体心立方固溶体合金中的"团簇+连接原子"结构模型

为体现固溶体合金中的溶质原子产生的化学短程序,文章提出了配位数为14的团簇在体心立方(bcc)点阵中的堆垛模式,并建立了基于bcc结构的"团簇+连接原子"结构模型,用团簇成分式[团簇](连接原子)_x表述. 此模型中,与基体组元具有相对大的负混合焓的溶质原子占据团簇心部,其他原子作为连接或者替代团簇壳层基体原子. 1 ∶1结构模型[团簇](连接原子)₁由于最大程度地保证了团簇与连接原子的近邻,构成了连接原子最有效的合金化方式. 在两个实用bcc固溶体合金体系中, 1 ∶1模型指导设计了低V含量的储氢合金 V₁和低弹性模量高强度的 Nb₁合金. 关键词： 体心立方固溶体成分设计 "团簇+连接原子"结构模型 Ti-Cr-V合金 Ti-Zr-Mo-Nb合金     

Structural relaxation and chemical decomposition in amorphous TM-M alloys

Small angle X-ray scattering experiments performed on amorphous two-component transition metal-metalloid alloys have shown that these materials decompose on annealing clearly before crystallization starts. This process influences the behaviour of physical properties in a similar way as structural relaxation does. To separate the two effects mainly composition dependent properties must be compared with others which are mainly sensitive to structural changes. In this paper it is shown that the ratio of the annealing-induced changes of the temperature coefficient of the resistivity and of the residual resistivity is determined by the underlying elementary process. The two values resulting for predominating structural relaxation and for predominating decomposition seem to be universal and are compatible with a theory of phonon-controlled conductivity in high-resistance conductors.     

Organotin anions in solution and in the solid state

Moessbauer data have been obtained for a series of organotin anions. The spectra of these anions in solution show the presence of two species, a singlet in the tin(IV) region and a doublet in the tin(II) region. The crystal structure of triphenyltin anion with potassium in 18-crown-6 as the cation has been determined. The Moessbauer spectrum of this sample is a doublet in the tin(II) region.     

Threshold stress for superplasticity in solid solution magnesium alloys

The effect of alloying elements on the threshold stress for superplasticity was investigated using two binary solid solutions, namely, Mg–Al and Mg–Y alloys. Both alloys exhibited superplasticity, and in spite of the absence of fine particles showed threshold-stress-like behavior. Different origins were suggested for the threshold-stress-like behavior after considering grain growth during deformation. The threshold-stress-like behavior in Mg–Al alloys originates from the effects of microstructural instability (grain-growth hardening). On the other hand, analysis of grain-boundary segregation suggested that the threshold-stress-like behavior in Mg–Y alloy originates from the segregation of yttrium in grain boundaries and its interaction with grain-boundary dislocations.     

Change in primary extinction during decomposition of supersaturated solid solution

The integrated intensity of the matrix reflection (200) was measured during the precipitation of the, and phases. The observed effects were explained as the consequence of lattice defects formed in the neighbourhood of precipitate particles.

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(200) , . .

     

Electronic origin of solid solution softening in bcc molybdenum alloys

The intrinsic mechanism of solid solution softening in bcc molybdenum alloys due to 5d transition metal additions is investigated on the basis of ab initio electronic-structure calculations that model the effect of alloying elements on the generalized stacking fault (GSF) energies. We demonstrate that additions with an excess of electrons (Re, Os, Ir, and Pt) lead to a decrease in the GSF energy and those with a lack of electrons (Hf and Ta) to its sharp increase. Using the generalized Peierls-Nabarro model for a nonplanar core, we associate the local reduction of the GSF energy with an enhancement of double kink nucleation and an increase of the dislocation mobility, and we reveal the electronic reasons for the observed dependence of the solution softening on the atomic number of the addition.     

Formation of lactose-mannitol molecular alloys by solid state vitrification

In this paper, we report the possibility to form glassy molecular alloys (α-lactose)_1−x(mannitol)_x for x<0.5 by co-milling two crystalline powders of pure α-lactose and pure mannitol β. The results have been established by differential scanning calorimetry and by powder X-ray diffraction. The concentration dependence of the glass transition temperature is found to obey the Gordon Taylor rule expected for regular solutions. It is also shown that the milling of pure mannitol β (x=1) leads to a polymorphic transformation towards the metastable form α of mannitol.     

Luminescence of poly(N-vinylcarbazole) in the solution and solid state

Fluorescence spectra of poly(N-vinylcarbazole) have been studied as a function of various environments and temperature. Fluorescence spectra have been excited with UV light from 200 W Xenon lamp or with second-harmonics of a pulsed ruby laser. Polymer films and powders show exclusively excimer fluorescence at room temperature. At high intensities of excitation the observed fluorescence spectra have a weak quasiperiodic structure.     

Thermodynamic analysis of the decomposition of a supersaturated solid solution of the Fe-Cu system

The structure of Fe-5.5% Cu alloy is studied after eutectoid reaction and aging of the martensite. The thermodynamic analysis of decomposition of the supersaturated solid solution with copper release is peformed for different types of crystal lattice. The free energy of solid solutions and phase separation diagrams of the BCC and FCC phases of copper is calculated. Barriers for the nucleation of copper particles with BCC and FCC lattices are evaluated. The factors that favor the appearance of intermediate structural forms of copper preceding the formation of stable FCC phase are discussed.     

