Recrystallization Characteristics Research Articles

A novel optical diagnostic technique for analyzing the recrystallization characteristics of polycrystalline silicon thin films following frontside and backside excimer laser irradiation

Excimer laser annealing (ELA) is a widely used technique for producing polycrystalline silicon (poly-Si) thin films. An optical inspection system with simple optical arrangements for rapid measurement of recrystallization results of poly-Si thin films is developed in this study. The recrystallization results after both frontside ELA and backside ELA can be easily visible from the profile of peak power density distribution using the optical inspection system developed with an optimized moving velocity of 0.312 mm/s of the specimen. The method of backside ELA is suggested for batch production of low-temperature polycrystalline silicon thin-film transistors due to higher laser beam utilization efficiency and lower surface roughness of poly-Si films.

An investigation to the microstructural evolution of Fe–29Mn–5Al dual-phase twinning-induced plasticity steel through annealing

The potential effects of twinning induced plasticity have been taken into consideration to further improve the mechanical properties of advanced high-strength steels. Accordingly the high-Mn twinning-induced plasticity (TWIP) steels with austenite–ferrite dual-phase microstructure have been developed. In the present study, the influence of cold rolling and post-annealing treatments on the microstructural evolution and mechanical behavior of a group of dual-phase TWIP steel as a function of annealing time have been investigated. The mechanical behavior of processed materials has been examined through applying a set of low strain rate (0.001 s −1) compression tests at room temperature. The austenite recrystallization characteristics during various annealing conditions are explained through proper microstructural examinations. The results indicate that as the recrystallization proceeds with annealing time the related yield stress deceases. The rapid drop of yield stress in short annealing periods is related to the onset of recrystallization. The yield stress variation diminishes as the annealing duration increases. This is attributed to the formation of austenite side-plates which may balance the softening effects of restoration processes.

Fractal studies on primary phase morphology of TA15 titanium alloy after hot compressive deformation

The goal of this study is to establish relationships between the hot compression deformation behaviors and the fractal dimension of primary phase morphology of TA15 titanium alloy using the analytical methods of metallurgical microscope and transmission electron microscope coupled with box-counting dimension method. The hot compression deformation behaviors vary with decreasing fractal dimension owing to the change of microstructure caused by different parameters of the hot compressive deformation. The results indicate that TA15 alloy shows dynamic recrystallization characteristics at deformation temperature lower than 850 °C while fractal dimension exhibits a moderate decreasing trend with the temperature increasing, and shows dynamic recovery characteristics at deformation temperature higher than 850 °C while fractal dimension reduces rapidly with the temperature increasing. The fractal dimension displays non-linear relationship with fraction of primary phase and with aspect ratio of primary phase.

Characterization of Austenite Dynamic Recrystallization under Different Z Parameters in a Microalloyed Steel

A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850–1100°C and strain rates 0.01–1 s −1 to study the influence of deformation conditions on the dynamic recrystallization characteristics of austenite. The results show that dynamic recrystallization occurs more easily with the decrease of strain rate and the increase of deformation temperature. The complete dynamically recrystallized grain size as a function of Zener-Hollomon parameter was established. It was found that dynamically recrystallized grain sizes decrease with increasing strain rate and decreasing deformation temperature. The effect of microalloying elements on peak strain was investigated and the solute drag corrected peak strain was determined. Also, the dynamic recrystallization map of austenite was obtained by using recrystallization critical parameters.

Effect of the Thermo-Mechanical Processing Characteristics on the Recrystallization of CuZn34 Brass

Abstract The effect of the temperature and the annealing time on the recrystallization characteristics of metals has been widely researched. However, the influence of the deformation history (involving changes in the strain path) on the recrystallization of metals is seldom studied. The aim of this paper was to investigate the effect of the mechanical loading sequence (monotonic and Bauschinger shearing tests), the amount of forward (effective strain of 0.10, 0.20, and 0.30) and reverse strain (effective strain of −0.35, −0.45, and −0.55), and the initial state (as-received and annealed) on the recrystallization kinetics of CuZn34 brass sheets. The softening caused by recrystallization is reduced as the forward and reverse strains applied during the Bauschinger shear tests are increased. For all investigated conditions, the data also revealed that the initial state and the amount of the reverse strain were the main variables on the recrystallization kinetics of CuZn34 brass.

