e) Variation of tbe tip vclocity of the calumnar dendrile in a·d during CET. Rcfined AJ-3.5wt% Ni, G = 30 °Clcm. Trans. Indian Inst. Met., Vol. 60, Nos.
Trans. Indian lnst. Met. Vo1.60, Nos. 2-3, April-JuDe 2007, :pp. 287-291
.< .:
TP 2141
Growth Structures, Interface Dynamics and Stresses in Metallic Alloy Solidification: In situ Synchrotron X-ray Characterisation ß. ßilIia', J. Gastaldi', H. Nguyen-Thi', T. Schenk', G. Reinbart', N. Mangelinck', ß. Grushko', H. Klein', J. Härtwig' and J. ßaruchel' IL2MP, UMR CNRS 6137, Universite Paul Cez.anneAix-Marscille m. Marseille, Franee lCRMCN, UPR CNRS 7251, Campus Luminy, Marseille, France lLaboratolrc de Physique des Materiaux. UMR CNRS 7556, &olc des Mines de Nancy, Naney, France 41FF. ForschuDgszentrum Juelich GmbH, Juelich, Gcrmany ~Laboratoi re de Cristallographic,UPR CNRS 5031, CNRS, Grenoble, France
.
'ESRF, Grenoble, Francc
E-Mail: hcmard.billia @L2MP.fr (Rcccivcd 30 June 2006 ; in revised form 20 November 2006 )
organic analogues has enabled direct optical observation cf the phenomena 3. Now, live synchrotron X-ray imaging has become the appropriate method 10 invesLigate the dynarrucs of solidification in metallic aUoys grown fram the melt 4,5. This paper reports results from OUf in situ and real-time invesLigation of the solidification of thin aluminium alloys by synchrotron X-ray imaging. Tbis study is carried out at the JD 19 beamline of ESRF (European Synchrotron Radiation Faci lity) using the protocols and the experimental set-up designed for upward Bridgman solidification monitored by synchrotron X-ray radiography orfand sy nchrotron X-ray topography, as dcscribcd in the companion paper by Gastaldi el al. 4. Our aim is 10 unveil the dynamical mechanisms co ntro llin g the solid - liquid interface morphology in solidification processing. First, disoricnting phenomella in columnar dendritic growth during direclional solidification of Al - 3.5 wt% Ni alloys are characterised. Thcn, the columnar to equ iaxed transition is analyscd. Finally. tbe solidification of AIPdMn quasicrystals from the melt is clarified.
... .
1.
INTRODUCTION
Thc properties and reliability of mat er ials processcd by commercial solidification techniques are governed by the microstructure left in the solid during elaboratioll. Thcrcfore, precise masteri ng of the sol idification is requircd to tailor finished products lO a spec ified quality. Directional solidification is tbe method of choice to study microslrUcture for:mation since Ihe process parameters (pu lling ra tc Vp ' temperattue gradient G, alloy composition) are independcntly and accurately controlled, and the response of the interface is quantitatively examined through thc evolution of the solid - liquid interface (smooth ~ cells ~ dendrites as growth rate is increased). The patterns forming at the solid-liquid interface during solidification also belong to the field of selforganization in sys tems far [rom thermodynamic equilibrium 1,2.
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For mctallic alloy s. the morphology of the solid- liquid interface 1S usually frozen by qucnching or decanting. and its evolution with time obtained by post-mortem anal)'sis of pictures taken on different sam pIes. Recollrse to transparent
2.
