MONOTONIC AND THERMOMECHANICAL TESTING OF P/M NiTi

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MONOTONIC AND THERMOMECHANICAL TESTING OF P/M NiTi

W. Johnson, J. Domingue, S. Reichman, F. Sczerzenie

To cite this version:

W. Johnson, J. Domingue, S. Reichman, F. Sczerzenie. MONOTONIC AND THERMOMECHAN- ICAL TESTING OF P/M NiTi. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-291-C4-296.

�10.1051/jphyscol:1982440�. �jpa-00222154�

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JOURNAL DE PHYSIQUE

Colloque C4, suppidment au n o 12, Tome 43, de'cembre 1982 page C4-291

MONOTONIC AND THERMOMECHANICAL TEST1 NG OF P/M N i T i

W.A. J o h n s o n , J . A . D o m i n g u e , S.H. R e i c h m a n a n d F . E . S c z e r z e n i e Special Metals Co~.poration, New Hartford, N . Y . , U.S.A.

( A c c e p t e d 9 A u g u s t 1982)

A b s t r a c t

-

P/M N i T i f o r t h e t e s t program was manufactured b y t h e S p e c i a l M e t a l s powder m e t a l l u r g y process and c o n v e r t e d t o w i r e b y h o t swaging and w i r e drawing. The mechanical t e s t program was undertaken i n o r d e r t o inves- t i g a t e t h e performance o f N i T i b o t h f o r s c i e n t i f i c reasons and f o r b e t t e r u n d e r s t a n d i n g o f m a t e r i a l performance as i t s p e c i f i c a l l y p e r t a i n s t o shape memory e f f e c t (SME) d e v i c e s . Two t y p e s o f m e c h a n i c a l t e s t s , m o n o t o n i c d e f o r m a t i o n l r e c o v e r y and thermomechanical c y c l i c d e f o r m a t ion, were used t o c h a r a c t e r i z e performance. As a n t i c i p a t e d , monotonic mechanical p r o p e r t i e s were e q u i v a l e n t t o cast/wrought p r o p e r t i e s . I n d i v i d u a l N i T i w i r e specimens were deformed i n an i n c r e m e n t a l s e r i e s up t o 30 p e r c e n t t o t a l e n g i n e e r i n g s t r a i n . These p r e s t r a i n e d specimens f o l l o w e d t h r e e e x p e r i m e n t a l t h e r m a l c y c l e s w h i c h m o n i t o r e d d e f l e c t i o n , l o a d , and e n e r g y d u r i n g t h e s h a p e r e c o v e r y - h e a t i n g c y c l e . The aforementioned t h r e e t e c h n i q u e s were used t o p r o v i d e a more complete u n d e r s t a n d i n g o f t h e shape r e c o v e r y response. Each measurement t e c h n i q u e demonstrated a maximum i n t h e r e c o v e r y r e s u l t i n g f r o m an 8 t o 1 2 p e r c e n t t o t a l i n p u t s t r a i n . Monotonic r e s u l t s i n d i c a t e t h a t t h e maximum r e c o v e r y s t r a i n i s a measure o f SME c a p a c i t y t o p r o v i d e a s i n g l e o u t p u t o f d e f l e c t i o n ( o r f o r c e ) , which a r e p o t e n t i a l l y u s e f u l d a t a f o r SME d e v i c e s p e r f o r m i n g m o n o t o n i c a l l y . Thermomechanical t e s t i n g was conducted u s i n g a s e r v o - h y d r a u l i c t e s t machine i n c o n j u n c t i o n w i t h a c o n t r o l l e d tem- p e r a t u r e e n v i r o n m e n t a l chamber i n an a t t e m p t t o s i m u l a t e a c y c l i c SME d e v i c e . A f t e r d e f o r m a t i o n t o a predetermined s t r a i n , s t r o k e p o s i t i o n was h e l d con- s t a n t as t h e specimen was t h e r m a l l y c y c l e d above Af and t h e r e s u l t i n g l o a d was m o n i t o r e d ( s t r a i n c o n t r o l ) . T h i s t e s t mode r e s u l t e d i n permanent damage and l o s s o f SME w i t h each thermomechanical c y c l e , m a n i f e s t e d as a decrease i n l o a d and a v a i l a b l e work d e l i v e r e d b y t h e specimen. I n l o a d c o n t r o l , t h e specimen was deformed t o a predetermined s t r a i n , t h e l o a d reduced, and main- t a i n e d a t a p p r o x i m a t e l y z e r o d u r i n g t h e t h e r m a l c y c l e above Af. T h i s l o a d c o n t r o l t e c h n i q u e proved t o b e much l e s s damaging t o t h e specimen. Load c o n t r o l t e s t i n g was c l e a r l y t h e s u p e r i o r t e c h n i q u e f o r e v a l u a t i n g p e r f o r - mance o f c y c l i c SME d e v i c e s .

