P/M PROCESSING AND CHARACTERIZATION OF CONTROLLED TRANSFORMATION TEMPERATURE NiTi

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P/M PROCESSING AND CHARACTERIZATION OF CONTROLLED TRANSFORMATION

TEMPERATURE NiTi

W. Johnson, J. Domingue, S. Reichman

To cite this version:

W. Johnson, J. Domingue, S. Reichman. P/M PROCESSING AND CHARACTERIZATION OF

CONTROLLED TRANSFORMATION TEMPERATURE NiTi. Journal de Physique Colloques,

1982, 43 (C4), pp.C4-285-C4-290. �10.1051/jphyscol:1982439�. �jpa-00222153�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppZdrnent au n o 12, Tome 43, dgcernbre 1982 page C4-285

P/M PROCESSING AND CHARACTERIZATION OF CONTROLLED TRANSFORMATION

TEMPERATURE N i T i

W.A. Johnson, J.A. Domingue and S.H. Reichman

Special Metals Corporation, New Hartford, N. Y . 13413, U.S. A.

(Accepted 9 August 1982)

A b s t r a c t

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Double vacuum a r c r e m e l t e d N i T i i n g o t s were vacuum i n d u c t i o n r e m e l t e d and s u b s e q u e n t l y atomized u s i n g a h i g h - p r e s s u r e gas stream. Powders were c o l l e c t e d , loaded i n t o cans, and h o t i s o s t a t i c a l l y pressed (HIP1ed)

.

P r i o r t o H I P 1 i n g , p o w d e r s o f v a r i o u s known 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 s (AS), were p r e c i s e l y blended t o achieve a d e s i r e d i n t e r m e d i a t e 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 (A,).. T h i s i n t e r m e d i a t e As f o l l o w s an e m p i r i c a l r e l a t i o n s h i p which approximates a r u l e o f m i x t u r e s based on w e i g h t f r a c t i o n . As were measured u s i n g d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC)

.

R e s u l t s f o r H I P 1 e d and h i g h t e m p e r a t u r e annealed specimens i n d i c a t e t h a t t h i s t e c h - n i q u e was a c c u r a t e and r e p r o d u c i b l e f o r measuring t h e t r a n s f o r m a t i o n tem- p e r a t u r e o f smooth, n e a r l y symmetrical DSC curves. DSC/DTA thermograms i n t h e l i t e r a t u r e t y p i c a l l y show a double peak e x o t h e r m i c - m a r t e n s i t i c reac- t i o n w h i c h was d i f f e r e n t t h a n t h e s m o o t h p e a k s o b s e r v e d f o r t h e H I P 1 e d c o n d i t i o n . The s p e c i f i c t h e r m o m e c h a n i c a l h i s t o r y p e r f o r m e d o n t h e P/M m a t e r i a l r e s u l t e d i n t h e d o u b l e peak e x o t h e r m i c r e a c t i o n . To i n t e r p r e t t h e peaks, thermal a r r e s t experiments were conducted on b o t h exothermic peaks, t h e r e s u l t s o f w h i c h c l e a r l y s u p p o r t t h e e x i s t e n c e o f a p r e m a r t e n s i t i c r e a c t i o n . Thermal a r r e s t experiments were a l s o performed t o analyze i n - complete t h e r m a l c y c l e s on b o t h h e a t i n g and c o o l i n g . Thermal a r r e s t o f t h e m a r t e n s i t e r e a c t i o n r e s u l t e d i n reduced e n e r g y absorbed f o r c o m p l e t i o n o f t h e a u s t e n i t e r e a c t i o n . Conversely, m a r t e n s i t e energy was reduced b y an a r r e s t o f t h e a u s t e n i t e r e a c t i o n . However, r e h e a t i n g t h e sample r e v e a l e d t h a t t h e a r r e s t s p l i t t h e a u s t e n i t e r e a c t i o n i n t o t w o d i s t i n c t p e a k s , t h e s p l i t o c c u r r i n g a t t h e t e m p e r a t u r e o f t h e p r i o r t h e r m a l a r r e s t . These r e s u l t s and r e s u l t s o f a d d i t i o n a l DSC experiments s e r v e t o emphasize t h e i n f l u e n c e o f thermomechanical h i s t o r y on t h e k i n e t i c s and e n e r g e t i c s o f t h e SME t r a n s - f o r m a t i o n . The P/M b l e n d i n g process t e c h n i q u e was f o u n d t o r e s u l t i n un- precedented c o n t r o l o f t h e AS temperature. S c i e n t i f ic / e n g i n e e r i n g cons i d - e r a t i o n s j u s t i f y P/M p r o c e s s i n g and mechanical b l e n d i n g as opposed t o con- v e n t i o n a l c a s t / w r o u g h t p r o c e s s i n g t o a c h i e v e a c c u r a t e As t e m p e r a t u r e s . I n t r o d u c t i o n

