HAL Id: jpa-00221038
https://hal.archives-ouvertes.fr/jpa-00221038
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
ULTRASONIC ATTENUATION AND VELOCITY
CHANGES DURING THE FCC-HCP MARTENSITIC
TRANSFORMATION IN COBALT-NICKEL
G. Kneezel, A. Granato
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C5, suppZQment au nO1O, Tome
42,octobre 1981
page
C5- 106 1ULTRASONIC ATTENUATION A N D VELOCITY CHANGES DURING T H E FCC-HCP
MARTENS ITIC TRANSFORMATION IN COBALT-NICKEL
*
3C
G.A. Kneezel and A.V. Granato
Physics Department and MateriaZs Research kboratory, University of IZZinois
a t Urbana-Champaign, Urbana, IL 61 801,
U. S . A.A b s t r a c t .
-
lleasurements were made t o s e a r c h f o r evidence of widening of ex- tended d i s l o c a t i o n s a s t h e t r a n s f o r m a t i o n temperature i s approached i n a s i n g l e c r y s t a l Co68Ni32 a l l o y . Specimens were p r e s t r e s s e d p l a s t i c a l l y i n s h e a r t o i n t r o d u c e d i s l o c a t i o n s p r i m a r i l y on one s l i p system. During t r a n s - f o r m a t i o n , measurements were made of l o n g i t u d i n a l and s h e a r s t r a i n a s w e l l a s v e l o c i t y and a t t e n u a t i o n a t 1 0 PWz. The sound waves were p o l a r i z e d t o e x c i t e t h e " o p t i c a l " mode of v i b r a t i o n of t h e two p a r t i a l d i s l o c a t i o n s u s i n g a (111) [ l l Z ] p o l a r i z a t i o n s h e a r wave. P r e - t r a n s f o r m a t i o n changes were found which a r e c o n s i s t e n t w i t h a simple model f o r t h e e f f e c t .1. I n t r o d u c t i o n .
-
A simple d i s l o c a t i o n modelL/ f o r t h e FCC-HCP m a r t e n s i t i c phase t r a n s f o r m a t i o n p r e d i c t s a n u l t r a s o n i c v e l o c i t y d e c r e a s e and a t t e n u a t i o n i n c r e a s e a s a p r e c u r s o r t o t h e t r a n s f o r m a t i o n . We looked f o r t h e e f f e c t i n t h e c o b a l t - n i c k e l2
/
system, which h a s been s t u d i e d extensively.-
By u s i n g t h e a l l o y C O ~ t h e t r a n s f o r m a t i o n temperature i s brought down t o ~ N ~ ~ ~ n e a r room temperature w i t h PIs%-10°C and A s % l l O O C . I n a d d i t i o n de Lamotte and
3
/
A l t s t e t t e r - showed t h a t by p r e s t r e s s i n g samples of t h e a l l o y t h e y could o b t a i n s i n g l e c r y s t a l t o s i n g l e c r y s t a l t r a n s f o r m a t i o n i n a .1 i n c h by 1 inch rod. Weston
2
/
and Granato-- used t h i s t e c h n i q u e t o o b t a i n a s i n g l e c r y s t a l cube of hcp phase l a r g e
3
enough (about 1 cm ) t o o b t a i n complete s e t s of e l a s t i c c o n s t a n t s i n both phases. 2. Experimental Procedure.
-
S e v e r a l samples of Co 38Ni32 were c u t t o approximate dimensions [ I l l ] = 4mm, [ I ~ O ] % [I121 % 12 mm from' a s i n g l e c r y s t a l i n g o t . The (111) f a c e s were p o l i s h e d f l a t and p a r a l l e l f o r good u l t r a s o n i c p u l s e r e f l e c t i o n . Samples were annealed a t 930°C f o r 1 6 hours t o r e l i e v e i n t e r n a l s t r e s s . The [ I l l ] dimension was made s h o r t t o minimize i n t e r n a l c o n i c a l r e f r a c t i o n e f f e c t s . This geometry i s a l s o f a v o r a b l e f o r p r e s t r e s s i n g t h e sample i n (111) [1?0] s h e a r t o i n t r o d u c e a l a r g e number of d i s l o c a t i o n s on t h e (111) p l a n e w i t h Burgers v e c t o r [ l i ~ ] . This b i a s e s t h e t r a n s f o r m a t i o n t o t a k e p l a c e p r e f e r e n t i a l l y on one of t h e twelve s l i p systems. These d i s l o c a t i o n s may be e x c i t e d i n o p t i c a l v i b r a t i o n by a (111) [112] u l t r a s o n i c8 2
s t r e s s wave. The y i e l d s t r e s s was about 1 . 2 x 1 0 dynes/cm
,
and samples were s t r a i n e d by a few t e n t h s of a p e r c e n t .*
Work supported by t h e N a t i o n a l Science Foundation, under g r a n t NSF DMR77-10556"
P r e s e n t a d d r e s s : Xerox Research Laboratory, Webster New York, U.S.A.JOURNfiL DE PHYSIQUE
Velocity and decrement measurements were made using the highly sensitive pulse- echo superposition system developed by ~ o l d e r ~ and modified by Read and Holder. 5 The system measures a change in superposition peak frequency related to the change in
7
velocity by A v - A f + E
.
