Microstructural Evolution Research Articles

Competing roles of microstructure and defects on the mechanical properties of laser-powder bed fused Ti-6Al-2Sn-4Zr-2Mo alloy

AbstractUsing different volumetric energy densities ($${E}_{\text{v}}$$ E v ), the microstructure, texture, and defect evolution in laser-powder bed fused (PBF-LB/M) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy is studied. PBF-LB/M Ti-6242 rods were manufactured using different $${E}_{\text{v}}$$ E v ranging from 41.67 to 66.67 J/mm3. The $${E}_{\text{v}}$$ E v is varied by setting the scan speed to 1000 mm/s, 1200 mm/s, 1400 mm/s, and 1600 mm/s. The mechanical properties (yield strength, tensile strength, and strain at fracture) were then studied under quasi-static loading conditions. It is observed that the strength of the sample printed using the lowest $${E}_{\text{v}}$$ E v is lower than the other conditions due to the formation of the lack of fusion defects. In addition, the sample printed with the highest $${E}_{\text{v}}$$ E v consists of redeposited process by-products that result in the lowest ductility. The microstructure and texture of the samples were studied using electron backscatter diffraction. The results show that microstructural features including α′ lath width, dislocation density, and lath orientation (texture) were almost identical under different $${E}_{\text{v}}$$ E v . Therefore, the variations in mechanical properties may not controlled completely by the microstructure. The defect analysis is conducted employing X-ray computed tomography. The defect characteristics change from keyhole to lack of fusion by varying the $${E}_{\text{v}}$$ E v . The volume fraction of defects in the samples is in the range of 0.0005–0.007%, which seems to be negligible. However, the fractography analysis shows the dominance of defects in controlling the mechanical properties. This study proves the sensitivity of PBF-LB/M Ti-6242 to defects as the mechanical properties were defect-driven rather than microstructure-driven.

Microstructure Evolution of Dissimilar Graded Joints of Ferritic P91 and Austenitic 347H Stainless Steels Manufactured with Directed Energy Deposition

Microstructure Evolution of Dissimilar Graded Joints of Ferritic P91 and Austenitic 347H Stainless Steels Manufactured with Directed Energy Deposition

Analysis of Microstructural Evolution during the Sintering Process of Aluminum Matrix Nanocomposites Reinforced with Al4C3, Using X-ray and HRTEM

Analysis of Microstructural Evolution during the Sintering Process of Aluminum Matrix Nanocomposites Reinforced with Al4C3, Using X-ray and HRTEM

Microstructural evolution of CrCuFeNiZn nanocrystalline high entropy alloy prepared by mechanical alloying

High Entropy Alloys (HEAs) have become the most researched structural materials in the scientific community during the last decade due to their attributes like excellent strength and wear resistance. In this research, CrCuFeNiZn HEA was prepared using mechanical alloying technique. Further, X-ray diffraction (XRD) and Scanning electron microscope (SEM) analysis of prepared HEA were carried out at various milling time intervals (10 min, 5 h, 10 h, 15 h, 20 h, and 25 h) to determine solid solution formation. Results manifest that CrCuFeNiZn HEA exhibited BCC + FCC (dual phase) structure after 25 h of milling. Moreover, the crystallite size as measured from the XRD analysis for the 25 h milled powder was found to be 32.8 nm and lattice strain at the end of milling is calculated to be 0.428%. SEM-EDS analysis further confirms the homogeneous distribution of the constituting elements in the as-fabricated alloy aggregate.

