Microstructural Evolution Research Articles

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.

The microstructure evolution and strengthening mechanism of fine-grained Al–Mn alloy with nanoscale precipitates during cryorolling

In this work, the fine-grained Al–Mn alloys with nanoscale precipitates was designed, and the texture evolution and strengthening mechanism for fine-grained Al–Mn alloy with nanoscale precipitates during cryorolling were investigated by EBSD and TEM analysis. The results show that the dislocation tangles gradually evolved into dislocation cells, and then into subgrains/ultrafine grains with boosting rolling reduction. The initial grains and precipitates can be significantly refined by cryorolling. Furthermore, the dominating P and M textures in the solution-treated Al–Mn alloy gradually converted to α-fiber and β-fiber textures during cryorolling. The formation of α-fiber and β-fiber textures is beneficial for improving the strength of the Al–Mn alloy. An excellent strength-ductility synchronization can be achieved in the Al–Mn alloy cryorolled at a rolling reduction of 50 %. The strengthening mechanism of Al–Mn alloy subjected to cryorolling is associated with texture strengthening, dislocation strengthening, grain refinement strengthening and precipitation strengthening.

The thermal stability and degradation mechanism of Cu/Mo nanomultilayers

ABSTRACT The microstructural evolution of Cu/Mo nanomultilayers upon annealing was investigated by X-ray diffraction and transmission electron microscopy. The isothermal annealing process in the temperature ranges of 300 – 850°C was conducted to understand the thermal behavior of the sample and follow the transformation into a nanocomposite. Annealing at 600°C led to the initiation of grain grooving in the investigated nanomultilayer, and it degraded into a spheroidized nanocomposite structure at 800°C. The sample kept the as-deposited Cu {111}//Mo{110} fiber texture up to 850°C. The residual stress was investigated to explain microstructure changes. The activation energy of degradation kinetics of Cu/Mo nanomultilayers was determined to understand the rate-determining mechanism for the degradation of nanolaminate structures.

Effect of hydrogen on microstructure evolution and properties of high-strain pipeline steel under cyclic strain loading

The microstructure evolution and the fatigue damage mechanism of hydrogen-charged high-strain pipeline steel during the cyclic strain loading is studied. Hydrogen promotes the formation of dislocation walls and cells, and causes the lattice distortion iron atoms to move to the grain boundary, resulting in grain refinement. The fatigue life is reduced to more than 40% when the strain amplitude reaches 0.5% after hydrogen-charged. The main reasons for the reduce are the decrease of the number and the change of propagation mode of secondary cracks, and the increase of cleavage fracture and the decrease of deflection angle in the main cracks.

Microstructure evolution and local mechanical properties of friction stir additively manufactured (FSAM) AA5083/AA6061/AA7075 gradient composite.

The current study is focused on the successful fabrication and characterization of microstructure and properties gradient composite made of 3 mm thick sheets of AA5083-O, AA6061-T6, and AA7075-T6 alloys using friction stir additive manufacturing (FSAM) technique. Two combinations of four layered FSAM builds (6756 and 7567) were fabricated successfully without any major defects at the optimized process parameters of 850 rpm (tool rotational speed) and 55 mm/min (tool transverse speed). The material mixing, interfacial bonding features, microstructure evolution, and mechanical performance within the stir zones (SZ) were thoroughly examined using advanced characterization techniques. Electron backscatter diffraction (EBSD) analysis confirmed a gradient microstructure along the build depth in both FSAM builds, with average grain sizes decreasing from 52 μm (AA5083), 46 μm (AA6061), and 40 μm (AA7075) in base metals to ∼ 4 to 7 μm within the SZ. The remarkable grain refinement within the SZ was mainly attributed to the dominant presence of the continuous dynamic recrystallization (CDRX) mechanism. Texture analysis corresponding to pole figures (PFs) and orientation distribution function (ODF) plots reveals the crystallographic texture evolution along the build depth. The prevalent existence of B/ and C textures throughout all regions of the FSAM builds affirms the presence of ample shear strain during the FSAM procedure. Through thickness miniature tensile and microhardness tests depict the gradient in mechanical performance for both the FSAM builds along the build depth. Microhardness gradients in the range from 80 HV0.1 to 145 HV0.1 were observed along the build depth. The axial sample in the build direction shows appreciable strength (265 MPa and 224 MPa) and uniform elongation (28% and 24 %) for 6756 and 7567 FSAM build, respectively. Furthermore, remarkable strain hardening behaviour corresponding to hardening capacity (Hc) and hardening exponent (n) was noticed for both the axial build samples compared to AA6061-T6 and AA7075-T6 base metals. These findings underscore the potential of FSAM in fabricating functionally gradient composite materials tailored to specific functional requirements.

