Numerical simulation of phase transitions in two-component materials upon noncongruent evaporation

The dynamics of noncongruent phase transitions in two-component materials upon evaporation in vacuum has been numerically simulated within a model based on the assumption of local thermodynamic equilibrium in the condensed phase. The results presented for nonstoichiometric uranium dioxide irradiated by a laser pulse have been obtained with allowance for the dependence of its thermodynamic and transport properties on the temperature and composition. It has been shown that, because of noncongruent evaporation changing the composition of the material in the surface region, its melting can begin in its interior even in the case of surface heating.     

In situ laser irradiation of WC-Co hardmetals inside an SEM

Samples of WC-6% Co hardmetals were irradiated with a pulsed CO₂ laser inside a scanning electron microscope in order to study the mechanisms and development of surface modification in microscopic detail. The photon emission of the irradiated surface was measured simultaneously. Samples with surfaces prepared in different ways were irradiated in a multiple pulse regime. The laser power density was somewhat lower than that which would lead to a melting of the polished surface. It is shown that under such irradiation conditions the decisive step in surface modification is the successive solution of the carbide in the lower melting binder phase. Above a critical number of laser pulses, in the order of a thousand, this leads to the onset of large-scale melting, pointing to a marked rise of absorbency. These alterations of surface state are well reflected in the photon emission measurements. The influence of the original binder distribution within the surface layer, changed by grinding or polishing, is investigated.     

Radiative Properties of Uranium Dioxide Near Its Melting Point

A new technique for measuring radiative properties of nuclear fuel materials at high temperatures has been developed. The technique is based on pulse diffuse optical probing of the sample surface and on pyroreflectometry used in measuring radiative properties of refractory materials during laser heating or cooling. Pulse diffuse optical probing of the sample has been realized for the first time in subsecond pyrometry of the open surface heated by laser radiation. Such a procedure of sample irradiation during sample laser heating or cooling enables reflectivity and emissivity measurements near high temperature phase transitions to be performed in spite of possible sharp changes of the reflection indicatrix at phase transitions in the investigated material. With the method developed in this study, the spectral emissivity and reflectivity of uranium dioxide near its premelting and melting points have been measured. It has been found that condensation of the vapor plume formed above the sample during laser heating influenced the melting and boiling temperatures of uranium dioxide. The first-order phase transitions in uranium dioxide, such as solid–vapor–solid and liquid–vapor–liquid, have been observed in uranium dioxide for the first time during laser heating. Also, new data on the spectral emissivity of uranium dioxide at a wavelength of 0.644 m and in the temperature range of 2000 to 4200 K are presented.     

Fast Melting of Amorphous Silicon Carbide Induced by Nanosecond Laser Pulse

We report the first experimental detailed study of laser induced surface melting on the nanoscale time scale of amorphous silicon carbide layers produced by ion implantation. Time-resolved reflectivity has been used to observe the fast liquid–solid–liquid transition features, and transmission electron microscopy (TEM) was used in order to study the structure resulting after the fast solidification following the laser induced melting. By means of the evaluation of the laser fluences required to induce melting of amorphous layers of different thickness on top of a crystalline substrate, we evaluated the thermal diffusion coefficient and the melting point of the amorphous material which occurred much lower than for crystalline material. Moreover, we give evidence of amorphous-to-crystal transitions occurring in the solid phase on the nanosecond time scale, for laser irradiation at fluences below the melting threshold. A quite different crystalline structure is observed for crystallization from the liquid phase than from the solid phase.     

In situ laser irradiation of WC-Co hard metals inside a scanning electron microscope

Cemented carbides with higher binder contents were irradiated with a CO₂-pulse laser within a SEM to investigate the structural development in more detail. The decisive medium is the melting binder which dissolves the refractory carbide phase. In this way a superficial layer of an oversaturated solid solution with highly dispersed secondary carbides is produced. At higher pulse numbers a periodic structure is engraved in the surface by local evaporation as result of a self-amplifying resonance phenomenon.     

Investigation of laser irradiation of WC-Co cemented carbides inside a scanning electron microscope (LASEM)

A combination of scanning electron microscope and laser enables direct observation of structural modifications by a high-energy input. With this new device melting phenomena and fracture processes in a WC-6% Co hard metal were investigated. The first laser pulse leads to melting of a thin surface layer with the formation of blisters and craters. Further pulses effect only small changes. This is attributed to exhaustion of the driving forces exerted from highly compressed pores. Cracking is induced by the relaxation of compressive surface stresses during the high-temperature stage and the appearance of tensile stresses during cooling. Besides crack formation and extension, complete welding of crack surfaces was observed after repeated laser irradiation.     