I n t r o d u c t i o n

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Work has been underway a t S p e c i a l M e t a l s C o r p o r a t i o n t o understand t h e mechanical/metallurgical performance o f powder m e t a l l u r g i c a l (P/M) N i T i . S i n g l e t h e r m a l c y c l e monotonic t e s t i n g demonstrates t h e e f f e c t of s t r e s s and s t r a i n on t h e t r a n s f o r m a t i o n temperature, f o r c e l d e f l e c t i o n , and e n e r g y o f t h e phase t r a n s f o r m a - t i o n . P r e s t r a i n i n c r e a s e s t h e t r a n s f o r m a t i o n temperature; e x c e s s i v e p r e s t r a i n a t t e n u a t e s t h e r e a c t i o n . P r o p e r t i e s degrade w i t h a s i n g l e p r e s t r a i n and t h e r m a l c y c l e . S t r a i n and l o a d c o n t r o l thermomechan i c a l c y c l i n g t e s t s a r e compared.

EXPERIMENTAL PROCEDURES

Method o f Manufacture

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N i T i f o r t h e t e s t p r o g r a was manufactured b y a powder m e t a l 1 u r g i c a l process developed a t S p e c i a l M e t a l s Y l . 2 ) P r e a l l o y e d Vacuum induc- t i o n M e l t e d (VIM) and Vacuum Arc Remelted (VAR) N i T i i n g o t s were VIM r e m e l t e d and d i s i n t e g r a t e d u s i n g a h i g h p r e s s u r e argon stream. R e s u l t i n g powder always remained i n an i n e r t environment. Powder was screened t o -60 mesh; loaded i n t o s t a i n l e s s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982440

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C4-292 JOURNAL DE PHYSIQUE

s t e e l cans and h o t i s o s t a t i c a l l y pressed (HIP'ed) i n t o b i l l e t s , which were t y p i c a l l y 13 mm diameter and 600 mm long. B i l l e t s were converted t o w i r e .

Mechanical Testing

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Mechanical t e s t i n g was conducted on a MTS servo-hydraulic machine. Three types o f t e s t s c h a r a c t e r i z e d N i T i response t o s t r e s s and s t r a i n : 1) monotonic/isothermal, 2) monotonic p l u s s i n g l e thermal cycle, and 3) thermomechanical c y l ing. An environmental temperature chamber was used f o r a l l t e s t i n g . A t y p i c a l p r o d u c t i o n heat was selected.

Monotonic Testing

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Specimens were t e n s i l e t e s t e d t o f a i l u r e i n an isothermal environment above (140°C) and below (-23°C) t h e zero s t r a i n Af and Mf, r e s p e c t i v e l y . Monotonic Testing w i t h S i n g l e Thermal Cycle

-

Specimens were cooled below t h e Mf temperature and subsequently deformed i n an incremental s e r i e s up t o 30 percent p r e s t r a i n . Specimens f o l l o w e d t h e same thermal path t o a u s t e n i t e w h i l e d e f l e c t i o n , load, and energy were monitored independently d u r i n g t h e h e a t i n g c y c l e . Trans- format i o n energy o f t h e specimens was measured w i t h t h e d i f f e r e n t i a l scanning c a l o r i m e t e r (DSC) module of t h e DuPont 990 Thermal Analyzer.

Thermomechanical C y c l i c Testing

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T h i s was conducted t o s i m u l a t e t h e performance o f a SME device. I n i t i a l l y specimens were cooled below Mf and p r e s t r a i n e d :

S t r a i n Control

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Grips were c o n t r o l l e d a t t h e f i x e d p o s i t i o n and t h e specimen t h e r m a l l y cycled above Af. This c y c l e was repeated t e n times, load monitored, and subsequently t h e area under t h e load-temperature curve measured.

