Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe–Mn–Al–C alloy studied by transmission electron microscopy and atom probe tomography

We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe–3.0Mn–5.5Al–0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.     

Microstructure changes during non-conventional heat treatment of thin Ni–Ti wires by pulsed electric current studied by transmission electron microscopy

Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1 mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50 nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200 nm.     

Quantitative analysis of stress-induced martensites by in situ transmission electron microscopy superelastic tests in Cu–Al–Ni shape memory alloys

Stress-induced martensite nucleation and further growing, in Cu–Al–Ni shape memory alloys, have been studied during in situ superelastic tests in the transmission electron microscope. Two kinds of martensite, ${\mathrm{\beta }}_3^{\prime }$ and ${\mathrm{\gamma }}_3^{\prime }$, are induced and can coexist under stress, both exhibiting in a high density of stacking faults. The interface plane and the orientation relationships between the different variants of such martensites have been determined, and the atomic configurations of the lattices across the interface have been described. Finally, in light of the results, selection rules for the stress-induced promoted martensites at the nano-scale have been established, being determined by the shear direction and the basal plane of the martensite lattice.     

Grain and grain boundary activities observed in alumina–zirconia–magnesia spinel nanocomposites by in situ nanoindentation using transmission electron microscopy

At room temperature, in situ nanoindentation experiments in a transmission electron microscope reveal the grain and grain boundary activities in fully dense ceramic nanocomposites composed of Al₂O₃:ZrO₂:MgAl₂O₄ (AZM) processed by spark plasma sintering (SPS). The composites have a bi-modal grain size distribution, where the larger grains (500 nm?1 μm in diameter) consist of Al₂O₃ and MgAl₂O₄ grains, and the smaller grains (100–300 nm in diameter) are primarily ZrO₂. In situ dynamic deformation studies show that the AZM nanocomposites undergo the deformation mainly through the grain-boundary sliding and rotation of small grains, i.e. ZrO₂ grains, and some of the large grains, i.e. MgAl₂O₄ grains. We observed both plastic and elastic deformations in different sample regions in these multi-phase ceramic nanocomposites at room temperature.     

Partitioning and site occupancy of Ta and Mo in Co-base γ/γ′ alloys studied by atom probe tomography

The detailed understanding of solute partitioning and site occupancies of these solutes within the ordered γ′ (L1₂) precipitates holds a key role in the development of cobalt-base γ/γ′ alloys with optimized properties. The present atom probe tomography study utilizes both structural and compositional information to determine the partitioning behavior of transition elements like Ta and Mo between γ matrix and γ′ precipitates and their site occupancy within the γ′ phase. The addition of Ta, which enhances the formation of γ′, to a ternary Co–Al–W alloy with stoichiometric Co₃(Al,W) precipitates, results in the substitution of only the W in the γ′ precipitates to form Co₃(Al, W, Ta) precipitates. Interestingly, Mo, typically considered a γ solid solution strengthener in nickel-base alloys, also partitions strongly to γ′ precipitates when added to the Co–Al–W alloy and displaces only the W atoms. The experimentally observed equal atomic substitution of W by both Ta and Mo, without any change in the Al content within the γ′ precipitates, gives insights into the energetics of relative site substitutions in this ordered compound.     

Magnetic domain structures in Co–Ni–Al shape memory alloys studied by Lorentz microscopy and electron holography

Magnetic domain structures in recently developed Co–Ni–Al ferromagnetic shape memory alloys were examined by Lorentz microscopy and electron holography, and relations of the martensite variants (crystallographic domains) and the magnetic domains were discussed. Direct observations of the magnetic domain walls by Lorentz microscopy and the magnetic lines of force by electron holography revealed that each martensite variant was divided into fine magnetic domains under a low magnetic field, e.g. about 0.2 mT. Although an applied magnetic field of about 0.4 T made each variant a large single magnetic domain, a similar configuration of multiple magnetic domains to the previous one appeared when the applied field was removed. In situ Lorentz microscopy studies have demonstrated that magnetic domain structures are sensitive to the crystal structure and/or microstructure in Co–Ni–Al alloys, i.e. a magnetic domain structure favorable to the parent phase is not inherited to the parent phase, but a distinct domain structure is observed in the martensitic phase.     

