Three-dimensional Atom Probe Tomography Research Articles

Quantitative study on the microstructural evolution and dimensional stability mechanism of 2024 Al alloy during long-term thermal cycling

During the start-stop process of high-precision instruments in service, the critical materials of instruments undergo thermal cycling, resulting in changes in microstructure and dimension. In this paper, the evolution and coupling effects of dislocations and precipitates within 2024 Al alloy during 500 cycles were investigated, and their influence on the dimensional change was quantitatively characterized and decoupling analyzed. Transmission Electron Microscopy (TEM)/aberration-corrected scanning TEM (CS-TEM), X-ray diffraction (XRD), and Three-Dimensional Atom Probe Tomography (3D-APT) were used for microstructural evolution analysis and quantitative statistics. The dislocation density increased from 3.32 × 1014 m−2 to 5.75 × 1014 m−2 after 500 cycles, which accelerated elemental diffusion and provided nucleation sites of precipitates, contributing to an increase in the number density of the S'/S phase from 1.04 × 1024 m−3 to 1.41 × 1024 m−3. Based on the quantitative statistics of precipitate types and corresponding volume fractions, the dimensional change induced by precipitate evolution was −1.25 × 10−4. The dimensional change caused by the free volume introduced by dislocation density was 3.32 × 10−6. Comparing these values with the experimental value of −1.94 × 10−4, it is clear that the precipitate evolution is the main factor that triggers the dimensional change.

Achieving 2.2 GPa Ultra-High Strength in Low-Alloy Steel Using a Direct Quenching and Partitioning Process.

Advanced high-strength steels (AHSS) have a wide range of applications in equipment safety and lightweight design, and enhancing the strength of AHSS to the ultra-high level of 2 GPa is currently a key focus. In this study, a new process of thermo-mechanical control process followed by direct quenching and partitioning (TMCP-DQP) was developed based on Fe-0.4C-1Mn-0.6Si (wt.%) low-alloy steel, and the effects of microstructure evolution on mechanical properties under TMCP-DQP process and conventional hot rolled quenched and tempered process (HR-QT) were comparatively studied. The results show that the TMCP-DQP process not only shortened the processing steps but also achieved outstanding comprehensive mechanical properties. The TMCP-DQP steel exhibited a tensile strength of 2.23 GPa, accompanied by 11.9% elongation and a Brinell hardness of 624 HBW, with an impact toughness of 28.5 J at -20 °C. In contrast, the HR-QT steel exhibited tensile strengths ranging from 2.16 GPa to 1.7 GPa and elongations between 5.2% and 12.2%. The microstructure of TMCP-DQP steel primarily consisted of lath martensite, containing thin-film retained austenite (RA), nanoscale rod-shaped carbides, and a minor number of nanoscale twins. The volume fraction of RA reached 7.7%, with an average carbon content of 7.1 at.% measured by three-dimensional atom probe tomography (3DAP). Compared with the HR-QT process, the TMCP-DQP process resulted in a finer microstructure, with a prior austenite grain (PAG) size of 11.91 μm, forming packets and blocks with widths of 5.12 μm and 1.63 μm. The TMCP-DQP process achieved the ultra-high strength of low-alloy steel through the synergistic effects of grain refinement, dislocation strengthening, and precipitation strengthening. The dynamic partitioning stage stabilized the RA through carbon enrichment, while the relaxation stage reduced a small portion of the dislocations generated by thermal deformation, and the self-tempering stage eliminated internal stresses, all guaranteeing considerable ductility and toughness. The TMCP-DQP process may offer a means for industries to streamline their manufacturing processes and provide a technological reference for producing 2.2 GPa grade AHSS.

