Enhancement of an intermediate phase in NiTi

S o l d State Commumcatlons, Vol 86, No 11, pp 755-758, 1993 Printed m Great Britain

0038-1098/93 $6 00 + 00 Pergamon Press Ltd

E N H A N C E M E N T OF AN I N T E R M E D I A T E PHASE IN NITI Hltoshl Matsumoto Department of Matermls Science and Engineering, The National Defense Academy, Hashlrlm~zu, Yokosuka 239, Japan

(Received 25 November 1992, m revtsed form 13 January 1993 by G Fasol) The generation of an intermediate phase on the transformanon of N150T150 and N148T152 was investigated by a calorimetric method Transformation cycles and the anneahng after deformation contribute to stabilization of the mtermedmte phase, so that the transformation behavior vanes from a one-stage to a two-stage transformation due to the generation of a new intermediate phase It is thought that lattice defects play an important role m the enhancement of intermediate phase and relate intimately to the transformation mechanism

1 INTRODUCTION I N V E S T I G A T I O N S on physical and mechamcal propernes of near eqmatomlc N f f l alloy have been often performed [1-10], because the NIT1 alloy shows a thermoelasnc martensltlC transformanon in the wclnlty of room temperature which leads to the mechanical features called "shape memory" and "pseudoelasnclty" Its transformation behavior is sensitive not only to the compositional varmtlon [11-14] but also to thermal cycling through the transformanon [15-18] The mtermedlate (rhombohedral) phase appears when the high-temperature phase (CsC1) of NIT1 ~s martensltlcally transformed into the lowtemperature (monochnlc) phase [19-21], which ~s characterized by an increase in electrical reslsnvlty or a negatwe temperature coefficient in resistivity [15, 16] With increasing thermal cycles through the transformanon, such a resistivity anomaly is enhanced, gradually, and the temperature region of the intermediate phase is broadened The premartensltlC phenomenon such as the formation of the lntermedmte phase is often observed in N1TI alloy containing an addmve element, this comphcates the transformanon behavior The author thinks that the formation of the lntermedmte phase with thermal cychng is an intrinsic phenomenon which presents a feature in the transformation mechanism of NIT1 and transformanon-mduced defects play an important role in the transformanon procedure In the present work, effects of plastic deformation m the transformation behawor of NIT1 have been studied by a dlfferentml scanning calorimetry (DSC)

It is emphasized that the intermediate phase is also stablhzed in the recovery process due to anneahng, m addition to cychng through the transformation 2 EXPERIMENTAL PROCEDURE NIT1 alloys were prepared from nickel (purity, 9 9 9 9 % ) and titanium (purity, 9 9 9 % ) by the electron-beam melting method After several remelts, the alloy mgots were annealed at 1273 K m a vacuum of 10 -4 Pa for homogenization The alloy compositions studied were NlsoT150 and NIasTI52 A plate was cut off the alloy ingot and was cold-rolled The transformation behavior was measured thermally, using a differential scanning calorimeter (Du Pont, TA 2000-910DSC) Details of the preparanon and the calorimetric measurement have been given elsewhere [13, 22] 3 E X P E R I M E N T A L RESULTS A N D DISCUSSION Figure 1 shows the DSC curves during cooling for NlsoTl50 where the number corresponds to the number of thermal cycles through the transformanon The exothermlc peak becomes spilt into two after 10 thermal cycles have been made This produces a two-stage transformation hightemperature phase ~ lntermedmte phase ~ lowtemperature phase [18] W~th increasing thermal cycles, the exothermlc peak due to the transformation to the intermediate phase shifts to a higher temperature region at the rate of about 0 5 K per thermal cycle, while the peak due to the transformation from the intermediate phase to the

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Fig 1 The exothermlc behawor m relation to the number of thermal cycles of transformation made in N15oTzso low-temperature phase moves towards the low temperature side at the rate of about 1K per thermal cycle Therefore, the intermedlated phase of NlsoT150 is stabilized with increasing transformation cycles or transformation-Induced defects because its temperature region is broadened, as shown in Fig 1 Figure 2 shows the DSC curves obtained during cooling for N148T152 The transformation temperature of N148TI52 is higher than that of Ni50Tlso Although the exothermlc peak shifts to the low temperature side, the transformation of NI48TI52 remains a one-stage transformaUon, even after 70 thermal cycles This suggests that the formation of the lnterme&ate phase caused by thermal cychng depends on the c o m p o s m o n or the transformation temperature Figure 3 shows the exothermlc behavior after anneahng at temperatures of 623, 673 and 723 K for NisoTls0 cold-rolled by the cold reduction percent of 27%, together with that for the as-rolled sample The broad hump is observed after cold-rolling, as shown in Fig 3, which is taken to be attributable to the broadening of the temperature regmn of transformation Two humps an the exothermlc curve appear after the annealing at 6 2 3 K and become sharper with Increasing the annealing temperature

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Fig 2 The exothermlc behavior in relation to the number of thermal cycles of the transformation made in N148T152

