Thermal expansion anomaly of MnB

The thermal expansion of MnB has been measured in the temperature range from 0 to 420°C. An Invar type thermal expansion is observed. The thermal expansion coefficient is ?42×10^-6 deg.^-1 at 295°C(≈T_c).     

Anharmonicity and quantum effects in thermal expansion of an Invar alloy

We have investigated the anharmonicity and quantum effects in the Invar alloy Fe(64.6)Ni(35.4) that shows anomalously small thermal expansion. We have performed Fe and Ni K-edge extended x-ray-absorption fine-structure spectroscopic measurements and the computational simulations based on the path-integral effective-classical-potential theory. The first nearest-neighbor (NN) shells around Fe show almost no thermal expansion, while those around Ni exhibit meaningful but smaller expansion than that of fcc Ni. At low temperature, the quantum effect is found to play an essentially important role, which is confirmed by comparing the quantum-mechanical simulations to the classical ones. The anharmonicity (asymmetric distribution) clearly exists for all the first NN shells as in normal thermal expansion systems, implying the breakdown of the direct correspondence between thermal expansion and anharmonicity.     

Low thermal expansion and broad band photoluminescence of Zr0.1Al1.9Mo2.9V0.1O12

A new material of Zr0.1Al1.9Mo2.9V0.1O12 is synthesized by the traditional solid state synthesis method.The phase transition,coefficient of thermal expansion,and luminescence properties of Zr0.1Al1.9Mo2.9V0.1O12 are explored with Raman spectrometer,dilatometer,and x-ray diffraction(XRD)diffractometer.The results show that the Zr0.1Al1.9Mo2.9V0.1O12 possesses the strong broad-band luminescence characteristics almost in the whole visible region.The sample is crystallized in a monoclinic structure group of P21/a(No.14)crystallized at room temperature(RT).The crystal is changed from monoclinic to orthorhombic structure when the temperature increases to 463 K.The material has very low thermal expansion performance in a wide temperature range.Its excellent low thermal expansion and strong pale green light properties in a wide temperature range suggest its potential applications in light-emitting diode(LED)and other optoelectronic devices.     

Mössbauer study of local environment effects in the ordered Fe70Al30 Invar alloy: Temperature dependence of isomer shift in the spin-glass phase

The systematic studies of the magnetic hyperfine field distribution for ⁵⁷Fe in the spin-glass (SG) phase of the ordered Fe₇₀Al₃₀ Invar alloy have been performed using Mössbauer spectroscopy technique in the temperature range from 5 to 80 K. Particular emphasis has been placed on the low-field component of the distribution, which is considered as corresponding to the Fe sites in the SG magnetic configurations. The main result is the observation of the pronounced temperature dependence of isomer shift for several atomic SG configurations. The temperature behavior of the local electron density is strongly correlated to the Invar properties of the Fe₇₀Al₃₀ alloy. We argue that the observed temperature dependence of the isomer shift due to a local volume effect. The temperature range, for which the pronounced decrease in atomic volume is observed, coincides with the range of the existence of the Invar effect. The influence of the competition between opposite in sign exchange interactions on the Invar properties is discussed.     

Influence of the structure of the Ti-49.8 at % Ni alloy on its thermal expansion during the martensitic phase transformation

The temperature dependence of thermal expansion of the Ti-49.8 at % Ni alloy has been measured after rolling at temperatures of 470, 570, 670, 770, and 870 K. The maximum dilatation jump during the martensitic phase transition has been observed for the samples rolled at 570 and 670 K. A fragmented structure, in which the phase transformations are hampered, is formed during the low-temperature rolling. An increase in the rolling temperature to 770–870 K leads to the return processes and dynamic recrystallization of the material; as a result, the slope of dilatation curves changes and the range of phase transitions narrows.     

