The Mechanics of Multifunctional Materials

Nano-structured NiTi instantaneously transforms from fcc austenite via an intermediate phase (the R-phase) into monoclinic B19’ martensite. At a grain size less than a critical limit of about 50nm no transformation can be observed even upon cooling down to cryogenic temperatures. In slightly larger grains the martensite appears as a laminate of alternating twinrelated Bain correspondence variants. For grain diameters in the order of 100nm it becomes more likely to observe two such martensite laminates whose arrangement relative to each other, as it appears in a micrograph, gives the impression of a “herring-bone pattern”. This term has widely been adopted in the literature for this particular formation. This paper focuses on the prediction of the resulting morphology, which is determined by the energies involved, i.e. the chemical and mechanical contributions as well as interface energies. The elastic strain energy as well as the interface energies generated by the transformation eigenstrains of the newly created phases are evaluated. Various arrangements are calculated and compared with each other in terms of their impact on the total energy balance. Such an approach allows to determine an optimum configuration minimizing the total energy introduced into the system. The herringbone morphology is also compared with the case of a single laminate as an alternative energy release mechanism. The parameters defining the geometry of the laminates such as the twin band thickness or the laminate width are computed and verified by means of high-resolution TEM images. Introduction NiTi alloys are the most important practically used shape memory alloys In contrast to their conventional coarse-grained counterparts, bulk nanocrystalline NiTi alloys attract considerable attention as advanced functional material [1]. In nanograins of NiTi a unique path of atomic scale twinning is encountered [2]. A variety of martensite arrangements allows to accommodate the newly created phase in order to maximize the energy release triggered by the transformation. It is the objective of the present study to investigate the dependence of the martensitic morphology on the grain size of nanocrystalline NiTi alloys by numerical as well as experimental means using transmission electron microscopy (TEM). Martensitic phase transformations are characterized by the formation of multiple symmetry related variants of the martensite building complex microstructures on different length scales [3]. The eigenstrains that occur during