Исхакова, Э И

Чуракова А.А., Исхакова Э.И.

В представленной статье проведены комплексные исследования микроструктуры и фазового состава сплава до и после электрохимических коррозионных испытаний в растворах 1М HCl, 3M HCl, 1M H2SO4, 3M H2SO4. Проведенные исследования показали, что коррозионный процесс в сплаве Ti49.1Ni50.9 в случае электрохимической коррозии протекал на всех образцах и во всех растворах в виде появления питтингов, а также с продуктами коррозии при испытании в растворах H2SO4 с разной концентрацией, в случае 3 M HCl продукты коррозии также были обнаружены на поверхности образцов. Было обнаружено изменение характера микроструктуры в крупнозернистом состоянии в 3M-растворах, в то время как в ультрамелкозернистом состоянии таких изменений не обнаружено.

Чуракова А.А., Исхакова Э.И.

В представленной статье проведены исследования влияния многократных мартенситных превращений В2-В19’ на структуру и температуры превращений в различных структурных состояниях сплава TiNi. Показано, что в крупнозернистом, ультрамелкозернистом и нанокристаллическом сплаве TiNi происходят последовательные изменения в микроструктуре и температурах фазовых переходов, при увеличении числа термоциклов до n=100 с быстрым нагревом и быстрым охлаждением до –196 °C. Температуры превращений в ультрамелкозернистом состоянии Ti49.15Ni50.85 более устойчивы к термоциклированию (ТЦ), чем в крупнозернистом состоянии. Обнаружено образование нанодвойников мартенсита в наноструктурном состоянии после многократных термоциклов.

Li H.F., Nie F.L., Zheng Y.F., Cheng Y., Wei S.C., Valiev R.Z.

TiNi alloys, with their unique shape memory effects and super elastic properties, occupy an indispensable place in the family of metallic biomaterials. In the past years, surface treatment is the main technique to improve the bioinert nature of microcrystalline TiNi alloys and inhibit on the release of toxic nickel ions to obtain excellent osteogenesis and osseointegration function. In the present study, nanocrystalline Ti49.2Ni50.8 alloy has been fabricated via equal channel angular pressing (ECAP), and the in vitro and in vivo studies revealed that it had enhanced cell viability, adhesion, proliferation, ALP (Alkaline phosphatase) activity and mineralization, and increased periphery thickness of new bone, in comparison to the commercial coarse-grained counterpart. These findings indicate that the reduction of grain size is beneficial to increasing the biocompatibility of Ti49.2Ni50.8 shape memory alloy.

Wang X., Pu Z., Yang Q., Huang S., Wang Z., Kustov S., Van Humbeeck J.

In this work, a new process is developed to improve the functional stability of Ni-rich NiTi alloys. Repetitive temperature- and stress-induced phase transformation is first conducted to generate dislocation networks in the grain interior. Dislocations serve as nucleation sites for Ni4Ti3 nanoprecipitates, which are formed after subsequent low-temperature (523 K) aging. With the presence of dislocations, a homogeneous distribution of nanoprecipitates in the grains is expected, enhancing the strength of the NiTi matrix and resisting plastic deformation during the martensitic transformation. As a result, an improved functional stability of NiTi alloys is achieved.

Churakova A.A., Gunderov D.V., Dmitriev S.V.

Natalia R., Sergey B.

Different methods for calculating the entropy change in the thermoelastic martensitic transformation were analysed. A new way of estimating the transformation entropy was proposed based on the simple relationship between the heat released during the forward martensitic transformation and the finish temperature of the forward phase transition. The value of the B2 → B19′ transformation entropy was calculated for Ti50Ni50 alloy using the different methods. It was shown that the values of ΔSA→M calculated based on the “entropy argument”, “Clausius–Clapeyron relation” and through the calorimetric data (proposed in the present work) were close to each other and gave values which were approximately equal to −0.105 J/(g K). As the method proposed in the present work is simpler than others, it might successfully be used to estimate the transformation entropy and the contributions of the latent heat, elastic and dissipative energies.

Bowers M.L., Chen X., De Graef M., Anderson P.M., Mills M.J.

