Metal–ceramic interfaces studied with high-resolution transmission electron microscopy

How the core structure of an interface dislocation network depends on both misfit and bond strength across the interface is investigated. It is shown that, in principle at least, it is possible to assess the bond strength by investigating the atomic structure of the dislocation cores. As examples, the misfit-dislocation structures at Ag/Mn₃O₄, Cu/MnO interfaces formed by parallel close-packed planes of Ag or Cu and O obtained by internal oxidation were studied using HRTEM and lattice static calculations. The lattice static calculations are instrumental in indicating the possible dislocation network and their results served as input for HRTEM image simulations which are then compared with experimental HRTEM images. In addition, the influence of dissolution of a segregating element (Sb) in these systems was also studied using HRTEM. The influence on Mn₃O₄ precipitates in Ag is distinct, namely: (i) the initial precipitates, sharply facetted by solely {111}, are changed into a globular shape with sometimes also short {220} and (002) facets, (ii) a partial reduction of Mn₃O₄ into MnO occurs for a part of the precipitates. Further Sb appeared to prevent Oswald ripening of the precipitates.     

In-situ TEM analysis of the reduction of nanometre-sized Mn3O4 precipitates in a metal matrix

The objective of the present work is the in-situ study of the transformation of small oxide precipitates in a metal matrix by conventional and high-resolution transmission electron microscopy (HRTEM). As an example the reduction of Mn₃O₄ into MnO for nano-sized oxide precipitates in a silver matrix was studied in detail. A convenient method for monitoring the reduction process is shown for a large number of precipitates simultaneously. It is based on two-beam dark-field images showing distinct Moiré patterns for the MnO and the various types of Mn₃O₄ precipitates embedded within an Ag matrix. A controlling factor of the transformation kinetics appeared to be the rate in which the system can relax the strains due to the accompanying volume reduction of the precipitates. Other interesting aspects of the Mn₃O₄ to MnO transformation scrutinized and explained were the shape change of the precipitates upon reduction and the fact that mixed Mn₃O₄/MnO precipitates were only detected within a small temperature/time interval. Ostwald ripening of the MnO precipitates was observed as well.     

Microstructure dependence of strength of Ir-base refractory superalloys

Precipitation hardening was investigated in Ir–Nb and Ir–Zr alloys with a two-phase structure consisting of the fcc matrix and L1₂ coherent precipitate phases, similar to that in Ni-base superalloys. Cuboidal L1₂ precipitates and plate-like L1₂ precipitates were formed with coherent interfaces in the fcc matrix in the Ir–Nb and Ir–Zr alloys, respectively. Effects of precipitate shape and coherency strains on precipitation hardening are discussed in terms of lattice misfit. Plate-like precipitates forming a 3-dimensional maze structure in the Ir–Zr alloys were profitable to precipitation hardening in both factors, that is precipitate shape and coherency strains.     

A model for evolution of shape changing precipitates in multicomponent systems

Recently the authors introduced a concept of shape factors to extend an already established model for the growth and coarsening kinetics of spherical precipitates in multicomponent multiphase environments to needle- and disc-shaped geometries. The geometry of the precipitates is kept in the original version of the concept to be self-similar with a given fixed aspect ratio. In the present treatment, the aspect ratios of individual precipitates are treated as independent evolving parameters. The evolution equations of each precipitate, described by its effective radius, mean chemical composition and the aspect ratio, are derived by application of the thermodynamic extremal principle. The driving force for the evolution of the aspect ratio of the precipitate stems from the anisotropic misfit strain of the precipitate and from the orientation dependence of the interface energy. The model is used for the simulation of the precipitation of Ti₃AlN and Ti₂AlN in Ti–Al–0.5 at.% N matrix.     

HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of δNb₁₁Si₄ usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss ⟶ δNb₁₁Si₄ + Nbss₁. In some cases, the δNb₁₁Si₄ precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr₂Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/γNb₅Si₃ interfaces. The coupling precipitation behaviors and precipitation reactions of δNb₁₁Si₄, Cr₂Nb and γNb₅Si₃ in the Nb-Ti-Si based alloys have been discussed.     

Predicting equilibrium shape of precipitates as function of coherency state

A general approach is proposed to predict the equilibrium shapes of precipitates in crystalline solids as function of size and coherency state. The model incorporates effects of interfacial defects such as misfit dislocations and structural ledges on transformation strain and on interfacial energy. Using α precipitation in α/β titanium alloys as an example, various possible equilibrium shapes of precipitates having different defect contents at interfaces are obtained by phase-field simulations. The simulation results agree with experimental observations in terms of both precipitate habit plane orientation and defect content at the interface. In combination with crystallographic theories of interfaces and experimental characterization of habit plane of finite precipitates, this approach has the ability to predict the coherency state (i.e. defect structures at interfaces) and equilibrium shape of finite precipitates.     

