Martensitic phase transformation in NiTi bi-crystals with symmetric ∑25 twist and tilt grain boundaries

Abstract

NiTi alloys attract a lot of attention of researchers for a number of reasons; among them are their practical importance and challenges for theoretical understanding. The most exciting feature of these alloys is the shape memory effect due to the martensitic transformation at temperatures close to the room temperature. There exist many factors affecting the transition temperatures in such materials, such as a deviation from stoichiometric composition, dislocation density, grain size, and the type of grain boundaries. The latter factor is one of the less explored, and we are aware of just a few studies in this direction. In the present work, molecular dynamics simulations are carried out to reveal the effect of symmetric tilt and twist grain boundaries in bi-crystals with nanosized grains on the forward and reversed martensitic transformations during cooling down from the austenite B2 phase and subsequent heating up from the martensite B19' phase. Phase composition, elastic strain components, relative change of volume, potential energy per atom, and shear stresses are calculated and analyzed as the functions of temperature. It is found that the type of grain boundaries in the bi-crystals strongly affects the transition temperatures. Start and finish temperatures of the forward and reverse martensitic transformations are much lower in the bi-crystal with twist grain boundaries as compared to that having tilt grain boundaries. Overall, the simulation results of this study are in a good qualitative agreement with the available experimental data. NiTi сплавы привлекают большое внимание исследователей по ряду причин; среди них их практическая ценность и сложность для теоретического понимания. Наиболее выдающаяся особенность таких сплавов это эффект памя-ти формы, который обусловлен мартенситным превращением, протекающим при температурах близких к комнат-ной. Существует множество факторов влияющих на температуру фазового перехода в этих материалах, например, отличие химического состава сплава от стехиометрического, плотность дислокаций, размер кристаллитов и тип границ зерен. Последний фактор, из перечисленных, наименее изучен, и мы осведомлены только о нескольких ра-ботах, посвященных этому вопросу. В данной работе проведено моделирование методом молекулярной динамики с целью изучения вопроса влияния симметричной границы наклона и кручения в нанокристаллических бикристал-лах на прямой и обратный мартенситные переходы при охлаждении с аустенитной B2 фазы и последующем нагреве с мартенситной B19' фазы. Определены и проанализированы фазовый состав, компоненты упругой деформации,

Grain boundary-induced martensitic transformations: A phase-field study of nucleation, size-effect, triple junction-effect, microstructures, and compatibility at the nanoscale

“…The experimental studies have shown that the grain boundaries (GBs) and triple junctions (TJs) are additional heterogeneities favourable for the M nucleation in the polycrystalline samples of such phase-changing materials [27][28][29][30][31][32][33][34][35][36][37], which was also confirmed through atomistic simulations [38][39][40][41][42]. In particular, the parameters such as the grain orientation distribution (see, e.g.…”

“…The temperatures of M nucleation from the GBs are usually significantly higher than the homogeneous transformation temperature of M [28,29]. The start and finish temperatures for both the forward (A → M) and the reverse (M → A) transformations from the tilt GBs are much higher than that for the twist GBs [28,32,38,39]. The reason for this was attributed to the local stress fields associated with the intrinsic dislocations in the low angle tilt GBs [28,32].…”

“…In particular, the parameters such as the grain orientation distribution (see, e.g. [1,36,42,43], and the references therein), average grain size [31,44,45], type of the GBs (tilt or twist) [28,32,39], GB energy change during MTs [27], stress concentration within the GBs during transformations [33,36,46], degree of reduction in symmetry from A to M [43,47], and direction of loadings [22] have been observed to play an important role on nucleation and evolution of very rich martensitic microstructures. The MTs in nanocrystalline materials, which contain a large volume fraction of the GBs and TJs, are greatly influenced by these heterogeneous sites [48,49].…”

Effect of grain boundary segregations on martensitic transformation temperatures in NiTi bi-crystals

“…Therefore, the current work aims to study the effect of Ni and Ti segregations to Σ25 tilt GBs of NiTi bi-crystals on MT characteristic temperatures by means of the molecular dynamics simulations. This paper can be regarded as a continuation of our previous work devoted to NiTi bi-crystals having tilt or twist GBs without segregations [39], and therefore it will be focused on the effect of GB segregations of Ni and Ti on MT temperatures in comparison with our previous results.…”

“…MD simulation of the equiatomic alloy without GB segregation is conducted for a computational cell in the form of rectangular parallelepiped that was used in our previous work [39]. Ni and Ti atoms of the rectangular cell (12.8 × 4.0 ×12.8 nm 3 ) form two B2 crystals separated by the symmetric Σ25 (710) tilt GBs with the <010> misorientation axis (Fig.…”