The electrical resistance of some lithium-magnesium alloys near the martensitic transformation

The electrical resistivity of four lithium-magnesium alloys with magnesium concentrations of 1.0, 5.0, 10 and 20 atomic percent has been measured as the specimens were slowly cooled from 300 K to ～3 K and warmed again to 300 K. From a hysteresis in the resistivity in the region of the martensitic transformation the M_s temperatures were found. M_s increases ～5 K per atomic percent of added magnesium and, while in agreement with Dugdale and Gugan the martensite in the dilute alloy is less resistive than the high temperature (bcc) phase, it has a larger resistivity in three less dilute alloys.     

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni-Mn-Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using ⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni-Mn-Ga-Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.     

Fe-Ni合金马氏体相变缺陷的正电子湮没研究

本文中用正电子湮没技术研究了Fe-Ni系合金的马氏体相变。实验结果表明,含镍量28.22—31.30wt％的六种退火态合金,正相变后,产生了大量的缺陷,使正电子湮没平均寿命及多普勒加宽线型参数S值分别约增加30％和20％。对28.93wt％Ni和31.30wt％Ni两种马氏体亚结构不同的合金,观测了正电子湮没参数与处理温度之间的关系,发现:缺陷主要产生在爆发马氏体形成阶段;在等时退火曲线上形成两个明显的台阶,是由于空位和位错恢复引起的。 关键词：     

Martensitic transformations in some ferrous and non-ferrous alloys under magnetic field and hydrostatic pressure

Martensitic transformations are extensively influenced by external fields, such as temperature and uniaxial stress, in transformation temperatures, crystallography and amount and morphology of the product martensites. Therefore, to clarify the effect of external fields on martensitic transformations it is very important to understand the essential problems of the transformation, such as thermodynamics, kinetics and the origin of the transformation, whose information is naturally useful in technological applications using the transformation. Magnetic field and hydrostatic pressure are important in such external fields because there exist some significant differences in magnetic moment and atomic volume between the parent and martensitic states. In the present paper, therefore, we summarizz the effects of magnetic field and hydrostatic pressure on martensitic transfonnations in some ferrous and non-ferrous alloys by referring to past and recent works made by our group and many other researchers. The transformation start temperatures of all the ferrous alloys examined increase with increasing magnetic field, but those of non-ferrous alloys, such as Ti-Ni and Cu-Al-Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu-Al-Ni alloys. The magnetic field and hydrostatic pressure dependencies of the martensitic start temperature are in good agreement with those calculated by our proposed equations.

During investigations of ferrous Fe-Ni-Co-Ti shape memory alloy, we found that a magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in a specimen of Fe-Ni alloy single crystal.

We further found that the isothermal process in Fe-Ni-Mn alloy changes to athermal under a magnetic field and the athermal process changes to isothermal under hydrostatic pressure. Based on these facts, a phenomenological theory has been constructed, which unifies the two transformation processes.     

Transformation strains in martensitic phase transitions of Co alloys

The thermally induced fcc → dhcp martensitic phase transformation was investigated in two different CoFe alloys (Co-5.75 and 6.0at.% Fe). Analysis by transmission electron microscopy methods yields that in both alloys the transformation proceeds by the movement of transformation dislocations (partials) that are correlated on an atomic scale; partials on adjacent close packed planes interact and combine to a paired partial. Two different and competitive modes of the transformation were observed. In Co-6.0 at. % Fe all the paired partials have the same Shockley partial Burgers vector adding up their long-range strain fields (transformation mode A). Contrary to this, in Co-5.75 at.%Fe paired partials of different Burgers vectors are compensating their long-range strain fields on an atomic scale (transformation mode B). The mode of the transformation seems to depend on both the parameters of the material and the experimental conditions.     

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.     

替代掺杂的MnNiGe1-xGax合金中马氏体相变和磁-结构耦合特性

研究了MnNiGe_1-xGa_x (x=0–0.30) 系列合金中成分、结构、马氏体相变性质和磁性的相互关系. 在较小的成分范围内, Ga取代Ge元素可有效地将马氏体相变温度降低近400 K. Ga的引入削弱了体系中的共价成键作用, 马氏体相显示出磁交换作用的增强. 相图显示, 掺杂使马氏体相变先后穿过T_N 和T_C 两个磁有序温度, 居里温度窗口效应在体系有存在的可能, 磁性对相变温度的成分关系有所影响. 实验观察到合金变磁转变的特性及相变行为对制备方法的敏感性. 这些特性的发现, 有利于进一步优化这类材料的磁结构和相变特性, 获得具有应用价值的新材料. 关键词： MM’X合金 马氏体相变 磁有序温度 变磁转变     

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni₅₀Mn₃₅In_{15 ? x} Si _x (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220?C280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150?C200 ??? cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.     

