Nucleation Mechanism Of DRX Research Articles

Effect of Deformation Conditions on the Dynamic Recrystallization of GH690 Alloy

Abstract Hot compression tests were conducted using a Gleeble–3500 thermomechanical simulator to investigate the effect of hot deformation conditions on dynamic recrystallization (DRX) of GH690 alloy. The results show that the DRX process of GH690 alloy is controlled by deformation temperature and strain rate. Under the investigated condition at a constant strain rate ranging from 0.001 to 1 s −1 , the temperature needed for fully dynamic recrystallization increases with the increases of strain rate, and the size of dynamically recrystallized grain is greatly affected by the increase of deformation temperature. The values of critical strain for the initiation of DRX can be determined from the strain-stress curves, and a equation related to the Zener-Hollomon Parameter is as follows: ɛ c =1.135×10 −3 Z 0.14233 . A discontinuous dynamic recrystallization (DDRX) mechanism with nucleation of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX.

Nucleation mechanisms of dynamic recrystallization in Inconel 625 superalloy deformed with different strain rates

The effects of strain rates on the hot working characteristics and nucleation mechanisms of dynamic recrystallization (DRX) were studied by optical microscopy and electron backscatter diffraction (EBSD) technique. Hot compression tests were conducted using a Gleeble-1500 simulator at a true strain of 0.7 in the temperature range of 1000 to 1150 °C and strain rate range of 0.01 to 10.00 s−1. It is found that the size and volume fraction of the DRX grains in hot-deformed Inconel 625 superalloy firstly decrease and then increase with increasing strain rate. Meanwhile, the nucleation mechanism of DRX is closely related to the deformation strain rate due to the deformation thermal effect. The discontinuous DRX (DDRX) with bulging of original grain boundaries is the primary nucleation mechanism of DRX, while the continuous DRX (CDRX) with progressive subgrain rotation acts as a secondary nucleation mechanism. The twinning formation can activate the nucleation of DRX. The effects of bulging of original grain boundaries and twinning formation are firstly gradually weakened and then strengthened with the increasing strain rate due to the deformation thermal effect. On the contrary, the effect of subgrain rotation is firstly gradually strengthened and then weakened with the increasing strain rate.

RESEARCH ON DYNAMIC RECRYSTALLIZATION BEHAVIOR OF INCOLOY 800H

In order to optimize the hot working technology of Incoloy 800H,the dynamic recrystallization(DRX) behavior of Incoloy 800H at temperatures ranging from 850°C to 1100°C and strain rates from 0.01 s~(-1) to 10 s~(-1) was investigated by single-pass compression tests on MMS- 300 thermo-mechanical simulator.The evolutions of microstructure and nucleation mechanisms of DRX were analyzed combined with the technique of EBSD and TEM.The results show that when the deformation temperature is below 950°C,the behavior of DRX is obviously restrained by precipitation of Cr_(23)C_6 and Ti(C,N).Therefore the hot deformation constitutive equations in two temperature intervals(from 850°C to 950°C and from 950℃to 1100°C) were established by regression analysis with the deformation activation energy of 465.394 kJ/mol and 427.360 kJ/mol respectively.The inflection points were determined by fitting a third order polynomial to the lnθ-ecurves,which makes the prediction for the ratios of critical stress to peak stress and critical strain to peak strain more accurately. Accordingly,the mathematical models of critical stress and critical strain vs Z parameter were deduced.The DRX nucleation mechanisms of Incoloy 800H during hot deformation mainly include strain induced grain boundary migration,grain fragmentation and subgrain coalescence.

The effect of deformation temperature on the microstructure evolution of Inconel 625 superalloy

Hot compression tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator between 900 °C and 1200 °C with different true strains and a strain rate of 0.1 s −1. Scanning electron microscope (SEM) and electron backscatter diffraction technique (EBSD) were employed to investigate the effect of deformation temperature on the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). It is found that the relationship between the DRX grain size and the peak stress can be expressed by a power law function. Significant influence of deformation temperatures on the nucleation mechanisms of DRX are observed at different deformation stages. At lower deformation temperatures, continuous dynamic recrystallization (CDRX) characterized by progressive subgrain rotation is considered as the main mechanism of DRX at the early deformation stage. However, discontinuous dynamic recrystallization (DDRX) with bulging of the original grain boundaries becomes the operating mechanism of DRX at the later deformation stage. At higher deformation temperatures, DDRX is the primary mechanism of DRX, while CDRX can only be considered as an assistant mechanism at the early deformation stage. Nucleation of DRX can also be activated by the twinning formation. With increasing the deformation temperature, the effect of DDRX accompanied with twinning formation grows stronger, while the effect of CDRX grows weaker. Meanwhile, the position of subgrain formation shifts gradually from the interior of original grains to the vicinity of the original boundaries.

Deformation Behavior and Dynamic Recrystallization of Hot Deformed GH625 Superalloy

Microstructure evolution during dynamic recrystallization (DRX) of hot deformed GH625 superalloy was investigated by optical microscope (OP) and transmission electron microscope (TEM). Hot compression tests were conducted using Gleeble-1500 simulator. It was found that the nucleation mechanism of DRX for the alloy deformed at 1150°C is composed of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) in the vicinity of the serrated grain boundaries. With the increasing strain, the fraction of the DRX grains increases, while the size of the DRX grains almost remains in the same range. As the deformation temperature increasing, the size and fraction of the DRX grains increase, and no precipitation of intergranular carbides are found when the deformation temperature increases to 1150°C. At lower strain rate, the size and volume fraction of DRX grains decrease with the increasing strain rates. However, the size and volume fraction of DRX grains increase at higher strain rates due to the deformation thermal effect.

