Dissociation of dislocations in Al67Mn8Ti26 deformed at ambient and high temperatures

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Scripta Metallurgica et Materialia, 28, 12, pp. 1519-1523, 1993-06-15

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Dissociation of dislocations in Al67Mn8Ti26 deformed at ambient and

high temperatures

Wu, Xiaohua; Rong, Yonghua; Chen, Shipu; Hu, Gengxiang

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S c r i p t a M E T A L L U R G I C A Vol. 28, pp. 1 5 1 9 - 1 5 2 3 , 1993 P e r g a m o n P r e s s Ltd. et M A T E R I A L I A P r i n t e d in the U . S . A . A l l r i g h t s r e s e r v e d

D I S S O C I A T I O N O F D I S L O C A T I O N S I N A h , M n . T i , , D E F O R M E D A T A M B I E N T A N D H I G H T E M P E R A T U R E S

Xiaohua W u , Yonghua Rong, Shipu Chen and Gengxiang H u Department of Materials Science. Shanghai Jiao Tong University,

Shanghai 200030, P.R.China ( R e c e i v e d J a n u a r y 14, 1993)

( R e v i s e d M a r c h 30, 1993) Introduction

The LI~ structured AljTi-based alloys are n o w of considerable interest because they have shown appreciable compressive ductility at room temperature. It has been determined that the slip systems in LI, A h T i alloys are (110){111}, but studies on the dislocation structures in the deformed alloys give conflicting results [l-SJ. Earlier observations by transmission electron microscopy ( T E M ) on the room temperature deformed AI,vNi,Ti. [1,21 or AhTi,Fe E3] samples suggested that the a(ll0) dislocations moving on {Ill) planes are undissociated. Further works on the Fe-modified LI, alloy have shown that at room temperature the a(ll0) dislocations tend to dissociate into a/2(ll0) partials on {Ill} planes separated by antiphase boundary (APB)[4-61. while others cited that the dissociation is of the a/3~ll2)-type with the superlattice intrinsic stacking faults (SISF) between two partials on {III} planes C7,8]. T E M studies on the dislocations in AhTi-based Ll, alloy deformed at elevated temperatures are limited. Lerf and Morris [31 reported that the majority of the dislocations in the Fe-modilied alloy after 500"C deformation are dissociated as pairs of a/2(110) on {111} planes with A P B between, and the results of Inui et al [81 showed, also, the APB-coupled a/2(110) superpartials at 600"C but on {001} planes.

In the present study, the dislocation structures and dissociation modes of superdislocations in the AI,vMn,Ti, alloy deformed at ambient and elevated temperatures were investigated by T E M . The temperature dependence of mechanical properties of this alloy are discussed in relation to the observed dislocation structures.

Experimental

A n intermetallic alloy with nominal composition Al.,Tiz,Mn, (ate) was prepared by arc melting in argon on a water cooled copper hearth. The button ingots were homogenized at 1373K for 60h. Samples with dimensions 4ram x 4ram x 7ram were cut from the homogenized ingots and deformed under compression to ~ 2 % plastic strain at room temperature, 673K,and 873K respectively. Thin disks of 3ram in diameter were spark cut from the deformed samples and then mechanically polished to ~ 1 0 0 u m in thickness. Final preparation of the thin foils for T E M observation was accomplished by twin jet polishing in a mixture of perchloric acid, butanol, and methanol (30:175:300 by volume) at 233K. Conventional and weak beam techniques were performed in T E M study.

Results

The general features of dislocation structure deformed at room temperature, 673K, and 873K are s h o w n in Fig.1 respectively. In c o n t r a s t to the long, straight dislocations observed in the room t e m p e r a t u r e deformed alloy (Fig.l(a)), the dislocations in 673K deformed sample axe curved, and more dipoles are seen (Fig.l(b)), while the 873K deformed sample shows random a r r a n g e m e n t of short, curved dislocation segments and dipoles (Fig.l(c)).

