STUDY ON RELATIONSHIP OF PHASES IN ALLOYS BY MEANS OF EPMA

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STUDY ON RELATIONSHIP OF PHASES IN ALLOYS BY MEANS OF EPMA

Xu Leying, Zhu Yaoxiao, Li Yiyi, Shi Changhsu

To cite this version:

Xu Leying, Zhu Yaoxiao, Li Yiyi, Shi Changhsu. STUDY ON RELATIONSHIP OF PHASES IN ALLOYS BY MEANS OF EPMA. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-625-C2-630.

�10.1051/jphyscol:19842146�. �jpa-00223817�

STUDY ON RELATIONSHIP OF PHASES IN ALLOYS BY MEANS OF EPMA

Xu Leying, Zhu Yaoxiao, Li Yiyi and Shi Changhsu

Institute of Metal Research, Academia Siniaa, Shenyang, China

Résumé - Les diagrammes de phase sont importants à connaître pour des raisons théoriques et pratiques. On diagramme pseudo- ternaire Ni-(Al + Ti)-(Cr + Mo) pour des super-alliages à base nickel ainsi que le diagramme Fe-Mn-Al ont été déterminés par microanalyse par sonde électronique. On a mis en évidence l'effet du bore sur la porosité des coulées de super-alliages à base nickel, par l'étude des déplacements des courbes de so- lidification du diagramme.

Abstract - Phase diagram is of both academic and technological interest. A pseudo-ternary phase diagram of Ni-(A1 + Ti ) -

(Cr + Mo) of Ni-base superalloy was constructed as well as Fe-Mn-Al ternary phase diagram by means of EPMA. The effect of boron on porosity of cast Ni-base superalloys was revealed through studying the solidification process. It was shown that EPMA is a very useful method for the studying relationship of phases in alloys.

Phase diagram is of both academic and technological interest. Electron- probe microanalysis (EPMA) is considered as an useful method for the determination of binary and ternary phase diagram. In order to provide the necessary phase equilibrium data for developing Fe-Mn-Al austeni- tic alloys, partial diagrams of this system have been constructed. It is difficult to determine the phase diagram of a superalloy containing more than 10 alloying elements. For understanding the rules governing phase formation in this alloy, EPMA was used to determine the composi- tion of each phase and to reveal the elemental segregation in the alloy. A pseudo-ternary phase diagram of Ni-(Al+Ti)-(Cr+Mo) was thus constructed. The investigation of elemental segregation can also re- veal the function of trace elements in the alloy. With low solubility in solid solution this kind of elements tends to concentrate in spe- cial areas, such as in the last solified liquid or at interdendrites.

Undoubtedly, EPMA is an useful means for studying such a problem.

Certain amount of boron added to superalloy may alternate its porosity tendency. This phenomenon has not been clarified so far. It is the third part of this paper to illustate the function of boron through studying the solidification process of the alloy and the behavior of boron by means of EPMA.

I. DETERMINATION OF Fe-Mn-Al PHASE DIAGRAM

The pineer work of phase diagram determination by means of EPMA was done by Adda et al.(l). The diffusion couple method was used in their work. However shortcomings restrict the use of this method in certain cases. An alternative method known as quench and anneal method had been widely used for determination of various phase diagrams(2).

The accuracy of phase diagram determination by means of EPMA is limi- ted by the uncertainties involved in the conversion of measured X-ray intensity ratio to chemical concentration. In order to avoid unexpec-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842146

C2-626 JOURNAL DE PHYSIQUE

t e d e r r o r caused by e x p e r i m e n t , s u c h a s t h e p u l s e a m p l i t u d e s h i f t i n f l o w p r o p o r t i o n a l c o u n t e r s ( 3 ) . which might o c c u r and would s e r i o u s l y i n f l u e n c e t h e measured r e s u l t , a s e r i e s of s i n g l e phase Fe-Mn-A1 a l l o y s was s e l e c t e d a s s t a n d a r d s . A c o n c e n t r a t i o n - i n t e n s i t y r a t i o ( C - K ) c a l i - b r a t i o n c u r v e was made t o a c q u i r e t h e c o n c e n t r a t i o n . Fig.1 shows two C-K c u r v e s of A 1 measured a t a c c e l e r a t i o n v o l t a g e of 10KV and 20KV r e s p e c t i v e l y , I t shows t h a t t h e K:C r a t i o i s a b o u t 1:2 a t 10KV and it changes t o a b o u t 1:4 when t h e a c c e l e r a t i o n v o l t a g e i n c r e a s e s t o 20KV.

