Thermomechanical treatment to achieve stable two-way shape memory strain without training in Ti-49.8 at.% Ni alloy

Babicheva, Rita I.; Mulyukov, Kharis Ya.

doi:10.1007/s00339-014-8345-z

Cited by 18 publications

(8 citation statements)

References 44 publications

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“…It is known that during the B19' → B2 transformation, the crystal lattice demonstrates extension in two normal directions and compression in the third one. Obviously, the change of bi-crystal dimensions observed in the study can be only realized when we deal with two or more martensite variants, and their sum effect leads to the opposite situation [7]. But unlike the bi-crystal with tilt GBs, where we can observe only one type of variants in each grain, in the case of twist GBs, FMT results in formation of two variants in each crystal separated by the domain boundary (Fig.…”

Section: Resultsmentioning

confidence: 89%

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

Babicheva¹,

Gunderov²,

Stolyarov³

et al. 2018

LoM

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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,

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Section: Resultsmentioning

confidence: 89%

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

Babicheva¹,

Gunderov²,

Stolyarov³

et al. 2018

LoM

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“…Recently, authors of [18] reported that Zr segregation to GBs and formation of amorphous complexion promote dislocation emission, and at the same time can facilitate the dislocation pinning at GBs, while Borovikov et al [19] showed that solute addition to GBs can suppress dislocation emission. Due to the ability to accommodate dislocation emission and adsorption, such intergranular films in GBs can strengthen materials [20 -25] and improve the structural thermal stability of nanocrystalline alloys [26], and they could be an alternative method to thermo-mechanical training upon material cycling [27]. On the contrary, there are works demonstrating that some complexions in GBs can cause severe embrittlement [28 -31].…”

Section: Introductionmentioning

confidence: 99%

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

2019

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NiTi alloys are very important in a number of applications since they demonstrate shape memory effect, which is due to the martensitic phase transition with the transition temperatures close to the room temperature. Many factors affect transition temperatures of the alloy, including a variation of its chemical composition and thermo-mechanical treatment, which affects grain size, dislocation density, and other crystal structure parameters. It is well-known that the chemical composition of the alloys in grain boundaries can differ significantly from that in the bulk due to segregation of certain elements from the matrix to the grain boundaries. The effect of grain boundary segregations on the martensite transformation temperatures is still poorly understood. In the present molecular dynamics study, the possible effect of segregation of Ti or Ni atoms along Σ25 tilt grain boundary on the forward and reverse martensitic transformations is analyzed. The segregation is simulated by replacing the monoatomic Ti or Ni layer in the grain boundary with Ni or Ti layer, respectively. The results are compared to the case of no segregations. We analyze the initial relaxed and thermalized structures of the bi-crystals in austenite state as well as the temperature dependencies of potential energy per atom and volumetric dilatation. It is found that segregations may significantly decrease the start and finish temperatures of the martensitic transformation, and this effect is more pronounced for segregations of Ni.

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“…It is of special interest to establish the influence of the UFG state of TiNi alloys on the structural changes during thermal cycling through martensitic transformation temperatures, determine the role of the grain boundaries in dislocation generation at phase martensitic transformations, and establish the possibility of enhancement of the mechanical and functional properties of these materials via phase hardening. There are a limited number of publications reporting on the studies of the effect of thermal cycling on UFG TiNi alloys [11][12][13]. The paper of Tong et al [11] is devoted mainly to the effect of mechanocycling on the superelastic deformation of ECAP-processed TiNi.…”

Section: Introductionmentioning

confidence: 99%

Transformation of the TiNi Alloy Microstructure and the Mechanical Properties Caused by Repeated B2-B19′ Martensitic Transformations

Churakova

Гундеров

Lukyanov

et al. 2015

Acta Metall. Sin. (Engl. Lett.)

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The influences of thermal cycling treatment in the temperature range of B2-B19 martensitic transformations (-150 to 150°C) on the TiNi alloy structure and properties were studied. Different states named the initial coarse-grained (CG) state, the ultrafine-grained (UFG) state after ECAP (with a grain size of 200 nm), the state after ECAP and cold upsetting by 30% were considered. The results show that the microhardness and the strength increase in all the three states. According to the XRD analysis, a more significant increment in the dislocation density, resulting from thermal cycling, is observed in the UFG alloy than in the CG alloy.

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Thermomechanical treatment to achieve stable two-way shape memory strain without training in Ti-49.8 at.% Ni alloy

Cited by 18 publications

References 44 publications

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

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

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

Transformation of the TiNi Alloy Microstructure and the Mechanical Properties Caused by Repeated B2-B19′ Martensitic Transformations

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