1 5 1 9

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1 5 2 0 D I S S O C I A T I O N O F D I S L O C A T I O N S V o l . 28, N o . 12

Detailed analyses were done to determine the slip system and dissociation mode of the superdislocations in the above mentioned samples. The dislocations in the sample deformed at room t e m p e r a t u r e consisting of paired partials, labeled A and B in Fig.2, were c h ~ e n l'or analysis. Trace analysis t h r o u g h a series of dislocation images t a k e n with different beam direcrions determined t h a t both A a n d B are lying on t h e (I11) plane. The Burgers vectors of the dislocations were examined by di/Traction c o n t r a s t analysis using various reflections. Table I gives the I r b values for the partials u n d e r different reflections and among them typical micrographs are shown in F i g ! I t was found t h a t the superdislocations formed during compressive deformation at room temperature are dissociated into two a/3<I12> superpartials on (111} planes, with SISF between them. For the dislocations A :

a [ 1 1 0 ] - , a / 3 [ 2 1 1 ] + S I S F + a / S [ 1 2 r ] , and for B: a [ 0 T I ] - , a / 3 [ ~ I ] + S [ S F + a / 3 [ I ~ 2 ; . T A B L E I . g'b Values for the Dislocations Formed by R ~ m Temperature Deformation under Different Reflections

22O 02~ 3Z~ 3Tl 131 i l l A z ( b = l / 3 [ 2 l t ] ) 2 ~=z/3f121]) obser, g'b obser, g'b V. 2 V. 2 I.V. 0 V. 2 V. 2 V. 2 V. 2 I.V. 0 I.V. 0 V. - 2 R.C. --2/3 V. - 4 / 3 l (b= I/3[i~l]) obser, g-b V, - 2 V. - 2 V. - 2 I.V. 0 V. 2 R-C. 4/3 B 2 (b= 1/311~2~) Fig. obser. I f b [.V o 2(a) V. - Z 2(b) [.V 0 V. , 2 V. ~- V. 2/3 2(e)

V. : visible, l,V. : invisible, R.C. : residual conu'ast

Analyses of the sample deformed at 6"/3K determined that the superdislocatiorm are dissociated in the s a m e m o d e as those in the r o o m temperature deformed sample.

The dislocations in the sample deformed at B73K are apparently dissociated, b u t the c o n t r a s t analysis shows t h a t b o t h partials are in c o n t r a s t or out st" c o n t r a s t simultaneously for all the operating reflections. This suggests t h a t the dissociation is of the a / 2 ( l 1 0 > type. Table !] lists the g ' b values and the weak-beam images used for the analysis are s h o w n in Fig.3. The Buygers vector of the superdislocation s h o w n in the mierograph was determined by the " i n v i s i b i l i ~ criterion" as all'01], end the dissociation is a[101]-*a/2[T01]+APB-I-a/2[T0t]. The dissociation plane was determined by the widest s e p a r a t i o n of t h e dissociated pairs t h r o u g h tilting of the sample, The result shows t h a t the APB is on the (111) plane (see Fig.3(b)), and the measured separation is ~ l T n m . The APB plane is checked f u r t h e r by observing the change of dislocation configuration when viewed in dilIerent orientations. The possible dissociation planes for the [~01] superdislocation are ( l U ) , (1TI) of (111} a n d (010) of {001}. Since the curved dislocations did not appear s t r a i g h t when the sample was tilted to a beam direction [001] or [ 0 I l l , t h e dislocation plane must n o t be (010) or (1i"1) respectively.

TABLE iI. g'b Values for the Dislocations in 873K Deformed Sample 020 111 fly ~-2O ~02 b = b , = b , = I/2[T01] obssr, g'b I.V. 0 I.V. 0 V. - 1 V. l V. 2 V. 2 Fig. 3(a) 3(b) 3(c) V. : visible, I.V. : invisible

Vol. 28, No. 12 D I S S O C I A T I O N OF D I S L O C A T I O N S 1521

Discussion

The T E M studies of dislocation structures in the deformed A I . , M n . T i . indicated that at ambient and intermediate temperatures, e.g. 673K, the a ( l l 0 ) superdislocations dissociate into a/3(I12) superpartials bounding SISF on {111) planes, and at higher temperatures, e.g. 873K, they dissociate into a/2(110) bounding APB on {III) planes. It has been shown by atomistic modeling [9] that the dislocation core structure of a/3(112) superpartials is nonplanar and is, therefore, sessile. Their motion requires a thermally activated process, so the yield stress will decrease rapidly with increasing temperature. But in the AI.vMn, Ti.-. alloy, the yield stress declines slowly from room temperature to 523K, and then shows a plateau with increasing temperature (Fig.4) [10]. This phenomenon may be related to the strong tendency of dipole formation with increasing temperature in this alloy (Fig.l). Thus, a high stress is required to break these dipoles and a plateau temperature dependence of the L1, AI.Ti alloy result s.

