Microstructural Evolution Research Articles

Superior high-cycle fatigue property through in-situ precipitation of α' martensite in additively repaired titanium alloy

TC4-DT titanium alloy is commonly used in aircraft airframe structures due to its high strength-to-weight ratio. However, these parts are susceptible to fatigue failure during their service life. In this study, the damaged TC4-DT titanium alloy components were repaired using laser directed energy deposition (LDED) by depositing TC4 powders with a side repairing method, and the effects of the repair process on microstructural evolution, residual stress together with high-cycle fatigue strength were comprehensively investigated through multiscale characterization techniques. Results showed that the high-cycle fatigue strength of the repaired specimen was significantly increased by 11.9% compared with the substrate material, from 404.3MPa to 452.6MPa, and this was ascribed to its unique microstructural characteristics, especially the in-situ precipitation of α' martensite resulting from the rapid cooling involved in the LDED process. Furthermore, the dense repaired microstructure, the surface residual compressive stress, and the numerous dislocations induced by phase transformation, combined with the Hall-Petch effect caused by the formation of α lath, hindered the dislocation motion and delayed the strain localization, which significantly inhibited crack initiation and greatly contributed to the fatigue performance. This work demonstrates the potential of additive manufacturing to repair alloys with superior mechanical properties for structural applications.

Kinetics and mechanisms of high-temperature oxidation in BCC and FCC high-alloy Fe-based alloys with high volume fraction of carbides

This study conducts a detailed analysis of high-temperature oxidation and microstructural evolution in two high-alloy Fe-based alloys, each characterized by a high-volume fraction of carbides but differentiated by their matrices − body-centered cubic (BCC) and face-centered cubic (FCC), which was achieved by the addition of nickel. By investigating the intricate interplay of factors such as phase composition, nickel content, and the presence of carbides, this research aims to elucidate the diverse oxidation kinetics and underlying mechanisms specific to each alloy type. Results show that the BCC alloy exhibits slower oxidation kinetics compared to its FCC counterpart, suggesting better performance in high-temperature environments. Moreover, while both alloys develop strong adhesive oxide scales primarily composed of Cr2O3, the FCC alloy experiences more pronounced scale spallation and cracking. A faster progression of decarburization was observed in the BCC alloy. This comprehensive comparison highlights how variations in matrix structure, along with nickel content and carbide behavior, critically influence oxidation kinetics, scale adhesion, and the overall integrity of oxide scales. Understanding these nuanced interactions is crucial for designing high-performance alloys tailored for extreme operating conditions.

Exploring the hydration feature and microstructure evolution of MK-Q-LS blended cementitious materials with electrochemical techniques

The hydration process of blended cementitious materials is primarily affected by the effect mechanism of supplementary cementitious materials. This study utilizes electrochemical impedance spectroscopy (EIS) method to investigate the hydration kinetics of cementitious materials incorporating metakaolin (MK), quartz (Q) and limestone powder (LS). The experimental results reveal the influence of MK+Q+LS on the hydration kinetics, phase assemblage, and microstructure of the blended systems. Besides, the electrochemical impedance response of blended cement materials exhibits variation based on the MK contents and the curing age. Furthermore, the resistance of the continuously connected micro-pores (Rccp) in the blended cementitious material progressively increases throughout the hydration process. The inclusion of MK reduces the Rccp values at early ages, while contributes to an increase in Rccp values from 7 days onward. Additionally, positive correlations are established between the Rccp value and compressive strength/heat release across different MK contents and curing ages. The combination of Rccp values and other characterization method can effectively reveal the microstructure evolution process. Therefore, as a nondestructive method, EIS can be effectively applied to predict the hydration of cementitious materials and the mechanical properties.

Microstructure evolution and property regulation of CoCrNiNbx laser cladding coatings

The influence of different Nb on the microstructure evolution, properties of the CoCrNi laser cladding coatings was investigated. With the increase of Nb addition, the eutectic structures showed a transition from planar crystals to cellular crystals to dendritic crystals. Besides, the eutectic lamellae gradually became dense. A large amount of Ni3Nb-HCP structure precipitates were precipitated in the coatings with excessive Nb. The introduction of Nb brought about an increase in heterogeneous nucleation cores, which in turn significantly refined the grain size in the secondary remelting zone of the cladding coating. As the Nb content increased, the hardness of coatings gradually increased, but both wear resistance and corrosion resistance showed a trend of first increasing and then decreasing.

