This document discusses creep mechanisms and equations for calculating creep rates. It provides an example calculation of creep rate for a Ni-base superalloy at 925°C and 350 MPa. It also calculates how the creep rate would change if stress or temperature were increased. Creep mechanisms discussed include diffusion, dislocation, and stress relaxation. Design strategies to reduce creep in metals, ceramics, and polymers are also summarized.

1. 530.352 Materials Selection
Lecture #26 High Temperature Creep - II
Tuesday
November 15th, 2005

2. Sherby-Dorn Equation
ss = C n exp (- Qdiffusion / RT)
.
Temperature dependence
Stress dependence
Constants
Use this equation to calculate creep rate at
any given or new stress or temperature !!

3. Example of creep based design:
Ni-base superalloys that are used for jet turbine
applications exhibit Qcreep = 320 kJ/mol and n=5.
• What is the creep rate at 925 oC and 350 MPa if C=1.7x10-7
and R=8.314 J/mol-oC ?
• What would the creep rate be if the stress were increased by
25 MPa ?
• What would the creep rate be if the temperature were increased
by 25 oC ?
• If your boss wanted to increase the operating temperature by
50 oC, how much would you have to decrease the stress to
maintain the same creep rate ?

4. Creep rate; T=925 oC and =350 MPa
ss = C n exp (- Q / RT)
.
ss = 1.7x10-7 3505 exp( -320,000/8.314 x 1198 K )
= 1.7x10-7 x 5.25x1012 x 11.1x10-15
= 10-8 sec-1
.

5. Is 10-8 sec-1 fast ?
year
per
or
year
year
days
day
hours
hour
ss
ss
%
31
31
.
0
25
.
365
24
sec
600
,
3
sec
10
1
8








Is short for a service life but long for a graduate student
-- must extrapolate from short tests to long times !!

6. Increasing by 25 MPa :
ss-1 = C 1
n exp (- Q / RT1)
.
ss-2 = C 2
n exp (- Q / RT2)
.
1 / 2 = (1/2 )n = (350/375)5 = 0.708
2 = 1.4 x 10-8 sec
. .
.

7. Increasing by 25 oC:
ss-1 = C 1
n exp (- Q / RT1)
.
ss-2 = C 2
n exp (- Q / RT2)
.
518
.
0
1223
1
1198
1
314
.
8
000
,
320
exp
1
1
exp
2
1
2
1
2
1












 

























T
T
R
Q
2 = 1.92 x 10-8 sec

8. Changing both T and  :
ss-1 = C 1
n exp (- Q / RT1)
.
ss-2 = C 2
n exp (- Q / RT2)
.
MPa
T
T
R
Q
n
307
1223
1
1198
1
314
.
8
000
,
320
exp
350
1
1
1
exp
2
5
2
2
1
2
1
2
1












 






































9. Creep Mechanisms (metals and ceramics)
Diffusion creep
Dislocation creep (power-law creep)
Stress Relaxation
Creep Fracture

10. Diffusion creep

bulk crystal
diffusion
d1
grain boundary
diffusion
d



11. Dislocation creep
glide
glide
climb
Diffusion assisted climb important:
climb
1. Annihilation:
poof !
2. By passing obstacles:

12. Dislocation Climb:

13. Stress Relaxation
total = el + pl = 0 so el = - pl
el =  / E and pl = c = A n (@ cont. T)
-n d = - AE dt
1-n | - AE t |0
. .
. .
. .

time

time

p
el
o
t

14. Tertiary creep :
c
time
tertiary
creep
Creep damage starts

15. Design against creep (metals)
Minimize T / Tmelting
to slow diffusion, climb, and creep.
Arrange for large grain sizes to slow
diffusion.
Use precipitates (oxide particles) and
solid solutions to slow dislocations.

16. Creep in ceramics :
Very little dislocation motion - mostly
diffusion creep or something else.
Glassy phases (oxides) that form at
grain boundaries soften and high T
and lead to grain boundary sliding.

17. Design against creep (ceramics)
Similar to metals, reduce diffusion and
dislocation motion, but must also ...
Reduce/control grain boundary phases.

18. Creep Mechanisms (polymers)
Tg replaces Tm at the critical T
and Tg is often close to RT !!!
Viscous flow is like creep:
 = C 1 exp (-Qv / RT)
Qviscous not QDiffusion,
Qvicsous is Q slide lumpy molecules past one another
n = 1 for Newtonian viscous flow
.

19. Design Against Creep (polymers)
Increased degree of cross-linking ->
increased Tg and less creep.
High molecular weight -> high viscosity
-> low creep rate.
Crystalline polymers better than glassy.
Add fibers or particles to make
composites !!