Transformation characteristics of aged NiTi shape memory alloy obtained by rapid solidification

Pergamon

Scripta Metallurg,ica et Materialia, Vol. 31, No. 1, pp. 53-56, 1994 Copyright ©1994 Elsevier ScienceLid Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00

TRANSFORMATION CHARACTERISTICS OF AGED NiTi SHAPE MEMORY ALLOY OBTAINED BY RAPID SOLIDIFICATION X . Z . W u , S . D . W a n g , J. P. Z h a n g , H . Q . S u . Analytic and T e s t i n g C e n t e r , Southeast U n i v e r s i t y , N a n j i n g , 210018, P. R. China. J. L. Jin, J. G. Zhang. Shanghai Iron and Steel Research I n s t i t u t e , Shanghai, 200940, P. R. China.

(Received January 25, 1994) (Revised March 14, 1994) Intr oduetion

P r e s e n t l y , the R and martensitic transformation of TiNi alloy can be separated by aging or adding a third e l e m e n t E 1 , 2 ] . It is generally accepted that in the cooling process the TiNi alloy experiences B2 (parent phase)--~IC(Ineommensurate)--*-R(Rhombohedral)--~M( M a r t e n s i t e ) , in which B2--~IC t r a n s f o r mation is second order while IC--~R is first order[3"]. H o w e v e r , other analyses using the charge density wave ( C D W ) model and L a n d a u ' s theory have deduced that the B2--~IC is also a first order phase change E4,5]. But it has not been experimentally confirmed yet. Our present study aims at investigating the three transformations in rapidly solidified TiNi alloy.

Experimental Details

The TiNi r i b b o n , the m o t h e r a l i o y ( T i - - 5 0 . 8at. °/~Ni) of which had been remelted in a BN composite crucible, was made by melt-spinning in Ar a t m o s p h e r e . The liner velocity of pure copper chill roller (dia. -----230mm) is 21m/s. T h e ribbon obtained is 0. 0 5 - - 0 . 0 7 r a m thick and 5 - - 6 m m wide. lts composition is almost identical to that of its mother alloy. Heat treatment adopted was to age at 450"0 for l h and then air-cool. The t r a n s f o r m a t i o n temperature has been determined by a Perkin-Elmer D S C - - 7 analyser using a heating and cooling rate of 10"C/min. T h e X-ray diffraction experiment has been conducted by Regaku D / m a x RB X-ray diffractometer. The selected experimental temperatures correspond to the t r a n s f o r m a t i o n peak and its completion. The crystalline parameters adopted are B2 (a---- 0. 3 0 l n m ) ['6-], R ( a = 0 . 903rim, a----89.3" )['77, M ( a = 0 . 2898nm, b = 0 . 4108nm, C = 0 . 4646nm, 13----97.78")E8].

Results and Discussion

Figure 1 s h o w s the DSC curve of aged TiNi ribbon. On its cooling curve there are two obvious exothermic p e a k s , corresponding to R and martensitic transformation respectively. The difference of two t r a n s f o r m a t i o n temperatures is about 70"0, this demonstrates that aging treatment can separate the two t r a n s f o r m a t i o n s effectively) which guarantees the exactness of the analyses of the two t r a n s f o r m a t i o n s . According to the DSC c u r v e , temperatures 40"0, 2 5 ~ , 10"C, --45"0 and - - 7 0 ~ , corresponding to various t r a n s f o r m a t i o n stages ) are selected as experiment temperatures.

