Elastic properties of metals and alloys before martensite transformations

A review of the elastic behavior of materials in the temperature region before martensitic transformations (MP) is carried out. The nature of changes in elastic constants before these transformations is extremely informative in studying both the mechanisms and the nature of structural transitions, since elastic modules determine the resistance of the crystal lattice to uniform shifts and influence the stability of structures during martensitic rearrangement. All metals and their alloys by the nature of the change of elastic properties can be divided into three groups. In the 1st group, the temperature dependence of the elastic moduli is normal (the temperature coefficient is less than zero) and phase transformations take place far from the point of loss of stability of metals and intermetallic compounds. The anomalous “softening” of elastic constants of single crystals of alloys before the MP is common for all compounds of the 2nd group. In this case, as a rule, the elastic constant “softens” $C'=1/2(c_{11}-c_{12})$, corresponding to the Bane deformation and lattice structural adjustment, it is smaller than all the other constants $c_{ij}$. Alloys of the 3rd group are characterized by an anomalous decrease in the modulus $C'$ and a sharp increase in the elastic anisotropy $A=c_{44}/C'$ to dramatic values. The part of the intermetallic compounds is characterized by the type of premartensitic instability, found in B2 compounds based on titanium nickelide and consisting in the simultaneous “softening” of all the shear and longitudinal moduli of elasticity and a decrease in the elastic anisotropy coefficient. As a result, the lattice becomes soft in all major shear systems, which ensures the diversity of structural phase transitions in this group of alloys and the unique properties of shape memory and super elasticity.