Integrated Fabrication of Cf/Al Composite and Joining with TC4 via Liquid-Solid Infiltration Extrusion: Microstructure Evolution and Fracture Behavior

Integrated Fabrication of Cf/Al Composite and Joining with TC4 via Liquid-Solid Infiltration Extrusion: Microstructure Evolution and Fracture Behavior

Effects of Laser Scanning Strategy on Bending Behavior and Microstructure of DP980 Steel

Laser bending is a kind of cumulative forming technology and bending efficiency is one of its most important indexes. This study investigates the bending behavior and the microstructure of DP980 steel plates under different laser scanning strategies, using an IPG laser system. Two sets of experiments varied the accumulated line energy density (AED) by altering the laser scanning velocity and number of scans. The results show that, for the single laser scanning process, the bending angle of the plate increases with AED, due to a larger temperature gradient through the thickness direction; however, this relationship is nonlinear. A higher AED led to a sharper initial increase in bending angle, which then plateaued. Under the same AED conditions, the bending angle of the plate undergoing multiple laser scans increases by at least 26% compared to the single one, due to the microstructure changes. It is revealed that the bending efficiency is affected by both the AED and the resultant microstructure evolution in the DP980 steel. Higher AED values and appropriate peak temperatures facilitate better bending behavior due to the formation of uniform martensite and grain refinement. Conversely, excessive peak temperatures can hinder bending due to grain growth.

Constitutive Model and Microstructure Evolution of Ti65 Titanium Alloy.

The hot deformation behavior and mechanism of Ti65 alloy with a bimodal microstructure were investigated by isothermal compression experiments conducted on the Thermecmastor-Z simulator equipment at temperatures ranging from 950 to 1110 °C and strain rates ranging from 0.01 to 10.0 s-1. The Arrhenius constitutive model, based on strain compensation, and Grey Wolf optimization-neural network with back propagation model (GWO-BP), were both established. The differences between the experimental and predicted value of flow stress were compared and analyzed using the two models. The results show that the prediction accuracy of GWO-BP in the two-phase region is higher than that of Arrhenius model. In the single-phase region, both methods demonstrated high prediction accuracy. Compared to the single-phase region, the flow stress of Ti65 alloy shows a higher degree of softening in the two-phase region. During deformation in the two-phase region, the initial lamellar α phase transformed from a kinked and elongated morphology to a globularized topography as the strain rate decreased. Boundary-splitting was the primary mechanism leading to the spheroidization process. The degree of recrystallization increased with the increase in strain rate during the deformation in the single-phase region, while dynamic recovery and strain-induced grain boundary migration were the main deformation mechanisms at a lower strain rate. Discontinuous dynamic recrystallization may be the dominant recrystallization mechanism under a high strain rate of 10 s-1.

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Tungsten inert gas-assisted surface modification of 1045 steel with SiC particles

AISI 1045 steel is a medium-carbon steel with good strength, hardness, machinability, and weldability. However, it has poor wear and corrosion resistance. This study is an attempt to investigate the feasibility of creating a hard and wear-resistant composite layer on the surface of AISI 1045 steel using SiC particles through the tungsten inert gas (TIG) welding. The SiC powder was mixed with sodium silicate glue and polyvinyl alcohol and then applied to the samples using the TIG welding process. The welding voltage was kept constant at 15 V, and the welding current and welding speed were varied from 85 to 95 A and ∼ 0.5 to 1.8 mm/s, respectively. The powder percentage varied between 10 and 30 wt. %. The microstructural evolutions were studied using an optical microscope, a field emission scanning electron microscope (FESEM) equipped with an energy-dispersive spectroscope (EDS), and X-ray diffraction (XRD). It was observed that the microstructure of the surface-modified samples consisted of a dendritic Fe–Si matrix with carbon masses remaining from the partial dissolution of SiC particles. The effect of heat input on the microstructure was also investigated. The hardness of the surface-modified samples increased by about eight times compared to the base metal, and their wear resistance was notably improved. The results of the electrochemical corrosion tests showed an overall improvement in the corrosion resistance of the coated samples compared to the base metal.

In situ investigation on the evolution mechanism of heterogeneous micro-scale structure in potassium-niobate microwave flash sintering

Heterogeneity in the final product of flash sintering is a significant limitation for massive development. Synchrotron radiation computed tomography (SR-CT) was initially used to investigate the in-situ experiment of potassium-niobate (KNbO3) prepared by microwave flash sintering, revealing the temporal evolution of KNbO3 microstructure. The study has revealed the critical role of spatial arrangement between incompact particles and sintered agglomerates in achieving homogeneity in flash-sintered samples. Particles located the angle formed by the sintered agglomerate are more likely to experience electric field concentration, resulting in densification, and the densification rate decreases as the increases of angle size formed by the dense sintered agglomerates. This letter highlights that the presence of particles between sintered agglomerates is a significant factor contributing to heterogeneous sintering. The phenomenon is explained through a combination of experiments and simulation, which holds promise for enhancing the homogeneity of the sintering process.

