Papers by Keyword: Dynamic Recovery

Abstract: According to the different softening mechanism, a flow stress model for magnesium alloy AZ31B is established. At the stage of dynamic recovery (DRV), the effect of work hardening (WH) and DRV on flow stress is described by dislocation evolution model. At the stage of dynamic recrystallization (DRX), the flow stress curve is obtained from Avrami equation denoting the recryatallization kinetics. Model parameter and its dependence on deformation condition are identified by the measured flow stress curve. The calculated curves agree well with the measured ones, which demonstrate the availability of the method.

Abstract: In the present work we develop a physically based model of strength evolution during hot deformation of Al-Mg-Si alloys. The goal is to predict a change of material strength taking into account the impact of microchemistry, i.e. the influence of solutes and precipitates on strengthening and softening mechanisms. The material strengthening is considered in the present work in terms of solid solution strengthening (the Labusch-Naborro model), work hardening (the advanced one-parameter Kocks model), as well as precipitation strengthening due to the stress contribution of non-deformable particles, i.e. dispersoids (the Orowan by-pass). The material softening is described by dynamic recovery through thermal activation of dislocation climb. For the precipitation kinetics the computational thermodynamics code MatCalc (Materials Calculator) was used. The model was validated by comparison with experimental data of compression tests of the 6xxx series aluminium alloys and a reasonable agreement of the simulated and measured flow stress curves was found.

Abstract: The flow behavior and microstructural evolution of an as-wrought duplex stainless steel has been investigated by Gleeble-3500 thermal-mechanical simulator within the temperature range of 950-1200°C and the strain rate range of 0.1-10s-1. The flow curves exhibited a peak stress characteristic followed by dynamic softening and the strain for appearance of steady stress is bigger at higher strain rate than at lower strain rate. The apparent activation energy (Q) and the apparent stress exponent (n) of the test steel are obtained to be about 462 kj/mol and 3.95, respectively. The relationship between peak stress (σp) and Zener-Holomon parameter (Z) is obtained, whereby the σp can be predicted at differern hot working conditons. The results of microstructural observation show that the austenite softens by the dynamic recrystallization (DRX) which can be dominantly responsib le for dynamic softening, while the ferrite phase mainly continues to exhibit dynamic recovery (DRV).

Abstract: Hot deformation characteristics of 316 stainless steel were investigated at elevated temperatures. Hot compressive tests were carried out in the temperature and strain rate ranges from 900 to 1100 °C and 1 × 10⁻¹ to 1 s^–1, respectively. Correlation between the flow behavior and the microstructural evolution was analyzed. The flow behavior showed that the softening mechanisms were related to the dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. Flow behavior analyses and microstructural observations indicated that DRV was the major softening mechanism at high strain rates and low temperatures. Dynamic softening proceeded via a combination of DRV and DRX at intermediate strain rates and temperatures. The contribution of DRV to the softening effect decreased with decreasing strain rate (or increasing temperature). Grain growth was the major softening effect at low strain rates and high temperatures.

Abstract: The dynamic process of grain evolution in a Super304H austenitic stainless steel was studied in compression tests. The tests were carried out to a strain of 0.7 at temperatures ranging from 700 to 1000°C and strain rate of 10^-3s^-1. In addition to single pass compression the multiple compressions with changing the loading direction in 90^o and decreasing the temperature with step of 100°C from 1000 to 700°C in each pass were utilized to achieve large cumulative strains. Under multiple compression the values of flow stresses were lower than those at single-pass compressions under the same temperatures. The fraction of dynamically recrystallized grains decreased from 1.0 to almost zero with decreasing temperature in single-pass compressions. On the other hand, almost fully recrystallized structure developed under conditions of multiple compressions. The size of dynamically recrystallized grains decreased with decreasing the deformation temperature, approaching a submicrometer scale level at 700°C. The relationship between the deformation conditions and operating mechanisms of dynamic recrystallization is discussed in some details.

