Twin boundary mobility in additive manufactured magnetic shape memory alloy 10M Ni-Mn-Ga

The additive manufacturing of magnetic shape memory (MSM) alloys is attracting increasing interest as a promising method for manufacturing the active parts of small devices with complex geometries and tailored properties. Recently, laser powder bed fusion (L-PBF) was demonstrated as a successful approach for manufacturing magnetically actuatable samples from Ni-Mn-Ga MSM alloys, presenting a giant repeatable magnetic-field-induced strain of 5.8%. The most important characteristics of an MSM material that define its application potential include its magneto-structural and twin boundary (TB) mobility properties. This research presents for the first time a detailed characterization of TB mobility in a mm-sized single crystalline grain of five-layered (10M) martensite of Ni-Mn-Ga manufactured via L-PBF. Both type 1 and type 2 TBs, which are characteristic of 10M Ni-Mn-Ga, were identified and their mobility was characterized under mechanical stress and magnetic actuation. The results show that the TBs of both types exhibit behavior similar to that observed in conventionally grown single crystals. However, their mobility is decreased, which is attributed to keyhole porosity defects characteristic of the L-PBF process. The average twinning stress for type 1 and type 2 TBs was measured as 1.4 and 0.6 MPa, respectively, resulting in a maximum output stress of about 1.3 MPa for type 1 and 2.1 MPa for type 2 TBs. The presented results demonstrate that L-PBF has good potential for manufacturing active parts from Ni-Mn-Ga. The stabilization of desired TB types and patterns in an active region to ensure reliable and repeatable device operation is required.