Assuming t h e e r r o r of i n t e n s i t y r a t i o K i s +0.1%, t h e e r r o r s of con- c e n t r a t i o n C a r e +0.2% and +O.4% f o r a c c e l e r a t i o n v o l t a g e of 10KV and 20KV r e s p e c t i v e l y . T h e r e f o r e , low a c c e l e r a t i o n v o l t a g e i s s u i t a b l e f o r l i g h t e l e m e n t a n a l y s i s , I t i s well-known t h a t t h e f.c.c. a u s t e n i t e p o s s e s s e s e x c e l l e n t p r o p e r t i e s e i t h e r a t h i g h o r low t e m p e r a t u r e . Most of t h e a u s t e n i t i e - s t e e l c o n t a i n c e r t a i n amount o f N i and C r . However, Mn c a n s t a b i l i z e h i g h t e m p e r a t u r e a u s t e n i t e and A 1 s t a b i l i z e low tem- p e r a t u r e a u s t e n i t e . By s e l e c t i n g a p p r o p r i a t e amount of Mn and A l , a s t a b l e Fe-Mn-A1 a u s t e n i t e c a n be o b t a i n e d .

Some p a p e r s a b o u t Fe-Mn and Fe-A1 system have been p u b l i s h e d , however v e r y few works on t h i s t e r n a r y system i s a v a i l a b l e , Schmatz(4) d e t e r - mined a n i s o t h e r m a l s e c t i o n of Fe-Mn-A1 a l l o y a t 760°c by m e t a l l o g r a - p h i c method, There i s no i n f o r m a t i o n a b o u t t h e e f f e c t of c a r b o n on t h e y r e g i o n . Fe-Mn-A1 phase diagrams a t h i g h t e m p e r a t u r e s which a r e impor- t e n t f o r h e a t t r e a t m e n t a r e l a c k i n g .

I n t h e p r e s e n t work t e r n a r y i s o t h e r m a l s e c t i o n have been c o n s t r u c t e d f o r Fe-Mn-A1 w i t h carbon c o n t e n t s ( 0 ~ 0 . 2 , 0.4%) a t t e m p e r a t u r e s 800, 1000, 1100, and 1200°C r e s p e c t i v e l y , Specimens were f o r g e d a t Il4OoC f o l l o w e d by homogenization t r e a t m e n t a t 12000C f o r 2h t o e l i m i n a t e s e - g r e g a t i o n , t h e n t h e y a r e k e p t a t d i f f e r e n t t e m p e r a t u r e s f o r d i f f e r e n t p e r i o d d e s i r e d . L i n e s c a n n i n g t e c h n i q u e w a s used t o check t h e homogen- e i t y of e a c h phase.

Some of t h e e x p e r i m e n t r e s u l t s a r e shown i n Fig.2 and 3. Fig.2 shows t h a t carbon e n l a r e s t h e y r e g i o n , ^Fig.3 i s a n i s o t h e r m a l s e c t i o n of 0.2% a l l o y a t 800 C , 6-Mn p h a s e a p p e a r s a t 8000C and a s t h e tempera- d

t u r e i s i n c r e a s e d t h e 3-phase f i e l d moves t o t h e r e g i o n w i t h h i g h Mn and A 1 c o n t e n t ,

11, SOLIDIFICATION OF NICKEL BASE SUPERALLOYS

C a s t n i c k e l b a s e s u p e r a l l o y s have been used a s m a t e r i a l f o r g a s t u r - b i n e b l a d e s f o r more t h a n twenty y e a r s . The p r o p e r t i e s of a c a s t a l l o y can be improved e i t h e r by a d j u s t i n g c h e m i c a l c o m p o s i t i o n o r by c o n t r o l - l i n g t h e s o l i d i f i c a t i o n p r o c e s s i n o r d e r t o minimize s e g r e g a t i o n of a l l o y i n g elements. U n f o r t u n a t l y , t h e r e a r e v e r y few r e s e a r c h works p u b l i s h e d a b o u t s o l i d i f i c a t i o n b e h a v i o r of c a s t n i c k e l b a s e s u p e r a l l o y , A n i c k e l a l l o y s i m i l a r t o IN-100 was u s e d a s specimen i n t h i s work, Specimens from t h e same m a s t e r a l l o y were m e l t e d a t 1400°C f o r 5 minu- t e s t o e n s u r e t h a t a l l c a r b i d e s w i l l be d i s s o l v e d and homogenization r e a c h e d . They were t h e n c o o l e d down i n f u r n a c e t o t h e t e m p e r a t u r e de- s i r e d , k e p t f o r 10 m i n u t e s and t h e n quenched i n t o a l k a l i n e w a t e r . The s t a n d a r d used i n EPMA was made i n t h e way a s f o l l o w i n g : t h e a l l o y spe- cimen was melted a t 1450°C and k e p t 5 minutes t h e n quenched i n t o a l k a - l i n e w a t e r , t h i n p l a t e w i t h t h i c k n e s s l e s s t h a n 1 mm was s e l e c t e d a s s t a n d a r d ,

