Laser compression of monocrystalline tantalum

Monocrystalline tantalum with orientations [1 0 0] and [1 1 1] was subjected to laser-driven compression at energies of 350–684 J, generating shock amplitudes varying from 10 to 110 GPa. A stagnating reservoir driven by a laser beam with a spot radius of ～800 μm created a crater of significant depth (～80 to ～200 μm) on the drive side of the Ta sample. The defects generated by the laser pulse were characterized by transmission and scanning electron microscopy, and are composed of dislocations at low pressures, and mechanical twins and a displacive phase transformation at higher pressures. The defect substructure is a function of distance from the energy deposition surface and correlates directly with the pressure. Directly under the bottom of the crater is an isentropic layer, approximately 40 μm thick, which shows few deformation markings. Lattice rotation was observed immediately beneath this layer. Further below this regime, a high density of twins and dislocations was observed. As the shock amplitude decayed to below ～40 GPa, the incidence of twinning decreased dramatically, suggesting a critical threshold pressure. The twinning planes were primarily {1 1 2}, although some {1 2 3} twins were also observed. Body-centered cubic to hexagonal close-packed pressure induced-transformation was observed at high pressures (～68 GPa).The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on the constitutive response, and the similarities and differences are discussed in terms of the mechanisms of defect generation.     

Strong morphological and crystallographic texture and resulting yield strength anisotropy in selective laser melted tantalum

Selective laser melting (SLM) makes use of a high energy density laser beam to melt successive layers of metallic powders in order to create functional parts. The energy density of the laser is high enough to melt refractory metals like Ta and produce mechanically sound parts. Furthermore, the localized heat input causes a strong directional cooling and solidification. Epitaxial growth due to partial remelting of the previous layer, competitive growth mechanism and a specific global direction of heat flow during SLM of Ta result in the formation of long columnar grains with a 〈1 1 1〉 preferential crystal orientation along the building direction. The microstructure was visualized using both optical and scanning electron microscopy equipped with electron backscattered diffraction and the global crystallographic texture was measured using X-ray diffraction. The thermal profile around the melt pool was modeled using a pragmatic model for SLM. Furthermore, rotation of the scanning direction between different layers was seen to promote the competitive growth. As a result, the texture strength increased to as large as 4.7 for rotating the scanning direction 90° every layer. By comparison of the yield strength measured by compression tests in different orientations and the averaged Taylor factor calculated using the viscoplastic self-consistent model, it was found that both the morphological and crystallographic texture observed in SLM Ta contribute to yield strength anisotropy.     

Texture evolution of high purity tantalum under different rolling paths

Deformation behavior and texture evolution of the material can be significantly affected by strain path change. For this reason, two rolling methods, unidirectional rolling (UR) and clock rolling (CR), were employed to manufacture tantalum plates. Texture evolution during unidirectional rolling and clock rolling was studied respectively by orientation distribution function (ODF). Related annealed microstructures were investigated by orientation image map (OIM). Usually, unidirectional rolling led to a strengthening of the main texture component with increasing strain, but for tantalum dominant texture component {0 0 1} θ-fiber was stable after 70% deformation, while minor texture component {1 1 1} γ-fiber was enhanced with increasing strain. In clock rolling, both of the two fibers were not stable any more for their intensity varied with rolling pass. After the final deformation, a similar texture was produced by the two rolling methods. However, recrystallization texture revealed a big difference. Such different texture development was contributed to microstructural change resulted from rolling path change.     

Corrosion behavior and surface characterization of tantalum implanted TiNi alloy

TiNi shape memory alloy has been modified by Ta plasma immersion ion implantation technology to improve corrosion resistance. The results of the polarization tests show that the corrosion resistance of TiNi alloy in Ringer's solution at 310 K has been improved by the Ta ion implantation and the Ta/TiNi sample with a moderate incident dose of 1.5 × 10¹⁷ ions/cm² exhibits the best corrosion resistance ability. The surface characterization and chemical composition of the Ta/TiNi samples were determined by Auger electron spectroscopy (AES), Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) methods. AFM images reveal that compact aggregates of nano-grains uniformly disperse on the surface of the Ta/TiNi samples. AES and XPS analyses on the Ta/TiNi sample show that the component of the surface layer is mainly composed of TiO₂ and Ta₂O₅, which is benefit to the corrosion resistance ability and biocompatibility.     