Hot deformation behavior of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting

TiAl-based alloys are known for their exceptional high-temperature properties but suffer from low hot workability. The emergence of additive manufacturing (AM) technology significantly decreases the workability requirement, while the microstructure heterogeneities in AMed TiAl are troublesome. This paper explores the hot deformation behavior and microstructure evolution of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting. The Arrhenius constitutive model with strain compensation was established, and hot processing maps were constructed. The microstructure showed that at the initial stage of deformation, a large number of dislocation slips occur preferentially in the soft coarse-grained layers. With the increase of strain, dislocation accumulation in heterogeneous interfaces, and dynamic recrystallization (DRX) occurs. Eventually, the coarse-grained region is constantly occupied by fine DRX grains. A high density of deformation twins at high strain rates further promotes the formation of tiny DRX grains. When the hot compression temperature reaches 1200 °C, the initial alternative fine-grained and coarse-grained hetero-structure transforms into duplex (DP) and near-lamellar (NL) microstructure, and microstructure heterogeneity is eliminated. The results enhance understanding of the hot deformation behavior of heterogeneous microstructure TiAl alloys.

Excess-sulphate phosphogypsum slag cement blended with magnesium ion: Part Ⅱ-the long-term microstructure characterisation and phase evolution

Magnesium ions, as the most destructive ions in seawater, significantly affect the durability of cement-based structure. Excess-sulphate phosphogypsum slag cement (ESPSC) with great resistance to chemical attack, exhibits a potential application in marine engineering. As a continuation of previous research, the long-term performance development and microstructure evolution of ESPSC mixed with water including various magnesium ion concentrations (0–0.5 mol/L), are investigated in this study. The results indicate that incorporating magnesium ions within 0.2 mol/L has optimised the later compressive strength, as well as the pore structure. Furthermore, the components and nanostructure of C-(A)-S-H gels change due to the charge balance and cation compensation caused by magnesium ions. The phase evolution of AlⅣ to AlⅥ and an increase in hydration degree have been detected, where the aluminium phase tends to exist stable as ettringite. The M-S-H gel production is detected in samples incorporating 0.5 mol/L magnesium ions at early stage, which is detrimental to the gelatinous structure and strength development of ESPSC. From the perspectives of technological feasibility and environmental requirements, ESPSC production is an effective method for CO2 emission reduction and phosphogypsum utilisation with scalable potential. Introducing magnesium ions through seawater is also regarded as a modification to enhance the mechanical performance of ESPSC.

Study of the microstructure evolution of alloy structural steel and inhomogeneity effect of the microscale pulsed currents during current-assisted plane strain compressions by modeling a novel cellular automata method

The current-assisted forming process is considered to be a novel process that utilizes the electroplasticity effect to reduce the deformation resistance of difficult-to-deform metals and to refine the grain size while improving the mechanical properties of the part. However, the lack of understanding of the mechanisms by which pulsed current affects grain refinement still makes it difficult to experimentally develop analytical or empirical models of the relationship between grain size, pulse current parameters, and deformation amount. It will seriously hinder the further development and application of the current-assisted forming process. To reveal the mechanism of grain refinement coupling with electroplasticity, a series of Electron Back Scattering Diffraction observation tests were carried out after current-assisted plane strain compressions. The results show that the grain orientation spread value of the pulse current condition is lower than that of the non-current condition, while the refined grain area percentage is higher than that of the non-current condition. Based on the microstructure observation results, a cellular automata model for grain refinement is proposed. This consists of a dislocation density evolution, sub-grains generation, grain fragmentation, and a grain orientation rotation algorithm. To study the electroplasticity of the grain refinement process, the cellular automata model is also embedded with a finite difference numerical algorithm for solving the microscopic current density distribution of the model. The cellular automata model simulation results show that the error of grain size accuracy of this cellular automata model under non-current and pulse current conditions is 10.48% and 8.55%, respectively. It shows that the model can accurately characterize the grain refinement behavior of the current-assisted plane strain compression. The simulation results reveal the deep mechanism of pulsed current promotion on grain refinement, i.e., an inhomogeneous microstructure leads to uneven distribution of pulse current density. The inhomogeneous current distribution will further increase the grain refinement rate in the coarse-grained regions, thus increasing the proportion of the refined grain regions in the microstructure and leading to a relatively more homogeneous microstructure under pulse current conditions. The cellular automata model accurately reveals the mechanism of electroplasticity on the grain refinement. The model will serve as a crucial theoretical reference in designing current-assisted forming process routes to ensure excellent microstructure and properties in manufactured parts. It will enhance the widespread utilization of current-assisted forming process.