Another interesting feature of the A I . , M n , T i , alloy is the rapid increase of compressive ductility with temperature above 773K, while the yield stress is almost constant (Fig.4) [10]. This behavior can be explained by the dissociation mode of superdislocations at higher temperatures. It has been generally accepted that the symmetry of the L1. structure assures the stability of a/2(110) APB on COOl) but not on {iii}. and, therefore, cress slip of a/2(l10) screw dislocations from {III} to {001} leads to the anomalous temperature dependence of yield stress in some Llm alloy8 such as Ni.AI. In the present case, the a/2(l10) superpa~ials are on {III} planes, and no {001} croas slip was observed by the weak-beam electron microscopy. It is also shown by atomistic modeling [9] that w h e n a/2(110) superpartials with A P B are on the (III) plane, one of the three core configurations of a/2(l10) is glissile, with the core spreading in the A P B plane. Thus a(ll0) superdislocation8 can move on the {III} planes and contribute to the plastic deformation of AI.Mn.Ti. at higher temperatures, and the ductility of this alloy is greatly improved.

Conclusions

In the LI. AI,TMn.Ti. alloy deformed at room temperature and 673K. the a(ll0) superdislocations dissociate into two a/3(112) superpaxtials with SISF between them on {111} planes, while at 873K. the dissociation m o d e changes into a/2(l10) partials separated by A P B on {111} planes. The temperature dependence of mechanical properties can be explained adequately by the dislocation structures and dissociation modes at different temperatures.

References

1. V.K.Vasudevan R.Wheeler and H.L.Fraser in High-Temperature Ordered Intermetallic Alloys, edited by C.T. Liu, A.I. Taub, N.S. Stoloff and C.C. Koch (Mater. Res. Soc. Syrup. Proc.133, Pittsburgh, PA, 1989)pp.705

2. C.D. Tuner, W.O. Powers and J.A. Wert, Acta Metall 37, 2635 (1989) 3. R. Lerf and D.G. Morris, Acta Metali $9, 2419 (1991)

4. E.P. George, J.A. Horton, W.D. Porter and J.H. Schneibel, J. Mater. Res. 6, 1639 (1990)

5. H.R.P.Inoue, C.V.Cooper, L.H.Favrow,Y Hamada, and C.M.Wayman in High-Temperature Ordered Intermetallic Alloys, edited by L. Johnson, D.P. Pope and J.O.Stiegler (Mater. Res. Soc. Syrup. Proc. 213, Pittsburgh, PA, 1991) pp.493

6. D.G. Morris, J. Mater. Res. 7, 303(1992)

7. Hu Gengxiang, Chen Shipu, Wu Xiaohua and Chen Xiaofu, J. Mater. Res. 8, 957 (1991)

8. H. Inui, D.E. Luzzi, W.D. Porters, D.F. Pope, V. Vitek and M. Yamaguchi, Phil. Meg. A. 60, 1245 (1992)

9. V. Vitek, Y. Sodani and J. Cserti in High.Temperature Ordered Intermeta]lic Alloys, edited by L. Johnson, D.P. Pope and J.O. Stiegler (Mater. Res. Soc. Syrup. Proc. 213, Pittsburgh, PA, 1991) pp.195.

10. Xiaohua Wu, Shipu Chen, Yonghua Rong, Xiaofu Chen and Geng'ziang Hu, Proc. MRS 1992 Fall Meeting, Nov.30.Dec.4, 1992, Boston, to be published

1522 DISSOCIATION OF DISLOCATIONS Vol. 28, No. 12

Fig.1 BF image of dislocation st'ruct~re after deformation at: (a) room temperature, (b) 673K, and (c) 873K.

Fig.2 TEM micrographs for contrast analysis of the dislocation formed by room temperature. Beam direction: (a) [I"11], (b) [011], and (c)

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