Microstructure and corrosion resistance of tantalum after nitrogen ion implantation

This paper investigates the effect of nitrogen ion implantation on surface structure as well as resistance against tantalum corrosion. Bulk Ta surface was implanted with 30?keV nitrogen ions at a temperature of 100°C with doses between 1?×?10¹⁷ and 1?×?10¹⁸?ions/cm². The implanted samples were characterised by atomic force microscopy, X-ray diffraction analyses and the corrosion test to identify structural, compositional and electrochemical changes at various doses. The experimental results indicate the formation of hexagonal tantalum nitride (TaN_0.43), in addition to the fact that by increasing the ion dose, nitrogen atoms occupy more interstitial spaces in the target crystal, a case which can significantly improve corrosion resistance. The maximum extent in the improvement of the micro hardness was 75% and the reduction in the corrosion current was 83%. According to scanning electronic microscopy and corrosion results, in the dose of 1?×?10¹⁸?ions/cm² the highest corrosion resistance was received against the H₂SO₄ corroding media.     

激光技术在材料加工中的应用现状与展望

简述了激光加工的特点，综合介绍了激光切割、激光打孔、激光标记、激光焊接、激光表面热处理、激光成形以及激光快速原型在材料加工中的应用现状，并结合现在的加工要求，对激光技术未来的发展趋势进行了展望。     

On the development of grain growth resistant tantalum alloys

During exposure of pure Ta to temperatures up to 1800 °C pronounced grain growth combined with embrittlement becomes a major problem. Doping with elements which form nanometer to submicron sized oxide or silicide particles is an appropriate way to prevent, or at least to hamper, uncontrolled grain growth. In the present paper the effects of doping with varying combinations of Si and Y on microstructure and mechanical properties of cold-worked and annealed Ta have been investigated. For these purposes methods including small-angle neutron scattering and transmission electron microscopy as well as light-optical microscopy have been applied. Ta samples doped with Si show a higher hardness and strength than those doped with Y or made from pure Ta powder. However, the grain growth behaviour of all alloy variants is rather similar. The particle size distributions of doped Ta change significantly with varying annealing treatments as coarsening and dissolution of the prevailing particles (oxides and silicides) take place. Therefore, particles do not play a significant role in grain growth kinetics at temperatures as high as 1800 °C. However, this loss in retarding force is partially compensated for by an increased solution drag stemming from elements in solid solution.     

Thermodynamic evaluation of the Ta–O system from pure tantalum to tantalum pentoxide

The partial thermodynamic functions of the tantalum–oxygen system (up to O/Ta=2.5) were evaluated from 873 to 3120 K. The pressure–composition–temperature (p–C–T) relationship in the single-phase region of solid Ta was investigated. Relationships for other phases where no data are available in the literature were proposed based on thermodynamic analysis. The results are presented as oxygen isobars superimposed on a phase diagram.     

Synthesis of FCC Mg–Ta hydrides using GPa hydrogen pressure method and their hydrogen-desorption properties

Magnesium–tantalum ternary hydrides with an FCC structure were prepared by an ultra high-pressure technique. The FCC Mg–Ta hydride phases with the Ca₇Ge-type super-lattice structure were formed at 4 and 8 GPa. Another new phase, Mg₃TaH_y, was also verified by XRD and SEM in the specimen prepared at 8 GPa. From the temperature-programmed desorption (TPD) measurements with a heating rate of 10 K/min under vacuum, the FCC hydride exhibited hydrogen desorption of ca. 3.1 mass% at an onset temperature of 583 K. When the FCC hydride phase coexisted with Mg₃TaH_y, simultaneous decomposition occurred and the hydrogen-desorption temperature was lowered to 543 K. The hydrogen-desorption temperatures of the Mg–Ta FCC hydrides were 130–170 K lower than that of MgH₂.     

Investigation of the initial stage of borating tantalum

The results of investigation of initial stage of the tantalum borating as well as the effect of oxygen and hydrogen on it are presented. The scheme of the initial stage of tantalum boron saturation is proposed.     

激光抛光机理及应用

介绍了一种新型材料表面处理技术——激光抛光，并对其机理、优点、应用实例等相关内容作了简要阐述。     

Microstructure Evolution of Intermetallics Produced by Laser Powder Deposition

NiAl intermetallic alloys are potential high temperaturestructural materials in aeronautical and astronauticalindustries[1].However,the poor ductility andtoughness at room temperature severely restrict theirengineering applications[2,3].It is important todevelop new technology to product intermetallicmaterials for the improvement of the comprehensiveproperties and the workability.Laser powder depositionhas been used to synthesize many new materials.Furthermore,if incorporated with rapid proto…     