Specimens s o l i d i f i e d a t v a r i o u s t e m p e r a t u r e s were o b s e r v e d by t h e op-

t i c a l microscope and t h e q u a n t i t i e s of v a r i o u s p h a s e s were d e t e r m i n e d

m e t a l l o g r a p h i c a l l y , t h e r e s u l t s a r e summarized i n Table 1 , The s e g r e -

g a t i o n of a l l o y i n g e l e m e n t s and t h e c o m p o s i t i o n of v a r i o u s p h a s e s were

d e t e r m i n e d by EPMA. Some of t h e r e s u l t s a r e i l l u s t r a t e d i n T a b l e 11.

t h e end of s o l i d i f i c a t i o n , but t h e main p a r t i s accomplished i n t h e t e m p e r a t u r e i n t e r v a l between 1340° and 1290°C;

T i c b e g i n s t o form a t 1330°C;

The e u t e c t i c r e a c t i o n of L + ( y t y t ) + L 1 o c c u r s i n t h e t e m p e r a t u r e r a n g e of 1 2 6 0 ~ - 1 2 3 0 ~ ~ . The amount of y t i n t h e ( y t y t ) e u t e c t i c i n c r e a s e s w i t h t h e d e c r e a s e of s o l i d i f i c a t i o n t e m p e r a t u r e , and f i n a l l y y' i s t h e main c o n s t i t u e n t of e u t e c t i c ;

Y phase ( T i 2 ( C S ) ) i s p r e c i p i t a t e d i n t h e t e m p e r a t u r e r a n g e of 1260°- 1 21 0% ;

Below 1 2 1 0 ~ ~ ~ a h i g h C r , Mo phase may be p r e c i p i t a t e d a s shown i n Fig.4.

According t o t h e d a t a i n t a b l e 11, a p s e u s o - t e r n a r y p h a s e diagram o f

~ i - ( ~ 1 t T i ) - ( C r t M o ) w a s c o n s t r u c t e d a s shown i n Fig.4. E u t e c t i c r e a c - t i o n b e g i n s a t 1260°C, a t t h e b e g i n n i n g of t h i s r e a c t i o n , o n l y few Y1 a p p e a r e d , p o i n t s C and C y r e p r e s e n t t h e c o m p o s i t i o n o f l i q u i d and Y phase. The t r i a n g l k B L B y B y t r e s p r e s e n t s t h e e u t e c t i c r e a c t i o n a t 12500C, p o i n t s BL, By and By( d e n o t e t h e c o m p o s i t i o n o f l i q u i d , Y and y 1 r e s p e c t i v e l y . T r i a n g l e ALAYA 1 r e p r e s e n t s t h e e u t e c t i c r e a c t i o n a t 12300C. I t shows t h a t t h e e u t e c x i c r e a c t i o n p r o c e e d s a l o n g t h e p a t h of i n c r e a s i n g (CrtMo) and d e c r e a s i n g ( A l t T i ) as t e m p e r a t u r e d r o p s con- t i n u o u s l y , If t a n g e n t l i n e s a r e drawn a t AL aQd BL t h e i n t e r s e c t i o n s o f t h i s l i n e w i t h AyAyr and B By, a t A ¹ and B r e s p e c t i v e l y r e - p r e s e n t t h e a v e r a g e c o m p o s i t i x n of t h e Y i Y t e c t i c x k a t t h e p r o p o r t i o n o f y and y1 can be r e p r e s e n t e d by t h e r a t i o o f Ayty,dgto A , A

and t h a t o f Byt , B y t o B I B , . Obviously y / y l l n c r e s e a g s X ~ J '

t e m p e r a t u r e i s lowered f F o m Y $ $ 6 ~ ~ t o 12300C. T h i s can e x p l a i n t h e s t r u c t u r e of t h e e u t e c t i c a s shown i n Fig.5,

