TEM studies of the martensitic transformation in a Ti50Ni40Au10 alloy

Scripta METALLURGICA

Vol. 21, pp. 83-88, 1987 Printed in the U.S.A.

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TEM STUDIES OF THE MARTENSITIC TRANSFORMATION IN A Ti5oNi4oAUlo ALLOY S. K. Wu* and C. M. Wayman Department of Materials Science and Materials Research Laboratory University of I l l i n o i s at Urbana-Champaign 1304 W. Green Street Urbana, I l l i n o i s 61801, USA

(Received October 30, 1986) Introduction The occurrence of the shape memory effect (SME) in equiatomic and near equiatomic TiNi alloys has been widely recognized and investigated (I-6). The effect of ternary elements on the nominal TiNi alloy has been less studied. In one report (7), the effect of substitutions of up to 2 at.% of Ni or Ti by Au was explored, and i t was demonstrated that SME behavior was retained. To understand how Au affects both the "premartensitic" transition and martensitic transformation in TiNi binary alloys, we have studied Ti5oNi50~xAux alloys with x in the range of 5 - 50 at%. Experimental •results for Ti 5 0 Ni 5 P - ~~Auv with x=~, ~u and 50 w i l l be reported in other papers ^ (8,9). In thls report, TEM experlments on a TisoNi4oAUlo alloy are presented. Experimental Procedure Experimental procedures for alloy preparation and TEM specimens are similar to those for a TispNi45Au5 alloy ( 8 ) . The martensitic transformation temperature was measured using a fourprobe ~lectrical r e s i s t i v i t y measurement technique. Measurements were carried out at temperatures ranging from -196°C to 300°C, Electron diffraction and transmission electron microscopy were carried out using a Hitachi H-800 TEM with operating voltage 200 kV. A t i l t i n g cooling stage permitted the specimen to be t i l t e d and cooled from room temperature to -120°C. Experimental Results and Discussion Figure i shows the experimental results on electrical r e s i s t i v i t y vs. temperature for the TisoNi4oAu10 alloy. The M~ and Af temperatures are 20°C and 48°C, respectively. There is an e l e c t r i C a l - r e s i s t i v i t y anomaly (increase) in the range of 20°C ~ 270°C. Figures 2(a) and (b) show d i f f r a c t i o n patterns (D.P.) from the [011]B2 and [111182 zones, repectively, for the Ti 50Ni4oAu10 alloy at room temperature. Diffuse I/3-reflections and general diffuse electron scattering can be observed. This means that the "premartensitic" R phase exists, but is p a r t i a l l y suppressed in this a l l o y , as with the Ti50Ni45Au5 alloy (10). Figure 3 shows (OOT)M twins in the TisnNi4oAu10 martensite. Figures 3(a), (b) and (c) are the bright f i e l d image and corresponding D.P.'s from the [120] M and [130] M zones, respectively. Note that there are streaks along c*. Figure 4 shows (lIT)u twins in the TisoNi4nAUln alloy. Figures 4(a) and (b) are dark f i e l d images using imaging with'1(OO2)M and (002)T sp~s, Xe) is the corresponding D.P._of (a), and (f) and (g) are index diagrams f o r - ( e ) . From Fig. 4 ( f ) , the martensite shows a [110] M zone. From Fig. 4(g), the orientation relationship between B2 and the martensite is as TomIows. [111]B2 m [ I [ 0 ] M (101)B2 about 7° from (001)M The above relation is in good agr_eement with that predicted f r o m the phenomenological crystallographic theory i f type I (111)M twins are taken as the l a t t i c e invariant shear ( I I ) . Note the (001) M stacking faults in the martensite which induce streaks along c* in Fig. 4(e). Figure 5 shows type I I [011] M twins in the martensite. Figure 5(a) is a bright f i e l d image, ~Present Address: Department of Mechanical Engineering and Department of Materials Engineering, National Taiwan University, Taipei, Taiwan, Republic of China.

83 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd.

