Transmission Electron Microscopy Of Thin Foils Research Articles

Transmission Electron Microscopy Specimen Preparation Method for Multiphase Porous Functional Ceramics

An optimum method is proposed to prepare thin foil transmission electron microscopy (TEM) lamellae of multiphase porous functional ceramics: prefilling the pore space of these materials with an epoxy resin prior to focused ion beam milling. Several advantages of epoxy impregnation are demonstrated by successful preparation of TEM specimens that maintain the structural integrity of the entire lamella. Feasibility of the TEM alignment procedure is demonstrated, and ideal TEM analyses are illustrated on solid oxide fuel cell and solid oxide electrolysis cell materials. Some potential drawbacks of the TEM specimen preparation method are listed for other samples.

Pseudoelastic deformation and size effects during in situ transmission electron microscopy tensile testing of NiTi

The stress-induced B2–B19′ transformation in aged 51 at.% NiTi was investigated using in situ straining transmission electron microscopy (TEM). Increased applied strain along [110]B2 transforms B2 into plates containing B19′ variants that are related by a (110)B2 compound twin plane. This atypical twin plane is explained by relaxing the invariant plane constraint in the crystallographic theory of martensite (CTM) to an invariant line constraint. The relaxation is rationalized from the thin foil geometry. The relaxed CTM approach, coupled with conditions to maximize transformation strain along the loading axis and minimize elastic energy, predicts the observed twin interface, diffraction patterns, and interface with the B2 austenite. These results demonstrate subtleties in interpreting thin foil TEM results regarding martensitic transformations, and translating those results to bulk response.

Electron holography study of remanence states in exchange-biased MnPd/Fe bilayers grown epitaxially on MgO(001)

We investigated magnetic remanence states of epitaxially grown, exchange-biased MnPd/Fe bilayers by electron holography emphasizing the crystallographic orientations of the layers. Thin-foil transmission electron microscopy (TEM) specimens were carefully prepared along both hard and easy axes of the Fe layer. The ex situ magnetization-reversal process was carried out using the TEM specimens, and magnetic flux densities of the ultra-thin Fe layers were evaluated at different remanence states. We show that a spin configuration in the TEM specimens is determined by the competition between an exchange coupling at the MnPd/Fe bilayer interface, shape anisotropy of TEM specimens and intrinsic magnetocrystalline anisotropy of Fe.

Characteristics of martensite as a function of the Ms temperature in low-carbon armour steel plates

The microstructure, morphology, crystal structure and surface relief of martensite in a number of experimental armour steel plates with different M s temperatures were analysed. Atomic force microscopy, thin foil transmission electron microscopy and scanning electron microscopy allowed the identification of three groups of low-carbon martensitic armour steels. The investigation showed that the size of individual martensite products (plates or packets, laths or blocks) increases as the M s temperature increases. Comparison of ballistic performances suggests that the morphology (plate or lath) and size of the individual martensite products dictate the effective “grain size” in resisting fracture or perforation due to ballistic impact.

Evolution of grain size in oxide dispersion strengthened tungsten alloys during their elaboration by ball milling and sintering

Oxide dispersion strengthened tungsten alloys have been elaborated from tungsten and yttrium powders by reactive ball milling, compaction and sintering. The present study focuses on the analyses of the nanostructure of the crystallites and second phases developed during ball milling and underlines the significant grain growth during sintering. The major causes of such an evolution are on the first hand, contamination by oxygen and cobalt during ball milling which lead to liquid state sintering and on the second hand, coarsening of the second phases. Numerous techniques were used, among which thin foil transmission electron microscopy of powder particles, so as to study the evolution of both the size and morphology of the crystallites and the second phases dispersion. At last, various ways of improvement of the elaboration process are suggested.