I n Fig.6, a bar-shaped h i g h C r , Ho phase appeared i n f r o n t o f t h e eu- t e c t i c , t h e s t r u c t u r e of which seemed changed, Normally a t t h e f r o n t o f t h e e u t e c t i c , t h e main c o n s t i t u e n t i s y l , Due t o t h e p r e c i p i t a t i o n of h i g h C r , Mo phase, t h e e u t e c t i c became s t r i p - s h a p e d y t s e p e r a t e d by y, T h i s can be e x p l a i n e d a s f o l l o w i n g : During t h e f i n a l s t a g e of s o l i - d i f i c a t i o n , a s t h e e u t e c t i c r e a a t i o n p r o c e e d s , C r and Mo a r e c o n t i n u - o u s l y r e j e c t e d i n t o t h e r e m a i n i n g l i q u i d and A 1 and T i i n t h e l i q u i d d i f f u s e t o t h e f r o n t i e r of t h e e u t e c t i c . A s t h e c o n t e n t s of C r , Mo a r e h i g h i n t h e r e m a i n i n g l i q u i d , t h e yc formed i n t h e e u t e c t i o r e a c t i o n i s much more p r e d o m i n a t e o v e r y r e s u l t i n g i n t h e f o r m a t i o n of massive y c . If t h e c o n t e n t o f C r and Mo i n t h e l i q u i d i s h i g h enough t o form t h e h i g h C r , Mo p h a s e , t h e n t h e c o n t e n t o f C r and Mo i n t h e r e m a i n i n g l i q u i d would be lowered and t h e c o n t e n t o f A 1 and T i i n c r e a s e d a c c o r - d i n g l y . According t o Fig.4, i n a l i q u i d w i t h low C r , Mo c o n t e n t and h i g h A l , T i c o n t e n t , t h e e u t e c t i c r e a c t i o n L + ( y t y l ) t L 1 w i l l p r o c e e d a l o n g t h e t a n g e n t l i n e B L B ~ + ~ , and t h e r a t i o of y t t o y i n c r e a s e s . Because of t h e i n c r e a s e of t h e amount of y i n t h e e u t e c t i c , t h e shape

of e u t e c t i c changed a s shown i n Fig.5.

Degree of s e g r e g a t i o n i s d e f i n e d a s t h e r a t i o of c o m p o s i t i o n of a c e r -

t a i n a r e a o r p o i n t t o t h e a v e r a g e c o m p o s i t i o n of a l l o y , t h e r e s u l t s i s

shown i n Fig.7, I n t h e f i g u r e , t h e c u r v e s a t t h e l e f t r e p r e s e n t t h e se-

g r e g a t i o n i n e u t e c t i c which i s dependent on t h e amount o f y t , t h e

c u r v e s a t t h e r i g h t r e p r e s e n t t h e d e n t r i t i c s e g r e g a t i o n . Element such

as Mo, C r , Co, which a r e c o n s i d e r e d a s t h e o-phase forming e l e m e n t s ,

e n r i c h e d i n f r o n t of t h e e u t e c t i c , t h e r e f o r e o p h a s e i s more pronoun-

ced i n t h i s a r e a ,

C2-628 JOURNAL DE PHYSIQUE

From t h i s e x p e r i m e n t , it i s concluded t h a t t h e r e i s no d e n t r i t i c s e - g r e g a t i o n of A 1 d u r i n g s o l i d i f i c a t i o n and T i . C r . Mo a r e p o s i t i v e den- d r i t i c s e g r e g a t i o n , T i i s t h e most s e r i o u s one. a phase can be a v o i d e d e i t h e r by a d j u s t i n g t h e r a t i o of A 1 t o T i o r c o n t r o l l i n g t h e c o o l i n g r a t e i n o r d e r t o d e c r e a s e s e g r e g a t i o n and r e f i n e t h e e u t e c t i c .

111. EFFECT OF BORON ON THE SOLIDIFICATION OF CAST NICKEL-BASE SUPER- ALLOYS

I t h a s been proved t h a t a d d i t i o n of s m a l l amounts of boron t o s u p e r - a l l o y can e x t e n d t h e l i q u i d u s and s o l i d u s gap, and t h u s i n c r e a s e s t h e t e n d e n c y of p o r o s i t y f o r m a t i o n . I t was d i s c o v e r e d r e c e n t l y t h a t when boron c o n t e n t i s i n c r e a s e d t o 0.1$, t h e gap i s f u r t h e r e x t e n d e d , on t h e c o n t r a r y , t h e t e n d e n c y of p o r o s i t y f o r m a t i o n i s d e c r e a s e d . The mechanism c o n c e r n i n g t h e e f f e c t o f boron on p o r o s i t y h a s n o t been a c c o u n t e d f o r s a t i s f a c t o r i l y s o f a r ,

I n o r d e r t o make a t h o r o u g h i n v e s t i g a t i o n of t h i s mechanism, t h e e f f e c t of boron on s o l i d i f i c a t i o n p r o c e s s was s t u d i e d .