Microstructure homogenization control of Nb-bearing X65 pipeline steel by the CSP process

In order to get the controlled methods of microstructure homogenization and high strengthening-toughening combination by compact strip production (CSP) rolling, the dynamic recrystallization characteristics of each pass were obtained during CSP rolling using a Gleeble-1500 thermal mechanical simulator. Then, the CSP process was simulated by laboratory rolling experiment. Through thermal mechanical simulation experiment and laboratory rolling experiment, a design idea of Nb-bearing pipeline steel by CSP can be obtained as the following: the level of dynamic recrystallization behavior should be increased through the reasonable balance of high deformation temperature and deformation amount in F1 and F2 passes, and F3-F7 passes should be controlled in the austenite non-recrystallization zone. Finally, X65 pipeline steel with microstructure uniformity and good properties was produced by CSP. The yield strength is up to 497 MPa, the tensile strength is up to 563 MPa, the elongation is 30% on average, and the toughness is very good whose Charpy impact value is 110 J on average and drop-weight tear test shearing areas are all 100% at −60, −40, −20, 0, and 20°C.

Hot Deformation Characteristics of 34CrMo4 Steel

Abstract The 34CrMo4 (AISI 4130) steel is extensively utilized in the compressed natural gas cylinders. Due to the importance of thermomechanical processing in the production of these cylinders, the dynamic recrystallization (DRX) characteristics of 34CrMo4 steel were investigated. The effect of hot deformation parameters such as temperature and strain rate on the dynamic restoration processes of a 34CrMo4 alloy was studied. Hot compression tests were performed in the temperature range of 900 to 1100 °C and the strain rate range of 0.001 to 0.1 s −1. Hot workability of this alloy has been analyzed by employing flow localization parameter. The flow curves observed are typical of occurrence of dynamic recrystallization and have exhibited single or multiple peaks in flow stress curve before reaching steady state, depending on the temperature and strain rates. The flow stress decreases with the increase of deformation temperature and the decrease of strain rate. Finally the activation energy obtained in this research is 490.749 kJ/mol.

Dynamic Recrystallization in Zirconium Alloys

Abstract In this investigation, the characteristics of dynamic recrystallization (DRX) in five Zr-alloys, namely, commercial pure zirconium (CP-Zr), Zircaloy-2 (Zr–Sn), Zr-2.5Nb (Zr–Nb), Zr-2.5Nb-0.5Cu (Zr–Nb-Cu), and Zr-1Nb-1Sn-0.1Fe (Zr–Nb–Sn–Fe), have been examined. For this purpose, processing maps were constructed using strain rate sensitivity (m) data as a function of temperature and strain rate. This has been done using compression tests carried out at constant true strain rate. The processing maps revealed the bounds of DRX domain as iso-contour of m in strain rate–temperature plane. Optical microscopy and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to ascertain the nature of dislocation mechanisms in deformed and quenched samples from the DRX domain. For all the alloys studied, TEM revealed complete modification of β-transformed microstructure to an equiaxed morphology containing dislocation substructure within. The variation of grain size with strain rate and temperature, high ductility, and a linear variation of log(grain size) with log(Z) (where Z is the Zener–Hollomon parameter) are some important characteristic features associated with DRX domains of Zr-alloys. While Nb exhibited strong influence on lowering the strain rate for the occurrence of DRX of CP-Zr, Sn addition (Zr–Sn) did not alter DRX characteristics. TEM observations neither revealed the nature of nucleation event nor identified the dislocation mechanism involved in the DRX process. In order to ascertain the rate controlling dislocation mechanism, the flow stress data were analyzed using a model of thermally activated flow. On the basis of the magnitudes of activation energy and apparent activation area, the cross-slip of screw dislocation has been identified as the rate controlling mechanism involved in the DRX process. The activation energy has been found to increase by alloying elements, which is expected to lower stacking fault energy (SFE) and increase separation between the partials. Further analysis of the data using Friedel’s model revealed that the activation energy for the cross-slip process can be decomposed into two parts—one related to the process of forming a constriction in the dissociated dislocations and the other needed for recombining the dissociated dislocations. While the former component (related to SFE) is dominant in Nb containing Zr-alloys, both the components are involved in controlling the deformation behavior of CP-Zr and Zr–Sn alloys. The effects of alloying elements on DRX characteristics and on the evolution of dislocation structure have been rationalized on the basis of cross-slip as the rate controlling dislocation mechanism in a qualitative manner. The importance of dynamic recovery in DRX process has been emphasized in view of this.

