Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 84

2 Mechanical Action
2.1 Principles of a Material's Mechanical Behavior
The stresses a material undergoes can result in different responses depending on the
material type and the stress, temperature, loading rate, and environmental conditions
(water, liquid metal, hydrogen, etc.). One of these responses is deformation and the
other is rupture (or fracture). Some ductile materials can change the behavior when
the temperature is lowered, becoming brittle.
When a material is stressed, it will deform elastically. During this deformation,
the material stores up internal energy corresponding to an elastic energy. When the
stress is released, the deformation reverses. When the elasticity limit of the material
is reached, it deforms under higher stress and exhibits plastic behavior. This defor-
mation mechanism corresponds to a local restructuring that is often irreversible.
These different stages are presented in Fig. 5.1.
Fig. 5.1 Graph showing the
different steps of deformation
( y-axis ) as a function of the
stress ( x-axis )
The plasticity zone contains two domains: a plastic deformation area and a dam-
age area. There are two methods of passing from elastic deformation to plastic
deformation: Either there is a smooth transition and no clear distinction can be seen
between the linear part and the plastic part (ductile samples) or there is a recess at the
start of the plastic deformation, the stress falls, and an irreversible defect is created
(brittle samples). This is called the damage area. Cracks in a material appear at the
end of the plastic deformation curve. In brittle materials, the damage rapidly causes
a rupture, whereas in ductile materials, the plasticity area can vary significantly up
to superplasticity.
In brittle materials, there is no macroscopic plastic deformation. The propagation
of cracks is very rapid and the rupture is neat, without a reduction of the localized
cross section (contraction) following the crystallographic planes. Plastic deforma-
tion quickly causes damage, creating defects. This is the case for ceramics, minerals,
a large number of vitreous thermoplastics above their glass transition temperatures
( T g ), thermoset polymers, and elastomers with low cross-linking rates. In the field of
biology, materials such as bone, tooth, and siliceous or calcareous skeletons present
this type of brittle structure. Brittle fractures can be transgranular, intergranular, or
interatomic.