Silver segregation to θ′ (Al2Cu)–Al interfaces in Al–Cu–Ag alloys

θ′ (Al₂Cu) precipitates in Al–Cu–Ag alloys were examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ′ (AlAg₂) precipitates were present. These dislocation loops were enriched in silver prior to θ′ precipitation. Coherent, planar interfaces between the aluminium matrix and θ′ precipitates were decorated by a layer of silver two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al–θ′ interfacial energy. The lateral growth of the θ′ precipitates was accompanied by the extension of this silver bilayer, resulting in the loss of silver from neighbouring γ′ precipitates and contributing to the deterioration of the γ′ precipitate assemblies.     

Three-dimensionally intercrossing Mn3O4 nanowires

Three-dimensional Mn₃O₄ nanostructures were synthesized by a soft chemistry templating process using a block copolymer as the structure-directing agent. This material has a unique structure of intertwining nanowires which are grown from faceted Mn₃O₄ particles and aligned spatially perpendicular to or along each facet. The magnetic measurements of such nanostructured films show clear signs of magnetic shape anisotropy, indicating that this templating synthesis approach could be a promising method to develop Mn₃O₄ nanostructures with desired anisotropic properties through dimensionality control.     

A TEM study of precipitates in an Al-Mn-Si alloy

Two types of intermetallic precipitates were found in aluminum alloy 3003 creep tested at 300°C. These precipitates are an Al₁₂(MnFeCu)₃Si₂ precipitate having a simple cubic structure with a = 1.2556 nm and an Al₆Mn-type precipitate with an orthorhombic structure with a = 0.6506 nm, b = 0.7501 nm, and c = 0.8800 nm. High resolution lattice imaging indicated the presence of a number of defect structures in the polycrystalline Al₁₂(MnFeCu)₃Si₂ precipitates.     

Effect of γ2-phase precipitates on the martensitic transformation of a β-CuAlBe shape memory alloy

The influence of γ₂ precipitates on the MT of a Cu–22.66Al–2.98Be (at%) polycrystalline alloy was analysed by compression test, differential scanning calorimetry and microscopy. As the precipitate content (or size) increases, the stress-induced martensitic transformation occurs at higher stress levels. The thermally induced martensitic transformation is only significantly affected when the γ₂ phase content exceeds 14%, the thermal peaks are shifted to higher temperatures and less β phase transforms to martensite. TEM observations suggest good coherency between both phases, and dislocation bundles remain in the β side of the γ₂/β interface after the compression-induced transformation to martensite. The evolution of σ_s with fvγ₂ is analysed considering a chemical and a mechanical contribution.     

Oxidation behavior of an austenitic stainless FeMnSiCrNi shape memory alloy

The oxidation behavior at 800 °C in air of an austenitic stainless Fe14.29Mn5.57Si8.23Cr4.96Ni (wt.%) shape memory alloy was investigated by thermogravimetry, OM, SEM (EDS), XRD, GAXRD, TEM, and DSC. The results showed that its oxidation process obeys a parabolic law. After 100 h exposure, the oxide scales are composed of outer Mn₂O₃, middle Mn₃O₄, and inner MnCr₂O₄. A ferrite layer exists between the oxidation layer and matrix. The ferrite results from the transformation of oxidation-induced Mn-depleted layer, and its thickness increases with increasing oxidation time. Profuse chi phase precipitates inside matrix and at the interface between matrix and ferrite layer.     

Microstructural evolution during the α2→α2+O transformation in Ti–Al–Nb alloys: phase-field simulation and experimental validation

The microstructural development during precipitation of a coherent orthorhombic phase (O-phase) from an α₂ matrix (DO₁₉) in a Ti–Al–Nb system is investigated through computer simulations using the phase-field approach and through experimental observations using transmission electron microscopy (TEM). Two compositions were considered in the simulations in order to examine the influence of volume fraction of the O-phase on the microstructure. It is found that in the alloy with higher volume fraction of the O-phase, the precipitates have either square or rectangular shapes on (0001)_α2 or (001)_O planes. All the particles are interconnected by sharing their corners. In the alloy with lower volume fraction, the dominant morphology for the precipitates is thin plate. The spatial distribution of precipitates is highly non-uniform with the precipitates aggregating together to form various unique patterns to accommodate the elastic energy arising from the lattice misfit between the α₂ and O-phase. All the interfaces between the α₂ and O-phase are found to be undistorted habit planes of the type {470)_O, and the domain boundaries between different orientation variants of the O-phase are twin boundaries which are the strain-free planes {110)_O or {130)_O. The simulation predictions agree remarkably well with existing experimental observations and the concurrent TEM study.     