Micromechanism insights and constitutive modeling for deformation in a novel high manganese austenitic steel

We explore the mechanical behavior and microstructural evolution of a novel high manganese austenitic steel (HMAS), extensively applied in underground engineering and structural construction. Our investigation focuses on two variants of HMAS, namely HMAS-1.9 and HMAS-10, which exhibit distinct yield strengths and elongation behaviors. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to elucidate the evolution of microstructures during plastic deformation. Based on these experimental results, we developed a new phenomenological multiscale constitutive model that captures the mechanical behavior of HMAS, integrating micromechanisms and microstructural evolutions. Optimized with the MATLAB genetic algorithm toolbox, this model was successfully implemented in a uniaxial tensile finite element model (FEM) within ABAQUS, proving its efficacy in predicting both macroscopic flow stress and microstructural evolutions. Our findings highlight the important roles of initial grain size, dislocation density, and twin fraction in determining the mechanical properties of HMAS. Throughout the deformation process, both HMAS variants demonstrate similar plastic deformation behaviors, characterized by comparable rates of increase in dislocation density and twinning fraction, accompanied by a consistent trend of grain refinement. These findings not only deepen our understanding of the complex relationship between microstructure and mechanical properties of the HMAS steel, but also provide an effective multiscale constitutive modeling approach for predicting its deformation behavior.

Evolution of microstructure and mechanical properties of Ti65 high-temperature titanium alloy after additive manufacturing and annealing

Ti65 was prepared by Laser Direct Energy Deposition (LDED) and subjected to Annealing Heat Treatment (AHT). The microstructure and mechanical properties of different specimens were evaluated. The results indicated that the prior β columnar crystals had a strong [0001] texture along the grain growth direction. The coarsening or spheroidization of α lath was attributed to the splitting of lamellar α, subsequent non-directional growth, element migration, and Ostwald ripening. The isotropy of the specimen annealed at 950°C was caused by trimodal microstructure, discontinuous grain boundary α (αGB), precipitates dissolution, and low-angle grain boundary. A mathematical model was established to predict yield strength (YS) and microhardness based on the α lath thickness. The microhardness-strength correlation of LDED Ti65 followed an empirically derived model (Tabor's relationship). Improvements to the original models of the microhardness-strength relationship for Ti65, in conjunction with the strain-hardening effect and α orientation, made it possible to use these models to predict the strength of LDED Ti65 parts and improve the accuracy of the predicted values. Simultaneously, the model can provide data support for the industrial application of Ti65 or, where appropriate, provide a reference for other technical production (material applications).

Evolution of Microstructure, Texture and Mechanical Properties of ECAP-Processed ZK60 Magnesium Alloy

Evolution of Microstructure, Texture and Mechanical Properties of ECAP-Processed ZK60 Magnesium Alloy

Long-term thermal aging of austenitic stainless-steel weld: Microstructure evolution in δ-ferrite and δ / γ phase boundary and apparent recovery of mechanical properties

This study aimed at investigation of thermal aging effects on evolution of microstructure and mechanical properties of ER316L austenitic stainless-steel weld (ASSW). The ASSW was subjected to thermal aging at 400 °C for up to 30,000 h. Initially, thermal aging triggered spinodal decomposition of δ-ferrite and clustering of Ni. Thermal aging up to 20,000 h further enhanced spinodal decomposition and G-phase formation. After 30,000 h of aging, precipitation of Mo-rich χ-phase occurred adjacent to the G-phase. Concurrently, thermal aging induced segregation of different elements and formation of nickel depletion zone was observed at the ferrite / austenite (δ / γ) phase boundary. The consequence of these microstructure changes was the hardening of δ-ferrite and loss of fracture toughness of ASSW. However, after 30,000 h of aging at 400 °C, degraded fracture toughness and tensile properties were partially recovered. This recovery in fracture toughness was attributed to the evolution of microchemistry and the formation of a softer shell in the δ / γ phase boundary region

A Visualized Microstructure Evolution Model Integrating an Analytical Cutting Model with a Cellular Automaton Method during NiTi Smart Alloy Machining

In this study, a visualized microstructure evolution model for the primary shear zone during NiTi smart alloy machining was established by integrating an analytical cutting model with a cellular automaton method. Experimental verification was conducted using an invented electromagnet rotation-type quick-stop device. The flow stress curve during the dynamic recrystallization of the NiTi smart alloy, the influence of relevant parameters on the dynamic recrystallization process, and the distribution of dynamic recrystallization in the primary shear zone were studied via the model. The simulation results showed that strain rate and deformation temperature significantly affect the relevant parameters during the dynamic recrystallization process. Three typical shear planes were selected for a comparison between simulation results and experimental results, with a minimum error of 3.76% and a maximum error of 11.26%, demonstrating that the model accurately simulates the microstructure evolution of the NiTi smart alloy during the cutting process. These results contribute theoretical and experimental insights into understanding the cutting mechanism of the NiTi smart alloy.

