<p>The effect of alloying the Ni_49.5Ti_35.5Hf₁₅ polycrystals with up to 15 at. % Nb on the stress-induced martensitic transformations, shape memory effect, and superelasticity of this alloy is studied. The initial Ni_49.5Ti_35.5Hf₁₅ polycrystals exhibit no shape memory effect due to a&#xa0;high volume fraction of (Ti, Hf)₂Ni particles, suppressing the stress-induced martensitic transformations. First, an addition of Nb results in the homogeneous distribution of Ni, Ti, and Hf, the formation of local β‑Nb phase regions, and the suppression of the (Ti, Hf)₂Ni particle precipitation. Second, the (Ni_49.5Ti_35.5Hf₁₅)₈₅Nb₁₅ alloy exhibits high-temperature shape memory and superelasticity effects, with a&#xa0;maximum transformation strain of 1.0% and a&#xa0;reversible strain of 0.3%–0.4% in compression. Third, despite their high strength, the Ni_49.5Ti_35.5Hf₁₅ polycrystals suffer brittle fracture near the yield stress during deformation in the austenitic state, while an addition of Nb increases the material plasticity up to 14.5%–18%. Moreover, a&#xa0;high resistance to the oriented martensite formation in the (Ni_49.5Ti_35.5Hf₁₅)₈₅Nb₁₅ polycrystals results in a&#xa0;mismatch between the curves of the σ(<i>T</i>) dependence derived from the experimental ε(<i>T</i>)_σ and σ(ε)_<i>T</i> curves.</p>