Mn/Cr掺杂对Ni-Fe-Ga合金相变和磁性转变的影响

系统研究了Mn和Cr元素掺杂对Ni_(55)Fe_(18)Ga_(27)合金马氏体相变温度和居里温度的影响.研究表明:随着Mn含量的增加,Ni_(55-x)Mn_xFe_(18)Ga_(27)系列合金的马氏体相变温度逐渐降低,居里温度有所增加;Ni_(55)Fe_(18-x)Mn_xGa_(27)系列合金的马氏体相变温度也逐渐降低,但居里温度变化并不明显.随着Cr含量的增加,Ni_(55-x)Cr_xFe_(18)Ga_(27)系列合金的马氏体相变温度明显降低,居里温度则小幅度降低;Ni_(55)Fe_(18-x)Cr_xGa_(27)系列合金的马氏体相变温度和居里温度均有规律的降低.     

Acoustic study of martensitic-phase aging in copper-based shape memory alloys

An acoustic technique was applied to study aging of the β ₁ ^′ martensitic phase in a number of copper-based shape memory alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Be) characterized by various degrees of martensitic-phase stabilization. The nonlinear anelasticity of the martensitic phase was studied in wide ranges of temperature (7–300 K) and vibrational strain amplitude (2 × 10^?7 ?2 × 10^?4) at vibrational-loading frequencies of ～100 kHz. It was shown that aging effects of the martensitic phase can have homogeneous and heterogeneous components. The homogeneous component is associated with a change in the degree of atomic order in the crystal volume. The basic heterogeneous mechanisms of martensitic-phase aging are associated with the formation of atmospheres of point defects and local changes (which are greater than those in the crystal volume) in the degree of atomic order in the vicinity of partial dislocations and the boundaries between martensite variants. It is concluded that various stabilization properties of the alloys at hand result not only from the different diffusion properties of quenching point defects but also from the different influence of these defects on the degree of atomic order and the different features of their interaction with partial dislocations and intervariant boundaries.     

Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

Structural and magnetic transition temperatures of ferromagnetic shape memory alloys present a strong dependence on slight departures from the stoichiometry, as does the mobility of twin boundaries responsible for the large magnetic field induced strains. In this work we study four non stoichiometric Ni-Mn-Ga polycrystalline alloys with compositions of 43–52 at.% nickel, excess manganese and deficient in gallium, and a single crystal of composition Ni₅₂Mn₂₆Ga₂₂. Those compounds are of technical interest due to the observed large room temperature magnetic field induced strains. Calorimetric and magnetic measurements determined the martensitic transition and Curie temperatures of the alloys (A_S = 331 K and T_Curie = 366 K for 52 at.% nickel alloy). Nickel defective alloys present a martensitic transition region broader than excess nickel ones. Neutron powder diffraction analysis confirmed orthorhombic martensitic structures for nickel defective alloys, and tetragonal for excess nickel ones. In the 52 atomic % nickel alloys case the crystallographic structure of the martensitic phase was also obtained on a single crystal with the same composition, trained to get a single variant in agreement with determined in the powder sample.     

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L2₁ cubic austenite by Cu substituting for Ni in the Ni_50-xCu_xMn₃₁Ga₁₉ (x=2--10) alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius (k_F) is applied to evaluate the change of the martensitic transformation temperature. The increase of k_F leads to the increase of the martensitic transformation temperature.     

The effect of Si content on the martensitic transformation temperature of Ni55.5Fe18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni55.5Fe18Ga26.5-xSix alloys is determined by x-ray diffraction (XRD),and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni55.5Fe18Ga26.5-xSix alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations,size factor,L21 degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

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.     

