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Ductile To Brittle Transition

The document discusses the ductile to brittle transition that occurs in metals, particularly body-centered cubic (BCC) metals. This transition happens at low temperatures or high strain rates, where the metal begins to fracture in a brittle manner rather than deforming plastically. The transition temperature can be determined using a notched bar impact test, which measures the temperature at which the metal switches from ductile to brittle fracture behavior. Below the transition temperature, the fracture stress is lower than the yield stress, resulting in brittle cracking rather than plastic deformation when stressed. Fine-grained metals have a lower transition temperature than coarse-grained metals of the same composition.

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Tushar Saini
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410 views2 pages

Ductile To Brittle Transition

The document discusses the ductile to brittle transition that occurs in metals, particularly body-centered cubic (BCC) metals. This transition happens at low temperatures or high strain rates, where the metal begins to fracture in a brittle manner rather than deforming plastically. The transition temperature can be determined using a notched bar impact test, which measures the temperature at which the metal switches from ductile to brittle fracture behavior. Below the transition temperature, the fracture stress is lower than the yield stress, resulting in brittle cracking rather than plastic deformation when stressed. Fine-grained metals have a lower transition temperature than coarse-grained metals of the same composition.

Uploaded by

Tushar Saini
Copyright
© Attribution Non-Commercial (BY-NC)
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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 of the 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 of grain size and these stresses increase with the decreasing grain size. Obviously, the fine-grained metals have a lower

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.

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