Microstructure Evolution Law Research Articles

Study on the macro-mechanical behavior and micro-structure evolution law of broken rock mass under triaxial compression

Study on the macro-mechanical behavior and micro-structure evolution law of broken rock mass under triaxial compression

Effect of multiple laser remelting on microstructure and corrosion resistance of Fe0.5CoCrNi1.5Nb0.68Mo0.3 high entropy alloy coatings

High entropy alloy (HEA) coatings of Fe0.5CoCrNi1.5Nb0.68Mo0.3 was prepared on 304 stainless steel using laser cladding and subsequently remelted multiple times. The effects of multiple thermal cycles on the phase composition, grain size, microstructure evolution, and corrosion resistance of the coatings are thoroughly investigated. The original coating consisted of face-centered cubic (FCC), Laves, and NbC phases, and the phase composition does not change obviously, but the distribution and microstructure of the phase change significantly after multiple remelting. As the number of remelting times increased, the bar-like eutectic structure decreased while the lamellar eutectic structure became more prominent. Laser remelting induced dynamic recrystallization in the coating, resulting in a transformation from columnar grains to equiaxed grains, and then back to columnar grains. The grain size also changes significantly with different remelting times. Potentiodynamic polarization and electrochemical impact spectroscopy measurements were conducted in a 3.5 wt% NaCl solution to evaluate the corrosion resistance of the coating. The results revealed both the original coating and the once-remelted coating exhibited lower corrosion current density and higher corrosion potential, but the latter had a higher Faraday impedance. Additionally, immersion tests were performed in 10 wt% FeCl3 solution, which demonstrated that the once-remelted coating displayed fine and uniform corrosion pits with shallow depth. This study provides theoretical support for the regulation of microstructure and the optimization of coating performance. Furthermore, the microstructure evolution law discovered in this research is also applicable to additive manufacturing.

Construction of a Predictive Model for Dynamic and Static Recrystallization Kinetics of Cast TC21 Titanium Alloy

In this study, hot compression experiments were conducted on cast TC21 titanium alloy using a Gleeble-1500D thermal simulation compression tester, and the hot-compressed specimens were heat-treated. The data obtained after analyzing the thermal compression of cast TC21 titanium alloy were analyzed to construct a thermal machining diagram with a strain of 0.8 and to optimize the machining window. This study investigated the microstructure of the alloy after hot pressing experiments and heat treatment, applying the study of the microstructure evolution law of cast TC21 titanium alloy. The analysis of the tissue evolution law established the dynamic and static recrystallization volume fraction as a function of heat deformation parameters. The results show that the optimal processing window for cast TC21 titanium alloy is a deformation temperature in the range of 1373 K–1423 K and a strain rate of 0.1 s−1. The increase in deformation volume and deformation temperature both favor recrystallization and make the recrystallization volume fraction increase, but the increase in strain rate will inhibit the increase in the recrystallization degree to some extent. The dynamic and static recrystallization equations for the cast TC21 titanium alloy at different temperatures were constructed. The experimental measurements of recrystallization volume fraction are in good agreement with the predicted values.

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.

Development of a novel fabricating thin-walled TA2 titanium tube via high-frequency induction welding

Due to the problems of low welding efficiency, large heat-affected zone, and poor welding quality in the process of welding thin-walled titanium tubes by argon arc welding, there are few studies on the use of high-frequency induction welding (HFIW) of thin-walled titanium alloy tubes. The evolution law of weld microstructure and mechanical properties of the thin-walled titanium tube needs to be further studied because of rapid welding speed and the small heat-affected zone of HFIW. Therefore, a novel manufacturing method via high-frequency induction welding is proposed in this paper to solve the existing problems. With an industrial-grade titanium TA2 tube (wall's thickness is 0.5 ​mm) as the research object, a comparative study is conducted in this research to examine the morphology, microstructure, microhardness, and tensile characteristics of welded joints at different welding power. The findings demonstrated a significant efficacy of HFIW in resolving these challenges. The mechanical properties and microstructue of heat-affected zone (HAZ) were characterized. The lowest hardness is measured at 202 HV, while the base material was recorded as 184 HV, when the welding speed of HFIW is set at 50 ​m/min. Meanwhile, the heat-affected zone has the highest hardness at 224 HV, a tensile strength of 446.8 ​MPa and a post-fracture elongation of 16%. The results showed that HFIW can not only greatly improve the welding efficiency, significantly improve the microstructure of weld joint and HAZ, and improve the mechanical properties of thin-walled titanium pipe, but also provide a highly feasible welding method for welding ultra-thin-walled pipes.

