Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
behaviour, with an optimum ageing time to give a maximum hardness, is
commonly observed in many commercial alloys. The time to peak hardness
depends on the solute diffusion rate and, thus, on the ageing temperature.
Combinations of strengthening mechanisms
Most commercial alloys owe their strength to a combination of several of the
strengthening mechanisms we have reviewed. We will return to this later and
Figs 3.22 and 3.26 illustrate how the strength of certain steels may be
understood in terms of the additive contributions of grain size strengthening,
solute strengthening and the presence of second phases.
Perhaps the most dramatic example of strengthening from several
mechanisms is the formation of martensite when steel is rapidly quenched
from a high temperature (see Chapter 1). An extremely hard (and brittle)
phase is formed in this diffusionless transformation. Martensite owes its
strength to the combination of a high dislocation density, a very fine grain
size and a high supersaturation of solute atoms (carbon).
3.1.4 Strength at high temperature – creep-resistant
alloys
A prerequisite for an engineering component operating at elevated temperatures
is that it should be resistant to degradation by oxidation and corrosion from
its environment. This aspect of material behaviour will be considered in
more detail later (see Section 3.4).
At low temperatures materials deform by the glide of dislocations and the
principle of designing strong materials is essentially one of introducing barriers
to this process. As the operating temperature increases, other mechanisms of
flow become possible: dislocations can climb over barriers, grains slide over
each other at grain boundaries and vacancies diffuse. Thus, in a polycrystalline
solid, there are many distinguishable mechanisms by which it can flow. In
one range of stress and temperature, one of these flow mechanisms is dominant,
in another range a different mechanism will obtain.
This ‘landscape’ of deformation mechanisms is most conveniently surveyed
with the aid of a ‘deformation mechanism map’, which summarizes, for a
given polycrystalline solid, information about the range of dominance of
each of the mechanisms of plasticity and the rates of flow they produce.
Figure 3.7 is such a map for pure nickel with a grain size of 100 µm; the
coordinate axes are temperature T normalized with respect to the melting
point T m , and normalized shear stress τ/G on a logarithmic scale.
At low temperatures, flow is confined to the dislocation glide field, slip
being the dominant mechanism. Above 0.3 T m , dislocations can climb and
the shear strain rate ( ˙ γ ) can be characterized by an equation of the form: