Martensitic phase transformation in short-range ordered Fe₅₀Rh₅₀ system induced by thermal stress and mechanical deformation

Metallic/intermetallic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] direction, 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 L1(0) phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe50Rh50 induced by quenching in water and liquid nitrogen from 1150 degrees C and 1250 degrees C. We demonstrated that samples quenched from 1150 degrees C into water and liquid nitrogen show the 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 proposed that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once Fe50Rh50 is quenched from 1250 degrees C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c/a ratios (similar to 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}< 110 >) is presented that describes the martensitic transformation of B2 to these tetragonal phases.