Study on microstructure and property evolution of additive manufactured 2219 aluminum alloy under unidirectional compression and multidirectional loading deformation

The additive manufacturing technology of high strength 2219 aluminum alloy has been widely used in aerospace field. In this paper, coarse microstructure and poor mechanical properties of as-deposited 2219 Al alloy was focused, unidirectional compression and multidirectional loading deformation were studied, and the evolution law of microstructure and tensile properties was analysed. The results show that the deformation mainly concentrated in the core of uniaxial compression sample. The dynamic recrystallization was dominant above 450 °C. The lamellar microstructure was com pletely eliminated after multidirectional loading deformation, and hydrogen pores were completely closed. The yield stress reached 224 MPa, the ultimate tensile stress reached 386 MPa, and the elongation reached 11%, indicating that the improvement effect on microstructure and properties was the best.

Effects of different scanning speeds on microstructure evolution and tribological properties of Inconel 718 alloy vacuum electron beam surface modification

In this paper, based on vacuum electron beam surface modification technology, the surface modification experiments of electron beams on Inconel718 plates at different scanning speeds (180 mm/min, 240 mm/min, 300 mm/min, 360 mm/min and 420 mm/min) were studied. The microstructure evolution law and friction and wear properties of electron back scatter diffraction (EBSD) and friction and wear testing machine RTEC (MFT-5000) were analyzed. Then, SEM and EDS were used to study the friction and wear mechanism. The results show that the microstructure of Inconel 718 sample after electron beam surface treatment is divided into basis material (BM), central zone (CTZ) and column zone (CLZ). The grains in the BM region are equiaxial, the grains in the CTZ region are fine strips, and the CLZ region is dominated by short coarse grains. After electron beam treatment, the annealed twins inside the specimen are eliminated, and when the scanning speed is 300 mm/min, the recrystallization ratio is the largest and the orientation difference becomes the most uniform. Different recrystallization textures appeared in different scanning speed specimens, and the textures in the original state are concentrated on copper{112}<111> and S{123}<634 > textures. After electron beam treatment, the texture of the specimen is mainly concentrated in the cubic texture. Experiments have shown that electron beam surface treatment can reduce oxidative wear and adhesive wear of specimens. When the scanning speed is 300 mm/min, the wear volume is the smallest, the wear resistance is the best, and the wear rate is reduced by about 15 % compared with the original specimen. Electrochemical corrosion experiments demonstrated that the relative corrosion rate of Inconel 718 was 1.3 times higher than that of the selected electron beam clad specimens.

Microstructure evolution law and dissolution time effect of bituminous coal under acetic acid acidification: Based on Micro-CT image analysis

