Influence of aging on the temperature of the beginning of a martensitic transformation in the intermetallide TiNi

The influence of aging at 250°C on the temperature of the beginning of a martensitic transformation in the alloy Ti + 51 at.% Ni is investigated after tempering from 600 and 800°C, as well as after preliminary thermal cycling. The temperature dependence of the electrical resistivity, x-ray diffraction analysis, and low-angle neutron scattering (LNS) were used as investigation methods. It is detected that M_s varies along the curve with a minimum which is at 1.5 h of annealing. The total effect of reducing M_s threefold exceeds the influence of thermal cycling. The LNS is observed 1.5 h after annealing, which indicates the origination of concentration inhomogeneities in the alloy. The M_s reduction stage is related to formation of zones with insignificant disturbances of the crystalline lattice of the matrix, and the stage of its magnification is related to liberation of particles of new nickelenriched phase.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 11–16, October, 1982.     

Lattice instability and martensitic transformation in LaAg predicted from first-principles theory

The electronic structure, elastic constants and lattice dynamics of the B(2) type intermetallic compound LaAg are studied by means of density functional theory calculations with the generalized gradient approximation for exchange and correlation. The calculated equilibrium properties and elastic constants agree well with available experimental data. From the ratio between the bulk and shear moduli, LaAg is found to be ductile, which is unusual for B(2) type intermetallics. The computed band structure shows a dominant contribution from La 5d states near the Fermi level. The phonon dispersion relations, calculated using density functional perturbation theory, are in good agreement with available inelastic neutron scattering data. Under pressure, the phonon dispersions develop imaginary frequencies, starting at around 2.3 GPa, in good accordance with the martensitic instability observed above 3.4 GPa. By structural optimization the high pressure phase is identified as orthorhombic B(19).     

Cu掺杂对TiNi合金马氏体相变路径影响的第一性原理研究

不同浓度的Cu元素掺杂会极大地影响TiNi二元合金的物理性质和相变行为.为了解释其中的物理机制,本文通过第一性原理计算,对TiNi和Ti_(50)Ni_(25)Cu_(25)的相变机制和相稳定性进行了计算和讨论.通过计算Cu掺杂前后立方相到正交相、再到单斜相过程中的相变路径和相变势垒,解释了Cu掺杂对二元合金TiNi相变过程的影响.计算结果表明:TiNi合金的正交相和单斜相之间存在一个大小为1.6meV的相变势垒;而对于Ti_(50)Ni_(25)Cu_(25),这两个相之间的相变势垒大小至少为10.3meV,如此大的一个相变势垒意味着Ti_(50)Ni_(25)Cu_(25)合金的正交相很难跨过势垒相变到单斜相.     

The ultrasonic equation of state in the vicinity of the martensitic transformation in Au-47.5 at.% Cd

The isotropic elastic constants of Au-47.5 at.% Cd, and their hydrostatic pressure derivatives, within the temperature interval (10–95°C) of the thermoelastic martensitic transformation, were determined. From the experimental data, using ultrasonic techniques at hydrostatic pressures (up to 5 kbar), the temperature dependence of the isothermal equation of state of the cubic β-phase was calculated. It was found that the low-temperature orthorhombic β′-phase is elastically softer than the cubic β-phase, consequently affecting the β&larr2;gb′ transformation kinetics upon application of hydrostatic pressure. The high values of the Grüneisen parameter, calculated from the pressure derivatives of the sound velocities, in the orthorhombic β′-phase are indicative of a high anharmonicity of interatomic potential in the vicinity of the β′?β phase transformation.     

动态压缩下马氏体相变力学性质的微观研究

使用分子动力学方法,模拟了活塞以恒定加速运动从一端压缩单晶铁（沿［001］晶向）发生马氏体相变的微观过程.根据模拟结果将上述压缩过程分为弹性压缩、晶格软化、相变（bcc至hcp）、超应力松弛和高压相弹性压缩五个阶段,对各阶段的原子滑移规律和应力变化特征做了详细分析.分析得出应力超过约10 GPa时,开始出现弹性常数软化行为;层错结构（fcc）和孪晶界为新相形核的两种缺陷,前者更为稳定;相变后粒子首先进入超应力松弛状态（即沿加载方向的偏应力呈现负值）,在应力超过约36 GPa粒子转变为高压相弹性压缩状态. 关键词： 分子动力学 单晶铁 相变 动态压缩     

The elastic behaviour of In-Tl alloys in the vicinity of the martensitic transition

To test further the prediction of collapse of the [110], q|| [11̄0] acoustic mode at the martensitic phase transition, pulse superposition measurements of ultrasonic wave velocities have been made in indium-thallium alloys containing 25 and 27 at. % thallium. These alloys are f.c.c. at room temperature and transform on cooling to the f.c.t. phase at 196 ± 2°K and 127 ± 2°K, respectively. Results show that $\text{1}\text{2}$(C₁₁ ? C₁₂) goes to zero within experimental error at the transition temperature. The room temperature elastic constants of the tetragonal 11.5 and 15 at. % thallium alloys are also reported.     

