Microstructure Evolution Features Research Articles

A unified constitutive model and microstructure evolution of a solution treated 7075 aluminum alloys under warm forming condition

The warm flow characteristics and microstructural evolution features in a solution-treated 7075 aluminum alloy are revealed. An elastic-visco-plastic constitutive model is developed to capture the flow stress behavior of 7075 aluminum alloy under warm deformation. The variations of microstructure features, including grain and subgrain size, are also analyzed under warm deformation, and are further incorporated in the proposed unified constitutive model. The results indicate that the dynamic recovery (DRV), related to dislocation annihilation and formation/growth of sub-grains, is the domain softening mechanism. The DRV behavior is enhanced at higher temperatures or lower strain rates. Warm external temperature field assists the plastic deformation by means of activating and accelerating the dynamic softening, while the reduced strain rate also gives sufficient time for the softening. Through experimental validation, the proposed constitutive model successfully captures the grain/subgrain features and flow behavior of the studied 7075 aluminum alloy during warm deformation. The predicted result is in a great agreement with the experimental stress-strain data, demonstrating the excellent predictive performance of the model. Moreover, the proposed model also successfully predicts the average grain/subgrain size.

Characterization of the microstructure evolution features of various aggregates based on confocal laser scanning microscope

To reveal the microscale structural evolution characteristics of aggregates, this study employs confocal laser scanning microscope (CLSM) to conduct the surface microstructures of six aggregates: limestone, basalt, granite, diabase, sandstone, and recycled aggregate (RA). Quantitative analysis was performed on the surface morphology, roughness parameters, and material ratio of the aggregates. The results indicate that the surface features of the aggregates vary with the type and treatment method, and these distinctions may be attributed to variations in diagenetic conditions. In the original state, the surface texture of limestone, basalt, and diabase is relatively minimal, with arithmetic mean height (Ra) distributed between 1.4 μm and 3.5 μm. Conversely, the surface texture of granite, sandstone, and RA is more complex, with Ra distributed between 7.5 μm and 9.5 μm. The roughness indices of basalt, limestone, and diabase show an increasing trend after lime treatment, indicating a more pronounced presence of microscopic peaks and pits. In contrast, all roughness indices of granite, sandstone, and RA exhibit a declining after lime treatment. After polishing, the Ra of the surfaces of the six different aggregates shows varying degrees of reduction. Specifically, Ra decreased by 43.89%, 33.11%, 26.52%, 30.14%, 29.80%, and 38.79% for limestone, basalt, granite, diabase, sandstone, and RA, respectively. Furthermore, the height distribution frequency and material ratio of aggregate surfaces vary with the type and treatment of aggregates. Through grey correlation analysis, the correlation degrees of root mean square slope (Rdq), kurtosis (Rku), and surface texture aspect ratio (Rtr) are 0.9524, 0.9522, and 0.9514, respectively, belonging to hybrid parameter, height parameter, and spatial parameter. This suggests a close relationship between these parameters and the polished stone value (PSV) of the aggregates. These findings offer valuable insights for improving aggregate selection and treatment processes, potentially leading to enhanced durability and performance of construction materials.

Features of Microstructural Evolution and Corrosion Behavior of Ti6Al4V Titanium Alloy Fabricated from Elemental Powder Blends

Sintered Ti6Al4V titanium alloys prepared from TiH2/60Al40V powder blends under various technological conditions were studied. The microstructural evolution was investigated by X-ray diffraction, scanning electron microscopy, optical microscopy, and energy dispersive X-ray analysis. The corrosion resistance of sintered titanium alloy was evaluated by the static immersion test in 40 wt.% H2SO4 acid, according to ASTM standard G31-72(2004). Depending on powder metallurgy processing parameters (compaction pressure or sintering temperature), the Ti6Al4V alloy was obtained with various structural features (porosity and structural heterogeneity). It was shown that those structural features of sintered Ti6Al4V titanium alloy are a key microstructural factor that determines their corrosion resistance. For instance, an increase in porosity leads to enhanced corrosion resistance. Based on the current research, the optimal manufacturing regimes of powder metallurgy of Ti6Al4V titanium alloy ensure the achievement of characteristics sufficient for practical use in aggressive conditions of the chemical industry were obtained.

