Microstructural Evolution Research Articles

Effect of processing sequences in multi-pass friction stir processing on microstructure, microtexture and microhardness of 6063 aluminum alloy

ABSTRACT Single and double direction stir processing sequences were employed and investigated by fixing all other FSP parameters. Optical microscopy (OM) was utilised to assess the size and geometry of the processed zone. EBSD was also employed to evaluate the microstructure and microtexture development in different regions of the processed zones. In addition, Vickers microhardness measurements were made to evaluate the hardness evolution across the processed zone for the two processing sequences. The results show that using a single direction processing sequence is preferable due to better grain refinement, mainly in all regions of the processed samples. Gradual improvement was observed in microhardness with each additional pass, stronger refinement, and homogeneous grains in the processed regions (stir and overlapping zones). In the first pass’s stir zone (SZ), the microhardness is approximately 42.5 Hv, while it increases to an average of 43.4 Hv in SZ2 of the second pass and 44.3 Hv in SZ3 of the third pass. Furthermore, more resistance to plastic deformation (slip) and lower intensity of heat distribution when compared to the double direction. Therefore, the application of a single direction processing sequence can be beneficial in the welding of such aluminium alloys.

Microstructure evolution of zirconium boride whiskers prepared by electrospinning and their toughening effect

AbstractZirconium boride (ZrB2) whiskers hold significant potential for toughening ultra‐high temperature ceramics (UHTCs). Yet, their fabrication remains challenging due to limited knowledge of the microstructure evolution. Herein, high‐purity ZrB2 whiskers were successfully prepared by electrospinning combined with carbothermal reduction. Specifically, a precursor solution containing Zr–O–C–B networks stabilized through hydrogen bonds with polyvinyl pyrrolidone was synthesized as a spinnable ZrB2 precursor. High‐purity ZrB2 whiskers with a diameter of ca. 200 nm and a length of 5–10 µm were obtained by precise heat treatment of ZrB2 precursor fibers. In this process, ZrO2 grains formed initially and transformed into ZrB2 grains through carbothermal reduction, which further converted into ZrB2 whiskers through a solid‐liquid‐solid mechanism. To testify to the toughening effect for UHTCs, the ZrB2 whiskers were added into ZrB2 powders and followed by sintering. This strategic addition of 5 wt% ZrB2 whiskers led to a 19% enhancement in the fracture toughness of ZrB2 bulk. This study illuminated the preparation strategy and formation mechanism for ZrB2 whiskers and their application for toughening UHTCs.

Microstructure evolution and mechanical properties improvement of (Ti8Zr6Nb4V5Cr4)100−xAlx lightweight high-entropy alloy by Laves phase transformation

Microstructure evolution and mechanical properties improvement of (Ti8Zr6Nb4V5Cr4)100−xAlx lightweight high-entropy alloy by Laves phase transformation

Effect of reinforcement particle size on the corrosion and mechanical properties of spark plasma sintered aluminium matrix composites.

In this study, the effects of different sizes of reinforcing particles on the corrosion behaviour and mechanical properties of aluminium (Al)-based composites produced by spark plasma sintering (SPS) are analysed. In the study, the effects of SPS parameters, including electrical power, applied pressure and sintering temperature, on the consolidation process and microstructure evolution of the composite are closely investigated. The results reveal a nuanced relationship between the sintering conditions and the properties of the particles, which in turn determine the sintering dynamics and the formation of the microstructural features. The evaluation of mechanical properties indicates a remarkable influence of particle size distribution on the hardness of the composites, showing an initial improvement with the introduction of nanoparticles, followed by a slight decrease as the balance between nano- and micron-sized Al2O3 particles shifts. A scanning electron microscopy (SEM) study demonstrates the influence of particle dimensions on the change of grain boundaries and the spatial arrangement of the composite matrix. Electrochemical experiments in a 0.1M NaCl solution show a consistent corrosion potential (Ecorr) across all samples, while the current densities associated with corrosion (icorr) show considerable variation. The presence of nano-sized Al2O3 particles was found to increase corrosion resistance, in contrast to the detrimental effects observed with larger microparticles. In particular, composites with a bimodal distribution of particle sizes showed a 3.5-fold increase in corrosion resistance compared to pure Al. The specific Al-2n8mAl2O3 composite that exhibited active electrochemical properties at elevated potentials without a defined passivation range emphasises the significant role of particle size. This study draws attention to bimodal microstructures as a promising route to achieving uniformity and improved corrosion resistance in Al matrix composites, while pointing to the need for further research to fully elucidate the operative mechanisms.

