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## 530.352 Materials Selection

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### Lecture #26 High Temperature Creep - II Tuesday November 15th, 2005 Sherby-Dorn Equation Example of creep based design: Ni-base superalloys that are used for jet ... – PowerPoint PPT presentation

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Title: 530.352 Materials Selection

1
530.352 Materials Selection
Lecture 26 High Temperature Creep - II
TuesdayNovember 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
n5.
• What is the creep rate at 925 oC and 350 MPa if
C1.7x10-7 and R8.314 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 T925 oC and s350 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 ?
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
.
.
?1 / ?2 (?1/ ?2 )n (350/375)5 0.708 ?2
1.4 x 10-8 sec
.
7
Increasing by 25 oC
?2 1.92 x 10-8 sec
8
Changing both T and s
9
Creep Mechanisms (metals and ceramics)
• Diffusion creep
• Dislocation creep (power-law creep)
• Stress Relaxation
• Creep Fracture

10
Diffusion creep
?
grain boundary diffusion ????d??
?
bulk crystal diffusion ????d??
?
11
Dislocation creep
Diffusion assisted climb important
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 (_at_ cont. T) ??-n d?
- A E dt ?1-n ? - A E t 0
.
.
.
.
t
s
o
?p
?
?
?el
time
time
14
Tertiary creep
15
Design against creep (metals)
• Minimize T / Tmelting to slow diffusion, climb,
and creep.
• Arrange for large grain sizes to slowdiffusion.
• 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 -gtincreased Tg
and less creep.
• High molecular weight -gt high viscosity-gt low
creep rate.
• Crystalline polymers better than glassy.
• Add fibers or particles to make composites !!