Polar state in a SrTiO3-KTaO3 solid solution

Dielectric properties of ceramic samples of a 0.85SrTiO₃-0.15KTaO₃ solid solution have been studied. The temperature dependence of the dielectric permittivity exhibits maxima with a strong frequency dispersion, which is described by the Vogel-Fulcher law with a characteristic temperature T _g=26 K. These features are apparently connected with transition to a polar glasslike state. Fiz. Tverd. Tela (St. Petersburg) 39, 1838–1840 (October 1997)     

Predicting the solid state phase diagram for glass-forming alloys of copper and zirconium

The free energies of six crystal structures associated with Cu-Zr alloys-Cu (face centred cubic), Cu(2)Zr, Cu(10)Zr(7), CuZr, CuZr(2) and Zr (hexagonal close packed)-are calculated using the embedded atom potential of Mendelev et al (2009 Phil. Mag. 89 967). We find that the observed low temperature stability of the Cu(10)Zr(7) and CuZr(2) phases is not reproduced. Instead, the model predicts that the CuZr phase remains stable down to T = 0 K. This discrepancy is largely removed when the interaction potentials are cut off at a short distance, such as that used by Duan et al (2005 Phys. Rev. B 71 224208). We present evidence, however, that the cut-off distance necessary to achieve the change in phase stability results in pathological artefacts in the energetics of some crystal phases.     

Structural characterization of the thermal transformation of halloysite by solid state NMR

Structural changes on thermal decomposition of the clay mineral halloysite (Al₂Si₂O₇·4H₂O) with progressive heating up to 1400°C have been followed by a combination of²⁹Si and²⁷Al solid state NMR, X-ray diffraction and transmission electron microscopy. Thermal transformations closely follow those previously outlined for the related mineral kaolinite. Clear evidence is presented here that the major exotherm at 1000°C is principally associated with the formation of γ-Al₂O₃ and results in phase separation of alumina-rich and silicarich regions on a scale of <5 nm. On further heating initially an extremely alumina-rich mullite forms. This progressively reacts with further silica, increasing the silica-content of the mullite, finally producing 3: 2 mullite with the residual silica crystallising as cristobalite.     

Structural modification of supersaturated BCC-Nd(Al) solid solution

The metastable Nd(Al) supersaturated solid solution with the BCC structure was synthesized by the melt-spinning method. The thermal stability of the supersaturated solid solution was studied by differential scanning calorimetry (DSC) and electrical-resistance measurements. The structural modification was determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A constrained BCC lattice was found due to the smaller Al atoms entering the BCC-Nd lattice. Two evident transformation stages were found during the heating process. The lower activation energy and negative temperature coefficient of the resistivity (TCR) suggest that a continuous structural transformation occur, which results from the softening mode of the highly metastable BCC structure. The strongly distorted and ultra-fine microstructure was observed directly by TEM.     

Constitution and magnetic properties of Ni-Mn-Sn alloys - solid and liquid state

Ternary Ni₅₀Mn_50-xSn_x alloys have been studied by means of the magnetothermal analysis. The high temperature phase of the Ni₂MnSn Heusler alloy is obtained directly from the melt (structure of B2 type). From the paramagnetic behaviour of the Heusler alloy in the liquid as well as solid states, the same magneton number $\text{n′}{}_{\mathrm{<mtext>eff</mtext>}}\text{? 5}$ per Mn atom results. At lower temperatures this phase forms the L2₁ structure and shows a resulting ferromagnetic magneton number $\text{n}{}_{\mathrm{rmf}}\text{? 4}$, a value which is well known.     

Solid solution decomposition and growth of precipitates in Be-Fe alloys from Mössbauer investigations

Mössbauer spectra of fine-grained hot-pressed beryllium and coarse-grained beryllium samples containing different amounts of impurities were obtained after homogenization and after annealing for different durations. Mössbauer spectra of solid solution of iron in beryllium and decomposed during isothermal annealing two different iron containing phases were fitted by a convolution equation of three Lorentz lines. The models of solid solution decomposition and growth of secondary particle precipitates were investigated. The average distance between dislocations and the average grain size were obtained from the application of the models. The dependencies between the decomposition mechanism, the average grain size, the impurity concentrations and the type of the secondary particles precipitates were revealed. The possibility of a coherent analysis of the decomposition process by means of a kinetic law classification and secondary particle precipitates growth processes based on diffusion models has been shown.     

Theory of spinodal decomposition in alloys

A statistical theory of the thermally driven composition fluctuations in a binary alloy is developed for the purpose of studying the phenomenon of spinodal decomposition. The theory can be stated in the form of a Fokker-Planck equation, which reduces, upon taking a suitable moment, to the nonlinear generalized diffusion equation which has been the basis of recent work in this field. Using the full Fokker-Planck equation, it is possible to compute the lifetime of the stationary solutions of the diffusion equation, and thus to study the rate at which the structure of the alloy coarsens during aging.     

Thermally induced decomposition of the disordered nanocrystalline supersaturated α-Fe(Pb) solid solution

The structural and phase transitions in the nanocrystalline supersaturated α-Fe(Pb) solid solution as a result of the thermally induced transition from a highly strained state to equilibrium have been investigated by differential scanning calorimetry, X-ray diffraction, and Mössbauer spectroscopy. Two stages of structural evolution are established.