Recent advances in understanding the origin of martensite aging phenomena in shape memory alloys

The martensite aging phenomena (martensite stabilization and rubber-like behavior) found in many shape memory alloys have puzzled material scientists for over 60 years without a definite answer. In this article we critically reviewed previous models to understand the aging phenomena, which include pseudotwin-type model, LRO model, SRO model, twinning dislocation model, and domain (twin) boundary pinning model. We showed that these models failed to meet generality criterion, although being able to explain the phenomena to some extent. Then we focused on a very recent general model (Ren and Otsuka, Nature, 389, 579–582, 1997) which makes use of only two common features of martensitic transformation and aging, i.e., diffusionless symmetry change during martensitic transformation and diffusion during aging. This model appears to be able to explain all of the available experimental observations on the aging phenomena. In view of recent development in this field, we have reason to believe that we are approaching the final solution to the aging problem.     

Neutron spectroscopy study of magnons in the austenite and martensite phases of a Ni-Mn-Ga Heusler alloy

The low energy region of magnon excitations of an off-stoichiometric Ni_49.1Mn_29.4Ga_21.5 single crystal has been investigated by neutron spectroscopy. The lowest magnetic exchange stiffness constant D of 97±2 meV Å² has been found in the cubic austenite phase. In the two martensitic phases the exchange stiffness constants are significantly larger with values of 149±4 meV Å² in the tetragonal phase and 198±7 meV Å² in the low temperature martensite. The large value of D in the low temperature phase compared to the other phases cannot be explained solely by renormalization effects due to magnon-magnon interaction and is attributed to a stronger magnetic coupling. In both the martensitic phases a gap of magnon excitation at the Γ-point of about 0.2 meV was observed.     

Helium bubble formation and evolution in NiMo-Y2O3 alloy under He ion irradiation

We report helium ion irradiation experiments for a new type of dispersion-strengthened NiMo-Y₂O₃ alloy with three different irradiation doses and varying irradiation dose rates at 750 ℃ to evaluate its helium-induced damage behavior. Transmission electron microscopy was used to reveal the evolution of helium bubbles after irradiation. The experimental results show that with increasing ion dose, the number density of helium bubbles increases continuously. However, the mean size of helium bubbles first increases and then decreases, mainly due to the varied ion dose rates. The volume fractions of helium bubbles in the three investigated samples after irradiation are 0.15%, 0.32%, and 0.27%, which are lower than that of the Hastelloy N alloy (0.58%) after similar irradiation conditions. This indicates that the NiMo-Y₂O₃ alloy exhibits better helium-induced-swelling resistance than the Hastelloy N alloy, highlighting its potential applicability to MSRs, from the perspective of irradiation performance.     

Influence of zirconium doping on the activities of zirconium and iodine co-doped titanium dioxide in the decolorization of methyl orange under visible light irradiation

Zirconium and iodine co-doped titanium dioxide (Zr-I-TiO₂) was prepared by the hydrolysis of tetrabutyl titanate, premixed with zirconium nitrate in an iodic acid aqueous solution, followed by calcination in air. The structure and properties of the resultant catalyst powders were characterized by X-ray diffraction, the Brunauer-Emmett-Teller method, X-ray photoelectron spectroscopy, transmission electron microscopy, and UV-vis absorption spectroscopy. The catalytic activity of the catalyst was evaluated by monitoring the photocatalytic decolorization of methyl orange under visible light irradiation. The results showed that the activities of Zr-I-TiO₂ catalysts were higher than that of TiO₂ doped with iodine alone (I-TiO₂), and the optimal doping concentration in the Zr-I-TiO₂ calcined at 400 °C was determined to be about 0.05 (molar ratio of Zr:Ti). In addition, the photocatalytic activity of Zr-I-TiO₂ calcined at 400 °C was found to be significantly higher than that calcined at 500 or 600 °C. Based on the physico-chemical characterization, we concluded that the role of zirconium on the I-TiO₂ surface is to increase the number of reactive sites by generating a small crystal size and large surface area. The inhibition of electron-hole pair recombination, by trapping photo-generated electrons with Zr⁴⁺, did not contribute markedly to the improved photocatalytic activity of Zr-I-TiO₂.     