Surface melt dynamics and super lateral growth regime in long pulse duration excimer laser crystallization of amorphous Si films

In this work, the excimer laser induced crystallization of a-Si films on SiO₂ was investigated, using a long pulse duration (200 ns) XeCl source. The microstructural analysis of the laser irradiated area, for incident energy densities comprised between the surface and full melting thresholds of the a-Si layer, respectively, was performed by scanning electron microscopy. A numerical simulation of the surface melt dynamics was also presented and compared to the experimental observations.     

激光熔化镁合金凝固组织及腐蚀行为

采用Nd:YAG毫秒脉冲激光器,在高纯氩气保护下扫描AZ91D镁合金样品,采用X射线衍射仪(XRD),光学显微镜,扫描电镜(SEM),原子力显微镜(AFM)等对处理后镁合金表面形貌、组织、成分进行研究。使用模拟改性体液和质量分数为3.5%的NaCl溶液对实验样品进行腐蚀,观察腐蚀表面并计算材料腐蚀率。结果表明:在相同腐蚀时间下,与未被处理样品相比,激光处理后镁合金由于其显微组织中细化的α-Mg相与β-Mg_(17)Al_(12)相,及选择性气化现象和基体合金化学成分共同导致表面Al元素富集提高了表层的抗腐蚀性能;通过测算激光熔化区枝晶晶胞尺寸与冷却速率的关系得到其凝固方程。     

Determination of the reflectivity of liquid semiconductors over a wide temperature range

A new method for the determination of the reflectivity of liquid semiconductors in the temperature range from the melting point to the boiling point is presented in the paper, The method is based on the pulsed laser irradiation of the semiconductor surface. the time-resolved reflectivity (TRR) measurement technique, and the numerical simulation of the process using a nonequilibrium thermal model, Matching the experimental and computed values of the maximum reflectivity of the cw probe laser and the surface melt duration in the dependence on energy density of the laser pulse and a least-squares-based fitting procedure lead to the determination of the reflectivity of the liquid at the wavelength of the primary laser beam, The method is illustrated by experimental data on XeCl (308-nm) and ArF (193-nm) excimer laser irradiation of Si(100 ), giving the results B,=0.67±0.01-(8±I) x10^–5 (T-1687) at 308 nm and R,= 0.755 ±0.010 - (7 ± I ) x 10^–5( T-1687) at 193 rim, where R, is the reflectivity of the liquid and T is temperature in K,Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Failure mechanism of optically transparent solids subjected to a local thermal laser pulse

Experimental data on the failure mechanism of glass plates subjected to a millisecond pulse of focused laser light are presented. It is established as a result of investigation of the failure kinetics of irradiated glass plates that their failure is caused by an increase in gas pressure in the cavity formed at the focal point of the light-collecting lens. This failure mechanism of glass differs from that described earlier for the case of the irradiation of a glass plate by a nanosecond pulse of laser light, according to which the glass fails as a result of the development of a high-amplitude acoustic pressure wave near the front surface of the irradiated plate. The failure mechanism described for the glass plates agrees with a thermodynamic failure model of transparent solids.Translated from Problemy Prochnosti, No. 4, pp. 88–93, April, 1993.     

Studies of graphite surface after laser irradiation

Graphite was irradiated by CO₂ laser of energy 10J and pulse width 4 μs. SEM and XRD examinations were carried out before and after laser irradiation. It was observed that XRD pattern changed after irradiation and new entities were seen on surface. Amorphous graphite was also irradiated and it was found that it became partly crystalline. The changes were explained on the basis of phase changes induced by high pressures and temperatures during laser irradiation.     

The dynamics of annealing of ion-amorphized silicon by nanosecond pulses of excimer laser UV radiation

Phase transitions in ion-amorphized silicon annealed by nanosecond UV radiation pulses of an ArF excimer laser were studied by time-resolved reflectivity measurements at λ=633 nm. It was established that epitaxial crystallization of a melted layer of silicon takes place at a laser energy density of W < 0.8 J/cm². At 0.2 J/cm² < W < 0.7 J/cm², silicon exhibits amorphization in the course of solidification, the initial and final amorphous phases being different. This amorphization of solidifying silicon is accompanied by the nucleation of separate crystallization centers. The presence of such centers changes the kinetics of phase transformations in the course of repeated laser action, leading to the formation of a polycrystalline structure, and provides for the possibility of obtaining intermediate crystallization states of Si in a sequence of laser-induced phase transitions.     

