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.     

First-principles study of martensitic phase transformation of TiRh alloy

Geometric structures and atomic positions were studied with plane wave pseudo-potential method based on density functional theory for cubic, tetragonal, and monoclinic phases of TiRh alloy. Their phonon dispersion curves were obtained with frozen phonon method at harmonic approximation using density-functional perturbation theory. Our calculations revealed that both B2 and L1₀ phases are thermodynamically unstable. Jahn-Teller effect triggers the occurrence of Bain transformation from cubic to tetragonal phase, and then soft-mode phonon further leads to the transition from tetragonal to monoclinic phase on cooling. The monoclinic phase was predicted to be P2/m space group through atomic vibrational movement along [001] direction of virtual frequency modes of L1₀ phase. The temperature from B2 to L1₀ and then to P2/m were predicted to be about T=1100.53 K and T=324.48 K through free energy calculations with the electronic plus vibrational energy of formation, respectively, which is in good agreement with experimental observations.     

Mössbauer study of the martensitic transformation in a Ni–Fe–Ga shape memory alloy

We present the results of an extensive Mössbauer study of the magnetic and martensitic transformation at room temperature of a polycrystalline alloy with a Ni₅₅Fe₁₉Ga₂₆ nominal composition. From calorimetric measurements, we have determined the martensitic transformation temperature of T _M ≈ 240 K, in good agreement with the one obtained by magnetic characterization. This sample has a Curie temperature of T _C ≈ 287 K. Additional Curie temperatures, belonging to a γ phase, have been also detected. Mössbauer spectroscopy performed at different temperatures monitored all these transformations and the fitting of the obtained spectrum at the highest temperature allow us to give percentages of the different phases in the sample.     

Diffuse neutron scattering from an ω-phase ZrNb alloy

A back-scattering neutron spectrometer with an energy resolution of ～2μeV (FWHM) has been used to measure the diffuse quasielastic scattering from the ω-phase of Zr_1-xNb_x (0.18 < x < 0.19). It is found that the intrinsic energy-width of the scattering does not change between room temperature and 1000 K. At both temperatures the width is less than ～0.2μeV and consistent with zero.     

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.     

Motion picture x-ray diffraction investigation of the martensitic transformation in nickel-titanium alloy

The thermoelastic martensitic transformation of an equiatomic nickel-titanium alloy is investigated by means of x-ray diffraction motion pictures using synchrotron radiation. The x-ray patterns are recorded by a parallel detector simultaneously in the range of angles 2=34°–51° at a wavelength of 1.4879 Å during heating and cooling with good temperature resolution. The basic parameters of the attendant B2, R, and B19 phases are determined simultaneously and compared. The variations of the parameters of the martensite B19 unit cell are traced, along with the evolution of the R phase {202}{202} reflection doublet during transformation. Volumetric transformation phenomena are determined, and it is shown that martensite originates in a more unconsolidated structure than in the equilibrium state.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–54, August, 1994.     

Neutron scattering by spin fluctuations in dilute Al-Mn alloy

Neutron scattering experiments were carried out on Al(0.5%Mn) alloy at T = 80°K and 300°K. Effects of spin fluctuation and conduction electron polarization on the incoherent magnetic scattering cross section were observed.     

Crystallographic analysis of martensitic transformation in an iron-nickel alloy with twinned martensite

A variant of the crystallographic theory of martensitic transformations is proposed, based on a mechanism of lattice deformation in which the angle of rotation of a martensite plate is reduced to a minimum. In an iron-nickel alloy with twinned martensite, the least angle of rotation corresponds to the deformation of the austenite lattice by shear on the (111) plane in the $\left[ {11\bar 2} \right]$ direction proposed by Kurdyumov and Sachs as the first shear in the two-shear theory of martensite formation in steel.     

Heusler合金Ni_2MnGa马氏体相变微观电子结构机理的研究

采用基于密度泛函理论的第一性原理框架下的投影扩充波函数(PAW)方法,研究了Heusler合金Ni2Mn Ga马氏体相变的微观电子结构机理.结果表明:Ni2Mn Ga发生马氏体相变时,其晶体发生了自正八面体结构朝z轴拉长型八面体结构的畸变.此畸变源自于由Ni的eg轨道电子的非均匀排布所引发的强带Jahn–Teller效应,其发生的机制是:通过孤对电子由dx2-y2轨道转移到dz2轨道,导致x、y方向的配体内移,z方向的配体外移,从而使dx2-y2、dxy能级升高,及dz2、dyz、dxz能级降低,消除和降低能级的简并度,体系能量降低,体系畸变到一个更稳定的状态(马氏体相).     

Hg2CuTi型Heusler合金Mn2NiB马氏体相变微观机理的研究

运用基于密度泛函理论的第一性原理方法,研究了Hg2 CuTi型Heusler合金Mn2 NiB马氏体相变的微观机理。研究表明：伴随着Mn2 NiB马氏体相变的发生,产生了拉长型八面体的畸变。此畸变先是由中心离子Mn的t2g轨道电子的非均匀占据而导致的弱Jahn－Teller效应来驱动。随着畸变的进行, Mn离子的孤对电子由dx2－y2轨道转移到dxy轨道, x、y方向的配体受到的推斥力减小而内移,进一步加剧这一畸变过程,使得dx2－y2和dxy轨道受到配体的推斥力增加,能级升高,而dz2、dyz和dxz受到的推斥力减少,能级降低,从而消除eg 能级和降低t2g能级的简并,体系能量降低,最终体系畸变到一个稳定的马氏体相。     

Neutron spin-wave scattering from a Heusler alloy Pd2MnSn

Spin-wave dispersion relations have been determined in three principal symmetry directions for Heusler-type alloy Pd₂MnSn. Analysis shows that the exchange parameters at large distances have an oscillatory character which can be interpreted in terms of the s-d interaction model. However the n.n and n.n.n. interactions have the opposite sign to that expected from the model and should be attributed to other mechanisms.     

Hg2CuTi型Heusler合金Mn2NiB马氏体相变微观机理的研究

运用基于密度泛函理论的第一性原理方法，研究了Hg2CuTi型Heusler合金Mn2NiB马氏体相变的微观机理．研究表明：伴随着Mn2NiB马氏体相变的发生，产生了拉长型八面体的畸变．此畸变先是由中心离子Mn的 轨道电子的非均匀占据而导致的弱Jahn–Teller效应来驱动．随着畸变的进行，Mn离子的孤对电子由 轨道转移到 轨道， 方向的配体受到的推斥力减小而内移，进一步加剧这一畸变过程，使得 和 轨道受到配体的推斥力增加，能级升高，而 、 和 受到的推斥力减少，能级降低，从而消除 能级和降低 能级的简并，体系能量降低，最终体系畸变到一个稳定的马氏体相．     

Study of stabilization of CuAlBe alloy during martensitic transformation by internal friction

In this study, the internal friction (IF) of a CuAlBe alloy during reverse martensitic transformation (MT) was investigated under isothermal and non-isothermal conditions. It was found that the IF decreases regularly with the aging time in the temperature range of phase transformation. It is thought that this phenomenon is caused by the defect movement. The defects diffuse to the interfaces between martensite and austenite and pin there gradually, thus leading to the IF decrease.     

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.