Effect of silicon and carbon content on shape memory in a single crystals of Fe–Mn–Cr–Ni–Si-based shape memory alloys

The effect of the Si concentration (2.8 and 3.89 wt. %) and carbon alloying 0.18 wt. % on the reversible FCC ↔ HCP martensitic transformation (MT) in a free state and under stress, as well as on the magnitude of the shape memory effect (SME), was studied in [1¯44]-oriented single crystals of Fe–Mn–Cr–Ni–Si-based shape memory alloys. Upon cooling and subsequent heating in a free state, a reversible FCC ↔ HCP MT occurred with a wide thermal hysteresis ΔTh = Af–Ms = 170–195 K, and the alloys exhibited an SME. For the first time, an SME of 17.5% was obtained at 77 K under tension in a carbon-doped alloy, equal in magnitude to the theoretical lattice deformation ε0=17.5% for the FCC–HCP MT. An SME with an ε0 = 17.5% was ensured by strengthening the FCC phase and developing one predominant variant of HCP martensite under stress, while suppressing the formation of irreversible BCT martensite inside the plates of reversible HCP martensite from the onset of transformation and with an increasing strain of up to 20%.