Microstructural Evolution Research Articles

In-depth understanding in the effect of hydrogen on microstructural evolution, mechanical properties and fracture micro-mechanisms of advanced high-strength steels welded joints

The hydrogen embrittlement (HE) mechanism of QP980 steels laser welded joints with complex microstructures is unclear. The joint with hydrogen pre-charging fractured at the inter-critical heat affected zone (ICHAZ) in slow strain rate tensile test, and the relative loss of elongation amounted to 84.8%. Hydrogen-induced microcracks mainly initiated inside martensite (~ 60%). Hydrogen-induced crack propagation paths of the joints were transgranular propagation of martensite and intergranular propagation of ferrite/martensite interface. The HE mechanism of the joint was the combination of the effects in hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE).

The strengthening mechanism of mechanical and tribological properties of TiC-reinforced 316 L matrix composites fabricated by selective laser melting

Selective laser melting (SLM) has been proved to be a promising additive manufacturing technique to fabricate high-performance metal matrix composites (MMCs) with unique microstructures. In this study, TiC-reinforced 316L MMCs were successfully prepared by SLM. The effects of TiC addition on the microstructure evolution, mechanical properties and tribological behavior were systematically investigated to study the strengthening mechanism. The TiC/316L MMCs fabricated by SLM exhibited a high relative density of over 99.6% and were close to full density. The TiC particles are effectively bonded with the 316L matrix, serving as a means of dispersion strengthening. In addition, the grains were significantly refined, and the dislocation density was greatly enhanced with the addition of TiC particles, which reinforce the strength of the material. Tensile tests show that the tensile strength of TiC/316L MMCs increased by 49.9% from 707 to 1060MPa and yield strength increased by 44.2% from 543 to 783MPa with 4wt.% TiC. Besides, the tribological behavior of TiC/316L MMCs was much superior to the pure 316L. The findings provide a new perspective on ceramic particles reinforced stainless steel MMCs with tailored microstructure and excellence performance.

A comprehensive mean-field approach to simulate the microstructure during the hot forming of Ti-17

Thermomechanical processing of titanium alloys involves several hot forming steps to shape the workpiece and achieve a final microstructure that meets the desired mechanical properties. While mean-field models are commonly used to design the process and understand the influence of the processing route on the final microstructure, they cannot provide detailed spatial information on the local microstructure and stress evolution during the deformation of a workpiece. To address this limitation, we propose a comprehensive approach in which the mean-field model is validated in a region of a cylindrical workpiece at a constant strain rate and temperature during deformation. The validated model is integrated as a material model into a finite element-based software to analyse the effect of the thermomechanical history on the local microstructural and mechanical responses of the α and β phases. The model robustly predicts the role of the α-phase in the microstructural evolution of the β-phase, the α-dynamic globularisation kinetics in regions with different thermomechanical histories, and the influence of strain rate jumps on β-microstructural and mechanical responses. This comprehensive approach provides valuable insights into the intricate interplay between processing parameters, microstructure, and thermomechanical response in titanium alloys.

Research on Hot Deformation Behavior of Cu-Ti Alloy Based on Machine Learning Algorithms and Microalloying

High strength and high elasticity copper alloys have good application prospects due to their excellent mechanical properties. The deformation and heat treatment in its preparation process are important steps to ensure the service performance. In this work, Cu-Ti based alloys were employed and hot compression experiments were conducted at 500-800℃ under four different strain rates. Their constitutive equations and thermal processing maps were constructed. The influence of microalloying elements on activation energy was also analyzed. The microstructure evolution and recrystallization behavior under hot deformation conditions were studied based on the results of TEM. At the same time, Arrhenius, back propagation neural network (BPNN) and support vector machine (SVM) algorithms were used to predict the flow stress of the alloy. The results indicate that the relationship between flow stress and the input factors (strain rate, temperature and deformation) can be well predicted using the support vector machine algorithm. Microalloying can increase the activation energy of copper alloys, and the sensitivity of activation energy to temperature changes is related to the interaction between precipitates and dislocations.