Effect of isothermal holding time on microstructure, precipitates and hardness of Ti–V–Mo microalloyed steel during coiling process

The microstructural evolution, precipitation behavior, and hardness variation of the Ti-V-Mo complex microalloyed steel during isothermal holding following austenite deformation have been investigated using thermal simulation tester (Gleeble-3800), optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), and Vickers hardness tester. The results indicate that the microstructure transforms from austenite to ferrite as the holding time increases from 0 s to 5400 s at 630 °C, with the microstructure primarily comprising martensite and ferrite. The proportion of ferrite continues to increase, while the proportion of martensite decreases. At a holding time of 600 s, the austenite has nearly completed the ferrite transformation (95%), and the average grain size is very fine (3.9 μm). At this point, the (Ti, V, Mo)C particles exhibit the highest density and a smaller size (7.7 nm), with the hardness reaching its maximum value (415 HV). Concurrently, numerous spherical precipitates particles with a size of < 10 nm formed in the ferrite region, conforming to the Baker–Nutting (BN) orientation relationship with respect to the matrix: (100)(Ti, V, Mo)C//(100)α-Fe, [011](Ti, V, Mo)C//[012]α-Fe. As the holding time increases from 30 s to 60 s, the hardness decreases rapidly due to the reduction of martensite and addition of soft-phase ferrite. However, the subsequent slow decrease in hardness is primarily attributed to weakened precipitation strengthening caused by precipitates coarsening and decreased grain refinement strengthening due to the slight increase in ferrite grain size as the holding time increases from 600 s to 5400 s.

Functional behaviour of novel multi-layered deposition via twin wire arc additive manufacturing: Microstructure evolution and wear tailoring characteristics

Twin wire arc additive manufacturing offers advantages such as high deposition rates and the ability to use a wide variety of materials, making it suitable for large-scale metal part production with enhanced efficiency and flexibility. Functionally graded deposition (FGD) based structure of dissimilar steels (SS316LSi and ER70S-6) fabricated via twin wire arc additive manufacturing and their detailed performance analysis are accomplished in the current research work. Microstructural, mechanical, and tribological characteristics of the deposited structure were investigated and analysed. Microstructure reveals polygonal ferrite, widmanstatten ferrite, skeletal ferrite, lathy ferrite, ferrite solidification, and ferrite to austenite (F/A) transformation. FGD structure possessed a microhardness of 371 HV. Average ultimate tensile strength, yield strength, and elongation percentage were 675.98 MPa, 414.21 MPa, and 38.37 %, respectively. In wear analysis, the average coefficient of friction value of WAAM-processed FGD samples is 0.44–0.49. 3D profilometer analysis reveals that average values of roughness, maximum height of roughness, and maximum roughness valley depth are significant with increasing applied loads. The findings of the present research focus on the quality and appropriateness of dissimilar steels for structural applications and provide a novel method for creating high-strength components.

Microstructural development in a hot rolled multi-phase steel

The present work describes the development of microstructures in multi-phase steels with minimum ultimate tensile strength (UTS) in the range of 750–800 MPa. A relatively leaner composition was designed to achieve the strength, and with two compositions, the UTS values of about 750 MPa and 800 MPa were achieved. In both cases, multi-phase microstructures were developed, which consisted of ferrite, bainite and martensite. Bainite was the major phase in these microstructures. The cooling rate played a vital role. Ferrite was observed to form as the major phase along with islands of bainite at a cooling rate of 2 °C/s or less than that. The desired amount of bainite phase could be obtained at and above a cooling rate of 20 °C/s. A double step cooling strategy was applied to achieve the desired multi-phase microstructure. After the completion of hot rolling at around 850 °C, a fast cooling to a temperature of 580 °C and holding there for a duration of 5–12 s led to the formation of ample bainite in the microstructure, followed by transformation of remaining austenite into martensite during the second step cooling at 250 °C.

