Effect of modes of metal transfer and microstructure on corrosion behavior of welded modified ferritic stainless steel in acidic environments

The effects of modes of metal transfer, i.e., short-circuit (SC) and spray (S) modes in single-pass gas metal arc welding of modified ferritic stainless steel using two types of austenitic stainless steel filler metals on microstructure as well as corrosion behavior of weld metal and high-temperature heat-affected zone (HTHAZ), were investigated. The results show that primary solidification modes (PSM) of the welds were exclusively dependent upon the Cr_eq/Ni_eq ratio of the respective welds. However, the amount of grain boundary austenite and martensite transformation in the welds were solely dependent upon the mode of metal transfer and the extent of cooling rate. Regarding the corrosion mechanism, grain boundary corrosion (GBC) behavior of welds and HTHAZ relied on the microstructural changes along the grain boundary due to the variation in mode of metal transfer. The results show that S-mode resisted grain boundary corrosion of the welds and both GBC as well as pitting corrosion of the HTHAZ. On the other hand, SC-mode improved only pitting corrosion resistance of the welds. Between the filler wires used, 316L welds, in general, provided better corrosion resistance compared with 308L welds.     

Influence of the Metal Particle Size on the Crack Growth Resistance in Mullite–Molybdenum Composites

The R -curve for mullite–molybdenum (32 vol%) composites, which were obtained at 1650°C under reducing conditions with three different Mo average grain sizes (1.5, 3, and 9 μm), was estimated by the indentation-strength method and compared with that monolithic mullite obtained under similar conditions. The composites material exhibited rising R -curve behavior. The composite with larger grain size, however, displayed better damage tolerance and higher resistance to crack growth. Microscopic observation of the crack path revealed, in the composites, the systematic presence of dispersoids acting as bridging sites in the crack wake. Therefore, the increased fracture toughness of these ceramic-matrix composites with adherent ductile phase can be attributed to clamping forces applied by metal ligaments that bridge the crack faces behind the crack front. These clamping forces retard the crack from opening as an external stress is applied. It was inferred that this superior performance of the larger Mo particle size composite can be attributed mainly to more effective bridging of the metal grains. Because of this, a higher applied stress intensity will be required to propagate the crack tip. These results suggest that the rising R -curve should be proportional to the metal grain size, since the grain bridging area is proportional to the metal grain size.     

Polytypes, Grain Growth, and Fracture Toughness of Metal Boride Particulate SiC Composites

In order to understand the relation between microstructure and toughening behavior in SiC materials, NbB₂, TaB₂, TiB₂, and ZrB₂ particulate SiC composites were fabricated with pressureless sintering. In the composites, 3(cubic)-SiC powder was used as starting material for the matrix. The p-SiC powder transformed to a(noncubic) phase during sintering. The transformation, the behavior of which was influenced by the existence of metal boride particles, was accompanied by normal or exaggerated grain growth. The metal boride particles suppressed large-scale exaggerated grain growth of SiC, and it had a tendency to simulate grain growth with a high aspect ratio of the SiC grains. Increase in the fracture toughness of the composites was observed when the grain size and the aspect ratio of the SiC grains increased together. The toughening behavior is discussed based on a grain bridging mechanism.     

Controllable fabrication and temperature-resistance characteristics of ordered nanoporous Au and Pt films

In this work, the ordered nanoporous arrays of Au and Pt films are fabricated using anodized aluminum oxide (AAO) template based on the sputtering method. The presented synthetic strategy is scalable to large area by incorporating the deposition of a thin layer of Au or Pt. In addition, the grain size of Au and Pt nanoporous films is controlled with sputtering time. The thorough study of electrical transport properties for these metal films enables us to infer the nanoporous film morphology, and the evolution of the grain size with the change of sputtering time. In fact, the different physical behaviors are observed to occur in these metal films. The negative temperature coefficient of resistance (TCR) is visible for Pt nanoporous films, while Au nanoporous films show the positive TCR. With the increasing of sputtering time, the Pt grain size gradually becomes bigger, and the negative TCR properties weaken because the interface scattering of the electrons reduces. Therefore, the fabrication of metal nanoporous films with well-controlled physical properties might open new pathways for the growth of metal electrodes on AAO substrates for nanoelectronic devices.     