Gas tungsten arc welding of a multiphase CoCuxFeMnNi (x=20,30) high entropy alloy system: Microstructural differences and their consequences on mechanical performance

With the current advancements in materials science, the development of high entropy alloys (HEAs) is progressively increasing. Hence, research on their processability is essential to make them competitive alternatives to common engineering alloys that are widely used in structural applications. One manufacturing technique commonly employed in this sector is gas tungsten arc welding (GTAW). This technique allows to obtain single monolithic parts from separate components, often at the cost of local microstructural and mechanical properties variation across the joint. As such, GTAW processing is capable of supplying relevant knowledge regarding the feasibility of joining new materials and their potential industry uptake. In this study, we present a comparative analysis on the use of GTAW on two distinct multiphase high entropy alloys: CoCu20FeMnNi and the CoCu30FeMnNi. Firstly, microstructural observations coupled with CalPhaD-based calculations and synchrotron X-ray diffraction analysis, allowed to delve, and compare, the microstructural evolution across both welds. It was possible to observe the dual phase nature of the microstructure throughout the welded joint alongside the nucleation of a B2 BCC phase in the heat affected zone (HAZ) of both HEAs. Considering the mechanical properties of the welded materials, results evidenced a poorer, yet still acceptable mechanical performance. The observed decrease in mechanical strength is attributed to the residual stress conditions and large grain size that developed owing to the process thermal cycle, which contrasted deeply with the microstructure of each base material.

Dissimilar low-temperature diffusion bonding of copper and titanium using a Zn interlayer: Interfacial microstructure and mechanical properties

In this study, a zinc (Zn) interlayer was applied in the diffusion bonding of pure copper (Cu) and titanium (Ti). The interfacial microstructure evolution and metallurgical reaction mechanisms of Cu/Ti joints with Zn interlayer were investigated by SEM, EDS and TEM. The addition of the Zn interlayer facilitated metallurgical bonding between Cu–Zn and Ti–Zn at low temperatures. The shear strength of joints exhibited a gradual increase followed by a sharp decline as the bonding temperature increased. An intimate bonding between Cu and Ti was achieved at 450 °C, and the highest average shear strength of 106.82 MPa was obtained at 490 °C. The weld seam is primarily composed of a β′-CuZn layer, which exhibited excellent plasticity and hindered the formation of brittle Cu–Ti intermetallic compounds. The τ1-Cu2TiZn layer, adjacent to β′-CuZn, demonstrated stronger resistance to crack propagation compared to the brittle TiZn3 and TiZn2 layers. Furthermore, the predominant β′-CuZn layer and τ1-Cu2TiZn layer formed a coherent interface, ensuring a reliable bonding. These results demonstrate that the addition of Zn interlayer is an effective strategy for preparing highly reliable Cu/Ti joints at low temperatures.

Surface Functionalization of Novel Work‐Hardening Multi‐Principal‐Element Alloys by Ultrasonic Assisted Milling

The development of multi‐principal‐element alloys (MPEAs) with unique characteristics such as high work hardening capacity similar to well‐known alloy systems like Hadfield steel X120Mn12 (ASTM A128) is a promising approach. Hence, by exploiting the core effects of MPEAs, the application range of conventional alloy systems can be extended. In the present study, work‐hardening MPEAs based on the equimolar composition CoFeNi are developed. Mn and C are alloyed in the same ratio as for X120Mn12. The production route consists of cast manufacturing by an electric arc furnace and surface functionalization via mechanical finishing using ultrasonic‐assisted milling (USAM) to initiate work hardening. The microstructure evolution, the hardness as well as the resulting oscillating wear resistance are detected. A pronounced lattice strain and grain refinement due to the plastic deformation during the USAM is recorded for the MPEA CoFeNi‐Mn12C1.2. Consequently, hardness increases by ≈380 HV0.025 in combination with a higher oscillating wear resistance compared to the X120Mn12. This shows the promising approach for developing work‐hardening alloys based on novel alloy concepts such as MPEAs.

Improvement on interfacial properties of CuW and CuCr bimetallic materials with high-entropy alloy interlayers via infiltration method