Effect of texture on bending behavior of Mg alloy sheets

The effect of Li elements on the microstructure and texture evolution of AZ31 alloy sheets during room-temperature three-point bending was investigated. The results showed that the conventional c-axis//ND basal texture of the extruded AZ31 sheet could be modified to a TD-elongated fiber texture with a 5wt.% Li addition. As a result, the room-temperature bendability of the Mg–3Al–5Li sheet was improved effectively, and this sheet showed a minimum bending radius of only 2mm. The superior bendability exhibited surpasses the room-temperature bending capabilities commonly observed in the majority of commercial Mg alloys. The improvement in bendability of the Mg–3Al–5Li sheet was attributed to the TD-elongated texture distribution. This type of texture distribution may offer more effective deformation modes to alleviate thickness strain during bending in comparison to the texture in extruded AZ31 sheets. This study could provide insights into developing Mg alloys with high room-temperature bendability by tailoring texture distribution rather than texture weakening.

Mechanical characterization and microstructural evolution of Inconel 718 and SS316L TIG weldments at high temperatures

This study examines the impact of both constant and pulsed current on the tensile characteristics, micro-hardness of weld surface, and metallurgical properties of the Inconel 718 and SS316L weldments. The Tungsten Inert Gas (TIG) welding process utilized two distinct arc modes to combine incompatible metals. The pulse frequency was systematically adjusted within the range of 2 Hz–6 Hz in order to investigate its impact on the welding parameters. A tensile test was performed using a universal testing equipment under various temperature conditions, including room temperature and an elevated temperature of 350 °C. When the samples were welded at a frequency of 4 Hz, both the Ultimate Tensile Strength (UTS) and Yield Strength (YS) were greater compared to when the samples were welded at 2 Hz, 6 Hz, or a continuous arc mode. Furthermore, it was noted that the YS-to-UTS ratio was higher for the weldments produced at 4 Hz pulse frequency. The UTS of current and pulsed (4 Hz) modes at a temperature of 350 °C were determined to be 505 and 524 MPa, respectively. Pulse welding demonstrated a higher hardness number compared to steady welding due to the regulated rate of heat. The TIG weldments of Inconel 718 and SS316L demonstrated superior mechanical and metallurgical qualities when exposed to elevated temperatures. The practical implications of these findings are relevant to the fabrication of turbine discs and shafts.

Oxidation mechanism and microstructure evolution of invar alloy with high temperature annealing process

The high temperature oxidation behavior of Fe–36Ni Invar alloy in the forged billet stage and hot rolled rod stage in the compressed air atmosphere has been systematically studied. The results show that the oxide layer can be divided into the external oxide layer composed of Fe2O3, Fe3O4 and NiO, and the inner transition oxide layer composed of FeO, Fe2O3, Fe3O4 and NiO. In addition, hot rolled rod samples treated by solution and rolling have higher oxidation resistance. The mechanism of O diffusion during oxidation is analyzed, and it is found that the O atom adsorbed on the surface of the substrate diffuses into the grain along the grain boundary. The grain boundary is the optimal path for O diffusion to the interior of the material, which results in the degree of intergranular oxidation of the alloy being greater than the degree of intragranular oxidation. In summary, the oxidation resistance of Invar alloy can be improved by proper solution treatment and avoiding the formation of grain boundary cracks during production.

ZK60 alloy deformation and damage at different temperatures using unified constitutive model

The deformation behaviour and damage mechanism of ZK60 magnesium alloy under temperatures of 25–140 °C and strain rates of 0.01–0.0001 s−1 were investigated by uniaxial hot tensile and characterization experiments. The results suggested that the main factor for damage in ZK60 magnesium alloy was the nucleation, growth and coalescence of microdefects such as microvoids and cracks around the second-phase particles and/or grain boundaries. Then, a unified constitutive model including damage evolution, work hardening and recovery mechanisms was established. The model, verified by experimental data, shows good predictability. It was implemented into ABAQUS software through the VUMAT subroutine, which then predicted and analyzed the microstructure evolution.

