Transformation In NiTi Alloy Research Articles

A model of the kinetics of the temperature-induced phase transformation in NiTi alloys and its experimental verification

A model, describing the kinetics of the temperature-induced austenite phase transformation processes in shape memory alloys (SMAs) is presented. The model is formulated for a general stress and temperature state in the solid body and allows the computation of the austenite phase fraction evolution in it, that is, the dependence of the amount of the austenite phase fraction on the values of the temperature and stress. It is shown that the predicted amount of the austenite phase fraction is not dependent on the control volume discretization. The model does not take into account microstructural changes in the material and the existence of different phases in the material (R-phase, twinned martensite). The experimental verification of the model was conducted using SMA nitinol wire that was prestrained (detwinned martensite state) and subjected to a heat load. It was shown that the austenite phase fraction predicted by the model agrees very well with the tabulated (reference) and experimental data.

Phase Transformation Temperature of NiTi Alloy Sheet Examined by Lamb Wave

In this study, the ultrasonic PZT transducers were used for exciting and receiving Lamb waves on NiTi alloy sheet. Lamb waves were measured when the temperature of the NiTi alloy changed. Analysis on frequency spectrums of the Lamb waves was also done. Some marked changes were observed in the dependence of the waveforms and the frequency spectrums of the Lamb waves versus temperature during phase transformation of NiTi alloy. The results show that phase transformation temperature of NiTi alloy sheet may be examined by Lamb wave method.

Atomistic Simulation on the Relation between Amorphization and Crystalline Transformation in Ni-Ti Alloy

Ni-Ti alloys are typical shape-memory materials. It is suggested that the atomic-scale phase transformation between martensite and austenite crystalline phases is responsible for such function. However, it is reported that these alloys sometimes show also amorphization together with nanocrystal under severe deformation. The mechanism of amorphization should compete against martensitic (crystalline) transformation (MT). This study focuses on the atomistic relation between amorphization and MT in Ni-Ti alloys. Molecular dynamics simulation is performed by using our simplified potential built on the MEAM framework. The change in crystalline state is identified and traced mainly by developing common neighbor analysis (CNA) method. Under amorphization from B2 cubic structure, the characteristic topological change in CNA cluster around a transforming atom is detected. By applying simple shear in periodic specimen, it is found that the martensite phases appear first then amorphous phases are formed. The nucleation of amorphous phases is strongly dependent on choice of shear plane, e.g. {100}, {110} or {111} plane. When the shear direction does not match to the orientation of inherent slip system, amorphous phase is nucleated with relative ease and remains for a long time. It is recognized that dislocation slip (plastic deformation), MT and amorphization are closely related in atomic scale. The direction of phase transition is mostly B2(austenite)→B19'(martensite)→amorphous, except for the case of {111} shear plane which possesses pre-amorphous structures. It is found that the pre-amorphous is once formed and then disappears, preceding martensite phase or principal and later formation of amorphous phase.

The features of the martensitic transformation in titanium nickelide

The features of the martensitic transformation in Ni-Ti alloys have been studied. The model of the structure formation resulting from the mechanochemical reactions caused by shear deformations of different magnitude during martensitic transformation of the B2 structure is proposed.

Thermoelastic phase transformation in TiNi alloys under cyclic instrumented indentation

Austenitic and martensitic TiNi shape memory alloys were studied by means of cyclic instrumented indentation with the purpose of documenting the phase transformation. Recovery indexes were determined from the energy absorbed during loading, unloading and reloading and their relation with the microstructure and shape memory properties established. Significant changes on the load–displacement curves, associated to the transformation phenomenon, were exhibited by both materials. Under spherical cyclic nanoindentation, a classic pseudoelastic behavior was observed in the austenitic alloy, which was characterized by high recovery indexes and constitutes the first experimental evidence of this property under such testing conditions. On the other hand, the martensitic alloy showed lower recovery indexes, as a consequence of a faster stabilization process of the martensite plates.

616 Ni-Ti合金における応力誘起非晶質化および結晶変態の原子レベル解析(G01-2 計算力学(2)破壊力学・材料解析,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

616 Ni-Ti合金における応力誘起非晶質化および結晶変態の原子レベル解析(G01-2 計算力学(2)破壊力学・材料解析,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Modeling of shape memory alloy shells for design optimization

A three-dimensional phenomenological constitutive model for the analysis and design optimization of shape memory alloy (SMA) structures is presented. This model specifically targets the pseudoelastic behavior due to the R-phase transformation in NiTi alloys, but also applies to similar SMA materials with low hysteresis. A history-independent formulation is presented, which allows cost-effective sensitivity analysis. The possibility to efficiently compute design sensitivities is essential for enabling the use of gradient-based optimization algorithms, which will allow design optimization of complex SMA structures. The use of the constitutive model in a problem of realistic complexity is illustrated by the analysis of a SMA miniature gripper, modeled using shell elements. The sensitivity analysis of SMA structures using the presented model is addressed in an accompanying paper.

