Growth Of Recrystallized Grains Research Articles

A novel strategy to enhance the interfacial bonding quality of CoCrFeMnNi high-entropy alloy joints produced by vacuum hot-compression bonding

The strain history of strain rate transient changes was introduced into the vacuum hot-compression bonding (VHCB) process of CoCrFeMnNi high-entropy alloy, which rapidly improved the interfacial bonding quality of the joint. The results of interfacial microstructure characterization indicated that the high-density dislocations generated in the interfacial region during the initial high strain rate stage not only promoted the nucleation of the interfacial recrystallized grains but also induced the transformation of the sub-boundaries and the interfacial grain boundaries (IGBs) to the high mobility twin boundaries (TBs). The accelerated discontinuous dynamic recrystallization process, the growth of TB-type continuous dynamic recrystallization grains, and the interfacial TB migration during the transient VHCB dramatically enhance the IGB migration level of the joints. This study provides an effective strategy for the interfacial regulation of high-quality VHCB joints.

Grain boundary migration behavior and microstructure evolution during multilayer additive hot-compression bonding

Multilayer additive hot-compression bonding (MAHCB) was a new technology to prepare large-size homogeneous materials, the strain hardening at the bonding interface was an urgent issue for its application. In this paper, the effect of grain boundary migration on strain hardening was studied by the MAHCB process of pure copper. The results show that prolonging the holding time can significantly promote both interfacial grain boundary migration (IGBM) and matrix grain boundary migration (MGBM). The bonding interface became tortuous due to the promotion of IGBM, accompanied by the reduction in dislocation density at the interface and the gradual elimination of interfacial defects. It occurred mainly by the evolution of the triple junction grain boundary from T-type to equal type, which was driven by the decrease in the interface energy. On the other hand, the growth of dynamic recrystallization (DRX) grains was improved with MGBM, and enlarged the low dislocation density region. With the combined effect of both, strain hardening was completely eliminated, when the strain was 0.15 and the holding time was 3 h. Then the optimal bonding properties were obtained, the average tensile-shear strength and elongation were 191.2 MPa and 212 %, and the recovery rates were 99.7 % and 98.9 %, respectively, compared to the as-cast pure Cu samples.

Effect of Ti particles on microstructure and mechanical properties of Mg–9Al–1Zn based composite sheets

It is a great challenge to simultaneously improve the strength and ductility of magnesium matrix composites (MMCs). In this work, AZ91 alloys reinforced with 1 wt%, 2 wt%, and 3 wt% Ti particles were produced using stir casting combined with ball milling process. The microstructure of the extruded composite sheets was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscatter diffraction (EBSD). Transmission electron microscopy (TEM) was utilized to observe the structure of the Ti/Mg interface. The mechanical properties of the composite sheets were also investigated. The results show that the adding Ti particles, the texture strength decreased, and the growth of recrystallization grains was also restrained. Due to the interfacial reaction between Ti and Al, the formation of β-phase (Mg17Al12) was inhibited. The tensile test demonstrated that the Ti particles could significantly improve the mechanical properties of the AZ91 alloy sheets. Compared to AZ91 sheet, 2Ti/AZ91 composite sheet along extrusion direction (ED) showed the best comprehensive mechanical properties. The improvement in mechanical properties was mainly attributed to grain refinement, texture weakening, mismatched coefficient of thermal expansion (CTE), and work hardening.

Dynamic Constitutive Relationship of Mg–Gd–Y–Zr–Ag Alloy during High Temperature Deformation Process

The thermal deformation behavior of the Mg-Gd-Y-Zr-Ag alloy was studied by isothermal hot compression tests at high temperatures. The flow stress increased with increased strain rates and decreased temperatures, first increasing and finally remaining stable with increased strain. A hot processing map was built. Using the processing map and microstructural analysis, the temperature should remain at 673-773 K for this alloy to ensure the deformation quality. The primary softening mechanism is discontinuous dynamic recrystallization (DDRX). Rising temperatures and declining strain rates facilitated the emergence and growth of Dynamic recrystallization (DRX) grains. An original JC (O-JC) model and a modified JC (M-JC) model were established. The M-JC model indicated a better prediction than the O-JC model. Still, it was deficient in predicting flow stresses with insufficient coupling effects. Hence, based on the M-JC model, a newly modified JC (NM-JC) model, which further enhances the interaction between strain and strain rate as well as strain and temperature, is proposed. Its projected values can better align with the tested values.

