Interfacial Defects and Lamellar Decomposition in Titanium Aluminides

The interfacial defect content of lamellar interfaces in Ti aluminide alloys has been evaluated by HREM and the data analysed using the Topological Theory of Interfacial Defects. It has been found that the defects observed are all perfect interfacial disconnections, and that the lamellar decomposition is diffusion-controlled. An analysis of the diffusive fluxes required for disconnection motion has been used to reconcile the apparent discrepancy between this conclusion and the martensitic crystallography exhibited by the TiAl lamellae. Moreover, this analysis has been used to explain why disconnections with Burgers vectors b = 1/3〈211〉 have been observed in these interfaces.     

Martensitic transformation in Ni-Mn-Ga alloys

Single crystals of different compositions in shape memory alloys Ni-Mn-Ga have been studied by electron and low temperature X-ray diffraction as well as by differential scanning calorimetry. It is shown that the cooling-induced martensitic phases are long-periodic ones modulated along the (110) directions by a transverse wave of atomic shifts with 5 and 7 atomic layers periodicity for the alloys studied exhibiting a martensitic transformation at 180 K and 446 K, respectively. The transformation heats appeared to be about 10 times different for both alloys.     

The influence of hexagonal anisotropy on the magnetic structure of antiferromagnetic rare earth metals and alloys

The effect of hexagonal base plane anisotropy on the antiferroniagnetic (AF) spiral spin structure of rare earth metals and alloys has been investigated. Minimization of the anisotropy energy is considered either by a reorientation of rigid coherent sections of the spiral or by a continuous modulation of the spiral periodicity. Both results indicate that when the spiral periodicity is commensurate with the hexagonal anisotropy the energy is greater than for spirals which are non-commensurate. The calculations predict that when the temperature is varied in the AF phase, the magnetic structure becomes unstable as a commensurate turn angle is approached, which may provide a qualitative explanation for the abrupt changes in the c-axis thermal expansion and temperature dependence of the elastic constant C₄₄ that have been observed experimentally in dysprosium and terbium-50% holmium. Further consequences of the spiral perturbation by hexagonal anisotropy are considered.     

Description of (hhz) habit planes in wave models of martensite growth for alloys based on copper,gold, and iron

A two-wave scheme was used to analyze crystal habits of the form (hhZ), observed during martensite transformations in Au-Cd, Cu-Al-Ni, Cu-Zn, Fe-Ni, and Fe-C alloys. In the case of gold- and copper-based alloys, preference is given to pairs of shears waves with polarization vectors perpendicular and parallel, respectively, to the (110) plane; for iron-based alloys it is concluded that pairs of longitudinal pairs play the leading role.     

Alloy-assisted Auger recombination in InGaN

It has been numerically investigated the effect of alloying on the Auger recombination rate in wurtzite type n-InGaN. In order to explicitly take into account the effect of alloy disorder, the calculations have been performed with a 256-atom supercell that includes In and Ga atoms randomly distributed over the supercell sites to obtain a given composition. A full band structure (no band scissors-shifting) and high-dense inhomogeneous k-point grid were used to improve the accuracy of the calculations. We show that the large number of allowed interband Auger transitions originated by the breaking of the translational periodicity plays a crucial role in the wide band gap InGaN alloys. The alloy-assisted Auger coefficients for these alloys are in the 1.0?×?10^?32–4.7?×?10^?31 cm⁶/s range     

Application of SSNTDs for study of boron distribution in alloys

A study of boron behavior and its distribution in boron doped iron-, nickel- and aluminum-base alloys subjected to various treatment conditions has been performed. For this purpose a technique based on the detection of ¹⁰B(n,)⁷Li reaction products with track etch detectors has been applied. High boron enrichment of grain boundaries of both iron base and ingot nickel base alloys has been observed. After hot and then cold rolling of the latter specimens boron rearrangement along with the proper deformation of the structure took place. Striation of boron in a hot rolled aluminum alloy along the rolling direction has been observed.     

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.     

