Diffusional creep and creep-degradation in dispersion-strengthened Ni-Cr base alloys
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Fig. 1--Schematic r e p r e s e n t a t i o n of diffusional c r e e p in a
dispersion-strengthened alloy. particles when annealed in hydrogen. During testing, ZrH~-free regions were formed around the grain boundaries tending to be normal to the applied tensile stress. This observation was interpreted as evidence that diffusional creep occurred. Also, Karim et al. s'D observed that voids formed in the precipitate-free zones, and these led to failure of the specimen. Additionally, several studies t1'taof high temperature ( T / T M ~ 0.7) creep in y' precipitation strengthened nickel-base superalloys have indicated that diffusional creep m a y be an active creep mechanism in these alloys. In these studies, y'-free regions were observed around grain boundaries generally lying at right angles to the applied tensile stress. Thoriated nickel-base alloys are, to date, the best examples of successful dispersi.on-strengthened systems for use at elevated temperatures. These alloys are intended for use in situations where high temperature creep strength is required." While considerable high temperature testing of thoriated Ni-base alloys has been conducted (e.g., Refs. 13 to 16), diffusional creep has not been reported for these materials. However, recent results ~7 of room temperature tensile tests of TD-NiCr (Ni-20 Cr-2 ThOa) specimens which VOLUME 4, JUNE 1973-1475
100( - r Nominal as received / / strength
%
T a b l e I. Dispersion-Strengthened
A l l o y s Evaluated
80( o
z
oO
Alloy
Heat
Composition
Form-Thickness, cm
TD-NiCr TD-NiCr TD-NiCr TD-NiCr TD-NiCr TD-NiCr TD-NiCrAI IN-853
3636 3637 3697 3090 3774
Ni-20.2Cr-0.02C-0,002S-2.2ThO2 Ni- 19.8 Cr-0.02C-0.002S-2.1ThO2 Ni-20.1Cr-0.02C-0.006S-2.1ThO2 Ni-20.2Cr-0.03C-0.004S-2.1ThO2 Ni-21.1Cr-0.02C-0,007S-2.3ThO~ Ni- 19.4Cr-0.003C-0.001S-2.4ThO2 Ni-I 6Cr-3.9A1-2ThO2 Ni-20Cr-2.STi-1.5AI-0.2Fe-0.05CO.07Zr-O.007B-1.3Y203
sheet-0.051 sheet-0.025 sheet 0.025 sheet-0,160 sheet-0.051 sheet 0.049 shee.t-0,025 b a r - 1.6 dia, 0.63D test
o
o
60(
=
" ~ Nominal as received elongation
o o
o
o
o
E ,,=, 20(
~o
-
o
.2
.4 ~ C reep strain
0 0 ~
I
.6
I
.8
.2
(a)
0000 0
I
o
.4 % Creep strain
I
section
.8
(b)
F i g . 2 - - E f f e c t s of c r e e p s t r a i n p e r a t u r e t e n s i l e p r o p e r t i e s of 0.075 cm) after being creep a t v a r i o u s s t r e s s l e v e l s , t7 (a) (b) T e n s i l e e l o n g a t i o n .
on subsequent
room tem-
TD-NiCr
sheet (0.025 t e s t e d at 1365 K f o r 100 h Ultimate tensile strength.
had been p r e v i o u s l y c r e e p t e s t e d at 1365 K to v a r i o u s strain l e v e l s r e v e a l e d that the t e n s i l e p r o p e r t i e s w e r e r e d u c e d by the e f f e c t s of even s m a l l c r e e p s t r a i n s , Fig. 2. Such r e d u c t i o n s would be e x p e c t e d if T D - N i C r u n d e r g o e s dfffusional c r e e p s i n c e the m a j o r i t y of the t e n s i l e d e f o r m a t i o n would be confined to the w e a k e r nonthoriated r e g i o n s that f o r m e d during
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