Ti-BN粉末冶金原位制备Ti(N)-TiBw复合材料

本文以纯Ti粉末和BN粉末为原料,采用放电等离子烧结技术(SPS)通过原位反应制备了Ti(N)-TiBw复合材料,研究了退火温度对Ti(N)-TiBw复合材料显微组织演化和力学性能的影响。结果表明:在1000 ℃进行烧结时,Ti与BN发生原位反应生成了TiBw和N固溶复合增强钛基复合材料。TiBw以针状形式呈网络状分布于一次颗粒边界处,随着热处理温度的升高,TiBw的长径比先增大后减小,在1100 ℃时达到最大值。而在1100 ℃以上退火处理时,TiBw逐渐发生粗化,微观形貌从针状变成短棒状,其对基体的钉扎效果明显减弱,Ti基体晶粒尺寸逐渐粗化,TiBw的形貌演变遵循Ostwald熟化机制。随着热处理温度的升高,材料的强度先提高后降低,在1000 ℃达到最大值,为908 MPa。材料强度的提升归因于晶粒细化、TiBw载荷传递和O/N的固溶强化。     

In situ technique for synthesizing multiple ceramic particulates reinforced titanium matrix composites (TiB + TiC + Y2O3)/Ti

In situ synthesized titanium matrix composites reinforced with multiple ceramic particulates including TiB, TiC and Y₂O₃ were fabricated by non-consumable arc-melting technique utilizing the chemical reaction among Ti, B₂O₃, B₄C and Y. The thermodynamic feasibility of the in situ reactions has been considered. X-ray diffraction (XRD) was used to identify the phases in the composites. Microstructures of the composites were observed by means of optical microscope (OM), scanning electron microscope (SEM) and electron probe. It is concluded that multiple reinforcements are synthesized and they show different shapes: TiB grows in needle shape; TiC grows in near-equiaxed and rod-like shapes; Y₂O₃ grows in near-equiaxed shapes when the content of Y is 0.6 wt.% and grows in dendritic shapes when the content of Y increases to 1.8 wt.%. Reinforcements TiB, TiC and Y₂O₃ are distributed uniformly in the titanium matrix.     

In situ observation and thermodynamic calculation of MnS in 49MnVS3 non-quenched and tempered steel

Based on thermodynamics calculation, the results of the formation temperature of MnS inclusions of non-quenched and tempered steel during heating process were discussed. It is shown that while the solid fraction is 0.9, MnS inclusions began to precipitate in the final stage of solidification. The solidification process of 49MnVS3 non-quenched and tempered steel during heating has been observed in situ using a confocal scanning laser microscope (CSLM), which agrees well with the thermodynamics calculation. MnS particles were coarsening during heating process, which would reduce the pinning effect on the austenite grain boundaries and bring about the sudden growth of some austenite grains in this stage.     

In situ observations and crystallographic analysis of martensitic transformation in steel

The development of a martensitic structure in a low-carbon and low-alloy steel was characterized using in situ confocal laser microscopy, high-speed photography and crystallographic analysis, including the nature of variant selection. The initial stage of transformation involves the partitioning of the austenite grain into packets, after which the rate of transformation is gradual. The crystallographic orientation of the plates that form is not random, but involves selection determined by the relationship between the shape deformation direction and the free surface. The vicinity of austenite grain and twin boundaries, and martensite/austenite interfaces also affect variant selection.     

球墨铸铁加热过程中渗碳体石墨化的原位观察

采用激光共聚焦扫描显微镜直接观察了球墨铸铁在加热过程中渗碳体的分解和石墨形态的演化。结果表明：球墨铸铁在加热过程中渗碳体逐渐溶解,加热速率越快,渗碳体的分解速率越快。加热过程中石墨形态的演变行为与加热速率有关。加热速率较慢,组织中出现了小石墨,原有石墨长大。加热速率较快,渗碳体周围的小石墨逐渐溶解变小。     

Zr—4合金单调和循环变形结构的扫描电镜原位观察

利用ＳＥＭ原位观察了Ｚｒ－４合金单调和循环变形过程中,试样表面滑移,裂纹萌生及扩展特征。结果表明：拉伸过程中,随着外应力增大,滑移程度加剧,从单滑移逐渐向多滑移转化;     

