Effect of thermomechanical parameters on dynamically recrystallized grain size of AZ91 magnesium alloy (original) (raw)
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Hot deformation behavior of AZ91 magnesium alloy in temperature ranging from 350°C to 425°C
Transactions of Nonferrous Metals Society of China, 2012
The flow behavior and microstructure evolution of AZ91 magnesium alloy during a thermomechanical process, hot compression test， was investigated. The specimens were hot compressed at a temperature ranging from 350 °C to 425 °C and at strain rate of 0.1 s −1 to the strains of 0.3, 0.5 and peak. Microstructural evolutions were studied using optical and scanning electron microscopes. The results show that during the compression process, the recrystallized grains nucleate along the pre-existing grain boundaries. The amount of dynamically recrystallized grains is increased with strain in a sigmoid scheme followed by Avrami equation. The size of dynamically recrystallized grains also increases at the beginning and decreases after reaching the maximum value.
Grain refinement in AZ91 magnesium alloy during thermomechanical processing
Materials Science and Engineering: A, 2003
Microstructural changes during high-temperature extrusion and torsion of an AZ91 alloy (Mg Á/9Al Á/1Zn, wt.%) were investigated. In the experimental domain studied, dynamic recrystallisation (DRX) occurs and the effect of temperature and strain rate on the resulting recrystallised grain size was investigated. Complete recrystallisation in torsion is associated with the development of a stress plateau after softening from the peak stress, which is systematically observed in the first steps of straining. The resulting grain size can be related to the value of the peak stress. It appears that the precipitation of the Mg 17 Al 12 phase does not affect significantly the torsion behaviour of the alloy in the experimental domain investigated here. This study supports the idea that very fine-scale microstructures (i.e. with a mean grain size smaller than 5 mm) can be easily produced by DRX during hightemperature extrusion of the AZ91 alloy.
Materials Science and Engineering: A, 2012
The present study deals with the recrystallization behavior of homogenized AZ81 magnesium alloy with emphasizing on the role of mechanical twins and g precipitates. Towards this end a set of hot compression tests were conducted in the temperature range of 250-450 1C under strain rates of 0.003, 0.03 and 0.3 s À 1. The results indicate that the dynamic recrystallization (DRX) has been occurring in the large scales thereby considered as the main factor affecting the related flow stress characteristics. The twin related dynamic recrystallization (including DRX within twin bands and DRX at twin-twin intersections) possesses a great contribution to the formation of new recrystallized grains. The obtained results also confirm that the dynamic precipitation of g phase has a significant effect on hindering the growth of new DRX grains thereby resulting in an outstanding grain refinement. The role of mechanical twinning and dynamic precipitation on the DRX behavior of the alloy is more pronounced at higher and lower Zener-Hollomon parameter, respectively.
Analysis of high-temperature deformation and microstructure of an AZ31 magnesium alloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2007
High-temperature plastic deformation and dynamic recrystallization of AZ31 extruded (EX) and heat treated (FA) alloy was investigated in the temperature range between 200 and 400 • C. High-temperature straining resulted in partial dynamic recrystallization above 250 • C; in the EX alloy recrystallization was complete at 300 • C, while a moderate grain growth was observed at 400 • C. The peak flow stress dependence on temperature and strain rate are described by means of the conventional sinh equation; the calculation of the activation energy for high temperature in the whole range of temperature deformation gives Q = 155 kJ/mol, i.e. a value that was reasonably close but higher than the activation energy for self diffusion in Mg. The microstructure resulting from high-temperature straining was found to be substantially different in EX and FA alloys; in particular, the EX alloy was characterized by a lower flow stress, a higher ductility and by a finer size of the dynamically recrystallized grains. These results are then discussed on the basis of the "necklace" mechanism of dynamic recrystallization.
Flow behavior and microstructure of ZK60 magnesium alloy compressed at high strain rate
Transactions of Nonferrous Metals Society of China, 2014
Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250−400 °C and the strain rate range of 0.1−50 s −1. The results showed that dynamic recrystallization (DRX) developed mainly at grain boundaries at lower strain rate (0.1−1 s −1), while in the case of higher strain rate (10−50 s −1), DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 °C, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10−50 s −1 and in the temperature range of 250−350 °C was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.