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.     

Kinetics of radiation damage and martensitic transformations in TiNi alloys irradiated with neutrons

The variation of the temperatures of martensitic transformations and the rate of radiation damage in TiNi alloys were studied upon irradiation with reactor neutrons. The irradiation was performed at temperatures of 120 and 335 K. In the process of irradiation, electrical resistance of the alloys was measured continuously and thermal cycling through the temperature range of martensitic transformations was carried out. The transformation temperatures were shown to decrease at different rates with increasing irradiation fluence. The electrical resistance increases linearly with increasing neutron fluence to 6.7×10¹⁸ cm^?2 irrespective of the irradiation temperature. Deviation from a linear dependence is only observed when the irradiation leads to a change in the phase state of the alloy. The rate of the resistance increase only slightly depends on the irradiation temperature. In martensite, it is greater by a factor of 2–4 than that in austenite. Mechanisms of irradiation-induced modification of the structure of TiNi alloys that explain the experimental data obtained are discussed.     

CoNiZ系列合金的结构和马氏体相变性质

利用X射线衍射研究了CoNiZ(Z=Si,Sb,Sn,Ga等)合金在不同热处理条件下的相组成.当Z元素为Sn,Sb时,材料是完全的B2结构;但Z为Si时,材料变成面心立方的γ相.形成B2还是γ相由电子浓度和原子尺寸效应两种因素共同决定.而CoNiGa的研究结果表明,在合金中除了形成B2结构的同时还容易形成γ相,常表现出两相共存的特性.对材料进行不同方式的热处理可以使合金中两相的含量有所消长,γ相含量的多少对CoNiGa合金的马氏体相变有很大的影响.分析指出,两相共存及其所带来的物性变化是CoNiGa铁磁性形状记忆合金非常有利用价值的物理性质.     

Influence of neutron irradiation on the martensitic transformations and shape-memory effect in a TiNi alloy

The paper reports measurements of the strains and electrical resistance of a TiNi shape-memory alloy under irradiation in the low-temperature helium circuit of a nuclear reactor. Irradiation of the alloy in martensitic state at 170 K revealed that the transition temperatures from cubic to rhombohedral and from rhombohedral to monoclinic phase decrease exponentially with increasing dose. No change in the shape-memory effects and transformation plasticity was observed up to a dose of 6.7×10²² n/m². Keeping the sample at 340 K without irradiation restores (increases) partially the transition temperatures. The relations observed can be assigned to a change in the degree of long-range order in the lattice caused by neutron irradiation. Fiz. Tverd. Tela (St. Petersburg) 40, 1705–1709 (September 1998)     

On the entropic nucleation barrier in a martensitic transformation

The nucleation of martensite in alloys is hindered by a free energy nucleation barrier, hence comprising contributions of the potential energy and the entropy. The leading effect is commonly attributed to the potential energy barrier due to strain fields. In this contribution, we investigate the nature of the entropic barrier by means of molecular dynamics (MD) simulations. We study a transformation process of an undercooled single crystal and examine two nucleation events observed under adiabatic conditions using vibrational mode analysis of the atomic trajectories. Our analysis shows that martensitic nucleations are indicated by transit from a state of uncorrelated into a state of correlated atomic motions. This correlation process is built up locally by a small group of atoms even before the product lattice can be recognized morphologically and it produces vibrational ‘soft’ modes along transformation paths. Phase space analyses unveil that the correlation process is characterized by narrow domains – ‘nucleation channels’ – the atomic trajectories have to pass, connecting the phase space domains of the parent and the product lattice. For a successful nucleation event, the nucleus atoms have to pass this channel collectively, which stochastically represents a rare event. Thermal fluctuations prevent finding the channel at elevated temperature and give rise for entropic stabilization of the parent phase. This ‘entropic nucleation barrier’ is reduced in the undercooled state but still effective, thus preventing the parent phase from collapsing into the product. The entropic barrier may be interpreted as the probability of a group of atoms to simultaneously pass the nucleation channel. Such group then represents a nucleus.     

