Transformation In TiNi Research Articles

Effect of prestrain on tensile property of TiNif/Mg composite

ABSTRACTA TiNi shape memory alloy fibre-reinforced AZ31 Mg alloy matrix composite was fabricated herein by spark plasma sintering. The microstructure of the composite and the effect of prestrain on its strength were analysed. The phase transformation of TiNi was observed in situ, and the effect of temperature was monitored. Metallurgical bonding formed at the interface between the TiNi fibre and AZ31 matrix. The prestrain strengthening effect on the yield stress of the composite at 298 K was more dramatic than at 373 K, and the effect decreased with increasing prestrain. The prestrain strengthening effect on tensile strength of the composite was effective at both 298 and 373 K. Because of the difference between recovery stress of TiNi and prestrain-induced compressive stress, higher temperature strengthened the material more significantly than did prestrain application.

Features of the Internal Friction in the Temperature Range of Martensitic Transformation in TiNi

The influence of the external parameters (deformation amplitude and oscillation frequency) on the components of the internal friction peak during a thermoelastic martensitic transformation in TiNi has been considered. Models describing the behavior of the transition components of the IF peak have been analyzed. On the basis of the best model experimental dependence of the transformed volume fraction on temperature has been established.

Amorphization of TiNi induced by high-pressure torsion

Using high-resolution electron microscopy, we investigated the initial stages of high-pressure-torsion-induced crystalline-to-amorphous transformation of TiNi. It is found that the deformation-induced amorphization initiated from dislocation core regions in the interior of grains and from grain boundaries. It is believed that both the energy stored in the dislocations and the energy stored in the grain boundaries contribute significantly to driving the crystalline-to-amorphous transformation.

In situ study of stress-induced martensitic transformation in TiNi

By means of synchrotron X-ray diffraction method the stress-induced martensite transformation in TiNi (with two different phase compositions) at room temperature was investigated in situ. It has been shown that in alloys with temperature-induced martensite in the initial state a non-periodic fluctuation of intensity of the X-ray reflexes appear due to anomalous transformation B2+B19′ T→B2→B19′ SI.

Time–dependent nature and origin of displacive transformation

We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni and Fe–Ni–Cr alloys under pulsed and static magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. Also, we have calculated electronic structures of B2 and ζ′2 phases in AuCd by FLAPW and/or LAPW methods in order to understand the origin of B2–ζ′2 transformation. The following results were obtained. (i) The two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process under a hydrostatic pressure and the isothermal process changes to the athermal one under a magnetic field. (ii) These findings of (i) can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group. (iii) The calculation of the generalized susceptibility, x(q), for the B2 phase of AuCd shows that there exists a nesting vector of near 1/3<110>2Π/a as in the B2 phase of TiNi calculated previously. The density of states at the Fermi energy of the ζ′2 phase is lower than that of the B2 phase, which is similar to the case of B2–R transformation in TiNi previously calculated.

In-Situ Investigations of the Martensitic Transformation in TiNi by Synchrotron Radiation

By means of synchrotron X-ray diffraction method the stress-induced martensite transformation in TiNi (with two different phase compositions) at room temperature was investigated in situ. It has been shown that in the alloys with temperature-induced martensite in the initial state a nonperiodic fluctuation of intensity of the X-ray reflexes appeared due to anomalous transformation B2 + B19′T → B2 → B19′Si.

Phase-Fraction Evolution During Incomplete Cyclic Transformation in TiNi: Correlation of Analytical Models with Magnetic Susceptibility Measurements

The response of titanium-nickel, and other alloys exhibiting thermoelastic martensite transformations, display phase-coexistence and hysteresis over a wide range of temperature, arising from the fourfold splitting of a single transformation temperature into M f , M s , A s , and A f . Attempts to precisely characterize the evolution of phase-fraction during temperature reversals that involve incomplete transformation has led to the formulation of various hysteresis descriptions. We examine a description based on the Duhem-Madelung framework, which takes complete transformation behavior as constitutive input, and on this basis formally predicts the hysteresis associated with transformation arrest and reversal. In conjunction with this effort, a series of magnetic susceptibility measurements on TiNi has been conducted using a SQUID susceptometer. The measured hysteresis in the temperature/susceptibility curves are qualitatively described by the Duhem-Madelung model.

An electron-microscopical investigation on the martensitic transformation in TiNi

An electron-microscopical investigation of the morphology of equiatomic TiNi martensite has revealed at least two types of martensite: the twinned plate martensite and a wavy martensite. To our knowledge the latter has never been reported in the literature before. In a thin foil, the plate martensite can be transformed into the wavy martensite within a few weeks at ambient temperature or by temperature cycling between +100° C and liquid nitrogen temperature.