The effect of hardening by annealing in ultrafine-grained Al–0.4Zr alloy: influence of Zr microadditives

Influence of annealing on the microstructure and mechanical properties has been studied for Al–Zr (0.4 wt.%) alloy with the ultrafine-grained (UFG) structure formed by high-pressure torsion (HPT) at room temperature. A drastic hardening effect by short-term annealing in the temperature range of 90–280°С was observed for the HPT-processed Al–Zr alloy. The effect of hardening by annealing for the HPT-processed Al–Zr alloy is compared with that for the HPT-processed commercial purity (CP) Al. It was shown that addition of 0.4 wt.% Zr in Al does not cause a significant impact on the magnitude of hardening by annealing up to 150°С, however it leads to a shift of its maximum to higher annealing temperatures and expansion of the thermal stability range of strength up to 280°С. The kinetics of hardening by annealing for CP Al and Al–Zr alloys in the UFG state has been studied for the first time. It was shown that in both materials the strength first increases linearly with the duration of annealing and then reaches saturation. The temperature dependence of the rate of hardening by annealing was analysed through an Arrhenius law, and apparent activation energy was extracted for both systems. The addition of Zr results in the reduction of the activation energy of annealing-induced hardening by ～2 times. Possible physical mechanisms controlling the kinetics of hardening by annealing are discussed for the ultrafine-grained CP Al and Al–Zr alloy.     

Invar合金的电子化学势均衡判据

用二元团簇作为描述Invar合金的基本结构单元,运用"团簇共振"结构模型,建立起以某一已知二元团簇与连接原子按1:x比例连接,描述Invar合金的电子化学势均衡判据.给出了Invar合金成分的经验分子式,即Invar合金成分=[团簇]1(连接原子)x.运用此判据,解析了部分Invar合金成分.发现典型的Invar合金实验成分与经验分子式相符合,说明基于"团簇共振"模型的Invar合金的电子化学势判据很好地解释了Invar合金成分的形成规律.     

Evidence of new high-pressure magnetic phases in Fe–Pt Invar alloy

To investigate the magnetic properties of disordered Fe₇₀Pt₃₀ Invar alloy under high pressure, measurements of the real part of the AC susceptibility (χ) were made under pressure up to 7.5 GPa in the temperature range 4.2–385 K using a cubic anvil high-pressure apparatus. The Curie temperature (T_C) decreased with increasing pressure, and then, two new high-pressure magnetic phases appeared. These results show that the ferromagnetism of Fe–Pt Invar alloy becomes weaker, and the antiferromagnetic interaction becomes dominant with increasing pressure.     

Thickness dependence of magnetization in FeNi invar alloy film

The thickness dependence of magnetization of FeNi Invar alloy films was observed by means of small angle Lorentz electron diffraction. A remarkable reduction of magnetization at room temperature was observed for films with thickness below 400 Å. This may be ascribable to the instability of the ferromagnetic state in Fe-Ni Invar alloys.     

Thermal-expansion anomalies and spontaneous magnetostriction of Lu2Fe17−xSix intermetallic compounds

The thermal expansion of Lu₂Fe_17−xSi_x solid solutions has been measured by X-ray powder diffraction. The magnetic ordering in all compounds within the homogeneity range (x3.4) is accompanied by a large spontaneous volume magnetostriction, distributed anisotropically over the principal axes of the hexagonal crystal structure. The volume effect ω_s in the ground state reaches 14.7×10⁻³ in Lu₂Fe₁₇ and decreases monotonously to 8.9×10⁻³ for x=3.4, following the reduction of magnetic moment. Despite a still large ω_s, the Invar behavior observed in Lu₂Fe₁₇ changes to a positive thermal expansion for x>1 due to an increasing Curie temperature.     

Electron irradiation effects on iron-nickel invar alloys

Electron irradiation was used as a means of accelerating the diffusion in Fe-Ni alloys. Mössbauer effect, X-ray and electron microscopy experiments on samples with 28 to 50% Ni show that the Invar character disappears after irradiation up to 250°C, in particular the variation of the lattice parameter versus temperature becomes linear around room temperature. At the same time, two phases appear in the alloy, one rich in nickel and ordered with FeNi (AuCu) superstructure, the other rich in iron and probably ordered (Fe₃Ni). The Invar state is therefore shown to be a metastable state. A diagram of the Fe-Ni alloy is proposed.     

