A transmission electron microscopy study of constituent-particle-induced corrosion in 7075-T6 and 2024-T3 aluminum alloys

To better understand particle-induced pitting corrosion in aluminum alloys, thin foil specimens of 7075-T6 and 2024-T3 aluminum alloys, with identified constituent particles, were immersed in aerated 0.5M NaCl solution and then examined by transmission electron microscopy (TEM). The results clearly showed matrix dissolution around the iron- and manganese-containing particles (such as Al₂₃CuFe₄), as well as the Al₂Cu particles. While Al₂CuMg particles tended to dissolve relative to the matrix, limited local dissolution of the matrix was also observed around these particles. These results are consistent with scanning electron microscopy (SEM) observations of pitting corrosion and are discussed in terms of the electrochemical characteristics of the particles and the matrix.     

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys in vacuum and air. There was evidence that crack growth occurs cycle-by-cycle in vacuum, but only for fa tigue in air was it possible to relate dislocation substructure band spacings, immediately below the fracture surface, to cyclic crack growth. A discussion of crack propagation mechanisms suggested that the Tomkins and Biggs model of striation formation comes closest to explaining the fractographic features.     

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Metallurgical and Materials Transactions A - Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys...     

An electron microscopy study of precipitation in Cu-Ti sideband alloys

Electron transmission microscopy and diffraction have revealed directly the fine-scale precipitation processes and the mechanism of formation of the periodic microstructures which characterize the decomposition of Cu-Ti “sideband” alloys. The initial stages of decomposition involve the development of «100» compositional waves giving rise to the well-known sideband or satellite reflections along the «100» directions. This «wave-like» clustering producing a triaxially modulated structure involves the formation of two disordered phases; ordering does not accompany the initial clustering. Prolonged aging produces a periodic array of interpenetrating rods along the «100» directions of the matrix. During aging the periodic structures coarsen according to a t^1/3 law. The experimental activation energy for the coarsening of the modulated structures is approximately 48 kcal/ mole. Maximum strength is associated with the formation of the transition phase β’ which forms byin situ transformation of the titanium-rich regions; this transformation is not accompanied by a loss of coherency in the modulated structures at the aging temperatures studied in this investigation. It is suggested that these periodic structures arise from spinodal decomposition.     

High-resolution analytical electron microscopy study of isothermal plate-shaped products in some β-phase alloys

The isothermal plate-shaped products in β-phase Cu-Zn-Al, Cu-Zn-Au, and Ag-Cd alloys have been studied from the viewpoints of morphology, crystal structure, and composition using a high-resolution analytical electron microscope equipped with a field-emission electron gun. It is shown that thin plate-shaped products, at early stages of the isothermal formation, exhibit crystallographic properties characteristic of martensites formed at subzero temperatures but that their compositions were definitely different from those of the surrounding matrices. Therefore, atom diffusion is conceivable to be involved in the plate formation from the nucleation stage and to occur across the coherent plate/matrix interfaces. In order for nucleation of the isothermal transformation to occur, two kinds of reactions should be thermally activated at intermediate temperatures above T_o for the martensitic transformation: (1) atom diffusion to achieve the observed composition differences between plates and matrices and (2) martensitic transformation in hypothetical alloys with compositions less than those of the original ones by the observed composition differences. The incubation periods experimentally observed should be associated with these two thermal activation processes. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

An analytical electron microscopy study of paraequilibrium cementite precipitation in ultra-high-strength steel

To support quantitative design of ultra-high-strength (UHS) secondary-hardening steels, the precipitation of cementite prior to the precipitation of the M₂C phase is investigated using a model alloy. The microstructure of cementite is investigated by transmission electron microscopy (TEM) techniques. Consistent with earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M₅C₂ (Hägg carbide). The concentration of substitutional alloying elements in cementite are quantified by high-resolution analytical electron microscopy (AEM) using extraction replica specimens. Quantification of the substitutional elements in cementite confirms its paraequilibrium (PE) state with ferrite at the very early stage of tempering. The implications of these results are discussed in terms of the thermodynamic driving force for nucleation of the primary-strengthening, coherent M₂C carbide phase. The ferrite-cementite PE condition reduces the carbon concentration in the ferrite matrix with a significant reduction of M₂C driving force. The kinetics of dissolution of PE cementite and its transition to other intermediate states will also influence the kinetics of secondary hardening behavior in UHS steels.     

