Impact of nitrogen atoms doping on elastocaloric effect of solution-treated Ti_50.5Ni_49.5 alloys

The microstructural characteristics, martensitic phase transformation behavior, and elastocaloric effect of (Ti_50.5Ni_49.5)_100-xN_x alloys with different nitrogen (N) contents (x = 0.01, 0.05, 0.1, 0.2, 0.3, and 0.5 at%) are investigated. The elastocaloric effect of the alloy is enhanced after doping with N, both in theoretical calculations and actual measurements. The entropy change of the alloy increases first and then decreases with the rising content of N. It reaches the maximum value of 64.56 J/kg·K in the alloy containing 0.2 at% N content, which is 23.7 % higher than that in the alloy of 0.01 N content. The enhancement of the alloy's entropy change is explained through the analysis of its thermomechanical responses using the Maxwell equation. Interstitial N atoms simultaneously reduce the phase transformation temperature interval and increase the transformation strain, thereby leading to a large dε/dT. According to the Maxwell relation, the elevated dε/dT directly correlates with an increased entropy change. N doping can also improve the superelasticity of TiNi alloys through grain refinement. Finally, a maximum adiabatic temperature change of 8.60 ℃ is observed in the alloy containing 0.2 at% N content, which is attributed to both the elevated ΔS and the increased recoverable strain enabled by grain refinement.