Instrumented micro-indentation of NiTi shape-memory alloys

We study the instrumented Vickers micro-indentation of single-crystal Ti–50.9 at% Ni shape-memory alloys with systematically varied surface normal orientations ([100], [210], [111] and [221]) and Ti₃Ni₄ precipitate sizes (0 nm, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelastic deformation via dislocation activity and a stress-induced martensitic transformation. The room-temperature hardness, Hv, and recoverable energy, E_r, of NiTi are shown to be a maximum for very small precipitate sizes, decrease for intermediate precipitate sizes, and increase for large precipitate sizes. The maximization of Hv and E_r at small precipitate sizes (10 nm) is attributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in Hv and E_r at intermediate precipitate sizes (50–300 nm) are attributed to a decrease in the resistance to dislocation motion and a measured increase in the transformation temperatures with respect to the indentation temperature. The increases in Hv and E_r at large precipitate sizes (500 nm) are attributed solely to measured decreases in the transformation temperatures with respect to the indentation temperature, since the resistance to dislocation motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate the highest and lowest values of Hv and E_r, respectively, an effect attributed primarily to orientation of favorable slip systems.     

Morphology of L10-TiAl(Ag) precipitation in Ag-modified L12–Al3Ti

Transmission electron microscopy (TEM) examinations of Ag-modified Al₃Ti with an L1₂-ordered structure have revealed the precipitation of L1₀–TiAl upon aging after quenching from higher temperatures. TEM observations revealed that plate-like L1₀–TiAl precipitates lie on {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. A quantitative X-ray microanalysis for determining the chemical compositions of precipitate and matrix has been done by using an analytical electron microscope. The L1₂ phase field in the Ti–Al–Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.     

Disorder strengthening of ordered L12 alloys by face centered cubic (A1) precipitates

We present a new theory of strengthening (disorder strengthening) of a crystallographically ordered L1₂ matrix by spherical, coherent crystallographically disordered fcc (A1) precipitates. There are two versions of the theory, each involving different approximations for the length of the chord, inside the precipitate, subtended by the trailing dislocation in the super-dislocation pair. Each of these versions incorporates two other assumptions involving the statistics of dislocation-precipitate interactions, which determine the average spacing of the precipitates along the super-dislocation. All versions of the theory predict a peak-strength condition at small particle sizes, followed by an extended period of overaging in which the strength decreases slowly with increasing particle size. We compare the quantitative predictions of the theory with both computer-simulated and experimentally generated data on inverse Ni-base γ′-type superalloy matrices strengthened by γ precipitates. In all cases at least one version of the theory is in excellent agreement with the data. The predicted strengthening is significant, the order of 85–110 MPa in the computer simulated results and 40–80 MPa in model precipitation strengthened inverse superalloys containing small volume fractions (≤0.04) of γ precipitates. The results are discussed in light of the statistics of dislocation-precipitate interactions and the resistance of individual γ particles to shearing by the super-dislocation pair; this resistance depends on the ratio of maximum force required to break free of the precipitate and the line tension of the dislocation pair.     

Near-coincidence-sites modeling of the edge facet dislocation structures of α precipitates in a Ti–7.26 wt.% Cr alloy

While much attention has been paid to the orientations and structures of the habit planes of various precipitates, the orientations and structures of the edge facets are less well understood. This work presents a new approach to the interpretation of edge facets. The orientations and dislocation structures of the edge facets of α precipitates in a β matrix in a Ti–7.26 wt.% Cr alloy were investigated with a near-coincidence-sites (NCS) model combined with the analysis of Moiré planes based on the O-lattice theory. The distribution of NCS clusters exhibits two orders of modulated periodicities, which can be defined by the approximate intersections between several sets of Moiré planes. Two possible edge facets are suggested, based on the NCS distribution. Based on the misfit analysis, the dislocation structures in the edge facets were calculated. The calculated orientations and dislocation structures of the edge facets are consistent with the experimental observations.     