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Figure 2 shows the X-ray diffraction s p e c t r u m of the specimen at 40"C, indicating a typical B2 s t r u c ture. The strongest peak in the s p e c t r u m , (200)B2, is selected in speculating B2 phase content by variation of the intensity of this peak. Compared with that at 40"C, the X-ray diffraction spectrum at 25"C (Fig. 3) is different. The intensity of (200)B2 decreases obviously while the half-width increases about 1.3 t i m e s , and other peaks also change. But the diffraction spectrum of the R phase has not appeared yet. When the temperature decreases from 25"C to 10"C, according to X - r a y diffraction spectrums at 10"C (Fig. 4 ) , the dramatically changed peak (200)B2 changes is r e v e r s e , its intensity increases slightly while the half-width decreases about 33°~0. Based on the X-ray diffraction spectrums at different t e m p e r a t u r e s , the following process can be inferred. When the temperature decreases from 40"C to 25"C, lattice distortion of B2 takes place while the c o m m e n s u r a t e R has not yet formed to a perceivable e x t e n t . So the first half part of the DSC curve has c o n t r i b u t e d to the B2--~IC transformation. The unconventional intensity decrease and widening of B2 diffraction peaks may result from periodic modulation of the lattice after IC transformation takes place. By comparing Fig. 3 and Fig. 4, we noted that a considerable amount of R phase formed when the t e m perature decreased from 2 5 ~ to 10"C. T h e existence of (200)B2 and ( l 1 0 ) B 2 peaks indicates that some B2 phase remains without taking part in the B2--~IC-'~R transformation. According to X-ray diffraction s p e c t r u m s , when the temperature decreases from 35"C to 2 5 ~ , the dominant transformation is B2--~IC, which is e x o t h e r m i c , indicating that this transformation is a first order one. When the t e m p e r a t u r e decreases from 25C to 10C the dominant t r a n s f o r m a t i o n is IC--*-R. Since the two transformations give a perfect exothermic peak, it seems plausible that the IC phase is an intermediate of the B2 and R and the B2--~IC a pre-transformation of the B2--~R. Figure 5 shows the X-ray diffraction s p e c t r u m at - - 4 5 " C , analyses of which indicate the coexistence of B2, R and M phases. Compared with Fig. 4, the diffraction intensity of the R phase remains nearly equal, which indicates that the percentile of R phase remains unchanged. H o w e v e r , the diffraction intensity of B2 decreases obviously and diffraction peaks ( 0 0 2 ) M , ( 0 2 0 ) M , (121)M and (003)M appear~ all these demonstrate a stage in which the B2--~M transformation takes place but it is not completed. Because of the low s y m m e t r y of the m a r t e n s i t e c h a n g e , it is not as obvious as that of the B2 phase. As shown in Fig. 6 in the X-ray diffraction spectrum at - - 7 0 " C , diffraction peaks of the B2 phase disappear completely! (424)R and (601)R p e a k s , formerly covered in (200)B2, come out. The diffraction intensity of each peak increases to its own maximum extent which d e m o n s t r a t e s the completion of the B2--~M transformation. M e a n w h i l e , t h e r e is little difference between the diffraction intensity of the R phase in Fig. 4, Fig. 5 and Fig. 6, suggesting the high stability of the R p h a s e , it does not transform to martensite when further cooled. Generally speaking, for an aged NiTi specimen obtained by rapid quenching in a cooling p r o c e s s , one part of its parent phase ( B 2 ) experiences a B2"--~M t r a n s f o r m a t i o n , while the other part experiences a B2--~IC--~R t r a n s f o r m a t i o n , after which the formed R no longer t r a n s f o r m s to martensite.

Conclusions

Transformation characteristics of an aged NiTi made by rapid solidification in cooling process are as follows : 1. One part of the parent phase (B2) experiences B2---~IC-~'R t r a n s f o r m a t i o n , while the other part experiences B2-~M t r a n s f o r m a t i o n , which needs lower temperatures. 2. B2--~IC t r a n s f o r m a t i o n is e x o t h e r m i c , suggesting that it is a first order phase change. 3. T h e two t r a n s f o r m a t i o n s B2--~IC and IC-~R form a perfect exothermic peak. T h u s , the IC phase may be thought as an intermediate s t r u c t u r e and B2-~IC transformation a p r e - t r a n s f o r m a t i o n of B2--*-R. 4. T h e R phase in this specimen is of high stability~ It normally does not transform to m a r t e n s i t e , suggesting that the aging t r e a t m e n t in the rapidly quenched NiTi can effectively depress the R---~M transformation.

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References

1. S. Miyazaki and k . O t s u k a , Met. Trans. , 1 7 : 1 ( 1 9 8 6 ) , A 5 3 . 2. O . M a t s u m o t o a n d T . H o n m a , Supplement to Trans. J I M , 1 7 ( 1 9 7 6 ) . 3. C . M . Wayman, The International Symposium on Shape Memory Alloys, Edited by Chu Youyi et al, Guilin, China, Sept. 4 - - 9 , (1986), 59. 4. Wang Weizhong, Qian H u a , Wang Zixiao, Chinese Phys. Letts. ,8(1989)363. 5. Wang Weizhong, Qian H u a , wang Zixiao, Chinese Phys. Letts. ,1 (1990)48. 6. C . M . J a c k s o n , H . J . Wagner and R . J . Wasilewski, N A S A - S P 5110,(1972). 7. K . C h a n d r a and G . P . P u r d y , 3. Appl. P h y s . , 39(1986)2176. 8. Y . K u d o h and M . T o k o n a m i , Acta Metall. , 33:11(1985),2049.

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