Zirconium Solubility in Aluminum Alloys

Transcription

1 roceedngs of the 9 th Internatonal Conference on umnum loys (2004) 1353 Edted by J.F. Ne, A.J. Morton and B.C. Muddle Insttute of Materals Engneerng Australasa Ltd Zrconum Solublty n umnum loys C. Sgl can, Centre de Recherches de Voreppe, B Voreppe Cedex, France Keywords: 3Zr, solublty, solvus, phase dagram, zrconum, 7050, 7108, 7449, 2196, 2098 Abstract It has been reported n the lterature that maor alloyng elements can have a very strong effect on the precptaton behavor of 3 Zr. Ths paper proposes a model that descrbes these effects: the strong zrconum-alumnum nteracton wthn the sold soluton nduces large varaton of the zrconum solublty from one alloy to another. The detal of these varatons also depends on all the other nteractons between the elements n sold soluton. The nteractons have been evaluated by a thermodynamc fttng procedure of phase dagrams or by frst prncple ab-nto calculatons. 1. Introducton Zrconum s added to alumnum alloys n order to prevent recrystallzaton. It can precptate n two forms havng the same 3 Zr chemstry but dfferent structure: D0 23 and L1 2 (see Fgure 1). The stable structure s D0 23 ; t forms sem coherent or ncoherent precptates n the alumnum matrx. The metastable L1 2 structure s found commonly n 7xxx alloys. After homogenzng t s fully coherent wthn the alumnum sold soluton and has a dense and fne dstrbuton. The L1 2 phase s responsble for the effectveness of Zr to control recrystallzaton n alumnum alloys. The solublty lmts of the stable D0 23 phase and of the metastable L1 2 phase are presented n Fgure 2. The solvus of the stable phase can be found n the complaton of hllps [1]. The solvus of the metastable L1 2 phase s less well known. At 660 C, values of 1wt% and 0.7 wt% have been calculated by, respectvely, Saunders [2] and Murray [3]. More recently, Clouet [4] has used an ab nto approach and found the metastable solublty lmt to be 0.87 wt% at 660 C. Clouet s metastable solvus has been adopted n ths study and s presented n Fgure 2. A typcal 7xxx homogenzng temperature s 480 C. Consderng an alloy contanng 0.1wt% Zr and the concentraton gradents generated after castng, the L1 2-3 Zr precptate free zones should occupy at least 70% of the volume (see Fgure3). Ths predcton s n contradcton wth observatons on 7xxx alloys that ndcate that the FZ fracton les n a 20-30% range. The obectve of ths paper s to show that the zrconum solvus (stable and metastable) s very senstve to L, Zn, Cu, Mg... addtons and to propose a model that descrbes ths effect. A predcton of the metastable zrconum solvus s then presented for some ndustral alloys.

2 1354 c Zr Temperature ( C) L L + AL 3 Zr (D0 23 ) D0 23 L a Fgure 1: DO 23 (left) andt L1 2 (rght) structures for 3 Zr. Zr (wt%) dendrte center Zr (n wt%) Fgure 2: Stable [1] and metastable [4] -Zr bnary phase dagram. dendrte border as cast (predcton from a Schel model) Metastable Equlbrum at 480 C Normalzed Dstance From Dendrte Center Fgure 3: redcted concentraton gradent wthn dendrtes n an as-cast bnary alloy contanng 0.1 wt% Zr (1D model) compared wth the metastable zrconum solublty 2. Generalzed Solublty roduct Concept The general expresson of the molar Gbbs free energy of a sold soluton n a regular soluton model s gven by the followng expresson: G = + x x Ω, > ( T ) RT x ln( x ) where the pure elements n the fcc structure are taken as reference and where the followng notatons have been used: x atomc concentraton of element n the alumnum sold soluton, T temperature (n Kelvn), R gas constant, Ω ( T ) fcc soluton nteracton parameter between elements and ; t may be wrtten as ( T) = A B T Ω wth A and B constant. + The chemcal potentals of elements are calculated usng the general equaton: (1)

