Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels

Complex multiphase microstructures were obtained in transformation induced plasticity C–Mn–Si–(Nb–Al–Mo) steels by simulated controlled thermomechanical processing. These microstructures were characterized using transmission electron microscopy, X-ray diffraction and three-dimensional atom probe tomography (APT), which was used to determine the partitioning of elements between different phases and microconstituents. The measured carbon concentration (∼0.25 at%) in the ferrite of carbide-free bainite was higher than expected from para-equilibrium between the austenite and ferrite, while the concentrations of substitutional elements were the same as in the parent austenite suggesting that incomplete bainite transformation occurred. In contrast, the distribution of substitutional elements between the ferrite lath and austenite in carbide-containing bainite indicated a complete bainite reaction. The average carbon content in the retained austenite (3.2 ± 1.6 at%) was somewhat higher than the T₀ limit. On the basis of the APT measured composition, the calculated M_s temperatures for retained austenite were above room temperature, indicating its low chemical stability.     

Transmission electron microscopy study of metallic multilayers

Transmission electron microscopy (TEM) becomes a necessary and routine technique in the examination of texture, epitaxy, metastable phases and interface structures in metallic multilayers. This paper presents a viewpoint of study of the structures in both micro and atomic scales by various TEM techniques.     

Bi-phase transition diagrams of metallic thin multilayers

Phase transitions of metallic multilayers induced by differences in interface energy are considered thermodynamically, based on a thermodynamic model for interface energy and the Goldschmidt premise for lattice contraction. Bi-phase transition diagrams of Co/Cr, Zr/Nb, Ti/Nb and Ti/Al multilayers are constructed, which are in agreement with experimental results.     

Deformation-induced synthesis and structural transformations of metallic multilayers

At large strains following repeated rolling and folding, stacked foil assemblies evolve into nanometer scale multilayers. The interfacial area per unit volume is a key microstructural metric and together with the interfacial mixing zone scale can be used to gauge alloying and transformation behavior.     

Interface-enabled defect reduction in He ion irradiated metallic multilayers

Metallic multilayers are good model systems to explore the effects of heterophase interfaces in reducing radiation damage in structural materials. We summarize recent studies on radiation damage in immiscible face-centered cubic/body-centered cubic metallic multilayers, in particular Cu/V and Cu/Nb. These multilayers have shown unique characteristics compared to bulk metals under irradiation, including several orders of magnitude higher He solid solubility, dramatic reduction of bubble density, interface confined growth of He bubbles, and much lower radiation hardening. The mechanisms for interface enhanced radiation tolerance are briefly discussed.     

Plasticity in fine scale semi-coherent metallic films and multilayers

Recent developments in the study of the plastic deformation of fine scale metallic multilayers are presented in this paper. Modelling using dislocation dynamic simulations adapted to the composite structure and experimental observations using large strain deformation are briefly reported.     

Cu-Cr纳米金属多层膜屈服强度/硬度的尺寸依赖性

采用磁控溅射方法分别在聚酰亚胺基体以及单晶硅基体上制备恒定调制比(η)以及恒定调制周期(λ)的Cu-Cr纳米金属多层膜;通过单轴拉伸试验以及纳米压痕试验系统研究Cu-Cr多层膜屈服强度及硬度的尺度依赖性。微观分析结果表明:基体对多层膜的微观结构无影响,Cu-Cr多层膜在生长方向上均呈现Kurdjumov-Sachs取向关系,即{111}Cu//{110}Cr和-110-Cu//-111-Cr。力学测试结果表明:调制比恒定的Cu-Cr多层膜的屈服强度及硬度随调制周期的缩短而增加;调制周期恒定的Cu-Cr多层膜的屈服强度/硬度随调制比的增加而增加。Cu-Cr多层膜变形机制在临界调制周期(λc≈25 nm)和临界调制比(ηc≈1)由Cu层内单根位错滑移转变为负载效应。     