Energy conversion in the Carnot cycle during martensitic transformation

A Carnot cycle is proposed for a working material undergoing martensitic transformation while operating a finite temperature interval. The size of the interval depends on the thermodynamic characteristics of the transformation. Diagrams are presented to decribe the deformation of titanium under adiabatic and isothermal conditions. The diagrams confirm the possibility of realization of the transformation. A comparison is made with other known cycles. It is shown that transformation hysteresis diminishes the efficiency of the actual cycle.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 91–94, February, 1989.     

Physical properties of polycrystalline Ni52Mn26Al22 around martensitic transformation

Electrical resistivity and magnetic properties on Ni₅₂Mn₂₆Al₂₂ polycrystalline Heusler alloy are investigated in detail. The progress of transformation of this alloy was also followed by heat flux measurements. The transformations were found to be broad and hysteretic. Using Deng & Ansell’s phenomenological theory, the energy consumed for phase boundary motion in this polycrystalline alloy was found to be (31.5 J/mol). The broad transition as compared to Ni–Mn–Ga observed in this polycrystalline alloy, is discussed in light of a phenomenological model.     

Modelling of dislocation-induced martensitic transformation in anisotropic crystals

The structural transformation caused by dislocation-induced heterogeneous nucleation in the fcc?→?bcc martensitic transformation in elastically anisotropic crystals is investigated by using the phase field microelasticity model. The three-dimensional microstructure of the dislocation-induced martensitic embryos is obtained. It is found that the embryos are not single-domain particles as is usually assumed but rather a complex self-organized assemblage of stress-accommodating twin-related microdomains. Sessile metastable martensitic embryos around the dislocation loops form in the prototype Fe–Ni alloy system above the temperatures of the martensitic transformation. A possibility that the presence of these pre-existing embryos could be responsible, at least, for a part of the elastic modulus softening with the temperature decrease observed in many martensitic systems is discussed. The effects of elastic anisotropy, the “chemical” energy barrier and structural anisotropy of the Landau free energy on the formation and growth of martensitic embryos are investigated. The assumptions of elastic isotropy and a choice of the anisotropic term in Landau polynomial do not significantly affect the microstructure of martensitic embryos but may appreciably change the undercooling that is necessary to eliminate the total nucleation barrier and start the athermal martensitic transformation.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Power-law statistics for avalanches in a martensitic transformation

We devise a two-dimensional model that mimics the recently observed power-law distributions for the amplitudes and durations of the acoustic emission signals observed during martensitic transformation [Vives et al., Phys. Rev. Lett. 72, 1694 (1994)]. We include a threshold mechanism, long-range interaction between the transformed domains, inertial effects, and dissipation arising due to the motion of the interface. The model exhibits thermal hysteresis and, more importantly, it shows that the energy is released in the form of avalanches with power-law distributions for their amplitudes and durations. Computer simulations also reveal morphological features similar to those observed in real systems.     

Acoustic emission and shape memory effect in the martensitic transformation

Acoustic emission signals are known to exhibit a high degree of reproducibility in time and show correlations with the growth and shrinkage of martensite domains when athermal martensites are subjected to repeated thermal cycling in a restricted temperature range. We show that a recently introduced two dimensional model for the martensitic transformation mimics these features. We also show that these features are related to the shape memory effect where near full reversal of morphological features are seen under these thermal cycling conditions.     

Fractal dimension of the surface upon martensitic transformation in titanium nickelide

The fractal dimension of the residual deformation relief on the surface of titanium nickelide is shown to be sensitive to the deformation mechanism. It decreases under phase-transformation-induced inelasticity and increases under plasticity. At the time the basic deformation mechanism changes, fluctuations are maximal. The experimental dependence of the microstrain on the fractal dimension for the initial austenitic phase is not a single-valued function: it consists of two groups of data points that correspond to different deformation mechanisms.     

First-principles study of martensitic transformation of IrTi alloy

Ab initio phonon calculations were performed to probe the martensitic transformation of IrTi. The details of the orthorhombic structure were obtained by the soft-phonon approach. We demonstrate that the tetragonal orthorhombic (Cmmm) transition is driven by the softening of a phonon at the R point (0 ) of the Brillouin zone. The energy landscapes between the various phases of IrTi show that the structural behaviors of IrTi alloy are from cubic to tetragonal, then to orthorhombic and thus the original thought of cubic to monoclinic transition is modified.