Microstructure Evolution and Molten Salt Corrosion Resistance Dependent on Pre-Oxidation Treatment of NiAl Coatings

The chloride-infused molten salt shows the most potential as the medium for the upcoming generation of concentrated solar power system. However, the molten salt put for ward higher corrosion resistance demand on the critical components of concentrated solar power system. The application of NiAl coating proves to be a highly efficient method for preventing corrosion. In this paper, the microstructure, adhesion, and corrosion resistance of NiAl coatings on 310S stainless steel with/without pre-oxidation treatment were studied. The coating predominantly consisted of β-NiAl phase nanocrystals with an Al content of 41.0 at.%. Continuous α-Al2O3 oxide film was formed and no obvious cracks were observed after pre-oxidation. The adhesion strength of NiAl coating was increased by about 70% to 40.0 N through pre-oxidation. Importantly, the corrosion results of NiAl coatings by pre-oxidation treatment in the mixed salt of NaCl/MgCl2/KCl at 1073.15 K for 10 h showed no obvious diffusion of molten salt elements into the coatings. Compared to 310S stainless steel, the pre-oxidized coating sample exhibited a significant reduction of 50.2% in corrosion mass loss. The pre-oxidation of the NiAl coating creates an Al2O3 oxidation layer that effectively blocks corrosion, offering a new method for protecting stainless steels in concentrated solar power system plants.

Microstructure evolution of dendrite and eutectic in undercooled Al70Ge30 alloy

Al-Ge eutectic alloy, as a desirable bonding material, has been widely used in microelectromechanical systems (MEMS) packaging. The bonding property of Al-Ge eutectic alloy is closely related to their microstructures. However, the microstructure evolution and underlying mechanisms in undercooled eutectic Al-Ge alloy remain unclear. In this work, Al70Ge30 alloy was prepared and subjected to undercooling using a cycled superheating purification technology. The maximum undercooling can reach 90K (0.13TL). The solidified Al70Ge30 alloy exhibits concomitant dendrites and eutectic microstructures. The dendrites undergo rapid refinement at low undercooling and subsequently maintain nearly constant size as undercooling increases. Intriguingly, undercooling has a significant influence on eutectic microstructures. The eutectic microstructure transformed from feather structures to fishbone structures as undercooling increases then transformed into a line-like structure in eutectic cells. The evolution mechanisms of dendrites and eutectic structures are further elucidated based on several classical thermodynamic models. This study can provide theoretical and experimental guidance for the microstructure evolution of undercooled eutectic alloys with similar eutectic structures.

Effect of carbon dioxide mineralization curing on mechanical properties and microstructure of Portland cement–steel slag–granulated blast furnace slag ternary paste

To investigate the synergistic effects of steel slag (SS) and granulated blast furnace slag (GBFS) on cementitious systems during carbon dioxide (CO2) mineralization curing. Ternary paste specimens containing SS, GBFS, and Portland cement (PC) were prepared to analyze the influence of drying pre-treatment time and SS–GBFS mixing ratios on mechanical properties, CO2 fixation rate, and microstructure evolution. Results emphasize the significance of appropriate drying pre-treatment in enhancing mineralization degree and compressive strength. At 60 °C drying pre-treatment temperature, extending drying pre-treatment time up to 12 h enhances both compressive strength and CO2 fixation rate. However, beyond 12 h, a decline is observed in both parameters. Moreover, under the optimal drying pre-treatment time, the compressive strength of ternary paste specimens post-mineralization increases with higher SS content, with SS exerting a more pronounced positive effect on CO2 fixation rate and compressive strength compared to GBFS. After 4 h of mineralization curing, specimens with 50% SS and 10% GBFS demonstrate a 13.16% increase in compressive strength and a 9.16% increase in CO2 fixation rate compared to specimens with 10% SS and 50% GBFS. Microscopic test results reveal that the primary crystalline product in ternary paste specimens after mineralization is calcite, with traces of aragonite. The SS–GBFS mixing ratio has minimal effect on the type of reaction products but has a predominant influence on the total content and crystallinity of calcarea carbonica.

