Microstructural Evolution and Age Hardening in Aluminium Alloys ...

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106 S. P. Ringer and K. Honoelements of the phase are common tothose of the m3m -matrix, the intersectionpoint group is the same as that of phase:4/mmm. Because the point group of thematrix phase contains 48 symmetry elementsand the intersection point groupcontains 16, the index of the intersectionpoint group in the -matrix is 48/16 3.Symmetry thus requires three crystallographicallyequivalent variants. The specialcrystal forms compatible with this pointgroup [33] include pinacoids normal to thefourfold axis, and tetragonal prisms parallelto twofold axes possessing the requiredmirror symmetry, which is consistent withobservation.The tetragonal phase (I4/mcm, a 0.6066nm, c 0.4874nm) [34] occurs in avariety of orientations and morphologies.These have been systematically examinedby Vaughan and Silcock, who showed thatthere are 159 known orientations for a particle relative to the -Al matrix [35].EFFECTS OF Sn, Cd, AND In ONPRECIPITATION IN Al–Cu ALLOYSTrace additions of Cd, In, and Sn increaseboth the rate and extent of hardening in Al–Cu alloys aged at temperatures between100 to 200C [36–38] (Fig. 2). Comparisonsbetween binary and ternary alloys haveshown that, whereas these elements suppressthe formation of the GP zones and the phase, they stimulate a finer and moreuniform dispersion of the semicoherent .Two types of proposals have been offeredto describe the mechanisms for this refinedprecipitate dispersion. One hypothesis isthat the trace elements are absorbed at the/matrix interfaces, resulting in a reductionof the interfacial energy required forprecipitate nucleation. This explanationwas first proposed by Silcock et al. [37] toaccount for weak X-ray reflections (designated“p-diffractions”) observed duringthe early stages of aging. This proposal receivedindirect experimental support fromcalorimetric measurements by Boyd andNicholson [39] and TEM observations bySankaren and Laird [40], who claimed todetect what they referred to as Sn (Cd or In)segregates and precipitates in associationwith the particle/matrix interface. An alternativeexplanation is that the trace elementsfacilitate heterogeneous nucleationof either directly at Sn (Cd or In)-richparticles [41–43], or indirectly at the dislocationloops present in the AQ microstructure[44].Random area 1DAP analyses obtainedfrom samples of an Al–1.7Cu–0.01Sn alloyindicate that the Sn atoms are not homogeneouslydistributed in the -matrix immediatelyafter quenching from the solutiontreatment temperature [45]. Rather, theyoccur in discrete clusters, and there was noevidence of enrichment of Cu at their locations.As is usual in the examination of AQAl–Cu-based alloys, occasional clusters ofCu atoms were also observed, but thesewere not spatially correlated with the Snclusters. A statistical test of the atom-probedata, known as contingency table analysis,also indicated that Cu and Sn did not havea preferential interaction in the clusteringstage.Figure 4(a) is an He FIM image of the Al–1.7Cu–0.01Sn alloy aged 3 min at 190C andshows brightly imaging precipitates, indicatedby arrows. The selected area 1DAPanalysis presented in Fig. 4(b) was obtainedby probing at or near these brightly imagingregions, which were found to be Curich nuclei, heterogeneously nucleatedon Sn particles. Figure 4(b) is an example ofan analysis through the /Sn interface.The Cu ladder is uniform through both theSn particle and the adjacent matrix, confirmingthat there is no Cu enrichment inthe particles. However, there is a sharp interfacebetween the Sn-rich and Cu-richprecipitates. Note that in Fig. 4(b), the compositionof the Cu-rich precipitate is 33 at.% Cu, which corresponds to the nominalcomposition of (Al 2 Cu). This observationand the fact that Al and Sn have extremelylimited solid solubility in each other suggestthat the particles are pure Sn. Microbeamelectron diffraction confirmed thepresence of the -Sn phase (I4 1 /amd, a 0.583nm, c 0.318nm) oriented such that
Microstructural Evolution in Aged Al Alloys 107(100} Sn // {111} and [010 Sn // 112 [45].[Mixed notation indices are used to representplanes (hkl} and directions [uvw oftetragonal structures.] Figure 5 shows themicrostructure after 1 h at 190C, where it isseen that precipitates were often foundassociated with these particles, suggestingthat they had provided sites at which heterogeneousnucleation of could occur. TheSn particles appeared to be incoherent withthe Al matrix, because they were visibleonly through diffraction and structure factorcontrast, and no strain was observedunder two-beam conditions. The Sn particlesare in contact with the narrow, noncoherentplanes of . Further 1DAP analysesrevealed that Sn was not segregated to the/matrix interfaces either across the coherentbroad face of the plate or across therim or edge. These results support the proposalby Kanno et al. [41–43] that Sn (Cd orIn) facilitates precipitation of at elevatedtemperatures by providing heterogeneousnucleation sites. The rapid clustering andprecipitation of Sn in this alloy has been explainedin terms of evidence for a preferredSn–vacancy interactions during, or immediatelyfollowing, quenching, and the factthat the diffusion rate of Sn in Al may exceedthat of Cu by at least two orders ofmagnitude [45].Figure 6(a) shows an FIM micrograph ofa general high-angle grain boundary in theAl–1.7Cu–0.01Sn alloy following aging 3min at 200C. Figure 6(b) shows an integratedconcentration–depth profile acrossthe grain boundary. The intragranular concentrationof Cu determined from this datais significantly larger than the equilibriumsolid solubility (0.1 at. %), and this is consistentwith the observation that is in theearly stages of precipitation. However, aCu–solute-depleted region is evident in theregion of the grain boundary, and there isno evidence of Sn segregation in theboundary. The solute depleted region wasestimated to be 10nm thick. Narrow precipitatefree zones (PFZ) approximately0.1m either side of the boundaries, to-FIG. 4. (a) Field ion micrograph (He) from the Al–1.7Cu–0.01Sn alloy aged at 190C, 3 min. The 1DAP resultsin (b) indicate that the brightly imaging regionsare Cu-rich nuclei, heterogeneously nucleated on Snparticles. After Ringer et al. [45].FIG. 5. Transmission electron micrograph of the Al–1.7Cu–0.01Sn alloy following aging for 1 h at 190C. Afine and uniform dispersion of precipitates is seen,which are nucleated on spherical Sn particles (arrowed).After Ringer et al. [45].
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Microstructural Evolution and Age Hardening in Aluminium Alloys ...

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