Mechanics of nanoscale metallic multilayers: From atomic-scale to micro-scale

Layered composites of Cu/Nb with incoherent interfaces achieve very high strength levels. Interfaces play a crucial role in determining this strength by acting as barriers to slip. Atomistic models of Cu/Nb bilayers are used to explore the origins of this resistance. The models clearly show that dislocations near an interface experience an attraction toward the interface. This attraction is caused by shear of the interface induced by the stress field of the dislocation. More importantly, atomistic simulations also reveal that interfacial dislocations easily move in interfaces by both glide and climb. Integrating these findings into a micro-scale model, we develop a three-dimensional crystal elastic–plastic model to describe the mechanical behavior of nanoscale metallic multilayers.     

A predictive model for microstructure evolution in metallic multilayers with immiscible constituents

Focussing on the thermal stability of layered structures, we developed a predictive model to study the microstructure evolution of metallic multilayers with different morphologies including aligned and classical staggered grain geometries. We found that the zig-zag microstructure experimentally observed in multilayers forms when grains in each upper layer have a relative shift less than half the in-plane grain size to the lower layer. During this formation process, the non-equilibrium triple junctions move, driven by the imbalance of tensions of interphase and grain boundaries, corresponding to an extension to classical grooving theory. Numerical simulations show that a finite mobility of the triple junction can effectively impede the development of grooves, suggesting that the classical t^1/4 dependence of groove depth with the assumption of an infinite triple junction mobility might be questionable at low temperatures to predict the time to pinch-off. Further, a map for the stability of layered structure in Cu/Nb system is developed in terms of the aspect ratio of grain dimensions and the ratio of the distance between two nearest triple junctions to the in-plane grain size. A criterion for this stability is also proposed for multilayers with similar grain boundary energies based on simplified geometrical consideration. Both the map and the simple criterion are in good agreement with the experiments for Cu/Nb multilayers.     

Calculation of the long-range order parameter from atom probe data

In addition to the known diffraction techniques, field-ion microscopy with atom probe is well established for determining the long-range order parameter. The site occupation probabilities of the chemical species on the different sublattices may be estimated from experimental profiles. However, this evaluation method demands that the superstructure planes be identified unequivocally from the data. This condition is not fulfilled in all cases. We propose a new analytical method for which this condition need not be met.     

Atom probe microanalysis and nanoscale microstructures in metallic materials

An atom probe field ion microscope (APFIM) is a unique microanalytical instrument with a single atom detection sensitivity. By employing a three-dimensional atom probe (3DAP), alloying elements can be mapped out in a three-dimensional space with near-atomic resolution. APFIM is particularly useful for the analysis of solute clusters and small precipitate particles, because it can collect atoms exclusively from nanoscale particles embedded in a matrix. These features are suitable for characterizing the complicated microstructures in commercial metallic materials. Recent atom probe investigations on the nanoscale microstructures of the metallic materials are overviewed to demonstrate the unique feature of the APFIM technique.     

Al diffusion in ZnO nanowalls investigated by atom probe tomography

Synthesis of ZnO nanowall structures using Ni catalyst was studied. ZnO nanowalls were grown by vapour-liquid-solid method. Ni as being a catalyst in the formation of ZnO nanowalls provided nucleation sites for the nucleation and the growth of ZnO nanowalls. Even though the sapphire system with ZnO has the high stability, the reactions between ZnO nanowalls and the sapphire substrate formed a 10-nm-thick interlayer during ZnO nanowall growth. Moreover, during the growth of the ZnO nanowalls, diffusion of Ni and Al was not expected as the Ni-Sapphire system is known to be non-reactive. Atom-probe tomography revealed that Al and Ni diffused into the NiO interface and sapphire substrate. Al diffused along the interface generated by the growth of ZnO nanowall, but Ni diffused into the interlayer between ZnO and sapphire.     

