PRECIPITATION AND MICROSTRUCTURE OF Cr MODIFIED Al3Ti

1. IntroductionThe tetragonal D022 compound A13Ti has recently received attention as a possible structural material because of its low density, 3.4g/cm', and its good oxidation resistanced'2].However, D0zz-AlsTi has the disadvalltages, that it is a line compound and is extremelybrittle at room temperature, presumably due to the lack of slip systems associated withthe crystal symmetry[2]. It is known that partially replacing some of the aluminum atomsin AlsTi with Cr[31, Ni[4], or Ag['] etc…     

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.     

铁铝黄铜中的Fe_3Al沉淀

在铁铝黄铜的显微组织中观察到3种形貌的Fe_3Al沉淀.通过显微组织发展的研究表明,Fe_3Al的3种形貌反映了其长大过程中的3个阶段.Fe_3Al初析出时和基体共格.粗化过程中,沉淀颗粒沿〈111〉方向生长,于是其形貌发生从立方到树枝状的变化,同时也失去了共格性.     

PbS组装:PbS纳米单晶的取向连接(英文)

在低温下通过金属有机物分解制备了两种PbS超晶格结构。研究了反应温度和反应时间对PbS纳米晶组装结构的形貌及粒度影响。对形貌的研究揭示了一种可能的生长机理:复杂的组装结构主要由大量单个纳米晶体定向组装而成,而{111}晶面产生的偶极矩是这种组装形成的驱动力。     

Transition metal co-precipitation mechanisms in silicon

Formation mechanisms of precipitates containing multiple-metal species in silicon are elucidated by nano-scale morphology and phase investigations performed by synchrotron-based X-ray microprobe techniques. Precipitates formed at low (655 °C) and high (1200 °C+) temperatures exhibit distinguishing features indicative of unique formation mechanisms. After lower-temperature annealing, co-localized single-metal silicide phases are observed, consistent with classical models predicting that dissolved, supersaturated metal atoms will precipitate into solid second-phase particles. Precise precipitate morphologies are found to depend on the local crystallographic environment. In precipitates formed during slow cooling from higher-temperature anneals, nano-scale phase separation and intermetallic phases are evident, suggestive of a high-temperature transition through a liquid phase. Based on experimental results and phase diagram information, it is proposed that under certain conditions, liquid metal–silicon droplets may form within the silicon matrix, possibly with the potential to getter additional metal atoms via liquid–solid segregation.     

Elastic strain energy due to misfit strains in a polyhedral precipitate composed of low-index planes

The dependence of precipitate shape on the elastic strain energy caused by anisotropy of the elastic moduli is discussed for polyhedral precipitates with purely dilatational misfit strains. By considering cubic structures for the precipitates and matrix phase, the elastic strain energy is calculated for the precipitate shapes of a {1 0 0} cube, a {1 1 0} rhombic-dodecahedron, a {1 1 1} octahedron, and the truncated shapes of all three polyhedra. The anisotropy of the elastic moduli of Cu is used for the numerical calculations. The polyhedral shapes of the {1 1 1} octahedron and {1 0 0} cube result in the highest and lowest values of the elastic strain energy per unit volume of precipitate. The calculated values of the elastic strain energy obtained for the truncated polyhedra are compared with values estimated using a geometrical method that considers the area changes of the {1 0 0}, {1 1 0} and {1 1 1} interfaces.     

Analysis of Gibbsian segregation at heterophase interfaces using analytical transmission electron microscopy: a novel approach

A new and general approach to analyze Gibbsian segregation at heterophase interfaces is proposed. It is tested on the possible segregation of indium dissolved in a copper matrix to interfaces between MnO precipitates and the copper matrix. In the present approach the actual concentration of the segregating element in a monolayer at the interface is obtained. This is in contrast to line-scans or maps where the concentrations determined are a convolution of the concentration profiles with the electron probe and where, for general interfaces, the deconvolution problem cannot be solved accurately. This is possible because the present approach uses explicitly the information offered by hetero-interfaces. The occurrence of indium segregation is clearly demonstrated and the indium concentration in the terminating copper monolayer at the parallel {111} Cu/MnO interface is determined to be 15±3 at%, whereas the average indium concentration in the copper matrix is 3.8±0.4 at%. A comparison between experimental high-resolution transmission electron microscopy images of the interface, atomistic calculations and image simulations indicates that indium atoms in the terminating monolayer of copper at the interface segregate to generalized oxygen nodes present between the dislocation lines of a trigonal misfit-dislocation network with 〈110〉 line direction and 1/6〈112〉 Burgers vector. Theoretically, an indium concentration at the interface of 14±7 at% is estimated to be favorable, which compares well with the experimental findings.     