Oxidation-induced recrystallization and damage mechanism of a Ni-based single-crystal superalloy during creep

The creep tests for Ni-based single-crystal superalloy are carried out under 980 °C, and the microstructure at the crack tip is characterized at micro- and nano- scales. Numerous γ’ phase dissolution, nucleation and growth of fine grains are observed near the crack. In addition, γ’ phase did not effectively hinder the nucleation and growth of recrystallization grains at the oxidation-affected zone. Instead, recrystallization promoted the degradation of the γ’ phase to dissolve along the recrystallized grain boundaries. Combining the sub-grain boundary growth model proposed by Winning et al. and the discontinuous recrystallization model suggested by D.G. Cram, a recrystallization and oxidative damage model for Ni-based single-crystal superalloy is proposed. • Local polycrystallization or recrystallization will greatly damage the high-temperature properties of the alloy. From the point of view of Ni-based single-crystal superalloy design, polycrystallization during medium-high temperature creep is very rare and should be strictly avoided. • Many researches have paid attention to the recrystallization of polycrystalline Ni-based superalloys. However, there is no report about the appearance of polycrystallization region in Ni-based single crystal superalloys during creep. Additionally, systematic research and explanation on the mechanism of discontinuous recrystallization of Ni-based SC superalloy are also very lacking. • In this paper, it is found for the first time that in the process of medium-high temperature (980 °C) creep under low stress level, polycrystallization region and a large amount of γ’ phase dissolve appeared at the crack tip, which leads to a sharp decline in the properties of the alloy. Researchers believed that the γ’ phase is the main factor that hinder the recrystallization of Ni-based single-crystal alloy. Nevertheless, this paper propose that the local strain and depletion of solute atoms makes the sub-grain boundaries tend to become the nucleation of recrystallization.

Microstructure evolution and hot deformation characteristics of 15Cr-22Ni iron-base superalloy

The hot deformation behavior and microstructure evolution of a newly designed 15Cr-22Ni iron-based superalloy containing 0.6 wt %Nb was studied. The strain-compensated constitutive equations were established, which can accurately predict the high-temperature flow behavior of the superalloy. The dynamic recrystallization kinetic model was developed. The degree of dynamic recrystallization gradually increases with decreasing strain rate and increasing deformation temperature. The hot processing map of designed superalloy was developed and the optimum hot processing range has been determined to be 1050–1150 °C/0.01 s−1. Hot deformation induces the precipitation of nano-sized NbC at grain boundaries and dislocations in the designed superalloy. Nano-sized NbC particles act as nucleation sites for dynamic recrystallization. The dynamic recrystallization behavior is compared with that of 15Cr-22Ni-1 Nb alloy. The starting of dynamic recrystallization of designed superalloy is later than that of 15Cr-22Ni-1 Nb alloy, which is attributed to the lack of active sites in the early recrystallization nucleation. However, the recrystallization kinetics of designed superalloy is faster than that of 15Cr-22Ni-1 Nb alloy. Due to the weak pinning force of nano-sized NbC precipitates to the grain boundaries and dislocations, the movement of grain boundaries and dislocations is less hindered, which is beneficial to the growth of dynamic recrystallization grains.

