Материаловедение: свойства металлов. Матросова Т.А - 29 стр.

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29
неармированных материалов и армированных стекловолокном смол?
12. Каково соотношение модулей упругости реактопластов и цветных
металлов?
Topic 10. Recovery and Recrystallization
Recovery. During a cold-working process most of the energy used is
dissipated as heat but a small percentage is stored within the distorted lattice
structure of the alloy. That energy is the thermodynamic driving force that tends to
return the metal to its original state provided there is sufficient thermal energy to
enable the reactions to occur. Cold forming is done above the yield point in the
work-hardening range, so internal stresses can be built up easily. Evidence of this
is the springback as the work leaves the forming operation, and the warpage in any
subsequent heat treatment. Even a simple washer might, by virtue of the internal
stresses resulting from punching and them flattenening, warp severely during heat
treating.
Recovery is a gradual change in the mechanical properties of an alloy, i.e.,
loss in brittleness, or marked increase in toughness brought about by controlling the
heat-treating times and temperatures so that there are no appreciable changes in the
microstructure. However, recovery does significantly reduce the residual stresses
within the distorted lattice structure.
When doubt exists as to whether internal stresses will cause warpage, a piece
can be checked by heating it to about 1100 deg. F and then letting it cool. If there are
internal stersses, the piece is likely to deform. Pieces that will warp sererely while
being heated have been seen, yet the heat-treater was expected to put them through
and bring them out better than they were in the first place.
Recrystallization. All the properties of a cold-worked metal are affected to
some degree by a yield behavior heat treatment, but the yield strength and ductility
can only be restored by recrystallization. This may be defined as the nucleation and
growth of strain-free grains out of the matrix of cold-worked metal. In general the
properties of the recrystallized alloy are those of the metal before the cold-working
operation. This is of commercial importance because if an alloy has become work-
hardened in a drawing operation, it can be recrystallized and the drawing operation
can be continued. Nucleation is encouraged by a highly cold-worked initial structure
and a high annealing temperature. The presence of alloying elements in solid
solution decreases the rate of nucleation.

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