新型产品表面激光处理控制系统的设计与实现

对产品表面质量进行检测和控制是产品加工中的重要环节。提出一种新型的产品表面激光处理智能控制系统的软硬件设计方法,实现产品表面光斑的三维能量图的计算、显示和处理。实际应用表明,使用该系统能有效提高加工效率和产品质量。     

Ultrahigh-pressure consolidation and deformation of tantalum carbide at ambient and high temperatures

The deformation mechanism of the ultrahigh-temperature ceramic, tantalum carbide (TaC), consolidated at room temperature at a very high hydrostatic pressure of 7.7 GPa is investigated using high-resolution transmission electron microscopy. The deformation behavior of TaC at room temperature is also compared with that consolidated at high temperature (1830 °C) at a similar pressure. TaC could be consolidated to a bulk structure (90% theoretical density) at room temperature. The deformation mechanisms operating at room temperature and 1830 °C are found to be significantly different. The room-temperature deformation is dominated by the short-range movement of dislocations in multiple orientations, along with nanotwinning, grain rotation, crystallite misorientation with low-angle grain boundary formation and lattice structure destruction at interfaces. In contrast, at high temperature, the strain is accommodated mostly by a single slip system, forming a parallel array of dislocations. The consolidation at room temperature occurs by heavy deformation with the support from short range diffusion, whereas the consolidation at high temperature is mostly diffusion dominated, indicating a classic sintering mechanism. The improved degree of consolidation with fewer defects results in significantly improved elastic modulus and hardness in the case of high-temperature consolidate.     

Model based optimization criteria for the generation of deep compressive residual stress fields in high elastic limit metallic alloys by ns-laser shock processing

Laser Shock Processing (LSP) is based on the application of a high intensity pulsed Laser beam (I > 1 GW/cm²; τ < 50 ns) on a metallic target forcing a sudden vaporization of its surface into a high temperature and density plasma that immediately develops inducing a shock wave propagating into the material.The main acknowledged advantages of LSP consist on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behavior, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Due to these specific advantages, Laser Shock Processing is considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials, and is being developed as a practical process amenable to production technology.In this paper, a model based systematization of process optimization criteria and a practical assessment on the real possibilities of the technique is presented along with practical results at laboratory scale on the application of LSP to characteristic high elastic limit metallic alloys, showing the induced residual stresses fields and the corresponding results on mechanical properties improvement induced by the treatment. The homogeneity of the residual stress fields distribution following the laser treatment spatial density will be specially analyzed.     

Laser surface hardening of martensitic stainless steel hollow parts

This paper deals with numerical simulation and experimental validation of laser heat treatment of an industrial axisymmetric hollow mechanical part by means of a strategy based on helical tracks. This component, made of AISI 420B martensitic stainless steel, features a low wall thickness and, according to this, it is not easily treatable by means of laser surface hardening. The work carried out was aimed at demonstrating that numerical simulation allows a drastic reduction of the experimental activity and that it makes possible to prove a very high sensitivity of the results to process parameters variation.     

激光造型技术在摩擦副表面处理中的应用

摩擦副表面的微细形貌深刻影响着摩擦副的摩擦学性能,因而人们利用各种加工方法对摩擦副进行表面处理以达到所需的摩擦学性能,激光表面造型技术以其诸多的优点而被认为是摩擦副表面处理的理想手段.首先阐述了激光表面造型技术的原理及其减磨机理.介绍了激光表面造型专用设备的基本组成及其控制原理,接着详细论述了激光表面造型技术在重要摩擦副处理中的应用及取得的效果,最后提出了激光表面造型技术的发展和应用前景.     

The formation of supersaturated solid solutions in Fe-Cu alloys deformed by high-pressure torsion

Fully dense bulk nanocomposites have been obtained by a novel two-step severe plastic deformation process in the immiscible Fe-Cu system. Elemental micrometer-sized Cu and Fe powders were first mixed in different compositions and subsequently high-pressure-torsion-consolidated and deformed in a two-step deformation process. Scanning electron microscopy, X-ray diffraction and atom probe investigations were performed to study the evolving far-from-equilibrium nanostructures which were observed at all compositions. For lower and higher Cu contents complete solid solutions of Cu in Fe and Fe in Cu, respectively, are obtained. In the near 50% regime a solid solution face-centred cubic and solid solution body-centred cubic nanograined composite has been formed. After an annealing treatment, these solid solutions decompose and form two-phase nanostructured Fe-Cu composites with a high hardness and an enhanced thermal stability. The grain size of the composites retained nanocrystalline up to high annealing temperatures.