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CALORIMETRIC EVALUATION OF THE
MICROSTRUCTURE OF Al-Cu-Li-Mg 2091 ALLOY
S. Abis, E. Evangelista, P. Mengucci, G. Riontino
To cite this version:
S. Abis, E. Evangelista, P. Mengucci, G. Riontino. CALORIMETRIC EVALUATION OF THE MICROSTRUCTURE OF Al-Cu-Li-Mg 2091 ALLOY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-447-C3-453. �10.1051/jphyscol:1987351�. �jpa-00226582�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6ment au n09, Tome 48, septembre 1987
CALORIMETRIC EVALUATION OF THE MICROSTRUCTURE OF A1-Cu-Li-Mg 2091 ALLOY
S. ABIS, E. EVANGELISTA*, P. MENGUCCI* and G. RIONTINO'"
Istituto Sperimentale Metalli Leggeri, Aluminia S.p.A. , Via Fauser, 4, I-28100, Novara, Italy
" ~ n i v e r s i t a di Ancona, Dipartimento di Scienze dei Materiali, Via Brecce Bianche, I-60131, Ancona, Italy
* * Universita di Torino, Istituto di Chimica Generale, Facolta di Farmacia, Via Giuria, 9, I-10125, Torino, Italy
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The precipitation of S ' , T1 and 5' phases during ageing at 160 OC after quench, has been studied in a commercial 2091 alloy by using Differential Scanning Calorimetry (DSC) technique, 2091 samples have been solubilized, quenched and aged at 160 O C for times ranging from 10 minutes up to 90 h o u r s ; DSC traces h a v e
been then obtained during continuous heating; for each temper several temperature rise speeds have been used.
kt short times of ageing (10 min) three phases precipitated during ageing have been detected, causing three endothermic effects of dissolution in the DSC traces. For larger times of annealing the peak at lower temperature tends to disappeare and after 2 hours it can never been detected, whilst another thermal effect at higher temperature becames more pronounced and rises up to YO hours.
The peak temperatures depend on the temperature rise speed (thermally activated peaks), and this effect has been used to determine the apparent activation energies of dissolution according to the Ozawa plot,
Transmission Electron Microscopy (TEN) and Selected Area Diffraction Pattern (SADP) have been per formed fnr microstructural characterization of precipitated phases,
It has been concluded that for short ageing times tup to 2 hours) three phases are present: the S' fk13 Li) T1 (G.1: CuLi) and
.- , (k12CuMg); the T1 precipitate has been always detected on defects (dislocations and grain boundaries). For larger times of annealing the Tl phase tends to redissolve while the S' and S' precipitates grow up to 9O hours,
Rge hardenable quaternary Rl-Cu-Li-Mg-Zr alloys have been demonstrated to be the most promising ones, in the field of commercial Aluminium-Lithium alloys, due to the best compromise of ductility and strength, Such a result has been achieved as the hardening sequence of this kind of alloys can comprise the 5'-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987351
J O U R N A L DE PHYSIQUE
CuMgAl2 phase able to deviate the coplanar deforniation niode in an Orowan-like one, s o obtaining better characteristics o f ductility C1-57. This effect cannot he obtained in binary Al-Li or ternary k1-Mg-Li and k1-Cu-Li alloys CS.73, and i n . this last, in particular, the good effect of the TI-CuLiA12 on strength, is not .ioined with a positive effect on the deformation path, due to the particular orientation o f hexagonal T1 with the R1 matpix C(0001)T1//'(111)A13 C % , 9 1 .
For such a reason a lot o f research has been addressed t o formulate a composition, inside the A1-C~-Li-klg system, able to produce the coprecipitation of 2'- LiA13 and 5: -(:uMgk12 in such a way to enhance ductility without a notable reduction in strength C1U,113. The precipitation of the 5' phase is greatly 'favoured by defects, but, a s the interaction energy between vacancies and Li atoms is high C123, and therefore thermal vacancies retained during quench are not released, a thermomechanical treatment is generally required to obtain a fine and homogeneous precipitation of 5' .
Nevertheless, in the industlrial practice, thermomechanical treatments are sometimes not practicable, The study of 5' precipitation has been therefore addressed on two lines:
- study of 5' precipitation induced by stretching, and
- study of 5' precipitation during different thermal treatments required when thermomechanical treatments are not allswed,
In this paper we report results about a study of the precipitation and/.or dissolution od S'-Likl, , 5'-CuMgAl, and T1- CuLiAls phases during 150 OC ageing after quench, in a coniniercial 2091 alluy, by means of Differential Scanning Calorimetry.