3 1355 For the sold soluton, Eq.2 becomes: G G µ = + G x (2) x x ( x ) + Ω ( T ) x Ω ( T ) x x µ = RT k k, k> ln (3) The Gbbs free energy for a compound s expressed by the smple equaton n terms of ts formaton enthalpy, H f, and ts formaton entropy, S f : G = x µ = H f T S f (4) The alumnum sold soluton s n equlbrum wth a stochometrc compound when the chemcal potentals of each element n the sold soluton and n the compound are equal. The followng expresson s then deduced: ( x ) + Ω ( T) x Ω ( T) x x = H T S x µ = x RTln k k f, k> The generalzed expresson for the solublty product s then obtaned: x x = e ( T ) x x + ( T ) H T S Ω Ω f f, k> RT k x x k f (5.a) (5.b) For an deal soluton, Ω ( T)= 0 and Eq.5.b smplfes to the classcal expresson of the solublty product, K p : ( x ) x = K ( T)= e For a suffcently dlute regular sold soluton, x ( x ) x = K ( T)= e H f H f T S f RT 1, x, 0 and Eq 5.b smplfes to: x A T S f + x B RT (6.a) (6.b) It has been shown that the classcal solublty product expresson (Eq 6.b) s accurate enough for numerous precptates n alumnum; furthermore a general algorthm has been proposed to handle phase competton and complex phase equlbra [5]. However, when the solute-alumnum nteractons are very strong the classcal expresson s not accurate enough and the generalzed expresson (Eq 5.b) must be used. Ths apples n partcular for ttanum, scandum and zrconum. In ths paper, the effect for zrconum s llustrated. 3. Applcaton of the Generalzed Solublty roduct Concept to Zrconum In order to predct the solublty of zrconum, t s necessary to know the formaton enthalpy and entropy for 3 Zr as well as all nteracton parameters Ω (see equaton Eq

4 b). Values for the formaton entropy, enthalpy, Ω -Sc and Ω -Zr are taken from the abnto calculatons of Clouet et al [4, 6]. The other nteracton parameters are evaluated from the sub-regular parameters of the COST 507 database [7] except for Ω Zr-Zn whch has not been evaluated n [7] and whch s taken from our nternal database. The values of the most mportant nteracton parameters are llustrated n Fgure 4. As can be seen, scandum, ttanum and zrconum have a very strong attractve nteracton wth alumnum whereas Mg and L have a strong repulsve nteracton wth Zr. Ω -x Interacton (k/mole) Zr Zr Sc T Mn Fe L Cu S Mg Zn Interacton (k/mole) Zn Ω Zr-x Fgure 4: Most mportant nteracton parameters (at 480 ) used to predct the solublty of zrconum n alumnum alloys. If the maor alloyng elements are assumed to leave unchanged the free energy of 3 Zr (D0 23 or L1 2 ), t s possble to compute ther effect on the metastable zrconum solublty lmt (see Table 1). As can be seen, the more postve the solute-zrconum nteracton, the lower the zrconum solublty lmt. Ths can be easly understood: whle addng L, Mg, Cu, Zn... to the alumnum sold soluton, the strong -Zr bonds wthn the fcc sold soluton are replaced by weaker L-Zr, Mg-Zr,.. bonds resultng n an ncrease of the alumnum sold soluton free energy and therefore n a reducton of the zrconum solublty (stable or metastable). The slcon-zrconum nteracton s larger than the alumnum-zrconum one [7, 8]. We have assumed however that ths effect s compensated by the tendency for slcon to go nto 3 Zr formng (,S) 3 Zr [9]. Obvously, a more detaled study for slcon wll be needed; t s beyond the scope of ths paper. More generally, the model must be extended when solute elements (such as S, L, Sc..) present substantal solublty n 3 Zr. Cu Mg L

