Effect of precipitate on dislocation pinning and actuation fatigue in single-crystal NiTi: independent phase-field and dislocation density study

Abstract

In this study, a 3D crystal plasticity phase field model was developed to simulate the cyclic degradation under actuation condition in the presence of misfit precipitate interface array in the single crystal NiTi shape memory alloy (SMA). A fixed array (2×2×2) of non-transforming coherent precipitate, introduced as a stress-free misfit eigenstrain source, incorporated into the model to theoretically examine its role in stabilizing the actuation response via coherency strain accommodation and dislocation pinning. An independent Schmid factor-gated dislocation density model was made to study the dislocation substructure evolution over 100 actuation cycles. Based on the phase field simulation, presence of precipitates shows a damped elastic strain energy density by up to ∼21% and ∼ 9.8% in two initial peaks, with minimum alteration in chemical driving force, and lowers the retained dislocation density by 33% relative to the precipitate free crystal. Consequently, the irrecoverable strain itself saturates to a nearly identical plateau (∼0.57%) in both crystals by cycle 20. The precipitate misfit interface promotes a stable, multi-variant B19’ accommodation mode by pinning dislocation at the interface, in contrast to the precipitate free crystal, which converges towards a single dominant twin-pair variant with unresisted plastic strain accumulation. Together, the results of both independent models indicate coherency strain accommodation, dislocation pinning, and multi-variant stabilization, explaining an improved actuation fatigue life resistance of aged NiTi alloys at the mesoscopic level. Copyright © 2026. Published by Elsevier B.V.