Phase field simulations for the crossover from sharp martensitic transformation into smooth strain glass transition by fine precipitates

Recent studies have shown an intriguing phenomenon that by introducing nano-precipitates, its sharp first order martensitic transformation (MT) can be changed into smooth strain glass transition which is characterized by sluggish evolution of nano-sized martensitic domains. However, it remains unclear how the precipitates can fundamentally change the nature of the normal MT. In the present study, we reveal the origin of this phenomenon and explore the novel properties associated with the precipitate-induced strain glass using phase field simulations. Our model and phase field simulations show that the shape and size of the Ni4Ti3 precipitates influence the geometrical confinement. The geometrical confinement caused by precipitates with disk shape and high density could produce strong resistance to the growth of large-scale martensitic domains, and can lead to the freezing of nano-sized martensitic domains. When the average size of the B2 pocket between adjacent Ni4Ti3 precipitates is below a critical value (∼16 nm), the avalanche-like behavior of the normal MT is altered to a smooth nanoscale MT with nearly zero transition hysteresis, which exhibits interesting hysteresis-free superelasticity over a wide temperature range. Our results further show that the nanoscale MT exhibits characteristic signatures of strain glass, such as broken ergodicity and Vogel-Fulcher frequency dispersion of internal friction peaks and suggest the non-ergodic nature of the geometrically confined MT. Through precipitate engineering, it is possible to regulate and modify the normal MT into the nanoscale MT and consequently achieve novel properties.