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(b) is a dark field image using the (I02) T spot, (c) is the corresponding D.P. of (a) and ~d) is the index diagram for (c). From Fig. 5, we find that the martensite matrix features the [213]M zone, the twin is in the [231]u zone, and the width ratio is about 2.5:1, as observed from Fig. 5(b). The ratio is in go'Bd agreement with the predicted ratio of 2.69:1 using the phenomenological theory for the TiNi binary alloy (11). From Figs. 3 - 5, we find that the martensite of Ti5oNi4oAuIN is monoclinic, 2H. Type I (lIT) M twins, type II [011] M twins, compound (OOT)M twin~ ~ d (OOT)M stacking faults were observed in this martensite. The orientation relationship is also ~n good agreement with theoretical predictions. One main difference between the Ti~nNi4nAu10 and the Ti~nNi4~Au6 martensites, although both have the same BIg'(2H) structure and s~ila-r ~s temperatures~-is £ha£ the former does not easily form thermal martensite in TEM specimens (perhaps from a thin foil effect), but the latter does. This difference requires forming stress-induced martensite in the Ti~nNianAuln specimen f i r s t and then electropolishing i t . This specimen preparation procedure 1ea -a s tov some .v difference in morphology comparing the Ti6oNi4oAu10 and Ti50Ni45Au5 martensites, especially as to the presence of type II [011] M twinning C8~. Acknowledgement The authors acknowledge the support of the Government of the Republic of China and the National Science Foundation through the Materials Research Laboratory at the University of I l l i n o i s , Grant NSF/DMR83-16981. References 1. W. J. Buehler, J. V. Gilfrich and R. C. Wiley, J. Appl. Phys., 34, 1475 (1963). 2. W. J. Buehler and F. E. Wang, Ocean Eng., 1, 105 (1968). 3. C. M. Jackson, H. J. Wagner and R. J. WasiTewski, NASAReport SP-5110 (1972). 4. L. Delaey, R. V. Krishnan, H. Tas and H. Warlimont, J. Mat. Sci., ~, 1521, 1536, 1545 (1974). 5. G. R. Edwards, J. Perkins and J. M. Johnson, Scripta Met., 9, 1167 (1975). 6. K. Otsuka and C. M. Wayman, in "Reviews on the Deformation Tehavior of Materials," ed. by P. Feltham, Freund Publishing House, Ltd., Israel, Vol. 2, No. 2, 1977. 7. E. K. Eckelmeyer, Scripta Met., I0, 667 (1976). 8. S. K. Wu and C. M. Wayman, to be--p-ublished. 9. S. K. Wu and C. M. Wayman, to be published. 10. S. K. Wu and C. M. Wayman, to be published. I I . K.M. Knowles and D. A. Smith, Acta Met., 29, 101 (1981).

Ti 50Nt 40Aulo Ms - 20 "C Af " 48 "C Resistivity ano~ly: 20'C - 270'C Ms

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Electrical resistivity vs. temperature p l o t f o r Ti 50Ni4oAUlo al l o y .

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(a) and (b) are [Oil]B2 and [111]B2 zones of the Ti5oNi4(}Au1(1 a11oy at room temperature. Diffuse I/3-reflec{i'ons and normal electron diffuse scattering can be observed.

twins in Ti&nNi4nAUlo site. (a) B~Tght-field (b) D.P. of [120] M (c) D.P. of [130] M zone

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¢

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(111) M t w i n s in TIBoNt4oAu]o m a r t e n s i t e . (a) and (b) b r i g h t f i e l d Images w t ~ d i f f e r e n t m a g n i f i c a t i o n s , (c) dark f i e l d image using (002) M r e f l e c t i o n s (d) dark f i e l d image using (002) T r e T l e c t i o n , (e) D.P. of ( a ) , ( f ) and (g) l n d e x i n g diagrams f o r ( e ) . Note (O01)M s t a c k i n g faults in m a r t e n s t t e .

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(IOI)B2 about 7" from (001)M

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(d) o•

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oo matrix . . . twin . . . . .

Fig. 5.

{213)M z o n e [~.3~ T z o n e

Type I I [ O i l ] M twins in TisoNi4oAUlo martensite. (a) bright f i e I d image, (b) dark f f e l d image using (I02) M spot, (c) D.P. of (a), (d) indexing diagram of

(c).