An electron microscopy study of modes of intermetallic precipitation in Ti-Cu alloys

The precipitation processes that accompany the aging of supersaturatedα-titanium solid solutions containing up to 6 wt pct Cu have been studied by thin foil electron microscopy and X-ray diffraction techniques. Two mechanisms of decomposition have been identified: i) the heterogeneous nucleation and growth of Ti₂Cu at interfaces and internal substructure such as dislocations, and ii) the uniform nucleation of thin, coherent, disk-shaped precipitates which lie on . The coherent precipitates can form with densities up to 10¹⁷ per cu cm; this and many morphological features of the precipitation process in Ti-Cu are analogous to the well-known behavior of Al-Cu alloys. The coherent precipitates first lose coherency along the edge of the disk and then along the flat faces. The mechanisms by which these two processes occur are considered in detail. Formerly Predoctoral Research Associate, University of Washington, Seattle, Wash.     

Modeling of the Effect of Temperature, Frequency, and Phase Transformations on the Viscoelastic Properties of AA 7075-T6 and AA 2024-T3 Aluminum Alloys

The viscoelastic response of commercial aluminum alloys 7075-T6 and 2024-T3 as a function of temperature is presented. Experimental data are obtained with a dynamic-mechanical analyzer (DMA) at different loading frequencies and compared with the available transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) data. The effect of successive microstructural transformations (particle precipitation and redissolution) is revealed. An analytical model is developed, which fits the mechanical response up to 573?K (300?°C). The model takes into account the concentration of Guinier-Preston Zones (GPZ) and metastable precipitates (???? in AA 7075-T6 and ????/S?? in AA 2024-T3), allowing us to determine the kinetic parameters of these transformations. The activation energies were previously obtained by several authors from DSC measurements and other techniques, showing considerable dispersion. The presented data, obtained with a completely different technique, allow us to reduce the uncertainty on these data and show the potential of DMA measurements in the study of microstructural transformations.     

Fatigue Crack initiation and microcrack growth in 2024-T4 and 2124-T4 aluminum alloys

A metallograph was mounted directly on a closed-loop electrohydraulic testing unit and initiation of fatigue cracks was directly observed on polished notches at magnifications up to 800 times in aluminum alloys 2024 and 2124 in the T-4 condition. The latter is a high purity version of 2024 and contains considerably fewer constituent particles. At high stresses on the notch surface the fatigue cracks initiated on coarse slip lines in both alloys. At low stresses almost all of the cracks in 2024 initiated in the matrix adjacent to constituent particles. In 2124 at low stresses 50 pct of the cracks initiated near constituent particles and 50 pct in the matrix not near constituent particles. The probability that a constituent particle in 2024 initiates a fatigue crack falls off very rapidly as the particle size decreases below 6 μm. Growth of microcracks is impeded by grain boundaries. This research was done at Northwestern University.     

Sintering study of silver particles by in situ electron microscopy

Micron size silver spherical particles were sintered in the hot stage of an electron microscope between 370 and 720°C. In the 370 to 520°C temperature range, sintering can be explained either by grain-boundary diffusion or dislocation slip. At higher temperatures, the controlling mechanism changes from surface or grain-boundary diffusion to volume diffusion from 570 to 720°C. These conclusions are based on observations of neck growth and shrinkage and on analyses of the shapes of the sintering curves.     

Characterizations of pore and constituent particle populations in 7050-T7451 aluminum plate alloys

Although qualitative relationships between fatigue lives and the sizes of the microstructural features, such as pores and particles, are well known, the quantitative models are lacking because of the unavailability of the required detailed microstructural data. The purpose of this work was to obtain such data for the high porosity (HP) and reduced porosity (RP) variants of the aluminum 7050-T7451 thick-plate alloys. Both alloys had similar tensile and fracture properties, but the reduced porosity variant showed superior fatigue performance attributed to the smaller sizes of the fatigue crack initiating particles and pores. Those size differences, as well as the differences in the through-thickness size gradients, have been characterized in this work. The sizes, shapes, and orientations of particles and pores were analyzed first on the plane sections and then converted to the true three-dimensional (3-D) characteristics using the moment method. In the conversions, the particle and pore shapes have been assumed as triaxial ellipsoids and their size distributions as lognormal. The spatial distributions were quantified using the nearest neighbor spacing method. Results confirmed that the reduced porosity alloy had smaller particles and pores than the high porosity variant. The size distributions in the former were also more confined. In both alloys, the largest particles and pores were at the plate centers and the smallest at the surface. Their spatial distributions could be categorized as random with clusters.     

An analytical electron microscopy study of the high temperature carbide formed in a V-5 Ti-C alloy

A finely dispersed high temperature carbide that precipitates at 1000°C in a V-5 pct Ti-C alloy has been studied using analytical electron microscopy. The crystal structure has been determined using electron diffraction as B1 (NaCl-type) with a lattice parameter of 4.29Å. The carbide develops a habit plane parallel to {001} of the matrix. The orientation relationship between the carbide and the matrix is [010] carbide ¦ | [ $$ \begin{gathered} [010] carbide || [0\bar 10] matrix \hfill \\ (001) carbide ||(101) matrix. \hfill \\ \end{gathered} $$ The elemental concentrations of the metal constituents of the carbide have been deduced using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy, through examination of core electron excitations. The composition of the carbide is found to be V₁₁Ti₃gC₅₀, assuming a stoichiometric carbide.     

