  MnMX (M = Co or Ni, X = Si or Ge) alloys, experiencing structural
transformation between Ni2In-type hexagonal and TiNiSi-type orthorhombic
phases, attract considerable attention due to their potential applications as
room-temperature solid refrigerants. Although lots of studies have been carried
out on how to tune this transformation and obtain large entropy change in a
wide temperature region, the crystallography of this martensitic transformation
is still unknown. The biggest obstacle for crystallography investigation is to
obtain a bulk sample, in which hexagonal and orthorhombic phases coexist,
because the MnMX alloys will fragment into powders after experiencing the
transformation. For this reason, we carefully tune the transformation
temperature to be slightly below 300 K. In that case, a bulk sample with small
amounts of orthorhombic phases distributed in hexagonal matrix is obtained.
Most importantly, there are no cracks between the two phases. It facilities us
to investigate the microstructure using electron microscope. The obtained
results indicate that the orientation relationship between hexagonal and
orthorhombic structures is [4-2-23]h//[120]o & (01-10)h//(001)o and the habit
plane 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 the
crystallography of hexagonal-orthorhombic transformation for the first time and
is helpful for understanding the transformation-associated physical effects in
MnMX alloys.
