强激光辐照下孔洞尺寸对铁相变过程的影响

 利用分子动力学模拟研究了完美单晶铁以及含不同尺寸孔洞的单晶铁相变过程,分析了孔洞尺寸对相变过程的影响。模拟结果表明：孔洞的存在降低了相变的阈值应力,加速了相变区域成核速率和相变传播速率;随着孔洞直径的增大,相变的阈值应力逐渐降低;孔洞也改变了相变的初始成核区域,使相变区域呈现出一个蝴蝶状的形貌;孔洞反射的稀疏波对相变成核区域的影响随孔洞体积增大而增大,导致孔洞周围出现大量的无序结构原子;孔洞体积对相变的影响也体现在了粒子速度空间分布上,压缩过程中孔洞周围出现的大量“热点”导致了更低的粒子速度空间分布。     

原位中子衍射研究两相NiTi合金的微力学相互作用和相变机理

NiTi合金的形状记忆效应与其微观结构特征密切相关,中子衍射技术可以在力学加载过程中原位观察块体NiTi合金的相变、晶间应变以及孪晶再取向等演化特征．结合两相NiTi合金宏观应力一应变曲线呈现的四种阶段性变形特征,利用原位中子衍射技术对其变形过程中的微观结构演化进行了分析．奥氏体初始体积份额约22％,在低应变硬化阶段,晶面（110）B2和（002）B19,的应变分别突然减小和增大表明出现了应力诱发马氏体相变,奥氏体体积份额迅速减小,产生了（011）II型孪晶;同时初始马氏体也开始发生再取向,随着应变量的增加,开始出现新的{20i}型马氏体孪晶,这种孪晶引起的应变卸载时不能回复．在高应变硬化阶段孪晶变形起主导作用,衍射峰半高宽变化较小;而在应变硬化饱和阶段则以滑移机制为主,大量位错的产生使衍射峰半高宽显著增加．     

Mn effect on wurtzite-to-cubic phase transformation in ZnO

Mn doping effect on a wurtzite-to-cubic phase transformation in ZnO has been investigated by in situ high pressure X-ray powder diffraction using synchrotron radiation. Unit cell expansion is clearly observed in Mn-doped ZnO samples. Mn ions sit at Zn site in the wurtzite structure. The onset transition pressure for the wurtzite-to-cubic phase transformation decreases from about 9.5 GPa for pure ZnO to 6 GPa for sintered 2at.% Mn-doped ZnO while the compressibility and volume collapse at transition pressures are not sensitive to the Mn doping in the wurtzite phase. The doping of Mn ions in ZnO increases the onset transition pressure for the cubic-to-wurtzite phase transformation. The results could be explained by a reduction of phase transformation barriers for both transition paths by the Mn doping. The observation of reduction of the wurtzite-to-cubic phase transformation pressure might point out a new direction to synthesize cubic wurtzite phase of ZnO by doping transition element(s).     

A study of the action of magnesium on iron oxide catalysts for dehydrogenation of ethylbenzene to styrene

Mössbauer study of the action of magnesium on iron oxide catalysts for the dehydrogenation of ethylbenzene to styrene has been carried out systematically. The presence of opportune amount of Mg enhances the stability and dispersivity of catalysts. Therefore Mg is one kind of useful structure promoter.     

EXAFS measurement of iron bcc-to-hcp phase transformation in nanosecond-laser shocks

Extended x-ray absorption fine structure (EXAFS) measurements have demonstrated the phase transformation from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometry. This is a direct, atomic-level, and in situ proof of shock-induced transformation in iron, as opposed to the previous indirect proof based on shock-wave splitting.     

EPR study of the symmetry breaking effect in ferroelectric cesium dihydrogen phosphate doped with Cr5+ ions

The (PO₄)^3? units in a CsH₂PO₄ (CDP) crystal were replaced in a small fraction of sites by (CrO₄)^3? groups and the EPR of the Cr⁵⁺ center was investigated. Splitting of the EPR line appears at T¹_c=245 K, 91 K higher that the ferroelectric transition temperature T_c=154 K. The electronic wave function of Cr⁵⁺ (3d¹) is identified as d_x2_?y2. The d_x2_?y2 function couples with the near protons and the reorientation of this unit in the two possible configurations occurs in the paraelectric phase and breaks the symmetry far above T_c. The observed correlation time 10^?9 sec and associated activation energy ΔU=0.215 eV are discussed.     

Investigation of the effect of pressure on thermodynamic properties and thermoelastic phase transformation of CuAlNi alloys: A molecular dynamics study

Thermoelastic phase transformations and thermodynamic properties of CuAlNi alloys at 0, 1, 2 and 3 GPa pressures were investigated by using MD simulation in this study. The interactions between atoms were modelled by Sutton-Chen type of embedded atom method (SCEAM) that is based on many-body interaction. It was observed that thermoelastic phase transformation in the ternary alloy system occurred at the end of thermal process. Radial distribution function (RDF) was used in order to analysis the structures obtained from MD simulation using the simulation techniques’ thermodynamic parameters. The transformation temperatures, enthalpy and entropy of the ternary alloy system have been observed to be changing with the applied pressure. In addition, it was found that the elastic energy has been decreased about 22% by applied pressure whereas Gibbs free energy has been increased about 60% by applied pressure. The values of the thermodynamical parameters obtained in this study were observed to be in close agreement with the experimental study.     