Influence of misfit and interfacial binding energy on the shape of the oxide precipitates in metals: ; Interfaces between Mn3O4 precipitates and Pd studied with HRTEM

Transmission electron microcopy (TEM) revealed Mn₃O₄ precipitates with two types of dominant shape in Pd–3 at.% Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn₃O₄. These observations were compared with earlier observations in the Ag/Mn₃O₄ system and the octahedrons show a relatively larger truncation by (002) in Pd than in Ag. Further, the second type of precipitate shape, comprising about 1/3 of all of the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure, showing an orientation relationship where the tetragonal axes of Mn₃O₄ are parallel to the cube axes of Pd, with the c-axis of Mn₃O₄ as habit plane normal. High-resolution TEM observations revealed the presence of a square misfit dislocation network with line direction 〈110〉 and Burgers vector 1/2〈110〉 at these interfaces with (002)Mn₃O₄6{200}Pd. The general conclusions of the present analysis are: (1) the anisotropy in interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well-defined shapes associated with the Wulff construction; (2) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in mismatch is present and if the matrix is sufficiently stiff; and (3) the stronger coupling strength due to electronic binding effects across the interface in Pd compared with Ag is responsible for formation of the dislocation network structures at larger misfit.     

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.     

Atomic-scale structure and chemistry of ceramic/metal interfaces—I. Atomic structure of {222} MgO/Cu (Ag) interfaces

Ceramic/metal (C/M) {222} MgO/Cu (Ag) heterophase interfaces, prepared by internal oxidation, are studied by scanning transmission electron microscopy (STEM). The observed spacing between misfit dislocations (1.45 nm) in a 〈110〉 projection is in agreement with the prediction of Bollmann's geometric O-lattice theory and experimental values in the literature for {222} MgO/Cu interfaces. It is concluded that the {222} MgO/Cu (Ag) interfaces are semicoherent and contain a trigonal network of pure edge misfit dislocations parallel to 〈110〉-type directions, with an (a/6)〈211〉-type Burgers vector. Misfit dislocations are also found in a standoff position at a distance of a single (111) spacing of the Cu (Ag) matrix. Extra intensity at the interface, in some angular dark-field images indicates silver segregation, in agreement with our atom-probe field-ion microscope results. On the metal side of the interface, extra intensity is observed in five atomic layers, which corresponds to a total silver enrichment of approximately 0.7 effective monolayers.     

Characterization of microstructural strengthening in the heat-affected zone of a blast-resistant naval steel

The influence of simulated heat-affected zone thermal cycles on the microstructural evolution in a blast-resistant naval steel was investigated by dilatometry, microhardness testing, optical microscopy, electron backscatter diffraction and atom-probe tomography (APT) techniques. Coarsening of Cu precipitates were observed in the subcritical and intercritical heat-affected zones, with partial dissolution in the latter. A small number density of Cu precipitates and high Cu concentration in the matrix of the fine-grained heat-affected zone indicates the onset of Cu precipitate dissolution. Cu clustering in the coarse-grained heat-affected zone indicated the potential initiation of Cu reprecipitation during cooling. Segregation of Cu was also characterized by APT. The hardening and softening observed in the heat-affected zone regions was rationalized using available strengthening models.     

镁合金铸造缺陷塑性变形弥合与修复过程

通过电子显微镜、透射电镜并结合塑性变形数值模拟,观察镁合金铸造缺陷(晶间缩松和缩孔)在变形过程中的形貌变化及缺陷附近应力分布,讨论塑性变形消除铸造缺陷的力学行为,提出镁合金铸造缺陷塑性变形弥合过程为:缺陷体积压缩→界面闭合→界面两侧基体相切变→界面附近再结晶→缺陷部分弥合.结果表明:变形过程中缺陷附近应力集中明显,变形后应力集中大大降低,部分铸造缺陷得以弥合与修复.     