Load Control Cycling

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Specimen load was reduced t o zero and t h e specimen t h e r - maJly cycled above Af. G r i p motion was monitored as a f u n c t i o n o f temperature.

RESULTS AND DISCUSSION

Monotonic, Isothermal Testing

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T e n s i l e curves f o r b o t h t h e m a r t e n s i t i c and a s t e n i t i c phases are approximately e q u i v a l e n t f o r PIM and castlwrought N i T i

.Y3i

A u s t e n i t e and m a r t e n s i t e s t a r t and f i n i s h temperatures, AS, Af and Ms, Mf, respec- t i v e l y i n t h e unstressed c o n d i t i o n are shown i n Table I.

Monotonic Testing w i t h S i n g l e Thermal Cycle

-

Specimens p r e s t r a i n e d t o 2, 4, 6, 8,

z n e e r i n g s t r a i n are c u t from t h e gage sec-

t i o n , then t e s t e d b y DSC t o measure t r a n s f o r m a t i o n temperature and energy (Table I ) . I n F i g u r e 1 As increases most d r a m a t i c a l l y between 4 and 6 percent p r e s t r a i n . As increases from 57°C i n t h e unstrained c o n d i t i o n t o 113°C f o r 30 percent pre- s t r a i n . Ms temperature increases a t a slower r a t e .

F i g u r e 2 shows endothermic-austenit i c r e a c t i o n energies constant up t o 4 percent p r e s t r a i n . Reaction energy reaches a maximum a t 8 percent. Above 8 percent t h e energy dramat i c a l l y decreases.

TABLE I

1 I I

React i o n Energy

" C

I I

c a l l g r

I

%

I

Pre-

I

As

I

Af

I

MS

1

Mf

I I

A

I

s t r a i n

I I I I

37

1

27

1 1 I I

5.14

1

5.14

1 I

0 5 7 69

1

12

1 2 1

⁵⁹

1

⁷⁷

¹

³⁹ ^{1 2 0}

^{1 1}

^5.13 1 6 . 1 9

1

18

1

4 [ 59 [ 75 ( 3 9 1 1 7

1 1

5.29 1 5 . 4 3

1

16

1

6

1

8 1

1

89 1 4 2

1

3

1 1

5.39 1 6 . 1 6

1

8

1

8

1

87

1

96 1 4 3

1

4

1 1

5.68 1 4 . 8 3

1

9

1

10

1

93

1

98 1 4 2

1

8

1 1

5.39 1 5 . 0 1

1

5

1

15 1 1 0 2

1

108 1 4 4 [ 15

1 1

4.85

1

3.40

1

6

1

20 1 1 0 6

1

115 1 4 4 1 1 6

1 1

4.24 1 2 . 7 7

I

9

1

25 1 1 1 1 1 1 2 1 [ 45 1 1 5

1 1

3.46 1 3 . 6 1

1

10

1

30

1

113

1

133

1

53

1

13

I1

2.08

1

1.94

1

20

1

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-

a

I

- -

I

I I I

I I

0 5 10 15 20 25 30

Total Prestrain, %

FIGURE 1. Onset o f A u s t e n i t e Reaction as a Function o f T o t a l P r e s t r a i n .

Austenite Reaction Energy, cal/g

6

0

0 5 10 15 20 25 30

Total Prestrain, %

FIGURE 2. A u s t e n i t i c Reaction Energy as a Function o f T o t a l P r e s t r a i n .

