Metastable Microstructure Research Articles

A large compressive recoverable strain induced by heterogeneous microstructure in a Ni50.6Ti49.4 shape memory alloy via laser powder bed fusion and subsequent aging treatment

Laser induced highly metastable microstructure provides the possibility for the tunable precipitation behavior and mechanical/functional properties via direct aging treatment. In this paper, Ni-rich NiTi-based samples were fabricated by laser powder bed fusion (LPBF) and then were directly aged for different aging times that spanning three time-scales (10 min-100 h). The results showed that through the aging treatment, a heterogeneous microstructure consisting of Ti 4 Ni 2 O x nanoparticles, nanoscale Ni-rich precipitates, dislocation structures, and martensitic twin variants formed in the matrix. Due to the change in the elastic strain field, the short time aging treatment tended to induce (001) compound twins, while the long time aging assisted in the formation of< 011 > type II twins. The temperature-induced and deformation-induced phase transformation behavior were further studied. With the progress of precipitates, it was found that a rapid evolution from a single-stage phase transformation to a multi-stage one occurred. During the cyclic compression, the sample aged for 1 h showed the most excellent superelasticity with initial recoverable strain of 0.089 and steady recoverable strain of 0.087, as well as good cyclic stability. • Microstructure evolution of LPBF-ed Ni-rich NiTi SMA with the aging time was disclosed. • Effects of aged microstructure on transformation behavior and pseudoelasticity were elaborated. • A large compressive recoverable strain and its good cyclic stability were achieved via the proper aging treatment.

Influence of Aluminum on the Wear Properties of High‐Carbon Metastable Austenitic Steels

The two‐body abrasive wear properties of metastable austenitic steels (MAS) against SiC abrasive paper are investigated at different wear loads. To ensure a metastable austenitic microstructure, the alloying compositions are chosen such that the martensite start temperature of the MAS is approximately at room temperature, while the proportions of carbon, manganese, and aluminum change. The abrasion test results are compared to martensitic (40MnB5) and austenitic steel (Hadfield steel). An up to four times lower weight loss is found for the MAS compared to the Hadfield steel and up to 6.7 times lower weight loss compared to the martensitic steel. It is found that the wear resistance of the MAS increases significantly with wear load. Wear resistance of over is achieved at the highest wear load of 32 N. The wear properties of the MAS are associated with an increase in the surface hardness resulting from a mechanically induced austenite to martensite phase transformation. It is shown that the addition of aluminum to the MAS reduces the wear resistance. This is explained by an increase in stacking fault energy and the associated restriction of the mechanically induced transformation to martensite.

Evolution of microstructure and mechanical property of Ti-47Al-2Cr-2Nb intermetallic alloy by laser direct energy deposition: From a single-track, thin-wall to bulk

The complex thermal history during the laser direct energy deposition (L-DED) can result in the inhomogeneous microstructure of the Ti-47Al-2Cr-2Nb (TiAl) alloy. This work investigates the grain structure and metastable microstructure evolution of L-DED TiAl samples with different deposit shapes of single-track, thin-wall, and bulk. Numerical simulations were performed for all samples to reveal the influence of thermal history on microstructural evolution. Among the three samples, the single-track sample has unique dendritic and cellular structures due to the highest cooling rate. Meanwhile, incompletely precipitated lamellar γ was found at the boundary and inside the α2 grains. And residual B2 phase is also present in the single-track sample. As for the thin-wall and bulk samples, they exhibit fully lamellar structure, and contain only γ and α2 phases at room temperature. During the L-DED process, the temperature gradient and cooling rate inside both samples decreased with the elevation of the deposition height. However, due to the different thermal histories, the temperature gradient and cooling rate do not change in the same way. This leads to a large difference in grain structure and metastable microstructure between the thin-wall and bulk samples. Also, these changes affect the mechanical properties of TiAl alloys. In summary, this study can provide potential guidance for a tailored microstructure in L-DED TiAl alloy.