Scaffold microstructure evolution via freeze‐casting and hydrothermal phase transformation of calcium phosphate

AbstractExtensive research efforts have been focused on customizing the microstructure, macrostructure, and phase composition of calcium phosphate for enhanced biocompatibility and bioactivity in scaffolds for bone substitutes. Despite significant progress, achieving precise phase composition and microstructure remains a challenge, primarily due to the necessity of scaffold sintering. This study addresses the challenges in developing customized patient‐specific bone substitutes by proposing a sequential approach that reduces processing steps while providing control over the phase and morphology of the scaffolds’ structure. The methodology utilizes freeze‐casting and sintering for highly porous the scaffolds’ preparation, followed by hydrothermal treatment to modify the microstructure. The introduction of CaCO3 induces a phase transformation of tricalcium phosphate, increasing the hydroxyapatite content, while the overall macrostructure retains the characteristics of freeze‐casting. The surface morphology undergoes a transition from equiaxial grains to whiskers‐like structures and hexagonal rods, impacting compressive strength. Following hydrothermal treatment, the formation of whiskers‐like hydroxyapatite grains leads to a notable strength increase from 2.8 to 5.7 MPa. Remarkably, the scaffolds undergo nearly complete phase transformation, shifting from 100% tricalcium phosphate to 99% hydroxyapatite, all while conserving the macrostructure.

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

Microstructural evolution of the inner oxide layer of a Si-modified austenitic stainless steel exposed to oxygen-saturated lead-bismuth eutectic (LBE): Toward the origin of LBE penetration

Microstructural evolution of inner oxide layer of a Si-modified austenitic stainless steel exposed to oxygen-saturated static lead-bismuth eutectic (LBE) at 550 °C up to 3000 h has been investigated. A dual-phase separated structure consisting of Fe-Cr spinel and (Fe, Cr)1-xNix phase is formed in inner oxide layer, attributed to partitioning of Ni and Cr associated with Fe-Cr spinel formation. SiO2 is preferentially formed preceding the Fe-Cr spinel at internal oxidation frontier, leading to dispersed SiO2 nanoparticles within Fe-Cr spinel. The mechanism of LBE penetration into inner oxide layer is further developed based on preferable dissolution process of (Fe, Cr)1-xNix phase.

Microstructure evolution and fluid transport in porous media: a formal asymptotic expansions approach

Microstructure evolution and fluid transport in porous media: a formal asymptotic expansions approach

Microstructural evolution in laser powder bed fusion of water-atomized high-carbon low-alloy steel: Analysis of melting mode effects

Medium/High‑carbon steels are considered susceptible to cracking during rapid cooling in laser powder bed fusion (LPBF) applications. Also, water-atomized (WA) steel powders, containing oxides, have been associated with porosity defects during this process. In this study, LPBF was employed to process WA high‑carbon low-alloy steel powders under three operational regimes – conduction, transition, and keyhole modes. Analyses based on the scaling law of keyhole stability, melt pool dimensions, and x-ray computed tomography (XCT) suggested that processing under the transition mode closely resembled a stable keyhole, enabling the successfully printing of water-atomized powders to a density of 99.93 %. Subsequent heat treatment, hardness, and residual stress assessments presented a prominent structural softening and relief of compressive stress under the transition mode. This was attributed to the intense in-situ tempering and re-austenitization that occurred within each layer. Under this condition, with no post heat treatment, an ultimate tensile strength of 1250 MPa with >2.6 % elongation was achieved. Finally, the ball-on-disk test showed that wear performance of the printed steels was primarily governed by the tribo-oxides, with a limited influence from the microstructural variation.

Microstructural Evolution and Mechanical Behaviors of Cf/Cm-SiC-(ZrxHf1−x)C Composites with Different Carbon Matrices