The influence of ultrasound on shape memory behavior

The ability of ultrasonic vibrations (UVs) to influence the thermo-elastic phase transformation in TiNi alloys has been reported earlier. Particularly it was shown that preliminary ultrasound influence changes characteristic temperatures and transformation kinetics. Also UVs allow to realize a “sliding” shape memory effect (SME) when the reverse transformation occurs in certain areas. In the present paper it is shown that UVs determine a reversion of storage deformation at the temperature range of direct and reverse transformation of TiNi. UVs allow forming small thermal–mechanical hysteresis loops in the temperature range M s– M f. Besides UVs can initiate SME in full measure. At the same time the austenite finish temperature A f is reduced. On the whole the SME initiation with the help of UVs is caused by thermal as well as power effects.

Effect of aluminum addition on the transformation of NiTi alloy

The transformation behavior of ternary NiTi(Al) alloy was investigated with a differential scanning calorimeter in order to clarify the effect of aluminum substitution in NiTi, showing the shape memory due to the thermoelastic martensitic transformation between a high temperature phase and a low temperature phase. An intermediate phase appears due to aluminum substitution and its temperature region is broadening with increasing number of thermal cycles. By replacing titanium by aluminum, a unique behavior is observed in the transformation from the intermediate phase to the low temperature phase. The transformation behavior of NiTi(Al) depends significantly on the concentration of the substituted aluminum. The temperature region of the intermediate phase is controlled by the substituted aluminum. The shape memory effect also occurred in the transformation between the high temperature and the intermediate phases. Therefore, the substitution of aluminum is useful for various applications of the transformation to the intermediate phase.

Predicting in vivo failure of pseudoelastic NiTi devices under low cycle, high amplitude fatigue.

Due to the large reversible strains achievable through the stress-induced austenite-martensite phase transformation in NiTi alloys, NiTi has replaced stainless steel in the majority of large-strain biomedical applications such as root canal enlargement. However, the pseudoelasticity of NiTi is currently overshadowed by the short fatigue life of NiTi wires used in this low cycle (200-2000 rpm), high amplitude (epsilon(a) > 2.5%) application, resulting in in vivo fracture or premature retirement of otherwise reusable NiTi-based wire devices. In this study, the failure of pseudoelastic 55.8 wt % Ni-Ti wire is investigated experimentally, as a function of experimental parameters that include the clinically relevant regime. The effects of radius of curvature, angle of curvature, wire diameter, strain amplitude, cyclic frequency, volume under strain, and specific heat of the surrounding environmental fluid are considered systematically. These data indicate that the lifetime or cycles to failure N(f) of a rotating NiTi wire can be predicted via a modified Coffin-Manson relation that is a strong function of both strain amplitude and volume under strain, and a weaker function of frequency and fluid specific heat. The resulting quantitative relation can be used to predict useful device lifetime under clinically relevant conditions and thereby reduce incidences of in vivo failure.

Effect of dislocation inhomogeneity on the martensitic transformation in NiTi alloys

The reversible martensite transformation in NiTi alloys subjected to a deformation with a degree e=10% and subsequent annealing at 300–700°C is studied by methods of internal friction using a relaxation oscillator operating in the sound frequency range and of differential thermal analysis (DTA).

Two-Stage Martensitic Transformation in NiTi Alloys Caused by Stress Fields

It has been proved that the two-stage martensitic transformation in the NiTi alloys preceded by the R-phase transition my be induced by: low temperature annealing of the deformed alloy, thermal cycling and precipitation caused by ageing. The TEM observations of samples with two stage transformation have shown that the dislocations are distributed inhomogeneously and form a small-angle subgrain boundaries. The Granato-Lucke analysis of the internal friction peaks related to the two-stage transformation confirmed the difference in dislocation density released in both stages. Thus the two stage martensitic transformation is caused by the inhomogenity in the stress field which results from the dislocation configuration during the previous mentioned processes.

Effect of hydrostatic pressure on the thermoelastic transformation of Ni-Ti alloy and the entropy of transformation

Abstract The thermoelastic transformation temperature of a polycrystalline 50·22 at.% Ni-Ti alloy has been determined by measuring its electrical resistance over the temperature range 270K–370K and hydrostatic pressures up to 250 bar. The electrical resistance reaches a peak value at 50% transformation during both cooling and heating of the alloy. The volume change at the parent ↔ martensite phase transformation was measured by dilatometry, and the various energy and entropy changes at the transformation were determined by calor-imetry. The transformation temperature increased with the hydrostatic pressure at the rate of 7 ± 2Kkbar−1, the volume decreased by 0·25 ± 0·05 mlmol−1 and the chemical or structural entropy by 3·6±0·7 J mol−1K−1. Analysis of the calorimetric data gave an entropy change of 3·1 J mol−1 K−1. The contribution from the elastic strain energy is 64Jmor' and from the frictional energy is 62J mol−1. The latent heat of transformation is 995 J mol−1. The change in the entropy at the transfo...