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JOURNAL DE PHYSIQUE

The maximum i n t h e a u s t e n i t i c e n e r g y a p p r o x i m a t e l y c o i n c i d e s w i t h t h e end o f t h e h o r i z o n t a l r e g i o n o f a n a i v e m a r t e n s i t e t e n s i l e c u r v e and t h e onset o f non- r e c o v e r a b l e p l a s t i c s t r a i n . A moderate amount o f p l a s t i c s t r a i n i n c r e a s e s t h e r e c o v e r a b l e t r a n s f o r m a t i o n energy. A c o n t r o l l e d amount o f d i s l o c a t i o n m o t i o n and d e f e c t d e b r i s w i l l a s s i s t s h e a r i n g and u n s h e a r i n g o f t h e m a r t e n s i t e , a l l o w i n g more energy s t o r a g e . Excessive s t r a i n impedes i t . S p e c i f i c s t r a i n and t h e r m a l c y c l i n g c l e a r s t h e m a t r i x o f d i s l o c a t i o n d e b r i s , and d e p o s i t s o r induces d i s l o c a t i o n n e t - work f o r m a t i o n a t m a r t e n s i t e - m a r t e n s i t e i n t e r f a c e s , p r o v i d i n g an easy t r a n s f o r m a - t i o n r e g i o n w i t h d i s t i n c t v o l u m e t r i c l i m i t s . D i s l o c a t i o n t a n g l e s p r o v i d e d i s t i n c t i n t e r f a c i a l demarcation between m a r t e n s i t e p l a t s b e x e r t i n g back s t r e s s e s on t h e l a t t i c e and t h u s i n f l u e n c i n g t h e t r a n s f ~ r m a t i o n . ~ ~ - ~ f

T a b l e I shows A TA - A versus p r e s t r a i n f o r t h e same specimens. The t r a n s - f o r m a t i o n t e m p e r a t u r e r 8 n g l reaches a minimum o f 5°C a t 1 0 p e r c e n t p r e s t r a i n , t h e n i n c r e a s e s t o 20°C a t 30 p e r c e n t . A u s t e n i t i c r e a c t i o n e n e r g y decreases o v e r t h e same 1 0 t o 30 p e r c e n t p r e s t r a i n range. C o n t r o l l e d i n p u t s t r a i n a l l o w s t h e e n t i r e t r a n s - f o r m a t i o n t o o c c u r o v e r a v e r y narrow t e m p e r a t u r e range, t h u s a s s i s t i n g i t s comple- t i o n . Excessive p l a s t i c f l o w decreases r e a c t i o n energy and broadens t r a n s f o r m a t i o n temperature range. Narrower t r a n s f o r m a t i o n t e m p e r a t u r e range imp1 i e s f a v o r a b l e d i s l o c a t i o n t a n g l e s , a l l o w i n g e a s i e r u n s h e a r i n g o f p r e f e r r e d m a r t e n s i t e v a r i a n t s . E v e n t u a l l y t h e d i s l o c a t i o n t a n g l e s a t t e n u a t e t h e r e a c t i o n . Thermal e n e r g y must i n c r e a s e t o induce t r a n s f o r m a t i o n ; AS increases, and f i n a l 1 y t h e r e a c t i o n tempera- t u r e range i n c r e a s e s .

S t r a i n C o n t r o l T e s t i n g w i t h S i n g l e Thermal C y c l e

-

A f t e r p r e s t r a i n i n g and upon h e a t i n g , t h e specimen c o n t r a c t s . L o n g i t u d i n a l c o n t r a c t i o n i s n o t p e r m i t t e d due t o f i x e d g r i p p o s i t i o n . The specimen t r a n s m i t s f o r c e t o t h e t e n s i l e machine and does work upon i t s e l f , r e s u l t i n g i n t r a n s f o r m a t i o n a l p l a s t i c damage and d e g r a d a t i o n o f SME. F i g u r e 3 shows t h a t r e c o v e r e d s t r e s s i n c r e a s e s w i t h t o t a l p r e s t r a i n up t o 8 percent; above which i t decreases.

Recovery Stress

Ksi MPa

Total Prestrain, % FIGURE 3. Recovery S t r e s s vs. T o t a l

P r e s t r a i n

Recovered Strain, %

0 5 10 15 20 25 30 Total Prestrain, %

FIGURE 4. Recovered S t r a i n vs. T o t a l P r e s t r a i n .