[1 1 0]-Oriented micropillars of 50.7 at.% NiTi were compressed to stress-induce a single martensite plate. Scanning transmission electron microscopy identified two distinct slip systems in the prior transformation zone with dislocation loops of alternating Burgers vectors and a doubly periodic dislocation substructure. Micromechanics-based modeling predicts these systems to be dominant for a specific martensite plate type and correlates these observations with the alternating stress state near twinned martensite plates. This combined experiment–modeling approach provides new mechanistic insight into defect generation during pseudoelastic cycling.

Tadaki T., Nakata Y., Shimizu K.

Etude experimentale par calorimetrie differentielle a balayage et par microscopie electronique en transmission dans l'alliage Ti-51% at Ni soumis a differents traitements de vieillissement et a differents cycles thermiques. Deux types de cycles sont adoptes: le cycle complet provoquant la transformation matrice B 2 ⇇phase intermediaire rhomboedrique R⇄martensite M, le cycle incomplet conduit seulement a la transformation B 2 ⇄R. On observe des variations significatives de la temperature de transformation et de la microstructure en fonction du cycle thermique. Discussion des resultats

Ionita D., Caposi M., Demetrescu I., Ciuca S., Gherghescu I.A.

The present paper is focused on three different aging treatment of a TiNi alloy in order to enhance corrosion resistance in oral cavity environment. The best corrosion resistance in artificial saliva after aging treatment was found in the case of sample which was heat treated at 700 °C/20 min/water cooled, followed by aging at 400 °C/1.5 h/water cooled. Subsequent to such a heat treatment, the EDX surface analysis indicated an increase of Ti content of the matrix. This effect is in connection with stable and metastable precipitates which appear during the aging. Also, based on the greater affinity to oxygen of Ti compared to Ni, a uniform Ti based oxide layer is predominant explaining the better electrochemical stability. The obtained precipitates morphology is also an important factor in the corrosion behavior.

Belyaev S., Resnina N., Zhuravlev R.

One of the difficulties for application of TiNi shape memory alloys in actuators and the heat engine is an accumulation of a residual strain during thermal cycling. This problem may be solved by using a TiNi alloys with high yield stress or development of a special way for working cycle. The aim of the present work is a study of deformation behaviour and work performance in Ti–51.5 at.%Ni alloy during thermal cycling under different thermal-mechanical conditions. The samples were subjected to 30 thermal cycles through the temperature range of martensitic transformations under three different ways in torsion. The first sample was cooled and heated under a constant stress of 50 MPa, the second sample was cooled under the stress of 50 MPa and heated under the stress of 200 MPa (asymmetrical cycle). The third sample was cooled under the stress of 50 MPa, heated under the stress of 200 MPa, then unloaded and loaded in an opposite direction up to stress of −50 MPa, cooled down and heated under the stress of −200 MPa (symmetrical cycle). The results showed that the accumulation of a residual strain was found in all studied samples. However, thermal cycling under a symmetrical cycle resulted in the smallest accumulation of the strain in the sample.

Belyaev S., Resnina N.

AbstractVariations in strain and work output in two Ti-47 at.%Ni-3 at.%Cu and Ti-51.5 at.%Ni alloys during thermal cycling in the mode of fixed stress acting on cooling and heating were studied. It was found that in the Ti50Ni47Cu3 alloy with a low level of yield stress, the values of transformation plasti city and shape memory effect and work output were changing during thermal cycling. It was observed that the mechanical behavior and work performance of Ti-51.5 at.%Ni alloy with a high level of yield stress was stable on thermal cycling irrespective the value of stress acting on cooling and heating. It was found that under the same stress conditions the value of work output in Ti-51.5 at.% Ni alloy was four times lower than in the Ti-47 at.%Ni-3 at.%Cu alloy. Moreover, the accumulated residual strain was approximately 40 times lower than in Ti50Ni47Cu3 alloy with a low level of yield stress.

Sibirev A., Belyaev S., Resnina N.

Variation in kinetics of martensitic transformation, in an equiatomic TiNi shape memory alloy, during thermal cycles was investigated. Samples annealed at 500 °C for 1 hour were subjected to repeated thermal cycles, through the temperature range of martensitic transformation. Unusual 4-stage martensitic transformation during cooling after 30 thermal cycles was observed. Moreover, a new unusual phenomenon was found in the preliminary thermal cycled TiNi alloy. It was observed that variation in the highest temperature Th of thermal cycles temperature interval resulted in the redistribution of released heat among four calorimetric peaks, observed on cooling. It was found that if the Th temperature did not exceed 240 °C the variation in kinetics was repeatable, and determined only by the value of Th. It was assumed that the defect structure induced on preliminary thermal cycling changes reversibly on cooling and heating.