Undeveloped pores and fractures in coal seams constrain coalbed methane mining and dust disaster prevention significantly. The existing hydraulic fracturing technology has achieved some effects in coal seam modification. Still, there are hidden dangers in inducing microseismicity and other aspects. Applying acidification technology for pore and fracture modification improves the efficiency of coalbed methane mining and dust suppression. In this paper, 75% concentration of acetic acid was utilized to treat columnar coal samples, and the experimental method of X-ray computed tomography was applied to study the role of dissolution time on the microcomponents and structure of samples. The experimental results showed that the porosity increased with the dissolution time; the effect of acetic acid on the pore expansion and mineral dissolution showed three periods, i.e., the beginning of the reaction period (0 ∼ 12 h), the full reaction period (12 ∼ 48 h), and the reaction slowing down period (>48 h). The mineral volume fraction showed an overall decreasing trend; before 36 h, the dissolution of acetic acid on the mineral structure within the coal was faster, making it loose, which in turn accelerated the dissolution of coal, and after 36 h, the dissolution effect of acetic acid weakened, and the mineral was gradually stabilized. Acetic acid first reacts with minerals in large quantities. With the increase of products, the solution's ionic complexity increases, so acetic acid accelerates the ionization of more H+ ions, and the solution's acidity increases. After a while of dissolution, the content of acetic acid decreases, the ionized H+ ions are not enough to continue to react with the minerals, and the rate of the mineral's crystal particle size reduction slows down gradually. In conclusion, acetic acid has the potential for pore and fracture modification of coal reservoirs. It can affect the mineral through dissolution, at the same time, improve the porosity. The research results provide an essential reference for using acidification technology to modify the pores and fractures in coal seams.

Mechanical properties and interface microscopic characterisation of fibre-reinforced slag-fly ash geopolymer agglutinated iron tailings filling materials

In order to study the deformation resistance of fibre-reinforced alkali activated slag-fly ash geopolymer agglutinated iron tailings filling materials, the mechanical properties test, scanning electron microscope (SEM) and X-ray diffraction (XRD) microscopic test are performed using the macro-micro cross-scale method. Then, the evolution law of mechanical behaviour and microstructure of samples are studied, revealing the internal relationship between macro-mechanical properties and fibre parameters. The results show that based on experimental results, as the fibre content increases, the compressive strength of the sample first increases and then decreases. When 6‰ 9-mm fibre is added to the sample, the compressive strength and flexural strength of the sample reach the maximum. Through SEM and XRD, it was found that fly ash and slag in an alkaline environment to generate polymerization products hydrated calcium silicate and hydrated calcium aluminosilicate filled in the basic skeleton formed by iron tailings sand particles, making the filling material more dense. When the fibre content is 4–5‰, the fibre exists as a single embedded fibre. When the fibre content is 6‰, the occurrence form shows a three-dimensional (3D) network structure, and when the fibre content exceeds 6‰, the fibre appears to be entangled in the sample. The mechanical properties of the filling material are improved by the single embedded fibre and the 3D reticular knot distribution fibre through the wrapping effect and the reticular inhibition effect, respectively. The results obtained can provide a reference for theoretical research and practical applications of polypropylene fibre-reinforced geopolymer materials.

Microstructural evolution and dynamic recrystallization model of extruded homogenized AZ31 magnesium alloy during hot deformation

Abstract Using a thermal simulation testing machine, hot compression experiments were carried out on extruded homogenized AZ31 magnesium alloy, and the hot deformation behavior was analyzed. Based on this, the constitutive equation of the alloy is constructed to explore the evolution law of microstructure during hot deformation of the alloy, which could provide theoretical guidance for the reasonable selection of parameter ranges during hot compression of extruded homogenized AZ31 magnesium alloy. The experimental result indicated that the flow stress of the alloy during the hot deformation decreases with increasing temperature, increases with the strain rate increasing, and the real stress–strain curves during deformation show dynamic recrystallization curves. According to the experimental results, the deformation activation energy Q is 126.882 kJ mol−1 and the stress exponent n is 4.36 calculated by the constitutive equation under the given parameters, which confirmed that the glide and climb of dislocations in the climb-controlled regime is the deformation mechanism in this work. Decreasing the compression temperature and increasing the strain rate are helpful to reduce the Zener–Hollomon parameter, control the dynamic recrystallization occurring, and refine grain size to improve the mechanical properties effectively. Moreover, the dynamic recrystallization model of the alloy was constructed using the work hardening rate method and regression method, including dynamic recrystallization critical condition model, dynamic model and grain size model.