Abstract In order to achieve metallurgical bonding in the form of solid solution at the Cu/W interface and avoid the formation of intermetallic compounds, the novel high entropy alloys (HEAs) were designed on the basis of the mature high entropy alloy criteria. The CuCrCoFeNi x Ti high entropy alloys interlayers were applied to weld the CuW and CuCr bimetals by sintering–infiltration technology. Scanning electron microscope, energy dispersive spectrometry, and X-ray diffraction were used to explore the interfacial microstructure evolutions and strengthening mechanism of CuW/CuCr joints with applied HEA interlayers. The interfacial characterization results show that HEAs were diffused and dissolved into bimetallic materials, and a diffusion solution layer of 2–3 μm thickness was formed at the Cu/W phase interface, and there is no new phase generated at the CuW/CuCr interface. When CuCrCoFeNi1.5Ti interlayer was infiltrated into the CuW/CuCr interface, the electrical conductivity of CuCr side is 71.6%IACS, and the interfacial tensile strength reaches 484.5 MPa. Compared with the CuW/CuCr integral material without interlayer, the interfacial bonding strength is increased by 43.1%. And the SEM fracture morphology presents a larger amount of cleavage fractures of W particles. It indicates the appropriate solid solution layer on edge of W skeletons formed at the Cu/W phases interface. The Cu/W phase interface is strengthened, and it can effectively transfer and disperse the external load. Tungsten phase with higher elastic modulus endures a large amount of load, resulting in enhancing the CuW/CuCr interfacial bonding strength. When CuCrCoFeNi2Ti high entropy alloy interlayer was applied, the W skeleton near the CuW/CuCr interface was eroded, the imperfect W skeleton cannot withstand the tensile load effectively, resulting in decrease in the CuW/CuCr interfacial bonding strength. In the interfacial fracture, appears some fragmentations of W particles, and fewer W particles occur at the cleavage fracture.

The Role of Deformation and Microstructure Evolution on Texture Formation of a TA15 Alloy Subjected to Plane Strain Compression

The Role of Deformation and Microstructure Evolution on Texture Formation of a TA15 Alloy Subjected to Plane Strain Compression

Reconciliation of pore structure characterization methods: The simple case of PC-limestone cement pastes

Almost all properties of hydrated cementitious materials depend strongly on their pore structure. Many methods to quantify porosity have been applied to cementitious materials, but there is a huge confusion in the literature about the utility of these methods. By comparing the results between different methods, including MIP, 1H NMR and nitrogen adsorption, we highlight that semi-quantitative information can reliably be obtained from these methods. In particular, we demonstrate that they are consistent in their relative range of validity. This range of validity is explained in terms of microstructure features. We also show how the results are linked to macroscopic observables such as microstructure development (degree of hydration) and transport (conductivity and apparent chloride diffusion).

The influence of hydrogen on the recrystallization texture of a face centered cubic metal

The evolution of an alloy's microstructure in a hydrogen atmosphere at high temperatures is one of the important factors contributing to the degradation in the alloy's strength. The present study aimed to clarify the influence of hydrogen on the recrystallization texture of a face centered cubic (FCC) metal. As a model of FCC metal, commercially pure nickel (Ni) was cold-rolled and then annealed separately in vacuum and hydrogen atmospheres. Subsequently, the recrystallization textures were analyzed using EBSD and XRD. Hydrogen promoted the recrystallization of 〈001〉 − oriented deformed grains while hindering the recrystallization of 〈101〉 − oriented deformed grains. It increased the content of {011} 〈1−11〉 texture, promoted the decrease of R-Cube texture content, hindered the increase of Cube texture content， and hindered the reduction in contents of Brass, Copper, Goss, and S textures. In addition, it had an insignificant effect on {011}〈011〉 texture contents. Hydrogen demonstrated an anisotropic effect on the dislocation movement and density of grains with different orientations. Specifically, it impeded the dislocation movement within <101> − oriented deformed grains while facilitating the dislocation movement within <001> − oriented deformed grains. As a result, hydrogen induced an increase in dislocation density in <101> − oriented grains and a decrease in dislocation density in <001> − oriented grains.

Single-Particle Imaging Reveals the Electrical Double-Layer Modulated Ion Dynamics at Crowded Interface.

The dynamics of ion transport at the interface is the critical factor for determining the performance of an electrochemical energy storage device. While practical applications are realized in concentrated electrolytes and nanopores, there is a limited understanding of their ion dynamic features. Herein, we studied the interfacial ion dynamics in room-temperature ionic liquids by transient single-particle imaging with microsecond-scale resolution. We observed slowed-down dynamics at lower potential while acceleration was observed at higher potential. Combined with simulation, we found that the microstructure evolution of the electric double layer (EDL) results in potential-dependent kinetics. Then, we established a correspondence between the ion dynamics and interfacial ion composition. Besides, the ordered ion orientation within EDL is also an essential factor for accelerating interfacial ion transport. These results inspire us with a new possibility to optimize electrochemical energy storage through the good control of the rational design of the interfacial ion structures.