Grain refinement and strain delocalization in TRIP high-entropy alloys during hot deformation

Transformation-induced plasticity (TRIP) high-entropy alloy (HEAs) have shown promising mechanical properties, making them potential candidates for various applications. The preparation of structures with fine grains and heterogeneous orientation is important to improve the strength and ductility of TRIP-HEAs. However, the evolution and mechanism of grain refinement with heterogeneously oriented structures of TRIP-HEAs during thermal deformation and its effect on strain delocalization are not clear. In this paper, the microstructure evolution and strain delocalization of a TRIP-HEA, Co35Cr25Ni15Mn15Fe10 (atomic percent, at%) was examined through hot compression tests conducted at temperatures ranging from 900 ℃ to 1000 ℃, and strain rates ranging from 0.001s-1 to 0.1s-1. The results revealed that the average grain size of the alloy decreases with increasing deformation temperature and reducing strain rate due to the appearance of dynamic recrystallization (DRX) and annealed twins. After deformation at 1000 °C-0.001s-1, the average grain size of the alloy decreases from 169.85 μm (initial sample) to 21.5 μm, and the volume fraction of DRX and twin boundaries (TBs) increases to 81.9% and 37.6%, respectively. The DRX mechanism of the TRIP-HEA transformed from discontinuous dynamic recrystallization (DDRX) at 900 °C-0.1s-1/0.01s-1 to geometric dynamic recrystallization (GDRX) at 900 °C-0.001s-1 and then to continuous dynamic recrystallization (CDRX) at 950 °C/1000 °C-0.001s-1. Furthermore, the occurrence of DRX promotes the formation of annealed twins, resulting high volume fraction of high angle boundaries (HAGBs) and twin-matrix pairs, and contributing to grain refinement. Encouragingly, these HAGBs and twin-matrix pairs not only contributes to grain refinement, but also facilitates strain delocalization via dispersing strain and forming strain gradient, thus leading to coordinated deformation. This result is of great significance for understanding and improving the mechanical properties of TRIP-HEAs, and it can help in the design of high-performance TRIP-HEAs for various applications.

The effect of crystallographic orientation on globularization and texture evolution during the annealing of a near α titanium alloy tube

In order to weaken the rolling texture of the cold-rolled tube and improve its secondary forming ability, this paper studies the influence mechanism of different annealing times on the microstructure and texture evolution of Ti-6Al-3Nb-2Zr-1Mo tube after intermediate-temperature annealing. The results show that as the annealing time increases from 30minutes to 120minutes, the degree of globularization of α/β phases significantly increases. The globularized structure mainly consists of equiaxed α particles, which are surrounded by necklace-like globularized β phases. The original rolling texture component did not change after 30-minute annealing. The intensity of the rolling texture was weakened, and two new components appear after 120minutes of annealing. The appearance of these two components was caused by the growing up of globularized α phases close to the Burgers orientation relationship (BOR) with surrounding β phases during a long annealing process. Moreover, it was observed that globularization to some extent broke the BOR between α phase and the adjacent β phase.

Research on microplastic deformation of Inconel718 under the conditions of high temperature and high strain rate

The compression experiments of the nickel-based superalloy Inconel718 were carried out with the help of split Hopkinson pressure bar device, and the microstructure evolution law in the compression deformation of the alloy was studied by means of optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Based on the dislocation theory, a three-dimensional discrete dislocation dynamics model is established. By simulating the dynamic evolution of the discrete dislocation, the interaction of the dislocation itself and the formation of the dislocation microstructure were captured, and further explore the microplastic deformation process of Inconel718. The results show that deformation conditions significantly affect the rheological behavior and dynamic recrystallization behaviour. The cross slip of dislocations is closely related to temperature and strain rate, which in turn affect the dislocation configuration. The deformation mechanism of Inconel718 under the conditions of high temperature and high strain rate is jointly dominated by dislocation slip and twin deformation. As the temperature increases, the average Schmidt factor increases, and more dislocation slip systems are activated. As the strain rate increases, the number of deformation twins increases, while the thickness of deformation twins decreases.