Selective Laser Melting of Ti6Al4V: Effects of Heat Accumulation Phenomena due to Building Orientation

Titanium alloy Ti6Al4V is one of the most utilized alloys in the field of additive manufacturing due to the excellent combination of mechanical properties, density and good corrosion behavior. These characteristics make the use of this material particularly attractive for additively manufacturing components with complex geometry in sectors such as aeronautics and biomedical. Selective Laser Melting (SLM), by which a component is fabricated by selectively melting of stacked layers of powder using a laser beam, is the one of most promising additive manufacturing technologies for Ti6Al4V alloy. Although this technique offers numerous advantages, it has some critical issues related to the high thermal gradients, associated with the process, promoting the formation of a metastable martensitic microstructure resulting in high tensile strength but poor ductility of the produced parts. The formation of microstructural defects such as balling and porosity can occur together with the presence of residual stresses that may significantly affect the mechanical characteristics of the component. Specific process parameters and geometries can determine heat accumulation phenomena that result in a progressive decrease in thermal gradients between layers. These heat accumulation phenomena are influenced by the number of layers deposited, but also by the building orientation that, for a given geometry, determines a variation of the deposited surface for each layer.

Atomistic understanding of extreme strain shear deformation of Copper-Graphene composites

Copper-Graphene (Cu-Gr) composites demonstrate high strength, lubricity, and enhanced electrical and thermal conductivity compared to pure copper. A homogeneous dispersion of graphene/graphitic domains in Cu is highly desirable, which can be achieved by solid-state shear-assisted processing. A detailed understanding of structure evolution of Cu-Gr under deformation is needed. Here we subjected Gr coated Cu foils to high strain shear deformation using a tribometer to observe the distribution and reallocation of Gr in Cu matrix. A rupture and smearing of Gr layer into Cu were observed reducing the Cu grain size from 67 ± 14 μm to <10 nm, in maximum shear region close to surface. While a gradual strain accumulation further below resulted in grain rotations and dynamic recrystallization. During deformation, the Gr film was first observed to fracture into flakes and then embed into the Cu matrix. Graphitic domains observed in the Cu evinced the metastable composite microstructure with a sandwich layer of Cu2O on the interface. The local conductance, measured by conductive atomic force microscopy, shows a fivefold increase in Cu-Gr composite region compared to pure Cu. Our study shows the feasibility of shear processing to create fine-grained Cu-Gr composites with enhanced conductivity.

Synthesis, characterization, and dealloying of Al-Cu alloys prepared using spark plasma sintering

A series of Al-Cu alloys were prepared using spark plasma sintering by adjusting the initial composition ratio and powder sizes. The resulting phase and microstructure in these samples were investigated by X-ray diffraction, as well as energy-dispersive spectroscopy and electron back-scatter diffraction of samples examined in the scanning electron microscope. Both the initial composition ratio and powder size affect the phase composition and phase distribution of the sintered samples. Smaller size powders lead to formation of near-equilibrium phase structures, while larger initial powders lead to metastable microstructures containing phases that deviate significantly from the phase diagram. Dealloying of a metastable microstructure was observed to result in the formation of hierarchical skeleton nanoporous Cu structure.

Effect of seepage conditions on the microstructural evolution of loess across north-west China

SummaryLoess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg2+ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na+ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.

Effect of Spatially Varying Thermokinetics on the Electrochemical Response of Laser Additively Manufactured Ti6Al4V

Complex thermokinetics associated with laser-based powder bed fusion additive manufacturing of Titanium alloy component resulted in evolution of nearly non-equilibrium metastable microstructures and spatially variable aqueous electrochemical response. Further details can be found in the article number 2100938 by Narendra B. Dahotre and co-workers.

Microstructural evaluation of maraging 300 steel surface treated by Nd:YAG laser / Avaliação microestrutural do aço maraging 300 tratado superficialmente pelo laser Nd:YAG

Maraging steels are Ni-Co-Mo-Ti alloys of ultra-high resistance and spread application, since defense and nuclear industry until aeronautic components, pressure vessels and sports industry. Maraging steels are composed by a metastable martensitic microstructure. This work aims to evaluate the microstructural behavior of a maraging 300 steel after superficial laser treatment to reduce the oxygen permeability in the structure, increasing the mechanical resistance at elevated temperatures. Microstructural characterization was evaluated by scanning electron microscopy (SEM), X-ray diffraction and roughness by profilometry. The laser path typically of 500 µm wide was observed. Surface roughness results were Ra = 634 ± 160 nm; Rq = 789 ± 185 nm and Rt = 8,2 ± 3,2 nm. Laser treated sample exhibited, in addition of diffraction peaks due to the martensitic phase, additional peaks related to austenite phase and Fe2N/Fe3N. Therefore, laser surface nitriding can be used to change the surface properties.

