Crystallography and elastic energy analysis of VN precipitates in Fe-Mn-Si-Cr shape memory alloys

High-resolution electron microscopy investigations are carried out to describe the morphology and crystallography of VN precipitates which are formed in an Fe-28Mn-6Si-5Cr (mass%) shape memory alloy. It is revealed that the shape change from a cube with {1 0 0} interfaces to an octahedral shape with {1 1 1} interfaces occurs on aging in the precipitate. In order to identify the equilibrium shape of the VN precipitate, elastic strain energy of the precipitate has been estimated on the basis of microscopic theory of elasticity. It is found that a coherent precipitate (∼4 nm in edge width) in a cube shape with {1 0 0} surfaces can be formed, at an early stage of precipitation, with no misfit dislocations existing at the interface of the precipitate. It is also shown that the octahedral-shaped precipitate (∼15 nm in edge width) has a minimum elastic energy, among the cube-shaped, sphere-shaped and octahedron-shaped precipitates, only when the misfit dislocations are introduced at the interfaces. The elastic interaction energy between the misfit dislocations and the precipitate-misfit dislocations is estimated for the first time using the Fourier transformed microscopic theory of elasticity.