Effect of Al: W value on the precipitation process of Co-Al-W alloy: A phase-field study

The microstructure evolution of Co-10Al-xW (x=8, 10, 12) alloys at different Al: W values was simulated based on the microscopic phase field dynamics model. The results show that: the Al: W value affects the γ′ phase precipitation mechanism of Co-Al-W alloy. When Al: W>1, the precipitation mechanism of γ′ phase of Co-10Al-8W alloy is non-classical nucleation and growth mechanism. When Al: W=1, the γ′ phase precipitation mechanism of Co-10Al-10W alloy is a mixture of non-classical nucleation and growth mechanism and instability decomposition mechanism, meanwhile, the formation time of γ/γ′ stable microstructure morphology is the shortest, and the stability of γ′ ordered phase is stronger. When Al: W<1, the γ′ phase precipitation mechanism of Co-10Al-12W alloy is a mixed mechanism of equal component ordering and instability decomposition mechanism. Under different Al: W values, the γ′ precipitate phase of Co-Al-W alloy has similar stable micromorphology in the late precipitation stage, and all of them show square blocks along the direction of [001] and [100]. The main antisite defects in Co-Al-W alloys are AlCo and CoAl defects, and the influence of Al: W value on the antisite defects at different sites is different.

Synergistic effects of Monel 400 filler wire in gas metal arc welding of CoCrFeMnNi high entropy alloy

Weldability plays a crucial role in the journey of high entropy alloys towards their engineering applications. In this study, gas metal arc welding was performed to join an as-rolled CoCrFeMnNi high entropy alloy using Monel 400 as the filler wire. The present research findings demonstrate a favorable metallurgical chemical reaction between the Monel 400 filler and the CoCrFeMnNi high entropy alloy, resulting in compositional mixing within the fusion zone that promotes a solid-solution strengthening effect, counteracting the typical low hardness associated to the fusion zone of these alloys. The weld thermal cycle induced multiple microstructure changes across the joint, including variations in the grain size, existing phases and local texture. The grain size was seen to increase from the base material toward the fusion zone. An FCC matrix and finely sparse Cr-Mn-based oxides existed in both base material and heat affected zone, while in the fusion zone new FCC phases and carbides were formed upon the mixing of the Monel 400 filler. The role of the filler material on the fusion zone microstructure evolution was rationalized using thermodynamic calculations. Texture shifted from a γ-fiber (in the base material) to a strong cubic texture in the fusion zone. Digital image correlation during tensile testing to fracture coupled with microhardness mapping revealed that, stemming from the process-induced microstructure changes, the micro and macromechanical response differed significantly from the original base material. This study successfully established a correlation between the impact of the process on the developed microstructural features and the resultant mechanical behavior, effectively assessing the processing-microstructure-properties relationships towards an improved understanding of the physical metallurgy associated to these advanced engineering alloys. In conclusion, this work provides an important theoretical framework and practical guidance for optimizing the engineering applications of high entropy alloys.

Influence of microstructure evolution on hot ductility behavior of a Cr and Mo alloyed Fe-C-Mn steel during hot deformation

Cr and Mo alloyed Fe-C-Mn steel was conducted by hot tensile experiments under different strain rates and deformation temperatures. The influence of microstructure evolution on fracture mechanism was studied. The principal failure mechanism was attributed to intergranular crack propagation and microvoid aggregation when deformed at 800-1100 ℃, changing to a mixed ductile-brittle intergranular fracture when deformed at 1200 ℃. After hot deformation, the microstructure transformed from a mixed phase of pearlite and ferrite to a lath martensite structure. Moreover, the lath martensite structure transitioned from disorder to order as the deformation temperature increases. The study revealed that in case of lower temperature when dynamic recovery (DRV) was dominated, the growth of voids was impeded by fine dynamic recrystallization (DRX) grains, resulting in an irregular void morphology. In case of higher temperature, the degree of DRX was high, voids were surrounded and impeded by newly formed DRX grains, which were gradually elongate with deformation. On the other hand, when there were DRX grains present between voids, void coalescence was less likely to occur, thereby inhibiting damage propagation.

Creep deformation behavior and microstructure of α-Mg/long-period stacking ordered (LPSO) duplex Mg-Y-Zn alloy prepared by hot extrusion and heat treatment

There exists a great demand for improving the creep resistance of α-Mg/long-period stacking ordered (LPSO) duplex alloys. In this study, microstructure control through hot extrusion and heat treatment was employed on Mg-Y-Zn duplex alloys with the aim of achieving superior creep resistance. The extruded Mg-Y-Zn alloy exhibited an enhancement in the creep resistance with increasing heat treatment temperature. Furthermore, the heat-treated samples demonstrated outstanding creep resistance even under high stress. Their creep behavior is characterized by a low minimum creep rate and a relatively small stress exponent (n ∼ 5) even at high stress. Microstructural observations revealed that heat treatment induces grain coarsening and an increase in solute concentration within the recrystallized grains. Additionally, during creep deformation, a plate-like LPSO phase and kink bands formed within the recrystallized grains. Therefore, it is affirmed that the exceptional creep resistance observed in the Mg-Y-Zn alloys in this study can be attributed to the microstructure evolution induced by heat treatment and creep deformation.