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Fig 3 The exothermlc behavior after annealing at temperatures of 623, 673 and 723K on the transformation of a N150Tl50 sample cold-rolled by the cold reduction percent of 27%, together with that for the as-rolled one

Vol 86, No 11

E N H A N C E M E N T OF A N I N T E R M E D I A T E PHASE IN NIT1

Therefore, the intermediate phase is enhanced not only by thermal cycling but also by the annealing treatment after cold-rolhng It can be speculated that the effect of anneahng which ~s shown in F~g 3 may be due to the annlhdatlon and rearrangement of dislocations introduced by the plastic deformation with rolhng Such an effect of the deformation ~s also observed for the Nla8T152 alloy which shows no lntermedmte phase with thermal cychng, as described below Figure 4 shows the exotherm~c behavior after annealing at temperatures of 673 and 723K for NiasT152 cold-rolled by the cold reduction percent of 23% [23] Two peaks are seen in Fig 4 though they overlap The intermediate phase has a tendency to d~sappear at a low annealing temperature in companson with that of NlsoT150, because each peak for NlsoT150 IS spilt, even under the conditions of the cold reduction percent of 12% and the higher annealing temperature of 723 K This behavior seems reasonable, because the lntermedmte phase of N148T152 is not formed even with thermal cycling and is relatively unstable It is concluded from these experiments that lattice defects play an important role in the occurrence of mtermedmte phase, though detads of defects have not yet been suffioently characterized and the contribution of defects to the stablhty of each phase has not been estimated energetically Recently, a modulated lattice relaxation model has been suggested m order to explain the transformation process for matermls hawng a soft mode or a dip in '

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the phonon dispersion relation, by which the behavior of local phonon near the defect such as the embryo of intermediate phase In the high-temperature phase is shown the temperature dependence of the resultant dlffract~on patterns has been widely studied m the premartensmc temperature region [24-26] It ss reasonable to assume that the formation behavior of the lntermedmte phase m th~s model ~s sensitive to the presence of various lattice defects Therefore, it is thought that the experimental results presented here support the modulated lattice relaxation model as a transformation mechanism m the premartensit~c phenomenon To clarify the contribution of defects to the Intermediate phase, detaded microscopic studies are needed REFERENCES 1 2 3 4 5 6 7 8 9

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Fig 4 The exothermlc behavior after anneahng at temperatures of 723 and 673 K on the transformation of NI48T152 cold-rolled with the cold reduction percent of 23%

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J E Hanlon, S R Butler & R J Wasllewskl, Trans Met Soc AIME239, 1323 (1967) N G Pace & G A Saunders, Phtl Mag 22, 73 (1970), Sohd State Commun 9, 331 (1971) O Mercier, K N Melton, G Gremaud & J Hagi, J Appl Phys 51, 1833 (1980) O Mercier, B Tlrbonod & E Torok, J de Phys C5, 1037 (1981) H TIetze, M Muller & B Renker, J Phys C17, L529 (1984) R Kolodzlej & J Soltys, J Mater Sct Lett 11, 349 (1992) H C L l n g & R Kaplow, Met Trans A 11A, 77 (1980) J L McNlchols Jr, P C Brookes & J S Cory, J Appl Phys 52, 7442 (1981) S Mlyazakl, K Otsuka & Y Suzuki, Scrtpta Metal 15, 287 (1981) O Mercier & E Torok, J de Phys C4, 267 (1982) C M Hwang & C M Wayman, Scrzpta Metal 17, 381,385 (1983) K Sugimoto, K Kamei, T Suglmoto & T Sodeoka, Proc lnt Conf on Martenstttc Transformattons, p 729 Japan Institute of Metals Sendai (1987) H Matsumoto & H Ishlguro, J Less-Common Met 144, L39 (1988), 153, 57 (1989) H Matsumoto, Physwa B160, 138 (1989) F E Wang, B F DeSavage, W J Buehler & W R Hosler, J Appl Phys 39, 2166 (1968) G D Sandrock, A J Perkins & R F Hehemann, Met Trans 2, 2769 (1971) C M Wayman, I Cornehs & K Shimlzu, Scrtpta Metall 6, 115 (1972) H Matsumoto, J Mater Sct Lett 10, 408 (1991) K Chandra & G R Purdy, J Appl Phys 39, 2176 (1968)

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ENHANCEMENT OF AN INTERMEDIATE PHASE IN NIT1 K Otsuka, T Sawamura & K Shlmlzu, Phys Status Sohdt (a) 5, 457 (1971) G M Mlchal & R Sinclair, Acta Cryst B37, 1803 (1981) H Matsumoto, J Mater Scl Lett 10, 596 (1991) H Matsumoto, Mater Lett 11, 40 (1991)

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S M Shapiro, Y Noda, Y FUJll & Y Yamada, Phys Rev B30, 4134 (1984)

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Y Yamada, Y Noda & M Taklmoto, Sohd State Cornmun 55, 1003 (1985) Y Yamada, Bull Jpn Inst Met 31, 697 (1986) (m Japanese)