A non-isothermal creep fracture prediction method of DS superalloy considering microstructural damage

Nonisothermal service conditions are one of the main abnormal failure modes of aero-engines. The creep tests of a Ni-based directionally solidified superalloy under nonisothermal creep conditions (Tover = 980 °C, 1000 °C and 1020 °C) and constant load (σ = 227 MPa) were analyzed by SEM. According to the creep tests, we calculate the key creep parameters. By studying tests interrupted at different stages of creep, the microstructure evolution law is obtained. Based on the above research, an improved constitutive model is proposed. The creep behavior under different Tover is simulated and calculated. The simulation results are in good agreement with the experimental results.

Research Progress on Microstructure Evolution and Strengthening-Toughening Mechanism of Mg Alloys by Extrusion.

Magnesium and magnesium-based alloys are widely used in the transportation, aerospace and military industries because they are lightweight, have good specific strength, a high specific damping capacity, excellent electromagnetic shielding properties and controllable degradation. However, traditional as-cast magnesium alloys have many defects. Their mechanical and corrosion properties cause difficulties in meeting application requirements. Therefore, extrusion processes are often used to eliminate the structural defects of magnesium alloys, and to improve strength and toughness synergy as well as corrosion resistance. This paper comprehensively summarizes the characteristics of extrusion processes, elaborates on the evolution law of microstructure, discusses DRX nucleation, texture weakening and abnormal texture behavior, discusses the influence of extrusion parameters on alloy properties, and systematically analyzes the properties of extruded magnesium alloys. The strengthening mechanism is comprehensively summarized, the non-basal plane slip, texture weakening and randomization laws are comprehensively summarized, and the future research direction of high-performance extruded magnesium alloys is prospected.

Research progress on microstructure evolution and hot processing maps of high strength β titanium alloys during hot deformation

High strength β titanium alloys are widely used in large load bearing components in the aerospace field. At present, large parts are generally formed by die forging. Different initial microstructures and deformation process parameters will significantly affect the flow behavior. To precisely control the microstructures, researchers have conducted many studies to analyze the microstructure evolution law and deformation mechanism during hot compression. This review focuses on the microstructure evolution of high strength β titanium alloys during hot deformation, including dynamic recrystallization and dynamic recovery in the single-phase region and the dynamic evolution of the α phase in the two-phase region. Furthermore, the optimal hot processing regions, instability regions, and the relationship between the efficiency of power dissipation and the deformation mechanism in the hot processing map are summarized. Finally, the problems and development direction of using hot processing maps to optimize process parameters are also emphasized.

Microstructure evolution and grain refinement mechanism of rapidly solidified single-phase copper based alloys

• The maximum undercoolings were achieved respectively by using the deep undercooling technology. • The recalescence behavior and microstructure refinement were systematically studied. • The relationship between solute composition and solidification process was established. • High-angle grain boundaries and twin boundaries exist in the microstructure at high undercooling. The Cu65Ni35, Cu60Ni40 and Cu55Ni45 alloys were undercooled by fluxing method, and the rapid solidification structure with different undercoolings were also obtained. At the same time, the interface migration process during rapid solidification was photographed by high-speed photography, and the relationship between the morphological characteristics of solidification front and undercooling was analyzed. The microstructures of the three alloys were observed by metallographic microscope, and the microstructure characteristics and evolution law were systematically studied. It was found that two grain refinement events occurred in the low undercooling range and high undercooling range, respectively. The EBSD test of grain refined microstructures showed that the microstructure in the low undercooling range has a high proportion of low-angle grain boundaries and high strength textures. However, there were a large proportion of high-angle grain boundaries and a high proportion of twin grain boundaries and more randomly oriented grains in the microstructure in the high undercooling range. The TEM test of the Cu55Ni45 alloy with the maximum undercooling of 284 K showed that there were high-density dislocation networks and stacking faults in the grains. Finally, the evolution relationship between microstructure hardness and undercooling was systematically studied. It was found that the microhardness of the three alloys decreased sharply near the critical undercooling. Combined with EBSD, TEM and microhardness analysis, it was confirmed that the grain refinement under low undercooling was caused by dendrite remelting, while the grain refinement under high undercooling was caused by stress-induced recrystallization.

