Reconstruction of martensite variant microstructures in grains of deformed NiTi shape memory alloy by TEM

Abstract

Two transmission electron microscopy (TEM) based methods were applied to reconstruct martensite variant microstructures in entire grains of plastically deformed nanocrystalline NiTi wires. First method consists in manual TEM analysis of composite electron diffraction patterns and dark field images of microstructures taken using carefully selected Bragg spots. Second method involves automated orientation/phase mapping of martensite variant microstructures in TEM (ASTAR). Application of these two methods helped us to discover new mechanism of the plastic deformation of monoclinic B19’ martensite in NiTi shape memory alloys called kwinking as it combines dislocation slip based kinking and deformation twinning in martensite.In this work, we describe how the martensite variant microstructures in nanograins of plastically deformed NiTi shape memory wire are reconstructed using the TEM based methods and discuss their advantages and disadvantages. The application of the first manual method requires tilting the TEM lamella so that all martensite variants in the analysed grain are oriented in a common low index zone. Such common crystal lattice direction exists in each grain of plastically deformed martensitic NiTi wire because the microstructures were created by the kwinking deformation mechanism. The requirement to orient the selected grain into a common unique direction in monoclinic lattice, on the other hand, is not unique to kwinking and represents a severe limitation of the SAED-DF method due to the constraints on tilting the TEM lamella. Application of the automated crystal orientation mapping ASTAR method enables detailed orientation analysis of nanoscale martensite variant microstructures in generally oriented grains of deformed NiTi wires as well as pointwise ex-situ analysis of interfaces with excellent resolution in real and orientation space.