The dual role of martensitic transformation in fatigue crack growth.

Deformation-induced martensitic transformation (DIMT) has been used for designing high-performance alloys to prevent structural failure under static loads. Its effectiveness against fatigue, however, is unclear. This limits the application of DIMT for parts that are exposed to variable loads, although such scenarios are the rule and not the exception for structural failure. Here we reveal the dual role of DIMT in fatigue crack growth through in situ observations. Two antagonistic fatigue mechanisms mediated by DIMT are identified, namely, transformation-mediated crack arresting, which prevents crack growth, and transformation-mediated crack coalescence, which promotes crack growth. Both mechanisms are due to the hardness and brittleness of martensite as a transformation product, rather than to the actual transformation process itself. In fatigue crack growth, the prevalence of one mechanism over the other critically depends on the crack size and the mechanical stability of the parent austenite phase. Elucidating the two mechanisms and their interplay allows for the microstructure design and safe use of metastable alloys that experience fatigue loads. The findings also generally reveal how metastable alloy microstructures must be designed for materials to be fatigue-resistant.

Metastable transformation behavior in a Ta-containing TiAl-Nb alloy during continuous cooling

This study experimentally investigates the metastable transformation behavior in a cast Ti-48Al-3Nb1.5Ta (at。%) alloy during continuous cooling by in-situ and ex-situ methods. A CCT diagram of the alloy was constructed. A small amount of Ta significantly promotes metastable transformations, e.g., Widmanstätten, feathery, and massive γ transformations during continuous cooling at different rates (50–1200 °C min−1). In particular, Ta can reduce the cooling rates required for metastable transformations, decrease the undercooling required and enhance the volume fraction of metastable microstructures. Crystallographic and kinetics studies reveal that the Widmanstätten transformation is a direct α→γ phase transformation that obeys the Σ11 CSL γ nucleation mechanism rather than {112̅2}α twinning. Furthermore, the effect of Ta on metastable transformations is detailed by thermodynamic analyses. Ta may potentially reduce the energy of the CSL boundaries of the γ phase to promote the nucleation of metastable structures. Consequently, uniform refinement of the microstructure can be achieved in low-Ta containing TiAl-Nb alloys by heat treatment alone.

Magnetic evaluation of accelerated aging degradation on 2.25Cr-1Mo steel with martensitic and bainitic microstructure

Accelerated aging-induced microstructure, mechanical properties, and magnetic properties were evaluated on martensitic and bainitic 2.25Cr-1Mo steel to correlate the properties for magnetic non-destructive evaluation (NDE) of aged Cr-Mo steel components. Decline in hardness and coercive force was observed with the increase in aging time for the microstructural modifications as the pinning of dislocations and magnetic domain wall corresponds to the mechanical and magnetic hardening. Although high hardness and coercive force was found for the martensitic steel compared to the bainitic steel due to its metastable microstructure with large amount of compressive residual stress, in longer periods of ageing the hardness and coercive force was found to be lower for the martensitic steel for its faster degradation. Strong correlations were found among the coercive force and microhardness in both the martensitic and bainitic steels, indicating a possibility of precise NDE of ageing degradation in Cr-Mo steel components by magnetic hysteresis loop measurement through a portable magnetic NDE device.

In-situ precipitation of metastable phases and microstructure control for additively manufactured Al alloy

The Al-Cu6.3-Mg1.3 alloy with high Cu content was designed and additively manufactured by using double wires and arcs. Key findings demonstrated the micro hardness, YS, and UTS achieve 115.6 HV, 206 MPa, and 307 MPa, respectively. Those values are notably increased compared with that of the WAAM Al alloys with traditional compositions. Dendritic grains dominate the microstructure of the designed alloy with Cu and Mg contained eutectic structures. The in-situ precipitated and homogeneously distributed nano-scale needle-shape metastable S′ phases, which are triggered by rapid solidification and intrinsic heat treatment during WAAM, contribute to the mechanical properties improvement of directly deposited alloy. This study provides a promising guidance for the development of additively manufactured components with excellent performance.