Tailoring high-temperature oxidation resistance of FeCrMnVSix high entropy alloy coatings via building Si-rich dendrite microstructure

In this work, a novel FeCrMnVSix (x = 0, 0.5, 1.0, 1.5, 2.0) high entropy alloy (HEA) coatings with dendrite structure were successfully designed and prepared by laser cladding technology. The evolution law of the coatings' microstructure and high-temperature oxidation resistance behavior were systematically studied, and the influencing mechanism of Si in FeCrMnVSix HEA was proposed. Results showed that the HEA coatings were dense and uniform, exhibiting the characteristics of metallurgical bonding. And the HEA coatings had a single BCC solid solution structure. Moreover, the addition of Si aggravated the degree of lattice shrinkage and lattice distortion of HEA. Based on the non-equilibrium solidification effect of the laser cladding, the Si-rich dendritic structure was formed in the coating, which was strengthened with the increase of Si content. Additionally, the high-temperature oxidation resistance of HEA coatings was significantly improved. The Si2.0 HEA coating reduced the mass gain by about 75%, compared to the substrate. The excellent oxidation resistance was attributed to the thermodynamically preferential oxidation of Si in Si-rich dendritic structures, further forming a dense SiO2 layer on the surface of the HEA coatings.

Investigating the multiscale effects of a novel post–weld composite processing treatment on minimizing weld softening in AA6061-T6 Al–alloy

The fusion welding of heat-treatable AA6061-T6 aluminum alloy results in significant joint softening which results in the ultimate tensile strength (UTS) and elongation (El) of the welded joint only reaching up to 60–70% and 40–50% of the base metal (BM), respectively. A post–weld composite processing treatment (PWCPT) that uses solution treatment (ST), artificial aging treatment (AT) and cold rolling (CR) has been proposed in this study to mitigate this issue. The effect of different composite process sequences and parameters on the microstructural characteristics and mechanical properties of the joints were investigated. The results show that the formation of the fine-grained microstructure in the fusion zone (FZ) and abnormal sandwich microstructure in the heat-affected zone (HAZ) was highly dependent on the dislocation density and fraction of the insoluble phases. By clarifying the strain characteristics of the joints, a strategy was developed to optimize the homogeneity of the microhardness distribution which improves joint ductility. The optimal PWCPT involved the ST and AT for the AA6061-T6 joint that was produced by tungsten inert gas (TIG) welding with filler wire, with subsequent cold rolling of the weld reinforcement to achieve a rolling reduction of 1.2 mm. The PWCPT could completely eliminate joint softening and improved the joint UTS and El to 100% and 88.6% of the BM, respectively. By systematically expounding the evolution law of microstructure and mechanical properties of the welded joint under the coupling of thermal and mechanical, the strengthening mechanism of the PWCPT is revealed.

Unconfined compressive strength and pore structure evolution law of structural clay after disturbance

