Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982428

CoZZoque C4, suppZ6rnent au n o 12, Tome 43, de'cembre 1982 page

NEUTRON D I F F R A C T I O N MEASUREMENTS I N E Q U I A T O M I C N i T i

-

A L L O Y S

W. Biihrer, R. ~otthardt: A. ~ulik*+and 0. ~ercier**

I n s t i t u t fur Reaktortechnik, % i s s Federa2 I n s t i t u t e of Technology, 5303 Wiiren Zingen, Switzer Zand

" ~ n s t i t u t de Ge'nie Atornique, Swiss Federal I n s t i t u t e of TechnoZogy, 1015 Lausanne, SwitzerZand

* * ~ w i s s Federal I n s t i t u t e for Reactor Research, 5303 WZirenZingen, Switzer Zand

(Accepted 9 August 1982)

Abstract.- Neutron diffraction spectrums of the different phases found in binary NiTi alloys are presented. From thesemeasurementsit is possible to separate the influence of precipitates and to differentiate between diffe- rent proposed models for the martensitic structure. No extra peaks due to the R-phase are observed.

Introduction.- It is now widely accepted that the unusual mechanical properties of the shape memory NiTi alloy are related to a martensitic phase transformation from a high temperature CsCl (B2) structure to one of lower symmetry. However, un- til now, no satisfactory structure of the latter was proposed, even if there is an agreement to interpret it as a monoclinic distortion of either the h.c.p. (A3) or AuCd (B19) structures (1,2,3). The disagreement is on the type and the direction of the atomic shuffles occuring during the transformation. In addition, interme- diate phases were also observed in the same range of temperatures ( 4 , 5 , 6 , 7 ) and little is known on their structures. Therefore it is necessary to improve our know- ledge in this field, specially on the experimental side. Indeed, one of the impor- tant obstacles is the absence of good experimental diffraction spectrums of the mar- tensitic phase. The only presently published diffractometry scans from NiTi powder are those of Wang et a1 (8) ; in their paper, no analyse (height, width and shape) of the experimental peaks are given and thus no elaborate comparison between cal- culated and measured structures can be accomplished. Therefore, a new experimental study was undertaken and the first results are presented here.

The experiments were carried out with the neutron diffraction technique, because the volume, which is examined by this technique, is large (>lcm3). This eliminates the problem encoutered in X-ray transmission method, where the measurement is often perturbated by precipitates, located in the small viewed volume. In addition, with the neutron technique, the observed effects are certainly bulk effects and not sur- face effects, as it could be in the electron microscopy specimens. The Ni atoms are well distinct of the Ti atoms, which is not the case with the other diffraction

techniques.

In a first step, the measurements were done on fine grains bulk specimens, in order to avoid the problem of the fine powders, where the martensitic transformationcould be suppressed by a size effect.

Experimental.- Specimens were cut out of 2 different ingots of nominal composition 45 wt% Ti and 55 wt% Ni (Table 1). A subsequent analysis (Johnson Matthey Chemicals Ltd, England) has shown an actual composition of 45.05 wt% Ti 54 wt% Ni 0.7 wt% Cu and 0.25 wt% C.

Neutron diffraction patterns were recorded by means of a double axis spectrometer at the reactor Saphir in Wgrenlingen. A vertically bent pyrolithic graphite mono- chromator was used, the wavelength was 2.34

8,

and a graphite filter was inserted in the monochromatic beam in order to remove higher order contaminations. The sam- ples were enclosed in cylindrical vanadium containers and placed in a CTI closed-

+on leave' from IPPT PAN, Fiarsaw, Poland

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982428

C4-220 JOURNAL DE PHYSIQUE

Fig.3 : Neutron diffractim spectrums measured in a partially transformed NiTi-al-

-

loy, specimen 2, (a) ; calculated for different models proposed by HS (b), MS (c) and OSS (d), (see also Tables 3 and 4).