Microstructure evolution of 7050 aluminum alloy during hot deformation

Hot compression of 7050 aluminum alloy was performed on Gleeble 1500D thermo-mechanical simulator at 350°C and 450°C with a constant strain rate of 0.1 s −1 to different nominal strains of 0.1, 0.3 and 0.7. Microstructures of 7050 alloy under various compression conditions were observed by TEM to investigate the microstructure evolution process of the alloy deformed at various temperatures. The microstructure evolves from dislocation tangles to cell structure and subgrain structure when being deformed at 350°C, of which dynamic recovery is the softening mechanism. However, continuous dynamic recrystallization (DRX) occurs during hot deformation at 450°C, in which the main nucleation mechanisms of DRX are subgrain growth and subgrain coalescence rather than particle-simulated nucleation (PSN).

Recrystallization Behaviors of an Al-Zn-Mg-Cu Alloy during Multi-Pass Hot Compression

Dynamic recovery (DRV) and dynamic recrystallization (DRX) play important roles during thermomechanical processes of light metals and alloys because they have obvious influence on microstructure evolution and finally on the mechanical properties of the worked material. Hot compression tests of 7050 aluminum alloy was carried out on Gleeble1500D thermomechanical simulator to modeling multi-pass hot rolling process. Microstructure evolution features of the alloy deformed to a reduction up to 80% were investigated through OM, TEM and EBSD observations. DRX behavior of the alloy during hot compression was emphasized. Some evidence of continuous DRX can be found in the alloy deformed at different temperatures and reductions. The main nucleation mechanisms of DRX are subgrain coalescence and subgrain growth. However, static recrystallization takes place in the material during slow cooling after hot compression.

Dynamic Recrystallization of Austenite in Ni-30 Pct Fe Model Alloy: Microstructure and Texture Evolution

The microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within the dynamic recrystallization (DRX) regime using hot torsion testing. The prominent DRX nucleation mechanism was strain-induced grain boundary migration accompanied by the formation of large-angle sub-boundaries and annealing twins. The increase in DRX volume fraction occurred through the formation of multiple twinning chains. With increasing strain, the pre-existing Σ3 twin boundaries became gradually converted to general boundaries capable of acting as potent DRX nucleation sites. The texture characteristics of deformed grains resulted from the preferred consumption of high Taylor factor components by new recrystallized grains. Similarly, the texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during the DRX process. The substructure of deformed grains was characterized by “organized,” banded subgrain arrangements, while that of the DRX grains displayed “random,” more equiaxed subgrain/cell configurations.

Flow behavior and microstructures of superalloy 718 during high temperature deformation

Flow behavior and microstructures of superalloy 718 were investigated by hot compression tests performed at temperatures ranging from 950 to 1100 °C with strain rates of 10 −3 to 1 s −1. The dependence of the peak stress on deformation temperature and strain rate can be expressed by a hyperbolic-sine type equation. The activation energy for superalloy 718 is determined to be 443.2 kJ mol −1. A power exponent relationship between the peak strain and the Z parameter is obtained. Microstructure analysis shows that the dynamically recrystallized grain size is inversely proportional to the Z parameter. The nucleation mechanisms of DRX are closely related to the value of Z parameter. Under low Z conditions, DRX nucleation and development are mainly assisted by the formation of twins near the original grain boundaries.

Nucleation of dynamic recrystallization and variant selection in copper bicrystals

Orientation-controlled copper bicrystals containing [001] symmetrical tilt boundaries aligned parallel to the loading axis were deformed in tension at 923 K and a strain rate of 4.2 × 10−4 s−1. The nucleation of dynamic recrystallization (DRX) was investigated along the grain boundary. For this purpose, both optical and orientation imaging microscopy methods were used. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply indented grain boundary regions. The critical strain for nucleation was about one-quarter to one-half of the peak strain and depended on the misorientation angle. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, all the primary twin traces were parallel to those of the inactive slip planes of the parent single crystals. Crystallographic analysis revealed the important role of the direction of GBM on twinning-plane variant selection. The characteristics of grain boundary nucleation depended sensitively on grain boundary character and on grain boundary mobility. The observed DRX nucleation mechanism is discussed in relation to the occurrence of GBM and twinning.

Dynamic Twin Nucleation at Triple Junctions in Fe-32%Ni Stainless Steel

The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Nucleation of dynamic recrystallization at grain boundaries in copper bicrystals

The nucleation mechanisms of dynamic recrystallization (DRX) were investigated in copper bicrystals deformed in tension at 923 K at a strain rate of 4.2 × 10 −4 s −1. The boundary plane was either parallel or inclined to the tensile axis. The grain boundaries were observed to migrate locally and to form bulges. Nuclei appeared preferentially at the migrating grain boundaries at a strain as low as ε = 0.15, i.e. only about half of the peak strain. All nucleated grains were identified as twins ( Σ3) to one of the two adjacent crystals. The twinning planes coincided preferentially with a secondary slip plane of the adjoining crystals. The DRX nucleation mechanisms at grain boundaries are discussed with respect to grain boundary migration and recrystallization twinning.