Unveiling the self-annealing phenomena and its effect on microstructure and texture evolution in the room temperature rolled Cu-0.13Sn-0.04Mg alloy

ABSTRACT In this study, we investigated the microstructural evolution of a Cu-0.13Sn-0.04Mg alloy that underwent heat treatment followed by room temperature rolling (RTR). Initially heat-treated specimens were rolled at room temperature (RT) with reduction ratios (RR) of 40% and 75% to perform microstructural and crystallographic textural analyses. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were utilise to examine both initial and deformed microstructures. Both exhibited heterogeneous microstructural distributions. Shear bands (SBs) existed in the initial specimen and became main sites for the nucleation of new grains during heat treatment. A denser presence of SBs was noted in the RTR specimens. Remarkably, an unusual self-annealing phenomenon was observed in these specimens, culminating in the emergence of recrystallized grains at RT. A plethora of such grains was observed in the severely deformed specimens, with numerous nucleating grains appearing proximal to the SBs or deformed grain boundaries (GBs). Time dependent microstructure evolution was observed via ex-situ EBSD technique in the compressed specimens. Formation of new grains, increase in high angle grain boundaries (HAGBs), and dislocation annihilations were observed with the progress of time. The crystallographic texture evolution in the initial specimen was predominantly influenced by the S component. At lower strains, the fractions of Copper component increased, whereas at higher strains, a greater fraction of Brass and S components were induced. Following a 40% RR, a saturation in hardness value was observed, which could be potentially attributable to an accelerated rate of RT recrystallization in the case of 75% RR.

Enhancing wear resistance of AZ61 alloy through friction stir processing: experimental study and prediction model

Abstract In this study, the friction stir technique is proposed to process AZ61 alloy and artificial neural network is built to predict and compare the experimental wear results. The effects of different processing parameters, including spindle speed (800–1200 rpm), travelling speed (05–15 mm/min), and depth of press (0.8-1.2 mm) on the microstructural evolution, mechanical properties and wear behaviour investigated. The grain size of the FSP1 samples found to be 14 ± 2 µm and shifting of peaks are observed in the X ray diffraction analysis chiefly due to texture development. As the spindle speed, travelling speed increases the surface roughness increases, it is observed that Ra – for FSP1 and FSP9 is found to be 68.4 nm and 116.3 nm respectively. Highest micro-hardness (113.36 Hv) values were observed for FSP1 sample and least is FSP9 sample (79. 51 Hv) and BM sample (65.92 Hv). Due to increased heat input into the processed regime, the hardness decreased as rotational and traveling speed increased, resulting in the development of coarse microstructure and a reduction in hardness. Based on wear results it has been observed that specimen FSP1 sample (0.003 g and 0.28) had the lowest weight loss and COF values compared to other FSP conditions. This is due to the combination of process parameters resulted in sufficient heat generation, grain refinement, and strain hardening of the stir zone, which enhanced the wear resistance of the specimen. Coefficient of friction is reduced to 0.28 from 0.68, and weight loss has been reduced to about 58% as compared to BM sample. This work shows that enhancing the wear resistance of magnesium alloys using friction stir processing is a successful approach.&amp;#xD;

Synergistic Improvement of Antibacterial and Osteogenic Differentiation of Thermomechanically Processed Mg-Zr-Sr-Ce Alloy: Insights into the Role of Precipitate Evolution Supported by AIMD Simulation Study.