-

Work 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 i s aimed t o w a r d t h e commercializa- t i o n o f c o n t r o l l e d 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 N i T i . T h i s paper o u t 1 i n e s t h e d e v e l - opment o f s o p h i s t i c a t e d m a n u f a c t u r i n g and t e s t procedures. I n c l u d e d i s c o n c i s e a n a l y s i s o f t h e m a n u f a c t u r i n g processes as t h e y a f f e c t t r a n s f o r m a t i o n temperature and a l l o y homogeneity. D i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) i s used t o d e t e c t a u s t e n i t e - m a r t e n s i t e phase changes and measure t r a n s f o r m a t i o n temperatures. DSC p r o v i d e s an a c c u r a t e method t o m o n i t o r p r o g r e s s i n a c h i e v i n g c o n t r o l l e d t r a n s f o r - m a t i o n temperature, as we1 1 as more fundamental i n v e s t i g a t i o n s o f a u s t e n i t e , mar- t e n s i t e , and p r e m a r t e n s i t i c r e a c t i o n s .

EXPERIMENTAL PROCEDURES

M a n u f a c t u r i n

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P r e a l l o y e d N i T i i n g o t s were prepared b y vacuum i n d u c t i o n m e l t i n g VIM f o l l o w e d b y d o u b l e vacuum a r c r e m e l t i n g (VAR), t h u s a s s u r i n g a c l e a n

+

homogeneous i n g o t p r i o r t o a t o m i z a t i o n . S t a r t i n g i n g o t s were VIM'ed i n an a t o m i z e r and d i s i n t e g r a t e d b y a h i g h p r e s s u r e stream o f argon. R e s u l t i n g powder was i n e r t l y c o l l e c t e d , screened, and s t o r e d t o a w a i t f u r t e p r o c e s s i n g . Screen a n a l y s i s and p a r t i c l e morphology were r e p o r t e d p r e v i o u s l y .

hr

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982439

C4-286 JOURNAL DE PHYSIQUE

The program aim was t o achieve a f u l l y c o n s o l i d a t e d p r o d u c t w i t h known, accu- r a t e and r e p r o d u c i b l e t r a n s f o r m a t i o n temperature. The convent i o n was assumed t h a t 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 i s e q u i v a l e n t t o a u s t e n i t e s t a r t temperature, As. T h i s approach q u a n t i f i e d a t e r m which was t y p i c a l l y q u a l i t a t i v e . C o n t r o l o f 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 was a c c u r a t e l y and r e p r o d u c i b l y achieved b y b l e n d i n g powders o f known 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 a c c o r d i n g t o a w e i g h t f r a c t i o n r u l e o f m i x t u r e s . T h i s b l e n d i n g approach u t i l i z e d 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 as t h e most i m p o r t a n t p r o d u c t i o n parameter.

Powders f r o m two "pure" h e a t s were i n t i m a t e l y combined i n a b l e n d e r u s i n g p r e c i s e l y c o n t r o l l e d w e i g h t f r a c t i o n s f o r s u f f i c i e n t t i m e t o achieve a homogeneous m i x . Powders f r o m h e a t s o r b l e n d s were p u t i n t o s t a i n l e s s s t e e l cans and h o t i s o s t a t i c a l l y pressed ( H I P 1 e d ) t o 100 p e r c e n t t h e o r e t i c a l d e n s i t y a t 900°C f o r t h r e e hours a t 103 MPa p r e s s u r e . T h i s r e s u l t e d i n b i l l e t s 13 mm i n d i a m e t e r and 600 mm l o n g w e i g h i n g a p p r o x i m a t e l y 0.45 Kg. Cans were removed f r o m t h e b i l l e t s b y c e n t e r - l e s s g r i n d i n g . B i l l e t s were h o t worked and drawn i n t o w i r e .

