Twin-jet Electropolishing Research Articles

Lateral stress induced blistering of tungsten exposed to deuterium plasma

Surface blistering on tungsten under deuterium plasma exposure has been well known and investigated intensively in the last decade. However, the mechanism of the blistering is still unclear. There have been mainly two different models proposed: the gas driven model and lateral stress model. In this work, we designed an experiment to address this issue. Tungsten disc samples were prepared using twin-jet electro-polishing. The specimens were under deuterium plasma exposure to study the thickness effect on the surface blistering. The results showed that blistering was rarely observed on the surface around the inner edge of the central perforation with thickness of ∼100–500 nm. The blisters started to appear on the surface when the thickness was about 10 μm. Both the number and size of the blisters increased further on the outer surface with further increase in thickness. This trend was not obvious as the thickness increased up to above 180 μm. The diffusion depth of D in this work was calculated to be 8.5 μm. These results affirmed the lateral stress model as the surface blistering mechanism.

Revisiting a Cu-rich layer on the aluminum surface after twin-jet electropolishing

Twin-jet electropolishing (TJE) is a well-known method to prepare Cu-containing Al alloys for transmission electron microscopy (TEM) observation. Previous studies have reported a Cu-rich layer usually formed between Al matrix and outermost alumina layer at the electropolished Al surface. However, characteristics and formation mechanisms of the Cu-rich layer remain largely unexplored. Herein, configurations, formation process and property of the Cu-rich layer formed during TJE of Al alloys are revealed detailly employing aberration-corrected scanning TEM (STEM), STEM image simulations and first-principles calculations. The results reveal a new orientation relationship between the Cu-rich layer (θ’-Al2Cu) and Al matrix, which produces a low interfacial Al strain field. Formations of ∼2-nm-thick θ’ layer and its beneath ∼6-nm-thick Cu diffusion zone at the electropolished Cu-containing Al surface are mainly associated with traces of Cu in the electrolyte and anodization. Selective oxidation of elements, Cu-emitting alumina layer and spontaneous solute Cu segregation at the Al surface are also responsible for formations of the θ’ layer and Cu diffusion zone. Further study reveals that the θ’ layer behaves a strong self-healing ability after damaged by high-energy electron beam. The obtained results provide a potential insight into the surface modification of metals and alloys using electrochemical method.

Thickness profiling of electron transparent aluminium alloy foil using convergent beam electron diffraction.

Convergent beam electron diffraction (CBED) was used to profile the thickness of aluminium alloys foils prepared by using the twinjet electropolishing method. The two-beam CBED condition was obtained by exciting the and aluminium diffracted g-vector. The aluminium alloy foil thicknesses were calculated at different distances from the sample hole edge. In areas where only one Kossel-Möllenstedt (K-M) minima fringe was obtained, the thickness was determined by matching the experimental with simulated convergent beam diffraction patterns. In areas far away from the sample edge, the thickness of foils was high enough to generate at least two (K-M) minima fringes, required for linear regression fitting.

An approximate in-situ method for investigating irradiation damage of grain boundary

Grain boundary plays an important role in irradiation damage. The presented work proposes an approximate in-situ method, mesh-covered ion irradiation (MCII) for investigating the irradiation damage. The method was applied to a Fe-9 wt.% Al model alloy. During irradiation, transmission electron microscopy (TEM) samples prepared by twin-jet electro-polishing were covered by 400-Cu gilder grids with a pitch of 62 µm, a hole width of 37 µm, and a bar width of 25 µm. After He irradiations, in both the high-angle boundary and the sub-grain boundary, the irradiated and non-irradiated zones can be simultaneously generated. The MCII method provides an approximate in-situ method to investigate irradiation damage of grain boundaries when separately using a conventional ion accelerator and a TEM.