The l i q u i d u s and s o l i d u s of a l l o y s w i t h d i f f e r e n t boron c o n t e n t was determined. The s o l i d i f i c a t i o n b e h a v i o r of y s o l i d s o l u t i o n was o b s e r - ved and t h e c o m p o s i t i o n of r e m a i n i n g l i q u i d was d e t e r m i n e d f o r a l l o y s w i t h d i f f e r e n t boron c o n t e n t .

T a b l e 111 shows t h e e f f e c t of boron on t h e m e l t i n g c h a r a c t e r i s t i c s . I t shows t h a t t h e m e l t i n g p o i n t of a l l o y w i t h O.I$B d o e s n o t change s o much, b u t t h e m e l t i n g p o i n t of a l l o y w i t h 1$B i s pronounced lowered, F o r a l l o y of normal B content(?.O.OI$), t h e e n r i c h m e n t of B i s n o t h i g h enough t o lower t h e m e l t i n g p o i n t of t h e r e m a i n i n g l i q u i d , While f o r a l l o y o f h i g h boron c o n t e n t (?.0,1$), t h e boron c o n t e n t a t t h e f r o n t i e r of t h e r e m a i n i n g l i q u i d may be a s h i g h a s I $ , s o t h a t t h e m e l t i n g p o i n t may pronouncedly lowered. F i g - 9 shows t h e d i s t r i b u t i o n c u r v e s of some a l l o y i n g e l e m e n t s i n a l l o y c o n t a i n i n g 0,1$B, C r , Mo, T i a s w e l l a s bo- r o n a r e e n r i c h e d a t t h e f r o n t i e r of t h e r e m a i n i n g l i q u i d . The e n r i c h - ment of boron h a s a l s o been proved by i o n - p r o b e a n a l y s i s , I t seems t h a t a l i q u i d l a y e r of low m e l t i n g p o i n t may s u r r o u n d t h e s o l i d i f i e d g r a i n s .

Specimens were h e a t e d t o 1400°C f o r 5 minutes, t h e n c o o l e d i n f u r n a c e t o 1290°C f o r 1 0 minutes, and quenched i n t o a l k a l i n e w a t e r , The quen- ched s t r u c t u r e s o f a l l o y s w i t h h i g h and low boron c o n t e n t a r e shown i n Pig.8. I t i s o b s e r v e d t h a t t h e y s o l i d s o l u t i o n grows c o n t i n u o u s l y i n t o r e s i d u a l l i q u i d d u r i n g quenching i n low boron a l l o y , I n h i g h bo- r o n a l l o y , t h e new y s o l i d s o l u t i o n n u c l e a t e s and grows mainly i n t h e r e m a i n i n g l i q u i d i t s e l f b e c a u s e of t h e e x i s t e n c e of t h e low m e l t i n g p o i n t l i q u i d l a y e r which h i n d e r s t h e growing of y s o l i d s o l u t i o n i n t o t h e r e m a i n i n g l i q u i d .

A s t h e y s o l i d s o l u t i o n i s u s r r o u n d e d by t h i s l i q u i d l a y e r i n h i g h bo- r o n a l l o y , t h e g r a i n s and d e n d r i t e s a r e s e p a r a t e d , Such l i q u i d l a y e r a p p e a r s t o be a "cobwebn l i k e c h a n n e l a s shown i n f i g u r e l o b which i s c o n n e c t e d t o t h e f i l l i n g s o u r c e s t o improve t h e f i l l i n g a b i l i t y of t h e a l l o y . By c o n t r a c t t h e d i s t r i b u t i o n of l i q u i d p h a s e of a l l o y w i t h low boron i s shown i n Fig.1Oa. The y s o l i d s o l u t i o n a r e c o n n e c t e d and t h e r e m a i n i n g l i q u i d s a r e i s o l a t e d w i t h each o t h e r , t h e y c a n n o t con- n e c t e d w i t h t h e f i l l i n g r e s o u r c e . T h e r e f o r e , t h e f i l l i n g a b i l i t y of h i g h B a l l o y i s much b e t t e r t h a n t h e low boron one.

REFERENCES

1. Y.Adda, J. P h i l i b e r t , Compt, Rend, 2&,(1956), 3081.

2. A,D,Romi J . I , G o l d s t e i n . M e t a l l . Trans. A , 8A(1977) , 309;

n n ( r 9 8 o f : 1151.

3. - Xu Leying, i n nScanning microscopyn ed. Yiao J i n g e n , Atomic Energy p r e s s , B e i j i n g , 1983.

4. D.J.Schmatz, Trans. AIME, = ( 1 9 5 9 ) , 112.

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JOURNAL DE PHYSIQUE

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