A n a l y t i c a l

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S e c t i o n s were c u t f r o m one o r more b i l l e t s i n each h e a t . Oxygen and n i t r o g e n c o n t e n t s were determined w i t h a LECO gas a n a l y z e r . N i c k e l and t i t a n i u m c o n c e n t r a t i o n s were measured v i a wet chemical t e c h n i q u e s . Wires were annealed a t 850°C f o r 20 m i n u t e s p r i o r t o DSC a n a l y s i s because t r a n s f o r m a t i o n temperatures on h e a t i n g and c o o l i n g were s t r o n g l y a f f e c t e d b y r e s i d u a l s t r e s s e s . Complete s t r e s s r e l i e f was n o t achieved f o r 500-600°C h e a t t r e a t m e n t s . T h i s was n o t a problem f o r t h e as-HIP1ed c o n d i t i o n due t o t h e s p e c i f i c n a t u r e o f t h e HIP c y c l e .

Thermal measurements were performed w i t h t h e h e a t f l u x DSC module o f t h e DuPont 990 Thermal A n a l y s i s System. Program r a t e was 1O0C/min. N i T i t r a n s f o r m a t i o n temperatures were c o n s i d e r e d t o be a c c u r a t e t o w i t h i n

+

^1°C. A DSC thermogram o f t h e N i T i a u s t e n i t e - m a r t e n s i t e t r a n s f o r m a t i o n i s shown i n F i g u r e 1. The tangen- t i a l e x t r a p o l a t i o n method was used t o d e f i n e t h e endothermic t r a n s f o r m a t i o n onset, As, and completion, Af, f o r " a u s t e n i t e s t a r t " and " a u s t e n i t e f i n i s h , " r e s p e c t i v e l y . For t h e c o o l i n g t r a n s f o r m a t i o n exotherm, t h e onset was l a b e l l e d Ms f o r " m a r t e n s i t e s t a r t " and t h e c o m p l e t i o n Mf f o r " m a r t e n s i t e f i n i s h . " Peak areas were measured

Exo

I I I ^I I I I ^I I I ^I I ^I

-20 0 20 40 60 80 100

T,"C

FIGURE 1. T y p i c a l DSC Thermogram f o r N i T i : a. Heating; b . C o o l i n g .

w i t h a compensating p o l a r p l a n i m e t e r . Areas were c l o s e d b y c o n n e c t i n g t h e b a s e l i n e s as shown i n F i g u r e 1. The c e l l c o n s t a n t f o r c a l c u l a t i o n o f h e a t o f t r a n s f o r m a t i o n was determined w i t h g a l l i u m .

RESULTS AND DISCUSSION

N i T i i s an e x t r e m e l y d i f f i c u l t i n t e r m e t a l 1 i c compound t o manufacture commer- c i a l l y . Manufacture i n c l u d e s m e l t i n g , r e m e l t i n g and a l l a t t e n d a n t h o t and c o l d f i n i s h i n g processes. A t t h e n u c l e u s o f t h e problem i s c h e m i s t r y , b o t h i t s c o n t r o l and i t s p r e c i s e measurement. These problems a r e d i s c u s s e d as f o l l o w s :

