  Metallic/intermetalic materials with BCC structures hold an intrinsic
instability due to phonon softening along [110] dirrection, causing BCC to
lower-symmetry phases transformation when the BCC structures are thermally or
mechanically stressed. Fe50Rh50 binary system is one of the exceptional BCC
structures (ordered-B2) that has not been yet showing such transformation upon
application of thermal stress, although mechanical deformation results in B2 to
disordered FCC (gamma) and L10 phases transformation. Here, a comprehensive
transmission electron microscopy (TEM) study is conducted on thermally-stressed
samples of Fe50Rh50 aged at water and liquid nitrogen from 1150 degree C and
1250 degree C. The results show that, samples quenched from 1150 degree C into
water and liquid nitrogen show presence of 1/4{110} and 1/2{110} satellite
reflections, the latter of which is expected from phonon dispersion curves
obtained by density functional theory calculation. Therefore, it is believed
that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once
Fe50Rh50 is quenched from 1250 degree C into liquid nitrogen, formation of two
short-range ordered tetragonal phases with various c/a ratios (~1.15 and 1.4)
is observed in line with phases formed from mechanically deformed (30%) sample.
According to our observations, an accurate atomistic shear model ({110}<1-10>)
is presented that describes the martensitic transformation of B2 to these
tetragonal phases. These findings offer implications useful for understanding
of magnetic and physical characteristics of metallic/intermetallic materials.
