Laser surface melting of a complex high alloy steel

The effect of laser surface melting (LSM) on the microscopic morphology of a complex high alloy steel has been examined in single track experiments. Different combinations of power (1–4 kW) and scan speed (500–1000 mm min⁻¹) were selected in order to obtain a range of depths of melt penetration. An increase in the depth of melt penetration has correlated with rise in the estimated surface temperature induced by laser melting. Within the strongly refined structure of the laser melted alloy, the interdendritic spacing, λ varied from 0.29 μm to 0.97 μm with increase in the depth of melt penetration. The microhardness in the laser surface melted region has been shown to decrease linearly with λ^−0.5.     

On the strength of a body at temperatures close to its melting point

On the example of frozen soils and ice existing at temperatures close to the melting point, the existence of two mechanisms (types) of destruction related to the phase transitions of water is shown. The first mechanism that operates near the melting point is plastic destruction, it gradually converts an ice body into a liquid-like state. The second mechanism operating far from the melting point is brittle destruction, disintegration into parts, with a slight preliminary deformation. For both types of destruction equations of long-term strength and of the behavior of deformation in time have been obtained.     

Extended investigation on high velocity pseudo surface waves

Recent communication equipment such as mobile and cellular phones, radio systems, pagers, LANs, have demanded high performance components. Among these components, the last generation of SAW filters and signal processing devices, presenting low loss, flexible frequency and phase response characteristics, control of spurious, and so forth, have played a major role in designing new equipment and redesigning existing systems. The highest frequency obtainable with SAW technology in practical devices is limited to a couple of GHz, usually due to restrictions in the fabrication process involved and SAW propagation characteristics. The pseudo-SAW and the shear horizontal mode, presenting phase velocities circa 40% superior than the SAW and low attenuation along certain directions, have permitted the construction of devices operating at higher frequencies. The high velocity pseudo-SAW, with phase velocities about 100% higher than the SAW and low attenuation in many materials along certain directions, extends the high operating limit of SAW devices even further. In this paper the major characteristics of this new type of wave are reviewed. Extended topics such as: the boundary function magnitude behavior, the relationship between the "growing tilted bulk-like partial waves" and the bulk slowness, the number of roots (uncoupling of modes), and the Poynting vector behavior with depth are explored, enlightening the solution and behavior of this new type of high velocity pseudo-SAW.     

Recovery of materials impacted at high velocity

Hypervelocity impact can produce unique effects in materials, including crystal structures, microstructures, and properties. Examples include impact-driven shock waves to synthesize novel materials 1 mm and 1 μm thin shocked to pressures up to 100 GPa (1 Mbar), preferential crystallographic alignment achieved by taking account the shape and size of powder particles, and high-pressure phase transitions quenched in geological materials. Thin speciments are used to performed by the highest quench rates. Methods are described which show that the experiments can be performed by precooling or preheating specimens in the range -170° to +1000° C. Calculational results for the quartz experiments show the importance of computational simulations to determine the pressure history in the specimen.     

MgO材料高压熔化特性的分子动力学模拟

用Shell模型分子动力学(MD)方法对MgO材料的高压熔化特性进行了研究,势函数取Buckingham形式.将计算得到的熔化曲线与实验以及其它理论结果在0～140GPa的压力范围内做了比较,通过修正得到的MgO熔化温度和实验测量能够很好地符合.MgO熔化模拟的结果对固体材料一阶相变分子动力学模拟的过热理论做了进一步的验证.另外,我们对反映体系微观结构的MgO径向分布函数(RDF)进行了分析.     

电子束端面熔炼法制备高纯钨的研究

采用250 kW的电子束熔炼炉对纯度为99.955%的钨棒进行提纯研究,利用扫描电镜、电感耦合等离子体光谱法(ICP-MS)和微米压痕仪分别对原料钨棒和熔炼后钨锭的形貌、纯度和显微硬度进行观察和测试.研究表明:在电子束轰击原料钨棒的过程中,钨棒端面的边缘最先熔化;经电子束熔炼后,钨棒的整体纯度显著提高,达到了99.975%,间隙杂质O和C的脱除率分别达到55.5%和45.8%;非间隙杂质的蒸汽压与脱除率密切相关,高蒸汽压的杂质元素Cd、As、K、Mg脱除较为完全,其脱除率分别为95%、90%、75%和71.4%,在钨棒中含量分别降至1、1、5和10μg/g.     

Estimation of the surface normal velocity of high frequency ultrasound transducers

This paper is concerned with the characterization of the true locally resolved surface normal velocity of an assumed piston-type ultrasonic transducer. Instead of involving a very complicated direct pointwise measurement of the velocity distribution, an inverse problem is solved which yields a spatially discretized weighting vector for the surface normal velocity of the transducer. The study deals with a spherically focused high frequency transducer, which is driven in pulse-echo mode. As a means of posing the inverse problem, the active transducer surface is divided into annuli of equal surface so that for each annulus the spatial impulse response can be calculated. An acrylic glass plate acts as a simple structured target. The resulting ill-posed nonlinear inverse problem is solved with an iterative regularized Gauss-Newton algorithm. The solution of the inverse problem yields an estimated weight for the surface normal velocity for each annulus. Experimental results for a thin copper wire target are compared to simulation results for both uniform and estimated surface normal velocities.     