Experimental investigation and modeling of the influence of microstructure on the resistive conductivity of a Cu–Ag–Nb in situ composite

A Cu–8.2 wt% Ag–4 wt% Nb in situ metal matrix composite was manufactured by inductive melting, casting, swaging, and wire drawing. The final wire (η=ln(A₀/A)=10.5, A: wire cross section) had a strength of 1840 MPa and 46% of the conductivity of pure Cu. The electrical resistivity of the composite wires was experimentally investigated as a function of wire strain and temperature. The microstructure was examined by means of optical and electron microscopy. The observed decrease in conductivity with increasing wire strain is interpreted in terms of inelastic electron scattering at internal phase boundaries. The experimental data are in very good accord with the predictions of an analytical size-effect model which takes into account the development of the filament spacing as a function of wire strain and the mean free path of the conduction electrons as a function of temperature. The experimentally obtained and calculated resistivity data are compared to those of the pure constituents.     

Experimental investigation and Ginzburg–Landau modeling of the microstructure dependence of superconductivity in Cu–Ag–Nb wires

The type-II superconducting properties of heavily deformed Cu–Ag–Nb wires, containing only 4 wt% (4.18 vol.%) of elongated Nb filaments as a separate superconducting phase, were investigated as a function of microstructure, temperature, total wire strain, and external magnetic fields. The microstructure of the wires was examined using optical and electron microscopy. The experimental observation of the proximity effect, i.e. of the penetration of the superconducting state into the normal resistive Cu–Ag matrix leading to bulk-superconductivity, is explained in terms of the experimentally determined topology of the microstructure in conjunction with Ginzburg–Landau theory. The pronounced drop of the critical temperature and of the critical magnetic field with increasing wire strain is explained in terms of the reduced thickness of the ductile Nb filaments which is at large strains of the order of the Ginzburg–Landau correlation length in Nb.     

Effect of annealing and doping with Zr on the structure and properties of in situ Cu–Nb composite wire

Heavily cold-drawn in situ Cu–Nb and Cu–Nb–Zr composites have been studied by transmission electron microscopy and mechanical testing. Ultimate tensile strength and microhardness decrease with introduction of intermediate annealing and increase with Zr doping. Besides, Zr results in formation of dispersed and coarse ZrO₂ particles, and changes the character of fracture from ductile to the brittle one.     

Three-dimensional atom probe microscopy study of interphase precipitation and nanoclusters in thermomechanically treated titanium–molybdenum steels

Atom probe microscopy was used to generate tomographic analyses of solute clustering and precipitation reactions in a Ti–Mo added microalloyed steel under simulated strip-rolling conditions. It was observed that the interphase row spacing of precipitates was reduced with the application of a pre-strain. The atom probe data also revealed the coexistence of nanoclusters and precipitate particles, even after isothermal holding for 3600 s. These microstructural features occurred both within 3-D interphase precipitate sheets, and in randomly selected fields of view. A bimodal distribution of larger (～8–10 nm) precipitates coexisted with smaller nanoclusters (～3 nm) within the interphase sheets/rows. Both the nanoclusters and the precipitates possessed a disc morphology, although nanoclusters with less than ～30 atoms were more irregular in shape. The size of the nanoclusters and the precipitates was expressed as a Guinier radius, and this varied between 0.5 and 8 nm for both strain conditions, with the average size ～1.8 nm. The composition of the nanoclusters varied over a wide range, yet was mostly rich in C. All of the nanoclusters and precipitates consisted of a mixture of Ti, Mo and C and the average precipitate composition was close to that of MC carbide stoichiometry, where M represents a mixture of Ti and Mo. In the majority of cases, the Ti/Mo ratio in the MC carbides was > 1. As the Guinier radius increased above 2.5 nm, the composition range became narrower, towards the MC carbide stoichiometry, with a small amount of Fe (～3–12 at.%).     

Direct observation of dislocation dissociation and Suzuki segregation in a Mg–Zn–Y alloy by aberration-corrected scanning transmission electron microscopy

Crystal defects in a plastically deformed Mg–Zn–Y alloy have been studied on the atomic scale using aberration-corrected scanning transmission electron microscopy, providing important structural data for understanding the material’s deformation behavior and strengthening mechanisms. Atomic scale structures of deformation stacking faults resulting from dissociation of different types of dislocations have been characterized experimentally, and modeled. Suzuki segregation of Zn and Y along stacking faults formed through dislocation dissociation during plastic deformation at 300 °C is confirmed experimentally on the atomic level. The stacking fault energy of the Mg–Zn–Y alloy is evaluated to be in the range of 4.0–10.3 mJ m^?2. The newly formed nanometer-wide stacking faults with their Zn/Y segregation in Mg grains play an important role in the superior strength of this alloy at elevated temperatures.     

