530.352 Materials Selection

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530.352 Materials Selection
Lecture 25 High Temperature CreepMondayNovember
14th, 2005
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High Temperature Creep
.
?creep f ( T, ?, t )

• Thermally activation deformation
• at constant stress
• over time.

Creep is slow, continuous deformation with time.
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What is a high temperature? RT
298K, Thair drier ?K, Tcar engine
?K Tjet engine ?K, Tlight bulb
?K, Tfusion reactor ?K What is a long time ?
Light bulbs ?, Jet engines ?, Automobile
engines ?, Steam pipes ?
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
??? 500K, Tcar engine 900K Tjet
engine 1,300K, Tlight bulb 2,300K, Tfusion
reactor ??? For metals and ceramics For
polymers high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine ??? 900K Tjet
engine 1,300K, Tlight bulb 2,300K, Tfusion
reactor ??? For metals and ceramics For
polymers high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine 900K Tjet engine
??? 10K, Tlight bulb 2,300K, Tfusion reactor
??? For metals and ceramics For polymers
high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine 900K Tjet engine
1,300K, Tlight bulb ???00K, Tfusion reactor
??? For metals and ceramics For polymers
high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine 900K Tjet engine
1,300K, Tlight bulb 2,300K, Tfusion reactor
??? For metals and ceramics For polymers
high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine 900K Tjet engine
1,300K, Tlight bulb 2,300K, Tfusion reactor
??? For metals and ceramics For polymers
high T gt 0.5 Tmelting high T gt Tg
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What is high temperature ??
300K? , 500K ?, 1,000K ?, 2,000K
? Applications RT 298K, Thair drier
500K, Tcar engine 900K Tjet engine
1,300K, Tlight bulb 2,300K, Tfusion reactor
??? For metals and ceramics For polymers
high T gt 0.5 Tmelting high T gt Tg
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Creep of lead at RT
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What are long times ??

• Light bulbsreplace every 500-1,000 hrs.
• Jet engines check every 1,000 hrs.
• Car engines2hrs/day x 5 days/wk x 50 wks/yr x
  5yrs 2,500 hrs.
• Steam pipes???

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What are long times ??

• Light bulbsreplace every 500-1,000 hrs.
• Jet engines check every 1,000 hrs.
• Car engines2hrs/day x 5 days/wk x 50 wks/yr x
  5yrs 2,500 hrs.
• Steam pipes???

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What are long times ??

• Light bulbsreplace every 500-1,000 hrs.
• Jet engines check every 1,000 hrs.
• Car engines2hrs/day x 5 days/wk x 50 wks/yr x
  5yrs 2,500 hrs.
• Steam pipes???

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What are long times ??

• Light bulbsreplace every 500-1,000 hrs.
• Jet engines check every 1,000 hrs.
• Car engines2hrs/day x 5 days/wk x 50 wks/yr x
  5yrs 2,500 hrs.
• Steam pipes???

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Types of Failure and Design Needs

• Displacement Limited
• maintain dimensions or clearance
• Rupture Limited
• fracture to be avoided
• Stress-relaxation Limited
• release of tension
• Buckling Limited
• in thin components

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Creep Testing and Curves
LOAD
?creep
steady-state creep
tertiary creep
primary creep
time
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Stages of Pure Metal Creep

• Primary creep
• deformation is fast until it strain hardens
• Steady-state creep
• linear response - microstructural SS
• strain hardening recovery (annihilation)
• Tertiary creep
• void formation / necking

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Effect of Stress
Keep T constant change ?
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Stress Exponents

• n 1 diffusion creep
• n 3-5dislocation creep
• n 20threshold stressesdue to particles

.
log ?
n20
n5
n1
log ?
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Effect of Temperature
Keep ? constant change T
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Qcreep and Qdiffusion
Diffusion controls power-law creep
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Sherby-Dorn Equation
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?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 !!
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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 ?

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Creep rate T925 oC and s350 MPa
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?ss C ??n exp (- Q / RT)
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?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
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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 !!
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Increasing by 25 MPa
.
.
?1 / ?2 (?1/ ?2 )n (350/375)5 0.708 ?2
1.4 x 10-8 sec
.
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Increasing by 25 oC
?2 1.92 x 10-8 sec
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Changing both T and s