Grain size dependence of stress-assisted two-way memory effect in Ti-50.04 at.% Ni shape memory alloy

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2022-10, Vol.856, p.143872, Article 143872
• Main Authors: Huang, Kai, Yin, Hao, Li, Mingpeng, Sun, Qingping
• Format: Article
• Language: English
• Subjects: Actuation; Actuation strain; Actuators; Convexity; Crystallites; Deformation effects; Free energy; Grain boundaries; Grain size; Intermetallic compounds; Martensitic transformations; Nickel base alloys; Nickel compounds; Nickel titanides; Operating temperature; Phase transition; Phase transitions; Reduction; Shape memory alloys; Strain; Temperature; Temperature dependence; Titanium compounds; Transformation temperature; Transition temperature
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2022.143872

The grain size (GS) effects on stress-assisted two-way memory behavior of NiTi shape memory alloys (SMAs) were investigated by thermomechanical actuation tests over a wide GS range (from 10 nm to 230 nm). The actuation strain and transformation temperatures are observed to decrease monotonically as the GS is reduced, but the transition temperature window is enlarged. The irrecoverable strain decreases monotonically with GS reduction overall. The underlying mechanisms were investigated through microstructural and thermodynamic analysis. It is demonstrated that, as the GS is reduced, the increasing volume fraction of dislocations and grain boundaries (GB) occupying the space of austenite crystallite suppress the phase transition (PT) and lower the entropy change, leading to the decrease of transformation strain. GS reduction converts the PT from first-order type to a continuous one, and greatly expands the operating temperature range of actuation, tuning the discontinuous leap in strain changes into a continuous variation. When GS > 40 nm, transformation induced plasticity dominates the irrecoverable deformation and is monotonically affected by GS dependent hall-patch effect. Further modeling analysis reveals that, the increase in the volume of GB caused by GS reduction drastically modifies the free energy density convexity of the crystallite-GB composite system and leads to the monotonic decrease of transformation temperatures. The near-linear temperature dependency of actuation strain and wider transition temperature window of the nanostructured NiTi, compared to traditional coarse-grained NiTi, provide the opportunity of improved actuation control of SMA actuator and have the potential for some unique applications in the actuation area. •Actuation effect of nanocrystalline NiTi alloys with different grain size (GS) were examined.•The decrease of actuation strain is mainly due to suppressed phase transition at nanoscale.•GS reduction transforms the phase transition from first-order type to a continuous one, widening the actuation operating range.•GS reduction causes the decrease of transformation temperatures by changing the free energy density.