Effect of grain size on the martensitic transformation in NiTi alloy

The effect of grain size on the martensitic transformation in Ni42Ti shape memory alloy has been studied. The kinetics of grain growth has been evaluated and the influence of different grain sizes on the transformation temperatures and the thermodynamic magnitudes has been reported. Image analysis and flow calorimetry techniques have been used. The study shows that grain boundaries favour the martensitic transformation and at the same time obstruct retransformation. Enthalpy and entropy variations are independent of grain size, but elastic energy decreases with the grain size.

Transformation characteristics of aged NiTi shape memory alloy obtained by rapid solidification

Presently, the R and martensitic transformation of TiNi alloy can be separated by aging or adding a third element. It is generally accepted that in the cooling process the TiNi alloy experiences B2 (parent phase) [r arrow] IC (Incommensurate) [r arrow] R (Rhombohedral) [r arrow] M (Martensite), in which B2 [r arrow] IC transformation is second order while IC [r arrow] R is first order. However, other analyses using the charge density wave (CDW) model and Landau's theory have deduced that the B2 [r arrow] IC is also a first order phase change. But it has not been experimentally confirmed yet. The present study aims at investigating the three transformations in rapidly solidified TiNi alloy.

Effects of hot working on the martensitic transformation of NiTi alloy

Among shape memory alloys, Ni-Ti alloys of near-equiatomic composition have been used for various commercial applications, since they have good mechanical properties and well-known shape memory effects. Processing disadvantages, however, include the difficulties and expenses of processing. Moreover, cold working to high reduction is difficult and expensive because limited cold workability and high work hardening, makes hot working indispensable. Therefore, optimization of hot working parameters and control of shape memory behavior is necessary. It is generally known that the influence of work hardening and subsequent isothermal annealing on the stability of the individual phases formed in Ni-Ti shape memory alloys has been investigated. Work hardening promotes the formation of the rhombohedral R phase. The matrix work hardening suppresses the phase transformation B2[leftrightarrow]B19[prime]. In this study, the equiatomic Ni-Ti alloy is plastic deformed by hot compression at various temperatures. The effects of hot compression on the martensitic transformation are investigated with differential scanning calorimeter measurements and hardness tests. The optimal hot working temperature range is also discussed.

The shape memory mechanism associated with the martensitic transformation in TiNi alloys—I. Self-accommodation

The martensitic transformation in TiNi alloys occurs by forming a typical triangular morphology consisting of three variants. By trace analysis of the three sides (habit planes) of the triangles in TiNi single crystals, it was found that these three variants form plate groups clustered about one of the {100} B2 poles. There are three junction planes between variants in the triangle, and they were also determined by trace analysis. The latter correspond to one of the (001) M Type I twinning planes and two of the 〈011〉 M Type II twinning planes. The calculated shape-strain matrix for the overall triangular morphology showed that the shear components of the shape-strain, although non-zero, are numerically quite small. The above observations and calculations support that the triangular morphology satisfies the self-accommodating conditions from both the macroscopic strain and microscopic crystallographic points of view. There are 16 groupings which form such a triangular self-accommodating morphology about each of the {100} B2 poles, resulting in 48 possible combinations in all to form such triangles.

The shape memory mechanism associated with the martensitic transformation in TiNi alloys—II. Variant coalescence and shape recovery

An in situ investigation of the shape memory effect was carried out using a TiNi single crystal with a specially designed tensile machine in conjunction with an optical microscope. Each variant in the thermally induced self-accommodating triangular martensite morphology, consisting of three variants, was characterized by determining the habit plane using two surface or one surface trace analysis. Upon loading, the triangular morphology changed into the most favorable variant with respect to the tensile axis by the movement of the interfaces between the variants. A model for variant coalescence upon loading is presented based on the observations made. Upon heating, the martensite phase reverse-transformed by the movement of the interface between the martensite and the R-phase, and finally the R-phase reverse-transformed to the B2 parent phase, these processes resulting in a perfect shape recovery.

Calorimetry and fractal analysis of stepwise martensitic transformation of NiTi alloys.

The stepwise character of the thermoelastic martensitic transformation in NiTi alloys is investigated in terms of the enthalpy changes connected with the transfomation. Differential scanning calorimetry has ben adopted to evidence the discontinuous heat flow developed during both the direct and reverse martensitic transformation. The results are examined at the light of a simplified fractal model advanced to describe the self-similarity of martensitic microstructures.

Internal friction transitory effects associated with martensitic transformation in NiTi alloys

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