The effect of elastic misfit strain on the morphological evolution of γ’-precipitates in a model Ni-base superalloy

The morphological evolution of coherent γ’-precipitates in a Ni-23.4Co-4.7Cr-4Al-4.3Ti (wt%) alloy has been observed under systematically varying heat-treatment conditions. By deeply etching the matrix and observing under a scanning electron microscope, the precipitate morphologies are determined accurately. By quenching after solution treatment and aging isothermally, high initial precipitate density is obtained, and the cuboidal precipitates cluster and align to form regularly spaced arrays. The dimensionality of the aligned structure increases with increasing precipitate volume fraction as the aging temperature decreases. By directly cooling to an aging temperature after the solution treatment intermediate precipitate density is obtained. During the isothermal aging, the cuboidal precipitates split into octets and clusters of several pieces which disintegrate subsequently into aligned cuboids. Upon further aging the coherency is broken. When directly cooled to aging temperatures very close to the solvus temperature, the dispersed precipitates grow dendritically along the 〈111〉 directions. Various thermodynamic and kinetic theories for the elastic strain effect stemming from the lattice misfit agree quantitatively with the observed clustering, alignment, and splitting, but presently there is no complete theory which accurately describes the observed morphologies.     

Effect of pre-straining on structure and formation mechanism of precipitates in Al–Mg–Si–Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al–Mg–Si–Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al–Mg–Si–Cu alloy. The precipitates formed along the dislocations exhibit three features: non-periodic atomic arrangement within the precipitate; Cu segregation occurring at the precipitate/α(Al) interface; different orientations presented in one individual precipitate. Four different formation mechanisms of these heterogeneous precipitates were proposed as follows: elongated precipitates are formed independently in the dislocation; string-like precipitates are formed directly along the dislocations; different precipitates encounter to form string-like precipitates; precipitates are connected by other phases or solute enrichment regions. These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.     

Influence of Heat Treatment on Precipitation Behavior and Mechanical Properties of Extruded AZ80 Magnesium Alloy

By means of annular channel angular extrusion technology and solution treatment,the AZ80 supersaturated solid solution was prepared.The β-Mg_(17)Al_(12) precipitate morphologies analysis along with its sizes and area fraction measurement was performed to investigate mechanical properties variation of alloy at various aging temperature.The results indicated that after solution treatment,the intergranular precipitate of as-extruded alloy was re-dissolved into the matrix,eff ectively decreasing the stress concentration,which was beneficial to the enhancement of elongation.At aging temperatures of 300 ℃,Widmanstatten structure,lath-shaped precipitate,and intergranular precipitate were observed,while at 175–200 ℃,cellular structure and oval-shaped particle were also identified.The morphology,area fraction,size of the precipitates,and PFZs(precipitate free zones) width worked together to aff ect the age-hardening behavior of the alloy.The strength and hardness of the peak-aged samples decreased with the increase in aging temperature,which is primarily attributed to the reduction in cellular structure area fraction and the corresponding increase in the interlamellar spacing,and the coarsening of the rhombus precipitate.     

Atomic scale investigation of Cr precipitation in copper

The early stage of chromium precipitation in copper was analyzed at the atomic scale by atom probe tomography (APT). Quantitative data about the precipitate size, three-dimensional shape, density, composition and volume fraction were obtained in a Cu–1Cr–0.1Zr (wt.%) commercial alloy aged at 713 K. Surprisingly, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction to the equilibrium Cu–Cr phase diagram. APT data also show that some impurities (Fe) may segregate along Cu/Cr interfaces. The concomitant evolution of the precipitate shape and composition as a function of the aging time is discussed. Special emphasis is given to the competition between interfacial and elastic energy, and to the role of Fe segregation.     