High-Temperature Compression Behaviors and Constitutive Models of a 7046-Aluminum Alloy.

High-temperature forming behaviors of a 7046-aluminum alloy were investigated by hot compression experiments. The microstructural evolution features with the changes in deformation parameters were dissected. Results indicated the formation of massive dislocation clusters/cells and subgrains through the intense DRV mechanism at low compression temperature. With an increase in deformation temperature, the annihilation of dislocations and the coarsening of subgrains/DRX grains became prominent, due to the collaborative effects of the DRV and DRX mechanisms. However, the growth of subgrains and DRX grains displayed the weakening trend at high strain rates. Moreover, two constitutive models involving a physically based (PB) model and a gate recurrent unit (GRU) model were proposed for predicting the hot compression features. By validation analysis, the predicted values of true stress perfectly fit with the experimental data, indicating that both the proposed PB model and the GRU model can accurately predict the hot compression behaviors of 7046-aluminum alloys.

Microstructure evolution mechanisms and a physically-based constitutive model for an Al–Zn–Mg–Cu–Zr aluminum alloy during hot deformation

High-temperature flow features of the Al−Zn−Mg−Cu−Zr aluminum alloy was revealed by hot compression tests. The evolution mechanisms of dislocation clusters, subgrain, and dynamic recrystallization (DRX) grains, are thoroughly explored by EBSD and TEM analysis. Experimental results suggest that the high strain rate can exacerbate dislocation clusters formation, as well as subgrain nucleation/accumulation, inducing the increasing of flow stress. Nevertheless, the noticeable annihilation of substructures, as well as the growth of DRX grains, emerge at the higher temperature, causing the descending of flow stress. Three types of DRX nucleating mechanisms, i.e., discontinuous DRX (DDRX), geometric DRX (GDRX) and continuous DRX (CDRX) are activated in the Al−Zn−Mg−Cu−Zr aluminum alloy during hot compression. Simultaneously, the GDRX often appears at a high compressed temperature or a low strain rate. A physically-based (PB) model is proposed to collaboratively reconstruct true stresses and microstructure evolution features. The estimated values of true stress, DRX fractions and average grain size preferably fit the experimental data, indicating the proposed PB model can precisely catch the thermal compression behaviors and microstructure evolution characteristics of the Al−Zn−Mg−Cu−Zr aluminum alloy.

Modelling of Microstructure Evolution during Laser Processing of Intermetallic Containing Ni-Al Alloys

There is a growing interest in laser melting processes, e.g., for metal additive manufacturing. Modelling and numerical simulation can help to understand and control microstructure evolution in these processes. However, standard methods of microstructure simulation are generally not suited to model the kinetic effects associated with rapid solidification in laser processing, especially for material systems that contain intermetallic phases. In this paper, we present and employ a tailored phase-field model to demonstrate unique features of microstructure evolution in such systems. Initially, the problem of anomalous partitioning during rapid solidification of intermetallics is revisited using the tailored phase-field model, and the model predictions are assessed against the existing experimental data for the B2 phase in the Ni-Al binary system. The model is subsequently combined with a Potts model of grain growth to simulate laser processing of polycrystalline alloys containing intermetallic phases. Examples of simulations are presented for laser processing of a nickel-rich Ni-Al alloy, to demonstrate the application of the method in studying the effect of processing conditions on various microstructural features, such as distribution of intermetallic phases in the melt pool and the heat-affected zone. The computational framework used in this study is envisaged to provide additional insight into the evolution of microstructure in laser processing of industrially relevant materials, e.g., in laser welding or additive manufacturing of Ni-based superalloys.