In this study, two types of porous Cf/Cm composites were obtained by introducing pyrolytic carbon (PyC) and pyrolytic carbon/furan resin carbon (PyC/FRC). Subsequently, Cf/Cm-SiC-(ZrxHf1−x)C composites with different carbon matrices were prepared by introducing SiC and (ZrxHf1−x)C matrices into the porous Cf/Cm composites via the reactive melt infiltration method, specifically termed as Cf/PyC-SiC-(ZrxHf1−x)C and Cf/PyC/FRC-SiC-(ZrxHf1−x)C composites. The microstructures of the porous Cf/Cm and Cf/Cm-SiC-(ZrxHf1−x)C composites with different carbon matrices were examined, and a comprehensive analysis was conducted on microstructural evolution and mechanical behaviors of the Cf/Cm-SiC-(ZrxHf1−x)C composites. The results indicate that both Cf/Cm-SiC-(ZrxHf1−x)C composites underwent similar microstructural evolution processes, differing only in terms of evolution kinetics and final microstructure. Differences in the pore structures of porous Cf/Cm composites, as well as in the reactivities of carbon matrices, were identified as primary influencing factors. Additionally, both Cf/Cm-SiC-(ZrxHf1−x)C composites exhibited “pseudo-ductile” fracture characteristics, with flexural strengths of 214.1 ± 8.8 MPa and 149.6 ± 12.2 MPa, respectively. In the Cf/PyC-SiC-(ZrxHf1−x)C composite, crack initiation during loading primarily originated from the ceramic matrix, while in the Cf/PyC/FRC-SiC-(ZrxHf1−x)C composite, failure initially arose from the residual FRC matrix. Excessive fiber corrosion and the presence of residual low-modulus FRC matrix resulted in lower mechanical performance.

Enhancing self-healing properties of engineered cementitious composites through the application of super-sulfated cement

To date, no prior research has addressed the self-healing performance of engineered cementitious composites (ECC) prepared with super-sulfated cement (SSC), despite its potential to reduce carbon emissions compared to Portland cement (OPC). This paper addresses this significant research gap in the literature by exploring the influence of SSC on the recovery ability of pre-cracked ECC samples. In addition to the mechanical characterization at the sound state, an exhaustive investigation was undertaken to comprehensively assess the self-healing ability of flexural strengths, deflections, ultrasonic pulse velocity (UPV), and rapid chloride permeability (RCPT) of preloaded SSC-based ECCs. Furthermore, scanning electron microscopy (SEM), coupled with energy-dispersive X-ray (EDX) was employed to evaluate the microstructural changes and development of self-healing products within the microcracks of SSC mixtures prepared with various amounts of FA. The findings indicated that SSC-based ECC, while maintaining comparable mechanical and ductility properties, exhibited significant improvements in the recovery rates of ECC, reaching more than 27%, 7%, and 76% for flexural strength, UPV, and RCPT, respectively, compared to the OPC-based control ECC. The microstructural SEM-EDS results confirmed the enhanced precipitation of ettringite as a new self-healing product related to the inclusion of SSC in ECCs, along with the conventional C-S-H/C-A-S-H gels.

Mechanical Properties and Martensitic Transformation Behavior of 316LN Stainless Steel Under Cryogenic Deformation

Digital image correlation (DIC) technology can capture strain anomalies and predict crack initiation providing early warning of material failure. Herein, DIC technique is used to calculate the full‐field strain by analyzing the grayscale patterns of speckle images during the tensile process. This allowed for an analysis of the microstructure evolution of the 316LN austenitic stainless steel (SS) at cryogenic temperatures. Deformation behavior of the 316LN SS at cryogenic temperatures is further analyzed using electron backscatter diffraction technology and transmission electron microscopy. Based on the strain field obtained by the DIC technique, a comprehensive analysis of the martensite volume fraction at different strains can be conducted. The results show that the strain localization under cryogenic deformation is related to martensitic transformation, while the random distribution of slip bands aligns with local strain peak values. Notably, fracture under cryogenic deformation occurs in regions where the strain field reaches its peak, rather than at locations with the maximum strain value.

Continuous Dynamic Recrystallization and Deformation Behavior of an AA1050 Aluminum Alloy during High-Temperature Compression

Continuous dynamic recrystallization (CDRX) forms a new recrystallized microstructure through the progressive increase in low-angle boundary misorientations (LAGBs) during the hot forming of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. The present work investigates the effect of deformation parameters on the evolution of the dynamic recrystallization microstructures of an AA1050 aluminum alloy during compression at elevated temperatures. The alloy microstructure is investigated at deformation temperatures and strain rates in the range of 300 °C to 500 °C and 0.001 to 0.8 s−1. A well-defined substructure and subsequent DRX grains provide indication that recrystallization can proceed with continued strain under high-temperature compression. At a strain rate of 0.1 s−1, the DRX fraction is observed to be 0.25 at a temperature of 300 °C. This fraction increases to 0.32 as the temperature rises to 400 °C. The recrystallization mechanism is identified by analyzing the flow stress, the evolution of the subgrain misorientation angle, and the distribution of recrystallized grains. The observations of discontinuous dynamic recrystallization (DDRX) and CDRX under various deformation parameters are discussed. Moreover, the main substructure evolution laws observed from the high-temperature compression of an AA1050 Al alloy are summarized.