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MRE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns.     

Effects of temperature and impurities on electron radiation damage in the high voltage microscope

This paper describes the results of irradiation of specimens of copper and copper containing up to 2 at % beryllium in the high-voltage electron microscope. The results of irradiation at temperatures ranging from 20–500 K are described and discussed in the light of recently proposed theories of defect nucleation. Although the results are difficult to interpret because of the many diverse factors which influence the nucleation and growth of the damage it was found that the values for the volume density of defects were in general agreement with the predictions, if the effects of the foil surfaces were taken into account.

The corrected densities were used to derive values for the activation energy for motion of an interstitial in copper, E_m , and the binding energy between an interstitial and a beryllium atom, E_b . The results obtained were 0.12±0.03 eV for E_m and 0.29±0.07 eV for E_b .     

Novel microstructural growth in the surface of Inconel 625 by the addition of SiC under electron beam melting

Electron beam melting is being used to modify the microstructure of the surfaces of materials due to its ability to cause localized melting and supercooling of the melt. This article presents an experimental study on the surface modification of Ni-based superalloy (Inconel 625) reinforced with SiC ceramic particles under electron beam melting. Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction techniques have been applied to characterize the resulted microstructure. The results revealed growth of novel structures like wire, rod, tubular, pyramid, bamboo and tweezers type morphologies in the modified surface. In addition to that fibrous like structure was also observed. Formation of thin carbon sheet has been found at the regions of decomposed SiC. Electron beam modified surface of Inconel 625 alloy has been hardened twice as compared to the as-received samples. Surface hardening effect may be attributed to both the formation of the novel structures as well as the introduction of Si and C atom in the lattice of Inconel 625 alloy.     

Effect of impurities on the growth of {113} interstitial clusters in silicon under electron irradiation

The growth and shrinkage of interstitial clusters on {113} planes were investigated in electron irradiated Czochralski grown silicon (Cz-Si), floating-zone silicon (Fz-Si), and impurity-doped Fz-Si (HT-Fz-Si) using a high voltage electron microscope. In Fz-Si, {113} interstitial clusters were formed only near the beam incident surface after a long incubation period, and shrank on subsequent irradiation from the backside of the specimen. In Cz-Si and HT-Fz-Si, {113} interstitial clusters nucleated uniformly throughout the specimen without incubation, and began to shrink under prolonged irradiation at higher electron beam intensity. At lower beam intensity, however, the {113} interstitial cluster grew stably. These results demonstrate that the {113} interstitial cluster cannot grow without a continuous supply of impurities during electron irradiation. Detailed kinetics of {113} interstitial cluster growth and shrinkage in silicon, including the effects of impurities, are proposed. Then, experimental results are analyzed using rate equations based on these kinetics.     

Study of temperature effects on the conduction and trapping of charges in the alkali-silicate glass under electron beam irradiation

A scanning electron microscope (SEM) is employed to investigate the temperature effect on the charging behaviour of alkali-silicate glasses under electron beam irradiation using electrostatic influence method (EIM). A modified special arrangement adapted to the SEM allows to study charging mechanisms and charge transport characteristics of these glasses using the simultaneous measurement of displacement and leakage currents. The trapping process during continuous electron irradiation can be directly determined by the EIM. The experimental results reveal that the charging ability of glasses decreases with increasing temperature. The variation of charge process has been confirmed by measuring the surface potential in response to the sample temperature. In this report, we introduce also the secondary electron emission (SEE) yield. It was found the strong dependence of the SEE yield on the temperature variation. The higher is the temperature and the lower is the SEE yield. The trapping ability is analyzed taking into account the regulation mechanisms involved under electron irradiation.     