Load C o n t r o l T e s t i n g w i t h S i n g l e Thermal Cycle

-

F i g u r e 4 shows t h a t l o w p r e s t r a i n s r e s u l t i n m i l d l y e f f e c t i v e r e c o v e r y . Recovered s t r a i n i n c r e a s e s t o 5.6 p e r c e n t a t 15 p e r c e n t p r e s t r a i n , above which p l a s t i c d e f o r m a t i o n a t t e n u a t e s SME and r e c o v e r a b l e s t r a i n . Recovered s t r a i n decreases r a p i d l y above 20 p e r c e n t p r e s t r a i n . R e s u l t s

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f o r n a i v e w i r e i n d i c a t e t h a t maximum r e c o v e r e d s t r a i n r e q u i r e s l a r g e p r e s t r a i n s . O v e r s t r a i n i n g may b e u s e f u l f o r d e v i c e s r e q u i r i n g a s i n g l e shape r e c o v e r y response.

Very l a r g e p r e s t r a i n s (15 p e r c e n t ) s h o u l d be avoided i n c y c l i c d e v i c e s .

Thermomechanical C y c l i c S t r a i n C o n t r o l T e s t i n g

-

P l a s t i c damage i s a n t i c i p a t e d f o r t h i s t e s t mode, an e x t r a p o l a t i o n of t h e s i n g l e c y c l e t e s t . Two specimens a r e t e s t e d u s i n g a c y c l i c 4 and 8 p e r c e n t t o t a l p r e s t r a i n . Areas under t h e f o r c e - t e m p e r a t u r e curves a r e p l o t t e d vs. c y c l e s i n F i g u r e 5 . While t h e specimen i s n o t a l l o w e d t o c o n t r a c t l o n g i t u d i n a l l y , i t c o n t r a c t s r a d i a l 1 y making t h e f o r c e - t e m p e r a t u r e c u r v e e q u i v a l e n t t o a f o r c e d e f l e c t i o n , o r work c u r v e . Both p r e s t r a i n s show a p p r o x i m a t e l y e x p o n e n t i a l decay o f t h e a r e a under t h e F-T c u r v e .

Force-Temperature Pound-OC

1 2 0

r

Newton-OC 8% Strain

A

4% Strain

0 2 4 6 8 1 0

Cycles

FIGURE 5 . Degradation o f U s e f u l Work w i t h Repeated Thermal Cycl i n g Load C o n t r o l Thermomechanical Cycl i n g

T o t a l i n p u t s t r a i n i s chosen a t f o u r p e r c e n t , c o i n c i d i n g w i t h t h e s t r a i n c o n t r o l t e s t y e t a v o i d i n g e x c e s s i v e nonrecoverable p l a s t i c damage. For a c o m p l e t e l y n a i v e specimen, a t o t a l i n p u t s t r a i n o f 4 p e r c e n t ( 3 . 6 p e r c e n t p l a s t i c ) r e s u l t s i n r e c o v e r a b l e s t r a i n o f o n l y 2.2 p e r c e n t on t h e f i r s t c y c l e . A d d i t i o n a l thermomechan-

i c a l c y c l e s i n c r e a s e r e c o v e r a b l e s t r a i n , which a s y m p t o t i c a l l y approaches t o t a l i n p u t s t r a i n . F i g u r e 6 shows t h e r a t i o o f r e c o v e r e d s t r a i n t o t o t a l s t r a i n vs.

t h e r m a l c y c l e s . A b r e a k - i n p e r i o d f o r t h e w i r e i s a n t i c i p a t e d ( s a t u r a t i o n o f r e c o v e r y s t r a i n , as a f u n c t i o n o f c y c l e s ) . I t i s f o r t u i t o u s t h a t s a t u r a t i o n occurs a t t h e t o t a l i n p u t s t r a i n . S a t u r a t i o n ( c r i t i c a l c y c l i c h i s t o r y ) s h o u l d c o i n c i d e w i t h t h e i n p u t p l a s t i c s t r a i n . The f o r t u i t o u s n a t u r e o f t h e d a t a r e s u l t s f r o m t h e f a c t t h a t s t r a i n r e c o v e r y i s confounded b y t h e r m a l expansion, e l a s t i c and a n e l a s t i c r e c o v e r y , two-way SME, and t r a n s f o r m a t i o n a l r a t c h e t i n g . F u t u r e l o a d c o n t r o l t e s t -

i n g w i l l s e p a r a t e t h e SME f r o m o t h e r t y p e s o f d e f o r m a t i o n .