The non-isothermal method of Orawa has been considered to calculate the appal-ent activation energies of dissolution of the phases precipitated during the 150 "C annealing, Transmission Electron Microscopy has been also used to connect DSC results with microstructure,
FI 2091 extruded plate has been supplied by Pechiney-Cegedur (France) in the T351 temper; the chemical analisys is reported in Tab I ,
Samples cut from the plate has been solubilized in an anhydrous argon atmosphere at a temperature of 530 OC for 8 hours and then quenched in a brine bath at 0 O C . Thermal treatments have been performed in a thermostatic silicone-based hath a t 160+ 1 *C for times ranging from 10 min to 90 hours.
TEM specimens have been prepared following the sequence:
mechanical polishing up to 200 pm of thickness, electrochemical thinning up to 5 0 pm and then double-jet thinning in a 1 : 3 HNO :
CH OH solution at -30 OC, 1 2 V . Prepared speciniens have been hold
at 0 " ( 2 t o avoid thermal effects deriving Srnm handling.
TEM e:.:aminations have been carried out by using a 120 k V Philips C H I 2 facility,
DSC speciniens have been prepared starting from treated cylinders 6 0 nim in length, 5 mm in diameter; discs of about 50 mg have been
slowly c u t under a continuous cooling flow for measuren~entk.
The heat evolution a t constant heating rate has been monitored in a 710 DuPont cell under a protective high purity argon atmosphere,
Samples treated a t 160 OC for 5 min and 90 h have been investigated using different scanning rates from 5 to 5n K/min, while a standard speed o f 50 K/min has been adopted for the other
specimens,
ps?its 2nd Discussion
The evolution of precipitated phases with respect to ageing time has been studied performing a DSC scan a t a standard speed o f 50 K J m i n for each temper ( 1 0 , 20 min, 1 , 2, 17, 48, 90 hoursf ; in some cases, after a first scan performed up to 530 *C, a second one has been executed for the same specimen at the same scan speed, obtaining a base-line of reference; results are shown in fig 1 ,
The trend of DSC traees appears t o be complex, but each one can be divided into three parts: firstly an endothermic effect in the range 190 - 29O * C , then an exothermic peak at about 330 OC, followed by a large endothermic one above 400 O C .
For the 10 min sample, the endothermic zone a t lower temperature can be resoloved into three different peaks marked D , E, E in fig 1 . The evolution nf thermograms a s a function o f ageing time shows a s the peak D tends to lover and it disappears after 2 h of annealing at 160 *C, while the peak E enhances and became= the main thermal effect at YO h ,
The peaks D , E and E must b e attributed t o the dissolution of three different phases precipitated during ageing; their evolution indicates a s the phase associated with the D-peak is stable for short ageing times, whilst these associated with E* and E peaks becames more and more stable for longer times of annealing,
The e:.:nthermic peak centered a t about 330 OC is associated with the precipitation, during the DSC scans, of high temperature phases, while the following large endothermic effect must be attributed to the complete d i s s o l u t i ~ n of precipitated phases,
As o n l y - D , B and E peaks are associated with precipitates formed during 1b0 OC annealing, and in particular with hardening phases, our attention has been addressed to perform a kinetic analisys of peaks in the range 1PO - 290 OC Different DSC runs have been therefore performed on 10 min and 90 h samples at different scanning rates, namely: 5, 10, 20, 30, 50 K/min, obtaining the results reported in figs 2 and 3 r e s p e c t i v l y .
k s can he seen, a notable shift of the peak temperature Tp is determined by changing the scan speed showing a s D , E and E peaks are thermally activated, Ey assuming that, during dissolution, the transformed fraction corresponding t o Tp is the same irrespective of the scanning speed, the Ozawa plot C133 can be used to obtain an estimate o f the apparent activation energy of dissolution associated with each thermally activated peak, The fig 4 shows the dependence of the scanning speed 9 o n the r.eciprncal of the peak temperature l,/Tp, referring to 0 , & and E peaks; obtained values o f activation energy are shown in tab 11.
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k TEM analisys has been also performed on 10 min and 9 0 h samples. The 10 min sample shows an homogeneous distribution of
X ' spherical particles and large spherical ZrA13 dispersoids (fig
5 ) surrounded b y nucleated S' and PFZ; another form of Zrkl,
nucleated on defects (grain boundaries and helicoidal dislocations) can be also observed, Moreover rare T1 precipitates having a he:.:agonal structure are also present (a diffraction pattern is reported in fig 5).