Atom probe tomography study of the decomposition of a bulk metallic glass

The microstructural evolution of a Pd₄₀Ni₄₀P₂₀ bulk metallic glass that was isothermally annealed at 260 °C for 14 h, and then aged at 340 °C for times up to 1280 min has been studied. Differential scanning calorimetry (DSC) curves of the aged samples show an endothermic peak at approximately 370 °C in addition to the ubiquitous glass transition. The endothermic peak appears after 20 min aging and disappears after 320 min aging. The corresponding X-ray diffraction (XRD) data show no Bragg peaks that could indicate the formation of a crystalline phase. Near-atomic-resolution atom probe tomography (APT) was used to study changes in the atomic spatial distributions as a function of aging time. The chemical environment around each of the atomic species, and the tendencies for solute clustering and chemical short range ordering, were determined from statistical analysis of the APT data. Clustering and possible phase separation are identified by APT after only 20 min aging at 340 °C, which correlates with the appearance of the peak in the DSC signal. Crystallization is apparent in the APT and XRD data after aging for 320 min. The study suggests that the amorphous Pd₄₀Ni₄₀P₂₀ annealed at a temperature 40 °C above T_g phase separates into two or more amorphous phases. The endothermic peak in the DSC trace is produced by the dissolution of the phase separation.     

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 °C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.     

Tribological studies of a Zr-based bulk metallic glass

In the present study, the tribological behavior of a Zr_52.5Cu_17.9Ni_14.6Ti₅Al₁₀ bulk metallic glass (BMG) was investigated using pin-on-disk sliding measurements under an argon atmosphere, rubbing against a type 303 stainless steel counterface. The tested pins and disk were examined using X-ray diffractometry, optical microscopy, profilometry, scanning electron microscopy and transmission electron microscopy. The results showed that the wear of the BMG pins was substantially larger compared with previous tests performed against a zirconia counterface. Strain softening was found in the near-surface region of the glassy pin due to the highly localized shearing. Frictional heating contributed to the occurrence of viscous flow and material transfer on the worn surface of the wear pin and the disk, respectively. Thus, the pin exhibited a severe adhesive-dominated sliding wear.     

Structural studies of metallic nanowires with synchrotron radiation

Mesoporous alumina membranes, with pore diameters between 5 and 100 nm, have been used as templates for the electrochemical deposition of nanowires of metals including gold, silver, iron and cobalt. Several absorption and scattering techniques using synchrotron X-ray radiation, including EXAFS, XANES, SAXS, WAXS and high-energy diffraction, have been used to study the structural features of the empty membranes and the confined metal nanowires. The results show that the membranes consist of arrays of highly-aligned parallel cylindrical pores with axes that are distributed on a disordered two-dimensional hexagonal lattice. Nanowires of gold, silver, and iron have the same lattice structures and almost identical interatomic distances as the corresponding bulk metals. Cobalt nanowires are composed of a mixture of h.c.p. and f.c.c. phases in a ratio depending on the pore diameter. The crystallites of the metallic nanowires show strong preferred orientation relative to the pore axis in the case of iron and cobalt but are more isotropic in the case of gold and silver. The results confirm the anisotropic behaviour of the materials but further theoretical work and additional modelling will be required to extract the full quantitative information from the available data. The powerful combination of complementary X-ray techniques shows the advantages of using third-generation synchrotron radiation for this type of structural investigation.     

3D atom probe characterization of temporal evolution of precipitates in aging Nb-V micro-alloyed steel

A 3D atom probe combined with TEM was applied to characterize carbides precipitation in Nb-V microalloyed steel aging at 550 °C for different times after being water-quenched from 1200 °C. The results indicated that the V- and Nb-containing ultra-fine carbides obtained by 4 h aging had the highest number density and moderate sizes, resulting in a peak precipitation strengthening value. C, V and Nb firstly co-segregated to form nano-sized clusters at the initial stage of precipitation, and then contributed to the formation of composition-steady carbides as the aging time was prolonged.