Grain Size Evolution Research Articles

Microstructural evolution and mechanical property of friction stir welded joints in AlSi10Mg alloys fabricated by laser powder bed fusion

Friction stir welding (FSW) was introduced to join laser powder bed fusion (LPBF) AlSi10Mg alloys in this study. Microstructure and texture evolution, microhardness distribution and tensile properties of the FSW joints were investigated. The evolution of grain size, grain boundary and misorientation angles distribution, dynamic recrystallization and crystallographic texture in various regions of the FSW joint concerning base metal (BM), thermo-mechanically affected zone (TMAZ), upper part of nugget zone (UPNZ) and low part of nugget zone (LPNZ) was characterized by using electron back-scattered diffraction (EBSD). The analysis reveals a strong texture evolution of shear components on the TMAZ, UPNZ and LPNZ as well as a fully recrystallized grain structure on the UPNZ and LPNZ. Additionally, a gradual evolution in Si-rich eutectic from continuous cellular networks in the BM to totally globularized particles in the UPNZ and LPNZ occurs in the FSW joint. The microhardness rapidly decreases to the TMAZ from the BM through heat affected zone (HAZ) reaching the lowest value of ∼65.0 HV at the TMAZ/NZ boundary. The FSW joint in LPBF AlSi10Mg alloys exhibits an ultimate tensile strength (UTS) of 247 MPa and an elongation-to-failure (EI) of 8.8%. The UTS of FSW joints is reduced compared to the BM, but the EI has been increased by 160%, implying an improvement in tensile ductility.

Microstructure evolution and EBSD analysis of multi-principal element alloy coatings after high-temperature oxidation

Multi-principal element alloy coatings (MPEACs) have great application potential for high-temperature diffusion barrier coatings. The sluggish diffusion and lattice distortion of this new coating material due to its own atomic size effect give the coating excellent resistance to high temperature oxidation and grain coarsening. In this study, the high-temperature oxidation properties, and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings (whose lattice distortion magnitude is linearly increased) at 1100 °C were studied. The evolution of grain orientation, size, and grain boundaries of alloy coatings after high temperature exposure were analyzed by EBSD. The results showed that from quaternary to denary alloy coatings, with the rise in lattice distortion magnitude of the alloy system, the sluggish diffusion ability of the coating at high temperatures was more obvious. Specifically, the oxidative activation energy of quaternary to denary alloy coatings is 355.41, 480.96, 478.76, and 654.03 kJ/mol, respectively. A high distortion magnitude is beneficial to increase the activation energy of the alloy coating, as a result, grain growth of alloy coatings with high activation energy is significantly inhibited, and under the combined action of storage strain energy and solute dragging, the grain boundary migration rate of the coatings also declines. Finally, the denary alloy coating shows good resistance to high temperature softening. This study provides a positive reference for promoting the application of multi-principal alloys in diffusion barrier coatings.

Effect of solute concentration on microstructure evolution during static recrystallization in Mg-0.2%Ce alloy using cellular automata

A new cellular automata model has been developed to evaluate the effect of solute drag on the microstructure and texture evolution during static recrystallization of Mg and its alloys in the temperature range of 200 – 400 °C. In the present model, the site saturation nucleation was assumed and all the nuclei are present at the grain boundaries. The growth of each nucleus depends on the solute concentration and dislocation density difference between neighboring grains. The microstructural analysis of the newly developed cellular automata model showed the evolution of grain size at different temperatures compared to the equilibrium microstructure containing the equilibrium concentration of solute atoms at the grain boundaries. Further, the texture evolution showed that there is an increase in the texture strength with an increase in the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries, whereas the texture strength decreased with increasing the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries. The present model shows that solute drag pressure increased continuously with increasing the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries. However, the solute drag pressure decreased with an increase in the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries.

Dislocation evolution mechanism of plastic deformation process of AZ31 magnesium alloy with different grain size

The evolution of grain size and dislocation has a great influence on the plastic deformation of the material. The variation of dislocation lines in the deformation process of magnesium alloys with different grain size was studied by molecular dynamics(MD) simulation technology combined with transmission electron microscope(TEM) experimental methods. The decrease in the size of grains will lead to an increase in the dislocation density during the plastic deformation of the magnesium alloy, and the dislocation line will become tangled. This provides a new means and theoretical basis for studying the plastic deformation of AZ31 magnesium alloy.

