8/2/2019 Ductile to Brittle Transition
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DUCTILE TO BRITTLE TRANSITION
This is commonly observed in BCC metals and almost missing in
most of the FCC metals. This
transition is observed at low temperatures, extremely high rates
of strain or notching the material. This
is very important when selecting materials for engineering
purposes. The notched bar impact test for
metals can be used to determine the temperature over which the
transition from ductile to brittle takes
place. Such a temperature is termed as transition
temperature.
One can explain the ductile to brittle transition with the help
of Fig. 8.42. Figure shows the plot
of brittle fracture stress (f) and the yield stress (y) as a
function of temperature or strain rate. We note
that the curve for brittle fracture stress rises slightly to the
left because the surface energy increases as
temperature decreases. We note strong temperature dependence in
the yield stress curve as in BCC
metals and metal oxide ceramics. From figure it is clear that
the two curves intersect and a vertical line
is drawn at the point of intersection, which is called the
ductile-brittle transition temperature.
Now, if a material is stressed at a temperature or strain rate
which is to the right side of line CD,
it will reach its yield point prior it reaches the brittle
fracture stress and will undergo some plastic
deformation prior to fracture. However, applying a stress under
conditions which lie left of the line CD
will result in brittle fracture. Obviously, at all temperatures,
below the transition temperature, the
fracture stress is smaller than that of the yield stress.
This reveals that fracture stress may be controlled by
the yield stress. As the applied stress reaches a value equal
to
the yield stress, the crack is nucleated at the intersection
ofthe slip planes and propagates rapidly. The temperature
range over which the rapid changes takes place is termed as
the transition region.
The yield stress as well as fracture stress is a function
ofgrain size and these stresses increase with the decreasing
grain size. Obviously, the fine-grained metals have a lower
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transition temperature as compared to the course grained
metals.
For mild steel, the consumption of energy in an impact test as a
function of temperature is
shown in Fig. 8.43. By fast loading, one can achieve a high
strain rate in impact testing machines. We
know that increasing the strain rate is equivalent to lowering
the temperature. This means the materials
which are ductile when strained slowly at a given temperature
will behave in a brittle manner when
subjected to a high strain rate.