Phase transformations in Au(Fe) nano- and microparticles obtained by solid state dewetting of thin Au–Fe bilayer films

Sub-micrometer-sized particles of Au–Fe alloys were obtained by solid-state dewetting of single-crystalline Au–Fe bilayer films, deposited on c-plane sapphire (α-Al₂O₃) substrates. Depending on the annealing parameters, precipitation of an Fe-rich phase occurred on the side facets of the particles in an interface-limited reaction. Based on the literature values of surface and interface energies in the system, the precipitates were expected to grow inside the Au(Fe) particles, resulting in an (Fe) core–(Au) shell morphology. However, more complex, time-dependent precipitate morphologies were observed, with faceted Fe-rich precipitates attached to the parent faceted Au-rich particles of the same height being dominant at the last stages of the transformation. Our high-resolution transmission electron microscopy observations revealed a nanometric segregation layer of Au on the surface of Fe-rich particles and at their interface with sapphire. This segregation layer modified the surface and interface energies of the Fe-rich particles. A thermodynamic transformation model based on the concept of weighted mean curvature was developed, describing the kinetics of precipitations and morphology evolution of the particles during the dewetting process. Employing the values of surface and interface energies modified by segregation resulted in a good qualitative agreement between theory and experiment.     

Localized corrosion mechanism associated with precipitates containing Mg in Al alloys

To clarify the localized corrosion mechanism associated with precipitates containing Mg in Al alloys, the simulated bulk precipitates of S and β were synthesized through melting and casting. Their electrochemical behaviors and coupling behaviors with α（Al） in NaCl solution were measured. Meanwhile, simulated Al alloys containing S and β particles were prepared and their corrosion morphologies were observed. It＇s found that there exist two kinds of corrosion mechanisms associated with precipitates containing Mg. The precipitate of β is anodic to the alloy base, resulting in its anodic dissolution and corrosion during the whole corrosion process. While, there exists a corrosion conversion mechanism associated with the S precipitate, which contains active element Mg and noble element Cu simultaneously. At an initial stage, S is anodic to the alloy matrix at its periphery and the corrosion occurs on its surface. However, during its corrosion process, Mg is preferentially dissolved and noble Cu is enriched in the remnants. This makes S become cathodic to a（Al） and leads to anodic dissolution and corrosion on the alloy base at its periphery at a later stage.     

Microstructures and mechanical behavior of Inconel 718 fabricated by selective laser melting

In this study Inconel 718 cylinders were fabricated by selective laser melting in either argon or nitrogen gas from a pre-alloyed powder. As-fabricated cylinders oriented in the build direction (z-axis) and perpendicular to the build direction (x-axis) exhibited columnar grains and arrays of γ″ (body-centered tetragonal) Ni₃Nb oblate ellipsoidal precipitates oriented in a strong [2 0 0] texture determined by combined optical metallography, transmission electron microscopy, and X-ray diffraction analysis. Fabricated and hot isostatic pressed (HIP) components exhibited a more pronounced [2 0 0] columnar γ″ phase precipitate architecture parallel to the laser beam or build direction (spaced at ∼0.8 μm), and a partially recrystallized fcc grain structure. Fabricated and annealed (1160 °C for 4 h) components were ∼50% recrystallized and the recrystallized regions contained spheroidal γ′ precipitates distributed in a dense field of fine γ″ precipitates. The γ″ precipitates were always observed to be coincident with {1 0 0} planes of the γ-fcc NiCr matrix. Some δ phase precipitates in the unrecrystallized/recrystallized interfaces and recrystallized grain boundaries were also observed in the annealed samples. The microindentation (Vickers) hardness was 3.9 GPa for the as-fabricated materials, 5.7 GPa for the HIP material, and 4.6 GPa for the annealed material. Corresponding tensile properties were comparable with wrought Inconel 718 alloy.     

Characterization of α_2 Precipitates in Ti–6Al and Ti–8Al Binary Alloys: A Comparative Investigation

Transmission electron microscopy (TEM) and atom probe tomography (APT) techniques were used to investigate the nanoscale orderedα_2 (Ti _(3 )Al) precipitates in Ti–Al binary alloys.Ti–6Al and Ti–8Al binary alloys were solution treated and aged to obtain Widmanstatten microstructure and promoteα_(2 )precipitates.The TEM results displayed strong short-range ordering ofα_(2 )precipitates in Ti–8Al alloy,while no evidence of the superlattice reflections ofα_(2 )in Ti–6Al alloy.The results acquired from APT showed theα_(2 )clusters and atoms distribution at the interface between the matrix andα_(2 )precipitates.The size and morphology ofα_(2 )particles in Ti–8Al alloy,respectively,obtained by TEM and APT are closely consistent.Meanwhile,the APT results displayed tiny size clusters in Ti–6Al alloy,which supposed to give evidence of the initial ordering process ofα_(2 )precipitates in the absence of correlative results from TEM.