Ni-Fe-Ga磁致形状记忆合金中间马氏体相变

采用差示扫描量热和x射线衍射技术研究Ni-Fe-Ga磁致形状记忆合金的马氏体相变行为.结 果发现,在多晶Ni565_56.5Fe190_19.0Ga245_{2 4.5}和Ni563_56.3Fe170_17.0 Ga267_26.7合金中除马氏体相变外,还观察到一次完整的、正相变和逆相 变对应出现、单 纯由温度诱发的中间马氏体相变.该中间马氏体相变与马氏体相变均为热弹性相变. 关键词： Ni-Fe-Ga 中间马氏体相变 磁致形状记忆合金     

Low-temperature specific heat of Ni–Mn–Ga ferromagnetic shape memory alloys

Results of the low-temperature specific heat measurements (2–80 K) for one austenitic and three martensitic Ni–Mn–Ga ferromagnetic alloys are presented. The alloy compositions are chosen to comprise a wide span of valence electron concentrations e/a=7.3–7.78. Debye temperature (261–345 K) is found to be an increasing function of e/a while the experimental values of the Sommerfeld coefficient (2.9–3.4 mJ/mol K²) appear to be increasing in the martensitic region only. Observation of those trends rekindles the discussion about the role of vibrational and electronic contributions to the lattice instability and transformation mechanism of studied alloys.     

Special Features of Behavior of Phenomenological Parameters of Thermoelastic Martensitic Transformations in Low-Stability Alloys Based on Titanium Nickelide

The paper presents the results of an analysis of the data on phase and atomic composition changes in the austenite and martensite phases obtained by an in situ x-ray diffraction examination of thermoelastic martensite transitions in the alloys based on titanium nickelide doped with different elements (Fe, Co, Rh, Cu, Pd, Pt, and Au). To this end, the parameters determining the martensitic transformation hysteresis loops were obtained. These were then classified using the x-ray diffraction and phenomenological data.     

The effect of Si content on the martensitic transfor-mation temperature of Ni55.5e18Ga26.5-xSix alloys

This paper investigates the effects of substitution of Si for Ga on the martensitic transformation behaviours in Ni-Fe-Ga alloys by using optical metallographic microscope and differential scanning calorimetry (DSC) methods. The structure type of Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys is determined by x-ray diffraction (XRD), and the XRD patterns show the microstructure of Ni-Fe-Ga-Si alloys transformed from body-centred tetragonal martensite (with Si content x = 0) to body-centred cubic austenite (with x = 2) at room temperature. The martensitic transformation temperatures of the Ni_55.5Fe₁₈Ga_26.5-xSi_x alloys decrease almost linearly with increasing Si content in the Si content range of x ≤ 3. Thermal treatment also plays an important role on martensitic transformation temperatures in the Ni-Fe-Ga-Si alloy. The valence electronic concentrations, size factor, L2₁ degree of order and strength of parent phase influence the martensitic transformation temperatures of the Ni-Fe-Ga-Si alloys. An understanding of the relationship between martensitic transformation temperatures and Si content will be significant for designing an appropriate Ni-Fe-Ga-Si alloy for a specific application at a given temperature.     

Phase and structural transformations in Ni-Mn alloys alloyed with aluminum

NiMn alloys alloyed with aluminum in a wide range of temperatures and compositions have been investigated by resistivity measurements, electron diffraction, and X-ray diffraction. It is found that the martensitic transformation temperature decreases with an increase in the aluminum content in an alloy. The martensitic transformation temperatures were determined and the full diagram of martensitic transformations for Ni-Mn-Al alloys was constructed. It is established that alloying with aluminum leads to a change in the type of the martensite crystal structure.     

Influence of the atomic order on the magnetic characteristics of a Ni–Mn–Ga ferromagnetic shape memory alloy

The influence of the atomic order on the magnetic properties has been analyzed in a polycrystalline Ni_49.5Mn_28.5Ga₂₂ ferromagnetic shape memory alloy prepared by arc melting under Ar atmosphere. Different thermal treatments have been performed to modify the order degree of the alloy. The effect of the different thermal treatments on the magnetic and structural characteristics has been analyzed by superconducting quantum interference device (SQUID) and differential scanning calorimetry (DSC) measurements. The magnetic and structural properties of the alloys are modified as a consequence of the atomic order change. The martensitic transformation temperatures increase as long as the order degree increases. On the other side, the Curie temperature and magnetization saturation also reflect the order degree of the alloy but seems to be linked to the particular order of the Mn sub-lattice.