Wire-arc directed energy deposition of high performance heat treatment free Al-6Mg-0.3Sc alloy

The release of residual stress during heat treatment of additively manufactured components could lead to significant deformation, particularly for those with thin-wall features and variable cross-sections. In this study, high performance heat treatment free Al-6Mg-0.3Sc alloy components were fabricated using the wire-arc directed energy deposition (DED) technique, achieving a yield strength of 223.0 ± 0.9 MPa, tensile strength of 408.5 ± 3.2 MPa, and elongation of 20.6 ± 1.7 %. The results indicate that compared to the CMT arc mode, the CMT + PA arc mode leads to a more pronounced bimodal microstructure, resulting in improved elongation. The mechanical property enhancement mechanism relies more on solid solution strengthening and grain boundary strengthening than on precipitation strengthening. Additionally, this study establishes for the first time a correlation between droplet transition and microstructure evolution. The research presented herein lays a theoretical foundation for wire-arc DED in the direct manufacturing of large thin-walled components without the need for heat treatment.

Heat Treatment Post-Processing for the Improved Mechanical Properties of Scalmalloy® Processed via Directed Energy Deposition

As high-strength aluminum alloys present several processability issues with additive manufacturing (AM), Scalmalloy®, an Al-Mg-Sc-Zr-based alloy, has been developed. This alloy is age-hardenable, allowing it to precipitate out a strengthening precipitate phase, Al3(Sc,Zr). The manufacturer recommends a single-stage aging treatment at 325 °C for 4 h; however, the majority of the literature studies utilize a powder bed processing known as selective laser melting (SLM) over powder-fed processing directed energy deposition (DED). This study addresses the lack of information on heat treatments for DED fabrication by exploring the application of artificial aging temperatures of 300–400 °C for 2, 4, and 6 h to: 1. determine the impact on the microstructural evolution and mechanical performance and 2. determine whether the recommended treatment for Scalmalloy® is appropriate for DED fabrication. Tensile testing determined that low-temperature treatments exhibited no visible dependence on time (2–6 h); however, time becomes influential at higher temperatures starting at 350 °C. The temperature plays a considerable role in the mechanical and microstructural behaviors of DED Scalmalloy®. The highest tensile strength was noted at 300 °C (384 MPa, 21.6% increase), but all heat-treated cases resulted in an improvement over the as-built case. This investigation established that increasing the treatment temperature resulted in a decreasing trend for the tensile strength that held over time. Elongation at 2 h displayed a near parabolic trend that peaks at 350 °C (20%) and falls with higher temperatures. At the 4 h treatment, a slight decreasing trend was noticed for elongation. No visible change was observed for elongation at 6 h, with elongation values remaining fairly consistent. The microstructural evolution, including micron-sized and nano-sized Al3(Sc,Zr) and grain size, was examined, and coarsening effects were noted with the increase in the temperature. It is recommended that treatment be conducted at 300 °C to achieve the precipitation of the strengthening Al3(Sc,Zr) phase while minimizing coarsening.

Ultrasonic wave diffusion-based investigations on microstructural development in multi-scale cementitious materials

Microstructural study of cement and its composites has immense significance from both fundamental and applied research points of view. Concrete, a mesoscale cementitious material, possesses high heterogeneity and inherent complex behavior. The multi-phases of concrete show an evolving nature at any instance of time, starting from the initiation of hydration to the complete maturity level. The hydration products and the aggregate-matrix Interfacial Transition Zone (ITZ) play a crucial role in defining the microstructural arrangement, which determines the mechanical and durability properties. Ultrasonic diffusion-based study has a unique advantage for characterising microstructural behavior in heterogeneous materials as it can decouple the different attenuating parameters- diffusivity (D) and dissipation (σ), where, D reflects the material microstructure and σ indicates the viscoelastic property of the base material. For that purpose, an ultrasonic diffusion study has been carried out in the present research for hydration monitoring in two scales, cement (micro-scale), and concrete (meso-scale) under different transmission modes. Ultrasonic measurements have been processed in the time-frequency domain to deduce the diffusivity and dissipation parameters using Spectral Energy Density distribution over the entire time window. The present research reveals the key findings as follows: (1) the influence of microstructure in ultrasonic wave propagation can be explicitly demonstrated using diffusion based analytical formulation and experimental results; (2) ultrasonic parameters (in terms of diffusivity and dissipation) can provide physical information on microstructural development in cementitious material during its hydration stage; and (3) the diffusion based method can distinguish the complex microstructural behavior across the scales of heterogeneity- micro-scale (cement paste) to meso-scale (concrete).