Multi-scale simulation of deformation and recrystallization of austenitic stainless steels with solidified columnar crystal structures

Columnar grain structures (CCS) often distinct in steel ingots, which have to be refined and homogenized during forging. In this investigation, simulation of deformation and dynamic recrystallization of austenitic stainless steels with CCSs were carried out by macroscopic, mesoscopic (microscopic) and nanoscale simulation techniques. (1) Using molecular dynamics simulation method, the nano-CGSs model with different loading directions was simulated. The results show that the deformation stresses are anisotropic with variation of angles between the loading direction and the columnar crystal growth direction. The higher stresses present at the 0° and 90° angles due to higher dislocation density; However, the lower stresses present at the angles from 30° to 60° due to higher stacking faults and twins. (2) The cellular automata (CA) fractal rules were proposed to simulate nucleation and grain growth of dynamic recrystallization by introducing weighted variables considering Σ3 twin nucleation rate for the twinning-promoted recrystallization. The CA method of nucleation at primary columnar crystal boundaries, secondary dendrites and deformation bands was proposed to simulate the joint nucleation. (3) The coupling simulation of macroscopic thermal parameters by finite element method, solidification of columnar structures and hot deformation dynamic recrystallization by CA was realized.

Effect of Nb on Hot Deformation Behavior and Microstructure of Fe–6.5 wt% Si High‐Silicon Electrical Steel

Hot rolling is one of the key processes in the new casting–rolling forming process of high‐silicon electrical steel. The hot deformation characteristics, microstructure, and defects after deformation strongly influence the formability in subsequent warm‐ and cold‐rolling processes. This study investigates the hot compression behavior of Fe–6.5 wt% Si–0.15wt%Nb and Fe–6.5 wt% Si samples at 800–1100 °C as well as the effect of Nb on the flow stress, dynamic recrystallization, and ordered structure of high‐silicon electrical steel. The results show that the grain size of the Nb‐containing as‐cast samples is small. The Nb‐rich second‐phase particles can inhibit the growth of dynamic recrystallization grains during hot compression and refine the dynamic recrystallization microstructure; consequently, the hot activation energy of Nb‐containing samples is 11.8% higher than that of Nb‐free samples. Microcracks appear in the Nb‐free samples at temperatures of 800 and 1100 °C and strain rates of 1 and 10 s−1; the addition of Nb can effectively inhibit the generation of microcracks during hot deformation. The addition of Nb reduces the ordered degree of high‐silicon electrical steel after hot compression, which is beneficial to the subsequent warm‐ and cold‐rolling processes.

Influence of Mo on the dynamic recrystallization behavior of Al–Cu–Mg–Ag alloy

High-throughput double cone specimens were used to study the influence of Mo on the microstructure evolution of Al–Cu–Mg–Ag alloy under a wide range of strain along the axis. The microstructure evolution was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results demonstrated that Al6(Fe, Mn, Mo) dispersoids formed in the Al–Cu–Mg–Ag alloy with the addition of 0.2% Mo during double-step homogenization. Meanwhile, rod-like Al20Cu2Mn3 and disk-like Al6(Fe, Mn, Mo) dispersoids segregated to opposite distribution in interdendritic regions and dendrite cores, respectively, which obviously eliminated the dispersoid-free zones and intragranular sparse regions. With different effective strains along the axis, Al6(Fe, Mn, Mo) dispersoids obviously reduced the transformation from Low-angle grain boundaries to High-angle grain boundaries. The nucleation and growth of dynamic recrystallization grains were effectively inhibited by the coupling of Al20Cu2Mn3 and Al6(Fe, Mn, Mo) dispersoids due to the obstruction of dislocation movement.

Internal friction and heat resistance of Al, Al–Ce, Al–Ce–Zr and Al–Ce–(Sc)–(Y) aluminum alloys with high strength and high electrical conductivity

To study the application performances of new aluminum alloy conductors, dynamic mechanical analysis and microstructure observation were used to characterize the heat resistance and internal friction. Severe recrystallization occurred in Al and Al–0.2Ce alloy above 320 °C. The hardness and electrical conductivity of Al decreased from 40.3 to 22.2 HV and 61.58 to 60.87 %IACS after temperature scanning from 50 to 500 °C, which was mainly attributed to the enhancement of electron scattering by recrystallized grain boundaries. The co-addition of Sc and Y element can obtain the best heat resistance. The diameter of Al3Sc precipitates enriched with Y atoms is lower than 40 nm under high temperature (400–500 °C) and the recrystallization behavior can be effectively inhibited by Al3Sc precipitates. The internal friction of Al, Al–0.2Ce and Al–0.2Ce–0.1Y alloys mainly originates from dislocation motion before PR peaks and the growth of recrystallization grains after PR peaks. The internal friction of Al–0.2Ce–0.2Sc and Al–0.2Ce–0.2Sc–0.1Y alloys increases evenly because of the strong pinning effect of Al3Sc precipitates on dislocations and boundaries. The internal friction is composed of weak dislocation motion and boundaries relaxation. Finally, the internal friction of Al–0.2Ce–0.12Zr alloy under high temperature is complicated and affected by the precipitation behavior of Al3Zr precipitates, dislocations motion, the interaction between Al3Zr precipitates and dislocations, recrystallization processes.