DlRECTIONAL SOLIDIFICATION OF AI 3.5 wt% NI ALLOYS
2.1 Disoricnting phenom.cna in dcndritic microstrllctnre The columnar · dendritic microstructure forms from th e morphological instability of the smooLh solid - liquid interface by the amp lifi catio n of corrugations and subsequellt sid ehra nchin g 2. Under the dynamical interacti.on of morphological instability and flu id flow, coupled growlh eveotua lly estab lishes. characterised by localisation of dendritic growth (left of Fig. lb) and transition 10 eutectic pattern (right of Fig. Ib), wiLh dendrites protruding ioto the liquid . Dendrite clustering al slow pulling velocity is the signature of convection induced under gravity in the meh by the horizontal tClUperature gradient 6 (Hg. lc). Indeed. evcn in Bridgman solidification stable .with respcct to fluid flow drivcn by the longitudinal tel.llperature and solute gradients (upward growth, heavy sOlute), a radial thermal gradient. 1S unavoidable because of the difference in thenna! conductivity bctween liquid, solid and crucible.
288 I Billia 81 a/. : Growth Struelures, Interface Dynamics and Stresses in Metallic Alloy Solldification
dendrites
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- CPig. 1 : Coupled growth or columnar dendrite with eUlcctic in Al-3.5 wt% Ni. Vp = 1 J.lIllls, G = 30 oe/ern. - a) 4233 sec, - b) t = 4254 sec after birth of morphological inscability. Sketches - c) of fluid flow in tbe meh snd Ni accurnulation. and - d) of a seeondary dendrite ~ growing perpendicrnarly to gravity.
Besides, the X-ray radiography video, frorn wruch the images in Fig. 1 are extracted, shows repcated 'disorienting of dendlite anns. This is illustmted by thc rotation of a secondary arm betwcen Figs la and lb. On the video, secondary arms are seen to switch ODe after the other from the initial position in Fig. Ia (dotted line 1) to a new position (dottcd line 2), rotating by about 50. After rotation each secondary arm becomcs parallel to the ·rums below. Trus ann disorienting can be attributed lO the effeet of growth-induccd mechanical constraint. Indeed, as thc growth direction of the secondary arms is horizontal, a bending moment is building under Earth gravity (Fig. ld) 7 MB = g n r' (ps - Pe) V. ' (I -
'0>' I 2
(I)
whcrc g is the gravity aceeleration, r the radial coordinate and t the time, Va the ann growth velocity. ,PL and Ps arc thc liquid and solid densities. Wirh time, Mn inC;feases and mechanical bending occurs at the thinner solid neck attaching the secondary arm to the primary dendrite stern, whcre the yield stress is overcome first. When there are equiaxed crystaJs growing in the melt around columnar dcndrites (Fig. 2), it happens that the bending of a secondary ann is precipitated by the weight added by cq uia xed grain(s) incidentaUy sitting on its extremity (arrows). Synchrotron X-ray topography is much more sensi tive to . microstructure disorienting than synchrotron X-ray radiography. Indeed, as topography is based on diffraction , any srnall change in cell or dendrite oricn tation with respec~t to thc incident X-ray hearn manifests itself by a displacement .
Fig. 2 : Bending of a secondary dendricc arm triggered by equiaxcd grains I and 2 wcighing TiB 2 particlcs as nuc1cation sites. Vp = 4 ~un/s. G = 23 oe/ern. Trans. Indian Inst. Met., Vol. 60, Nos. 2-3, April-June 2007
00
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AJ ~3 .5
wt% Ni refined by adding
BiIJ;a et al. :. Growth Structures, Interface Dynamics and Stresses in Metallic Alloy SolJdification I 289
region wilh · equiaxed grains growing in all space diTcctions. Equiaxed and columnar mieroslructures lead 10 materials with respeetively isotropie and oriented macroseopie mcehanieal propenies.
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Fig. 3 : Morphological inSlll.bility and dendrite grOwth aoalysed by combined synchrotron X-ray radiogmphy (top row)
and topography (b0I10m row) using (O-2-2)-reflection . AI-3.5 wt% Ni. V p = 1 ~s . G :: 30 °C/cm.