Evolution of nanosized Cu-rich clusters in a Fe–15Cu–15Ni alloy produced by laser powder bed fusion

Designing a metastable microstructure with a coherent nano-sized precipitation phase in the matrix is an effective strategy in improving the strength of materials. Recently, the rapid fusion and solidification cycle associated with laser-based additive manufacturing (AM) has emerged as a promising strategy to design unique microstructures with lattice distortion, solute segregation, and nano-sized precipitations. In this study, the evolution of nano-sized Cu-rich clusters in an AM-processed Fe–15Cu–15Ni alloy (wt.%) was investigated by conducting multiscale microstructural characterization. The results reveal that nano-sized Cu-rich clusters were generated inside the matrix due to a phase decomposition induced by the intrinsic heat treatment during the AM process. The heat energy generated by the laser beam not only initiated Cu-rich cluster formation, but also induced precipitation growth. Therefore, the average Cu-rich cluster size increased with an increase in the volumetric energy density. The hardness of the AM-processed Fe–15Cu–15Ni alloy at first increased with an increase in the energy density until a medium energy density level (140 J/mm3), due to formation of Cu-rich clusters. The hardness decreased with further increase in energy density (185 J/mm3), due to the Cu-rich cluster growth and retained austenite. The results reveal that laser-based AM successfully induces nano-cluster without the need for post-treatment and that the mechanical properties of materials can be optimized by adjusting the processing parameters in a way to enable nano-sized cluster and phase formation.

First-principles study of helium-vacancy complexes in Be12Ti

First-principles calculations have been conducted to study the stability of He-vacancy clusters and the trapping behavior of vacancies for He atoms in Be12Ti. We considered the various metastable microstructures of four types of He-vacancy clusters, and the most stable configurations were determined by the calculations of solution energy. Some He-vacancy clusters in Be12Ti have specific geometrical shape, and grow along specific crystal plane. For example, three types of He2V clusters (two He atoms trapped by vacancy) form dumbbell-shaped structure, three He atoms of He3V clusters (three He atoms trapped by vacancy) are located in the specific crystal plane. The solution energies of four types of clusters increase with the increase of number of He atoms, and the increasing rate is basically identical. The solution energies of HenVBe1 clusters are always the largest, and the solution energies of the HenVTi clusters are always the smallest, which mainly attribute to the larger vacancy volume and perfect spatial symmetry of Ti vacancies. The binding energies of a single He atom with HenV (n=0-3) clusters are all positive, and the segregation of He atoms on the inner surface of the vacancy is closer to the center of the vacancy, indicating that the He-vacancy has a stronger trapping ability for He atoms. The present research results provide a theoretical foundation of vacancy trapping mechanism for He atoms in Be12Ti, which is extremely important for the nucleation and growth mechanism of He bubbles in Be12Ti.

Nanoscale chemistry and atomic-scale microstructure of a bulk Ni3Sn material built using selective laser melting of elemental powder blends

Cubic specimens of the intermetallic Ni3Sn compound were built using selective laser melting of elemental powder blends. A specimen built at a laser power of 200 W and a scanning speed of 0.5 m/s was determined to have a homogeneous distribution of Ni and Sn on a mesoscopic scale in spite of a 2 at.% Sn deficiency. Characterization of the microstructure using the HAADF-STEM technique reveals a dispersion of ultrafine Ni particles, nanoscale chemical inhomogeneity and the formation of antiphase nanodomains in the matrix of equiaxed Ni3Sn grains. While a mesoscopic homogeneity of the specimen demonstrates a prospect of additive manufacturing of a bulk intermetallic material using selective laser melting, the nanoscale chemical inhomogeneity indicates a need for a better balance between the melting time and the liquid mixing time of melt pools, perhaps by the use of a scanning speed below 0.5 m/s. The formation of the antiphase nanodomains indicates that melt pools attain a liquid undercooling of 165 K leading to disorder trapping during rapid solidification followed by a disorder-order transition. This progress can be helpful to an understanding of metastable microstructure formation in the selective laser melting technology of other intermetallics.