5 1357 Table 1: Effect of a 2wt% addton of Zn or Mg or L or Cu on the metastable zrconum solublty lmt. Case consdered Metastable Zr solublty lmt at 480 C (wt%) Bnary -Zr wt% Zn wt% Cu wt% Mg wt% L The combned effect of solute elements on the metastable Zr solublty s llustrated n Fg.5. A large effect s observed wth L contanng alloys where the solublty lmt s reduced by a factor 10; a sgnfcant effect s also seen wth hgh strength 7xxx alloys. Temperature ( C) Metastable Zrconum Solublty (related to L1 2 ) Fgure 5: Metastable Zr solublty lmt for dfferent alumnum alloys. Zr bnare Table 2: loy composton (wt%) n the calculaton presented n Fg.5 S Fe Cu Mn Mg Zn L loy Zr bnare < 0.10 < 0.10 <0.05 < < 0.12 < < < 0.12 < < < 0.12 < < < < 0.12 < < < The predctons for 7050 are compared wth Murray's predctons reported n [10]; as shown n Fgure 6 both sets of predctons agree well. 4. Conclusons and erspectves The metastable zrconum solublty lmt correspondng to 3 Zr n the L1 2 structure must be consdered n order to evaluate the ablty of zrconum to block recrystallzaton. Ths metastable solublty lmt s strongly nfluenced by maor alloyng addtons such as Zn, Cu, Mg, L... We can rank the elements wth respect to ther effect as (ncreasng effect): Zn<Cu<Mg<L. The effect of S stll remans to be evaluated n detal; n addton the model must be extended to non-stochometrc precptates. The same phenomenon,.e. a large varaton of solublty wth maor alloyng element content, s occurrng for Sc, Cr and T and can be treated wth the same formalsm.

6 1358 Temperature ( C) Ths work J. Murray Metastable Zr solvus (wt%) Fgure 6: Metastable Zr solublty lmt for at 7050 alloy: comparson between present work and the predcton of Murray as reported by Robson [10]. References [1] H. W. L. hllps, Annotated Equlbrum Dagrams of Some umnum loy Systems, vol. 25: Insttute of Metals, [2] N. Saunders, Calculated Stable and Metastable hase Equlbra n -L-Zr loys, Z. Metallkd., vol. 80, pp. 894, [3] J. Murray, A. eruzz, and J.. Abrata, The -Zr (umnum-zrconum) System, Journal of hase Equlbra, vol. 13, pp. 277, [4] E. Clouet, M. Nastar, C. Sgl, and J. M. Sanchez, Frst rncple Study of the Solublty of Zr n, hys. Rev. B, vol. 65, [5] C. Sgl, L. Maenner, C. Sztur, and R. Shahan, hase Dagram, Soldfcaton and Heat Treatment of umnum loys, presented at umnum loys: Ther hyscal and Mechancal ropertes (ICAA6), Toyohash, Japan, [6] E. Clouet, M. Nastar, and C. Sgl, Nucleaton of 3 Zr and 3 Sc n alumnum alloys: from knetc Monte Carlo smulatons to classcal nucleaton theory, To be publshed n hys. Rev. B, [7] I. Ansara, A. T. Dnsdale, and M. H. Rand, Thermochemcal database for lght metal alloys, vol. 1-3: COST 507, European Commsson, [8] H. Bakker, Enthalpes n loys, Medema's Sem-Emprcal Model: Trans Tech ublcatons LTD, [9] T. Sato, A. Kamo, and G. W. Lormer, Effects of S and T addtons on the Nucleaton and hase Stablty of the L1 2 type 3 Zr hase n -Zr alloys, presented at umnum loys: Ther hyscal and Mechancal ropertes (ICAA5), Grenoble, France, [10] J. D. Robson and. B. rangnell, 3 Zr Dspersod recptaton n Multcomponent umnum loys, Mat. Scence and Eng., vol. A352, pp. 240, 2003.