1. Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

2. Temperature Dependence
Some material properties have a linear
dependence on temperature while
others may have an exponential
relationship
Figure 13.1
Figure 13.2
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

3. Viscous Flow and Creep Strain rate in very viscous fluids
from an applied tensile stress
Creep – slow, continuous deformation of a material
at elevated temperatures, ending in fracture
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

4. Creep Testing Specimen is loaded in tension or compression,
usually at a constant load, inside a furnace
that is maintained at a constant temperature
Figure 13.3
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

5. Steady-state creep rate
The constants ε0, σo, n, and Qc are experimentally
found and vary from material to material
Figure 13.4
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

6. Stress-Rupture Curve
Figure 13.5
Design data based on creep is generally presented in a
stress-rupture curve – allows you to identify either the
design stress or rupture life at a given temperature
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

7. Melting Point The temperature at which a material starts
to creep depends on its melting point
Figure 13.6
Polymers can start
to creep at room
temperature
Metals Tm
Ceramics Tm
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

8. Strength – Maximum Service Temperature
Figure 13.7
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

9. At room temperature, material selection requires only
Figure 13.8
At room temperature, material selection requires only
a single strength-density chart
For high-temperature design, charts are needed
that account for temperature and an acceptable
strain rate
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

10. Diffusion Diffusion is the spontaneous intermixing of
atoms over time – the rate of diffusion
is expressed by Fick’s law:
D: diffusion constant
dc/dx: concentration gradient
Figure 13.9
In a crystalline solid,
two things are needed
for an atom to switch
sites:
Enough thermal energy
An adjacent vacancy
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

11. Interdiffusion Diffusion of chemically different atoms
Figure 13.10
Qd – activation energy per mole
Do – constant based on
oscillation of atoms and
atomic size
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

12. The mean distance that one type of atom
Figure 13.11
The mean distance that one type of atom
travels from diffusion is given by
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

13. Diffusional Flow Diffusion can change the shape
of polycrystalline materials
Figure 13.12
Grain boundaries act as sources
and sinks for vacancies
If a vacancy joins a boundary,
an atom must leave it – if a
vacancy leaves a boundary,
and atom must join it
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

14. Dislocation Climb Diffusion can allow a dislocation to
Figure 13.13
Diffusion can allow a dislocation to
move beyond particles in its path
The half-plane of atoms is eaten
away by diffusion, allowing the
dislocation to “climb” over the
impeding particle
This is the basis of power-law-creep
which is defined by:
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

15. Deformation Mechanisms
Materials can deform by dislocation plasticity, or
at high temperatures, by diffusional flow or
power-law creep
Figure 13.14
Deformation mechanism maps
show the range of stress
and temperature in which
we expect to find each
sort of deformation and the
strain rate that any combination
of them produces
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

16. due to creep by creating voids that nucleate on grain boundaries
Creep Fracture
Diffusion can cause creep as well as fracture
due to creep by creating voids that nucleate on grain boundaries
Figure 13.15
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

17. Creep and Diffusion of Polymers
As in crystalline solids, polymers creep and the creep
is often related to diffusion – diffusion requires free
volume (vacancies for metals) which is found dispersed
among all atoms in a polymer since there is no lattice structure
Figure 13.16
Free volume increases with
temperature and does so
most rapidly at Tg
Polymers behave in a
visco-elastic manner
around their Tg meaning they act neither as an elastic solid or viscous liquid
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

18. The visco-elastic nature of polymers reduces the
Figure 13.17
The visco-elastic nature of polymers reduces the
rate of creep while being loaded and allows for a
small amount of reverse creep upon unloading
The creep modulus Ec is used when designing
polymers against creep
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

19. Materials to Resist Creep
Figure 13.18
Materials to Resist Creep
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

20. High-Temperature Pipework
Figure 13.19
Typical operating conditions of 650 °C at
a pressure of 15 MPa
Figure 13.20
Design
For a known design life,
use the chart to find the
stress below which
fracture will not occur –
then plug the stress value
into the equation to find
the minimum pipe
thickness
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

21. Turbine Blades At typical stress and temperature levels,
Figure 13.21
At typical stress and temperature levels,
pure nickel would deform by power-law
creep at an unacceptable level – the impact
of strengthening mechanisms on MAR-M200
nickel alloy reduces this rate by a factor
of 106 – diffusional creep can then be slowed
by increasing the grain size
Figure 13.22
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

22. Thermal Barrier Coatings
Figure 13.23
Design against creep can
include the use of thermal
barrier coatings
For the turbine blade shown
in Figure 13.23, a ceramic
coating is applied to the
metal surface allowing for
an increase in gas temperature
with no increase in that of the
blade itself
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

23. Airframes Aircraft flying above speeds of
Figure 13.24
Aircraft flying above speeds of
Mach 1 are subject to creep due
to high temperatures and thermal
expansion caused by the ΔT of
the jet and the atmosphere
Material selection for this application
has several potential limiting factors –
a combination of tensile strength
and high temperature performance
is required, but weight often
forces a material with lower
than desired values to achieve
optimal speed
Figure 13.25
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

24. Creep causes pre-tensioned components
Creep Relaxation
Creep causes pre-tensioned components
to relax over time
Figure 13.26
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon