Nucleation centers for martensite with habitus {110} in the shape memory alloys

Abstract

The dynamic theory of martensitic transformations (MT) considers the formation of habit planes of martensite crystals as a consequence of the propagation of a controlling wave process (CWP). The general ideology makes it possible, by comparing the observed habits with calculations of the elastic fields of defects (as a rule, dislocations), to identify nucleation centers. In a number of cases (In-Tl alloys, Ni50Mn50 alloys, Heusler alloys …) under MT in the shape memory alloys, {110} habits are observed (in the basis of the initial cubic phase), which often have a fine twin structure with twin boundaries of the same type. This highly symmetric structure is described by the CWP containing longitudinal waves (both relatively long-wavelength ℓ and short-wavelength s) propagating along the 4-order symmetry axes. In this paper, it is shown that such habits are associated with rectilinear segments of dislocation loops with directions Λ along <001> and Burgers vectors along <010> (or <110>) orthogonal to Λ, both for sliding and for prismatic loops. The tetragonality, the relative volume change during the MT, and the dependence of the start temperature M s on changes in the concentration of alloy components are also briefly discussed.