In situ synchrotron tomography estimation of toughening effect by semi-ductile fibre reinforcement in a tungsten-fibre-reinforced tungsten composite system

Tungsten-fibre-reinforced tungsten composites (W_f/W) are supposed to enable enhanced toughness owing to extrinsic energy dissipation mechanisms such as interface debonding and plastic deformation of fibre. In particular, the latter is an effective source of toughening, since ductile tungsten fibres can absorb a considerable amount of plastic work. For a precise evaluation of the toughening capability, the energy dissipation mechanisms need to be analysed in detail. To this end, single-fibre tungsten composite specimens are fabricated and the stress–strain behaviour of the tungsten fibre bridging a matrix crack is measured by means of in situ high-energy synchrotron microtomography during a uniaxial tensile test. Despite the high X-ray attenuation in tungsten, a sufficiently high resolution is achieved and clear images of crack extension and deformation are obtained. The amount of absorbed energy due to plastic deformation of the tungsten fibre is determined and compared with values obtained conventionally from single-fibre tensile tests.     

Impact of He and Cr on defect accumulation in ion-irradiated ultrahigh-purity Fe(Cr) alloys

The effect of He on the primary damage induced by irradiation in ultrahigh-purity (UHP) Fe and Fe(Cr) alloys was investigated by transmission electron microscopy (TEM). Materials were irradiated at room temperature in situ by TEM in a microscope coupled to two ion accelerators, simultaneously providing 500 keV Fe⁺ and 10 keV He⁺ ions. Single Fe ion and dual Fe and He ion beam experiments were performed up to a dose of 1 dpa and to a He content of up to 1000 appm. Defects appear in the form of nanometric black dots with sizes between 1 and 5 nm. Defocused images reveal a dense population of sub-nanometric cavities after both single-beam and dual-beam irradiation. In Fe(Cr) alloys, the number densities of visible black dot defects still resolved in TEM are significantly higher after single than after dual-beam irradiation. In UHP Fe, conversely, the presence of He strongly increases the defect number density. The presence of He changes a a₀〈1 0 0〉 dominated defect population to a 1/2a₀〈1 1 1〉 dominated one in all materials, and the more so in UHP Fe. It appears that Cr increases the number of visible defects relative to UHP Fe. The dependence with increasing Cr content is weak, however, showing only a slight decrease in the number densities. The decrease in the density of visible a₀〈1 0 0〉 loops and increase in the visible 1/2a₀〈1 1 1〉 loops in all materials when He is present supports the idea that visible a₀〈1 0 0〉 loops are formed by the interaction between mobile 1/2a₀〈1 1 1〉 loops, as the latter would be immobilized by He already at sub-microscopic sizes. It is concluded that the primary loop population is dominated by 1/2a₀〈1 1 1〉 loops.     

Self-assembly of polystyrene sphere colloidal crystals by in situ solvent evaporation method

The three-dimensional (3D) ordered colloidal crystals were fabricated on the surface of polystyrene (PS) emulsion through an in situ solvent evaporation, which was controlled by altering both emulsion temperature and environmental pressure. The evaporation rate serves as an important role in intensifying the sphere transfer through solution flux and the capillary force between spheres for assembly, as well as supporting a stable close packing of colloidal crystals. The self-assembly process behaves a multi-nucleus site evolution, and the plane packing undergoes a transformation from square to hexagonal arrangement. The colloidal crystal array achieves an ordered close packing with multi-layer structure at evaporation temperature of 50 °C at normal pressure or proper pressure range of 160–500 mmHg at room temperature. The in situ solvent evaporation by decreasing the environmental pressure at room temperature could reduce disturbing effect of Brownian diffusion for an ordered PS colloidal crystal arrangement. The relationship between layer thickness and in situ evaporation time is a linear trend in time range of 20–75 min.     

