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APL 102
Creep- & Cyclic- deformation in Materials
Concepts: Diffusional creep, Grain boundary sliding, cyclic loading
Lecture 31
Recap
Creep (Next Lab) Pb creeps at RT
Why d ?
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Creep is a time dependent “high temperature” plastic deformation
Frank – Read source at constant load or stress
Damage nucleation
T 0.4 Tm Creep is a concern
Accumulation of dislocation
vs.
Relaxation of dislocation
stress and temperature
dependence Creep
mechanisms
b / L
Cross-slip
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Screw dislocations can cross-slip (by thermal activation) and can give rise
to plastic strain [as f(t)]
Slip plane 1
b
1 2
3
This happens at low temperatures when resolved shear stress on other system is enough
Dislocation climb
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Edge dislocations piled up against an obstacle can climb to another slip
plane and cause plastic deformation [as f(t), in response to stress]
Rate controlling step is the diffusion of vacancies
2
1
High temperature is key
for this mechanism
Dislocation Creep: Edge dislocation
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Climb unlocks dislocations from the precipitates that pin them and
further glide then can take place
Lock-unlock explains the progressive nature of creep
Diffusional Creep
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Nabarro-Herring creep →
high T → lattice diffusion
Diffusional creep
Coble creep → low T →
Due to GB diffusion
Atomic diffusion: Mechanism
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How atomic diffusion takes place in crystalline solids??
Bulk diffusion takes place by two mechanism:
Jump from one interstice to another Movement requires vacancy to sit next to it
Interstitial diffusion Vacancy diffusion
C, O, N, B and H diffuse interstitially in most crystals Zn atom diffuses in brass
Fast diffusion paths: Grain boundary
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and dislocation core
Grain boundary diffusion
Dislocation-core diffusion
Diffusion Creep
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In response to the applied stress vacancies preferentially move from surfaces/interfaces
(GB) of specimen transverse to the stress axis to surfaces/interfaces parallel to the stress
axis→ causing elongation
This process like dislocation creep is controlled by the diffusion of vacancies → but
diffusional does not require dislocations to operate
Flow of vacancies
Diffusional Creep
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Grain elongation under mechanical driving force, but this time atoms move from one
grain faces to other
This does not involve any dislocation motion
At high T/Tm the diffusion takes place by bulk diffusion
Grain Boundary Sliding
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After deformation
Before deformation
11
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Grain boundary sliding
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Grain boundary regions can slide past
Log (viscosity Pa-s) each other under stress
High temperature and stress both
crystal
30 required
glass
Grain boundaries in materials
were good at low temperature but
18
they become detrimental at high
12 Undercooled liquid
temperatures
Stable liquid
Tg Tm
Grain boundary sliding
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At low temperatures the grain boundaries are ‘stronger’ than the
crystal interior and impede the motion of dislocations
Being a higher energy region, the grain boundaries melt before the
crystal interior
Above the equi-cohesive temperature grain boundaries are weaker
than grain and slide past one another to cause plastic deformation
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High temperature materials
• Metals and alloys Applications
• Ni and Fe-Ni based alloys Gas Turbine and Jet
• Co based alloys Turbine
• Ti-alloys (poor oxidation resistance) Nuclear reactors
• Few grades of steel, Power plants
particularly austenitic steels Spacecraft
• Tungsten based materials Chemical processing
• Dispersion strengthened materials
• Ceramics Problems
• Toughened zirconia
Mechanical degradation
(thermal barrier coatings)
Chemical degradation
• Tungsten carbide (Cutting tools)
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Question for thought
What form of microstructure or material is being used for
Turbine Blades?
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Which
microstructure
will show high
creep resistance?
16
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manufacturing technique: -> Less grain boundaries
-> Better for creep application
Single Crystal
-> No grain boundaries
-> Best for creep application
Nanocrystalline materials
-> not good for creep applications!
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18
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How to make materials Creep Resistant??
Adopt means to minimize diffusion and dislocation and grain boundary movement
High melting point → E.g. Precipitates, Ceramics, W
Dispersion hardening → ThO2 dispersed Ni (~0.9 Tm)
Creep
resistance Ni3Al in Ni by addition of Al
Solid solution strengthening
Minimize grain boundaries
like Single crystal / aligned (oriented) grains
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Question for thought
What other mode of deformation possible in
materials/applications?
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Dynamic loading - deformation
Fatigue
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Fatigue is a deformation that happen in materials/structures subjected to
repeated cyclic loads
For example, bridges and connecting rods etc.,
Bridge Connecting rod
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Question for thought
What are the different types of fatigue that is possible?
Thermal fatigue &
Mechanical fatigue
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Mechanical fatigue: Types of Cyclic Loading
(a) Low amplitude acoustic vibration
(b) High-cycle fatigue: cycling below
the yield strength
(c) Low-cycle fatigue: cycling above
the yield strength but below the
the tensile strength
High-cycle fatigue loading is most significant in engineering terms
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Fatigue
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Fatigue failures occur due to cyclic loading at stresses below or above a material’s yield strength
Depends on the amplitude of the stress
and the number of cycles
Loading cycles can be in millions for an
aircraft
Fatigue testing must employ millions of
fatigue cycles to provide meaningful
design data
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Fatigue test
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A common method of testing fatigue life is the Wohler rotating rod test or
rotating bending method
One end of the specimen is mounted in a rotating chuck and a load suspended
from the other end
The specimen experiences cyclic forces, from tension to compression in a
sinusoidal cycle, as it rotates
Announcement
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Quiz 2: Tentative date: Nov. 1st (Wednesday) @ 5:30 pm
One Compensating Class: 25th October 5:30 to 6:20 pm ?
Attendance uploaded: Check it and contact for any discrepancies,
medical reason leave will be considered when need arises
