d. Flexibility and Elasticity. These terms differ in their technical definition, but
they are very closely related. Flexibility is the characteristic of a metal that allows it to
deform temporarily. The elasticity of a metal is used when it returns to its original shape
when the load or force is removed.
e. Fatigue. Fatigue is the property of a metal to tire and to fracture after
repeated stressing at loads below its proportional limit.
f. Structure (Crystalline or Grain Structure). Metals are crystalline and many
of their physical properties depend largely upon the size and arrangement of their
minute crystals called grains.
(1) Grain size. The size of the grains in a solidified metal depends upon the
number of nuclei of crystallization present and the rate of crystal growth. In the practical
sense, the faster a molten is cooled to solidification, the greater will be the number of
nuclei and the smaller will be the grain size. Generally speaking, small grains arranged
in an orderly fashion give the most desirable properties.
(2) Grain shape. The shape of the grains is also formed at the time of
crystallization. If the metal is poured or forced into a mold before cooling, the grains will
be in a flattened state. Metal formed by this method is known as cast metal. If the
metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal
becomes a wrought wire.
g. Crushing Strength. Crushing strength is the amount of resistance of a
material to fracture under compression.
h. Thermal Conductivity. Thermal conductivity is defined as the ability of a
material to transmit heat or cold. A low thermal conductivity is desired in restorative
materials used on the tooth whereas a high thermal conductivity is desirable where the
material covers soft tissue.
a. Cold Working. This is the process of changing the shape of a metal by
rolling, pounding, bending, or twisting at normal room temperature.
b. Strain Hardening. This occurs when a metal becomes stiffer and harder
because of continued or repeated application of a load or force. At this point, no further
slippage of the atoms of the metal can occur without fracture.
c. Heat Softening Treatment (Annealing). This treatment is necessary in
order to continue manipulating a metal after strain hardening to prevent it from
fracturing. The process of annealing consists of heating the metal to the proper
temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by
immersing in cold water. Annealing relieves stresses and strains caused by cold
working and restores slipped atoms within the metal to their regular arrangement.