New insights into the formation mechanism of the multicomponent carbides (Nb, M)C (M = Ti, Cr and Mn)

Combining first-principles calculations and classical nucleation theory, the results of elemental doping on the structure, electrons and stability of niobium carbides are investigated. Ti, Cr and Mn doping altered the physical characteristics of the carbides, resulting in lower mismatch strains and interfacial energies between the carbide and the substrate. Elemental doping reduced the critical nucleation size of carbides and contributed to the promotion of carbide nucleation. However, excessive doping with Cr and Mn increases the formation energy of carbides and unfavorable to carbide stabilization. Therefore, Cr and Mn are involved in the early elemental aggregation of carbides, while Ti and Nb are the main carbide-forming elements. The nano-precipitated phases of various compositions produced in niobium microalloyed steel were experimentally validated utilizing transmission electron microscopy (TEM) and three-dimensional atom probe tomography (3D-APT). The carbides primarily composed of Nb, Ti and C elements, with the presence of Cr and Mn in the core region leading to the formation of a core-shell structure, which is consistent with theoretical calculations.

D022 precipitates strengthened W-Ta-Fe-Ni refractory high-entropy alloy

Refractory high-entropy alloys have recently emerged as promising candidates for high-temperature structural applications. However, their performance is compromised by the trade-off required between strength and ductility. Here, a novel W30Ta5(FeNi)65 refractory high-entropy alloy with an outstanding combination of strength and plasticity at both room and elevated temperatures is designed, based on the multi-phase transitions design strategy. The alloy comprises a body-centered cubic dendrite phase, a topologically close-packed μ rhombohedral phase, and a high-density coherent nano-precipitate γ″ phase with the D022structure (Ni3Ta type) embedded in a continuous face-centered cubic matrix. Owing to precipitation strengthening of D022, the yield stress of the alloy is determined as high as 1450 MPa, which is a significant improvement (∼100%) in comparison with the D022-free alloy, without a loss of ductility. This alloy exhibits an excellent high-temperature strength, with the yield strengths of 1300 MPa at 600 °C and 320 MPa at 1000 °C. Detailed microstructural characterization using transmission electron microscopy, high-angle annular dark-field imaging, and three-dimensional atom probe tomography analyses indicated that this superior strength–plasticity combination stems from the synergy of a multiple-phase structure. These results provide a new insight into the design of RHEAs and other advanced alloys.

Atomic-scale mechanism of effect of co-separation of elements at interface of ferrite and austenite on Cr-depleted zone of duplex stainless steel

Three-dimensional atom-probe tomography and first-principles calculation combined with density functional theory were used to study the effect of the co-segregation of different elements formed during the solidification process of S32205 duplex stainless steel on the Cr-depleted zone at the interface between ferrite and austenite. It was found that the co-segregation of different elements formed during the solidification process of duplex stainless steel can also form Cr-depleted zone at the interface between ferrite and austenite. Moreover, Mo, Si, B, C and P atoms promote co-segregation with Cr atoms, which promotes the formation of Cr-depleted zone at the interface between ferrite and austenite in duplex stainless steel. Mo and Si strongly promote the segregation of Cr at the interface between ferrite and austenite, thereby promoting the formation of Cr-depleted zone. B, C and P elements also promote the segregation of Cr element at the interface between ferrite and austenite and the formation of Cr-depleted zone, but their effect is weaker than that of Mo and Si elements. These conclusions provide a new theoretical basis for improving the intergranular corrosion performance of duplex stainless steel.

Atomic-scale imaging of ytterbium ions in lead halide perovskites.

Lanthanide-doped lead halide perovskites have demonstrated great potential for photoelectric applications. However, there is a long-standing controversy about the existence of lanthanide ions, e.g., whether the doping of Ln3+ is successful or not; the substituting sites of Ln3+ in lead halide perovskites are unclear. We directly identify the doped Yb3+ in CsPbCl3 perovskites by using the state-of-the-art transmission electron microscopy and three-dimensional atom probe tomography at atomic scale. Different from the previous assumptions and/or results, we evidence that Yb3+ simultaneously replace Pb2+ and occupy the lattice interstitial sites. Furthermore, we directly observe the cluster phenomenon of CsPbCl3 single crystal at near atomic scale. Density functional theory modeling further confirms and explains the mechanisms of our findings. Our findings thus provide an atomic-level understanding of the doping mechanism in perovskites and will stimulate a further thinking of the doping effect on the performance of perovskites.

