Evaluation of structural defects in brazed ceramic-to-metal joints with C-mode scanning acoustic microscopy

Structural defects and the integrity of brazed aluminium nitride-to-metal joints were studied by using C-mode scanning acoustic microscopy. Lack of bonding, cracks at the nitride/braze interface and surface opening cracks were clearly detected. The comparison of C-SAM results with those of scanning electron microscopy showed good correlation and demonstrated the high resolution of acoustic microscopy especially for detecting surface cracks.     

On the use of atomic force microscopy for structural mapping of metallic-glass thin films

In this article, we discuss the recent use of the high-resolution dynamic atomic force microscopy (DAFM) in mapping the nano-scale dynamical structural heterogeneity in thin film metallic-glasses (TFMGs). Our focus is laid on the major factors which can influence the structural contrast in the DAFM images, such as tip radius, free-amplitude, set-point amplitude and surface roughness. Finally, through a comparative study of different TFMGs and single-crystal silicon, we demonstrate that the DAFM technique is effective in distinguishing different nanostructures through their energy dissipation spectra.     

Contribution to the elucidation of corrosion initiation through confocal laser scanning microscopy (CLSM)

In this work, an electrochemical minicell has been developed to study the microscopic changes that occur on the electrode surface at the first steps of the corrosion process by means of optical and confocal laser scanning microscopy. Duplex stainless steel (UNS 1.4462) specimens (1.6 mm in diameter) have been used to check this technique. It has been observed that the corrosion process happens in a sequential way: first, a pit appears on the tested surface; later, the corrosion product spreads and catalyzes new corrosion processes. The spread corrosion product grows in a three-dimensional way. The used technique has demonstrated to be very useful for the observation of the initiation of corrosion processes.     

Evaluation of corrosion behaviour in a 317L stainless steel strip welding using scanning Kelvin probe force microscopy

Scanning Kelvin probe force microscopy and magnetic force microscopy have been used in combination with SEM/EDS and immersion tests to study a 317L electroslag strip weld which contains austenite and interdendritic ferrite and sigma phase. The individual phases can easily be recognized from the compositional contrast, magnetic pattern and Volta potential variation. Austenite, which is paramagnetic, exhibits the highest Volta potential followed by non‐magnetic sigma phase and ferromagnetic ferrite, respectively. Corrosion testing in acidic chloride solutions indicates that the Volta potential measured in air can be related to the tendency to uniform corrosion, while pitting corrosion shows different dependence. In both cases ferrite and sigma phase behaved in a similar manner, indicating that there was no specific detrimental effect of sigma phase on corrosion properties in this material.     

Micro-cells beneath organic lacquers: a study using scanning Kelvin probe and scanning acoustic microscopy

The mechanism of degradation of epoxy-phenolic lacquer coated thin electro-chrome-coated mild steel (food can material) has been investigated in brine using a novel combination of experimental techniques. A scanning Kelvin probe (SKP) was used to map electrochemical potential distributions beneath the coating at several stages of exposure to 0.17 M (1% w/w) NaCl solution. An interesting sequence of events was observed, especially at defects and blisters in the lacquer. These results have been correlated with those obtained in parallel by electrochemical impedance spectroscopy (EIS), scanning acoustic microscopy (SAM) and optical microscopy. The SKP results showed coating defects to be negative (anodic) with respect to the surrounding area in the initial stages of exposure (?36 h). This was followed by a gradual shift in the relative potential to values of the defect and the surrounding coated metal, with the defects becoming the cathode and the corrosion products formed in the defects becoming the cathode reactant. The reduction of red rust to magnetite was confirmed using the optical microscope. SAM revealed blisters, within which new anodes were detected by SKP.     

钛合金表面等离子喷涂Y2O3稳定的ZrO2涂层的 扫描热显微镜分析

以纳米结构Y2O3稳定的ZrO2热喷涂粉末为原料,采用等离子喷涂法在Ti-6Al-4V合金上制备了纳米结构的热障涂层。利用扫描电镜(SEM)及扫描热显微镜(SThM)对涂层的微观组织及热性能进行了分析。在实验基础上建立了理论模型,并对涂层及基体的热导率进行了估算。结果表明:采用SThM分析方法估算的涂层厚度及涂层上的缺陷尺寸与采用其它分析方法测得的结果一致;虽然热导率的估算结果与采用其它方法得出的结果差异较大,但显示出扫描热显微镜分析是估算材料热导率潜在的方法。     

ZrV1.5Ni1.5 as electrode material in nickel-metal hydride batteries An in situ scanning tunnelling microscopy investigation

We have investigated the alloy ZrV_1.5Ni_1.5 by means of scanning electron microscopy (SEM) with electron probe X-ray microanalysis (EPMA), X-ray diffraction (XRD), in situ STM (scanning tunnelling microscopy in an electrolyte under controlled electrochemical potential) and electrochemical charge discharge measurements. By means of EPMA we found that the alloy is composed of three different crystallographic phases. The main phase (≈75 vol.%) is ZrV_0.81Ni_1.47, the second phase (≈20 vol.%) is V₉₂Ni₈ and the third (≈5 vol.%) is a ZrNi based phase. Using in situ STM we investigated the different corrosion behaviour of the phases. At a potential of −600 mV versus the Hg/HgO reference electrode we observed the corrosion of the vanadium rich phase while the other two phases passivated.     