Fig.1 : Neutron diffraction spectrum of I

A

-

a a NiTi alloy containing precipitates (spe-

-

.- N

.-

20 30 40 50 60 28 +

6 0 cimen 1)

-

6

7 -

_{o Fig.2 :} Neutron diffraction spectrums

- g ?

measured in an austenitic NiTi Alloy :

5 0 4

(a) specimen 2; (b) specimen 3 ; (c)

I - - e

.-.

7

- -

.-.

- - -

-

1 ,

^,

:,

^{8 2c)}

30 60 90 2 8

-

^I

-

calculated.

- -

-

-

7 .-

.-

i

^I

4 I

^ I

-

7

-

R e s u l t s and d i s c u s s i o n s

P r e c i p i t a t e s peaks.- F i g . 1 shows t h e n e u t r o n d i f f r a c t i o n s p e c t r u m o f t h e N i T i s p e c i - men l ( f i r s t i n g o t ) , w h e r e b o t h m a r t e n s i t e (M) and p a r e n t p h a s e (A) a r e p r e s e n t a t t h e measurement t e m p e r a t u r e (220K). The h e i g h t of t h e p e a k s due t o t h e s e 2 p h a s e s chan- g e s when t h e specimen t e m p e r a t u r e i s v a r i e d between Ms and MF o r As and AF. A d d i t i o - n a l r e f l e c t i o n p e a k s a r e a l s o f o u n d , w i t h a h e i g h t , which do n o t v a r y w i t h tempera- t u r e ; t h e y a r e i d e n t i f i e d w i t h p r e c i p i t a t e s of t h e T i 2 N i and T i c t y p e s ( T a b l e 2 ) . I t i s c l e a r t h a t t h e p r e s e n c e of s u c h p r e c i p i t a t e s p e a k s c o m p l i c a t e s t h e i d e n t i f i c a - t i o n of e a c h p e a k , s p e c i a l l y a t l a r g e 8 ; i t changes a l s o t h e i n t e n s i t y o f t h e g a r - t e n s i t e p e a k s , l o c a t e d a t t h e same v a l u e of 8 ; ( f o r example, t h e (100) M and (101)M p e a k s i n F i g . 1 ) . F i g . 2a shows t h e s p e c t r u m o f specimen 2 measured a t room tempera- t u r e . T h r e e main p e a k s a r e o b s e r v e d , l o c a t e d a l m o s t e x a c t l y a t 28 v a l u e s , c a l c u l a - t e d f o r t h e CsCl s t r u c t u r e ( F i g . 2 c ) . The s m a l l a d d i t i o n a l p e a k , o b s e r v e d a t a b o u t 28 = 5 6 . 1 0 ~ i s due t o T i c p r e c i p i t a t e s and i t s h e i g h t i s t e m p e r a t u r e i n d e p e n d a n t . T h i s peak i s l o c a t e d a t a n a n g l e where no o t h e r CsCl o r m a r t e n s i t e p e a k s a r e p r e - s e n t and b e c a u s e t h e o t h e r T i c p e a k s a r e t o o weak t o b e v i s i b l e i n o u r s p e c t r u m s , a l l t h e o t h e r measurements were done o n s p e c i m e n s , c u t o u t of t h i s i n g o t . However, i n comparing t h e h e i g h t of t h e p e a k s of F i g . 2a and 2 c , one n o t i c e s t h a t t h e y d o n o t match. The measurement ( F i g . 2b) of specimen 3 , c u t i n a d i r e c t i o n p e r p e n d i c u - l a r t o t h e one o f specimen 2 shows s t i l l a n o t h e r s e t of i n t e n s i t i e s f o r t h e s e 3 CsCl p e a k s . T h i s i n d i c a t e s t h a t o u r s p e c i m e n s were n o t t e x t u r e f r e e , which had b e e n con- f i r m e d by X-ray measurements.