Chemistry Measurement

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N i T i i s n o m i n a l l y 55 W.% n i c k e l and 45 W.% t i t a n i u m . I n t h i s c o n c e n t r a t i o n range, t h e e r r o r a s s o c i a t e d w i t h t h e chemical a n a l y s i s i s approx- i m a t e l y rf. 0.25 W.% f o r n i c k e l o r t i t a n i u m . Moreover, t h i s i s a r e l a t i v e e r r o r o f l e s s t h a n 0.5%, which i s w e l l w i t h i n a c c e p t a b l e l i m i t s f o r most analyses. Such i s n o t t h e case f o r N i T i because m i n o r c h e m i s t r y v a r i a t i o n s r e s u l t i n l a r g e v a r i a - t i o n s i n t h e 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 . Best ava.il a b l e d a t a r e l a t i n g 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 t o c h e m i s t r y i n d i c a t e t h a t one w e i g h t p e r e n t v a r i a t i o n i n n i c k e l o r t i t a n i u m r e s u l t s i n a s h i f t o f a p p r o x i m a t e l y 150°C.(i) I t i s apparent t h a t e r r o r i n t h e accuracy o f t h e chemical a n a l y s i s , a maximum 0.5 W.%, r e s u l t s i n a 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 v a r i a t i o n o f 75°C. The i n v e r s e r a t i o i s a more impor- t a n t measure o f c o m p o s i t i o n a l c o n s t r a i n t t o t h e manufacturer; i.e., a v a r i a t i o n o f o n l y 70 ppm o f e i t h e r n i c k e l o r t i t a n i u m w i l l s h i f t t h e 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 one degree c e n t i g r a d e . S e n s i t i v i t y t o c h e m i s t r y w i l l a f f e c t m e t a l p r o c e s s i n g due t o n i t r i d e , c a r b i d e , and o x i d e f o r m a t i o n which r e d u c e n i c k e l o r t i t a n i u m .

Chemistry C o n t r o l

-

M e t a l l u r g i c a l

M e l t i n g

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I t i s t e m p t i n g t o m a n u f a c t u r e N i T i b y preweighed charge c h e m i s t r y . d i f f i c u l t i e s w i t h t h i s approach a r e m e l t - c r u c i b l e and melt-atmosphere r e a c t i o n s .

Ef

Even under c o n d i t i o n s o f VIM w i t h i n e r t c r u c i b l e s t h e s e i n t e r a c t i o n s scavenge elements p r e f e r e n t i a l l y and change t h e 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 . C r u c i b l e o u t - g a s s i n g f o r m s T i 0 2 and T i N . P/M b i l l e t m a t e r i a l t y p i c a l l y c o n t a i n s 1260 ppm oxygen and 55 ppm n i t r o g e n . On a w e i g h t f r a c t i o n b a s i s and assuming a l l o f t h e oxygen and n i t r o g e n compound w i t h t i t a n i u m , a n e t l o s s o f 685 ppm o f t i t a n i u m r e s u l t s . T h i s t r a n s 1 a t e s t o a 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 d.ecrease o f 1 0 ° C . T i t a n i u m o x i d e s and n i t r i d e s t r a p p e d on t h e c r u c i b l e w a l l s f u r t h e r a l t e r 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 . Such a problem a l s o e x i s t s w i t h powder p r o d u c t i o n and may be enhanced due t o t h e l a r g e s u r f a c e area o f powder. T h i s m e l t i n g r e l a t e d c h e m i s t r y problem and t h e i n a c c u r a c y o f t h e chemical a n a l y s i s p r e v e n t t h e manu- f a c t u r e r from m e l t i n g an aim c h e m i s t r y t o achieve a p r e c i s e t r a n s f o r m a t i o n tem- p e r a t u r e .

A1 l o y m e l t r e a c t i o s and nominal c h e m i s t r y become r e l a t i v e l y u n i m p o r t a n t i n t h e S p e c i a l M e t a l s ' process?4) because 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 i s c o n t r o l l e d d i r e c t l y b y powder b l e n d r a t i o s . Therefore, t h e aim c h e m i s t r y needs t o be o n l y approximate f o r a s u c c e s s f u l powder campaign. The p u r e powder h e a t s a r e blended u s i n g a w e i g h t f r a c t i o n r u l e o f m i x t u r e s t o achieve t h e d e s i r e d i n t e r m e d i a t e t r a n s f o r m a t i o n temper- a t u r e . The o n l y c o n s t r a i n t f o r t h e two p u r e powder h e a t s i s t h a t t h e t r a n s f o r m a - t i o n temperatures b r a c k e t (above and below) t h e d e s i r e d t r a n s f o r m a t i o n temperature.