一种反射型有机/无机复合高能激光防护涂层制备及性能研究

以酚醛树脂、环氧树脂为粘结剂,采用单一因素考察法,分别考察了ZrO₂和Al₂O₃共2种具有高反射率的耐热填料对高能激光防护涂层性能的影响,分别对其抗高能激光烧蚀性能、耐激光辐照时间、激光辐照前后涂层表面反射率等性能进行了表征,开展了辐照前后涂层表观形貌观察及XRD扫描,从微观角度确定了耐热填料晶型变化情况。结果表明,ZrO₂在高能激光辐照过程中具有良好的耐高能激光烧蚀性能,可耐受高能激光辐照功率密度达到2.35 kW/cm²,激光辐照前后涂层表面反射率提高了46.28%。     

The optical properties of graphite under intense picosecond laser illumination

Picosecond laser ellipsometry has been used in the visible to estimate for the first time the index of refraction of highly oriented pyrolitic graphite samples at high temperatures. While recent experiments seem to indicate that molten graphite behaves in many respects as a metal, investigation of optical properties with picosecond laser pulses consistently shows a reduced reflectivity value as soon as the threshold value of laser fluence for surface melting is exceeded. This occurrence may be interpreted as a reduced conductivity of the liquid phase. The experimental conditions are discussed, and in particular, the possible occurrence of optically thick layers of matter in front of the hot surface during the measurements is evaluated.Paper presented at the Second Workshop on Subsecond Thermophysics, September 20–21, 1990, Torino, Italy.     

Comparison of corrosion behaviour of friction stir processed and laser melted AA 2219 aluminium alloy

Dissolution of second phase particles (CuAl₂) present in AA 2219 aluminium improves the corrosion resistance of the alloy. Two surface treatment techniques, viz., solid state friction stir processing and fusion based laser melting lead to the reduction in CuAl₂ content and the effect of these processes on the corrosion behaviour of the alloy is compared in this study. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were carried out to compare corrosion behaviour. The corrosion resistance achieved by friction stir processing is comparable to that obtained by the laser melting technique.     

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaO_x thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaO_x films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.     

Picosecond laser pulse-driven crystallization behavior of SiSb phase change memory thin films

Transient phase change crystallization process of SiSb phase change thin films under the irradiation of picosecond (ps) laser pulse was studied using time-resolved reflectivity measurements. The ps laser pulse-crystallized domains were characterized by atomic force microscope, Raman spectra and ellipsometrical spectra measurements. A reflectivity contrast of about 15% can be achieved by ps laser pulse-induced crystallization. A minimum crystallization time of 11 ns was achieved by a low-fluence single ps laser pulse after pre-irradiation. SiSb was shown to be very promising for fast phase change memory applications.     

Dynamic fracture behaviour of Fe78Si9B13 metallic glass ribbon under laser shock loading

The fracture behaviour of Fe₇₈Si₉B₁₃ metallic glass under laser shock loading was investigated. Morphologies of the fracture surface and laser irradiated surface were characterized using scanning electron microscope. The results show that the fracture surface consists of sliding region and final fracture region with crack propagation. Liquid droplets and melted belts are scattered on the fracture surface as the notable features compared with fracture surface morphology under quasistatic loading, indicating the significant temperature increase in shear bands during dynamic loading. The primary and secondary shear bands are distributed on the specimen surface resulting from the simultaneous operation of multiple shear bands at high strain rates. Ripples with the characteristic spacing of about 1 µm are generated on the laser irradiated surface because of the interaction of laser pulse with solid surface.     

ZrO2陶瓷颗粒在激光作用下组织形貌演变规律

采用激光表面处理技术制备ZrO_2复合热障涂层可使陶瓷颗粒与金属之间实现冶金结合,增加结合强度,但由于陶瓷颗粒和金属热膨胀系数之间的差异,制备大面积复合热障涂层仍存在比较严重的开裂问题.为降低复合材料层的开裂倾向,更好地控制ZrO_2陶瓷颗粒的离散规律,本文针对激光熔注过程中陶瓷颗粒的熔化问题,采用高速摄像、扫描电子显微镜研究了氧化锆陶瓷颗粒进入熔池前后激光对其形貌与离散规律的影响规律.结果表明,在激光能量密度较低的情况下,激光直接作用于氧化锆陶瓷颗粒会导致单个ZrO_2陶瓷颗粒发生裂解现象;当激光能量密度高于2.5 J/mm~2时,ZrO_2陶瓷颗粒熔化严重并发生球化现象.在激光熔注过程中,由于受到激光和高温熔池的双重作用,氧化锆陶瓷颗粒内部晶粒长大,高能量大尺寸晶界促进了Ti熔体向ZrO_2陶瓷颗粒内部扩散.     

The influence of coatings on subsurface mechanical properties of laser peened 2011-T3 aluminum

High power Q-switched laser systems are currently being developed for use in a process known as laser shock processing or laser peening which results in significantly improved fatigue properties in aluminum components. An ablative, sacrificial coating such as paint or metal foil is used to protect the aluminum component from surface melting by the laser pulse, which adversely affects fatigue life. This paper, using nano-indentation, analyzes the effect of the paint and foil coatings on the shock wave propagation into the aluminum specimen and the resulting change in mechanical properties versus depth. Near the surface, hardness was found to be increased by the laser peening, however this process decreased the measured elastic modulus. The laser pulse energy density and properties of the foil including its adhesion to the aluminum alloy were found to influence the change in surface mechanical properties.