Formation of amorphous precipitates in a corroded over-aged Al-Mg-Si alloy

Coarse intermetallics and fine precipitates play an important role in the corrosion of Al alloys. So far, there is no systematic experimental investigation about the corrosion of nano-precipitates. In the present work, the morphology and chemical composition of the corroded MgSi nano-precipitates in an Al-Mg-Si alloy with different aging times have been studied by transmission electron microscopy. Coarsened βʹ precipitates occurring in the over-aged state were found to be corroded through dissolution of Mg and oxidation of the remnant Si during twin-jet electropolishing. They finally transformed to amorphous SiO2 precipitates, showing an almost unetched interface with the Al matrix. The proportion of amorphous precipitates increased with prolonging aging time, as larger precipitates were preferentially corroded. This newly discovered crystalline-to-amorphous phase transformation depletes acid electrolyte but shows no damage. Such an intragranular corrosion behavior partly accounts for why over-aged Al-Mg-Si alloys are less susceptible to pitting and intergranular corrosion.

Phase stability of the amorphous phase and non-equilibrium phase in a β Ti-Zr-based shape memory alloy

This work presents the amorphization of metastable β phase and the phase stability of the amorphous phase and non-equilibrium phase in a Ti-Zr-Nb-Sn shape memory alloy (SMA). The amorphous phases with lenticular shape uniformly embedded in β matrix are observed in the twin-jet-electropolished thin foil specimen for transmission electron microscope (TEM) observations. Meanwhile, a co-existence of non-equilibrium athermal ω phase and nanodomain phases are found to be formed in the β matrix. These non-equilibrium phases could favorably compete with the amorphous phase formation and render the amorphous phases are not formed, leading to that the amorphous phase is not observed in the bulk specimens for X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) and also TEM specimen prepared by a focused ion beam (FIB). The formation of the amorphous phase in the twin-jet-electropolished TEM specimen is thought to be due to the hydrogen-related artifacts introduced during twin-jet electropolishing.

Twin-jet electropolishing for damage-free transmission electron microscopy specimen preparation of metallic microwires.

A method to prepare TEM specimens from metallic microwires and based on conventional twin-jet electropolishing is introduced. The wire is embedded in an opaque epoxy resin medium and the hardened resin is mechanically polished to reveal the wire on both sides. The resin containing wire is then cut into discs of the appropriate size. The obtained embedded wire is electropolished in a conventional twin-jet electropolishing machine until electron transparency in large areas without radiation damage is achieved.

Ion Irradiation-Induced Microstructural Evolution of Ni⁻Mo⁻Cr Low Alloy Steels.

As leading candidates of sheet steels for advanced nuclear reactors, three types of Ni–Mo–Cr high-strength low alloy (HSLA) steels named as CNST1, CNST2 and CNSS3 were irradiated by 400 keV Fe+ with peak fluence to 1.4 × 1014, 3.5 × 1014 and 7.0 × 1014 ions/cm2, respectively. The distribution and morphology of the defects induced by the sample preparation method and Fe+ irradiation dose were investigated by transmission electron microscopy (TEM) and positron-annihilation spectroscopy (PAS). TEM samples were prepared with two methods, i.e., a focused ion beam (FIB) technique and the electroplating and twin-jet electropolishing (ETE) method. Point defects and dislocation loops were observed in CNST1, CNST2 and CNSS3 samples prepared via FIB. On the other hand, samples prepared via the ETE method revealed that a smaller number of defects was observed in CNST1, CNST2 and almost no defects were observed in CNST3. It is indicated that artifact defects could be introduced by FIB preparation. The PAS S-W plots showed that the existence of two types of defects after ion implantation included small-scale defects such as vacancies, vacancy clusters, dislocation loops and large-sized defects. The S parameter of irradiated steels showed a clear saturation in PAS response with increasing Fe+ dose. At the same irradiation dose, higher values of the S-parameter were achieved in CNST1 and CNST2 samples when compared to that in CNSS3 samples. The mechanism and evolution behavior of irradiation-induced defects were analyzed and discussed.