M a r t e n s i t i c p h a s e

At 70K, t h e specimen 2 p r e s e n t s a m i x t u r e of m a r t e n s i t e and p a r e n t p h a s e ( F i g . 3 a ) . The i n t e n s i t y of t h e m a r t e n s i t i c p e a k s a r e c e r t a i n l y changed by t h e t e x t u r e , b u t a l l t h e p e a k s a r e n e v e r t h e r l e s s p r e s e n t and a c a r e f u l 1 comparison of t h e i n t e n s i t y of o u r p e a k s w i t h t h o s e of Wang e t a 1 (8) ( T a b l e 4) shows t h a t t h e d i f f e r e n c e s i n h e i g h t a r e always s m a l l e r t h a n 50%. T h e r e f o r e , o u r d a t a c a n b e u s e d t o v e r i f y t h e e x i s t i n g models of t h e m a r t e n s i t i c s t r u c t u r e * , e v e n i f i t i s n o t p o s s i b l e t o r e s o l - ve models, where t h e o n l y d i f f e r e n c e i s i n t h e r e l a t i v e i n t e n s i t v o f t h e p e a k s . The m a r t e n s i t i c p e a k s were i n d e x e d a c c o r d i n g t o t h e s e c o n d s e t t i n g ( s e e T a b l e L.)Dneobser- v e s a g e n e r a l agreement between t h e e x p e r i m e n t a l and c a l c u l a t e d peaks ( T a b l e 4 and F i g . 3 ) , e x c e p t t h a t

1 ) t h e (010) peak, p r e d i c t e d by O t s u k a e t a 1 (OSS) ( I ) , i s n o t o b s e r v e d 2) t h e (101) peak i s p r e s e n t , a s p r e d i c t e d by Hehemann e t a 1 (HS) ( 2 ) and

Michal e t a 1 (MS) ( 3 )

3 ) t h e (110) peak, p r e d i c t e d by MS and OSS i s n o t o b s e r v e d 4) h a l f of t h e c a l c u l a t e d peaks a r e h i g h e r t h a n t h e measured o n e .

The f a c t t h a t t h e (010) peak i s a b s e n t and t h a t t h e (101) peak i s p r e s e n t shows t h a t t h e model OSS h a s t h e wrong symmetry. The two o t h e r models, which b e l o n g t o t h e same s p a c e group and which o n l y d i f f e r by t h e t y p e of s h u f f l e a r e n e i t h e r one n o r t h e o t h e r i n good agreement w i t h t h e measurement, e v e n i f t h e i r symmetry i s p r o b a b l y c o r r e c t . T h i s i s c o n f i r m e d by t h e low v a l u e s of t h e r e l a t i o n f a c t o r R c a l c u l a t e d by ( 3 ) , b o t h f o r X-ray s p e c t r u m (R = 0.17 - 0.19) and f o r n e u t r o n spec- trum (R = 0 . 3 7 ) . P r o b a b l y t h e c o r r e c t s t r u c t u r e of t h e m a r t e n s i t e p h a s e h a s n o t b e e n found y e t .

*

I n T a b l e 3 , t h e 3 models (HS, OSS and MS) a r e c o r r e c t l y w r i t t e n i n t e r m of s p a c e group symmetry, which was n o t t h e c a s e i n ( 3 ) .

C4-222 JOURNAL DE PHYSIQUE

Specimen Ingot Type

1 1 plate

2 2 plate

3 2 plate c u t 1

4 2 wire

5 2 single

crystal (5) Table 1 : Origin and form

of the specimens

Alloy Structure Parameters Tic F.C.C. a,b,c = 4.3186

2

a,$,y = go0 Ti,Ni F.C.C. a,b,c = 11.32

2

a,$,y = 90°

Table 2 : Crystallographic data on Tic (10) and TieNi (11)

OSS (1) System

Space group a

(8)

b

(8

C

(8)

a

^(O,

V (O)

2 (e)

2(f)

Monoclinic

Table 3 : Crystallographic data on proposed NiTi martensite structures (1,2,3) HS (2)

Monoclinic P 112 l/m

2.883 4.623 4.117 96.8 - - ( X ~ Y , ~ ) (x,y,314)