微乳法制备纳米催化剂研究进展

微乳法作为制备纳米微粒的有效方法,具有操作简单、制备粒子尺寸均匀、颗粒大小及形状可控等优点,在纳米催化剂制备领域具有广阔的应用前景。综述了近年来采用微乳法制备纳米催化剂的研究状况,尤其是对利用微乳法制备金属纳米粒子、金属氧化物及负载型金属催化剂进行了综述。     

晶粒细化对金属Cr在含Cl-介质中腐蚀电化学行为的影响

采用机械合金化通过热压烧结工艺制备了纳米晶金属Cr,利用动电位扫描法和交流阻抗技术,与常规尺寸金属Cr对比研究了它们在含Cl-介质中的腐蚀性能以及晶粒细化对其腐蚀电化学行为的影响。结果表明：随着Cl-浓度的增加,不同晶粒尺寸金属Cr的腐蚀电流密度增大,腐蚀速度加快。晶粒尺寸降低后,在相同条件下,腐蚀电流密度变大,其抗腐蚀性能降低。两种尺寸金属Cr的极化曲线均出现了钝化由强到弱交替变化的趋势,反映了Cl-对钝化膜的破坏与修复作用。两种尺寸金属Cr的交流阻抗谱均呈单容抗弧特征,表明腐蚀过程受电化学反应控制。     

Structure and Chemistry of Yttria-Stabilized Cubic-Zirconia Symmetric Tilt Grain Boundaries

The atomic structures of two symmetric [001] tilt grain boundaries in yttria-stabilized cubic-zirconia, Σ5 (310) and near-Σ13 (510), are studied by Z -contrast scanning transmission electron microscopy. Both boundaries are composed of periodic arrays of highly symmetric structural units, with a distinct unit for each boundary. Oxygen K -edge electron energy-loss spectra show that the oxygen coordination is similar between the bulk and grain boundary, indicating that oxygen ions within the grain boundary reside in distorted tetrahedral sites. Atomic models of the grain boundaries are proposed that are consistent with the experimental data. The core structures are different from previously studied metal or oxide grain boundaries and are unique to the fluorite structure. Yttrium segregation to the grain boundaries is also investigated by electron energy-loss spectroscopy. Yttrium is found to segregate preferentially to the Σ5 grain boundary, and the spatial distribution of the segregation layer is confined to within 1 nm of the boundary plane.     

脉冲放电高能转化制备纳米粉体

综合介绍了脉冲电流通过细金属丝放电(pulsed wire discharge,PWD)制备纳米粉体的方法.讨论了影响纳米粉体,特别是晶粒尺寸的因素,以防止形成亚微米颗粒.因为达到电压峰值的丝的沉积能相当于丝的汽化能,因此,能夠计算出沉积能.随着所施加的能量增加,气体压力降低,介质气体的热扩散率增大,晶粒尺寸变小.在惰性气氛中,采用PWD工艺,由金属蒸气急冷可制备金属粉体.如果介质气体变为氧气或者氨气,就能制备氧化物、氮化物纳米粒子.要制备双金属合金、双氧化物或氮化物纳米粒子就必需采用双金属丝和不同的介质气体.采用PWD工艺,在有机气体或烟气中,能制备电磁屏蔽和导电浆料和其它用途的钝化纳米粒子.采用丝输送器而实现大量生产纳米粉体的PWD工艺一个实例证明了PWD工艺生产纳米粉体的可行性.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Route and Regulation of Zinc,Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters,Metal Speciation,Grain Cd Reduction and Zn and Fe Biofortification

Zinc (Zn) and iron (Fe) are essential but are sometimes deficient in humans, while cadmium (Cd) is toxic if it accumulates in the liver and kidneys at high levels. All three are contained in the grains of rice, a staple cereal. Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes. To clarify the mechanisms underlying such different metal contents, we synthesized information on the routes of metal transport and accumulation in rice plants by examining metal speciation, metal transporters, and the xylem-to-phloem transport system. At grain-filling, Zn and Cd ascending in xylem sap are transferred to the phloem by the xylem-to-phloem transport system operating at stem nodes. Grain Fe is largely derived from the leaves by remobilization. Zn and Fe concentrations in phloem-sap and grains are regulated within a small range, while Cd concentrations vary depending on xylem supply. Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2′-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations. The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.     