A quantitative study of blister growth on lacquered food cans by scanning acoustic microscopy

Scanning acoustic microscopy was used to image blistering of samples from food cans, internally coated with an epoxy-modified polyester lacquer, during exposure to aerated brine. Results were subjected to quantitative image analysis and have been discussed in the light of different models of blistering. The samples were taken from the side wall of the can, which had been formed by the draw-redraw process from pre-lacquered electro-chrome-coated steel sheet, thus providing insights into the effects of lacquer deformation.     

Effect of dispersion agent on the degradation of a water borne paint on steel studied by scanning acoustic microscopy and impedance

Scanning acoustic microscopy (SAM) was used as a complementary method to electrochemical impedance spectroscopy to characterize delamination processes at a water borne epoxy coating/steel interface. The nature of the dispersing agent incorporated in the water borne epoxy coating was found to have a strong influence on the coating performance. SAM results supported the impedance data, the same ranking of the specimens being obtained. SAM observations also gave complementary information on the initiation sites of the delamination, on the morphology of the blisters and qualitatively on their growth rate.     

A scanning force microscopy study of grain boundary energy in copper subjected to equal channel angular pressing

We employed a scanning force microscopy technique to determine the ratio of grain boundary and surface energies in copper using the thermal grooving method. Samples of ultrafine grain copper obtained by four passes of equal channel angular pressing were heat treated in a reducing atmosphere at 400 °C for 15 min and at 800 °C for 2 h. The average dihedral angles of the grain boundary grooves after the former and the latter heat treatments were 152.4 ± 6.3° and 164.2 ± 4.3°, respectively, which can be translated into the difference by a factor of 1.8 in average grain boundary energies. This difference implies that the grain boundaries in ultrafine grain copper produced by equal channel angular pressing are in a state of high non-equilibrium that cannot be fully relaxed after a short annealing at 400 °C, but that undergoes significant relaxation after annealing at 800 °C.     

A comparison of the corrosion behavior of WC-Co-Cr and WC-Co HVOF thermally sprayed coatings by in situ atomic force microscopy (AFM)

The corrosion behavior of WC-Co-Cr and WC-Co high velocity oxygen fuel (HVOF) sprayed coatings were examined in static saline conditions. Direct current (DC) polarization tests were conducted and the electrochemical corrosion behavior was shown, by in situ atomic force microscopy (AFM) and scanning electron microscopy (SEM), to be complex because of the composite ceramic-metal nature of the coating. The addition of chromium to the matrix greatly enhanced the corrosion resistance of the coating.     

A new steel with good low‐temperature sulfuric acid dew point corrosion resistance

In this work, new steels (1#, 2#, and 3#) were developed for low‐temperature sulfuric acid dew point corrosion. The mass loss rate, macro‐ and micro‐morphologies and compositions of corrosion products of new steels in 10, 30, and 50% H₂SO₄ solutions at its corresponding dew points were investigated by immersion test, scanning electron microscopy (SEM) and energy‐dispersive spectrometry (EDS). The results indicated that mass loss rate of all the tested steels first strongly increased and then decreased as H₂SO₄ concentration increased, which reached maximum at 30%. Corrosion resistance of 2# steel is the best among all specimens due to its fine and homogeneous morphologies of corrosion products. The electrochemical corrosion properties of new steels in 10 and 30% H₂SO₄ solutions at its corresponding dew points were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results demonstrated that corrosion resistance of 2# steel is the best among all the experimental samples due to its lowest corrosion current density and highest charge transfer resistance, which is consistent with the results obtained from immersion tests.     

Plastic deformation of single crystals of VSi2 and TaSi2 with the C40 structure

The deformation behavior of (0001)110 basal slip in single crystals of VSi₂ and TaSi₂ with the C40 structure has been investigated in the temperature range from room temperature to 1500 °C in compression. Plastic flow is observed only above 400 and 600 °C for VSi₂ and TaSi₂, respectively. The critical resolved shear stress (CRSS) for basal slip decreases with increasing temperature but exhibits a moderate peak around 1100 and 1400 °C for VSi₂ and TaSi₂ respectively. Dislocations with b = 1/3110 gliding on (0001) basal planes both in VSi₂ and in TaSi₂ are observed to dissociate into two identical partials with b = 1/6110 involving a stacking fault. High-resolution transmission electron microscopy of 1/3110 dislocations indicates that basal slip both in VSi₂ and in TaSi₂ occurs through a conventional single shearing mechanism, unlike in isostructural CrSi₂ and Mo(Si,Al)₂ in which basal slip occurs through a synchroshear mechanism.