Stress-induced transition from modulated 14M to non-modulated martensite in Ni-Mn-Ga alloy

Observation of in situ straining of seven-layered (14M) martensite of Ni-Mn-Ga alloy was carried out by a transmission electron microscope (TEM). During straining, the intermartensitic transition from 14M to non-modulated martensite (NM) occurred via motion of partial dislocation with a Burgers vector b = 1/6[1 (1) over bar 0] along the basal plane of 14M. High resolution TEM studies revealed that the moving partial at the end of the fault is actually a pair of partial dislocations leading to a two-layered fault on the basal plane. This fault is twin related to the adjoining five-layered basal plane forming complete seven-layered nanotwinned structure. The results demonstrate that the 14M to NM transition is a detwinning process of these nanotwins, and seems to confirm the nanotwin nature of the long-period stacking-order structure of 14M martensite. The lattice parameters of the average structure are strongly affected by the regularity of the stacking sequence, i.e., by the volume fraction of the two-layer nanotwins and their distribution. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.