A study of thermal parameters of Ag50Pd50 alloy using X-ray diffraction

Thermal parameters of Ag₅₀Pd₅₀ alloy were studied using the in-situ X-ray diffraction (XRD) technique. Diffraction experiments in the temperature range 308-1178 K showed that the alloy forms a face centered cubic (fcc; A1-type) structure. Temperature dependence of the lattice parameter was investigated using the Bragg line displacement method. Results show that the lattice parameter for the alloy increases with increase in temperature. The temperature dependent XRD data was utilized to determine the mean linear thermal expansion (MLTE (%)) and coefficient of thermal expansion (CTE) ‘α’ in the mentioned temperature range. It was observed that the values of CTE vary between 9.3 and 14.6 [K⁻¹][10⁻⁶]. The same data was also utilized to determine the Debye temperatures Θ_D and mean square amplitude of vibration (MSAV) (u²(T)) for this alloy. Values of thermal parameters as determined in this investigation are in good agreement with the estimated values as well as with those for other alloys of noble metals.     

Al-Cu-Ge合金的热物理性质与快速凝固规律研究

Al-Cu-Ge合金是典型的三元共晶体系,在工业上有重要的应用价值,对其进行研究有助于了解该合金的热物理性质和提高该合金的结构性能.本文选择了Al55Cu10Ge35,Al70Cu10Ge20和Al80Cu10Ge10三种成分合金作为研究对象,对合金的固态比热和热膨胀系数进行了测量,并对比分析了合金在近平衡凝固和落管快速凝固条件下的组织特征和凝固路径.研究发现,合金比热随Al含量的增大和Ge含量的减少而增大.这三种成分合金的软化温度均为666 K,物理热膨胀系数α在370—650 K温度范围内基本一致,约为1.5×10-5K-1.近平衡凝固条件下合金凝固过程中最后一步反应生成的均为(Al)+(Ge)二相共晶而不是三元共晶,这表明(Al)、(Ge)和CuAl2相在这三种成分的Al-Cu-Ge合金中难以同时形核并协同生长.然而,在快速凝固条件下,初生相的形核和生成受到抑制,合金中更易于形成二相共晶和三元共晶组织.     

Structure and magnetic properties of CoPtCu:Ag nanocomposite films with (001) texture

Nanocomposite (001) textured CoPtCu:Ag films consisting of well separated L1₀ structure CoPtCu nanoparticles have been prepared using a CoPt/Cu/Ag multilayer precursor on SiO₂/Si(100) substrate by magnetron sputtering and subsequent vacuum thermal annealing. With a Cu concentration of 6–10% and Ag of 10–20% in atomic ratio, the films start ordering at an annealing temperature of 450 °C, which is roughly lower by 150 °C than that needed for most CoPt-based films especially with (001) texture. The perpendicular coercivities for the film are in the range from 5 kOe to 7 kOe after annealing at 500 °C in vacuum. The (001) texture for the film is partially due to the formation of an Ag underlayer after annealing, the decreasing of the ordering temperature is most-likely related to the formation of CoPtCu alloy nanoparticles in the film. PACS 75.30.Gw; 75.50.Kj; 75.70.Ak     

Predicting macroscopic thermal expansion of metastable liquid metals with only one thousand atoms

Results of thermal expansion prediction from atomic scale for metastable liquid metals are reported herein. Three pure liquid metals Ni, Fe, and Cu together with ternary Ni₆₀Fe₂₀Cu₂₀ alloy are used as models. The pair distribution functions were employed to monitor the atomic structure. This indicates that the simulated systems are ordered in atomic short range and disordered in long range. The thermal expansion coefficient was computed as functions of temperature and atom cutoff radius, which tends to maintain a constant when the cutoff radius increases to approximately 15 Å. In such a case, slightly more than 1000 atoms are required for liquid Ni, Cu, Fe and Ni₆₀Fe₂₀Cu₂₀ alloy, that is, the macroscopic thermal expansion can be predicted from the volume change of such a tiny cell. Furthermore, the expansion behaviors of the three types of atoms in liquid Ni₆₀Fe₂₀Cu₂₀ alloy are revealed by the calculated partial expansion coefficient. This provides a fundamental method to predict the macroscopic thermal expansion from the atomic scale for liquid alloys, especially in the undercooled regime.     