Formation of Face-Centered Cubic Phase in Ti35 Alloy Under In Situ Heating Transmission Electron Microscopy

A thermally induced hexagonal close-packed (HCP) to face-centered cubic (FCC) phase transition was investigated in an α-type Ti35 alloy with twinned structure by in situ heating transmission electron microscopy (TEM) and ab initio calculations. TEM observations indicated that the HCP to FCC phase transition occurred both within matrix/twin and at the twin boundaries in the thinner region of the TEM film, and the FCC-Ti precipitated as plates within the matrix/twin, while as equiaxed cells at twin boundaries. The crystallographic orientation relationship between HCP-Ti and FCC-Ti can be described as: $\left\{ {111} \right\}_{{{\text{FCC}}}} //\left\{ {0002} \right\}_{{{\text{HCP}}}} \;{\text{and}}\; < 110 >\,_{{{\text{FCC}}}} //\, <1\overline{2} 10>\,_{{{\text{HCP}}}}$. The HCP to FCC phase transition was accomplished by forming an intermediate state with a BB stacking sequence through the slip of partial dislocations. The formation of such FCC-Ti may be related to the thermal stress and temperature. Ab initio calculations showed that the formation of FCC-Ti may also be related to the contamination of interstitial atoms such as oxygen.     

Preparation of Al-Al3Ti in situ composites by direct reaction method

Al-Al3Ti composites were prepared by a direct reaction method, in which Al3Ti was formed by the reaction of Ti and Al in aluminum alloy melt. The morphology of Al3Ti changes apparently from the fine particle, needle-like to large block with the increase of Al3Ti content. The addition of magnesium can markedly change the morphology of Al3Ti and reduce their size. Short rod-like Al3Ti was formed and homogeneous distribution was obtained with the addition of 3 wt.% Mg. The effect of Al3Ti and Mg on the microstructure of Al-Al3Ti composites and the mechanism were also discussed.     

Direct TEM Observation of Phase Separation and Crystallization in Cu_(45)Zr_(45)Ag_(10)Metallic Glass

The structural evolution of Cu_(45)Zr_(45)Ag_(10) metallic glass was investigated by in situ transmission electron microscopy heating experiments. The relationship between phase separation and crystallization was elucidated. Nucleation and growth-controlled nanoscale phase separation at early stage were seen to impede nanocrystallization, while a coarser phase separation via aggregation of Ag-rich nanospheres was found to promote the precipitation of Cu-rich nanocrystals.Coupling of composition and dynamics heterogeneities was supposed to play a key role during phase separation preceding crystallization.     

Microstructure Characteristics and Mechanical Properties of Nb-17Si-23Ti Ternary Alloys Fabricated by In Situ Reaction Laser Melting Deposition

Fabrication of ternary Nb–17 Si–23 Ti alloys was attempted by in situ reaction laser melting deposition(LMD)with dual powder feeding method from Nb-28 at.% Ti powder mixture and pure Si powder. The microstructures of the asdeposited alloys were examined with scanning electronic microscope, and the phase constituents were analyzed by X-ray energy-dispersive spectrometer and X-ray diffraction. Furthermore, the effect of laser power on microstructure characteristics, microhardness and indentation fracture toughness was also investigated. The in situ reaction LMD process resulted in remarkable refinement of the microstructure. The as-deposited samples mainly consisted of NbSS,metastable(Nb Ti)3 Si and Ti-rich NbSS. With the increase in the laser power from 1000 to 2000 W, the NbSSmorphology changed from discontinuous dendritic to near equiaxed, but the Ti-rich NbSSphase tended to vanish. Furthermore, with the increase in the laser power, the microhardness of as-deposited samples increased from 822 to 951 HV, while the indentation fracture toughness was improved from 12.3 to 14.1 MPa m1/2. The corresponding mechanism is also discussed.     

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

Neutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.     