Sol i d i f icatio,n

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T y p i c a l VIM-VAR i n g o t s s o l i d i f y i n a r e a s o n a b l y p r e d i c t a b l e den- d r i t i c mode. L ~ ) R e s u l t s o f t h i s s o l i d i f ic a t i o n produce m i c r o - and macrosegrega- t i o n . M u l t i p l e VAR and e x t e n s i v e h o t w o r k i n g reduce s e g r e g a t i o n b u t cannot e l i m - i n a t e it, e s p e c i a l l y f o r a l l o y s which a r e v e r y s e n s i t i v e t o c h e m i s t r y v a r i a t i o n s . M i c r o s c o p i c s e g r e g a t i o n r e s u l t s i n a broad ATA -A

,

s l u g g i s h c o m p l e t i o n o f t h e reac- t i o n and an a t t e n u a t e d SME. Macroscopic seg?eg$tion r e s u l t s i n end t o end v a r i a - t i o n s o f 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 over t h e l e n g t h o f t h e f i n a l p r o d u c t , e.g.

w i r e . Long range macrosegregations a r e n o t removed b y a n n e a l i n g and h o t work. For most m e t a l l u r g i c a l a p p l i c a t i o n s s e g r e g a t i o n can b e removed w i t h i n d e t e c t a b l e l i m i t s . T h i s i s n o t t r u e f o r N i T i due t o h i g h s e n s i t i v i t y t o s m a l l chemical changes. Macro- s c o p i c s e g r e g a t i o n i s e l i m i n a t e d b y P/M p r o c e s s i n g because each p a r t i c l e i s an i n d i v i d u a l c a s t i n g . M i c r o s e g r e g a t i o n i s d r a m a t i c a l l y reduced due t o t h e r a p i d quench and s m a l l e r d e n d r i t e s i z e , an i n h e r e n t advantage o f P/M p r o c e s s i n g .

c4-288 JOURNAL DE PHYSIQUE

Transformation Temperature as a Function o f Blend Ratios

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Figure 2 shows temper- a t u r e versus powder b l e n d r a t i o . A r b i t r a r y numbers o f zero f o r lower t r a n s f o r m a t i o n temperature pure atomized heat (A) and one f o r t h e h i g h e r t r a n s f o r m a t i o n temperature pure atomized heat ( 5 ) are assigned. Note t h e absence o f chemical i n f o r m a t i o n .

"Rule o f m i x t u r e " values f o r blends are represented b y dashed l i n e s . Experimental t r a n s f o r m a t i o n temperatures v a r y by no more than 4°C f o r As and Ms and 6°C f o r Af and Mf. V a r i a t i o n from a 1 inear r u l e o f mixing a r i s e s from incomplete homogeniza- t i o n d u r i n g c o n s o l i d a t i o n . Inhomogeneity may also be observed as wider a u s t e n i t i c (ATA) and m a r t e n s i t i c (ATM) t r a n s f o r m a t i o n temperature ranges f o r blends than f o r t h e pure heats as shown i n Table I. I n p r a c t i c e t h e d e s i r e d t r a n s f o r m a t i o n tem- p e r a t u r e i s achieved v i a c a r e f u l l y c o n t r o l l e d compaction parameters.

-

i

A

1

I ---

Weighted Average

0 0.5 1

.o

A Weight Fraction Heat B

-

^B

FIGURE 2. Transformat i o n Temperatures o f N i T i Powder Blends Approximate a Linear Rule o f Mixing.

TABLE I

DSC THERMAL ARREST ANALYSES

I

AS

I

Af MS

I

M f

I

ATA

I

ATM

I

I

Heat-Blend

I ^{I I I I I I}

I I ^("C) I

("C)

I

('C)

I

('C)

I

('C)

I

("C)

I

Thermal a r r e s t s are b e s t understood by f i r s t examining F i g u r e 1. This f i g u r e represents t y p i c a l h e a t i n g and c o o l i n g thermograms f o r a specimen as-HIP'ed o r as-HIP'ed

+

850°C/20 min. Both energy curves are smooth and continuous.

Thermal A r r e s t o f t h e Austenite, Endothermic Reaction

A f t e r c o o l i n g below i t s Mf, t h e specimen i s heated t o t h e minimum o f t h e endothermic r e a c t ion; thermal l y arrested; cooled below t h e Mf and a1 lowed t o e q u i l i b r a t e . Reheating produces t h e doublet shown i n F i g u r e 3 . The f i r s t minimum i s a t t h e thermal a r r e s t temperature. Reaction energies f o r t h e standard and doublet peaks are equal.