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in Laue images 7, Yet. whcn the interface microstructurc is cumulating disorientations its Laue images break inta several pieces with time, making analysis intricate. Tben, clearer insight is obtaincd from combined synchrotron X-ray radiography and topography. In Fig. 3, the radiographs show the inception of morphological instability a11 over the solid - liquid interface and its evolution inta dendrites. while the topographs follow the same features but on a single grain (the other grain s are diffracting elsewhcre). At Ip28 S of growlh, [hc interface morphology is similar on hoth images. At 26 16 s, strains have dcveloped enough to send away parts of the image in thc topograph, in particular lhe dendrite top. At 3287 s, the topograph reveaJs that anothcr secondary ann has moved away (1) aod the dendrite top came back (2), which suggests that mechanical effects may be reversible. As bending due to gravity is irreversible and the dendrite axis is elose to vertical, tbe round trip of the dendrite top image eaD be interpreted as resulting from the action of the torque indueed by sheae stress thaI builds up with thc growth shapc 7.
2.2 Colunmar - equiaxed transition Cast ingots generally exhibit a region perpendicular 10 the mould walls wirh columnar microstructure, and an inner
During the columnar-to-equiaxed transi ti on (CET), th e equiaxed grains interaet between therilSelvcs and with the columnar front. The block.ing of the eolumnar mierostrueture is charactcriscd by synchrotron X-ray radiography on AI-3.5wt% Ni alloys refined by adding TiB 2 partieIes (Fig . 4). First, a pulling velocity V p = 2 ).Ulusec is applied to obtain a columnaT dendritic microstructure (Fig. 4a). To provoke CET, the pulling rate is jumped to 14 fUl1Is at 10. The dendrite tip ve!ocity increases (Fig. 4b,e) tilL the [lIst equiaxed grains form in the melt around the dcndrite. After a while, columnar growth is stopped (Fig. 4c,d,e). which marks the transition to an equiaxed microstructure. The fact that eolunmar dendrites ce ase to grow th before effective con tact with cquiaxed crystals confml1s that the blocking mechanism in CET has a strong solu tal component 8. After a transien t following CET. equiaxcd growth proceeds by the propagation of the forefront of an equiaxed mushy zone formcd by closely packed grains (Fig. 5a,b). The equiax.ed grai ns belonging to this effective front grow with a lcading-tip velocity near 10 the pulling rate. In the melt just ahead, new grains continuously nucleate (visible when their diameter gel~ larger than the resolution of X-ray radiography, i.e. 14.9 I-lffi with the CCD camera used). This propagation mechanism keeps repcating until the end of the sampIe. Thc absence of euteetic border in equiaxed growth and the flatncss of thc effcclive front are signs Ihat convection cffects are not disturbing, probabty becausc the pulLing rate is high enough. Figures Sa,b furthemlOre indicate that in OUf parameter range the loeation (and thus temperature) of the effective front increases with the pulling velocity. Also, the location of the fIrst visible cquiaxed grains docs not change, which suggests that nucieation oeeurs at a given undercooling. In Fig.5c, the equiaxed grain density N is ploued as a funetion of V p. N is a key parameter for CET and equiaxed growth. It first rapidly increases be fore approaching an asymptotic valuc as refinement efficicncy is reaching its limit , in agreemCl1t wirh models and experiments 9.10. Indeed, above a critical value of the growth velocity, the amount and extent of constitutional uodercooliog ahead of the effective front
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: Scquence of synchrotron X-ray radiographs showing ihe columnar-equiaxed transition resulting from a sbarp increasc at ta of thc pulling velocity from 2 10 14 jlJnJs: - a) 1 = 10 - 192 s, - b) t = to + 90 sec, • c) t ' 0 + 215 sec, - d) t to + 300 sec. - e) Variation of tbe tip vclocity of the calumnar dendrile in a·d during CET. Rcfined AJ-3.5wt% Ni, G = 30 °Clcm .
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Trans. Indian Inst. Met., Vol. 60, Nos. 2-3, April-June 2007
290 I Biflia el al. : G rowth Struclures . Inlerface Dynamics and Stresses In Melallic Alloy Solidlfication
