Heusler-type Fe₂VAl thermoelectric materials have been processed by high-pressure torsion (HPT) to improve their thermoelectric performance through a drastic reduction of the thermal conductivity. While the thermal conductivity for Fe₂VTa_0.05Al_0.95 is about 10 W/m K at 300 K, it is reduced to about 5.0 W/m K due to HPT processing. Furthermore, even after annealing at 873 K for recurrence of the Heusler-type structure, Ta-doped Fe₂VTa_0.05Al_0.95 possesses an ultra-fine grained structure with an average grain size of about 80 nm, in contrast to the presence of large grains of about 270 nm for non-doped Fe₂V_1.05Al_0.95. Since scanning transmission electron microscopy combined with energy-dispersive X-ray analysis reveals the solute segregation of Ta along grain boundaries and recrystallization interfaces, the retardation of recrystallization could be caused by the solute drag effect, leading to a suppression of grain coarsening. Thus, it is possible to retain the low thermal conductivity of about 5.0 W/m K at 300 K for Fe₂VTa_0.05Al_0.95, in parallel with the restoration of a large power factor, so that the dimensionless figure of merit reaches ZT = 0.30 at around 500 K.

Improvement of p-type thermoelectric performance for Heusler-type Fe₂V_1.08−yTi_yAl_0.92 alloys with y = 0–0.34 has been investigated, focusing on the valence electron concentration (VEC) effect for tuning the Fermi level across the center of the pseudogap via Ti doping, combined with a constructive modification of the electronic structure around the pseudogap through the off-stoichiometric V/Al composition change. The Seebeck coefficient changes its sign from negative to positive at around VEC = 6.0 due to Ti doping, and the peak value reaches approximately 120 μV/K at 350 K for y = 0.22. As a result of a drastic reduction in the electrical resistivity at 300–600 K, the power factor enhances up to 3.7 × 10⁻³ W/mK² at 300 K for y = 0.30. The thermal conductivity increases with Ti doping to 15.5 W/mK at 350 K for y = 0.16, because of an increased stability of the L2₁ structure for VEC closer to 6.0, but then decreases to approximately 12 W/mK for y = 0.34. While the dimensionless figure of merit ZT = 0.13 is achieved at 500 K for y = 0.34, the codoping of Ta (Fe₂V_0.69Ti_0.34Ta_0.05Al_0.92) leads to a further enhancement of up to ZT = 0.18 at 500 K.