N* i s d e f i n e d as t h e c r i t i c a l c y c l i c h i s t o r y t o o b t a i n complete SME r e c o v e r y . The e n g i n e e r now has a p r a c t i c a l d e s i g n t o o l t o d e t e r m i n e t h e number of c y c l e s f o r component b r e a k - i n . C r i t i c a l c y c l i c h i s t o r y t o g e n e r a t e complete break- i n w i l l be a s t r o n g f u n c t i o n o f m e t a l w o r k i n g and m e t a l l u r g i c a l parameters. Metal working h i s t o r y w i l l m a n i f e s t i t s e l f as r e s i d u a l s t r e s s e s and c r y s t a l t e x t u r e . M e t a l l u r - g i c a l parameters w i l l a l t e r performance due t o g r a i n s i z e and geometry v a r i a t i o n s , powder p a r t i c l e s i z e , second phase p a r t i c l e s , e t c . Hence i t i s i m p o r t a n t t o d e t e r - mine c r i t i c a l c y c l i c h i s t o r y under s i m u l a t e d component o p e r a t i n g c o n d i t i o n s .

(7)

JOURNAL DE PHYSIQUE

Strain Recovery

i

Strain Inout

Total Input Strain =

^4%

**N*= Critical Cyclic History to Generate Full SME Recovery**

I

^I I I I

I

^I

1

0 2 4 6 8

A,

¹⁰ ¹²

Cycles

FIGURE 6 . C r i t i c a l C y c l i c H i s t o r y f o r SME Recovery.

CONCLUSIONS

Monotonic I s o t h e r m a l T e s t i n g

T e n s i l e p r o p e r t i e s o f P/M N i T i are e q u i v a l e n t t o cast/wrought p r o p e r t i e s Monotonic T e s t i n g w i t h a S i n g l e Thermal Cycle

As temperature increases w i t h i n c r e a s i n g i n p u t s t r a i n .

A u s t e n i t i c r e a c t i o n energy passes through a maximum b e f o r e decreasing r a p i d l y w i t h i n c r e a s i n g i n p u t s t r a i n .

ATA -A

,

t h e a b i l i t y t o s t a r t and complete t h e a u s t e n i t i c r e a c t i o n , r a p i d l y d e d e a f e s , t h e n r a p i d l y increases w i t h p r e s t r a i n .

C o n t r o l l e d p r e s t r a i n i s b e n e f i c i a l t o s t r e s s and s t r a i n recovery; excessive p r e s t r a i n r e s u l t s i n l o s s o f SME.

Therrnomechanical C y c l i n g - S t r a i n C o n t r o l

R e p e t i t i v e c y c l i n g r e s u l t s i n t h e l o s s o f SME and c o n c e i v a b l y c a t a s t r o p h i c component f a i l u r e . T h i s mode o f o p e r a t i o n i s ill s u i t e d f o r h i g h c y c l e devices.

Thermomechanical C y c l i n g

-

Load C o n t r o l

Test mode r e s u l t s i n improved shape r e c o v e r y .

N* i s conceived f o r a unique thermomechanical h i s t o r y .

Use o f N* f o r mechanical design r e q u i r e s t e s t i n g based upon r e a l d e v i c e o p e r a t i n g c o n d i t i o n s o r an a c c u r a t e t h e o r e t i c a l model.

ACKNOWLEDGEMENTS: The authors wish t o express thanks t o T. D r e s s e l f o r h i s thermomechanical t e s t i n g experiments, and t o T. L. Rowlands, S. L. P r a t t and R. W. M a r t i n . REFERENCES

1. Johnson W.A., Domingue J.A., and Reichrnan S.H. ( i n p r e s s ) (1982).

2. Podob M.T., Johnson W.A., and Reichman S.H., Proc. N i t i n o l Heat Engine Conf., MP 79-441, S i l v e r Spring, MD (1978).

3. Jackson C.M., e t a l . , N i t i n o l - T h e A l l o y w i t h a Memory, NASA-SP 5110 (1972).

4. P e r k i n s J., e t a l . , Shape Mem. E f f e c t s i n A l l o y s , Proc. I n t . Symp. on Shape Mem. E f f e c t s and Appl., Plenum Press, NY (1975) 273.

5 . B a l l A., e t al., Proc. I n t . Conf. S t r . Met. and A l l o y s , Trans. Jap. I n s t . Met. - 9 Supp (1968) 291.