Temperature dependence of the mössbauer effect in iron uranyl phosphate

Two samples of iron uranyl phosphate have been studied with the⁵⁷Fe Mössbauer effect technique as a function of temperature between 80 and 300 K. The samples clearly contain about 10% ferric ions relative to total iron. The ferrous hyperfine parameters exhibit unusual variations in the temperature region 240–260 K which are ascribed to a transition from an ordered to a quasi-liquid state of the water layer.     

First-principles study of A7 to simple cubic phase transformation in As

First-principles plane-wave pseudopotential approaches were used to investigate the structural phase transition of As from the rhombohedral structure to the simple cubic (sc) one under high pressure at the transition pressure of 22 GPa, which is also found to be accompanied by a volume reduction of 0.8%. The detailed structural changes during the phase transition were analyzed.     

High pressure effect on the thermopower of cesium

The thermopower of cesium has been measured in the region of existence of four phases (I, II, III, and IV) at high pressures up to 7 GPa. The initial phase of cesium and three phases formed under pressure with various crystal structures have a positive thermopower. The thermopower of all three high-pressure phases decreases in magnitude as the pressure increases.     

Nature of the effect of magnetic fields on the starting temperature of martensitic transformation in iron alloys

The effect of a magnetic field on martensitic transformations, which is satisfactorily described by the Krivoglaz–Sadovskii formula, has been analyzed taking into account the nonequilibrium of the martensitic transformation, the possible adiabatic conditions, and the magnetostriction of the paraprocess in ferromagnetic austenite.     

The effect of the interaction between the minority phase droplets on the nucleation behavior during the liquid-liquid phase transformation

The microstructure evolution during the liquid-liquid phase transformation of Al-Pb alloy was calculated. The numerical results indicate that the interaction between the minority phase droplets has effect on the nucleation process of the droplets, and the effect increases with the cooling rate and the content of Pb. Supported by the National Natural Science Foundation of China (Grant Nos 50395104, 50671111 and 50620130095)     

The effect of dendritic shapes of goethite particles on their transformation to iron oxide and iron particles

The development of particle morphology and crystalline structure during the dehydration, oxidation, and reduction processes of goethite to iron needles was investigated by HREM. It is shown that the typical geometry of the particles is preserved during all transformation steps, and that initial internal defects of the crystalline structure caused by twinning of the goethite vanish, while some new defects are created.     

The effect of codeposited nitrogen on the amorphous-polycrystalline resistivity transformation temperature of thin iron films

Results are presented for the dependence of the amorphous-polycrystalline transformation temperature, T_t, on the concentration of codeposited gaseous nitrogen in thin iron films. These are compared with the effect of other impurities on T_t for the case of amorphous iron films. It is found that there is an approximate logarithmic dependence of T_t on impurity concentration.     

X-ray photoelectron spectroscopy and calculation of the electronic structure in iron based alloys under phase transformation

An experimental method which allows us to survey alterations in the X-ray photoelectron spectra under

- to

- transition is suggested. The densities of states of valence electrons calculated in the multi-band coherent potential approximation (CPA) reflect general features of XPS measurements. Our calculations have allowed us to determine contributions from electrons which occupy e_g- and t_2g- orbitals, oriented along different directions in crystal lattice to the cohesive energy.     

利用表面磁光克尔效应监测Ni薄膜生长的物相转变

利用磁控溅射方法制备了不同厚度和退火处理的Ni薄膜.表面磁光克尔效应研究表明:冷基底无退火处理的Ni薄膜为软磁性,退火处理的Ni薄膜为铁磁性质,且无退火处理的Ni金属膜磁性随膜厚的增加而增强.分析认为,冷基底条件下Ni膜是非晶态生长,且膜内存有大量缺陷,经退火处理的Ni膜发生了重融再生长,形成了大颗粒结晶,膜内晶粒内缺陷大量消失.     

The effect of additive noise on the signal phase noise in radio over fiber links

The effect of additive noise on the signal phase noise in radio over fiber links was studied and the theory model of the output signal phase noise was proposed. The theory model was verified by experiments. The results suggest that the signal phase-noise performance decreases due to enhancing the average photocurrent of the photodiode. In practical applications, in order to reduce the deterioration of reference's phase noise, it is necessary to improve the average photocurrent when the opto–electro modulator works at quadrature points.     

CEMS study of strain induced phase transformation in manganese Hadfield steel

A Conversion Electron Mössbauer Spectroscopy, (CEMS), study of phase transformations in a Hadfield steel induced by high rate strains is reported. Hadfield steel samples were impact deformed and the ensuing changes in the magnetic properties at the deformed zone and its surroundings have been studied by CEMS. The CEMS results are compared with wear tests and optical microscopy and show a formation of martensite by impact deformation only at the surface. Martensite is not produced by compression or tensile stresses but appears after wear tests in proportions that depend on the load and velocity conditions of test. The understanding of martensite phase formation and its evolution during deformation processes is also addressed.