Synergistic effect of thermomechanical processing and boron doping on achieving a large and stable elastocaloric effect in a (Ti₄₉Ni₄₁Cu₁₀)₉₉.₈B₀.₂ alloy

Elastocaloric cooling represents a highly promising solid-state refrigeration technology. TiNiCu alloys with high Cu content are attractive candidates due to their superior crystallographic compatibility (B2-B19 transformation). However, their poor ductility severely limits applicable cold deformation, hindering effective grain refinement and performance enhancement. In this work, we propose a novel synergistic strategy via moderate thermomechanical treatment combined with interstitial B micro-alloying in a Ti₄₉Ni₄₁Cu₁₀ alloy. Remarkably, with only 0.2 at.% B doping, an ultrafine-grained structure with an average grain size of ∼15 nm was achieved under a relatively low cold-rolling reduction of 30%. By comparing the microstructure with the B-free alloy, the grain-refining effect of B is attributed to an increased nucleation rate and the inhibition of grain growth during recrystallization. The resultant nanocrystalline (Ti₄₉Ni₄₁Cu₁₀)_99.8B_0.2 alloy exhibits excellent superelasticity with a large recoverable strain of 5.5% and a narrow stress hysteresis of ∼100 MPa. More importantly, it delivers a giant adiabatic temperature change of 21.3 K at 5.5% strain, along with improved cyclic stability compared to the B-free alloy (only 4.6% ΔT_ad decay after 10 cycles). This exceptional elastocaloric performance is attributed to the synergistic combination of favorable lattice compatibility, a low fraction of second phases, and the strengthening effect provided by the ultrafine-grained structure combined with dislocation networks. This work establishes B micro-alloying as an effective strategy for developing high-performance shape memory alloys under limited deformation processing.