Dislocation driven nanosample plasticity: new insights from quantitative in-situ TEM tensile testing

Intrinsic dislocation mechanisms in the vicinity of free surfaces of an almost FIB damage-free single crystal Ni sample have been quantitatively investigated owing to a novel sample preparation method combining twin-jet electro-polishing, in-situ TEM heating and FIB. The results reveal that the small-scale plasticity is mainly controlled by the conversion of few tangled dislocations, still present after heating, into stable single arm sources (SASs) as well as by the successive operation of these sources. Strain hardening resulting from the operation of an individual SAS is reported and attributed to the decrease of the length of the source. Moreover, the impact of the shortening of the dislocation source on the intermittent plastic flow, characteristic of SASs, is discussed. These findings provide essential information for the understanding of the regime of ‘dislocation source’ controlled plasticity and the related mechanical size effect.

Quantitative in-situ TEM nanotensile testing of single crystal Ni facilitated by a new sample preparation approach

Twin-jet electro-polishing and Focused Ion Beam (FIB) were combined to produce small size Nickel single crystal specimens for quantitative in-situ nanotensile experiments in the transmission electron microscope. The combination of these techniques allows producing samples with nearly defect-free zones in the centre in contrast to conventional FIB-prepared samples. Since TEM investigations can be performed on the electro-polished samples prior to in-situ TEM straining, specimens with desired crystallographic orientation and initial microstructure can be prepared. The present results reveal a dislocation nucleation-controlled plasticity, in which small loops induced by FIB near the edges of the samples play a central role.

Investigation of mechanical properties and proton irradiation behaviors of SA-738 Gr.B steel used as reactor containment

The proton irradiation behaviors of two kinds of SA-738Gr.B steels prepared by different heat treatment used as AP1000 reactor containment were investigated by transmission electron microscopy and positron annihilation lifetime spectrum (PAS). The mechanical properties of as-received steels were also measured. In the unirradiated conditions, the SA-738Gr.B steels had high tensile strength and excellent impact fracture toughness, which met the performance requirements of ASME codes. Both kinds of SA-738Gr.B steels were irradiated by 400keV proton from 1.07×1017H+/cm2 to 5.37×1017H+/cm2 fluence at 150 ºC. Some voids and dislocation loops with several nanometers were observed in the cross-section irradiated samples prepared by electroplating and then twin-jet electropolishing technology. The number of irradiation defects increased with increasing of displacement damage, as well as for the mean positron lifetimes. The stress-relief annealing treatment improved irradiation resistance based on open volume defect analysis from proton irradiation. SA-738Gr.B (SR) steel had higher proton irradiation resistance ability than that of SA-738Gr.B (QT) steel. The mechanism of irradiation behaviors were also analyzed and discussed.

An improved preparation method for cross-sectional TEM specimens of films deposited on metallic substrates.

Cross-sectional TEM analysis is one of the most important techniques to characterize microstructures of films. However, the complex process, low efficiency, and low success rate of specimen preparation limit its application. This paper analyzed the main causes of low success rate and proposed an improved method for specimen preparation of films deposited on metallic substrates. This method consisting of twin-jet electropolishing and one-sided rocking ion milling is high in efficiency and success rate.