Synergistic Effect of Alloying on the Strength and Ductility of High Carbon Pearlitic Steel

In this work, the effects of the micro-alloying of Mn, Ni, and Si on the microstructure and mechanical properties of high-carbon pearlite steels were investigated. The results indicated that the addition of solely Ni to high-carbon pearlitic steel can enhance the strength through the refinement of interlamellar spacing, but work-hardening in the ferrite of the pearlite colony may be delayed, leading to a reduction in area. The multiple additions of Ni and the increase in Mn and Si contents in high-carbon pearlitic steel were beneficial to obtaining a balance between ultimate tensile strength and reduction in area. Three-dimensional atom probe tomography results showed Si partitioning into ferrite and Mn and Ni elements partitioning into cementite. The addition of Si inhibited the formation of a continuous network of grain-boundary cementite, leading to high strength and high ductility through optimization of the microstructure.

Insights into hydrogen-induced strengthening mechanism of TiC/Ti6Al4V via experiments and first-principles calculations

This study is dedicated to summarize the strengthening mechanism of hydrogen microalloying for titanium matrix composites, i.e., 5 vol% TiC/Ti–6Al–4V, which is manufactured employing a unique technique called “melt hydrogenation” that involves melting alloys under a gas mixture of hydrogen and argon. This process enables the direct integration of trace hydrogen atoms into composites. The experimental results demonstrate that melt hydrogenation not only refines the microstructure of matrix but also promotes the network-like distribution of reinforcements. The three-dimensional atom probe tomography analysis of TMCs shows that the TiC/matrix interfaces can effectively trap hydrogen. Combining the high-resolution transmission microscopy observation and ab-initio calculation on the interface between matrix and reinforcement, it suggests that the addition of hydrogen improves the interfacial adhesion work, which has important implications for the improvement of load-bearing capacity of interface. Therefore, thanks to the above-mentioned structural changes, the room-temperature strength and plasticity can be simultaneously improved with the minor addition of hydrogen. These findings are of great significance for the further development of high-performance titanium matrix composites.

Solute drag-controlled grain growth in magnesium investigated by quasi in-situ orientation mapping and level-set simulations

Critical properties of metallic materials, such as the yield stress, corrosion resistance and ductility depend on the microstructure and its grain size and size distribution. Solute atoms that favorably segregate to grain boundaries produce a pinning atmosphere that exerts a drag pressure on the boundary motion, which strongly affects the grain growth behavior during annealing. In the current work, the characteristics of grain growth in an annealed Mg-1 wt.%Mn-1 wt.%Nd magnesium alloy were investigated by advanced experimental and modeling techniques. Systematic quasi in-situ orientation mappings with a scanning electron microscope were performed to track the evolution of local and global microstructural characteristics as a function of annealing time. Solute segregation at targeted grain boundaries was measured using three-dimensional atom probe tomography. Level-set computer simulations were carried with different setups of driving forces to explore their contribution to the microstructure development with and without solute drag. The results showed that the favorable growth advantage for some grains leading to a transient stage of abnormal grain growth is controlled by several drivers with varying importance at different stages of annealing. For longer annealing times, residual dislocation density gradients between large and smaller grains are no longer important, which leads to microstructure stability due to predominant solute drag. Local fluctuations in residual dislocation energy and solute concentration near grain boundaries cause different boundary segments to migrate at different rates, which affects the average growth rate of large grains and their evolved shape.