Analyzing and Extending Cellular Automaton Simulations of Dynamic Recrystallization

A polycrystalline system corroborating experimentally observed microstructure is developed in this paper by employing cellular automata techniques. Moreover, through appropriate transition functions, microstructural changes accompanying dynamic recrystallization, a manufacturing technique associated with the production of a wide range of components, is simulated. The grain growth that characterizes this change is analyzed to explicate the trend in the temporal evolution of mean grain size and its kinetics. As opposed to a progressive increase in mean grain size, which generally typifies a conventional grain growth, in dynamic recrystallization it is observed that the mean grain size increases and decreases sequentially, thereby rendering an oscillating pattern. A perceptive investigation of the fluctuating trend unravels individual growth events, characterized by monotonic increase in mean grain size, whose kinetics follows the third-order power law. Additionally, the existing approach of modeling dynamic recrystallization is extended to facilitate unrestricted yet energetically favored growth of defect-free nuclei across their neighboring grains.

Grain growth of natural and synthetic ice at 0 °C

Abstract. Grain growth can modify the microstructure of natural ice, including the grain size and crystallographic preferred orientation (CPO). To better understand grain-growth processes and kinetics, we compared microstructural data from synthetic and natural ice samples of similar starting grain sizes that were annealed at the solidus temperature (0 ∘C) for durations of a few hours to 33 d. The synthetic ice has a homogeneous initial microstructure characterized by polygonal grains, little intragranular distortion, few bubbles, and a near-random CPO. The natural ice samples were subsampled from ice cores acquired from the Priestley Glacier, Antarctica. This natural ice has a heterogeneous microstructure characterized by a considerable number of air bubbles, widespread intragranular distortion, and a CPO. During annealing, the average grain size of the natural ice barely changes, whereas the average grain size of the synthetic ice gradually increases. These observations demonstrate that grain growth in natural ice can be much slower than in synthetic ice and therefore that the grain-growth law derived from synthetic ice cannot be directly applied to estimate the grain-size evolution in natural ice with a different microstructure. The microstructure of natural ice is characterized by many bubbles that pin grain boundaries. Previous studies suggest that bubble pinning provides a resisting force that reduces the effective driving force of grain-boundary migration and is therefore linked to the inhibition of grain growth observed in natural ice. As annealing progresses, the number density (number per unit area) of bubbles on grain boundaries in the natural ice decreases, whilst the number density of bubbles in the grain interiors increases. This observation indicates that some grain boundaries sweep through bubbles, which should weaken the pinning effect and thus reduce the resisting force for grain-boundary migration. Some of the Priestley ice grains become abnormally large during annealing. We speculate that the contrast of dislocation density amongst neighbouring grains, which favours the selected growth of grains with low dislocation densities, and bubble pinning, which inhibits grain growth, are tightly associated with abnormal grain growth. The upper 10 m of the Priestley ice core has a weaker CPO and better-developed second maximum than deeper samples. The similarity of this difference to the changes observed in annealing experiments suggests that abnormal grain growth may have occurred in the upper 10 m of the Priestley Glacier during summer warming.

Grain size and temperature evolutions during linear friction welding of Ni-base superalloy Waspaloy: Simulations and experimental validations

This research study was aimed at investigating the influence of linear friction welding parameters on grain size alteration and temperature distribution of Ni-base superalloy Waspaloy. A 3D finite element model was developed to predict average grain size and peak temperature as responses. The linear friction welding parameters consisted of oscillation amplitude, oscillation frequency, and applied pressure. Initially, the evolution of the average grain size as a function of the most influential process parameters was subsequently modeled based on the Johnson-Mehl-Avrami-Kolmogorov recrystallization model and were then validated with experimental results. Then, D-optimal design of experiments and analysis of variance were conducted to determine the most influential process parameters that affect the average grain size and peak temperature of the welded joint. Thereafter, response surface method was employed to obtain the regression models of the responses. The analysis of variance demonstrated that the P-value of the regression models was smaller than 5% and R2, Radj2, and RPred2 were between 87% and 97%, which showed that the predictive regression models of PT and AGS can be used with a high level of confidence. The regression models were then validated by selecting two extra LFW tests in the space of the DoE. The optimum values of the welding parameters were determined to minimize the responses. The multi-criteria optimization analysis showed that both average grain size and peak temperature were more dependent on pressure than oscillation amplitude and frequency. The developed finite element and regression models can be utilized as a predictive tool for the design of joining industrial components, which minimize expensive and time-consuming experimental tests and measurements.