In the present study, we have discussed the influence of forging temperature (623 K (FT623), 723 K (FT723) and 823 K (FT823)) on microstructure and texture evolution and its implication on mechanical behavior, in vitro-in vivo biocorrosion, antibacterial response, and cytocompatibility of microalloyed Mg-Zr-Sr-Ce alloy. Phase analysis, SEM, and TEM characterization confirm the presence of Mg12Ce precipitate, and its stability was further validated by performing ab initio molecular dynamic simulation study. FT723 exhibits strengthened basal texture, higher fraction of second phases, and particle-stimulated nucleation-assisted DRX grains compared to other two specimens, resulting in superior strength with comparable ductility. FT723 also exhibits superior corrosion resistance mainly due to the strengthened basal texture and lower dislocation density. All the specimens exhibit excellent antibacterial behavior with Gram-negative E. coli, Gram-positive Staphylococcus aureus, and Pseudomonas aeruginosa bacteria. 100% reduction of bacterial growth is observed within 24 h of culture of the specimens. Cytocompatibility was determined by challenging specimen extracts with the MC3T3-E1 cell lines. FT723 specimen exhibits the highest cell proliferation and alkaline phosphatase activity (ALP) because of its superior corrosion resistance. The ability of the specimens to be used in orthopedic implant application was evaluated by in vivo study in rabbit femur. Neither tissue-related infection nor the detrimental effect surrounding the implant was confirmed from histological analysis. Significant higher bone regeneration surrounding the FT723 specimen was observed in SEM analysis and fluorochrome labeling. After 60 days, the FT723 specimen exhibits the highest bone formation, suggesting it is a suitable candidate for orthopedic implant application.

The role of microstructure in the thermal fatigue of solder joints

Thermal fatigue is a common failure mode in electronic solder joints, yet the role of microstructure is incompletely understood. Here, we quantify the evolution of microstructure and damage in Sn-3Ag-0.5Cu joints throughout a ball grid array (BGA) package using EBSD mapping of localised subgrains, recrystallisation and heavily coarsened Ag3Sn. We then interpret the results with a multi-scale modelling approach that links from a continuum model at the package/board scale through to a crystal plasticity finite element model at the microstructure scale. We measure and explain the dependence of damage evolution on (i) the β-Sn crystal orientation(s) in single and multigrain joints, and (ii) the coefficient of thermal expansion (CTE) mismatch between tin grains in cyclic twinned multigrain joints. We further explore the relative importance of the solder microstructure versus the joint location in the array. The results provide a basis for designing optimum solder joint microstructures for thermal fatigue resistance.

Design of advanced steels by integrated computational materials engineering

AbstractThe integrated computational materials engineering (ICME) has achieved great success in accelerating the rational design and deployment of new materials. It is a new route of designing new materials and processes and highlighted by Materials Genome Initiative/Engineering that stresses the high‐throughput computation in addition to high‐throughput experimentation and materials informatics. This article presents a brief review on the basic theories and multi‐scale computational tools of ICME to design advanced steel grades, including the first‐principles calculations, the CALPHAD method (i.e., computational thermodynamics) fueled by dedicated databases, diffusion and phase‐field simulations, as well as finite analysis methods and machine learning. In the ICME scheme to deal with steels, the CALPHAD method is considered as the core to readily consider multi‐component systems and integrated to link the microscopic simulations (such as diffusion and phase field method to predict microstructure evolutions in response to external conditions) and macroscopic finite analysis method to deal with mechanical properties. Two applications are also presented to address the new routes to carry out materials design, especially for advanced steels.

Effects of multi-axial compression and double-step aging on the microstructure and mechanical properties of Al alloy 7075

This study investigates the combined effect of severe plastic deformation via multi-axial compression (MAC) and subsequent double-step aging on the microstructure and mechanical properties of annealed 7075 Al alloy. Three distinct processes were employed: (1) double-step aging on the annealed sample, (2) double-step aging on the six-MAC pass processed sample, and (3) double-step aging without solutionization on the six-MAC pass sample. Process (2) yielded the most significant improvements in mechanical properties. Compared to the annealed sample, the six-MAC pass, double-step aged sample exhibited an average ultimate tensile strength increase of 134% and an average Vickers micro-hardness (HV) increase of 209%. This superior performance is likely attributable to the synergistic effect of grain refinement induced by MAC and precipitate hardening achieved through double-step aging. A comprehensive analysis of microstructure evolution, mechanical properties, fractography, and the relationship between them was conducted for all three processes. This in-depth examination provided valuable insights into the mechanisms governing the observed property enhancements, particularly in process (2). Additionally, a variety of characterization techniques were employed to comprehensively evaluate the material's mechanical and microstructural characteristics.