Atomistic insights into the inhomogeneous nature of solute segregation to grain boundaries in magnesium

In magnesium alloys with multiple substitutional elements, solute segregation at grain boundaries (GBs) has a strong impact on many important material characteristics, such as GB energy and mobility, and therefore, texture. Although it is well established that GB segregation is inhomogeneous, the variation of GB solute composition for random boundaries is still not understood. In the current study, atomic-scale experimental and simulation techniques were used to investigate the compositional inhomogeneity of six different GBs. Three-dimensional atom probe tomography results revealed that GB solute concentration of Nd in Mg varies between 2 and 5 at.%. This variation was not only seen for different GB orientations but also within the GB plane. Correlated atomistic simulations suggest that the inhomogeneous segregation behavior observed experimentally stems from local atomic rearrangements within the GBs and introduce the notion of potential excess free volume in the context of improving the prediction of per-site segregation energies.

Polyamorphism mediated by nanoscale incipient concentration wave uncovering hidden amorphous intermediate state with ultrahigh modulus in nanostructured metallic glass

Comprehending the pressure-/temperature-induced structural transition in glasses, as one of the most fascinating issues in material science, is far from being well understood. Here, we report novel polyamorphic transitions in a Cu-based metallic glass (MG) with apparent nanoscale structural heterogeneity relating to proper Y addition. The low-density MG compresses continuously with increasing pressure, and then a compression plateau appears after ∼8.1 GPa, evolving into an intermediate state with an ultrahigh bulk modulus of ∼467 GPa. It then transforms to a high-density MG with significantly decreased structural heterogeneity above ∼14.1 GPa. Three-dimensional atom probe tomography reveals concentration waves of Cu/Zr elements with an average wavelength of ∼5–6 nm, which promote the formation of interconnected ringlike networks composed of Cu-rich and Zr-rich dual-glass domains at nanometer scale. Our experimental and simulation results indicate that steplike polyamorphism may stem from synergic effects of the abnormal compression of the Zr–Zr bond length at the atomic scale and the interplay between the applied pressure and incipient concentration waves (Cu and Zr) at several nanometer scales. The present work provides new insights into polyamorphism in glasses and contributes to the development of high-performance amorphous materials by high-pressure nanostructure engineering.

Nano-alloying and nano-chemistry of the immiscible elements Fe and Cu in a FeSc–Cu nanoglass

In this study, the alloying of immiscible elements, Fe and Cu, at the atomic level was studied using three-dimensional atom probe tomography (3D APT).

A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying.

The essences of the quenching and partitioning (Q&P) process are to stabilize the finely divided retained austenite (RA) via carbon (C) partitioning from supersaturated martensite during partitioning. Competitive reactions, i.e., transition carbide precipitation, C segregation, and decomposition of austenite, might take place concurrently during partitioning. In order to maintain the high volume fraction of RA, it is crucial to suppress the carbide precipitation sufficiently. Since silicon (Si) in the cementite θ (Fe3C) is insoluble, alloying Si in adequate concentrations prolongs its precipitation during the partitioning step. Consequently, C partitioning facilitates the desired chemical stabilization of RA. To elucidate the mechanisms of formation of transition η (Fe2C) carbides as well as cementite, θ (Fe3C), besides the transformation of transition carbides to more stable θ during the quenching and partitioning (Q&P) process, samples of 0.4 wt% C steels tailored with different Si contents were extensively characterized for microstructural evolution at different partitioning temperatures (TP) using high resolution transmission electron microscopy (HR-TEM) and three-dimensional atom probe tomography (3D-APT). While 1.5 wt% Si in the steel allowed only the formation of η carbides even at a high TP of 300 °C, reduction in Si content to 0.75 wt% only partially stabilized η carbides, allowing limited η → θ transformation. With 0.25 wt% Si, only θ was present in the microstructure, suggesting a η → θ transition during the early partitioning stage, followed by coarsening due to enhanced growth kinetics at 300 °C. Although η carbides precipitated in martensite under paraequilibrium conditions at 200 °C, θ carbides precipitated under negligible partitioning local equilibrium conditions at 300 °C. Competition with the formation of orthorhombic η and θ precipitation further examined via ab initio (density functional theory, DFT) computation and a similar probability of formation/thermodynamic stability were obtained. With an increase in Si concentration, the cohesive energy decreased when Si atoms occupied C positions, indicating decreasing stability. Overall, the thermodynamic prediction was in accord with the HR-TEM and 3D-APT results.

