Материаловедение: свойства металлов. Матросова Т.А - 26 стр.

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26
only the structure, there are thermomechanical treatments that also alter both the
structure and shape, and thermochemical treatments that may modify both the
structure and the surface chemistry. All three of these processes for the enhancement
of properties can be classified as heat treating. Many alloys are heat-treated, the more
important being ferrous and aluminum alloys. The mechanical properties of
metallic materials can be increased by strain-hardening.
When a metallic alloy is plastically deformed, its yield strength increases
with increase in strain as long as the recrystallization temperature is not exceeded.
Thus, controlled amounts of cold working may be used to increase the mechanical
properties of a metallic material (Fig. 9.1). The true stress-true strain curves show
that on a log-log plot, the strain-hardening equation is indeed a straight line and its
slope is denned as the coefficient of strain-hardening. Through the strain-hardening
equation, an engineer can
predict the improvement in
properties that a given
operation will impart to a
material.
In polycrystalline alloys,
the mutual interference of
adjacent grains causes slip
to occur on many
intersecting slip planes with
accompanying strain-
hardening. This progressive
strengthening with
increasing deformation
stems from the interaction
of dislocations on
intersecting slip planes.
The ultimate tensile strength of metals far exceeds that of
polymers – by a
factor of 25 for unreinforced materials and a facor of 8 for the woven glass-
reinforced resins. In the case of the modulus of elasticity, the best thermosets are
below the lowest nonferrous materials (fig. 9.2).
Fig. 9.1. The rate of strain-hardening
in three metallic materials

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