Study on the Dynamic Recrystallization Behavior of 47Zr-45Ti-5Al-3V Alloy by CA-FE Simulation.

The dynamic recrystallization (DRX) behavior of 47Zr-45Ti-5Al-3V alloy was studied by using the experiment and numerical simulation method based on DEFORM-3D software and cellular automata (CA) over a range of deformation temperatures (850 to 1050 °C) and strain rates (10−3 to 100 s−1). The results reveal that the DRX behavior of 47Zr-45Ti-5Al-3V alloy strongly depends on hot-working parameters. With rising deformation temperature (T) and decreasing strain rate (), the grain size () and volume fraction () of DRX dramatically boost. The kinetics models of the and of DRX grains were established. According to the developed kinetics models for DRX of 47Zr-45Ti-5Al-3V alloy, the distributions of the and for DRX grains were predicted by DEFORM-3D. DRX microstructure evolution is simulated by CA. The correlation of the kinetics model is verified by comparing the and between the experimental and finite element simulation (FEM) results. The nucleation and growth of dynamic recrystallization grains in 47Zr-45Ti-5Al-3V alloy during hot-working can be simulated accurately by CA simulation, comparing with FEM.

Microstructure and mechanical properties of Mg-2Zn-0.3Ca-0.5La magnesium alloy extruded with different temperatures

The extrusion tests of the Mg-2Zn-0.3Ca-0.5La (ZXLa200) alloy were carried out at different extrusion temperatures (260 °C–320 °C), and the mechanical properties and microstructure were studied and analyzed by means of optical microscope, scanning electron microscope, x-ray diffraction, transmission electron microscope, and tensile testing. The research have demonstrated significant influence of Extrusion temperature towards grain size, recrystallization grains and texture. With the extrusion temperature increases, the recrystallization grain of the extruded ZXLa200 alloy increases, but the texture intensity decreases. During the extrusion process, a large amount of Ca2Mg6Zn3 (50–200 nm) precipitates are precipitated, which inhibited the growth of dynamic recrystallization grains and improved the yield strength of the alloys. The tensile yield strength and elongation of extruded ZXLa200 alloy were acquired to be 375 MPa, 5.9% at 260 °C and 310 MPa, 19.2% at 320 °C.

Evaluating compressive property and hot deformation behavior of molybdenum alloy reinforced by nanoscale zirconia particles

The paper deals with the fabrication of molybdenum alloy bars with different ZrO2 content via a series of processes, involving hydrothermal synthesis, co-precipitation, co-decomposition, powder metallurgy and rotary swaging. The compressive properties of molybdenum alloy bars were tested at various temperatures from room temperature to 1400 °C, and hot deformation behavior at different temperatures and strain rates was studied. The results show that ZrO2 particles with the size of tens of nanometers are evenly dispersed in Mo matrix, and ZrO2(110) and Mo(110) have a non-coherent interface due to a larger lattice misfit 13.57%. ZrO2 particles could significantly reduce grain size of molybdenum alloy and improve its compressive properties. When the volume content of zirconia increases from 0% to 2.0%, the grain size of molybdenum alloys annealed at 1200 ℃ decreases sharply from 55.9 µm to 3.5 µm. The compressive yield strength (CYS) of Mo-2.0 vol%ZrO2 annealed at 1200 °C is 674 MPa at room temperature, which is 65.6% higher than that of pure Mo. The high temperature CYS of Mo-2.0 vol% ZrO2 at 1400 °C is 176 MPa, increasing 49.6% higher than that of pure Mo. Compared with the previous works, the peak flow stress of the new Mo-2.0 vol%ZrO2 has been significantly improved. Adding of 2.0 vol%ZrO2 increases the initial temperature of dynamic recrystallization to 1200 °C, and effectively prevents the grain growth of dynamic recrystallization even at the increased deformation temperature of 1400 °C.

