Materials Science Forum, Vol. 94, 1992, pp

Transcription

1 RECRYSTALLISATION AND MICROTEXTURE DEVELOPMENT IN AN ALUMINIUM-IRON ALLOY Nicla Deards and H.K.D.H. Bhadeshia University f Cambridge Department f Materials Science and Metallurgy Pembrke Street, Cambridge, CB2 3Q, U. K. Recrystallisatin, Dispersids, Micrtexture Materials Science Frum, Vl. 94, 1992, pp ABSTRACT A high-purity AI-1.41 Fe wt.% ally has been studied with a view t revealing the rle f FeAl 3 particles in the develpment f recrystallisatin. Transmissin electrn micrscpy f defrmed and partially annealed samples revealed the early stages f recrystallisatin at the particles. Crystallgraphic rientatins f individual grains have been measured using electrn diffractin methds. Thus, the misrientatins f newly recrystallised grains with respect t the surrunding defrmed matrix, and als with respect t adjacent recrystallised grains, have been determined. An analysis f these data reveals that the behaviur f matrix grains at the ends f needle shaped precipitates tends t be quite different when cmpared with the grains at the sides f the precipitates. The implicatins f the results are discussed in the cntext f particle stimulated nucleatin f recrystallisatin. INTRODUCTION A majr aspect f the physical metallurgy f aluminium allys is cncerned with the develpment f crystallgraphic texture during defrmatin and heat-treatment. The aim f the wrk presented here is t study sme specific effects f precipitate particles in influencing recrystallisatin behaviur. Use is made f micrtextural data, in which the misrientatins between adjacent grains in the vicinity f the precipitate can give valuable infrmatin n the early stages f recrystallisatin. EXPERIMENTAL Slabs (4 cm thick) f a super purity AI-1.41 Fe wt. % cast ally ( <.1 wt. % impurity cntent) were hmgenised by annealing at 4 C fr 1hr, and ht-rlled dwn t 15mm thickness. After annealing at 38 C fr 1 hr, the samples were cld rlled dwn t 4 mm, a thickness large enugh t extract specimens n a plane nrmal t the rlling plane, while at the same time including the lng transverse directin. Further heat treatments were carried ut at 25 C t study the recrystallisatin f the cld rlled micrstructure. Thin fils fr transmissin electrn micrscpy (TEM, Philips EM4T at 12 kv) were prepared frm the lngitudinal sectin f the rlled billet using electrplishing in a slutin cnsisting f.5% nitric acid, 3% perchlric acid and distilled water at -1 C. Diffractin patterns were btained withut

2 sample tilting whenever measurements were made n a grup f sme 5-15 grains. Bulk texture was measured using X-ray diffractin n a Siemens diffractmeter. Hardness measurements were made n each specimen using a 1 kg lad. AS-DEFORMED MICROSTRUCTURE The FeAl 3 particles were characterised as being abut 1-7j.lm in length and.2-.5j.lm in width. They were nt unifrmly dispersed, but ccurred in bands apprximately 2-4 j.lm apart in the transverse directins. The mean particle spacings within the bands was fund t be abut 1-6j.lm. It is expected that such particles are nt shearable. Since they d nt change shape during defrmatin, the matrix arund each particle is expected t underg quite cmplex defrmatin, whse character varies with distance frm the particle/matrix interface [1]. Figure 1: Transmissin electrn micrgraphs taken in the lng transverse sectin shwing the micrstructure f Al-1.41 wt.%fe Ally. (a) In the as-rlled cnditin away frm particles, micrstructure in the prximity f a particle f FeAl 3. Nte that the particle has in fact drpped ut f the fil. Areas free f large precipitates, illustrating elngated subgrains and grains tgether with sme light shear banding are illustrated in Fig. la. The regin surrunding a FeAl 3 precipitate is shwn in Fig. 1b; there is sme indicatin that the micrstructure has mre intense defrmatin near the ends f the elngated precipitate when cmpared with the regins adjacent t its lnger dimensin. There will, in general, be a defrmatin and crystallgraphic rientatin gradient in a directin away frm the particle/matrix interface. T assess the effect f the particle in the develpment f such gradients (and hence n recrystallisatin behaviur), it was deemed necessary t measure the rientatin f the grains adjacent t the particle relative t thse lcated in the particle-free matrix sme 1-2j.lm frm the particle/matrix interface. Such measurements are hencefrth dented "remte rientatins". Fig. 2a shws a histgram f experimentally measured remte rientatins, expressed in the frm f a cincidence-site lattice parameter E. Nte that the values f E d nt imply exact rientatins, but are intended t represent the nearest value t the actual rientatins. This methd allws a ready classificatin in terms f the energy f bundaries. It is evident that the misrientatins are quite large. Fr cmparisn purpses, Fig. 2b shws the misrientatins between the adjacent elngated subgrains in the remte matrix; since all these measurements indicate very small misrientatins, the elngated grains are true subgrains, and a cmparisn f Figs. 2a and 2b demnstrates the majr perturbatin caused by the presence f particles.