First Observation of a Composition-Controlled Low-Moment/High-Moment Transition in the Fcc Fe-Ni System: Implications Regarding Invar and Anti-Invar Behaviors

We report the first direct observation of a high-moment (HM)/low-moment (LM) transition occurring in face centered cubic (FCC) Fe-Ni alloys. 57 Fe Mössbauer isomer shifts (ISs) give local electronic densities that exhibit a large discontinuity of , 0.4 el./ $ a_{0}^3 $ at the transition that spans the concentration range ～ 65-75 apc (atomic percent) Fe, in agreement with ab initio predictions. In the most Fe-rich alloys that have LM ground states (including n -Fe), we show that thermal stabilization of the HM state occurs at high temperatures, thereby providing an experimental proof that anti-Invar behavior is due to such HM stabilization. In Invar (Fe 65 Ni 35 ) and at near-Invar compositions, we observe temperature-induced changes in electronic density that follow the spontaneous magnetization curves and find that Invar is predominantly a HM phase at all temperatures where an Invar effect occurs. We show that LM phase thermal excitation cannot cause the Invar effect and that such excitation would cause a contraction instead of the required expansion, relative to normal behavior.     

Thermal properties of the alloys Ni,Ru and Cr with Fe

The thermal expansion coefficient α of the Fe-Ni, Fe-Cr and Fe₇₀Ni_30-xRu_x alloys was studied over the temperature range from 4,2 to 300 K. The magnetic contribution to α for all investigated alloys is obtained. The stabilization of the γ-phase in Fe₇₀Ni₃₀ alloys by small additions of ruthenium is shown to lead to the appearance of Invar anomalies of the thermal expansion coefficient. The magnetic contribution to α of Fe-Ni and Fe-Cr alloys is shown to have the same character despite the difference of crystal structure of these alloys.     

Structure–phase transformations and physical properties of ferritic–martensitic 12% chromium steels EK-181 and ChS-139

The thermophysical properties (specific heat, thermal diffusivity, thermal conductivity, linear thermal expansion coefficient, density) of 12% chromium ferritic–martensitic steels EK-181 (RUSFER-EK-181) and ChS-139 and the structure–phase transformations that occur in them upon heating and cooling in the temperature range 20–1100°C are studied. The temperatures of the start and finish of the α → γ and γ → α transformations in these steels and the Curie temperature are determined by differential scanning calorimetry. Peaks in the temperature dependence of the specific heat and jumplike changes in the linear thermal expansion coefficient and the density and the minimum of thermal diffusivity are detected in the α → γ transformation range. Specific heat peaks, thermal conductivity minima, and inflection points in thermal diffusivity curves are also observed near the Curie temperature.     

Thermal expansion, elastic properties and the Mössbauer effect of stainless Invar

The thermal expansion, the elastic moduli (E, G, B) and the specific heat of three Co-Fe-Cr alloys are reported together with the dependence of these properties. Alloys with fcc structure exhibit the Invar anomaly, i.e., a large spontaneous volume magnetostriction and an elastic softening below T_c. Data for the spontaneous volume magnetostriction and for the ΔE-Effect are given. The Mössbauer effect was measured at 4.2, 77 K and room temperature.     

Macrosegregation driven by movement of minor phase in (Al0.345Bi0.655)90Sn10 immiscible alloy

Formation of macrosegregation structure in (Al_0.345Bi_0.655)₉₀Sn₁₀ (mass percent, the same below) immiscible alloy was investigated by spraying its melt into silicone oil. Two kinds of typical macrosegregation structures were obtained in the dispersed alloy spheres: core/shell structure and crescent structure. Based on the estimated temperature field inside the alloy spheres, the velocities of thermal Marangoni, solutal Marangoni, and Stokes motions of the Bi-rich minor droplets were calculated. Analysis shows that surface segregation, Soret effect, thermal Marangoni motion, solutal Marangoni motion, and Stokes motion play a key role in the formation of Al/Bi–Sn core/shell structure. If the liquid alloy spheres solidify on the condition that the radius of the Bi-rich minor droplets is smaller than a critical value, it will form Al/Bi–Sn core/shell structure, while the crescent structure will be formed when the liquid alloy spheres frozen on the condition that the radius of the Bi-rich minor droplets exceeds the critical value.