We performed nanostructural characterization of TiNiSn-based half-Heusler materials with respect to materials processing and YSb doping. The samples were prepared using radio frequency melting, followed by the liquid quench (LQ) process. It is found that the LQ process employing a BN nozzle enhances the compositional homogeneity and reduces impurities, such as Si and O. Correspondingly, the power factor of (Ti0.5Zr0.25Hf 0.25)NiSn alloys is significantly improved over those fabricated using a silica nozzle and without the LQ process. The nanostructure of the YSb-doped samples is investigated with a laser-assisted local-electrode atom-probe tomography. The results demonstrate homogeneous distributions of Ti, Zr, Hf, Ni, and Sn over the analyzed volume of material, while a one-dimensional chain-like feature of Y are observed with increasing the Y doping concentration, which is the key for controlling the nanostructure for improvement of the thermoelectric properties in these multi-component systems.