Effect of temperature field on microstructural stability of Al–Cu–Mg alloy hemispherical component

Al–Cu–Mg alloy hemispherical components can be widely utilized in gyroscope navigation systems by virtue of low density, high deformability and fatigue strength. However, the nonuniformity in residual stress and microstructure of the component after deep drawing may lead to a complex microstructural evolution under the temperature environment, thereby decreasing the navigation precision. In this work, the evolution of grain sizes, dislocations, second phases and mechanical properties of Al–Cu–Mg alloy hemispherical components were evaluated under a temperature field. From the edge to the central part of the component, the grain size first decreased and then increased, while the hardness and residual stress first increased and then decreased. With the extension of exposure time, the dislocation density decreased and the coarsened Al2CuMg phases appeared, while the grain size of the component remained stable. Furthermore, the dislocation density declined faster and Al2CuMg phases precipitated rapider in the die fillet area in comparison with the situation at the bottom of the hemisphere. The higher dislocation density could provide more nucleation sites for Al2CuMg phases. Thereafter, more dislocations were consumed, thus accelerating the reduction of stored energy and lattice distortion under the temperature field. As a result, investigating the microstructural stability and mechanical properties of Al–Cu–Mg alloy hemispherical components can make a contribution to fabricating highly stabilized navigation devices to be further applied in the field of space exploration.

Uncovering Dislocation- and Precipitate-Induced Viscoplastic Damage in Al-Zn-Mg Alloy.

The existing phenomenological theories of plastic forming of sheet metal lack the predictability of the influences of dislocations and precipitates on viscoplastic damage in Al-Zn-Mg alloys. This study examines the evolution of grain size that occurs when the Al-Zn-Mg alloy undergoes a hot deformation process, specifically concentrating on dynamic recrystallization (DRX). The uniaxial tensile tests are conducted at deformation temperatures ranging from 350 to 450 °C and strain rates of 0.01-1 s-1. The intragranular and intergranular dislocation configurations and their interactions with dynamic precipitates are revealed by transmission electron microscopy (TEM). In addition, the MgZn2 phase induces microvoid formation. Subsequently, an improved multiscale viscoplastic constitutive model is established that emphasizes the effect of precipitates and dislocations on the evolution of microvoid-based damage. Using a calibrated and validated micromechanical model, the simulation of hot-formed U-shaped parts is conducted through finite element (FE) analysis. During the hot U-forming process, the formation of defects is expected to have an impact on both the distribution of thickness and the level of damage. In particular, the damage accumulation rate is influenced by temperature and strain rate, and local thinning is caused by the damage evolution of U-shaped parts.

A Comparative Study of Deterministic and Stochastic Models of Microstructure Evolution during Multi-Step Hot Deformation of Steels.

Modern construction materials, including steels, have to combine strength with good formability. In metallic materials, these features are obtained for heterogeneous multiphase microstructures. Design of such microstructures requires advanced numerical models. It has been shown in our earlier works that models based on stochastic internal variables meet this requirement. The focus of the present paper is on deterministic and stochastic approaches to modelling hot deformation of multiphase steels. The main aim was to survey recent advances in describing the evolution of dislocations and grain size accounting for the stochastic character of the recrystallization. To present a path leading to this objective, we reviewed several papers dedicated to the application of internal variables and statistical approaches to modelling recrystallization. Following this, the idea of the model with dislocation density and grain size being the stochastic internal variables is described. Experiments composed of hot compression of cylindrical samples are also included for better presentation of the utility of this approach. Firstly, an empirical data describing the loads as a function of time during compression and data needed to create histograms of the austenite grain size after the tests were collected. Using the measured data, identification and validation of the models were performed. To present possible applications of the model, it was used to produce a simulation imitating industrial hot-forming processes. Finally, calculations of the dislocation density and the grain size distribution were utilized as inputs in simulations of phase transformations during cooling. Distributions of the ferrite volume fraction and the ferrite grain size after cooling recapitulate the paper. This should give readers good overview on the application of collected equations in practice.