Abstract: Microstructural design of a new generation of 9%Cr steels for fossil power plants is considered. It was shown that microstructural stability of 9%Cr steels impairs their creep resistance. Two types of restoration processes can occur in the heat resistance steels under creep conditions: (i) normal grain growth and (ii) dynamic recovery. The first process associates with the migration of high-angle boundaries (HAGB) of blocks of tempered martensite lath structure (TMLS). However, their migration is negligible even during creep deformation. Boundaries of packets and prior austenite boundaries (PAB) are effectively pinned by precipitations of M₂₃C₆ and Laves phase Fe₂(W,N). The second process consists of transformation of lath boundaries to subboundaries and their subsequent migration (subgrain coarsening) under creep. Under aging the migration of low-angle boundaries (LAGB) is retarded by uniformly distributed nanoscale M(C,N) dispersoids and particles of M₂₃C₆ precipitated on these boundaries under tempering. Under creep the dissolution of M₂₃C₆ carbides located along LAGBs and coagulation of uniformly distributed M(C,N) carbonitrides facilitates LAGB migration. It was shown that the normal grain growth is not important for deterioration of creep strength. Conversion of the lath boundaries into subgrain boundaries strongly decreases creep rate. In contrast, continuous subgrain coarsening is the main process restricting the ability of the 9%Cr steel for long-range service under creep conditions. Tertiary creep is attained due to the occurrence of subgrain coarsening.

Abstract: The efficiency of gas turbine engines can improved by an increase of the working temperature. As a consequence Allvac® 718Plus™ was developed to enhance the high temperature properties. Since the performance of this alloy is strongly related to the microstructure the knowledge of the softening processes is important to develop precise microstructure evolution models. Specimens were deformed at different temperatures (950°-1050°C) and strain rate (0.1s^-1 – 10s^-1) to strains of 0.2-1.5. The microstructures obtained were analyzed by electron backscatter diffraction (EBSD) in the scanning electron microscope to investigate the softening mechanisms at the respective forming conditions.

Abstract: The dynamic process of grain evolution in an S304H-type austenitic stainless steel was studied in multiple forging tests at temperatures of 500°C, 600°C and 700°C. The deformation microstructure with a grain size of about 100 to 400 nm resulted from continuous dynamic recrystallization. The size of new grains and the recrystallization kinetics decreased with decreasing the deformation temperature. The dynamically equilibrium grain size evolved at large strains followed a power law function of the flow stress with a grain size exponent of about-0.2. The formation of new fine grains was assisted by dynamic recovery, which leads to an apparent steady state flow at large total strains.

Abstract: TMP of Al alloys includes hot working with dynamic substructures and deformation bands for texture components combined with static recovery or recrystallization as well as cold working altered by annealing. The above processes are separately tailored for solute (Al, Mg), dispersoid (Al-0.7Fe) and precipitation hardening alloys; aging combined with deformation can raise strength or improve fatigue or corrosion resistance. Hot and cold rolling with suitable holding intervals are managed to combine deformation and annealing textures for planar anisotropy or for producing less fibrous grains to avoid delamination corrosion; grains may be severely refined by discontinuous or continuous recrystallization for superplastic sheet. In hot-billet and impact extrusion as an addition to substructure and texture strengthening, the intense heating near the die may be employed for precipitate solution with exit quenching for press heat treatment to T5 temper. Similarly, friction stir surface treatment and welding provide intense hot straining with additional softening as metal is swept behind the pin. In combination with some of the above, forging provides grain and dispersoid fibering oriented for crack retardation; semi solid forming competes with this.

Abstract: Hot deformation behavior of Fe-3%Si steel within temperature range of 1073~1473K and strain rate range of 0.01~5s⁻¹ was investigated by isothermal compression test using thermo-simulation method. Over the applied deformation conditions, steady state flow behavior was well described by the power law relationship with dislocation climb as the rate-controlling mechanism, and the high apparent activation energy can be attributed to the high yield stress. A modified Bergström model was proposed by introducing yield stress, and consequently the whole stress-strain curves can be accurately predicted.