Interphase and fibrous precipitation in a thermomechanically processed V–Nb–Mo microalloyed steel

A novel V–Nb–Mo steel was thermomechanically processed to promote ferrite grain refinement and precipitation during the austenite to ferrite transformation. Transmission electron microscopy and three-dimensional atom probe tomography confirmed the formation of two main types of precipitates: nano-scale interphase precipitates and fibrous precipitates (FP). Both the precipitates had a Baker–Nutting orientation relationship with the ferrite matrix, but with different variants. Interphase precipitates exhibited a disc shape with an average distance between precipitate rows of 15 ± 2 nm and an average Guinier radius of 3.9 ± 1.7 nm, whereas FP had a rod-shape 2.5 ± 0.5 nm in radius and 8–25 nm in length. It appeared that the FP are formed along the plane perpendicular to the interphase precipitates plane.

Strengthening enhanced by Ru partitioned to γ' phases in advanced Nickel-based single crystal superalloys

Two Nickel-based single crystal superalloys (one Ru-free and one containing 3 wt.% Ru) were comparatively investigated by the three-dimensional atom probe tomography and the nanoindentation atomic force microscope to reveal the effect of Ru addition on the microstructure and the mechanical properties of γ' phases after standard heat treatment. The results showed that the Ru addition neither altered the morphology and the volume fraction of γ' phases, nor changed the interface structure and the partition coefficients of alloying elements between γ and γ' phases, however, it enhanced the hardness and elastic modulus of γ' phases. First-principles calculation indicated that no matter whether the Ni site or Al site was occupied by Ru, the elastic modulus of γ' phases increased, which might be ascribed to the solution strengthening effect of Ru on γ' phases.

Suppression and reactivation of transformation and twinning induced plasticity in laser powder bed fusion additively manufactured Ti-10V-2Fe-3Al

Laser powder bed fusion (LBPF) was employed to fabricate a strain-transformable β-Ti alloy, Ti-10V-2Fe-3Al (wt%). While the alloy is known to exhibit transformation induced plasticity (TRIP), the as-fabricated alloy, under tensile loading, did not show the same TRIP effects, even though it exhibits the same β + ω microstructure. The repeated heating-cooling cycles experienced during the LBPF process leads to the early stages of rejection of solute elements (Fe, V, and Al), forming isothermal omega (ω) precipitates, which were captured via detailed investigations coupling transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). While these homogeneously distributed isothermal ω precipitates lead to a higher yield strength, the TRIP/TWIP effects within the β matrix were suppressed, leading to very low ductility and virtually no strain-hardenability. Interestingly, after a simple β-solution heat treatment followed by quenching, leading to a β + ω (athermal) microstructure, the TRIP/TWIP effects were reactivated in the same LPBF Ti-10 V-2Fe-3Al alloy. The alloy exhibited substantial recovery of tensile ductility and a very large strain hardening (tensile strength minus yield strength ~500 MPa), with a high average strain hardening rate ~15000. Such a very high strain hardening rate in case of LBPF processed Ti-10 V-2Fe-3Al, appears to arise from a rapid strain-induced transformation from β to α" at the early stages of plastic deformation, leading to a high-volume fraction of the martensitic phase, coupled with hierarchical twinning within the martensite plates.

Growth behavior and stress distribution of bulk GaN grown by Na-flux with HVPE GaN seed under near-thermodynamic equilibrium