Numerical Prediction of Microstructure Evolution of Small-Diameter Stainless Steel Balls during Cold Skew Rolling.

The wear resistance and hardness of stainless steel (SS) balls formed by cold skew rolling are effectively improved due to the change in internal microstructure. In this study, based on the deformation mechanism of 316L stainless steel, a physical mechanism-based constitutive model was established and implemented in a subroutine of Simufact to investigate the microstructure evolution of 316L SS balls during the cold skew rolling process. The evolution of equivalent strain, stress, dislocation density, grain size, and martensite content was studied via simulation during the steel balls' cold skew rolling process. The corresponding skew rolling experiments of steel balls were carried out to verify the accuracy of the finite element (FE) model results. The results showed that the macro dimensional deviation of steel balls fluctuates less, and the microstructure evolution agrees well with the simulation results, which proves that the established FE model has high credibility. It shows that the FE model, coupled with multiple deformation mechanisms, provides a good prediction of the macro dimensions and internal microstructure evolution of small-diameter steel balls during cold skew rolling.

Analysis of microstructure evolution and deformation mechanism of nano-oxides Al2O3 dispersion strengthened copper alloy during compression at room temperature

The refinement of grains to the size of ∼ 600 nm is observed in a nano-scaled Al2O3 dispersion-strengthened (ODS) Cu alloy under compression distortion. The plastic deformation mechanism of the ODS Cu alloy is studied by the observation and analysis of the evolution of grain size, dislocation density and distribution, and texture orientation during compression process. The deformation process can be divided into four stages, i.e., leaner deformation stage, plastic-strengthening transition stage, relative steady state stage and softening stage under ∼ 90 % compression, according to the stress-strain curves. The grains are found split into refined sub-grains by the net-like entangled dislocations due to the pinning effect of nano-scaled Al2O3 particles. The dislocation density of ODS Cu is found increasing within 30 % compression, and then remaining to a plateau under ∼ 30 %−60 % compression, but then under ∼ 60 %−80 % and ∼ 80 %−90 % compression, the dislocation density increases firstly and then decreases. The crystal orientation is transformed from the Goss texture {110}< 001 > to the inhomogeneous Shear texture {001}< 110 > during the compression process, due to the rotation of grains. This study provides a further understanding of the strengthening mechanism and a guidance for the design of ODS Cu alloys, which is significant for the prospective future structural applications.

Preparation and characterization of Cu2ZnxFe1−xSnS4 thin ‎films deposited on intrinsic silicon substrates

The present paper reports the study of the effect of zinc content in Cu2Zn1−xFexSnS4 thin films. To achieve this purpose, CZTS, CZ0.5F0.5TS and CZTS thin films were grown at room temperature via thermal evaporation on unheated silicon substrates, which was followed by sulfidation at 400 °C. Analysis by x-ray diffraction indicates the polycrystalline nature of the CZFTS films with a preferential orientation along the (112) plane with structural transition from stannite (x = 0) to kesterite (x = 1) as the zinc content increases. The elemental composition of all thin films was analyzed by the EDAX technique. Morphological patterns were also explored in order to better understand the evolution of grain size and thickness as a function of zinc content. Hall effect measurements showed that the highest conductivity was obtained for CZTS. Impedance spectroscopy (IS) was performed between 5 Hz and 13 MHz. The complex impedance plots of the different samples revealed a single semicircle, suggesting that the response comes from a single capacitive element consistent with the grains. This measurement (IS) confirmed the enhancement of the conduction mechanism with increasing x-fraction. Experimental data suggested that AC conductivity in thin films C(Z, F)TS is proportional to ωs (s < 1). This is consistent with the correlated barrier hopping (CBH) model.