-

3 14 29 43

1

A

1

^{50% A}

-

50% B

1

^{25% A}

-

^{75% B}

1

B

49 80 92 98 3 1

5 1 6 7 79

15 40 50 57

18 29 25 19

18

1

26

1

21

1

14

1

Exo

FIGURE 3. a. D o u b l e t Produced i n N i T i H e a t i n g Thermogram b y Thermal A r r e s t ; b. F i r s t H e a t i n g a f t e r ( a ) .

Thermal a r r e s t appears t o d i v i d e t h e m a r t e n s i t e v a r i a n t s i n t o two d i s t i n c t p o p u l a t i o n s upon c o o l i n g below Mf. Reheating t h e specimen i n t o t h e a u s t e n i t i c r e g i o n produces two endothermic r e a c t i o n s . The f i r s t r e p r e s e n t s t h e p o r t i o n o f t h e m a r t e n s i t e p o p u l a t i o n p r e v i o u s l y unsheared ( t r a n s f o r m e d ) d u r i n g t h e t h e r m a l a r r e s t . The second r e p r e s e n t s t h e p o r t i o n which remains sheared d u r i n g thermal a r r e s t . C o o l i n g below Mf f o r t h e t h i r d t i m e and h e a t i n g t h r o u g h Af r e t u r n s t h e endothermic r e a c t i o n t o a s i n g l e peak.

Peak s p l i t t i n g phenomena p r o b a b l y r e s u l t f r o m d i s l o c a t i o n motion, t a n g l e forrna i n and r e s u l t i n g back s t r e s s e s generated b y t h e m a r t e n s i t i c transforma- t ion.f6!

'

D is l o c a t i o g e n e r a t i o n due t o u n s t r e s s e d t h e r m a l c y c l i n g has been addressed p r e v i o u s l y . ~ 7 ) D i s l o c a t i o n t a n g l e s a r e a b l e t o s u r v i v e t h e c o o l i n g c y c l e , b u t o n l y one h e a t i n g c y c l e . D i s l o c a t i o n d e n s i t y w i t h i n t h e t a n g l e s i s p r o b a b l y v e r y low because o n l y one thermal a r r e s t c y c l e and no p r e s t r a i n a r e used. T h i s p r o v i d e s t h e m a r t e n s i t e v a r i a n t s w i t h a d i s t i n c t s t r e s d s t r a i n t e n s o r which a s s i s t s s h e a r i n g and unshearing. "Low" t e m p e r a t u r e v a r i a n t s a r e a s s i s t e d w i t h unshearing, a l l o w i n g them t o s t a r t and f i n i s h t h e i r r e a c t i o n b e f o r e t h e second p o p u l a t i o n s t a r t s unshearing.

Use o f a s i n g l e h e a t i n g and subsequent c o o l i n g c y c l e t o r e s t o r e t h e s i n g l e peak suggests l o w d i s l o c a t i o n d e n s i t y t a n g l e s which a r e e a s i l y removed b y an

"avalanche1' o f s h e a r i n g m a r t e n s i t e p l a t e s . An avalanche o f m a r t e n s i t e p l a t e s a t t e n u a t e s o r e l i m i n a t e s t h e s t r e s d s t r a i n t e n s o r o f t h e d i s l o c a t i o n t a n g l e and p o s s i b l y t h e t a n g l e i t s e l f . End r e s u l t i s t h e l o s s o f two d i s t i n c t m a r t e n s i t e p o p u l a t i o n s .

Thermal a r r e s t o f t h e m a r t e n s i t i c r e a c t i o n produces no d o u b l e t . DSC RESPONSE TO THERMOMECHANICAL HISTORY