Recrystallization characteristics of a fine grained copper alloy

Recrystallization characteristics of severe cold rolled Cu–0.23 vol%Al 2O 3 alloy were investigated through the examination of properties and microstructure. A special change of mechanical and physical properties was observed during the annealing process, which is different from previous reports of particle-containing alloys. On the basis of metallographic, fracture observations and TEM microstructure, nature of recrystallization behavior and kinetic analysis, it was deduced that its diffusion activation energy in the nucleation process is higher than that in the growth process for the high temperature annealing.

Dynamic Recrystallization Behavior of GH4586 Superalloy during Hot Compression

Compression test for GH4586 superalloy was carried out at 1273 K and 1373 K with stain from 20% to 60%. The characteristics of dynamic recrystallization were investigated by confocal laser scanning microscope, TEM and EBSD. It was found that at 1273 K dynamic recrystallization mainly took place along preexisting grain boundaries creating a necklace structure. It was confirmed by TEM observations that the first layer of the necklace structure was formed by the mechanism of bulging of preexisting grain boundaries and the following layers were nucleated at the triple junctions of former dynamic recrystallization grains. When specimens were deformed with different strains at 1373 K, dynamic recrystallization was almost completed. Our observation showed that twinning played an important role during dynamic recrystallization by promoting dynamic recrystallization grains growing. Substructures of grains were constructed by the reconfiguration of dislocations hindered at large ' particles in interior of initial grains during dynamic recovery.

Amorphization of Crystalline Phase Change Material by Ion Implantation

AbstractGermanium ion implantation at an energy of 30 keV was used as a different method to re-amorphize thin films of crystalline phase change material Ge2Sb2Te5 (GST). It was found that rather low doses of 5×1013 cm-2 were sufficient to re-amorphize GST. Amorphization was determined by X-ray diffraction (XRD) as well as reflectivity measurements. Re-crystallization characteristics of ion-implantation-amorphized samples was studied using time-resolved XRD. It showed that samples re-crystallize at an increased crystallization temperature with increasing dose compared to as-deposited material. A static laser tester was applied to measure the crystallization times of material that was (1) as–deposited amorphous; (2) crystallized by annealing and re-amorphized by melt-quenching using a laser pulse; and (3) crystallized by annealing and re-amorphized by ion implantation. It was found that as-deposited amorphous and high-dose ion implanted samples had longer crystallization times while melt-quenched amorphous and low-dose ion implanted samples had shorter crystallization times.

Concentration characteristics of diffusion-induced recrystallization in Cu/CuAu multilayers of varying lattice mismatch

New grains appear in Cu/Au(Cu) thin film interdiffusion couples by diffusion-induced recrystallization. These grains are grouped into different generations of characteristic concentration levels. In this study, the lattice mismatch between initial diffusion partners is systematically varied. It is demonstrated that the intermediate concentrations shift parallel to the concentration of the initial layers. Thus, the characteristic compositions are controlled by the requirement of a characteristic lattice mismatch between parent layer and new grain rather than by metastable phases or highest chemical driving forces.

