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Phase transformation at controlled locations in nanowires by in situ electron irradiation
摘要： Solid state phase transformations have drawn great attention because they can be effectively exploited to control the microstructure and property of materials. Understanding the physics of such phase transformation processes is critical to designing materials with controlled structure and with desired properties. However, in traditional ex situ experiments, it is hard to achieve position controlled phase transformations or obtain desirable crystal phase on nanometer scale. Meanwhile the underlying mechanisms of the reaction processes are not fully understood due to the lack of direct and real-time observation. In this paper, we observe phase transformation from body-centered tetragonal PX-PbTiO3 to monoclinic TiO2(B) on the atomic scale by in situ electron irradiation during heat treatment in transmission electron microscope, at pre-defined locations on the sample. We demonstrate that by controlling the location of the incident electron beam, a porous TiO2(B) crystal structure can be formed at the desired area on the nanowire, which is difficult to achieve by traditional synthesis methods. Upon in situ heating, the Pb atoms in the crystal migrate out of the pristine nanowire through inelastic scattering under incident electrons while high temperature(> 400 °C) provides energy for the crystallization of TiO2(B) and the volatilization of a substantial number of Pb atoms, which makes the resultingTiO2(B) nanowires to be porous. In contrast, at temperatures < 400 °C, the segregated Pb atoms form Pb particles and the TiOx nanowires remain in the amorphous state. This work not only provides in situ visualization of the phase transition from the PX-PbTiO3 to monoclinic TiO2(B), but also suggests a crystallography engineering strategy to obtain the desired crystal phase at controlled locations on the nanometer scale.
关键词： controlled locations,phase transformation,in situ,transmission electron microscopy
更新于2025-09-16 10:30:52
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Cracking behavior and control of β-solidifying Ti-40Al-9V-0.5Y alloy produced by selective laser melting
摘要： A β-solidifying Ti-40Al-9V-0.5Y (at.%) alloy with a high cracking sensitivity has been successfully fabricated by selective laser melting (SLM) in this study. The influence factors for cracking sensitivity, cracking behavior and crack inhibition mechanism were investigated. The results show that the effects of process parameters on cracking sensitivity strongly depend on the cooling rate in molten pool with different heat transfer modes. The conduction mode with higher cooling rates exhibits a higher cracking sensitivity in comparison to the keyhole mode. Microstructure characteristics and phase transformations controlled by cooling rate determine the inherent ductility of β-solidifying γ-TiAl alloys during SLM. On this basis, the formation and inhibition mechanism of solidification and cold cracking are proposed. Finally, the crack-free Ti-40Al-9V-0.5Y sample with fine equiaxed microstructures and favorable mechanical properties (microhardness of 542±19 HV, yield strength of 1871±12 MPa, ultimate strength of 2106±13 MPa and ultimate compressive strain of 10.89±0.57 %) can be produced by SLM. The strengthening mechanism can be attributed to grain refinement and precipitation strengthening.
关键词： Phase transformation,Selective laser melting,Cracking behavior,Microstructure,β-solidifying γ-TiAl alloy,Cracking control
更新于2025-09-16 10:30:52
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Surface polishes of the SKD 61 tool steel by a femto pulse laser operating in a wide range of powers
摘要： SKD 61 tool steel workpieces without prior heat treatment are adopted for pulse femto laser polish with the power in a range of 0.1-20 mW. Single-pass tracks are provided first to evaluate the width (W) and depth (D) of groove's lateral profile. With the characteristics exhibited in the nearly constant D/W data and the noticeable changes in the crystal species and the Fe content in this powers range, the three power subregions and the powers required for the solid-(plasma) ablation and the ablation-evaporation plus plasma breakdown transitions can be identified clearly. Surfaces polished with 0, 50, and 70 % as the overlap ratio are presented to investigate the mean areal roughness (Sa) and the highest roughness amplitude (HA) transition, (Sa)0% >> (Sa)50% > (Sa)70% is presented. When the power is applied beyond the threshold (14 mW, 1.7224 J/cm2) for material evaporation and plasma breakdown, affected by the laser fluence (F) in these three subregions and the overlap ratio. As power is applied below the solid-ablation transition (9~10 mW, F: 1.3777~1.5239 J/cm2), (Sa)0% >> (Sa)70% > (Sa)50% is obtained; if it is applied higher than this frequency analysis; phase transformation.
关键词： D/W ratio,femtosecond laser polishing,phase transformation,spatial frequency analysis,ablation threshold
更新于2025-09-12 10:27:22
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Laser-induced Si3N4–TiN ceramics degradation
摘要： This paper presents the process of degradation of the surface layer of Si3N4?TiN ceramics induced by a pulsed Nd:YAG laser. The dissociation of silicon nitride and titanium nitride in the irradiation zone is turned out to yield in a silicon depletion and so the zone becomes enriched with titanium, which, in turn, leads to the formation of TiSi2. This process of the TiSi2 formation can be considered as a ceramics metallization and its concentration depends on the energy delivered by the laser. The expansion of dissociation/evaporation products through the atmosphere is accompanied by the formation of SiO2 and TiO2, which are then deposited on the substrate as an amorphous ?lm of complex composition.
关键词： Surface phase transformation,(Si3N4 ? TiN):(Al2O3+Y2O3) ceramics,Film formation
更新于2025-09-12 10:27:22
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Direct Laser Patterning and Phase Transformation of 2D PdSe <sub/>2</sub> Films for On-Demand Device Fabrication
摘要： Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe2 thin films using high power or trigger the local transformation of PdSe2 into a metallic phase PdSe2?x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe2 and a variety of stable phases compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe2 material. The process sensitivity to the laser power allows patterning flexibility for resist-free device fabrication. The laser-patterned devices demonstrate that a laser-induced metallic phase PdSe2?x is stable with increased conductivity by a factor of about 20 compared to PdSe2. These findings contribute to the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.