Soft Phonon Modes and the Propagation Velocity of Martensite

THE interpretation of martensitic phase transformations remains a central problem in physical metallurgy. Studies of lattice properties such as elastic and anelastic behaviour, thermal expansion and thermal conductivity have resulted in knowledge of the way in which such transformations proceed at the atomic level1–8. In particular, the martensitic phase changes in Au–Cd (ref. 1), In–T1 (refs. 2 and 7) and TiNi (refs. 5 and 6) are preceded by “softening” of certain elastic coefficients and the transitions result from the development of lattice instability evidenced as a soft acoustic mode; as the transition temperature is approached and the incipient mechanical instability rises, certain lattice vibration modes undergo a considerable energy decrease and as their frequency falls the wavelength increases and the interatomic binding forces are decreased. Ultimately, the vibration amplitude and anharmonicity are so large that the atoms adopt new sites. On the basis of this model properties which depend on the lattice vibration spectrum are expected to exhibit marked anomalies in the vicinity of the transformation. One property directly manifesting anharmonicity of lattice vibrations is the thermal expansion; a pronounced peak in the thermal expansion coefficient occurs centred around the martensitic transformation in TiNi (ref. 6), the attendant anharmonicity being reflected in the Gruneisen parameter which rises from the normal background value of about 2 to more than 40 in the transition region.

Homogeneity range and the martensitic transformation in TiNi

It is shown that the structure homogeneity range of the B2 TiNi compound is very restricted below 500°C. A new phase, containing 58 at. pct Ni and structurally related to Ni3Ti, forms peritectoidally at 625° ± 20°C. The precipitation in nickel-rich TiNi is very slow, and a metastable, supersaturated B2 structure can be readily retained. At equilibrium the martensitic transformation takes place at ∼60°C, independent of the overall composition. Quenching from 600°C and above raises the transition temperature in titanium-rich, and lowers it markedly in nickel-rich compositions. The resulting martensite structure in the latter is also believed to change with increasing the prior anneal temperature. The observations reported previously on the transformation behavior must be considered as obtained on nonequilibrium, metastable materials, which may account for the discrepancies in the observed property variation. Close control of the processing variables is necessary to ensure reproducibility, particularly if nonequilibrium transformation behavior is desired.

The effects of applied stress on the martensitic transformation in TiNi

The deformation behavior of TiNi at 20°C has been investigated as a function of composition and of the prior heat treatment. Wide mechanical property variation and significant differences between the effects of tensile and of compressive loading were observed. Under some conditions anelastic behavior, characterized by a broad hysteresis loop, was reproducibly obtained. The effects of heating the deformed materials aboveA f on the subsequent stress-strain behavior indicate the anomalies observed to be directly related to the martensitic transformation. The effects of stress application on the martensitic transformation are discussed. It is shown that, under some conditions, the stress-assisted transformation structures may be unstable on the removal of the stress. At test temperatures outside theM f toA f range, this can result in anelastic behavior. More complex behavior expected at temperaturesM f < T < Af, is discussed in some detail. It is shown that both the anelastic behavior and the “shape memory” can be accounted for by the effects of applied stress. It is also shown that the mechanical properties in this temperature range can vary markedly with the prior heat treatment, even at temperatures not normally considered of significance. Though based on observations made on TiNi, the phenomena discussed are inherent in materials undergoing a martensitic transformation over a narrow range of temperatures.

The observation of surface relief during the martensitic transformation in TiNi

The martensite transformation in TiNi occurs athermally in the vicinity of room temperature. The surface relief produced during the transformation was observed by light and scanning electron microscopy. The role of repeated cycling through the transformation on the martensite start temperature and distribution of martensite plates was determined.

Characteristics of the martensitic transformation in TiNi and the memory effect

The process of the martensitic transformation in a Ti-49.75 at. pet Ni alloy has been observed continuously by optical microscopy, and it was found that the transformation proceeds thermoelastically. The transformation has been determined to be first order by examining the change in lattice discontinuity upon transformation. Based on the present and previous results, the origin of the memory effect is discussed.

Lattice properties in the vicinity of the martensitic transformation in TiNi

A pronounced peak in the thermal expansion coefficient — and thus in the Grüneisen parameter — has been found at the temperature corresponding to the martensitic transformation in TiNi. This finding, together with results of ultrasound wave propagation studies, attests to anomalous lattice-dynamical behaviour in the vicinity of the phase transition. Ultrasonic wave attenuation and velocity data are interpreted on the basis of Akhieser-type, phonon viscosity damping. Results obtained are consistent with the hypothesis that a soft phonon mode plays a dominant role in the lattice properties of TiNi near the transition.