An electron microscopy study of chip formation

Chip formation or metal cutting is a unique large strain, high strain rate plastic deformation process. Almost all the previously reported studies of chip formation have examined the problem from the point of view of the mechanics of the deformable bodies using the mathematical theory of plasticity. This study, recognizing the heterogeneous nature of chip formation as encountered in course of machining metals, examines the problem from the metal physical or metallurgical view point. Electron microscopy studies were carried out on steel as well as nonferrous metal chips produced by shop machining conditions and compared to those chips produced by ultramicrotomy. This thin film orthogonal cutting process was employed to produce chips for microscopic examinations under well controlled and repeatable experimental conditions. The experiments carried out were designed to clarify the details of the heterogeneous plastic deformation activity occurring on the microscopic level during machining. The morphological (external surface) characteristics of the chips observed with the scanning electron microscope were correlated with the internal, dislocated structure of the chips observed by transmission electron microscopy methods. The effect of a stacking fault energy (SFE) change in an Ag-Sn alloy on chip thickness ratio(ν _t) is presented for the first time, demonstrating that this deformation process is sensitive to changes in SFE. The essentially discontinuous nature of the chip formation process observed by scanning and transmission electron microscopy is analyzed with a model involving dynamic dislocation behavior in a metal in the presence of large energy dissipation arising from plastic flow to account for the observed instability. N. Y. J. T. BLACK, formerly with the Department of Mechanical Engineering, University of Vermont, Burlington Vt.     

An analytical electron microscope study of the kinetics

The detection and quantification of the segregation of Bi to grain boundaries in Cu using Analytical Electron Microscopy is demonstrated and the effects of time and temperature are observed. The experimental data are compared with the theoretical prediction of McLean’s segregation isotherm. Special grain boundaries are also considered. The analysis shows that the detection of the variation of segregation with time and temperature as well as grain boundary characteristics is possible using analytical electron microscopy and that the quantification techniques used here agree well with theoretical calculations of the segregant levels.     

An electron microscopy study of carbide precipitation in vanadium

The precipitation of carbon from supersaturated solid solution in vanadium has been investigated using transmission electron microscopy techniques on thin foils. High purity vanadium strip was doped to a carbon content of about 0.2 at. pct and rapidly quenched in high vacuum, followed by aging treatments for various times in the temperature range from 300° to 600°C. The strip was then thinned for transmission electron microscopy. The carbon is observed to precipitate initially as very finely dispersed carbides visible through structure factor contrast. With increasing aging the precipitate distribution coarsens, and the carbides appear as very thin coherent platelets on {310} planes, showing a pronounced displacement fringe contrast. The coherent precipitate appears to have a bcc structure closely related to that of the matrix. With further aging these platelets are observed to thicken and partially lose coherency, punching out prismatic dislocation loops and helices having axes in 〈111〉 directions. This semicoherent precipitate is found to be the hexagonal V₂C phase described by other researchers, and its orientation relationship with the matrix may be expressed: (110)v‖(00.1)v₂c, [•111]v‖[•110] v₂C. The dislocation loops are a result of the specific volume difference between the V₂C precipitation and the matrix, and have Burgers vector b = a/2〈111〉. Formerly Research Assistant, Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana Ill.     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.     

A transmission electron microscopy study of the mechanisms of strengthening in heat-treated Co-Cr-Mo-C alloys

Microstructures produced in the Co-Cr-Mo-C alloy H.S.21 were observed by transmission electron microscopy in cast specimens following solutionizing at 1230°C and aging at 650°C and in low-carbon wrought specimens following solutionizing and aging at 650°C and 750°C. In all cases, aging was found to promote the formation of fcc stacking faults and to cause an initial martensitic transformation from the fcc phase to a heavily faulted hep structure. Precipitate formation was observed in hcp areas of the cast material after 20 h at 650°C and in hcp areas of wrought material after 20 h at 750°C. Prolonged aging at 750°C produced a transformation in the hcp structure of wrought specimens, with a relatively fault-free structure replacing the heavily faulted martensitic form. Interruption of fcc slip by both fcc stacking faults and bands of hcp phase was found to be the principal strengthening mechanism activated by aging. Precipitate formation in the hcp plays an increasingly significant role as aging time is increased. This microstructural information is used to explain the observed tensile properties of these alloys after the heat treatments mentioned.