A new criterion for elasto-plastic transition in nanomaterials: Application to size and composite effects on Cu–Nb nanocomposite wires

Nanocomposite wires composed of a multi-scale Cu matrix embedding Nb nanotubes are cyclically deformed in tension under synchrotron radiation in order to follow the X-ray peak profiles (position and width) during mechanical testing. The evolution of elastic strains vs. applied stress suggests the presence of phase-specific elasto-plastic regimes. The nature of the elasto-plastic transition is uncovered by the “tangent modulus” analysis and correlated to the microstructure of the Cu channels and the Nb nanotubes. Finally, a new criterion for the determination of the macroyield stress is given as the stress to which the macroscopic work hardening, θ_a = dσ_a/dε₀, becomes smaller than one third of the macroscopic elastic modulus.     

Mechanical behaviour and in situ observation of shear bands in ultrafine grained Pd and Pd–Ag alloys

An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x = 5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals.     

Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations

Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260 °C and only θ precipitates form in upper bainite isothermally held at 500 °C. ε and θ precipitate under paraequilibrium condition at 260 °C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 °C. In order to theoretically study ε and θ precipitation and the ε → θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol^?1 at room temperature and still by 4 kJ mol^?1 at 500 °C. Hence, the thermodynamic predictions agree well with the atom probe tomography results.     

Forced mixing and nanoscale decomposition in ball-milled Cu–Ag characterized by APFIM

Ag₅₀Cu₅₀ alloys are prepared by high energy ball milling at different controlled temperatures, 85 K, 315 K and 453 K, with milling times long enough to reach steady-state. Atom probe field ion microscopy (APFIM) is used to characterize the atomic mixing forced by low temperature milling and to study the nanocomposite materials stabilized by elevated temperature milling. A new method is devised that makes it possible to prepare sharp tips from ball-milled powders. Statistical analysis of the APFIM composition profiles is used to determine the degree of mixing as a function of the length scale. These results are compared with the ones obtained from kinetic Monte Carlo simulations. Cryo-milling results in nearly random mixing of copper and silver, whereas the mixing achieved by milling at 315 K is calculated to be around 70%. 453 K milling results in the decomposition into copper-rich and silver-rich regions at a scale of ≈25 nm.     

Defect-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys studied by in situ small-angle neutron scattering

Nanoscale Au precipitation in high-purity Fe–Au and Fe–Au–B–N alloys has been studied by in situ small-angle neutron scattering during isothermal aging at 550 °C and complementary ex situ transmission electron microscopy. The high temperature precipitation behavior in samples having received different degrees of cold deformation has been studied to explore the potential self-healing of deformation-induced defects by Au precipitation. It is found that dislocations induced by prior plastic deformation strongly facilitate the formation of Au precipitates, as no significant precipitation is observed for undeformed samples. Defect-induced Au precipitates are formed both at dislocations and along grain boundaries where the defect density is high. The fact that the Au atoms only precipitate on deformation-induced defects demonstrates that solute gold atoms act as efficient self-healing agents in the ferrous matrix. The addition of B and N is found to retard the Au precipitation.     

Premartensitic phenomena and other phase transformations in Ni–Mn–Ga alloys studied by dynamical mechanical analysis and electron diffraction

Ni–Mn–Ga ferromagnetic ordered shape memory alloys are known to exhibit phonon softening and soft mode condensation into a premartensitic phase prior to the martensitic transformation itself. In the present work, this unique behaviour of Ni–Mn–Ga system has been studied as a function of electron concentration (i.e., alloy composition) and a region on the phase transformations diagram which corresponds to the stability of the intermediate phase has been determined, being completely in the ferromagnetic zone. The results were mainly obtained by means of dynamical mechanical analysis and transmission electron microscopy. The two types of lattice instability which might occur in the parent phase driving it to either the soft mode condensed intermediate phase or to the martensitic phase are discussed in this work, together with precursor phenomena and the intermartensitic transformation observed in alloys with the highest electron concentration.