At 90 h larger 6' particles can be observed and only the spherical form of ZrAl dispersnids if ig 6 ) ; in this temper a large 5' precipitation is present while Ti precipitates are completely absent,
Following TEM indications The D peak can be therefore assigned to.T1 dissolution, while the E peak refers to the dissolution of the 5' phase,
The E effect must be finally linked out to 6' dissolution; this last result is in agreement with a recent nne obtained for a 2090 alloy by DSC measurements C141.
From DSC and TEM results here reported the precipitation sequence at 160 OC in a commercial G1-Li 2091 alloy has been determined.
For short times of ageing (see 10 min - 1 houri, S' TI and ZrA13 precipitates can be detected by both DSC and TEM. A little amout of 5' is also evidentiated by DSC even if it cannot be seen at TEM .
For larger ageing times the T1 phase disappears whilst the 5' one growths and at 90 h of ageing it is clearly visible at TEM.
Moreover two kinds of ZrA13 have been evidentiated for short times of ageing.
A k n p w l g d q e g e q t s - The authors are kingly indebited to Cegedur
Pechiney (France) for material supplying.
This work has been partially supported by CNR funds,
C13 C.J. Peel et Al: "Gluminium-Lithium Alloys 11" page 363 CZI K . K . Sankaran, N . J , Grant: Mat, Sci. Eny, 4 4 , 213 (1980) C31 P , Meyer, E , Dubust: "Rluminiurn-Lithium Alloys III", C ,
Eaker et A l , eds. The institute of Metals,London,l986 Fag 3 7 E43 F . E , Bretz, R . H . Satwell, ibid, page 4 7
C51 P , J , Gregson, H . M , Flower: kcta Met, 33, 527 (1985)
C63 K , Dinsdale, 5 . J . Harris, E , Noble: in "Fatigue and Microstructure, M , Meshu e d . , k . S , M . , Metals Park, 1979 page 20 5
- - -
~ 7 1 ' F . G . Gayle in "Aluminium-Lithium Alloys II", page 119 C81 J . M , Silcock: J . Inst, Metals W , 3 5 7 (1959)
E91 K , K . Sankaran, J , E . 0' Neal: in "kluminium-Lithium Alloys
I1 " , page 393
C101 J . White et kl.:"kluminium-Lithium Alloys 111" C , Eaker et Al. Eds. The institute of Metals, London, 1986, page 3 7 C111 M. Fridham et Al.: Ibid. page 547
C121 5 , Ceresara et A1 : Phil. May. 35,97 (19773 El31 T , Qzawa: J . Therm. Anal. 3 , 301 (19712i
El43 H , J , Hin-ia, E , k . Ludwiczak: "Gluminium-Lithium Rlloys 111°C.
Baker et Al. Eds. The Institute uf Metals, London, 1986, page 471
Tab I - Chemical Gnalysis of the 2091 alloy
2 . 0
] 2 . 1 I
I." .. .. ................
Tab I1 - Rpparent Activation Energies obtained from the Qzawa plot for the D, E arid E peaks. (Values are in KJ/Mole) ( * Values not detectable)
...
...
" ". "
Temper D e. E
- ... ... - ... "" ... "" ... -- ".. ... "" .- .-.- "" " ... " "".""
160 * / 10 min / 110
1
163 I /I)1
Fig 1 - DSC traces obtained a t a scan speed of 50 K/"min for the investigated samples, The baseline for the 1 hour sample is also reported
(dotted).
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F i g 2 - DSC t ~ a c e s n b t a i n r d fnr t h e 113 min s a m p l e at d i f f e r e n t s c a n n i n g r a t e s 1 5, 10, 20, 30, 50 K/min j ,
T( C)
F i g 3 - D s c t r a c e s o b t a i n e d a t d i f f e r e n t s c a n n i n g r a t e s f o r the 90 h s a m p l e ,
Fig (b)
4 - Ozawa plot i log 9 V S samples,
1 / ~ - 1 0 ~ K-'
for the 10 min (a) and
Fig 5 - TEM image of the 1 0 Fig L - TEM image of the 90 h rnin sample; a T I particle is sample; large S' particles and well visible (arrowed top); 5' CuMgF112 precipitates are the diffraction pattern shows visible; spherical ZrA13 parti- the typical hez:agonal trend cles can be also detected,
(right) , Lamellar Zrfil dispersoids can be also detected (arroued bottom) having the same diffraction pattern of the matri:.: (left),