Microstructure development of Ti scaffold by laser powder bed fusion with chemical polishing and its mechanical properties, biocompatibility

AbstractTitanium (Ti) dental scaffolds are widely used in dental prosthetics due to their excellent mechanical properties and biocompatibility. However, conventional Ti scaffolds manufactured through machining often do not fit perfectly with the bone defect site. Laser powder bed fusion (LPBF) technology enables the personalised manufacturing of custom‐made Ti scaffolds. A custom‐made Ti scaffold was prepared using LPBF and its surface roughness was improved through chemical polishing. To enhance the surface roughness, a nitric acid mixed solution with a specific composition of HF: HNO3:C3H6O3 = 2:2:3 was used. The polishing mechanism was investigated by adjusting the F/Ti ratio to control the formation and dissolution of the oxide film. As a result, the surface of the Ti scaffold after polishing exhibited a smooth and flat appearance compared to the LPBF part, with a reduced surface roughness (Ra) of 1.23 ± 0.19 μm. The custom‐made Ti scaffold also demonstrated favourable mechanical properties, with a bending strength of 335.18 ± 33.62 MPa and stiffness of 2.13 ± 0.21 GPa. Furthermore, in vitro cell tests confirmed the excellent biocompatibility of the custom‐made Ti scaffold. The authors present a feasible strategy for the further clinical application of custom‐made Ti scaffolds, offering enhanced surface properties and addressing the limitations of conventional machining methods.

Compositionally graded titanium to aluminum processed by laser powder bed fusion process: Microstructure evolution and mechanical properties

A compositionally graded Ti–Al alloy with a designed composition range from pure Ti to pure Al was successfully produced by a laser powder bed fusion (L-PBF) system equipped with two powder feeders. The compositionally graded Ti–Al profile is confirmed by scanning electron microscopy, X-ray diffraction analysis, electron backscatter diffraction microscopy, and transmission microscopy. The graded material is free of macro-horizontal cracks, but micro-cracks appear at the center of the sample. Varying the Ti and Al ratio along the build direction induces microstructural changes, transforming α+β-Ti into solid solution, creating intermetallics (Ti and Al), and yielding an Al phase with diverse morphologies. Additionally, the Ti–Al ratio's influence on microstructure formation and phase evolution results in varying mechanical properties among the graded material's layers. The results demonstrate that the L-PBF process can achieve continuous compositional grading of Ti–Al materials in a single run. Moreover, this study hold the potential to facilitate the design and production of numerous compositional variants for high entropy alloys and multi-materials, enabling future investigation into microstructural evolution, phase transformation, and physical and mechanical properties.

Computationally Efficient Algorithm for Modeling Grain Growth Using Hillert's Mean-Field Approach.

To investigate the interconnected effects of manufacturing processes on microstructure evolution during hot-rolling, a through process model is required. A novel numerical implementation of the mean-field approach was introduced to efficiently describe the grain growth of larger systems and extended durations. In this approach, each grain is embedded within an average medium and interacts with the average medium, thus avoiding the complexities of individual grain interactions. The proposed upsampling approach dynamically adjusts the simulation grain ensemble, ensuring efficiency and accuracy regardless of the initial number of grains present. This adaptation prevents undersampling artifacts during grain growth. The accuracy of the model is verified against analytical solutions and experimental data, demonstrating high agreement. Moreover, the effects of different initial conditions are successfully investigated, demonstrating the model's versatility. Due to its simplicity and efficiency, the model can be seamlessly integrated into other microstructure evolution models.

Effect of Overheating on the Tensile Properties of Nickel-Based Superalloy GH4720Li.

Aero-engines can be exposed to One Engine Inoperative (OEI) conditions during service, and the resulting overheating effect may significantly impact their structural integrity and flight safety. This paper focuses on the influence of overheating on the microstructural evolution and tensile properties of the GH4720Li alloy, a nickel-based polycrystalline superalloy commonly used in turbine disks. Based on the typical OEI operating conditions of a real aero-engine, a series of non-isothermal high-temperature tensile tests involving an OEI stage of 800 °C were conducted. The effects of OEI-induced overheating on the microstructure and tensile properties of the GH4720Li alloy were investigated. The results showed that, after OEI treatment, the primary γ' phase in this alloy was partially dissolved. The GH4720Li superalloy also exhibited numerous microcracks at the grain boundaries, resulting in complex effects on its tensile properties. The alloy's yield strength and ultimate tensile strength were slightly decreased, whereas its ductility decreased considerably. The OEI-induced embrittlement phenomenon was mainly caused by the non-uniform distribution of the tertiary γ' phase within grains. The formation of microcracks nucleated at the interfaces between the primary γ' precipitates and γ matrix phase was another key factor.