Ductility

The ductility indicates the capacity of a Matériau to be become deformed plastically without breaking. The rupture is made when a defect (crack or cavity), induced by the plastic deformation, becomes critical and is propagated. Ductility is thus the aptitude which has a material to resist this propagation. If it resists it well, it is known as ductile, if not it is known as fragile.

Ductility in geology

One speaks about ductile Roche when the latter can be deformed without “break”. For example, the rocks at the origin of the folds are ductile.

Measurements

Two independent measurements are taken:

the tests tensile measuring lengthening before the rupture and the constriction (reduction of the diameter of the test-tube on the level where it breaks);
the test sheep of Charpy measuring the energy spent to break a test-tube.

A material is ductile if:

its lengthening and its constriction with the rupture are important;
energy spent to break it is important.

Conversely a material is fragile if:

its lengthening and its constriction with the rupture are weak;
energy spent to break it is weak

Conditions

This ductility depends on the temperature and the speed of deformation. Indeed the mechanisms implied during the tests depend on these parameters:

the mobility of the Dislocation S,
restoration and the Dynamic recrystallization ,
the diffusion and the Creep.

Characteristics

For crystalline materials, intrinsic ductility (i.e. related on the Material and not to the conditions of deformation) is determined by:

the number of systems of slip available: indeed, the plastic deformation is done by slip of dense crystallographic plans according to dense directions, certain structures have some more than others; that explains the ductility of the crystals having a cubic symmetry with faces centered (CFC) like the Or, the Plomb or the Aluminum; in addition, in the case of the Alloy S ordered (Oxide S, intermetallic…), certain modes of deformation are blocked (required to respect chemical alternation constantly);
purity: the foreign atoms (intersticiels or in substitution) come to pin dislocations and obstruct their movement;
the unicity of the phase: if there are Précipité S, one has a age hardening (the precipitates block dislocations);
size of the Crystallite S: the grain boundary block dislocations, plus crystallites are small, more there are grain boundaries (cf law of Hall-Petch, the yield stress is proportional contrary to the square root of the average diameter of crystallites).

Simple: Ductility

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