Crystallographic insights into Ni-Co-Mn-In metamagnetic shape memory alloys

In the present work, the morphological and crystallographic features of the 6M modulated martensite in the Ni45Co5Mn37In13 alloy were investigated by electron backscatter diffraction and high-resolution transmission electron microscopy (HRTEM) at room temperature. The 6M modulated martensite is in plate form and organized in colonies within which the plates stretch roughly in the same direction. Each colony has four types of orientation variants that are related to three kinds of twin relations, i.e. {1 (2) over bar(3) over bar}(M) type I, <(3) over bar(3) over bar1 >(M) type II and {103}(M) compound twins. The twinning shears of type I and type II twins are the same and equal to 0.2681, being about one order of magnitude higher than that of the compound twin (0.0330). Variant interfaces are microscopically defined by their corresponding twinning plane K-1. The HRTEM investigations show that the interfaces of the type I twin are straight and coherent at atomic scale, whereas those of the type II and compound twins are 'stepped'. The step height of the compound twin interfaces is much larger than that of the type II twin interfaces. In view of variant organization, there is only one oriented type I interface and one compound twin interface, but there are two oriented type II interfaces which have an angular deviation of +/- 5.32 degrees with respect to the type I twin interface. The results of the present work provide comprehensive information on morphological and crystallographic features of Ni-Co-Mn-In metamagnetic shape memory alloys.