We used Na-flux method to grow Ga-polar GaN (Ga-GaN) and N-polar GaN (N-GaN) on a double-sided epitaxially polished HVPE-GaN seed. The growth behavior and stress distribution of Ga and N-polar GaN grown by Na-flux method was systematically studied for the first time. Under near-thermodynamic equilibrium growth conditions, the growth rate of the N-polar plane is higher than that of the Ga-polar plane, which is related to the high surface energy of the N-polar plane. The lower overall stress level of Ga-GaN/N-GaN bulk single crystals is confirmed by Raman spectroscopy and low-temperature PL. Three-dimensional atom probe tomography (3D-APT) directly proves that C and O impurities are enriched at the atomic level at the growth interface, resulting in the mismatch of the interface lattice, resulting in compressive stress and reduction of free carriers. Under near-thermodynamic equilibrium growth conditions, the dislocation density of GaN bulk single crystals can be further reduced. Ga-GaN and N-GaN dislocation density values were 4 × 105/cm2 and 8 × 105/cm2, respectively. The mechanism of dislocation reduction is discussed. In addition, we can achieve control of stress and free carriers by adjusting the behavior of impurity incorporation.

Grain boundary segregation and toughness of friction-stir-welded high-phosphorus weathering steel

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A3 and below A1. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A1 (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A3. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A1 was larger than that in the stir zone formed above A3 is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.

Effect of the microstructural characteristics of die-cast ADC12 alloy controlled by Na and Cu on the mechanical properties of the alloy

The mechanical properties of a die-cast Al–Si–Cu alloy (DC ADC12) were improved using new processing techniques that induce grain refinement and precipitation hardening. Several gravity-cast ADC12 (GC ADC12) samples with different microstructural sizes were also evaluated to better understand the material properties of DC ADC12. The material properties of DC ADC12 indicate that the alloy has both advantages (fine grains) and disadvantages (defects). The mechanical properties of ADC12 can be improved by adding Na before the T5 process. The grain refinement of ADC12 was achieved by introducing Na using benign and inexpensive NaHCO3 rather than metallic Na. The Na elements including sodium oxide retained in the melt cause fine spherical eutectic structures to form in the spherical α-Al matrix by interrupting the Si phase growth. The phenomenon can be detected via a three-dimensional atom probe tomography analysis. Precipitation hardening of the die-cast Al alloy was achieved by subjecting it to T5 treatment and a large amount of Cu, replacing the T6 treatment, for 3 h at 175 °C, because the die casting process can be replaced with the solid solution process in T6 owing to its high cooling rate. Precipitation hardening occurred in the grain-refined DC ADC12-Na sample during the T5 process, which was verified via in situ observation of its microstructure using laser microscopy during the heating and cooling processes. Al2MgCu, CuAl2, and fine Si particles, which were different from the precipitates obtained using the T6 process, were produced in the spherical α-Al phase. The demonstrated techniques resulted in remarkable improvements in the ultimate tensile strength and fracture strain of DC ADC12.

High density of strong yet deformable intermetallic nanorods leads to an excellent room temperature strength-ductility combination in a high entropy alloy

This paper introduces a new microstructural template for high entropy alloys (HEAs), where the face centered cubic (FCC) complex concentrated solid solution is reinforced with a high density of strong, yet deformable, nanorods of an ordered multi-component intermetallic L12 compound. Thermodynamic modeling has been employed to design this HEA with a large L12 volume fraction. Thermo-mechanical processing by isothermal annealing of the conventionally processed bulk cold-rolled alloy directly at precipitation temperatures, has been applied to produce a high density of uniformly distributed L12 nanorods within refined FCC grains, resulting from concomitant recrystallization and discontinuous precipitation processes. The nanorod morphology of the discontinuous L12 product has been established from three-dimensional atom probe tomography. The refined grains result in a complete coverage of the microstructure with discontinuously precipitated intermetallic nanorods. This nanorod strengthened HEA exhibits an exceptionally high room temperature yield strength of ∼1630 MPa, good tensile ductility of ∼15%, and an ultimate tensile strength of ∼1720 MPa. Furthermore, a single L12 phase alloy, melted based on the precipitate composition in the two-phase FCC + L12 HEA, exhibits very high compressive deformability and strain hardenability, unusual for ordered intermetallic compounds. These results open a new strategy for design of fine-grained microstructures strengthened via ordered intermetallic phases, exploiting the beneficial effects of discontinuous precipitation, for achieving very high room temperature tensile strengths while maintaining good ductility.