Additive manufacturing of a high strength Al-5Mg2Si-2Mg alloy: Microstructure and mechanical properties

A novel Al-5Mg2Si-2Mg alloy was processed by selective laser melting (SLM) to understand its representative features of microstructural evolution and mechanical properties during additive manufacturing (AM). The as-SLM fabricated Al-5Mg2Si-2Mg alloy is found to deliver much less hot cracks and other defects. Meanwhile, excellent mechanical property is also achieved, i.e. 452 ± 11 MPa for ultimate tensile strength, 295 ± 14 MPa for yield strength, and 9.3 ± 2.5% for elongation. Clearly, these mechanical properties are better than that obtained by high pressure die casting (HPDC), and better than some other alloys obtained by SLM. The as-SLM fabricated Al-5Mg2Si-2Mg alloy is featured by the significantly refined microstructures in the compact primary α-Al, the divorced Mg2Si eutectic networks distributing at α-Al grain boundaries, and some α-AlFeMnSi phase in association with eutectic Mg2Si phase. The high strength and ductility of the alloy are attributed to its unique features including (a) the reduced solidification range, (b) the possible increase of the eutectic level in the microstructure, and (c) the shift of eutectic point and the maximum solubility point of Mg2Si in Al matrix.

Hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel

The hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel were investigated at the temperature range of 950–1150 °C and strain rate varying from 0.01 to 10 s−1. A strain compensated constitutive equation based on the Arrhenius-type model was established to predict the flow stress. The hot processing map based on the dynamic materials model was achieved to identify the optimum processing parameters. In addition, the features of microstructure evolution combined with the processing map were systematically investigated. The experimental results revealed that the flow stress increased with decreasing deformation temperature or increasing strain rate. Dynamic recovery was confirmed to be the predominant softening mechanism. The values of flow stress predicted by the strain compensated constitutive equation agreed well with the experimental values. The extent of dynamic recrystallization and recrystallized grain size increased with increasing deformation temperature or decreasing strain rate, and the continuous dynamic recrystallization was attributed to be the predominant mechanism of recrystallization during hot deformation. The optimum hot working parameters were determined to be the deformation temperature of 1070–1150 °C and strain rate of 0.1–1 s−1 with a peak power dissipation efficiency of 42%.

Comparative Study of Microstructural Evolution and Mechanical Properties of Inconel® 718 and Waspaloy® Welds

This research work focuses on the mechanical behaviour comparative assessment in conjunction with microstructural evolution characterization of Waspaloy® and Inconel® 718, following TIG and EB welding. Both of the forth-mentioned alloys are precipitation strengthened Ni-based superalloys, widely used in chemical, petrochemical and aerospace industries. More specifically, Waspaloy® is strengthened by the precipitation of the ordered fcc gamma prime intermetallic phase, γ΄ - Ni3(Al,Ti), while Inconel® 718 is mainly hardened by the ordered bct gamma double prime phase, γ΄΄ - Ni3Nb, in addition to γ΄. After both welding processes, samples of the above superalloys were subjected to appropriate post-weld heat treatment, according to SAE Aerospace Material Specifications. The mechanical response of the tested specimens is assessed via uniaxial tensile tests, combined with fractography. Furthermore, the microstructural characterization of TIG and EB welds is conducted by Scanning Electron Microscopy (SEM), coupled with Energy Dispersive Spectroscopy (EDS), while phase identification was performed through X-Ray Diffraction (XRD). The main objective of the present research work is to examine the influence of post-weld heat treatment on the Waspaloy® and Inconel® EBW and TIG welds microstructural evolution features, correlating them with their corresponding mechanical behaviour.

Revisiting the lamellar globularization behavior of a two-phase titanium alloy from the perspective of deformation modes

Abstract The correlation between lamellar globularization and deformation modes during the primary hot processing of titanium alloy was evaluated by means of uniaxial compression, tension, torsion and plane strain compression tests. The microstructure evolution features and underlying globularization mechanisms were comparatively studied via Electron Backscattered Diffraction technique. Noticeable divergences of globularization response are captured in different deformation modes, especially for the globularized fractions and the critical strain. In the face of the strain applied is less than the critical globularization value (0.4−0.63) under uniaxial compression, the lamellae can be still globularized under tension or torsion deformation. But the growth rate of globularized fraction is more sluggish under tension than that under torsion deformation. The lamellae separation at early tension deformation preferentially occurs at junctions of lamellae without destroying Burgers Orientation relationship and significant orientation fluctuation. Shear deformation can facilitate the formation of intra-α boundaries and its distribution uniformity, which could make the lamellae grow up into globularized particles and improve the globularization heterogeneity. It is envisaged that the results obtained can provide guidance for the technology scheme and route design of primary hot working of titanium alloy aiming at the homogeneous equiaxed grains.