Ti : x = 0.5 y = 0.3125 Ni : x = 0.0

y = 0.8125

Fig. 4 : Neutron diffraction spectrums showing the transition from Aus- tenitic to Martensitic

MS(3) Monoclinic P 112 /m

1 2.885 4.622 4.120 96.8

-

- (x,Y,*) (x,y,3/4) Ti : x = 0.3274

y = 0.279 Ni : x = 0.9476

y = 0.807

50.0

INTERNAL .80

TEMPERATURE (K)

Fig. 5 : Phase transformations during cooling and heating, measured by the evolu- tions of Resistance, Frequency and Internal Friction (M = Q210K)

The detail of the parent-martensite transformation was studied, in order to deter- mine if some premartensitic peaks could be seen. Indeed, on a single crystal of the same ingot, a softening of the transverse acoustical phonon mode in the (110) direc- tion and a new reflection peak (0,%2/3,%2/3) have been observed around room tkmpe- rature (5). Fig. 4 shows the decrease of the (100) A peak, simultaneously to the growth of the (001)M, (0ll)M and (100)M peaks, when the specimen temperature de- creases from 150K to 120K. No additional peak, due to the R-phase was found. How- ever,measurements made with other techniques (resistivity, anelasticity) on speci- men 4 show clearly that the premartensitic phases are present in this alloy. Indeed, one observes (Fig. 5) the increase of resistivity between 300 and 200K, the inter- nal friction peak Q-' and the minimum of the Young's modulus at 270K attributed to the R-phase (9) and preceeding the martensitic transformation.

We observe no supplementary peak in the neutron powder diagram, whereas a soft mode condensation was measured in a single crystal of the same alloy. The explanation is that on the one hand new reflection peaks due to the intermediate phases are too small to be seen (the measurement on single crystal are much more sensitive than the neutron diffraction measurement on polycrystal or powder) and that on the other hand the distortion of the CsCl lattice, due to the R-phase (6), which could broaden the existing CsCl peaks, is too weak to be measurable by the conven- tional neutron diffraction techniques. An alternative explanation is that the R- phase exists only locally, around lattice defects.

References

(1) OTSUKA K., SAWAMURA T. and SHIMIZU T., Phys. Stat. Sol. A

5

(1971) 457.

(2) HEHEMANN R.F. and SANDROCK G.D., Scripta Met.

5

(1971) 801.

(3) MICHAL G.M. and SINCLAIR R., Acta Cryst.

2

(1981) 1803.

(4) SALAMON M.B., MEICHLE M.E., WAYMANN C.M., HWANG C.M. and SHAPIRO S.M., Proc.

Int. Conf. on Modulated Structures, Amer. Phys. Soc. Proc. No

53

(1979) 223.

(5) MERCIER O., BRUESCH P. and BUEHRER W., Helv. Phys. Acta 5 3 (1980) 243.

(6) LING H.C. and KAPLOW R., Met. Trans. A (1980) 77.

(7) MICHAL G.M., MOINE P., SINCLAIR R., Acta Met.

30

(1982) 125.

(8) WANG F.E., PICKART S.J. and ALPERIN H.A., 3 . Appl. Phys.

43

(1972) 97.

(9) MERCIER O., MELTON K.N., GOTTHARDT R. and KULIK A., to be published in

"Proceedings of the International Conference on Solid.

-

Solid Phase Transitions", Pittsburgh (1981).

(10) WYKOFF R.W.G., Crystal Structure, Interscience Publishers, New York (1960).

(11) DUWEZ P. and TAYLOR J.L., Trans. of AIME

188

(1950) 1173.

Table 4 : Intensity and positionof the martensitic-, CsCl

-

and Tic peaks for different measurements and different models. Equal unit cell para- meters (from MS) and the second setting ( 6 = 96.8) for the coordina-

tes have been chosen to facilitate the comparison.

(See next page) Acknowledgement

This work is partially supported by a special fund of the "Board of the Swiss Federal Institutes of Technology".

C4-224 JOURNAL DE PHYSIQUE

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