A microstructure evolution model for numerical prediction of austenite grain size distribution

In this study, a mathematical model has been developed to predict austenite grain size (AGS) of hot rolled steel. Using the compression test, the static (SRX) and metadynamic (MDRX) recrystallization characteristics of medium carbon steel were studied. Compression tests were carried out at various temperatures in the range 900–1100 °C with strain rates ranging from 0.1 to 10 s −1. The time required for 50% recrystallization for the SRX and MDRX was determined by carrying out double compression tests, respectively. Grain growth equation after full recrystallization was also derived by compression tests with various interpass times. The currently determined microstructure model has been integrated with a three-dimensional non-isothermal finite element program. The predicted results based on the model proposed in the present investigation for hot bar rolling processes were compared with the experimental data available in the literature. It was found that the proposed model was beneficial to understand the effect of recrystallization behavior and control the microstructure evolution during the hot bar rolling.

Texture and substructure characteristics of dynamic recrystallization in a Ni–30%Fe austenitic model alloy

The substructure and texture development during dynamic recrystallization (DRX) of an austenitic Ni–30%Fe model alloy was investigated using hot torsion testing. The current results revealed that the DRX texture was dominated by grains with a low Taylor factor component. This was related to the preferred nucleation and lower consumption rates of these grains during DRX. The substructure of DRX grains was “random” in character and displayed complex subgrain/cell arrangements that largely depended on grain orientation.

Analysis on flow stress of magnesium alloys during high temperature deformation

The flow stress of magnesium alloys during hot compression at different temperatures and strain rates was studied by experiments. Materials used were AZ91D alloys in as-cast, homogeneous treatment states, AZ31 and ZK60 alloys in as-cast state. The results show that the thermal simulation curves of different alloys differ from one another at the same deforming condition. The general curves of AZ31 and AZ91D alloys have the character of dynamic recrystallization. There are increase of true stress, drastic falling of true stress and increase of true stress in most curves of ZK60 alloy, while the other curves have the characteristics of dynamic recrystallization. From the analysis the reasonable deforming temperature should be selected from 523 to 673 K for AZ31 and the unhomogenized AZ91D alloy, from 473 to 673 K for the homogenized AZ91D alloy, and it was concluded to be 473 K or 673 K for ZK60 alloy.

Deformation behavior and dynamic recrystallization of Mg-Y-Nd-Gd-Zr alloy

The characteristics of dynamic recrystallization (DRX) in Mg-Y-Nd-Gd-Zr-RE magnesium alloy were investigated by compression tests at temperatures between 523 and 723 K and at strain rates ranging from 0.002 to 1 s −1 with maximum strain of 0.693. The strain- hardening rate can be obtained from true stress-true strain curves, plots of θ-σ, —(∂̸ θ/∂̸ σ)- σ and ln θ-σ in different compression conditions were obtained by further study. The critical condition of the onset of DRX process was determined as ((∂̸/∂̸ σ)(-∂̸ θ/∂̸ σ))=0. In the range of the above deformation temperature and strain rate, the ratio of critical stress ( σ c) to peak stress ( σ m) and critical strain ( ɛ c) to the peak strain ( ɛ m) stood at σ c/ σ m=0.62–0.89 and ɛ c/ ɛ m=0.11–0.37, respectively. DRX could be observed during hot deformation process, microstructure evolution of the magnesium alloy at different temperatures and strain rates were studied with the aid of optical microscope(OM), and the average recrystallized grain size was measured by means of intercepts on photomicrographs. It was shown that the average dynamically recrystallized grain size ( d rec) changed with different deformation parameters, the natural logarithm of the average recrystallized grain size varied linearly with the natural logarithm of Zener-Hollomon parameter; the peak stress changed with the average recrystallized grain size, and the natural logarithm of the average recrystallized grain size varied linearly with the natural logarithm of the peak stress.