Specific Features of Microstructural Evolution During Hot Rolling of the As-Cast Magnesium-Rich Aluminum Alloys with Added Transition Metal Elements

This study addresses specific features of microstructural development during the rolling of the as-cast magnesium-rich aluminum alloys with added transition metal elements, such as Zr and Sc. For investigation purposes, three magnesium-rich aluminum alloys were chosen: 5182 without added Zr and Sc, 1565 ch with added Zr and 1570 with added Zr and Sc. Optical microscopy, X-ray texture analysis, electron microscopy, and electron backscattered diffraction methods were used in this study. This study demonstrates that two completely different patterns can be observed in the microstructure during deformation. Two zones with different subgrain sizes are formed when the alloy tends to recrystallize. Without recrystallization, the structure develops homogeneously. Recrystallization also has a significant effect on the texture formation. If recrystallization does not occur, a strong β-fiber texture is formed. However, this type of texture tends not to form during inter-deformation intervals when recrystallization occurs. Second-phase particles have the strongest effect on microstructural evolution during the hot deformation of the as-cast structure. Fine particles are capable of inhibiting recrystallization. Coarse particles initiate the PSN (particle-stimulated nucleation) mechanism and suppress the growth of the cubic texture component during recrystallization. An adequate combination of coarse and fine particles enables either better homogeneity of the structure distribution in the deformation center or better grain structure refinement.

Stress-induced detwinning and martensite transformation in an austenite Ni-Mn-Ga alloy with martensite cluster under uniaxial loading.

Stress-induced martensitic detwinning and martensitic transformation during step-wise compression in an austenite Ni-Mn-Ga matrix with a martensite cluster under uniaxial loading have been investigated by electron backscatter diffraction, focusing on the crystallographic features of microstructure evolution. The results indicate that detwinning occurs on twins with a high Schmid factor for both intra-plate and inter-plate twins in the hierarchical structure, resulting in a nonmodulated (NM) martensite composed only of favourable variants with [001]NM orientation away from the compression axis. Moreover, the stress-induced martensitic transformation occurs at higher stress levels, undergoing a three-stage transformation from austenite to a twin variant pair and finally to a single variant with increasing compressive stress, and theoretical calculation shows that the corresponding crystallographic configuration is accommodated to the compression stress. The present research not only provides a comprehensive understanding of martensitic variant detwinning and martensitic transformation under compression stress, but also offers important guidelines for the mechanical training process of martensite.

Microstructure evolution and grain refinement during hot deformation of Ti-44Al-4Nb-1.5Cr-0.1Mo-0.1B alloy

The principle of the forging and rolling combined process (FRCP), along with the major microstructure evolution features and grain refinement mechanism of Ti–44Al–4Nb–1.5Cr–0.1Mo–0.1B alloy, was investigated thoroughly in this study. A crack-free rolled sheet characterized by a bimodal microstructure with an average grain size of approximately 3.52 μm was produced after FRCP. The microstructure per deformation stage was further examined via transmission electron microscope (TEM) observation and electron backscatter diffraction (EBSD) analysis. The grain size of the material gradually decreased from 120 μm in the initial structure to 3.52 μm during hot deformation. Dynamic recrystallization (DRX) and deformation twinning should account for continuous grain refinement during FRCP. The refinement effect of the microstructure was quantitatively evaluated by explaining the mechanism of the grain boundary distribution. The fraction of the high-angle grain boundaries induced by DRX exceeded 80%, thereby indicating that the nucleation and growth of DRX were the dominant mechanisms for grain refinement. Annealing at 1150 °C after forging stimulated the activity of 89 ± 3° misorientation angles mostly located within the matrix grains. Dislocation slipping and deformation twinning played synergistic roles in the refinement process. Several possible mechanisms for microstructure refinement via deformation twinning were identified, including twin intersection, formation of secondary twins, and parallel twinning.