The influence of solution treatment on the phase evolution and tensile properties of binary Ti-Mo alloys

The influence of solution treatment on the phase evolution and tensile properties of Ti-Mo alloys was investigated to assess their potential use in biomedical applications. Phase formation and microstructural evolution were studied using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The mechanical properties were characterized by means of tensile tests and bending strength. XRD analysis showed that solution treatment increased the volume fraction of ß phase and supressed the α" phase. The microstructures of the as-cast alloys consisted of ß equiaxed grains with sub-grain structures of different sizes, while the solution treated alloys comprised ß equiaxed grains only except for Ti-10.02Mo, which comprised needle-like a" structures. EBSD showed an increase in the volume fraction of the ω and α" phases in all the alloys after solution treatment. The elastic modulus and UTS of all the alloy significantly decreased after solution treatment, except for Ti-15.05Mo, whereas the elongation significantly increased. The fracture surfaces of all the alloys after solution treatment indicated more ductile behaviour than brittle.

A Comparative Study on the Performance and Microstructure of 304NG Stainless Steel in Underwater and Air Laser Welding

In order to facilitate the application of underwater laser welding technology in in situ repairs of nuclear power plants, this study conducted comparative experiments between local dry underwater laser welding and laser welding in air on 304NG nitrogen-controlled stainless steel. The aim was to explore its microstructural evolution and mechanical properties in underwater environments. It was found that, near the fusion line of laser welding in air, columnar dendrites gradually evolved into cellular dendrites toward the weld center, eventually disappearing, resulting in a skeletal ferrite and serrated austenite structure. The underwater laser welding joints exhibited similar characteristics yet with more pronounced alternation between columnar and cellular dendrites. Additionally, the size of cellular dendrites decreased significantly, and needle-like ferrite was observed at the weld center. The hardness of underwater laser welded joints was slightly higher than that of in-air laser welded joints. Compared to laser welding in air, the strength of underwater laser welding joints increased from 443 MPa to 471 MPa, and the displacement increased from 2.95 mm to 3.45 mm, both types of welded joints exhibited a mixed mode fracture characterized by plasticity and brittleness.

Magnetic field-assisted electrochemical additive manufacturing of nickel structure: Growth mechanism and microstructural evolution

The microstructure and crystal texture of magnetically assisted electrochemical additive manufacturing parts play crucial roles in their mechanical properties and applications. This paper elucidates the electrocrystallization process of electrochemical additive manufacturing by constructing a magnetically assisted setup and exploring both nucleation mechanisms and growth kinetics. The electrochemical behavior of the system was analyzed using electrochemical methods under different magnetic field strengths. Nickel structures with aspect ratios of 7.0 and 8.2 were fabricated using the experimental setup, and their morphology, crystal texture, and microstructure were characterized. The results showed that without applying a magnetic field, the nucleation mechanism followed three-dimensional instantaneous nucleation under diffusion control. Under magnetic field conditions, forced convection, double layer charging, and hydrogen evolution reactions were combined to modify the S-H nucleation model, demonstrating that the magnetic field can significantly enhance the nucleation rate and number of nuclei. The initial growth of the microstructure did not exhibit preferential solid orientation, primarily forming refined equiaxed grains. Subsequently, a characteristic pentagonal structure developed, followed by spiral dislocation growth, with a significant transformation of equiaxed grains into columnar crystals. The crystal orientation eventually evolved to <110>, with many twin boundaries, among which Σ3 grain boundaries accounted for more than 55 %. Applying a magnetic field improved processing efficiency, altered the morphology and texture of the material, refined the grains, and increased the types of twins. These results provide theoretical and experimental support for electrochemical additive manufacturing.

Thermokinetics driven microstructure and phase evolution in laser-based additive manufacturing of Ti-25wt.%Nb and its performance in physiological solution

A bio-compatible Ti-25 wt% Nb alloy fabricated from a blend of pure elemental powders using laser powder bed fusion additive manufacturing technique. The present work investigated the effects of processing conditions on the evolution of microstructures and its consequential material attributes, such as mechanical properties and corrosion performance. Thermal management strategies comprising laser powers of 200 W and 300 W in complement with a shorter scan length (1 mm) and substrate preheating above β-transus temperature (1123 K) were considered to achieve complete dissolution of niobium particles. The microstructure in the 200 W sample showed thin α′′ martensite needles in β matrix while martensite laths in the 300 W condition appear coarse and were twice the area fraction compared to that in 200 W build. On the other hand, microstructures in the heated substrate sample exhibited the evolution of α and β phases. A multi-scale finite element method based thermo-kinetic model spanning from melt pool scale to the component scale was incorporated to understand the mechanism of the evolution of microstructures during liquid–solid and solid–solid state transformation. Electrochemical performance in the simulated body fluid of the printed alloys was found to be significantly affected by the presence of martensite fractions. Both mechanical and corrosion behaviors were favorably influenced by adoption of the substrate preheating during additive manufacturing due to promotion of diffusional transformation of β to α at the expense of martensitic transformation.