Comparative Study of Prior Particle Boundaries and Their Influence on Grain Growth during Solution Treatment in a Novel Nickel-Based Powder Metallurgy Superalloy with/without Hot Extrusion

The prior particle boundaries (PPBs), as one of the typical defects in the nickel-based powder metallurgy superalloy, largely affect the microstructure and thus properties/performance of alloys. However, the effect of PPBs on the microstructure evolution in nickel-based powder metallurgy superalloy during heat treatment is still unclear. In this paper, a comparative study of PPBs and their influence on grain growth during solution treatment in a novel nickel-based powder metallurgy superalloy FGH4113A (i.e., WZ-A3 from Shenzhen Wedge, China) with/without hot extrusion (HEX) was conducted. Firstly, through a combination of scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) techniques, PPBs in FGH4113A alloys were characterized to be Al2O3, carbides (TiC, M6C, M23C6) and large-size γ′ particles. After HEX, the oxides broke, carbides deformed, and γ′ phase redistributed. After solution treatment at 950 °C, the TiC decomposed to M6C and M23C6, while no such decomposition occurred in FGH4113A alloys after solution treated at 1050 °C and 1150 °C. Secondly, the evolution of grain size in FGH4113A alloys was analyzed using the electron backscattered diffraction (EBSD) technique. At 950 °C, the decomposition of carbide TiC resulted in the increase of PPBs and the enhancement of their pinning effect on grain boundaries, thus inhibiting grain growth. At 1050 °C, the nucleation rate due to recrystallization is comparable to the grain growth rate, leading to the stable distribution of grain size. While at 1150 °C, the higher temperature can induce a higher content of PPBs. However, the driving force for grain growth surpassed the pinning force of PPBs, making the grains quickly coarsen. Finally, it was concluded that the HEX process is an effective method to modify the microstructure of powder metallurgy superalloy after HIP that can heavily refine the grains in the powder metallurgy superalloys. Furthermore, based on the present experiment and analysis, an appropriate solution treatment mechanism (i.e., 1050 °C for 2 h) was proposed for FGH4113A alloys.

Size, shape and orientation matter: fast and semi-automatic measurement of grain geometries from 3D point clouds

Abstract. The grain-scale morphology and size distribution of sediments are important factors controlling the erosion efficiency, sediment transport and the aquatic ecosystem quality. In turn, characterizing the spatial evolution of grain size and shape can help understand the dynamics of erosion and sediment transport in coastal, hillslope and fluvial environments. However, the size distribution of sediments is generally assessed using insufficiently representative field measurements, and determining the grain-scale shape of sediments remains a real challenge in geomorphology. Here we determine the size distribution and grain-scale shape of sediments located in coastal and river environments with a new methodology based on the segmentation and geometric fitting of 3D point clouds. Point cloud segmentation of individual grains is performed using a watershed algorithm applied here to 3D point clouds. Once the grains are segmented into several sub-clouds, each grain-scale morphology is determined by fitting a 3D geometrical model applied to each sub-cloud. If different geometrical models can be tested, this study focuses mostly on ellipsoids to describe the geometry of grains. G3Point is a semi-automatic approach that requires a trial-and-error approach to determine the best combination of parameter values. Validation of the results is performed either by comparing the obtained size distribution to independent measurements (e.g., hand measurements) or by visually inspecting the quality of the segmented grains. The main benefits of this semi-automatic and non-destructive method are that it provides access to (1) an un-biased estimate of surface grain-size distribution on a large range of scales, from centimeters to meters; (2) a very large number of data, mostly limited by the number of grains in the point cloud data set; (3) the 3D morphology of grains, in turn allowing the development of new metrics that characterize the size and shape of grains; and (4) the in situ orientation and organization of grains. The main limit of this method is that it is only able to detect grains with a characteristic size significantly greater than the resolution of the point cloud.

Effect of strain of cryorolling on structure and strength of nickel

The effect of isothermal rolling at liquid nitrogen temperature with reductions up to 90 % on the structural-mechanical behavior of coarse-grained nickel was evaluated by TEM, SEM-EBSD and X-ray analysis methods, and microhardness measurements. The highest hardening rate of the metal was found at the initial stage of rolling (up to about 50 %), after which it decreased slightly with further reductions. Detailed analyses of the evolution of dislocation density, grain size and grain boundary spectra have shown that the behavior found was attributed to an extremely low deformation temperature, resulted in a decreased rate of accumulation of crystalline defects, as well as the formation and rearrangement of dislocation structure. Due to the strong suppression of dynamic recovery and recrystallization, even when rolling to a reduction of 90 %, a relatively homogeneous coarse-fibered structure with a developed nanocellular substructure and less than 5 % fraction of nanoscale dynamically recrystallized grains was formed. It was concluded that the main structural strengthening factors in cryogenically rolled nickel was the formation of low-energy dislocation structures and their transformations into a well-developed substructure with nanosized crystallites separated predominantly by low-angle boundaries.