关键词： device fabrication,phase transformation,PdSe2,FET,laser patterning,2D materials,graphene
更新于2025-09-12 10:27:22
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Diffusion of Oxygen and Nitrogen into Titanium under Laser Irradiation in Air
摘要： This paper examines the dynamic interactions between pure titanium and ambient air under transient laser processing conditions. The microstructures and composition distributions of the reaction products on and within the titanium samples are reported, which lead to the estimation of high-temperature diffusion coefficients for nitrogen and oxygen into the titanium substrate. In-situ synchrotron X-ray diffraction testing is conducted to reveal the high temperature reaction steps between titanium and air. The formation mechanisms of both titanium nitrides and titanium oxides are discussed.
关键词： titanium oxidation,titanium nitridation,synchrotron X-ray diffraction.,in-situ phase transformation,laser irradiation
更新于2025-09-12 10:27:22
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Modeling the Temperature Distribution during Laser Hardening Process
摘要： A mathematical model was developed and is presented here to calculate the temperature distribution on the surface and bulk of a steel plate under the laser hardening process. The model starts from the basic heat equation, is then developed to a volumetric form and is connected to the various solid phases existing. The model is based on three strongly influencing parameters of the laser hardening process: velocity of the laser spot and irradiation time. The results are compared to the available experimental data reported in the literature. The volumetric model provides an assessment of temperature distribution in both the vertical and horizontal axis. Laser irradiation at sufficiently high fluence can be used to create a solid state phase change on the surface. Primary calculations show that the temperature profile has a Gaussian distribution in horizontal x-and y-axis and presents an exponentially decreasing distribution in the horizontal and vertical depth directions.
关键词： Laser hardening,heat treatment,Solid phase transformation,Temporal-temperature profile
更新于2025-09-12 10:27:22
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Exploring the Ti-5553 phase transformations utilizing in-situ high-temperature laser-scanning confocal microscopy
摘要： Recently, the β metastable Ti-5553 alloy has attracted significant interest owing to the possibility of achieving excellent mechanical properties. In this study, for the first time, the Ti-5553 phase transformations were investigated using high-temperature laser-scanning confocal microscopy (HT-LSCM). During a continuous heating of an aged sample, the α phase dissolution was not clearly visible owing to the remaining grooves of former grain and interphase boundaries. However, a continuous cooling from the β field showed that the α phase precipitation started preferentially at grain boundaries. With the increase in cooling rate, the α precipitates became more refined and the supercooling necessary to promote allotriomorphic precipitation increased. At a cooling rate of 100 °C min-1, no α phase was observed in the analyzed field of view. A higher oxygen content was suggested to contribute to the microstructural differences between the free surface and bulk. The volume fraction of the α phase was lower than that obtained on the original surface at cooling rates of 5, 25, and 50 °C min-1, while only minor differences were observed at cooling rates of 100 and 250 °C min-1. The α phase precipitation was analyzed in detail. The HT-LSCM technique can be a useful method for the analyses of solid-state phase transformations in Ti alloys.
关键词： Titanium,In-situ,Ti-5553 alloy,HT-LSCM,Phase transformation,Microstructure
更新于2025-09-12 10:27:22
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Titanium and nitrogen interactions under laser additive manufacturing conditions
摘要： To understand how to make bulk titanium parts or coatings with desired levels of titanium nitrides, this paper investigates the dynamic interactions between titanium and nitrogen under representative laser-based additive manufacturing (AM) conditions. Under a set of gas environments containing different concentrations of nitrogen, the titanium and nitrogen reaction products—formed under typical Selective Laser Melting (SLM) and Laser Engineered Net Shaping (LENS) AM scanning conditions—are examined for compositions, phases, and microstructures. In-situ synchrotron X-ray diffraction (SXRD) test is performed to reveal the high temperature reaction steps between titanium and nitrogen.
关键词： Synchrotron X-ray diffraction,in-situ phase transformation,Titanium nitridation,Laser melting
更新于2025-09-11 14:15:04
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Effect of tungsten carbide partial dissolution on the microstructure evolution of a laser clad surface
摘要： Laser surface modification has been a way to promote wear resistance in dies and molds application. Previously, researchers have succeeded in enhancing surface properties through laser surface modification. The addition of particles in laser cladding process enhanced the surface properties by strengthening the modified AISI H13 tool steel structure. Controlled parameter was laser power, pulse repetition frequency (PRF) and scanning speed with a range of 1.7–2.5 kW, 30–70 Hz and 10.5–24.5 mm s?1 respectively. The powder addition executed by a preplaced method. The grain size and hardness properties of treated samples were characterized using scanning electron microscope (SEM) and hardness Vickers indenter respectively. Surface roughness was characterized using roughness tester. From the findings, tungsten carbide (WC) particles were dissolved homogenously within refined substrate grain structure at higher laser energy. Higher laser scanning speed contributed in uniform particles distribution. The grain refinement with W element in modified layer resulted in maximum hardness of 660 HV. Rapid solidification during laser processing produced metastable phase formation grain refinement, and a higher fraction of grain boundary which resulted in grain boundary strengthening, grain refinement and metastable phase formation. In this paper laser cladding of AISI H13 tool steel with micron size WC particles addition for enhanced surface properties was investigated. These findings are important to design high precision modification of die surface for high temperature forming process.
关键词： Laser cladding,Nd:YAG laser,Grain refinement,Phase transformation,Carbide dissolution,Tungsten carbide particle
更新于2025-09-11 14:15:04