Changes of 1 part in 10 are detectable if the temperaturev f R
is suitably controlled.
Because the model makes predictions about the velocity change, and because the sample length changes by .39% during complete transformation, it is important to monitor the [Ill] dimension with a longitudinal strain gage. Typically it was found that in the temperature range of interest the length changes were relatively small so that (Av/v)%(Af/f) within about 15%. Shear strain was also measured to see if the shear transformation strain was a function of prestressing shear strain.
Because cobalt-nickel is ferromagnetic there is a large background attenuation of the ultrasonic wave due to vibrating domain walls and eddy current damping.6 This can be greatly reduced by pinning the domain walls with a saturation magnetic field. Due to shape demagnetization it is easier to magnetize a sample along one of its long dimensions. Using the magnetization curves7 for C O ~and calculating the ~ N ~ ~ ~ demagnetization factor for an ellipsoid8 of our sample dimensions suggested that the external field needed to be 1.5 kilogauss along one of the long dimensions. (e.g.,
[lie]).
Measurements were made in an external field of 6.5 kilogauss.Results of early runs were perplexing. Even though the samples had been pre- stressed to bias transformation on (Ill), the transformation lines were more prominant on (171), and (ill) and absent on (lli). Using small cube-shaped samples with magnetic field applied along different directions we found that the magnetic field was responsible for biasing transformation to occur on the sets of (1111 planes most nearly perpendicular to the magnetic field direction. The reasons for this biasing are crystal anisotropy10 and magnetostriction.l1 For our sample geometry the shape demagnetization factor along [Ill] would have required a field of 13 kilogauss for saturation, which was twice the field available. However, by putting chunks of Armco iron on the same lateral dimensions as our sample on either side, the sample was made to look more like a rod than a plate to the field so that magnetization to saturation along [Ill] could be achieved.
3 . Results and Discussion.
-
The sample designated M4 was prestressed plastically in [Ill] (1x0) shear by .7%. The superposition peak frequency is plotted versus temperature in Figure 1. The frequency is linear in temperature down to -6OC (normal background). Between -6'C and -10°C a dip in frequency with respect to theextrapolated line occurs. The dip is max- imized near -9°C and begins to recover. Before recovery is complete however, just below -lO°C the dramatic effects associated with transformation occur. The maximum
-4 fractional change at the dip is 1.7x10
.
Sample M was cut from the section of
5
ingot adjacent to M4. It was prepared in the same way but was not prestressed after annealing. The superposition peak
frequency is plotted versus temperature in Figure 2. The major difference between M4 and M is the absence of the dip immedi-
5
ately preceeding transformation in M5.
Fig. 1 - Pulse superposition frequency versus temperature for a (111) [I121 shear stress wave in sample 114 pre- stressed in shear to .7% strain.
C5-1064 JOURNAL DE PHYSIQUE
To e l i m i n a t e t h e p o s s i b i l i t y t h a t t h e d i p was due t o a n i n t r i n s i c d i f f e r e n c e between M 4 and M5, t h e l a t t e r was a n n e a l e d a g a i n and s h e a r e d t o .2% s t r a i n . F i g u r e 3
shows t h e s u p e r p o s i t i o n peak f r e q u e n c y and a t t e n u a t i o n v e r s u s t e m p e r a t u r e . A s m a l l d i p i n f r e q u e n c y i s found w i t h maximum f r a c t i o n a l change b e i n g 5 x The r i s e i n a t t e n u a t i o n which o c c u r s o v e r t h e same
'.
t e m p e r a t u r e r a n g e a s t h e d i p i s .075 db/l.~sec, o r a n i n c r e a s e i n decrement of 8 . 2 x The r a t i o of t o t a l decrement i n c r e a s e t o t h e z10.57r\
maximum f r a c t i o n a l change i n v e l o c i t y i s 1 6 g10.570 which i s g r e a t e r t h a n t h e 4 ~ d e r i v e d from t h eH
model. The d i s c r e p a n c y may b e a t t r i b u t e d t o\ l e n g t h changes a n d / o r i n c r e a s e d s c a t t e r i n g 10.5651 n e a r t h e t r a n s f o r m a t i o n .