The clay in the Zhanjiang Formation has thixotropic properties, which has greatly influenced the foundation engineering in the Zhanjiang area. The evolution law of macroscopic strength and clay microstructure during thixotropy can be used to explain the practical engineering problems caused by thixotropy. For undisturbed and reconstituted soil curing for a different period, unconfined compressive strength test, scanning electron microscopy, and mercury injection porosimetry test were carried out to obtain the unconfined compressive strength and pore structure evolution law in the thixotropic process. The results indicate that the Zhanjiang Formation structural clay is very sensitive to disturbance and its unconfined compressive strength decreases from 180.29 to 11.73 kPa after the natural structure is completely destructed. After 300 d of curing, the unconfined compressive strength of clay increased from 11.73 to 53.43 kPa because of thixotropy, which increased by 3.55 times. The stacking flaky flocculation structure of the undisturbed soil is destructed by reconstituting, turning to flaky flocculation structure, and the large pores are homogenized, the small pores develop into medium pores, and there is a decrease in soil strength. In the process of thixotropy, the soil particles gradually coagulate and form an aggregates flocculation structure, and the strength of clay increases with the increase in the degree of cementation. Based on the results, the thixotropic pattern of clay was established and its thixotropic mechanism was explained.

Microstructure evolution and process optimization of molybdenum rods during loose tooling forging

Defects such as abnormal grain growth and cracking are easy to occur in the process of loose tooling forging of large-size molybdenum rods prepared by powder metallurgy. In this work, the forging process of molybdenum rods was simulated, which were heated to 1350 °C, and one end of them was subjected to loose tooling forging with deformation of 20%, 30% and 40%, and then they were subjected to secondary heating at 1200 °C, 1250 °C, 1300 °C and 1350 °C for 60 min. The microstructure of the molybdenum rods was characterized by optical microscope (OM), and the equivalent stress, equivalent strain and damage distribution characteristics of during the forging process were calculated by Deform-3D finite element software. The microstructure evolution law and crack formation mechanism during deformation were analyzed. The results show that when the two ends of the molybdenum rods were separately forged and heated, incomplete static recrystallization was easy to occur in the middle gradient deformation zone due to the uneven distribution of stress and strain, resulting in the abnormal growth of local grains. In the subsequent forging process, the surface of the molybdenum rods was cracked due to the excessive stress at the grain boundary of the coarse grains in the transition zone. The initial temperature and softening temperature of static recrystallization in tempering process were different with the deformation of molybdenum rods. Finally, the process optimization of molybdenum rods forging was put forward.

Study on the microstructure and properties of iron-based composites locally reinforced by in-situ submicron TiC particles

This study discussed the strengthening mechanism of in-situ submicron TiC particles on high chromium cast iron matrix composites. By changing the Ti content, the microstructure evolution and performance change law of the composites are analyzed. The results show that the average particle size of the TiC particles will grow with the increase of the Ti content in the preforms. After the in-situ reaction, there will be a transition layer of 20–30 μm in the interfacial zone of the composite, and a high density of dislocations will appear at the same time. Furthermore, the presence of submicron TiC particles in the microstructure can significantly improve the strength and wear resistance of the composites. By contrast, only the Ti content in preforms is 50%, the microstructure and performance of the test steel (S2 steel) are the most ideal. The TiC particles in the composite area account for more than 70%, and the hardness is as high as 1236 HV, which is more than twice the hardness of the matrix. Under the same wear conditions, the mass loss and wear rate of S2 steel are the smallest, which are only 0.2 ± 0.1 mg and 3.08 × 10−6 ± 0.05 mm3/Nm, respectively.

Microstructural evolution and mechanical properties of pure Zn fabricated by selective laser melting

There are few studies on the microstructure formation mechanism and microstructure evolution of selective laser melting (SLM) of pure Zn. In this study, the process parameters of SLM pure Zn were optimized first, and the effect of process parameters on its mechanical properties was explored. With the increase of laser energy density, the strength and ductility of the SLM Zn were improved. The formation mechanism and evolution law of SLM Zn were also explored. The microstructure of SLM pure Zn was equiaxed grains and lamellar structure. The formation of equiaxed grains was due to the low temperature gradient and high scanning speed of SLM pure Zn, which is finally located in the E area of the CET transition. This paper mainly clarifies the microstructure formation mechanism and evolution law of selective laser melting of pure Zn, and provides theoretical bases for the application of SLM pure Zn in medical implants.