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MnMX (M = Co or Ni, X = Si or Ge) alloys, experiencing structuraltransformation between Ni2In-type hexagonal and TiNiSi-type orthorhombicphases, attract considerable attention due to their potential applications asroom-temperature solid refrigerants. Although lots of studies have been carriedout on how to tune this transformation and obtain large entropy change in awide temperature region, the crystallography of this martensitic transformationis still unknown. The biggest obstacle for crystallography investigation is toobtain a bulk sample, in which hexagonal and orthorhombic phases coexist,because the MnMX alloys will fragment into powders after experiencing thetransformation. For this reason, we carefully tune the transformationtemperature to be slightly below 300 K. In that case, a bulk sample with smallamounts of orthorhombic phases distributed in hexagonal matrix is obtained.Most importantly, there are no cracks between the two phases. It facilities usto investigate the microstructure using electron microscope. The obtainedresults indicate that the orientation relationship between hexagonal andorthorhombic structures is [4-2-23]h//[120]o & (01-10)h//(001)o and the habitplane is {-2113.26}h. WLR theory is also adopted to calculate the habit plane.The calculated result agrees well with the measured one. Our work reveals thecrystallography of hexagonal-orthorhombic transformation for the first time andis helpful for understanding the transformation-associated physical effects inMnMX alloys.

Crystallography of the martensitic transformation between Ni2In-type hexagonal and TiNiSi-type orthorhombic phases