Effect of Different Deformation Methods on Microstructure Evolution of TC4 Titanium Alloy Prepared by Spark Plasma Sintering

TC4 titanium alloy powder was made into the test piece by spark plasma sintering. The hot deformation of TC4 titanium alloy was performed on the Gleeble-1500 experiment machine. The effects of microstructure were studied. The results show that the number of passes increased with the increasing of the relative density at the same amount of deformation. The relative density have an important influence in one-pass deformation. At the deformation temperature of 950°C and a strain rate of 0.1s-1, the microstructure of TC4 titanium alloy was the weave-basket structure, and the widmanstatten structure existed. The increase in the number of passes could promote the growth of dynamic recrystallization grains under the same amount of deformation.

Evolution of microstructure, texture, and mechanical properties in a twin-roll cast AA6016 sheet after asymmetric rolling with various velocity ratios between top and bottom rolls

In this paper, we report the evolution of microstructure, texture, and mechanical properties of an AA6016 sheet prepared by twin-roll casting and subsequent asymmetric rolling (ASR). By controlling the velocity ratio (VR) of upper and lower rollers, it is determined that a strong shear strain is induced along the sheet thickness during the ASR process, which results in the formation of shear texture with a {111}//normal direction (ND). When the deformation sheets are annealed at 330 °C, the number and size of grain nuclei are increased with an increase in VR after partial recrystallization. When the sheets are completely recrystallized, different recrystallization mechanisms in symmetrically rolled (SR) and ASR sheets can be determined. The main recrystallization mechanism of the SR sheet is characterized as discontinuous recrystallization (DRX), while continuous recrystallization (CRX) contributes to the development of recrystallization in the ASR sheet, which is accompanied by the formation of finer-grain-band/ultrafine grains. Moreover, compared to that of annealed SR sheets, the shear texture ({111}//ND) of the annealed ASR sheet is enhanced during the nucleation and grain growth of recrystallization while the cube texture weakens, which results in a distinct increase in the Lankford coefficient for the annealed ASR sheet.

A Mechanism for the Evolution of Hot Compression Cracking in Inconel 625 Alloy Ingot With Respect to Grain Growth

Hot compression cracking of as‐cast Inconel 625 alloy is investigated utilizing high temperature and high strain rate compression tests. The formation mechanism of the hot crack is systematically investigated based on grain growth and crystal slipping. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analysis are used to characterize the crack morphology and microstructure. SEM results showed that the hot cracking during hot compression of Inconel 625 alloy is determined to be quasi‐cleavage fracture. The formation of carbide (NbC) plays a key role in the formation and propagation of the hot crack. The various slip systems activation strongly contributes to the overall growth of the dynamic recrystallization (DRX) grains. In addition, subgranular boundary formation within the grown grains is attribution to various slip systems activation in different local regions. The initiation and propagation of the hot compression cracks in Inconel 625 alloy depend on the development of the subgranular boundary within the grown grain.

Effects of Ce-Rich Mischmetal on Microstructure Evolution and Mechanical Properties of 5182 Aluminum Alloy.