1
A f t e r a t o t a l o f 2 1 u l t r a s o n i c measure- ment r u n s d u r i n g t h e fcc-hcp t r a n s f o r m a t i o nz'.6t
\
t h e f o l l o w i n g o b s e r v a t i o n s can b e made: .?-
1 . 4 1\
a ) The f r e q u e n c y d i p i n t h e o p t i c a l mode%
1.L
f
o c c u r r e d immediately p r i o r t o t h e t r a n s f o r -m a t i o n d u r i n g t h e f i r s t c o o l i n g r u n a f t e r -17' -16- -15O -14" -13O -lZO -11° -100
T o r
p r e s t r e s s i n g . I f t h e sample was h e a t e d and cooled a g a i n , t h e d i p was n o t o b s e r v e d .
b) The s i z e o f t h e d i p was r o u g h l y propor- F i g . 3
-
P u l s e s u p e r p o s i t i o n f r e q u e n c y t i o n a l t o t h e p l a s t i c p r e s t r a i n : and a t t e n u a t i o n v e r s u s t e m p e r a t u r e f o r a (111) [I121 s h e a r s t r e s s wave t o .2% s t r a i n . A f y % - 0 2 5 E.
c ) There was a s c a t t e r o f more t h a n 25°C i nM'
even f o r t h e f i r s t 5t r a n s f o r m a t i o n of samples c u t from t h e same i n g o t . d) The l a r g e s t s h e a r s t r a i n ob- s e r v e d was 5 x and t h e s i z e and d i r e c t i o n of s t r a i n was i n d e p e n d e n t o f p l a s t i c p r e s t r a i n . The s h e a r s t r a i n was, however, a u s e f u l i n d i c a t i o n of t h e o n s e t of t r a n s f o r m a t i o n . e ) It was shown t o b e p o s s i b l e t o a c h i e v e n e a r l y complete f c c s i n g l e c r y s t a l t o hcp s i n g l e c r y s t a l t r a n s f o r m a t i o n i n a l a r g e sample by p r e s t r e s s i n g i n
[ I l l ] ( 1 i 0 ) s h e a r and o r i e n t i n g a s a t u r a t i o n magnetic f i e l d a l o n g [ I l l ] .
The v e l o c i t y and decrement i n t h e v i c i n i t y of t h e d i p a r e r e m i n i s c e n t of t h e model c u r v e s of F i g u r e 2 of r e f . 1. The magnitude of t h e d i p i n F i g u r e 1 c a n b e f i t by assuming t h e d e n s i t y of extended d i s l o c a t i o n s r e s p o n s i b l e f o r t h e e f f e c t was
A = 2 h. 1 0 ~ c m - ~ . The h a l f maximum p o i n t s of t h e d i p o c c u r n e a r -7.0' and -9.5"C. From F i g u r e 2 of r e f . 1, we f i n d t h e s e p o i n t s c o r r e s p o n d t o
w
= 3 x 1 0 ~ s e c - ' andR
1 x l ~ ~ s e c - l , o r e q u i l i b r i u m w i d t h s o f a p p r o x i m a t e l y 3 x 1 0 - ~ c m and 1 0 - ~ c m . Assuming -4
The values found above for the parameters are reasonable. The dislocation density is smaller than is typically found in fcc metals, but it is not the total dislocation density which is important here - only those largest dislocations on the verge of transformation. The chosen dislocation length is a reasonable choice for a long dislocation. Note too that the fault widths at the dip half maximum points are nearly as large as the assumed length as the instability is approached.
Due to the instability at transformation the entire dip was rarely traced out before the dramatic transformation effects occurred. The determining factor as to how much of the dip is seen is whether there are wide equilibrium spacings at which the dislocations can vibrate at low enough frequencies to be detected at 10 MHz. In the freshly annealed sample (Figure 2), there are relatively few dislocations and these are strongly pinned. Rather than quasi-statically breaking a few pins to assume successively wider spacings, the dislocations remain firmly pinned until cat- astrophically breaking away at the transformation.
In conclusion, our measurements in prestressed C O ~were consistent with ~ N ~ ~ ~ the dislocation optical mode vibration model using reasonable values for the para- meters of the model.
References
1)
A.
V. Granato, R. B. Schwarz and G.A.
Kneezel. This conference. 2)W. F.
Weston andA.
V. Granato, Phys. Rev.B
12,
5355 (1975). 3)E.
de Lamotte and C. J. Altstetter, Trans. AU4E245,
651 (1969). 4) J. Holder, Rev. Sci. Instrum.41,
1355 (1970).5) D. Read and J. Holder, Rev. Sci. Instrum.
43,
933 (1972). 6) R. M. Bozorth, Ferromagnetism, (Van Nostrand, New York, 1951).7)
J . W. Shih, Phys. Rev.50,
376 (1936). 8) J .A.
Osborn, Phys. Rev.67,
351 (1945). 9)E.
C. Stoner, Phys. Rev.67,
351 (1945).10) R. S. Tebble and D. J . Craik, Magnetic Materials, (Wiley-Interscience, London, 1969).