Size-induced grain boundary energy increase may cause softening of nanocrystalline yttria-stabilized zirconia

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size-induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria-stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra-fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.     

Gradient microstructure development and grain growth inhibition in high-entropy carbide ceramics prepared by reactive spark plasma sintering

High-entropy carbide ceramics (Ti_0.2Hf_0.2Nb_0.2Ta_0.2W_0.2)C is prepared from five transition metal oxides and graphite by reactive spark plasma sintering. X-ray diffraction indicates the synthesized ceramics with the single-phase face-centered cubic structure. The elemental distribution maps by energy dispersive spectroscopy demonstrate homogeneous distribution of the five metal elements in both central and circumferential regions of the sample. SEM and corresponding back scattered electron observations show the residual graphite particles locating at the grain boundaries of high-entropy carbide ceramics. Moreover, the content of the residual graphite decreases and the grain size of the high-entropy carbide phase increases from central to circumferential region of the sample. Thermodynamic calculation results indicate that gradient gas pressure inside the sample affects the carbothermal reduction reactions during sintering and consequently results in the existence of residual graphite with gradient distribution feature. This study points out an effective way to inhibit the grain growth of high-entropy carbide phase during sintering process by the incorporation of graphite as the second phase particles acting as grain growth inhibitor.     

The Linear Thermal Expansion of Bulk Nanocrystalline Ingot Iron from Liquid Nitrogen to 300 K

The linear thermal expansions (LTE) of bulk nanocrystalline ingot iron (BNII) at six directions on rolling plane and conventional polycrystalline ingot iron (CPII) at one direction were measured from liquid nitrogen temperature to 300 K. Although the volume fraction of grain boundary and residual strain of BNII are larger than those of CPII, LTE of BNII at the six measurement directions were less than those of CPII. This phenomenon could be explained with Morse potential function and the crystalline structure of metals. Our LTE results ruled out that the grain boundary and residual strain of BNII did much contribution to its thermal expansion. The higher interaction potential energy of atoms, the less partial derivative of interaction potential energy with respect to temperature T and the porosity free at the grain boundary of BNII resulted in less LTE in comparison with CPII from liquid nitrogen temperature to 300 K. The higher LTE of many bulk nanocrystalline materials resulted from the porosity at their grain boundaries. However, many authors attributed the higher LTE of many nanocrystalline metal materials to their higher volume fraction of grain boundaries.     

Grain dependent electrochemical investigations on pure iron in acetate buffer pH 6.0

The influence of grain orientation on the electrochemical behaviour of iron in acetate buffer pH 6.0 was analyzed using a capillary microcell. Cyclic voltammetry and simultaneous capacity measurements were performed on the two crystal faces of an iron (1 0 0)/(1 1 1) bi-crystal, on the grain boundary between these two orientations and on several single grains of a coarse grain iron sample. The combination with electron back scattered diffraction (EBSD) yields the surface orientation of the analyzed grains and makes it possible to perform orientation dependent experiments on the polycrystalline metal. Clear differences in the electrochemical behaviour, e.g. in corrosion tendency and oxide formation rate were found between the orientations (1 0 0) and (1 1 1) of the iron bi-crystal and between several grain orientations of the polycrystalline sample.     