An electron microscopic study of spheroidal graphite nodules formed in a medium-carbon steel by annealing

The detailed evolution of the graphitized microstructure of carbon steel during annealing has been studied by thin-foil transmission electron microscopy, enabled by the refined dispersion of small graphite nodules produced. The graphitization annealing of 0.38 wt.% carbon steel in the quenched martensitic state resulted in the formation of graphite nodules, of the order of 2–5 μm in diameter, after 2–4 h. Two types of graphite nodules were observed to form: one possesses an irregular morphology and exhibits a coring particle (either a nitride or an oxide), whilst the other is more regularly spheroidal and apparently without a nucleating particle at its core. Analytical electron microscopy has been employed to characterize the microstructure of the graphite nodules formed, in order to provide a better understanding of their nucleation and growth, in particular, the mechanism of formation of the spheroidal graphite nodules without an apparent nucleating particle. For these more spherical nodules, the results showed that the outer mantles had a strong graphitic character, whilst the core regions exhibited a more amorphous carbon structure. It is deduced from the available experimental evidence that the nucleation sites most likely to result in this structure are dissolving cementite particles in the martensite.

Scanning electron microscopy imaging of dislocations in bulk materials, using electron channeling contrast

The imaging and characterization of dislocations is commonly carried out by thin foil transmission electron microscopy (TEM) using diffraction contrast imaging. However, the thin foil approach is limited by difficult sample preparation, thin foil artifacts, relatively small viewable areas, and constraints on carrying out in situ studies. Electron channeling imaging of electron channeling contrast imaging (ECCI) offers an alternative approach for imaging crystalline defects, including dislocations. Because ECCI is carried out with field emission gun scanning electron microscope (FEG-SEM) using bulk specimens, many of the limitations of TEM thin foil analysis are overcome. This paper outlines the development of electron channeling patterns and channeling imaging to the current state of the art. The experimental parameters and set up necessary to carry out routine channeling imaging are reviewed. A number of examples that illustrate some of the advantages of ECCI over thin foil TEM are presented along with a discussion of some of the limitations on carrying out channeling contrast analysis of defect structures.

Martensitic transformation and ordering in heat-resistant Ni-Co-Cr-AI β/γ eutectics

The method of thin-foil transmission electron microscopy was used to examine phase transformations in the β-phase, representing the main heat-resistant component, of β/γ eutectics of the Ni-Co-Cr-Al system (mass %: 0-15 Co, 14-20 Cr, 8.5-10.5 Al). The martensitic transformation β(B2)→L1 0 took place in all the alloys, except those with 20 mass % Cr, when they were quenched in water from 1200°C. It was found for the first time that high-rank superstructures A 2 B (Ni 2 Al) and A 5 B 3 (Ni 5 Al 3 ) could be formed in a complex-alloyed β solid solution. Phase transformations L1 0 →A 5 B 3 , L1 0 →14M, L1 0 →B2→A 2 B, and L1 0 →B2→A 5 B 3 were observed in the β-component of the eutectics when isothermal annealing temperature was increased from 250 to 600°C. At a temperature of 400 °C and higher those transformations were accompanied by the decomposition of the β solid solution and the appearance of dispersed α-Cr particles. Structural features of the aforementioned phase transformations in heat-resistant quaternary eutectics were considered.

Submerged arc welding of stainless steel and the challenge from the laser welding process

The welding of austenitic and duplex stainless steels has been reassessed by questioning traditional requirements of the weld metal and/or the heat affected zone (HAZ). The use of high dilution submerged arc welding of austenitic and duplex stainless steels has been shown to produce acceptable properties, despite the high heat input used in some instances. Corrosion characteristics have been established as being acceptable too. These findings have been further validated by examination of the weld region material using thin foil transmission electron microscopy (TEM). This showed that while some intermetallic phases were present, they did not adversely affect the weld metal properties. In addition, an examination has taken place of Nd:YAG laser-welded austenitic and duplex stainless steels, to establish the potential viability of this route compared to the submerged arc welding process. The material properties, including the relevant corrosion testing, have been found to be acceptable. TEM has again shown that some intermetallic phases are present in the weld metal. It has been suggested that a segregation mechanism is responsible for their presence in this case.

