>> The martensitic transformation:

The word "martensite" stands for the quenching component in steel, it comes from the famous German metallurgist Adolf MARTENS. When the steel is subject to a quenching from the high temperature phase (austenite), its hardness increases.

The martensitic transformation is a solid phase transformation:

· It is a transformation without any diffusion (the atoms do not move). There is no change in the chemical formation of the material.
· It implies a bending of the crystalline network that leads to a macroscopic change seen on picture 1 :

§ A weak volumetric variation (in the SMA)
§ An important cutting following a precise direction.

Picture 1: simplified representation of the macroscopic change of shape associated to the appearance of a martensite variant

· Different variants of equivalent orientations are created (picture 2) in order to minimize the interaction energy between the created martensite and the still present austenite.

Picture 2: Appearance of the martensite variants (Variants M1 and M2)

. With this transformation, there is a hysteresis phenomena (irreversibility).
. With the transformation, there are an emanation of heat (exothermic transition) and absorption of heat (endothermic transition).