Achieving grain refinement and enhanced mechanical properties in Ti–6Al–4V alloy produced by multidirectional isothermal forging

This study investigated the principle of multidirectional isothermal forging (MDIF) and determined the major microstructural evolution features and unique room-temperature mechanical properties of extra-low interstitial-grade Ti–6Al–4V alloy. The grain refinement mechanism, grain boundary characteristics, and phase transformation during MDIF were explored. After three-step MDIF, a homogeneous microstructure with a grain size of about 0.5µm was produced. The ultrafine grained Ti–6Al–4V alloy exhibited high yield strength (1170MPa), high ultimate tensile strength (1190MPa), and good ductility (10.4%). The mechanism of grain refinement during MDIF included continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Grain subdivision resulted from CDRX or the transformation of cellular dislocation substructures into new ultrafine grains. The necklace of new DDRX grains formed along the initial grain boundaries of the Ti–6Al–4V alloy. The main strengthening mechanisms were grain boundary and dislocation strengthening. The strength and grain size followed the typical Hall–Petch relationship.

Coarsening Kinetics and Morphological Evolution in a Two-Phase Titanium Alloy During Heat Treatment

The effects of alpha/beta heat treatment on microstructure evolution of Ti-17 alloy with a lamellar colony structure are established. Heat treatment experiments are conducted at 1103 or 1063 K for times ranging from 10 min to 8 h. The main features of microstructure evolution during heat treatment comprise static globularization and coarsening of primary alpha phase. Such behaviors can be accelerated by higher heat treatment temperature. Furthermore, globularization and coarsening behaviors show a faster rate at higher prestrain. In order to better understand the microstructure evolution of Ti-17 alloy during alpha/beta heat treatment, static globularization and coarsening behaviors are modeled in the theoretical frame of the Johnson-Mehl-Avarmi-Kolmogorov (JMAK) and Lifshitz-Slyozov-Wagner (LSW) theories, respectively. The JMAK and LSW kinetics parameters are derived under different experimental conditions. Agreements between measurements and predictions are found, indicating that the JMAK and LSW theories can be used to predict and trace static globularization and coarsening processes of Ti-17 alloy during alpha/beta heat treatment.

Effect of specific features of the phase formation on structural transformations and the formation of properties of solid solutions of the PbZr1 − x Ti x O3 (0 < x < 1.00) system

The x-T phase diagram (T = 25°C) of the PbZr1 − xTixO3 system has been constructed in the entire solubility range of components 0 ≤ x ≤ 1.00. It has been found that the phase composition of solid solutions, the cell volume decreasing rate, the electrical parameters, and density of ceramics periodically vary with increasing x. It has been shown that the nonmonotonic behavior of the electrical parameters is caused by different contributions of the spontaneous strain dominant in the tetragonal phase and domain reorientations different from 180-degree ones in the rhombohedral phase to their formation. Detailed multifractal parametrization of the grain structure of ceramics has been performed. The microstructure evolution features associated with phase changes in the system have been revealed. The data of other studies that tetragonal phase clusters appear in the range 0.11 < x ≤ 0.12 are confirmed; it has been assumed that rhombohedral phase clusters appear in the range 0.675 < x < 0.80.