Effect of straw reinforcement on the shearing and creep behaviours of Quaternary loess

In addition to the shearing behavior of soil, the creep character is also considered crucial in determining the long-term shear strength. This especially holds true for the loess that possesses the metastable microstructure and is prone to landslide hazards. This study explored the potential application of straw reinforcement to enhance the shearing and creep properties of the Quaternary loess. The mechanism responsible for the straw reinforcement to elevate the peak shear strength was revealed. Furthermore, three creep characters, namely attenuating creep, non-attenuating creep, and viscous flow were identified in this study. The unreinforced and reinforced specimen behaved in a different manner under identical shear stress ratio condition. The reinforced specimen was superior in limiting the particle relative movement within the shear plane than the unreinforced specimen. The chain reaction of interparticle contact loss, accompanied with excessive viscous displacement, rapid weakening of creep resistance, and eventually accelerated creep displacement, provided an evidence for the formation mechanism of slow-moving landslide. The long-term shear strength using the isochronal stress–strain relationship may be used for optimising the design of high-fill embankment works.

Segregation engineering of grain boundaries of a metastable Fe-Mn-Co-Cr-Si high entropy alloy with laser-powder bed fusion additive manufacturing

Laser-powder bed fusion (L-PBF) additive manufacturing offers unprecedented microstructural fine-tuning capabilities. Naturally, benefitting from such capability requires alloys that are amenable to microstructural heterogeneity and hierarchy (MHH) and that exhibit a low hot-cracking susceptibility (HCS). However, columnar growth, which is characterized by capillary effects and poor strain accommodation capabilities, is prevalent in L-PBF and increases the HCS of the processed alloys. Further, while solute segregation is prominent in cellular and dendritic growth modes during L-PBF, the effects of solute segregation on the alloy HCS and L-PBF processing window remain widely unexplored. Here, we demonstrate that solute segregation affects columnar growth, grain coalescence behavior during solidification, MHH and mechanical properties of a metastable Fe40Mn20Co20Cr15Si5 (at.%) high entropy alloy (CS-HEA) doped with 0.5 wt.% B4C (termed CS-BC). A theoretical framework is proposed, which reveals that a boundary-strengthening segregant may reduce the alloy HCS during L-PBF. In as-built CS-BC, boron, a boundary strengthener, segregated to the solidification cell boundaries, whereas carbon remained in the solid solution. The as-built CS-BC exhibited suppressed columnar growth, more random texture, smaller cell size and higher strength as compared to the as-built CS-HEA. Further, a wide crack-free L-PBF processing window of CS-BC allowed fine-tuning of its MHH and thus the mechanical properties. Upon annealing, as carbon-containing precipitates formed, CS-BC exhibited a metastable microstructure and transformation induced plasticity effect, which led to high synergistic strength-ductility. These findings will foster design of alloys that facilitate application-specific manufacture with L-PBF and thus, an extended outreach of L-PBF for structural applications.

Impact of semi-solid processing and cryogenic treatment on wear resistance of X210Cr12 tool steel

Semi-solid processing is type of process where metastable microstructure with carbide network may be obtained. Metastable austenite is embedded in carbide-austenite network even in ordinary tool steels. The paper describe semi-solid processing of the ledeburitic steel X210Cr12, with 2% carbon and 12% chromium. The semi-solid processing temperature was 1250 °C with dwell time 60 minutes. Two states for comparison were evaluated, with or without subsequent deformation. Another process used in this study is cryogenic treatment. Cryogenic treatment defined as an add-on process to conventional heat treatment, during which the material is brought to a temperature close to the liquid nitrogen temperature. The cryogenic treatment were used in this study for improving and comparing of impact on wear resistance combined with semi-solid processing. Wear resistance were evaluated based on pin-on-disc testing at ambient temperature.

Microstructural effects on the wetting-induced collapse in compacted loess

This paper presents the results of an experimental study aimed at evaluating the effects of soil microstructure on volume change and wetting-induced collapse of a compacted loess from Xi’an, China. One-dimensional (1D) compression tests are combined with Mercury Intrusion Porosimetry (MIP) tests and Scanning Electron Microscopy (SEM) analysis to examine the collapse behaviour for different compaction states and applied stresses. A phenomenon of partial collapse occurs upon full saturation (wetting), whose magnitude depends on the as-compacted suction, the as-compacted microstructure and the stress level applied. Following partial collapse upon full saturation some of the initially meta-stable microstructure of the compacted soil is preserved which leads to higher compressibility in subsequent loading stages. Additional collapse tests carried out under isotropic conditions show that partial collapse upon full saturation takes place only under zero-lateral deformation (1D) conditions due to the residual (‘locked-in’) horizontal stresses maintained in the sample after compaction. Microstructural results and a simple macroscopic model for soil compaction are used to qualitatively explain the phenomenon of partial collapse observed in compacted loess.