Influence of Heat Treatments on Microstructure Evolution and Indentation Response of Equiatomic AlCrFeMoNbNi High-Entropy Alloy

Influence of Heat Treatments on Microstructure Evolution and Indentation Response of Equiatomic AlCrFeMoNbNi High-Entropy Alloy

Exploring the Macroscopic Behavior and Microstructure Evolution of Lightly Cemented Sand in the Post-Liquefaction Process Using DEM

Exploring the Macroscopic Behavior and Microstructure Evolution of Lightly Cemented Sand in the Post-Liquefaction Process Using DEM

Many-Body MYP2 Force-Field: Toward the Crystal Growth Modeling of Hybrid Perovskites.

Hybrid perovskites are well-known for their optoelectronic and photovoltaic properties. Molecular dynamics simulations allow the study of these soft and ionic crystals by including dynamical effects (e.g., molecular rotations, octahedra tilting, ionic diffusion and hysteresis), yet the high computational cost restricts the use of accurate ab initio forces for bulk or small atomic systems. Hence, great interest exists in the development of classical force-fields for hybrid perovskites of low and linear scaling computational cost, via both empirical methods and machine-learning. This work aims at extending the transferability of our MYP0 model, which has been successfully tailored to methylammonium lead iodide (MAPI) and applied to the study of molecular rotations, vibrations, diffusion of defects, and many other properties. The extended model, named MYP2, improves the description of inorganic or hybrid fragments and the processes of crystal formation while preserving a good description of bulk properties. More importantly, it allows for the direct simulation of the crystal growth of cubic MAPI from deposition of PbI and MAI precursors on the surfaces. Our findings pave the way toward classical force-fields able to model the microstructure evolution of hybrid perovskites and the crystalline synthesis from deposition in vacuo.

Recycled Concrete Fines as a Supplementary Cementitious Material: Mechanical Performances, Hydration, and Microstructures in Cementitious Systems

Using recycled concrete fines (RFs) as a supplementary cementitious material in blended cement systems offers potential benefits for the construction industry. This study investigated the mechanical properties, hydration development, and microstructural evolutions of blended Portland cement systems incorporating different particle-size distributions of untreated RFs. The results highlighted that finer RFs improved the compressive strength of mortar and influenced hydration by promoting monocarbonate formation. The incorporation of RFs optimizes the pore structure in the early stage but increases porosity. RFs affected the distribution of capillary pores and created a rougher and more complex surface compared to inert mineral admixtures. Furthermore, the particle size of RFs impacted the fractal dimension of multi-scale pore sizes.

Friction stir welding of constrained groove pressed Al1050 using Al2O3 nanoparticles

In this investigation, two passes Constrained Groove Pressed (CGPed) Al1050 sheets went through Friction Stir Welding (FSW) by adding nano-sized Al2O3 particles. Then, the microstructure and mechanical properties of the welded joints, undergone various pass numbers and rotational to traverse speed (ω/ν) ratios, were presented. Findings showed all joints were fractured from the SZ, although HAZ softening happened due to grain growth in this region. The presence of the Al2O3 particles in the stir zone led to a fine and equiaxed grain structure, resulting from continuous dynamic recrystallization (CDRX) during the high-temperature welding process. This grain refinement contributed to an improvement in the mechanical properties of the joints, such as strength and microhardness. Employing two different ω/ν ratios resulted in a variety of thermal cycles during the welding procedure such that the peak temperature in the ω/ν of 70 and 25 r/mm reached ∼423 °C and ∼285 °C, respectively. Microstructure studies via optical observation and EBSD analysis illustrated that employing a higher ω/ν ratio and weld pass number resulted in more uniform particle distribution in the stir zone thanks to facilitating materials flow and incorporation during FSW. As a result, the three passes FSWed sample at the higher ω/ν ratio of 70 r/mm showed the most homogeneous particle distribution and finest grain structure in the SZ. This microstructure evolution was in reliable agreement with the mechanical performance of the joints. The three passes FSWed sample at the higher ω/ν ratio of 70 r/mm demonstrated the highest weld strength, microhardness, and %elongation compared to other FSWed joints. This sample showed an ultimate tensile strength (UTS) of ∼97 MPa and a %elongation of ∼16%, and its mean microhardness value had a ∼3.5% increase compared to the CGPed base metal. Conversely, the one pass FSWed sample at the lower ω/ν ratio of 25 r/mm represented the weakest mechanical properties due to cavities, defects, and agglomerated Al2O3 nanoparticles. The presence of nanoparticles was a decisive factor in microstructural evolution during the welding, thereby increasing the mechanical quality of the joints dependent upon their uniform dispersion, obtained by increasing the pass number and ω/ν ratio.