Temperature dependences of the electron-phonon coupling, electron heat capacity and thermal conductivity in Ni under femtosecond laser irradiation

The electron temperature dependences of the electron-phonon coupling factor, electron heat capacity and thermal conductivity are investigated for Ni in a range of temperatures typically realized in femtosecond laser material processing applications, from room temperature up to temperatures of the order of 10⁴ K. The analysis is based on the electronic density of states obtained through the electronic structure calculations. Thermal excitation of d band electrons is found to result in a significant decrease in the strength of the electron-phonon coupling, as well as large deviations of the electron heat capacity and the electron thermal conductivity from the commonly used linear temperature dependences on the electron temperature. Results of the simulations performed with the two-temperature model demonstrate that the temperature dependence of the thermophysical parameters accounting for the thermal excitation of d band electrons leads to higher maximum lattice and electron temperatures achieved at the surface of an irradiated Ni target and brings the threshold fluences for surface melting closer to the experimentally measured values as compared to the predictions obtained with commonly used approximations of the thermophysical parameters.     

Correlation between electron state density change and the electrical resistivity and magnetic permeability changes in the nanostructured powder of the NiMo alloy

The nanostructured powders of the Ni_95.4Mo_4.6 and Ni₉₉Mo₁ alloys (average crystallite dimensions of 14 and 21 nm) were obtained by the electrochemical deposition from ammonium solutions of nickel and molybdenum salts. The method of differential scanning calorimetry (DSC) and measurement of temperature dependence of the powder's electrical resistivity, magnetic permeability and the thermoelectromotive force were employed to examine structural changes of the powders. The nanocrystalline alloys Ni_95.4Mo_4.6 and Ni₉₉Mo₁ were stable up to about 460 K. The thermal stabilization of the alloys takes place within the temperature interval of 460–570 K. As a result of this process, a decrease in the electrical resistivity and increases in magnetic permeability as well as electron state density in the proximity of the Fermi level are observed. The crystallization temperature depends upon the current density of powder formation. The nanocrystalline alloy Ni_95.4Mo_4.6 obtained at j=70 mA cm⁻² becomes crystallized in the temperature range between 650 and 840 K, while the Ni₉₉Mo₁ alloy obtained at j=180 mA cm⁻² crystallizes in the 580–950 K temperature interval. The electrical resistivity and magnetic permeability of the nanocrystalline alloy decreased while the alloy's electron state density near the Fermi level increased after the process of crystallization took place. The electrical resistivity decrease recorded during the structural changes was due to an increase in the electron state density in the proximity of the Fermi level, as well as to an increase in the mean free path of the conducting electrons.     

Energy level of an electron in a saddle-potential quantum dot under a uniform magnetic field obtained by the invariant eigenoperator method

We show that the recently proposed invariant eigenoperator method can be successfully applied to solving the energy levels of an electron in a saddle-potential quantum dot under a uniform magnetic field. The Landau diamagnetism decreases with the value ω_y² - ω_x² due to the existence of the saddle potential.     

Energy level of an electron in a saddle-potential quantum dot under a uniform magnetic field obtained by the invariant eigenoperator method

We show that the recently proposed invariant eigenoperator method can be successfully applied to solving the energy levels of an electron in a saddle-potential quantum dot under a uniform magnetic field. The Landau diamagnetism decreases with the value ω_y² - ω_x² due to the existence of the saddle potential.     