A liquid-like picture for the interpretation of Raman scattering in superionic conductors

Raman spectra of superionic conductors are studied in a model which takes into account polarizability changes due to ionic motions between lattice sites. The scattered intensity is determined by a generalized dynamical structure factor at wave vectors depending on the periodicity of the lattice. The structure factor is evaluated in a model which for the mobile ions assumes Brownian motion in a sinusoidal potential. The results are compared with the spectrum of α-AgI. The observed broad Rayleigh wings can be explained by quasielastic scattering due to diffusive motions.     

傅里叶变换在差分吸收光谱技术气体浓度计算中的应用

在实际测量得到的气体吸收光谱中,发现大多数气体的吸收光谱具有明显的周期性,而傅里叶变换正是用来寻求信号的频率特征。在加窗的条件下,通过对不同气体的吸收光谱进行傅里叶变换,来寻求光谱信号对应的特征频率。在数据分析过程中,发现这样一个规律：在气体吸收光谱经傅里叶变换后的频谱图中,其对应特征频率的幅值与所测的气体浓度成明显的线性对应关系。因此,提出一种新的差分吸收光谱浓度解析方法,即利用气体吸收光谱傅里叶变换后其对应特征频率的幅值与浓度的关系,建立一种浓度反演计算的线性关系式,从而由气体吸收光谱傅里叶变换后特征频率的幅值直接求出气体的浓度。该方法完全摆脱了差分吸收光谱技术的理论基础,大大减少了光谱分析和气体浓度反演计算的过程,是一种值得进一步去探究的光谱分析方法。     

Radiation-induced structural changes in alloys

Development of alloys for reliable performance in extreme radiation environments is vital for the viability of advanced nuclear reactor systems. Over the past decade, there has been a considerable growth in our understanding of the basic processes of radiation damage, the nature of the induced defects, their interaction and migration, and the influence of these on the mechanical behaviour of metals. This understanding has however come mainly from studies in pure metals and dilute alloys, and there are difficulties when applying these concepts to concentrated alloys, particularly of technological interest. The present article, which attempts to bridge this gap, discusses recent research developments and some of the emerging new concepts as applicable to alloy systems. Interstitialcy transport; percolation effects in defect migration; short range and long range ordering and restructuring of alloys; defects and damage behaviour of metallic glasses; synergetic processes and phase instabilities; and finally, swelling, irradiation creep and ductility behaviour of alloy systems are the topics discussed.     

Classical many-body potential for concentrated alloys and the inversion of order in iron-chromium alloys

Atomistic simulations of alloys at the empirical level face the challenge of correctly modeling basic thermodynamic properties. In this Letter we propose a methodology to generalize many-body classic potentials to incorporate complex formation energy curves. Application to Fe-Cr allows us to correctly predict the order vs segregation tendency in this alloy, as observed experimentally and calculated with ab initio techniques, providing in this way a potential suitable for radiation damage studies.     

Lorentz microscopy in melt-spun R-Fe-B alloys

The magnetic domain structure of R-Fe-B alloys has been examined in several alloys from the less anisotropic Y-Fe-B to the most anisotropic Tb-Fe-B. In samples with larger grains the domain walls are observed to be pinned at grain boundaries. When the size of the grains is very fine, some very large domains are observed to coexist together with the much finer single domains. These large interaction domains are formed because of strong interactions between spins of adjacent domains.     

Kinetics of dispersion hardening and change in slip mechanism in Cu-Ti and Cu-Ti-Al alloys

It is shown in this paper that the hardening of Cu-Ti and Cu-Ti-Al alloys at the coherent-decomposition stage can be described in terms of the basic Mott-Nabarro mechanism. The overaging stage in Cu-Ti alloys is due to the partial loss of coherence of the -phase, though cutting of the particles by dislocations continues. In Cu-Ti-Al alloys bypassing of the particles in the way suggested by Orowan offers a possible deformation mechanism at the overaging stage. These conclusions are supported by the slip patterns obtained by electron microscopy of replicas. The small difference between the slip patterns of specimens having been quenched and those having been aged to give maximum mechanical properties should be particularly noted, since it is at variance with the data already available, mostly from aluminum alloys.     