Characterization of dislocation loops in hydrogen-ion irradiated vanadium

Vanadium alloys are considered as the candidate materials for structure application in fusion reactors because of their low radiation-induced activation, high resistance to radiation damage, high thermal conduction, and low thermal expansion coefficient. Before these materials, which will be exposed to high-flux hydrogen and helium isotopes, may be safely used in fusion device much more data based on irradiation damage are required. The study of dislocation loops in vanadium is designed to indicate the mechanism of void growing under irradiation. The mechanism is that different types of dislocation loops have different bias which represent their abilities to absorb point defects. It is possible to explain the irradiation swelling performance in the material with the bias of loops. The thin disks samples used in this experiment are made of pure vanadium and vanadium alloy (V-4Cr-4Ti) by twin-jet electro-polishing. Electrolyte of H2SO4-CH3OH (1 : 6 by volume) at -20 ℃ is used in a current of 80～120 mA. To get a clear view of dislocation loops, the SRIM code is used to simulate the implantation of hydrogen ions into vanadium. The ion irradiation is carried out to a dose of 51016H+/cm2, at an energy of 30 keV. Microstructure observations are performed on a Tecnai G2 F20 (transmission electron microscope, TEM) at an accelerating voltage of 200 kV. The Burger's vectors and nature of the dislocation loops formed in pure vanadium by hydrogen implantation are confirmed by TEM. This experiment has focused on as many as 76 dislocation loops, lots of images are taken under different diffraction conditions from the same areas of interest. Results show that most of the dislocation loops have a Burger's vectors of 1/2111 (90%), and a few of 110. No loops with b= 100 loops can be found in this study. The nature of dislocation loops is determined by the inside-outside method. The number of the dislocation loops that can make sure of their nature is 29, and all of them are conformed to be interstitial type, their habit planes are from {110} to {112}. No vacancy type loops are found. The density and average size of dislocation loops in vanadium and vanadium alloy are also analysed. Compared with the pure vanadium, the loops in vanadium alloy of V-4Cr-Ti are formed in a smaller size and higher number density. As a future work the difference of the loops nature between pure vanadium and vanadium-based alloys should be investigated to illustrate their behaviour of irradiation swelling.

A Personal View of the History of Twin-jet Electropolisher

Abstract Way back in 1966, when I first started my industrial career at U.S. Steel's Edgar C, Bain Laboratory for Fundamental Research (which is what it was called before it became a grease spot in Corporate America) Gene Fischione and Dick Glenn were engaged in a "moon race" of sorts to bring out some of the earliest electrolytic jet polishers. Remy Schoone helped Gene build his model; and Bob Sober helped Dick make the competition, I haven't used either one of these fine instruments in nearly thirty years, so the only connection is historical and the good feelings I have about the four nice guys who made them.

On the recrystallization of electrodeposited zinc during mechanical polishing to electron transparency

A primary concern involving transmission electron microscopy (TEM) analysis is whether the electron transparent region under investigation is representative of the bulk material. TEM is frequently employed to examine the microstructure of electrodeposited materials due to their small grain size and high dislocation density. Previous work in this laboratory on palladium electrodeposits has shown that deformation twins and diffusion induced recrystallization may be induced during preparation of thin foils using both twin jet electropolishing and ion beam thinning. Recent developments in TEM sample preparation in the physical sciences include a procedure for the cross-section of heterogeneous layered materials which reduces or eliminates the need for ion milling. In this sample preparation technique, a tripod polisher device is used to mechanically polish the specimen to electron transparency. The purpose of this paper is to report on the influence of the tripod polisher sample preparation technique, on the microstructure of zinc electrodeposits.

Microstructural characterization of α2 + B2 titanium aluminide intermetallic (Super- α2) using transmission electron microscopy

Dislocation structures in as-received and plastically deformed commercial α 2 + B2 and homogeneous ordered B2 intermetallic titanium aluminide alloys (“ super-α 2”) were characterized with transmission electron microscopy. Thin foils prepared by twin-jet electropolishing and ion beam thinning were found to yield different deformation structures. Electropolished foils were found to contain lath-like bands of dislocation loops with 1 2 〈111〉 Burgers' vectors ( b) in the B2 phase that were actually thinning artifacts. Dislocations in lightly deformed ion-beam-thinned foils appeared to be predominantly screw dislocations with b = 1 2 〈111〉

Channelling Enhanced X-Ray Microanalysis of the Preferred Substitution of Ti, Cr and Co Atoms in Sendust (Fe-9.6wt.%Si-5.5wt.%Al) Alloys