M5C2 carbide precipitates in a high-Cr martensitic steel

The precipitate phases in an advanced 11% Cr martensitic steel, expected to be used at 650 °C, have been investigated to understand the effect of precipitates on the creep-rupture strength of the steel. M₂₃C₆ and MX precipitates were dominant phases in this steel. Needle-like precipitates with a typical length of 180 nm and width of 20 nm; and metallic-element compositions of 53–74Fe, 16–26Cr, 3–18Ta, 2–8W, and 2–4Co (at%); were observed mainly within the martensite laths of the normalized-and-tempered steel. The needle-like precipitates have been identified as monoclinic carbide M₅C₂, which is not known to have been reported previously in high chromium steels, or in heat-resistant steels those have been normalized-and-tempered. This indicates that the formation of M₅C₂ carbides can occur in heat-resistant steels produced under appropriate tempering conditions, and that this does not require long-term isothermal aging or creep testing, in all cases.     

Jarosite-type precipitates mediated by YN22, Sulfobacillus thermosulfidooxidans, and their influences on strain

To have a better understanding on the properties of the jarosite-type precipitate synthesized by Sulfobacillus thermosulfidooxidans, the evolution of the S. thermosulfidooxidans-mediated precipitation and the influence of the precipitate on this species, a newly isolated strain （YN22） ofS. thermosulfidooxidans was cultured in a medium containing Fe^2＋ as energy source under optimal conditions （pH 1.5, 53 ℃, 0.2 g/L yeast extract, 30 g/L Fe2SO4·7H2O and 170 r/min）, added with or without glass beads. Remarkable differences were found in the oxidation rate of Fe^2＋, the precipitate yield ofjarosite-type compounds and the population density between the two groups of cultures. The group with glass beads has a 6 h faster Fe^2＋ oxidation, 6 h earlier precipitation, 78% higher precipitate yield and much lower population density than those without glass beads. XRD, EDS, FTIR and SEM analysis reveals that the precipitates originated from both groups are a mixture of potassium jarosite and ammoniojarosite, with morphological features similar to the latter. The results of the test referring to influence of the precipitates on YN22 show that the precipitate from the group without glass beads has no apparent influence on Fe^2＋ oxidation rate of YN22 and only a limited influence on growth of the strain, whereas that from the group with glass beads remarkably inhibits the growth and Fe^2＋ oxidation ability of YN22 when a precipitate content over 4 g/L is used.     

Effects of precipitates in Cu upon impact fracture: an ultra-high-vacuum study with local probe Scanning Auger/Electron Microscopy

In situ fracture under ultra-high vacuum (UHV) conditions of copper-alloys containing copper sulfide precipitates exhibits areas in the form of pits. The wide variety of morphologies depends significantly on the size of the existing precipitate. For large precipitates, the fractured surface reveals loops surrounded by step-terrace like structures with sulfur remaining preferentially on straight ledges as nanometer scale Auger images reveal in comparison with scanning electron images. For smaller precipitates (<3–4 μm), only well defined facets are formed having an almost uniform distribution of sulfur. Qualitatively the loop and facet formation could be explained due to accommodation of the Cu sulfide to the Cu matrix, leading for small precipitates to facet formation, and for large ones to step-terrace structures. Asymmetry in loop structures could be attributed to asymmetry of precipitate shape, while in some areas plasticity effects are also present. Finally, a comparison with distinctly different behavior in Cu–Bi alloys is made.     

Precipitation behavior of (Al,Ag)3Ti and Ti3AlC in L10-TiAl in Ti-Al-Ag system

A transmission electron microscopy (TEM) investigation has been performed on the morphologies of L1₂-(Al,Ag)₃Ti and Ti₃AlC precipitates in L1₀-ordered TiAl(Ag). During aging at temperatures around 1073 K after quenching from 1273 K, TiAl(Ag) hardens by the precipitation of (Al,Ag)₃Ti and Ti₃AlC. TEM observations revealed that plate-like (Al,Ag)₃Ti precipitates lie on {001} planes of TiAl(Ag) matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or high-temperature aging and finally to {112} of the matrix lattice. At the same time needle-like precipitates Ti₃AlC, which lie only in one direction parallel to the [001] direction of the TiAl(Ag) matrix, appear after long period aging or high-temperature aging. The anisotropic misfits between TiAl(Ag) matrix and L1₂-(Al,Ag)₃Ti and Ti₃AlC are considered to explain the morphologies of L1₂-(Al,Ag)₃Ti and Ti₃AlC precipitates.