二维介质柱光子晶体波导的优化设计

通过改变靠近导光区域介质柱的直径和周期,优化设计了二维介质柱光子晶体波导.结果表明：单独增大和减小柱的直径以及增大柱的周期不能提高波导的传输特性;单独减小柱的周期能提高波导的传输特性,获得传输系数小于1 dB,带宽80 nm的波谱;同时减小柱的直径及周期能够极大的提高波导的传输特性,得到了带宽为150~250 nm,传输系数小于1 dB的波谱.     

Thermal and microstructural investigation of Cu–Al–Mn–Mg shape memory alloys

There are many studies to improve the properties of Cu–Al–Mn shape memory alloys, such as high transformation temperatures, ductility and workability. Most of them have been performed by adding a quaternary component to the alloy. In this study, the effect of trace Mg addition on transformation temperatures and microstructures of three different quaternary Cu–Al–Mn–Mg alloys has been investigated using thermal analysis, optical microscopy and XRD techniques. The transformation temperatures are within the range of 120–180 °C, and they have not changed significantly on decreasing the Mn content, replacing with Mg. The fine precipitates have been observed in the alloys with the Mg content up to 1.64 at%. Calculated entropy change and XRD analysis reveal that the alloys with high Al content have mainly 18R-type structure which could be responsible for good ductility and workability.     

Shape memory alloys — characterization techniques

Shape memory alloys are the generic class of alloys that show both thermal and mechanical memory. The basic physics involved in the shape memory effect is the reversible thermoelastic martensitic transformation. In general, there exists two phases in shape memory alloys, viz., a high-temperature phase or austenitic phase (A) and a low-temperature phase or martensitic phase (M). In addition, an intermediate R phase exists in some special cases. The M↔A transformation is associated with a recoverable strain of about 6.5–8% and the R↔A transformation is associated with a recoverable strain of about 1%. The former transformation has been widely used in the applications like antenna deployment of satellite, aerospace couplings, orthodontic arch wires, medical guide wires for diagnostic and therapeutic catheters and other industrial applications. Our group has been giving emphasis to the characterization techniques for R phase, using differential scanning calorimetry (DSC), electrical resistivity probe (ER) and thermomechanical analyzer (TMA). R phase is found to have attractive features like stability against thermal cycling, a small thermal hysteresis and a negligible strain recovery fatigue. DSC has been used successfully to characterize the recoverable strain parameters, apart from the determination of transformation temperatures. ER is used, for the systematic study of the dependence of various phases on heat-treatment temperatures. TMA has been effectively employed for the study of the mixed phases. A space-rotating platform is designed and fabricated, using an actuator of shape memory spring, for obtaining controlled rotations. The efficiency and the reliability of this actuator has been tested, over a million thermal cycles.     

Plane-wave analysis of the near field of light diffracted by ultrasound.

Although the phenomenon of light diffraction by ultrasound has been studied very extensively during the last 40 years, almost all investigations were concentrated on the individual far field (Fraunhofer) diffraction orders. In the present paper, the basic theory is developed for studying the near field (Fresnel region) of light diffracted by an arbitrary plane ultrasonic wave and the fundamental periodicity properties are stated. The general plane-wave theory of Raman-Nath has been taken as a starting point. From the analysis, the near field of the diffracted light is seen to be highly sensitive to variations of the ultrasonic amplitude and this feature provides a useful technique for observing weak ultrasonic waves. In particular, for the specific case of Raman-Nath-type diffraction, a procedure is presented allowing the reconstruction of the time waveform of the ultrasonic wave from the diffracted light intensity signal.     

Decagonal quasicrystals

Following the discovery of two dimensional quasicrystals in rapidly solidified Al-Mn alloys by us and L. Bendersky in 1985, a number of fascinating studies has been conducted to unravel the atomic configuration of quasicrystals with decagonal symmetry. A comprehensive mapping of the reciprocal space of decagonal quasicrystals is now available. The interpretation of the diffraction patterns brings out the comparative advantages of various indexing schemes. In addition, the nature of the variable periodicity can be addressed as a form of polytypism. The relation between decagonal quasicrystals and their crystalline homologues will be explored with emphasis on Al₆₀Mn₁₁Ni₄ and ‘Al₃Mn’. It will also be shown that decagonal quasicrystals are closely related to icosahedral quasicrystals, icosahedral twins and vacancy ordered phases.