Static recrystallization of warm-rolled pure Ti influenced by microstructural inhomogeneity

The recrystallization characteristics of commercially pure titanium (CP-Ti) rolled at various temperatures between 25 and 600 °C was investigated. Increasing the rolling temperature up to 450 °C enhanced the progress of recrystallization and refined the grain structure in the as-recrystallized state, although it reduced the overall stored energy in the as-rolled state. This peculiar phenomenon was attributed to characteristic features of the microstructure that developed during warm rolling, such as deformation-induced high-angle grain boundaries and steep in-grain orientation gradients. These microstructural inhomogeneities, which originate from diverse slip systems – pyramidal 〈 c + a〉 in addition to the main prism 〈 a〉 – activated at the working temperature, are expected to promote the subsequent nucleation of recrystallization. It is proposed, therefore, that the rate of static recrystallization will accelerate when the factor of the microstructural inhomogeneity outbalances that of the stored energy that is reduced by dynamic recovery.

Effect of Initial Microstructure under Different Solidification Conditions on Hot Working Behavior of AZ31 Alloy

In order to investigate the effect of the as-cast microstructure on the hot working behavior of an AZ31 magnesium alloy, specimens were cast in copper moulds with and without water cooling. A series of compression tests were performed at a temperature of 350 °C, a strain rate of 0.01 s-1, and at strains up to 1.0. It was found that as-cast microstructure is very sensitive to the solidification conditions, which leads to a significant difference in flow behavior and dynamic recrystallization (DRX) characteristics. It appears that more uniform and refined as-cast grain size promotes dynamic recrystallization and reduces the flow stress. It is also possible that second phases (>1+m in size) contribute to DRX by acting as nuclei.

Recrystallization Characteristics of Ti-50.9at%Ni Alloy Processed by Equal Channel Angular Extrusion

The initial coarse grains of Ti-50.9at%Ni alloy were refined into submicron grains, small than 0.5 um in size, after eight passes Equal Channel Angular Extrusion (ECAE) at 500°C. Optical microscopy and high temperature DSC tests were applied to investigate recrystallization behavior. It is found that the recrystallization start (Rs) and recrystallization peak (Rp) temperatures of Ti-50.9at%Ni specimens processed by eight passes ECAE are lower than that of the samples processed by one pass ECAE. Ti-50.9at%Ni specimens processed by eight passes ECAE with submicron grains are characterized by higher stability and need less energy to finish recrystallization process.

Recrystallization Characteristics of Commercially Pure Titanium Rolled at Elevated Temperatures

Effect of deformation temperature on subsequent recrystallization characteristics of commercial purity titanium (CP-Ti) was investigated using EBSD and transmission electron microscopy. CP-Ti plates were rolled to at various temperatures between 25°C and 600°C to achieve 60% reduction, and were subsequently annealed at 600°C for various time periods. Increase in rolling temperature resulted in more refined microstructures, which could be attributed to coactivation of secondary slip systems such as basal <a> slip or pyramidal <c+a> slip in addition to prism <a> slip. During subsequent annealing, a large number of recrystallization nuclei formed in the warm-worked (below 450°C) CP-Ti while nucleation of recrystallization was slower for coldrolled one. This led to the faster recrystallization kinetics and a finer microstructure in asrecrystallized state for warm-worked CP-Ti. Meanwhile, the rate of recrystallization in CP-Ti rolled at 600°C was significantly retarded because a good portion of strain energy stored in deformed matrix was released by dynamic recovery. In the present work, we show that the widely accepted explanation for the influence of the deformation temperature on the recrystallization characteristics, i.e., increase of the deformation temperature retards the recrystallization process, is not always true for the hcp metals since deformation at elevated temperature activates secondary slip systems, providing more sites for nucleation of recrystallization.