The coupling machine learning for microstructural evolution and rolling force during hot strip rolling of steels

Microstructural evolution during hot rolling, which is complex and unperceptive but has direct effects on rolling forces, determines the final microstructure and mechanical properties of steel products. In this paper, a comprehensive set of machine learning (ML) models was developed through rolling forces to reveal the evolutions of recrystallization and grain size of austenite, in which the grain size was estimated by considering the grain shape effect after each rolling pass. By using the backpropagation neural network (BPNN) and genetic algorithm (GA) combined with industrial data, the quantitative connections of parameters in the models for static recrystallization (SRX), meta-dynamic recrystallization (MDRX), and mean flow stress (MFS) were established with steel chemical compositions and deformation conditions. Through the machine learning (ML) approach, softening kinetics and rolling forces can be more accurately predicted than traditional models. Also, the recrystallization behavior and evolution of austenite grain size during hot rolling were analyzed by using the proposed ML models, which are consistent with experimental results.

A calculation method of solidification parameter threshold for welding microstructure evolution

The relationship between the evolution of microstructure morphology (planar, cellular, columnar and equiaxed) and grain size (2.5 μm, 5.0 μm, 7.5 μm and 10.0 μm) of a specified metal and the calculated solidification parameter (G/R, G × R) of temperature gradient (G) and solidification rate (R) is calculated by comparing the “solidification process percentage” of the welding experiment and simulation. Taking pulsed laser single spot welding as the object, the varying solidification parameters and corresponding microstructure of liquid–solid interface are obtained by adjusting the pulsed closing shapes. The results show that the calculating method of the solidification parameter threshold of the welding microstructure evolution by using the solidification process percentage is reliable and the evolution threshold of microstructure morphology and the grain size of AZ31 magnesium alloy are obtained. The research lays a theoretical and practical foundation for the establishment of the solidification parameter threshold system of the welding microstructure evolution.

Recent Advances in the Equal Channel Angular Pressing of Metallic Materials

Applications of a metallic material highly depend on its mechanical properties, which greatly depend on the material’s grain sizes. Reducing grain sizes by severe plastic deformation is one of the efficient approaches to enhance the mechanical properties of a metallic material. In this paper, severe plastic deformation of equal channel angular pressing (ECAP) will be reviewed to illustrate its effects on the grain refinement of some common metallic materials such as titanium alloys, aluminum alloys, and magnesium alloys. In the ECAP process, the materials can be processed severely and repeatedly in a designed ECAP mold to accumulate a large amount of plastic strain. Ultrafine grains with diameters of submicron meters or even nanometers can be achieved through severe plastic deformation of the ECAP. In detail, this paper will give state-of-the-art details about the influences of ECAP processing parameters such as passes, temperature, and routes on the evolution of the microstructure of metallic materials. The evolution of grain sizes, grain boundaries, and phases of different metallic materials during the ECAP process are also analyzed. Besides, the plastic deformation mechanism during the ECAP process is discussed from the perspectives of dislocation slipping and twinning.

Thermal stability of nanostructured uranium within a surface layer processed using shot peening

A nanostructured α‑uranium surface layer with an average subgrain/grain size of 140 nm was obtained by using shot peening treatment (SPT). While, the nanostructures might be unstable in thermal condition due to dense lattice defects, high density of dislocations and ultrafine grains induced by severe plastic deformation. In this work, the thermal stability of the nanostructured surface layer was investigated using quasi-in situ electron backscatter diffraction (EBSD) after annealing treatments at temperatures ranging from 400 °C to 500 °C. Results show that the nanostructured surface layer has reasonably good thermal stability below 400 °C. Recrystallization and subsequent growth occurred when the temperature increased to 450 °C. The recrystallization nuclei are rooted in the initial nanostructured sites in the top surface layer and from dense dislocation walls in the subsurface consisting of high density of twin boundaries. At 450 °C, the exponent of recrystallization subsequent growth is determined to be 2.86, and the apparent activation energy value is 112.24 kJ/mol. Recrystallization occurs predominantly in a continuous manner. Furthermore, the evolution of dislocation density and grain size accounts for the decaying of surface mechanical properties.