<p>This study employs a combined approach of first-principles density functional theory (DFT) calculations and experimental characterization to investigate the atomic-scale origin of the enhanced martensitic transformation in as-cast Ni₄₇Ti₄₄Nb₉ (at.%) shape memory alloy. Calculations of formation enthalpy reveal that Nb solute atoms preferentially occupy Ti sublattice sites within the B2 NiTi matrix. This site preference is further corroborated by analyses of elastic constants and electron charge density distributions. Specifically, Nb substitution on Ti sites significantly reduces the critical shear moduli (c′ and C₄₄), thereby lowering the energy barrier for martensitic transformation (MT). This mechanism provides a fundamental understanding for the experimentally observed increase in martensite start temperature (Ms) with higher Nb solid solution, as confirmed by differential scanning calorimetry (DSC). The Coulomb repulsion between Nb at Ni sites and neighboring Ti atoms, visualized via charge density difference, makes Nb occupation of Ni sites energetically unfavorable. These atomic-scale insights reconcile existing debates on Nb site occupancy and establish a foundational understanding for strategically tailoring the functional properties of NiTiNb alloys through microstructural engineering.</p>