Grain Growth in Sintered Uranium Dioxide: I, Equiaxed Grain Growth

Grain growth was investigated in a UO₂ sinter of 94%) theoretical density over the temperature range 1555° to 2440°C. The results were in close, but not exact, agreement with a theoretical expression describing grain growth with a poly-crystalline matrix. For the material studied the mean grain diameter D (μm) after annealing for t hours at a temperature T (°K) was given by the equation

where D₀ and K₀ are, respectively, the initial grain size and a proportionality constant. Uranium metal was found in all specimens annealed above 2000°C. This was taken as evidence that the UO₂ lattice can be oxygen-deficient at high temperatures.     

Prebreakdown Conduction in ZnO Varistors

A model which quantitatively accounts for the important features of prebreakdown conduction in ZnO-based metal oxide varistors in presented. The prebreakdown current dependence with applied voltage and temperature enables us to extract several features for the grain boundary. Estimations have been made for the local electron concentration in polycrystalline ZnO near the grain boundary and for the grain boundary potential barrier height.     

Influence of vanadium content on the microstructural evolution and mechanical properties of (TiZrHfVNbTa)C high-entropy carbides processed by pressureless sintering

A series of (TiZrHfVNbTa)C high-entropy ceramics with different vanadium contents have been fabricated by pressureless sintering at 2300 °C–2500 °C for 1 h, utilizing self-synthesized carbide powders obtained by carbothermal reduction. The addition of vanadium is beneficial to promote densification process and refine grain, as well as facilitate the homogeneous distribution of metal elements. The distribution of pores is also modified, almost entirely existing at grain boundary, and the integral mechanical properties achieve optimization. However, excess adding vanadium does not favor forming a single-phase (TiZrHfVNbTa)C high-entropy ceramic. The optimal (TiZrHfVNbTa)C high-entropy ceramic sintered at 2300 °C possesses a high relative density of 97.5 % and homogeneous microstructure with small grain size of 1.2 μm. The flexural strength and Vickers hardness reach 473 MPa and 24.9 GPa, respectively. This work has established a cost-effective and convenient preparation of novel (TiZrHfVNbTa)C high-entropy carbide ceramics.     

Abnormal Grain Growth and New Core–Shell Structure in (K,Na)NbO3-Based Lead-Free Piezoelectric Ceramics

A unique core–shell structure was observed in coarse grains in (K,Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity-induced core–shell grain structure reported previously in BaTiO₃-based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger-sized but similar self-assembled subgrains. The electron-backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single-crystalline-like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN-based systems showed a similar grain growth transformation: from self-assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline.     

某云爆弹装药金属接触腐蚀性研究

为了探索某新型云爆弹装药对接触金属产生腐蚀的原因,对3种典型的新型云爆弹装药进行了与金属接触腐蚀性实验,并对装药各组分进行了内相容性分析。实验结果表明,在标准实验条件下,含有AP(高氯酸铵)成分的云爆弹装药对接触金属试片产生明显的腐蚀性穿孔现象。这主要是AP分解产生HClO4溶于水形成的高氯酸溶液与金属发生了强烈的氧化还原反应的缘故。对金属表面进行防腐处理以及对药柱进行密封都能有效地降低装药对金属的腐蚀作用。其中,对药柱密封,使之与外界的水分充分隔离具有较强的可操作性,是行之有效的一种方法。     

The influence of additives on microstrucutre of sub-micron alumina ceramics prepared by two-stage sintering

For various systems two-stage sintering has been reported as a successful way of suppressing the grain growth in the final stage of densification of polycrystalline ceramics. Our previous results on two-stage sintering of high purity submicrometre polycrystalline alumina indicate limited efficiency of the process with respect to suppression of grain growth. The present work deals with the influence of deliberate additions of various metal oxides (500 ppm of MgO, Y₂O₃ or ZrO₂) whose grain growth retarding effect in conventional sintering has been well documented, on two-stage sintering of submicrometre alumina ceramics. The addition of MgO was observed to enhance densification. Addition of yttria and zirconia impaired densification, but addition of all three dopants resulted in suppression of the grain growth and microstructure refinement in comparison to undoped alumina.