Thermal measurements at the body surface in a supersonic nitrogen flow

Measurements of the heat flux toward the surface of a body of revolution in a supersonic nitrogen flow with Mach number 4 were carried out. Inside the body, an electromagnetic facility is mounted which generates a gas-discharge plasma between electrodes flush-mounted at the body surface. The plasma rotates around the body surface and significantly influences the supersonic flow. During operation of the electromagnetic facility, the heat flux toward the body surface increases and depends on the direction of the electric current flowing through the plasma. The heat flux was measured by fast-response gradient heat flux sensors based on anisotropic bismuth crystals.     

Toward generalization of calculations of the stationary velocity of diffusion flame propagation over the surface of a fuel

The results of numerical investigations of the laws governing the spreading of a two-dimensional diffusion flame over thermally thin layers of a cellulose material using the algorithm of calculation of stationary velocity of flame propagation, which is based on the principle of minimum entropy production, are presented. The obtained dependences of the velocity of flame propagation on thermophysical parameters (thickness of the fuel, oxygen concentration and pressure of the ambient, velocity of the blowing flow) agree quantitatively with the well-known experimental data. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 3, pp. 103–111, May–June, 2007.     

Formation of a snow-firn layer in surface melting of snow

We consider the problem of formation of a snow-firn stratum in a mass of snow. We assume that initially on the surface melt water is formed, which subsequently seeps through a porous medium and crystallizes due to cooling. The change in the porosity during the crystallization of the water in the mass of snow is analyzed.     

Computational analysis of a sandwich shield with free boundary inserted fabric at high velocity impact

In this paper, a novel hybrid composite shield to protect space structures from hypervelocity impact of micrometeoroid and space debris is proposed. The finite-element model of the proposed shield was constructed and finite-element analysis was conducted to approximate the energy absorption rate. Before the final model analysis, analysis of each component including the aluminum plate (front plate), PMMA plate (rear plate), and intermediate layer of fabric was performed, verifying the finite-element model of each component. The material properties used in the analysis were predicted material property values for high strain rates. The analysis results showed that, other than the fabric, the energy absorption rate of each component was in agreement. Afterwards, the finite-element model of the hybrid composite shield was constructed, where it was analyzed for the constrained and unconstrained fabric boundary condition cases. Through the finite-element analysis, the fiber pullout mechanism was realized for the hybrid shield with free boundary inserted fabric, and it was observed that this mechanism led to energy absorption increase.     

Superfluid transport over a perspex surface

Transport phenomenon has been investigated over the He II film on a perspex surface. Some peculiarities have been established that were not observed before with other substrate materials. Emptying a perspex container takes place not only with positive but with negative level differences as well. In filling the container the transport starts only when the outer liquid surface is near the ampoule rim and ceases long before the levels become equal. A qualitative explanation of the observed phenomena has been suggested, namely, as a result of electrification of the substrate.     

A surface microwave discharge at high pressures of air

An investigation is performed of a microwave discharge generated on the external surface of a quartz antenna at high pressures of air, where the electron-molecule collision frequency is much higher than the circular frequency of electromagnetic field. Under these conditions, a surface microwave discharge consists of a system of thin plasma channels with transverse dimensions of 0.1–0.2 mm. The dynamics of development of surface microwave discharge are studied, and the dependences of longitudinal dimension of discharge and of longitudinal velocity of its propagation on microwave power input and on duration of stimulation are measured. It is demonstrated that, at the initial stage of forming of discharge, the longitudinal velocity of its propagation reaches a value of 2–30 km/s and, at high pressures of air p > 30 torr, the main mechanism providing for the propagation of discharge is the breakdown wave.     

The surface tension of liquid silicon at high temperature

The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO₂ and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.     

Position, velocity profile measurement of a moving body bymicrowave interferometry

A telemetric sensor that uses microwave interferometry to determine the velocity profile of a moving body following a known linear trajectory is proposed. By a slight modification of the design, it is possible to determine both the velocity profile and the distance to the trajectory. The principle of the method and the preliminary experiments carried out in the laboratory before extending the process to full-sized outdoor applications are described. The experimental results confirm the feasibility of the concept     

Errors in the measurement of the steady-state flow of heat at the surface of a body

We have undertaken an analysis of the distortions in the flow of heat in an object, said distortions attributable to the presence of a heat-metering thermometer, and we have derived formulas to estimate this measurement error.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 4, pp. 667–674, October, 1989.     

Mathematical model of the ice protection of a human body at high temperatures of surrounding medium

This paper presents a mathematical model describing the heat transfer inside of a human body at high temperatures of surrounding medium. The human body is considered as a slender body with heat transfer dominating in horizontal planes neglecting the heat fluxes in the vertical direction. Each body cross section is represented as an ellipse with five typical layers: inner and outer cores, muscles, fat layer and skin. The body is covered by the textile with embedded ice layer to protect the human from the overheating. The heat transfer within each horizontal plane is modelled using a simple sector model. With these simplifications the problem is reduced to the solution of the one dimensional heat conduction equation with certain initial and boundary conditions. The equation is solved numerically using inhouse code based on the finite differential method. Numerical calculations are performed with the aim to determine the ice layer thickness necessary to keep the temperature of the human body core at the temperature of 36.7^°±1^° within one hour. Results of simulations were utilized to design a special protection clothes for rescue team working in the mining industry. The experiments with clothes supported the simulation prediction.