initial precipitation stages is preserved evenduring quite long ageing. Such a network formedby globular and plate shaped precipitates whichacts as a strong barrier against dislocation motion,is indeed not altered at 250 °C in spite of theβ�→β� transformation. This morphology stabilityshould contribute to preserving a high level ofmechanical properties and more specifically goodcreep resistance at 250 °C.

4.3. Precipitation sequence at 250 °C

4.3.1. Transformation β�→β� at 250 °CExamination of samples aged at 250 °C has

brought evidence of a transformation of β� plateletprecipitates into β� ones having a bco structure.Such a β�→β� transformation accounts for theappearance of β� phase in form of platelets in the(1 1 �2 0) planes after long ageing. This trans-formation provides an understanding for the obser-vation reported by [3] of unidentified fringed plate-shaped precipitates in the (1 1 �2 0) planes. Actu-ally at 4 h ageing at 250 °C, the precipitates, whichhave the bco structure, are observed in differentmorphologies. This transformation, which occurscontinuously along the platelets, emphasizes thestrong structural relationship between the β� andthe β� phases, which was already suggested by thefact that the DO19 (1 �1 0 0) plane acts as a pre-cursor of the β� (bco) structure. Besides, accordingto our description of the bco structure, theβ�→β� transformation requires only chemical dif-fusion; it can then occur on the existing networkwithout upsetting the microstructure. Since the β�phase is solute-rich in comparison with the β�phase, the β�→β� transformation provides soluteelements available for further precipitation.


Eventually, in samples aged at 250 °C, the size ofprecipitates prevents SAXS analysis which isdevoted to very small particles.

4.3.2. Overageing at 250 °CFrom our observation, we pointed out the exist-

ence of the β1 phase in WE43 alloys, in agreementwith observations of Nie and Muddle in WE54alloys. Details of its formation, as well as its placein the precipitation sequence, have not beendetermined in this study. However, according to arecent study [12], it is undoubtable that this β1

phase has a place in the precipitation sequence ofMg–RE–Nd alloys at temperatures above 250 °C.

5. Conclusion

Evidence of nanoscale precipitation has beendemonstrated in samples aged at 150 °C, by TEMstudy and by SAXS measurements. The formerbrings information namely concerning nature, mor-phology and size of precipitates, and the latter addsqualitative and quantitative information concerningpopulations of precipitates in terms of size and vol-ume fraction. Precipitation at 250 °C has beenstudied here only by TEM, the size of precipitatesbeing beyond the limits of the SAXS technique.

SAXS analysis demonstrates that precipitationoccurs in WE43 alloys after solution treatment andquench, coherent with Guinier Preston zoneassumption. The average size of detected particlesis around 0.7 nm. They were not detected by theTEM examinations alone.

The first stages of precipitation at 150 °C (864h) have been studied by both techniques and theyprovide results consistent with each other. DO19

precipitates form from magnesium solid solutionand reach a volume fraction of 5.4%, as evaluatedby the SAXS analysis. The hypothesis of M3Xcomposition, necessary for the determination ofvolume fraction is justified in this ageing state. TheDO19 precipitates, which are monoplanar, form in(1 �1 0 0) and (1 1 �2 0) magnesium planes.According to the present study, they are the precur-sors for the development of later β� and β� precipi-tates.

With prolonged ageing times at 150 °C, there

is still good correlation between both techniques,especially in terms of the characteristic dimensionsof particles. At 1324 and 1896 h, TEM obser-vations reveal the existence of two families of pre-cipitates with different morphologies. As evalu-ation of volume fractions of precipitates is notpossible by TEM analysis, the SAXS measure-ments are essential to have an idea of populationof precipitates. The results obtained from analysisof Iq2 curves, although they are made with theassumption of a single family of precipitates, arein agreement with the wide estimate of volumefractions from alloy composition. At 1324 and1896 h at 150 °C, monoplanar precipitates developin both β�-DO19 and β�-bco precipitates; the formerare plate-shaped in (1 1 �2 0) magnesium planesand the latter are globular in (1 �1 0 0) magnes-ium planes and result from ordered stacking ofDO19 monoplanar precursors. So, the initial stagesof precipitation of DO19 precursors have a stronginfluence on final morphologies. Hence, the earlieststages affect the distribution of precipitates insidethe magnesium solid solution.

The whole study of precipitation in WE43 alloyhas proved the complementary need of techniquessuch as TEM and small angle X-ray scattering.

Acknowledgements

The staff of ID01 Beamline and especiallyMyles Hamilton and Peter Boesecke are gratefullyacknowledged for technical support and for help-ing with the SAXS experiments.

References

[1] Polmear IJ. Light alloys/metallurgy of the light metals,metallurgy and materials science, 3rd ed. 1995:96–206.

[2] Nie JF, Muddle BC. Acta Mater 2000;48:1691–703.[3] Nie JF, Muddle BC. Scripta Mater 1999;40(10):1089–94.[4] Lorimer G, Azari-Khosroshaki R, Ahmed M. In: Proceed-

ings of the International Conference on Solid–Solid PhaseTransformations. The Japan Institute of Metals1999:185–92.

[5] Ahmed M, Pilkington R, Lyon P, Lorimer G. In: Magnes-ium alloys and their applications: Proceedings volume.DGM Informationsgesellschaft 1992:251–7.

[6] Azari-Khosroshashi R. In: Magnesium alloys and their


applications: Proceedings volume. DGM Informationsge-sellschaft 2000:711–5.

[7] Pike TJ, Nobble B. J Less Com Met 1973;30:63–74.[8] Hisa M, Barry JC, Dunlop GL. In: Proceedings of the

Third International Magnesium Conference. London: TheInstitute of Materials 1997:369–79.

[9] Glatter O, Kratky O. Small angle X-ray scattering. Lon-don: Academic Press, 1982.

[10] Deschamps A, David L, Nicolas M, Bley F, Livet F, Seg-uela R et al. Adv Eng Mater 2001;3(8):579–86.

[11] Hisa M, Barry JC, Dunlop GL. Phil Mag A2002;82(3):497–510.

[12] Apps PJ, Karimzadeh H, King JF, Lorimer GW. Scr Mater2003;48:1023–102.

Hardening precipitation in a Mg–4Y–3RE alloy

Documents

Transcript of Hardening precipitation in a Mg–4Y–3RE alloy