Sendust (Fe-9.6wt%Si-5.5wt%Al) is a well-known soft magnetic material. Attempts have been made recently to improve the corrosion resistance of this alloy by adding a fourth and/or a fifth element. Since the excellent magnetic properties of Sendust have been reported to be deeply related to its crystal structure, the DO3 structure, it is of vital importance to determine the crystallographic sites those additional elements occupy. In the present work I will report the results obtained for the site occupation of Ti, Cr and Co in Sendust alloys using the ALCHEMI method.Sendust containing 4.0%Cr, 3.0%Co+0.5%Nb and 2.0%Ti alloys were arc melted, then furnace-cooled from 1250°C. TEM specimens were prepared in a twin jet electropolisher using 5% perchrolic-acetic acid and the final thinning was done using an ion milling machine. The ALCHEMI experiment was performed in a Philips EM420T equipped with an EDAX 9900 system.All the specimens observed exhibited the diffraction spots which are associated with the DO3 structure (Fig. 1).

TEM study of order, structure and defects of β and martensite in Cu-Al-Mn shape memory alloys

The shape memory effect (SME) shown by Cu-Al-Mn alloys stems from the thermoelastic martensitic transformation occuring between a β (L2,) metastable phase and a martensitic phase. The TEM study of both phases in single and polycrystalline Cu-Al-Mn alloys give us greater knowledge of the structure, order and defects.The alloys were obtained by vacuum melting of Cu, Al and Mn and single crystals were obtained from polycrystalline alloys using a modified Bridgman method. Four different alloys were used with (e/a) ranging from 1.41 to 1.46 . Two different heat treatments were used and the alloys also underwent thermal cycling throughout their characteristic temperature range -Ms, Mf, As, Af-. The specimens were cut using a low speed diamond saw and discs were mechanically thinned to 100 μm and then ion milled to perforation at 4 kV. Some thin foils were also prepared by twin-jet electropolishing, using a (1:10:50:50) urea: isopropyl alcohol: orthophosphoric acid: ethanol solution at 20°C. The foils were examinated on a TEM operated at 200 kV.

Structural changes in Pd electrodeposits during TEM sample preparation

The small grain size and high dislocation density of electrodeposits make it necessary to employ TEM analysis for microstructural evaluation. It had been generally accepted that twinned microstructures of electrodeposits observed by TEM were annealing or growth twins, and subsequently inherent to the properties of the electrodeposit. However, in a recent study, deformation cross-twinning in Pd electrodeposits was unambiguously identified. The purpose of this paper is to report on the influence of TEM sample preparation on the structure of Pd electrodeposits.Electrodeposited Pd (courtesty J. Vanhumbeeck, Siemens NV, Belgium) TEM foils were thinned by the standard techniques of twin jet-electropolishing and ion beam milling. The Pd foils were examined in a Philips EM420T TEM with analytical capabilities operating at 120 kV.FIG. 1 is a series of three dark field micrographs from the same region showing deformation crosstwinning. The dislocation density increases with increasing foil thickness and decreasing deformation cross-twinning.

Aging behaviour of U-6.0Wt.% Nb at temperatures above 350°C

Large strength increments accompanied by severe ductility losses are observed upon ageing U-6Nb at temperatures between 350°C and 450°C. This study attempts to characterize the fine scale morphological and compositional changes observed upon ageing this alloy in this temperature regime. Precipitation of Nb-rich particles having a tetragonal crystal structure is believed to be the mechanism responsible for age hardening in this alloy.Material supplied by AWE, Aldermaston, U.K. was received in the as-water quenched (WQ) condition in the form of 3 mm. diameter rods. Material was aged in sealed silica tubes containing a partial pressure of argon. The aged rods were then sliced to 0.5 mm. thick disks and subsequently ground down to a thickness of 90 μm. using 1200 grit SiC paper. Electropolishing was carried out in a Fischione twin-jet electropolishing unit using a solution of 5% HClO in methanol cooled to -60°C at a voltage of 40V.