Tracking microstructural evolution and hardening in Al–Zn–Mg–Cu alloys aged artificially via electrical conductivity measurements

Precipitation hardening, a crucial mechanism for strengthening aluminum alloys, involves stages like Guinier–Preston (GP) zone formation, precipitation, peak aging, and precipitate coarsening. This study focuses on the aluminum 7050 alloy, proposing a method to gauge artificial aging through electrical conductivity measurement. The evolving microstructure and time to peak hardness during aging are vital for creating high-strength alloys. The electrical conductivity variation over time is utilized to analyze the diffusion process governing the clustering and growth of specific phases (η′, η, and S) during artificial aging. The paper demonstrates the impact of GP zones, precipitate formation, and grain growth on electrical conductivity, correlating these factors with hardness, microstructure, and tensile strength to determine the hardening stage. Differential electrical conductivity plots, highlighting aging stages, assist in identifying the hardening phase. Tensile strength and hardness plots differentiate the precipitation phases. The Johnson–Mehl–Avrami–Kolmogorov equation models particle growth kinetics, determining growth rates for AA 7050 alloy. The overall activation energy for precipitate growth is 40.77 kJ/mol, with a growth constant ( m) of ∼4, indicating S phase nucleation during η′ and η growth.

Shock responses of nanocrystalline molybdenum via molecular dynamics simulation: Grain size and shock intensity effects

Molybdenum (Mo) is a strategic metal for the manufacture of aerospace equipment, satellite components, and vehicle armor. Thus, understanding its behaviors under harsh conditions like shock compression is crucial for its practical utilization. Through molecular dynamic simulations, we have explored the mechanical responses and microstructural evolutions of nano-polycrystalline (NPC) Mo under different shock intensities, with grain sizes ranging from 5 to 33 nm. Our study reveals that grain size considerably influences the Hugoniot data and waveform of NPC Mo. NPC Mo with a smaller grain size exhibits higher compressibility and lower Hugoniot shear stress. As the grain size increases, the presence of a double-wave structure becomes more pronounced. Additionally, with the increase in shock intensity, there is a reduction in the shock front width. Significantly, when the shock stress ranges from approximately 60 to 100 GPa, twinning structures are detected in samples with grain sizes ranging from 10 to 33 nm. Moreover, the elevated temperature behind the shock wave further promotes detwinning reactions. When the shock stresses exceed 100 GPa, twinning–detwinning as well as amorphization-recrystallization become the predominant deformation mechanisms, almost unaffected by grain size. As the shock stress exceeds 250 GPa, the atoms in the samples become completely disordered. These findings provide new insights into the mechanical responses as well as the microstructural evolutions of NPC Mo under shock compression.

Microstructure evolution and mechanical properties of Q690D steel repaired by wire based laser directed energy deposition

ABSTRACT A wire-based laser directed energy deposition process was developed to repair Q690D steel. The microstructure evolution, phase transformation and mechanical properties of the repaired specimens were investigated in detail. Excellent metallurgical bonding was achieved between the repaired zone and the substrate free of macroscopic defects. Typical columnar dendrites with grains grew perpendicularly from the fusion line to the deposition zone (DZ) centre. Microstructure in the DZ and heat-affected zone (HAZ) of the repaired specimen was predominated by lath martensite. The tensile properties of the repaired specimens were comparable to those of the substrate. The repaired specimens presented ∼200 MPa higher flexural strength than that of the substrates, while they have inferior ductility compared to the substrates under the three-point bend test. The average impact absorbed energy value for the repaired specimens achieved ∼85–90% of the substrates. This study introduces a novel restoration technique for offshore engineering platform maintenance.

Nano-lamellar Ti3Al/TiAl alloy prepared via dual-wire-fed electron beam-directed energy deposition: microstructure evolution and nanohardness enhancement

Nano-lamellar Ti3Al/TiAl alloy prepared via dual-wire-fed electron beam-directed energy deposition: microstructure evolution and nanohardness enhancement