High dilution submerged arc welding of Cr–Ni–Mo austenitic stainless steel

Improvements in the submerged arc welding (SAW) of Cr–Ni–Mo austenitic stainless steel have been achieved from a combination of simplified edge preparation, intermediate heat input, and high dilution (75–80%). The cost benefit is 83% compared with the previously used practice. Mechanical properties of the weld metal in a square edge preparation showed a decrease in yield and tensile strengths, but a significant improvement in toughness compared with the Y preparation weld metal. Thin foil transmission electron microscopy confirmed the presence of χ phase in some areas of the Y preparation weld metal, and M23C6 in some areas of the square edge preparation. This has been related to the incidence of fewer reheated and cooled areas in the square edge preparation weld. The reduced precipitation has influenced the toughness characteristics of the square edge preparation weld metal.

High dilution submerged arc welding of Cr–Ni–Mo austenitic stainless steel

Improvements in the submerged arc welding (SAW) of Cr–Ni–Mo austenitic stainless steel have been achieved from a combination of simplified edge preparation, intermediate heat input, and high dilution (75–80%). The cost benefit is 83% compared with the previously used practice. Mechanical properties of the weld metal in a square edge preparation showed a decrease in yield and tensile strengths, but a significant improvement in toughness compared with the Y preparation weld metal. Thin foil transmission electron microscopy confirmed the presence of χ phase in some areas of the Y preparation weld metal, and M23C6 in some areas of the square edge preparation. This has been related to the incidence of fewer reheated and cooled areas in the square edge preparation weld. The reduced precipitation has influenced the toughness characteristics of the square edge preparation weld metal.

Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures

The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {10\(\bar 1\)0} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal sli {\(\bar 1\)2\(\overline {11} \)} is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873 K. The {10\(\bar 1\)2}, {11\(\bar 2\)1}, and {11\(\bar 2\)2} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.

Transmission electron microscopic observation of precipitates in and aged Pb0.1wt.%Ca0.3wt.%Sn alloy

Samples of a Pb0.1wt.%Ca0.3wt.%Sn alloy were cast and either air-cooled or ice-water-quenched. They were then age-hardened for 400 days at ambient temperature and examined by thin foil transmission electron microscopy. In the air-cooled and aged condition, the alloy exhibits a sluggish ageing response, while a rapid ageing response occurs in the water-quenched and aged samples. In both types of sample, precipitate dispersions were observed. These are tentatively identified as Pb 3Ca with an ordered L1 2 crystal structure. In the air-cooled and aged condition, the particles were about 100 nm in diameter and occurred in planar arrays consistent with precipitation on migrating grain boundaries. In the water-quenched and aged samples, the particles were about 10 nm in diameter and uniformly distributed with a smaller interparticle spacing, which is probably due to nucleation on tangle dislocations. The more rapid ageing kinetics in the water-quenched and aged condition are attributed to the excess vacancy concentration produced by quenching.

Mechanism of enhanced consolidation of Nb3Al powders by ball-milling

The present paper examines dominating factors in enhanced consolidation by ball-milling. Crushed pieces of Nb[sub 3]Al castings were ball-milled to produce fine-grained structures of about 10nm. The milled powders were characterized using ordinary means such as thin-foil transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Ball-milling was found to enhance consolidation of Nb[sub 3]Al powders at temperature as low as 0.5Tm. The calculation of the densification rate using Ashby's HIP mechanism map showed that the fine-grain structures of about 10nm rather than the small powder size have significantly increased low temperature creep and was responsible for the enhanced consolidation. A good coincidence was found between the measured and calculated density vs time curves by using the power-law creep rate equation with n = 3.

Microstructure of polycrystalline aluminum nitride formed in shock waves

Shock wave treatment of aluminum nitride particles produces compacted specimens with a density equal to 0.93 to 0.94 of the theoretical. An examination of the microstructure using thin foil transmission electron microscopy indicates that polycrystalline AlN forms under these conditions as a result of structural transformations that are typical of AlN sintering at high temperatures and pressures due to static compression, and particularly, plastic deformation, relaxation dislocation transformations, primary and collective recrystallization. Plastic deformation of AlN is achieved by means of full basal dislocations which describe both the absence of deformation polytypism and the transition to the sphalerite modification.

