Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 81
Normalized shear sress (τ/G)
10 –1
10 –2
10 –3
Temperature (°C)
–200 0 200 400 600 800 1000 1200 1400
Ideal strength
Dislocation glide
10 –4 Typical turbine
operation
Diffusion flow
10 –5
10 –6
Boundary diffusion
˙ γ = 10 –10 s –1
Pure nickel
d = 100 µm
Power-law creep
10 –1 s –1
1 s –1
10 –3 s –1 10 –2 s –1
10 –5 s –1 10 –4 s –1
10 –7 s –1
10 –9 s –1
10 –8 s –1
10 –6 s –1
Lattice
diffusion
0 0.2 0.4 0.6 0.8 1.0
Homologous temperature (T/T m )
10 3
10 2
10
1
10 –1
3.7 Deformation mechanism map for nickel of grain size 100 µm.
Shear stress at 300 K (MPa)
˙ γ ∝ (τ/G) n [3.6]
This regime is called power-law creep.
If the stress is too low to permit dislocation movement, the material may
still undergo strain by diffusional flow of single ions at a rate which depends
strongly on the grain size (d ):
˙ γ ∝ τ/d m [3.7]
In this regime two subgroups exist, corresponding to diffusion occurring
predominantly through the grains (m = 2) (Nabarro–Herring creep) or round
their boundaries (m = 3) (Coble creep).
Intrinsically high creep strength is expected in materials in which diffusion
is slow, i.e. in materials of high melting point and in covalent elements and
compounds. Further reduction in power-law creep may be achieved by
metallurgical control of the microstructure: solute and particle hardening,
together with a grain-boundary precipitate to suppress grain-boundary sliding
are all commonly encountered in creep resistant materials.
The most sophisticated family of alloys exemplifying these principles are
the age-hardening nickel-based ‘superalloys’, which are widely used in gas
turbines. If power-law creep is effectively suppressed by these means, diffusion