激光金属汽化汽离混合区状态参量的稳定性及其影响因素分析

在汽离混合区激光强度时空修正的基础上,近似求解了流体力学模型下的状态参数,找出了稳定状态参数的共同自洽参量,分析了自洽参量的取值方法,研究了相关参数对自洽参量的影响机制,结果发现:(1)汽离混合区内,状态参量的稳定性可用自洽参量的稳定性来表征;(2)当入射激光强度小于108W.cm-2时,自洽参量随入射激光强度变化相对较快,稳定性差;当激光强度大于108W.cm-2时,自洽参量随入射激光强度变化相对较慢,稳定性高;(3)汽离混合区各处的约化自洽参量随辐照激光的波长成变周期增加的变化,波长越长,约化自洽参量越大;(4)要使自洽参量稳定,纹波较小,宜采取短波激光辐照;(5)当蒸汽压强小于2.7×106Pa时,自洽参量随蒸汽压强成指数递减规律变化;当蒸汽压大于2.7×106Pa时,自洽参量与蒸汽压强之间没有对应关系.     

Model and theory for the determination of diffusion coefficients by Auger electron spectroscopy measurements and an application to oxygen diffusion along the [0001] and [101¯0] axes in single crystal zirconium

A simple hopping model of the diffusion of adsorbed species from a surface into the bulk of a material has been formulated and solved mathematically. The difference in the energy barriers for an atom moving between the atomic layers at the surface and in the bulk are explicitly considered. This model is also capable of describing the initial stages of diffusion, something that conventional solutions of the continuum diffusion equation cannot handle. Auger electron spectroscopy has been used to measure the dissolution rate of oxygen from Zr(0001) and Zr(101¯0) surface into the bulk. Satisfactory results were obtained by applying our model to the diffusion data for these two zirconium surfaces for two different heating schedules: (i) rapid temperature ramp-and-hold and (ii) continuous linear heating with respect to time. The resulting Arrhenius expressions for diffusion are: D = (0.115 ± 0.031)exp[(−44.45 ± 4.82)kcal/RT]cm²/s along Zr[0001] and D = (1.07 ± 0.26)exp[(−46.18 ± 4.22)kcal/RT]cm²/s along Zr[101¯0].     

Investigation of changes in properties of water under the action of a magnetic field

The properties of water and their changes under the action of a magnetic field were gathered by the spectrum techniques of infrared, Raman, visible, ultraviolet and X-ray lights, which may give an insight into molecular and atomic structures of water. It was found that some properties of water were changed, and a lot of new and strange phenomena were discovered after magnetization. Magnetized water really has magnetism, which has been verified by a peak shift of X-ray diffraction of magnetized water + Fe₃O₄ hybrid relative to that of pure water + Fe₃O₄ hybrid, that is a saturation and memory effect. The properties of infrared and ultraviolet absorptions, Raman scattering and X-ray diffraction of magnetized water were greatly changed relative to those of pure water; their strengths of peaks were all increased, the frequencies of some peaks did also shift, and some new peaks, for example, at 5198, 8050 and 9340 cm⁻¹, occurred at 25°C after water was magnetized. In the meanwhile, the magnetized effects of water are related to the magnetized time, the intensity of an externally applied magnetic field, and the temperature of water, but they are not a linear relationship. The study also showed a lot of new and unusual properties of magnetized water, for example, the six peaks in 3000–3800 cm⁻¹ in infrared absorption, the exponential increase of ultraviolet absorption of wave with the decreasing wavelength of light of 200–300 nm, the frequency-shifts of peaks, a strange irreversible effect in the increasing and decreasing processes, as well as a stronger peak of absorption occurring at 50°C, 70°C and 80°C, the existence of many models of motion from 85°C to 95°C in 8000–10000 cm⁻¹, and so on. These results show that the molecular structure of water is very complicated, which needs further study. Furthermore, the macroscopic feature of mechanics, for instance, surface tension force of magnetized water, was also measured. Experiments discovered that the size in contact angles of magnetized water on the surface of hydrophobic materials decreases, thus the surface tension force of magnetized water decreases relative to that of pure water. It is seen from the above results that the clustering structure of hydrogen-bonded chains and polarization effects of water molecules are enhanced after magnetization. These results are helpful in revealing the mechanism of magnetization of water. Supported by the National Basic Research Program of China (Grant No. 2007CB936103)