Abstract: In this work, a new strategy is proposed to improve the pseudoelasticity stability of NiTi SMAs under strains beyond the martensite transformation stress plateau by introducing homogeneous Ni₄Ti₃ precipitates, multiple martensite variants, and high-density austenite twins. Our experimental results show that this new strategy achieves excellent and stable pseudoelasticity with a recoverable ratio up to 85.5% larger than the highest value ever reported of 63.4% after 40 cyclic loading under a fixed strain of 14% due to the introduction of high-density austenite twins. The mechanisms of this promising property are revealed with the aid of subregional (grain boundary and grain interior) EDS, TEM, and in-situ BSE analysis. First, an appropriate forging and aging process introduces homogenous Ni₄Ti₃ precipitates, which reduce the energy dissipation required for one pseudoelastic loop from 4.49 to 2.48 J g^-1 by changing the phase transformation path and temperature. Second, the high-density austenite twins induced by deformed martensite twins during cyclic loading enhance the strength of the matrix. The reduced energy dissipation and stronger matrix improve the pseudoelasticity and its stability. We attribute these beneficial microstructure features to the specially designed processing routes: forging and subsequent aging. On the one hand, the dislocations induced by forging provide homogeneous nucleation sites, leading to Ni₄Ti₃ homogeneously precipitating during aging; on the other hand, the dense and homogeneously distributed precipitates accelerate the martensite transformation and increase the elastic modulus of the martensite, resulting in the advancement of the second stress plateau from 38% to 12%, in which part of the martensite is reoriented to generate multiple martensite variants, resulting in the formation of deformation martensite twin. It transforms into austenite twin after unloading. These findings pave a feasible avenue for tailoring the functional properties of SMAs.

Key words: NiTi shape memory alloys, Pseudoelasticity, Ni₄Ti₃, Deformation martensite twin, Austenite twin