Microstructure evolution during isothermal forging and subsequent heat treatment of Ti-17 alloy with a lamellar colony structure

The effect of isothermal forging and subsequent heat treatment on microstructure evolution was investigated for Ti-17 alloy with a lamellar colony structure. For this purpose, Ti-17 alloy was isothermally forged at 820°C or 860°C and subsequently heat treated for times ranging from 10min to 8h. The principal features of microstructure evolution were found to be strain and temperature dependent. Microstructure evolution during deformation included the rotation of alpha lamellae toward the metal flow direction. The orientation distribution of alpha lamellae changed from the two symmetrical peaks to the single one with the increasing strain. The angle (θ) in the single peak position was about 90°. Additionally, length of the lamellar alpha decreased with strain and deformation temperature. Thickness of the lamellar alpha increased with deformation temperature but was independent on strain. During heat treatment, microstructure observations suggested that the process of evolution can be divided into two stages. The first included segmentation of the lamellae in the initial stage of heat treatment, whereas microstructure coarsening characterized the second one during prolonged heat treatment. Furthermore, the segmentation and coarsening processes could be accelerated by the increasing prestrain and temperature.

Phase transitions during artificial ageing of segregated as-cast U–Mo alloys

U–Mo alloys are a promising alternative in high-density fuel materials for use in research and test reactors, due to its resistance to swelling associated with the presence of the metastable γ-phase in the microstructure. It has been reported that increasing additions of Mo cause a beneficial delay in the decomposition of the γ-phase in U–Mo alloys during isothermal heat treatments. Analyses emphasized high temperature aging (>400°C), where the microstructural evolution features were followed by low resolution light microcopy, but little information is available for lower temperature treatments. High resolution microstructural characterization techniques allow a more in depth analysis of the decomposition, as already shown for U–Nb alloys. In this work, phase transitions resulting from the decomposition of the γ-phase, or its variants, were investigated in as-cast U–Mo alloys with 5, 7 and 10wt.% Mo, aged at 300 and 500°C. Characterization techniques employed light microscopy, hardness, high resolution FE-SEM and EFM, which establishes contrasts for domains with different physical properties. The ageing of Mo-segregated regions, typical of the as-cast state, allowed the consideration of a broad range of Mo contents. For the low Mo regions, results were similar to those reported for U–Nb alloys, involving transformation twins and disorder-ordering mechanisms in the low temperature ageing and the formation of a non-lamellar constituent (NL3) in the high temperature ageing. The precipitation of oriented acicular or Widmanstätten γ′ platelets over a γ-phase matrix was observed for the hypereutectoid compositions of the U–Mo system, while the interdendritic regions with Mo contents close to the eutectoid composition were initially untransformed. This indicates a maximum in the γ stabilizing effect for the eutectoid composition, in opposition to the commonly accepted increased stabilizing effect for raised Mo additions.

Influence of upscaling accumulative roll bonding on the homogeneity and mechanical properties of AA1050A

Accumulative roll bonding (ARB), as a method for production of ultrafine grained materials, is frequently supposed to be easily transferable to established industrial production lines. In literature, however, common sheet dimensions used for ARB in a laboratory scale are between 20 and 100 mm in width. In order to quantify the potential of upscaling the ARB process to a technological relevant level, sheets of AA1050A with an initial sheet width of 100–450 mm were accumulative roll bonded up to 8 cycles. In this regard, three different rolling mills of distinct dimensions were used for processing of the sheet material. The influence of process parameters and the reproducibility of the process, in terms of mechanical properties and homogeneity of the sheets, were studied by means of mechanical and microstructural characterization. Both appear to be largely independent on the sheet size and the rolling mill utilized for production. Only small deviations after the first cycles could be detected, vanishing in subsequent cycles due to the features of microstructural evolution. The finally obtained results indicate a high potential for industrial application of ARB and illustrate the possibility to upscale the process to a level necessary for that purpose.

Progress in mesoscopic modeling of microstructure evolution in steels

The mesoscopic modeling developed rapidly in the past three decades is a promising tool for predicting and understanding the microstructure evolution at grain scale. In this paper, the recent development of mesoscopic modeling and its application to microstructure evolution in steels is reviewed. Firstly, some representative computational models are briefly introduced, e.g., the phase field model, the cellular automaton model and the Monte Carlo model. Then, the emphasis is put on the application of mesoscopic modeling of the complex features of microstructure evolution, including solidification, solid-state phase transformation, recrystallization and grain growth. Finally, some issues in the present mesoscopic modeling and its perspective are discussed.