Progress with FIM and atom-probe investigations of nickel-based superalloys

385

Abstracts of 25th International Field Emission Symposium

We now b e l i e v e that p h o t o c o n d u c t i v i t y is m a i n l y r e s p o n s i b l e for this effect. The p o t e n t i a l drop across an oxide layer depends on the layer thickness, the tip temperature, and the light intensity. It may amount to a few kV. This quantity can be d e r i v e d quite a c c u r a t e l y from I-V curves. In a d d i t i o n the elect r o n - h o l e r e c o m b i n a t i o n rate, w h i c h again depends on both the tip temperature and the oxide layer thickness, and which may amount to several seconds, can be m e a s u r e d from the r e l a x a t i o n time of the FI enhancement. By c h a n g i n g the oxide layer thickness the effect of band b e n d i n g on the e l e c t r o n - h o l e rec o m b i n a t i o n rate can be investigated. It is i m p o r t a n t to note that the effect of b a n d b e n d i n g is exactly the same as that of field p e n e t r a t i o n , and can t h e r e f o r e be c o n t r o l l e d by a p p l i e d field. Our study should y i e l d significant i n f o r m a t i o n on physics of semic o n d u c t o r s and i n s u l a t o r surfaces. (3) We also found that field e m i s s i o n from a Si tip d e v i a t e s s i g n i f i c a n t l y from the F o w l e r - N o r d h e i m plot by light illumination. Such effect p e r s i s t s even w h e n a m o n o c h r o m a t i c light of 12,000 (% 1 eV < Egap) is used. The effect is e x p l a i n e d in terms of field p e n e t r a t i o n into the near surface layers. This work was DMR-76-11418.

supported

with a s u b s t a n t i a l l y lower cobalt content than in the primary y' precipitates. (2) I n v e s t i g a t i o n of the initial stages of d e c o m p o s i t i o n of the solutiont r e a t e d alloy. C o m p u t e r m e t h o d s have been d e v i s e d to follow the l o c a l i z e d comp o s i t i o n f l u c t u a t i o n s o c c u r r i n g during the e a r l y stages of y' formation. Fluctuations in the c o n c e n t r a t i o n s of any nulaber of elements can be f o l l o w e d simultaneously. (3) D e t e r m i n a t i o n of the c o m p o s i t i o n of the p r i m a r y MC carbide phase. All the major c a r b i d e - f o r m i n g elements (Ti, Zr, Ta, Nb, Cr and W) were found to be present in carbide p a r t i c l e s of o v e r a l l comp o s i t i o n MC0. SN i t r o g e n and boron were also p r e s e n t , 8 £ h e latter b e i n g e n r i c h e d at the c a r b i d e / m a t r i x interface. Work is also in p r o g r e s s to investigate the effects of p r o l o n g e d exposure, under skress, at e l e v a t e d t e m p e r a t u r e s on the m i c r o s t r u c t u r e and p r o p e r t i e s of superalloys. Highly a n i s o t r o p i c ¥' particle shapes nave been o b s e r v e d by FIM in a crept specimen of NIMONIC 90. Similar w o r k is b e i n g u n d e r t a k e n on IN939. i. P. A. Beaven, M. K. Miller, G. D. W. Smith, 24th International Field Emission Symposium, Oxford, England, 1977.

by NSF Grant

i. T. T. Tsong, Yee S. Ng, A. J. Melmed, to be p u b l i s h e d in Surface Sci.

FIM AND A T O M - P R O B E STUDIES OF DEFECTS IN DOPED T U N G S T E N LAMP W I R E S P.

A.

Beaven,

Miller, P R O G R E S S W I T H F I M AND A T O M - P R O B E I N V E S T I G A T I O N S OF N I C K E L - B A S E D SUPERALLOYS K. M. Delargy, P. A. Beaven, Miller, G. D. W. Smith

M.

K. M.

Delargy,

P. R. Williams,

M.

G. D. W.

K.

Smith

university of Oxford, Department of Metallurgy a n d S c i e n c e of M a t e r i a l s , P a r k s R o a d , O x f o r d OXI 3PH, E n g l a n d

K.

university of O x f o r d , D e p a r t m e n t of Metallurgy and Science of Materials, P a r k s R o a d , O x f o r d OX! 3PH, E n g l a n d

F u r t h e r i n v e s t i g a t i o n s of the micros t r u c t u r e and fine-scale c o m p o s i t i o n of n i c k e l - b a s e d s u p e r a l l o y s have been carried out u s i n g TEM, FIM, T.O.F. A t o m Probe and Imaging A t o m - P r o b e techniques. These studies follow the initial work on the d e t e r m i n a t i o n of y and y' phase c o m p o s i t i o n in alloy IN939, r e p o r t e d last year. New results to be p r e s e n t e d c o n c e r n ing alloy IN939 include the following: (i) D e t e r m i n a t i o n of the c o m p o s i t i o n of the u l t r a - f i n e secondary p r e c i p i t a t e s formed during the low t e m p e r a t u r e (700 ° C) final aging treatment. The phase p r e s e n t has been i d e n t i f i e d as ¥', but

P r e v i o u s FIM o b s e r v a t i o n s of defects in a l u m i n u m - p o t a s s i u m - s i l i c o n ('AKS') doped tungsten lamp wire I'2 have been s u b s t a n t i a l l y extended. The a c c u m u l a t e d e v i d e n c e shows that a large n u m b e r of o b s e r v a t i o n s in the e a r l i e r F i M literature of t u n g s t e n c o n c e r n i n g the "crossover effect", " d i s s o c i a t e d d i s l o c a t i o n loops", "horseshoe d i s l o c a t i o n s " etc. can be a s c r i b e d to the imaging of small defects i n t r o d u c e d into the t u n g s t e n wire during the doping process. These defects fall into two main classes: small voids (believed s t a b i l i z e d by the p r e s e n c e of potassium) and small solid particles, of h i t h e r t o unknown composition. Each of these classes of defect has been i n v e s t i g a t e d u s i n g the T.O.F. AtomProbe. In the case of the small voids, attempts at chemical analysis have been u n s u c c e s s f u l , owing to i o n - o p t i c a l aberration effects. In some cases, no substrate ions at all w e r e c o l l e c t e d from