This paper addresses the effects of Ce-rich mischmetal on the microstructure evolution of a 5182 aluminum alloy during annealing and rolling processes. The Ce-rich mischmetal was added to an as-cast 5182 aluminum alloy in an induction furnace, and this was followed by homogenized annealing at 450 °C for 24 h and a rolling operation. The microstructure evolution and mechanical properties’ analysis of the 5182 Al alloy were characterized. The results show that the Ce-rich mischmetal could modify the microstructure, refine the α-Al grains, break the network distribution of Mg2Si phases, and prevent Cr and Si atoms from diffusing into the Al6(Mn, Fe) phase in the as-cast 5182 Al alloys. Ce-rich mischmetal elements were also found to refine the Al6(Mn, Fe) phase after cold rolling. Then, the refined Al6(Mn, Fe) particles inhibited the growth of recrystallization grains to refine them from 10.01 to 7.18 μm after cold rolling. Consequently, the tensile strength of the cold-rolled 5182 Al alloy increased from 414.65 to 454.34 MPa through cell-size strengthening, dislocation density strengthening, and particle strengthening. The tensile strength of the recrystallization annealed 5182 Al alloy was increased from 322.16 to 342.73 MPa through grain refinement strengthening, and this alloy was more stable after the recrystallization annealing temperature.

Enhanced Performance of Mg–Zn–Y–Mn Alloy via Minor Ca Addition

Herein, the effect of calcium (Ca) microalloying on the microstructure and mechanical properties of Mg94Zn2.5Y2.5Mn1 alloy is identified. The long‐period stacking‐ordered (LPSO) phase transformation mechanism during solid‐solution treatment and hot deformation behavior during extrusion process are analyzed. The role of minor (0.34 at%) Ca addition in the as‐cast alloy is related to the fine microstructure and abundant 18R‐LPSO phase. Furthermore, the precipitation of 14H‐LPSO phase is greatly induced by Ca addition in subsequent solid‐solution treatment. As for as‐extruded alloys, the dynamic precipitation behavior of W phase nanoparticles is found. The high‐density W phase nanoparticles suppress the dynamic recrystallization (DRX) behavior and inhibit the growth of dynamical recrystallized (DRXed) grains. The as‐extruded Ca‐modified alloy shows a superior mechanical property with yield strength (YS), ultimate tensile strength (UTS), and elongation of 400, 434 MPa, and 19.5%, respectively. The tiny microstructure, W phase nanoparticles, and kinky LPSO (both 18R and 14H type) phase are the reasons for the outstanding mechanical properties.

The microstructure and mechanical properties of as-extruded Ca-containing Mg–6Zn–3La alloys

Effects of Ca additions (0, 0.3 and 0.5 wt%) on the microstructure and mechanical properties of the as-extruded Mg–6Zn–3La alloys are investigated by OM, XRD, SEM, TEM and tensile testing. With the addition of Ca, a small number of Ca atoms is dissolved in Mg–Zn–La phase, fine Ca2Mg6Zn3 particles with diameter of 50–150 nm are also found in the Ca-containing Mg–6Zn–3La alloys. These precipitates can inhibit growth of the dynamic recrystallization grains during extrusion, which play an important role on the improvement of yield strength. The ultimate tensile strength, yield strength and elongation of the as-extruded Mg–6Zn–3La–0.5Ca alloy are 357 MPa, 345 MPa and 13%, respectively. Compared with the Ca-free alloy, the ultimate tensile strength, yield strength and elongation are improved by about 37%, 53% and 24% respectively.

Microstructure Evolution and Surface Cracking Behavior of Superheavy Forgings during Hot Forging

In recent years, superheavy forgings that are manufactured from 600 t grade ingots have been applied in the latest generation of nuclear power plants to provide good safety. However, component production is pushing the limits of the current free‐forging industry. Large initial grain sizes and a low strain rate are the main factors that contribute to the deformation of superheavy forgings during forging. In this study, 18Mn18Cr0.6N steel with a coarse grain structure was selected as a model material. Hot compression and hot tension tests were conducted at a strain rate of 10−4·s−1. The essential nucleation mechanism of the dynamic recrystallization involved low‐angle grain boundary formation and subgrain rotation, which was independent of the original high‐angle grain boundary bulging and the presence of twins. Twins were formed during the growth of dynamic recrystallization grains. The grain refinement was not obvious at 1150°C. A lowering of the deformation temperature to 1050°C resulted in a fine grain structure; however, the stress increased significantly. Crack‐propagation paths included high‐angle grain boundaries, twin boundaries, and the insides of grains, in that order. For superheavy forging, the ingot should have a larger height and a smaller diameter.