Microstructure evolution in bituminous-coal pyrolysis under in situ and stress-free conditions: a comparative study

A self-made triaxial testing machine with thermal–hydraulic–mechanical–chemical (THMC) coupling and a tubular heating furnace, combined with in situ (IS) micro-computed-tomography technology was utilized in this study. The evolution of pore-fissure (PF) structure parameters (porosity, PF scale distribution, effective PF volume ratio, and permeability) of bituminous coal under stress-free (SF) and IS conditions with temperature was investigated, and then the mechanism of experimental results was analyzed. Results showed that (1) under SF conditions, at 300–550 °C, the coal samples after pyrolysis are dominated by elongated large fissures, with PF structure parameters positively correlating with temperature. After 400 °C, the number of PFs increases, with most PFs having equivalent diameter (R) ≤ 100 μm. (2) Under IS conditions, coal sample fissures are dominated by elongated large fissures at 300–350 °C and by holes at 350–600 °C. (3) Under IS conditions at 300–600 °C, the PF structure parameters of coal samples initially decrease with temperature and subsequently increase. The number of PFs fluctuates within a certain range, and the PF scale distribution dynamically shifts with temperature. (4) After 300 °C, the PF structure parameters of bituminous coal under SF and IS conditions show a bipolar distribution with temperature. Therefore, the weakening effect of stress on the PF structure of coal samples should not be overlooked during IS pyrolysis mining of coal bodies.

Coordinated deformation characteristics and its effect on microstructure evolution of LA103Z Mg-Li alloy in reciprocating rotary extrusion

The uneven metal flow and inhomogeneous microstructure on the cross-section of the extruded bar are mainly induced by the uncoordinated deformation during the traditional extrusion process, which seriously restricts its production and application. These defects are more prominent for the dual-phase Mg-Li alloy due to the phase transformation and the difference in flow between soft and hard phases. In order to solve the uncoordinated deformation in traditional extrusion, the reciprocating rotary extrusion (R-RE) process based on harmonic oscillation of die is proposed. The experiment and numerical simulations of the reciprocating rotary extrusion process were carried out at rotating frequency of 2.5 and 5 Hz, extrusion velocity of 1 mm/s, forming temperature of 290℃, die extrusion ratio of 12 and die rotating angle of ±6°. The coordinated deformation mechanism from macroscopical flow and microstructure in reciprocating rotary extrusion was investigated deeply. Meanwhile, a novel theoretical method was proposed to describe coordinate deformation characteristics quantitatively. The results indicated that the reciprocating rotary extrusion significantly reduces the forming load and accumulates more strain. The more uniform metal flow contributes to coordinated deformation. The extrusion deformation factors are proposed to reveal the coordinated deformation mechanism. In addition, the deformation body characteristic zone is novelly divided into six zones by combination of flow pattern and microstructure evolution.

Numerical simulation for β/α transformation of Ti–6Al–4V alloy using a lattice Boltzmann - Cellular automata method

This paper considers the beta/alpha transformation of Ti–6Al–4V alloy using a lattice Boltzmann method (LBM) – cellular automata (CA) coupled method in terms of microstructural evolution during phase transformation. Particularly, the effects of the cooling rate on microstructures such as beta grain size, alpha colony size, and alpha lath thickness were examined as well as the overall morphologies. The LBM and CA were used to implement the diffusion of alloy components and phase transformation, respectively. Additionally, the thermodynamic and kinetic data for simulating the ternary alloy system were obtained from CALPHAD software to utilize the equilibrium phase diagram calculations. The initial states of the beta grain and its composition fields affect the processing of beta/alpha phase transformation and the final alpha + beta phase morphologies. Validation of the proposed method was conducted to compare the simulation results with experimental trends for microstructures of Ti–6Al–4V from the literature. The error in prediction of microstructural morphologies were 20% in the average alpha thickness with deviation of up to 5 μm.