Alloying of iron powder base alloys with chromium and its compounds

The creep deformation of a 2.5%Cr-1%Mo-0.11%C steel was investigated as a function of various thermal treatments. The substructural changes that occurred during deformation were examined using transmission electron microscopy. The high creep resistance observed in both bainite and martensite was analyzed in terms of the high dislocation densities, the fine carbides and the ultrafine matrix lath sizes in these structures. The activation energy and other deformation parameters measured during the the creep process indicate that the deformation mechanism is mainly diffusion controlled. The changes in dislocation density and spacing occurring during the creep process were monitored by following the changes in internal stress and creep rate and by using thin foil transmission electron microscopy.

Process parameters for bonding copper powder to oxygen free high conductivity (OFHC) copper substrate by CIP and sintering

The creep deformation of a 2.5%Cr-1%Mo-0.11%C steel was investigated as a function of various thermal treatments. The substructural changes that occurred during deformation were examined using transmission electron microscopy. The high creep resistance observed in both bainite and martensite was analyzed in terms of the high dislocation densities, the fine carbides and the ultrafine matrix lath sizes in these structures. The activation energy and other deformation parameters measured during the the creep process indicate that the deformation mechanism is mainly diffusion controlled. The changes in dislocation density and spacing occurring during the creep process were monitored by following the changes in internal stress and creep rate and by using thin foil transmission electron microscopy.

Formation of B2 antiphase domains in rapidly solidified Fe-Al alloys

Abstract Fe-Al-X (X = Cr, Mo) alloys in the composition range 50 to 80 at.% Fe, 0 to 20 at.% X were rapidly solidified by chill-block melt-spinning. Thin foil transmission electron microscopy of the rapidly solidified ribbons revealed that antiphase domains occur in the B2 structure in the limited composition range 22 to 37 at.% Al. The B2 antiphase domain size was found to increase with the increasing Al-content of the ribbon. The criterion for antiphase domain (APD) formation is investigated in terms of mechanisms for formation of the B2 structure.

Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

The effect of microstructure on the slow bending stress and fracture energy in 0.5C-steel processed by high temperature thermomechanical treatment (processed by forging) (HTMT) was studied to understand which microstructural factors contribute to the strength and toughness of a HTMT steel. Significant improvement was achieved in the slow bending fracture energy, with moderate increase in the slow bending stress when the steel was deformed by 50% at 1473K followed by direct water quenching and subsequent tempering at 453 K. When the steel was deformed by 50% reduction at 1173 K followed by direct water quenching and subsequent tempering at 423 K, the slow bending stress significantly increased though the increase in the fracture energy was not as great as that of the 1473 K forged steel. However, an abrupt reduction occurred in the fracture energy above suitable tempering temperatures, so above these temperatures, there was little difference between the properties of the HTMT and conventional heat-treated steels. Microstructural factors contributing to the mechanical properties are discussed in terms of thin-foil transmission electron microscopy, non-isothermal dilatometry, and X-ray measurements.

An electron metallographic study of the commercial zinc-based pressure-die-cast alloy 3

The solidification structure of pressure-die-cast commercial alloy 3 was examined using scanning electron microscopy and thin foil transmission electron microscopy techniques. It was found that the rapidly cooled alloy commenced solidification by the formation of small rounded primary zinc (η) particles followed by pseudo-primary β particles and then a fine eutectic of β + η. The high temperature β phase subsequently decomposed into α-aluminum and η, the phases stable at ambient temperature, but an intermediate transitional phase was found in alloys examined shortly after casting. Chemical analysis by energy-dispersive spectroscopy showed that this phase contained zinc with 11.8 wt.% Al, suggesting that it was a transitional phase since none of the stable phases in the AlZn system has that composition. This transitional phase had almost entirely disappeared after aging for a period of 5 years. Precipitates had formed during or immediately after casting within the central regions of the zinc-rich primary dendrites. These were identified as the aluminium-rich α solid solution with f.c.c. structure. The orientation relationship between the phases was determined as [0001] ƞ[111] α (11 20) ƞ(1 10)α Each grain contained two families of precipitates with a common (0001) η habit plane, each adopting one of the two non-equivalent variants of the orientation relationship. Since this work had shown that aluminium precipitation from η was completed rapidly, the long-term dimensional changes found in zinc alloy castings on aging are considered to be due to the gradual replacement of the zinc-rich metastable phase by equilibrium α and η.