3 12 Cl 1... C'd '"' '"' 4 e : 2 (a) II U U n n n = 1... C'd '"' '"' 4 e: II U U n n n Figure 2: (a) Misrientatins between grains adjacent t the particles and remte grains in the unperturbed matrix. Misrientatins between adjacent elngated subgrains in the matrix remte frm large particles. PARTIAL RECRYSTALLISATION Annealing the cld-defrmed micrstructure prduced the usual recvery effects, tgether with the early stages f recrystallisatin. Figure 3a shws a grwing recrystallised grain lcated at the tip f a FeAl 3 particle. It is apparent frm the data presented in Figs. 3b & 3c that the recrystallised grains have mre r less the same remte rientatins as the defrmed grains adjacent t the particles. Cnsistent with a lt f published wrk [2,3], this implies that recrystallisatin invlves strain-induced subgrain bundary migratin, rather than the frmatin f entirely new grains. Thus, any differences in the develpment f crystallgraphic texture during recrystallisatin in particle-free and particlecntaining allys f this kind must be attributed t the defrmatin gradients caused by the presence f particles. It wuld therefre be f interest in future wrk t cmpare micrtexture develpment in super-pure aluminium withut the particles. Figure 4a shws the changes in hardness as a functin f annealing time at 25 C; it is evident that nly small changes in hardness ccur, cnsistent with the fact that nly partial recrystallisatin and recvery is bserved. Nte that the fully annealed ally has a hardness f abut 3 Rv. In fact, recvery is the dminant prcess fr time perids less than abut 3 min. The bulk texture measurements (Fig. 4b and Table 1) als shw little change, partly because the degree f recrystallisatin is rather small. In additin, the mechanism f recrystallisatin appears t be strain-induced grain bundary migratin, in which case, much f the riginal rientatins in the defrmed micrstructure are expected t be preserved, especially at the early stages f recrystallisatin. It was ften fund that fr a given particle, the L: values (crrespnding t the remte rientatins) fr clusters f grains at the particle tended t be similar. This might be expected since the defrmatin gradient may nt vary much alng the length f the particles. Each cluster was identified by the dminant L: value fr the cluster, irrespective f whether it cntained recrystallised r defrmed grains. A distinctin was made between the clusters lcated at the ends f the particle and thse alng the particle length. The end clusters were always fund t give large L: values (Fig. 5a), indicating relatively intense defrmatin at the particle ends. This is cnsistent with previus calculatins f the strain field arund particles in defrmed matrices [3]' and with the cmmn bservatin that the ends f particles ften are preferential sites fr the nucleatin f recrystallisatin [4]. Annealing has the effect f eliminating many f the large L: misrientatins present in the defrmed micrstructure at the particle ends (figures 5b and c cmpared t figure 5a). This is presumably

4 because the large misrientatin grains are the first t grw int large recrystallised grains, and hence are eliminated frm the measurements, where nly n thse particles where recrystallisatin is at an early stage are fcussed upn. It) = f: Grains _ Rcc:rJataJliac:d IDefrmed = It) &.. I) 1 Grains _ Rcc:r,atalliaed _Defrmed &.. I) 1. 8:= 5,.Q 8:= (c) U rr n n Figure 3: (a) Transmissin electrn micrgraph taken in the lng transverse sectin shwing a grwing recrystallised grain at the tip f a particle, during annealing at 25 C fr 6 min. Histgram shwing the remte misrientatins fr the recrystallised and unrecrystajlised grains adjacent t a particle, after annealing at 25 C fr 3 min. (c) Similar data fr 6 min at 25 C. CONCLUSIONS In the AI-Fe ally studied, the presence f large, needle shaped, nn-defrming particles f FeAl 3 influence the develpment f crystallgraphic texture during recrystallisatin by causing inhmgeneus defrmatin n a micrscpic scale. The particle shape als has cnsequences, in that the particle ends are assciated with the develpment f regins f larger misrientatin relative t the remte matrix, when cmpared with the matrix alng the particle length.

5 .- LOAD = lokg Lng Transverse Sectin / H Cl c.> >Jl D (a) Annealing Time (minutes) Cl H ::s - Cl t-2! Cl e::s - > -IllD Annealing 1Cl2llIDfD./S /Brass & ===/./ /Cu +- I!! I!I /Gss,/Cube I!l Time (minutes) Figure 4: Table 1: Changes in (a) hardness and bulk texture, during annealing at 25 C. Bulk texture changes during annealing at 25 C. Texture Annealing Time (minutes) Type 3 6 S-type Brass Cu-type Gss Cube ACKNOWLEDGMENTS The authrs are grateful t Prfessr Clin Humphreys fr the prvisin f labratry facilities at the University f Cambridge, and t Alcan Internatinal fr facilitating this research and fr prviding the aluminium ally used in the investigatin as well as the X-Ray diffractin measurements. Special thanks g t Dr. G.J. Marshall and Dr. R.A. Ricks f Alcan Internatinal fr their advice n technical cntent. REFERENCES (1) Humphreys, F.J.: Acta. Met., 1977, 25,1323. (2) Bellier, S.P. and Dherty, R.D.: Acta. Met., 1977, 25,521. (3) Bay, B. and Hansen, N.: Metall. Trans., 1984, 15A, 287. (4) Es-Said, O.S. and Mrris, J.G.: in Aluminium Allys, Their Physical and Mechanical Prperties., 1986, EMAS, 451.

6 U '" - S u ' (a) Clusters Side ii End II U U n D 8 Clusters Side bided 6., n -U '" IIU U n D (c) :.:.... Clusters Side REnd IIU U n D Figure 5: The frequency f grain cluster remte rientatins. (a) Defrmed sample. Annealed at 25 QC fr 3 min. (c) Annealed at 25 QC fr 6 min.