原位合成TiC和TiB增强钛基复合材料的微观结构与力学性能

利用钛与Ｂ４Ｃ之间的自蔓延高温合成反应经普通的熔钐工艺原位合成制备了ＴｉＣ、ＴｉＢ增强的钛基复合材料。光学金相、ＥＰＭＡ、ＴＥＭ和Ｘ射线衍射的研究结果表明：存在匠两种不同形状的增强体,即短纤维状ＴｉＢ晶须和等轴、近似等轴状ＴｉＣ粒子。ＴｉＢ、Ｔｉ基体界面洁净,没有明显的界面反应,而ＴｉＣ、Ｔｉ基体界面有非化学配比的ＴｉＣ过度层存在。由于增强体承受载荷,基体合金晶粒细化以及高密度位错的存在,制备钛基     

Deformation mechanisms of a 20Mn TWIP steel investigated by in situ neutron diffraction and TEM

The deformation mechanisms and associated microstructure changes during tensile loading of an annealed twinning-induced plasticity steel with chemical composition Fe–20Mn–3Si–3Al–0.045C (wt.%) were systematically investigated using in situ time-of-flight neutron diffraction in combination with post mortem transmission electron microscopy (TEM). The initial microstructure of the investigated alloy consists of equiaxed γ grains with the initial α′-phase of ～7% in volume. In addition to dislocation slip, twinning and two types of martensitic transformations from the austenite to α′- and ε-martensites were observed as the main deformation modes during the tensile deformation. In situ neutron diffraction provides a powerful tool for establishing the deformation mode map for elucidating the role of different deformation modes in different strain regions. The critical stress is 520 MPa for the martensitic transformation from austenite to α′-martensite, whereas a higher stress (>600 MPa) is required for actuating the deformation twin and/or the martensitic transformation from austenite to ε-martensite. Both ε- and α′-martensites act as hard phases, whereas mechanical twinning contributes to both the strength and the ductility of the studied steel. TEM observations confirmed that the twinning process was facilitated by the parent grains oriented with 〈1 1 1〉 or 〈1 1 0〉 parallel to the loading direction. The nucleation and growth of twins are attributed to the pole and self-generation formation mechanisms, as well as the stair-rod cross-slip mechanism.     

Characterization of (TiB + TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition

Preliminary characterization of microstructure and mechanical properties of (TiB + TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition is reported in this paper. The results indicate that in situ reaction occurred during laser direct deposition of coaxially fed mixed powders from TC4 and B₄C. Reinforcements of TiB and TiC with a fraction of about 25 vol.% were formed with feeding 5 wt.% B₄C. The morphology of TiB tended to be needle-like and prismatic, while TiC appeared as granular. Small amount of un-reacted B₄C with reduced size remained within the composites. A thin skull of reaction product formed around the un-reacted B₄C weakened its interface bonding with the titanium alloy matrix, resulting in less outstanding properties of the composites.     

AISI304不锈钢熔化过程中夹杂物在固-液糊状区漂移与聚集行为的原位观察

利用Confocal激光扫描显微镜原位观察了AISI304不锈钢熔化过程中夹杂物在固-液（S-L）糊状区内的漂移与聚集行为,结果显示,夹杂（SiO2-Al2O3-CaO-MgO复合氧化物）在未熔高温铁索体（δ）相间的液相通道中漂移,未熔δ相体积分数为70％时,夹杂的平均漂移速率为80μm／s;近完全熔化时,远离S-L界面的夹杂漂移速率高达1500μm／s,而S-L界面附近的夹杂速率在50-200μm／s之间;随着熔化的进行,未熔δ相表面逐渐凹凸不平,出现“类亚晶界”,δ相对其附近漂移的夹杂表现出较强的吸附作用;夹杂多在δ-L界面处聚集、自球化;δ相吸附提供了夹杂去除的潜在途径,分析了夹杂运动的动力学机制。     

Synthesis of copper–niobium carbide composite powder by in situ processing

The in situ formation of niobium carbide (NbC) particles was investigated during mechanical alloying process as well as subsequent heat-treatment. Initial starting powders of Cu, Nb and C were used with a composition of Cu–40 wt% Nb–10 wt% C which was corresponding to Cu–40 vol% NbC. A mixture of this powder system was milled in an argon atmosphere for four different durations, i.e., 15 h, 31 h, 42 h and 54 h. X-ray diffraction was used to verify the formation of NbC. No NbC was formed after the milling process. The in situ formation of NbC in a copper matrix only took place after heat-treatment at 900 °C. Longer milling time resulted in the formation of NbC phase with more intense X-ray diffraction peaks due to powder refinement.