Belyaev S., Resnina N., Sibirev A.

► Fraction of forward martensite transition influences accumulation of residual strain. ► If less than 50% of forward transition takes place then, no residual strain is found. ► If 75% and more of forward transition occur then residual strain is observed. ► Accommodation of martensite variants occurs elastically in the 50% of MT. ► Plastic accommodation takes place only in the last quarter of temperature range of MT. Strain variation on thermal cycling of the TiNi alloy under a constant stress of 50 MPa through the temperature range of complete as well as incomplete forward martensitic transformation was studied. The thermal cycling of the alloy through the temperature range of incomplete forward martensitic transformation was carried out to realize 25%, 50% or 75% of temperature range of forward phase transition. The thermal cycling of the TiNi alloy through the temperature range of the complete forward martensitic transformation resulted in observation of “training effect” (an increase in values of transformation plasticity and shape memory effects strain) and accumulation in residual plastic strain. It was shown that observation of “training effect” as well as accumulation of residual strain depended on the completeness of the forward martensitic transformation. As higher the fraction of temperature range of forward martensitic transformations on thermal cycling as stronger variation in values of shape memory effects and higher the accumulated plastic strain on the thermal cycling of the TiNi alloy. These phenomena were due to that the martensite accommodation process was elastically in the early stage of forward martensitic transformation. The plastic accommodation of martensite took place on the final stage of forward phase transition.

Valiev R.Z., Gunderov D.V., Lukyanov A.V., Pushin V.G.

Nanocrystalline (NC) Ti494Ni50:6 alloy with a different mean grain size in a range of 20–100 nm has been successfully produced using severe plastic deformation by high pressure torsion and further annealing at various temperatures. The features of microstructure and martensitic transformation of the NC TiNi have been studied. During tensile tests the effects of the grain size on mechanical properties at room and elevated temperatures are studied and discussed. The NC TiNi exhibits an unusual increased strength but low ductility at room temperature. However, it demonstrates enhanced strength and ductility at higher temperatures.

Chan C.W., Man H.C., Yue T.M.

In the present study the tensile and super-elastic behaviours of laser-welded NiTi wires in Hanks’ solution at open-circuit potential (OCP) were investigated using tensile and cyclic slow-strain-rate tests (SSRT). In comparison with NiTi weldment tested in oil (non-corrosive environment), the weldment in Hanks’ solution suffered from obvious degradation in the tensile properties as evidenced by lower tensile strength, reduced maximum elongation, and a brittle fracture mode. Moreover, a larger residual strain was observed in the weldment after stress–strain cycles in Hanks’ solution. In addition to the microstructural defects resulting from the welding process, the inferior tensile and super-elastic behaviours of the NiTi weldment in Hanks’ solution could be attributed to the trapping of a large amount of hydrogen in the weld zone and heat-affected zone.

Chan C.W., Man H.C., Yue T.M.

In this study, the susceptibility to stress corrosion cracking (SCC) of laser-welded NiTi wires in Hanks’ solution at 37.5 °C was studied by the slow strain-rate test (SSRT) at open-circuit potential and at different applied anodic potentials. The weldment shows high susceptibility to SCC when the applied potential is near to the pitting potential of the heat-affected zone (HAZ). The pits formed in the HAZ become sites of crack initiation when stress is applied, and cracks propagate in an intergranular mode under the combined effect of corrosion and stress. In contrast, the base-metal is immune to SCC under similar conditions. The increase in susceptibility to SCC in the weldment could be attributed to the poor corrosion resistance in the coarse-grained HAZ.

Chan C.W., Man H.C., Yue T.M.

Post-weld heat-treatment (PWHT) was applied to NiTi weldments to improve the corrosion behaviour by modifying the microstructure and surface composition. The surface oxide film on the weldments is principally TiO 2 , together with some Ti, TiO, and Ti 2 O 3 . The surface Ti/Ni ratio of the weldments after PWHT is increased. The oxide film formed in Hanks’ solution is thicker on the weldments after PWHT. The pitting resistance of the weldments is increased by PWHT. The galvanic effect in the weldments is very small. The weldment with PWHT at 350 °C shows the best corrosion resistance among other heat-treated weldments in this study.