Residual s t r e s s and h e a t t r e a t m e n t have a s i g n i f i c a n t e f f e c t upon t h e DSC thermogram. A specimen i s cooled t o -23"C, w e l l below t h e Mf, and t e s t e d t o f a i l - u r e i n t e n s i o n . DSC r e v e a l s no exo- o r endothermic r e a c t i o n . T h i s i s f u r t h e r d i s c u s s e d w i t h work p e r t a i n i n g t o r e a c t i o n e n e r g y and p r e s t r a i n . ( 8 ) Two specimens a r e c u t from t h e gage s e c t i o n o f t h e t e n s i l e w i r e . The f i r s t i s annealed a t 850°C f o r 20 minutes. The r e s u l t i n g thermogram i s i d e n t i c a l i n f o r m t o F i g u r e 1. The second specimen i s annealed a t 500°C f o r 20 m i n u t e s . I n F i g u r e 4 t h e As and Af temperatures have s h i f t e d . I n t h e m a r t e n s i t i c r e a c t i o n a second peak i s apparent a t a h i g h e r t e m p e r a t u r e t h a n t h e Ms and i s b e l i e v e d t o b e t h e p r e m a r t e n s i t i c

C4-290 JOURNAL DE PHYSIQUE

r e a c t i o n . Thermal a r r e s t a n a l y s i s i s conducted on t h i s h i g h t e m p e r a t u r e peak.

Reheating t h e specimen a f t e r t h e a r r e s t r e v e a l s

2

a u s t e n i t i c r e a c t i o n , c l e a r l y i n d i c a t i n g t h a t t h e h i g h t e m p e r a t u r e peak does n o t d i r e c t l y c o n t r i b u t e t o t h e aus- t e n i t i c r e a c t i o n . Moreover, i t s e x i s t e n c e i s r e l a t e d t o a thermomechanical h i s t o r y which r e t a i n s some r e s i d u a l s t r e s s / s t r a i n . To c o n f i r m t h i s h y p o t h e s i s , t h e spec- imen e x h i b i t i n g t h e d o u b l e peak i s annealed a t 85O0C/20 minutes, r e s u l t i n g i n e l i m i n a t i o n o f t h e d o u b l e peak and r e t u r n o f t h e t y p i c a l thermogram.

FIGURE 4 . a. H e a t i n g Thermogram Showing T y p i c a l Form.

b . Same Specimen Showing P r e m a r t e n s i t i c R e a c t i o n on C o o l i n g . CONCLUSIONS

C o n t r o l l e d 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 N i T i i s c o m m e r c i a l l y achieved.

Heat t o h e a t c o n t r o l o f 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 i s a c c u r a t e t o

+

5°C.

Transformation temperatures o f N i T i blended powders a c c u r a t e l y f o l l o w a l i n e a r w e i g h t f r a c t i o n r u l e o f m i x t u r e s .

DSC i s a u s e f u l t o o l f o r measuring t r a n s f o r m a t i o n temperatures.

A s i n g l e thermal a r r e s t on t h e DSC i m p r i n t s i t s h i s t o r y upon a n a i v e N i T i sample b y s p l i t t i n g t h e a u s t e n i t i c peak.

P r e m a r t e n s i t i c phenomena i s observed u s i n g DSC.

Thermal a r r e s t experiments r e v e a l t h a t t h e p r e m a r t e n s i t i c e f f e c t does n o t d i r e c t l y a f f e c t t h e a u s t e n i t e r e a c t i o n .

ACKNOWLEDGEMENTS: The a u t h o r s thank T. L . Rowlands f o r f i r s t o b s e r v i n g t h e thermal a r r e s t e f f e c t and S. L . P r a t t and R. W. M a r t i n f o r p r e p a r a t i o n of m a n u s c r i p t . REFERENCES

1. Podob M.T., Johnson W.A.. and Reichman S.H., P r o c . N i t i n o l Heat Engine Conf

.,

MP 79-441, - s i l v e r s p r i n g s ; MD ( 1 9 7 8 ) .

2. Jackson C.M., e t a l . , 5 5 - 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)18.

3 . S u t t o n W.H. and Johnson W.A.. Proc. F o u r t h I n t . Sym. on S u p e r a l l o y s , ASM (1980).

4 . U.S. P a t . No. 4,310,354.

5 . Fleming M.C., S o l i d i f i c a t i o n Processing, McGraw H i l l , New York (1974) 134.

6 . B a l l A.. e t a1

..

P r o c . I n t . Conf. S t r . Met. and A l l o y s , Trans. Jap. I n s t . Met.

2

( ~ u p p ) (1968) 291.

7 . 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., Plenem Press, N.Y. (1975) 273.

8 . Johnson W.A., Domingue J.A., Reichman S.H., and S c z e r z e n i e F.E. ( i n p r e s s ) .