Materials Science and Engineering: A, 2013
Hot workability of Zr alloy has been investigated by means of hot compression test using Gleeble-3800. Hot compression test was performed in the temperature and strain rate range of 700 to 925 o C and 0.01-10s-1 , respectively. Deformation behavior was characterized with the help of processing maps using Dynamic Material Model (DMM). From the power dissipation map peak efficiency of 48-55% was obtained at temperature of around 750 o C and strain rate of 1×10-2 s-1 , in (α +β) phase field. High efficiency suggests dynamic recrystallization and therefore is preferred processing condition. A domain of unstable flow was obtained at temperature range 700-730°C and strain rate range of 8×10-2-1s-1 in which instability parameter is negative. This region should be avoided during hot working. Effect of Cu in addition to Zr-2.5Nb also studied by power dissipation and instability map in this present study.
HIGH TEMPERATURE PLASTIC DEFORMATION OF A HEAT-TREATED AZ31 MAGNESIUM ALLOY
High-temperature plastic deformation and dynamic recrystallization were investigated in an extruded and heat-treated AZ31alloy in the temperature range between 200 and 400°C. High-temperature straining resulted in partial dynamic recrystallization above 250°C; at 400°C recrystallization was complete and a moderate grain growth was observed. The peak flow stress dependence on temperature and strain rate was described by means of the conventional sinh equation; calculation of the activation energy for high temperature in the whole range of temperature deformation gave Q=155 kJ/mol, i.e. a value that is reasonably close to, but greater than, the activation energy for self-diffusion in Mg. When the data obtained at the lowest temperature were excluded from the calculation, the activation energy increased to 180 kJ/mol. This difference in the activation energy value can be explained by the occurrence of dynamic recrystallization in the high-temperature regime; this observation was substantially confirmed by the plots of strain-hardening rate as a function of stress that were used to identify the onset of dynamic recrystallization.
Metadynamic Recrystallization Kinetics of Twin Roll Cast AZ31 Alloy during Hot Deformation
Procedia Engineering, 2014
Metadynamic recrystallization takes place during isothermal annealing of materials in which an incomplete dynamic recrystallization occurred. In magnesium alloys this question has been investigated less until now. The metadynamic recrystallization behavior of twin-roll-cast and annealed AZ31 strip after hot deformation was investigated under conditions corresponding to hot strip rolling process of twin-roll-cast Mg coils. Plane strain compression tests were performed using double hit compression tests at temperatures of 250-400 °C, strain rates of 0.1-5 s-1 and interpass time of 0.5-10 s. Based on the experimental results, the kinetic equations and grain size model were established. Results show the effects of deformation parameters, including forming temperature and strain rate, on metadynamic recrystallization softening fractions and grain size in the two-pass hot deformed AZ31 alloy. Results also reveal that the pre-strain (beyond the peak strain) has little influence on the metadynamic recrystallization behavior in AZ31 alloy. Comparisons between the experimental and the predicted results were carried out. A good agreement between the experimental and the predicted results was obtained.
Journal of Materials Research and Technology, 2022
The microstructure and microtexture of an extruded and annealed GWZ (Mg-8.2Gd-3.6Y-1.6Zn-0.5Zr, wt.%) magnesium alloy was recorded strain by strain in the course of thermomechanical processing. The specimens were compressed down to various interruption strains of 0.1, 0.3, and 0.5 under the strain rate of 0.001s-1 at 400°C, the temperature at which the material was capable to be recrystallized extensively. Appreciable refinement was recognized even at low imposed compressive strain of 0.1, and the recrystallization process was completed at true strain 0.3 where the mean grain size of 4.3μm was attained. The LPSO stimulated nucleation (LSN) and conventional continuous dynamic recrystallization (CDRX) mechanism were contributed in grain refinement. Consequently, the initial basal texture was considerably weakened which was mainly ascribed to the formation of RE-texture components. At higher imposed strain of 0.5, the majority of grains were found in deformed states, the capability of strain softening was decreased and the microtexture only changed in respect of intensity compared with true strain of 0.3. These indicated that the imposed strain was mainly accommodated through dislocation multiplication and tangling within the previously recrystallized grains. Complementary, the slip / twin activity, and the sequence of strain accommodation was investigated through Schmid analysis of the various systems.
Advanced Materials Research, 2012
The effects of heat treatment on the microstructure, tensile property and fracture behavior of as-extruded AZ91 magnesium alloy were studied with OM and SEM. The results show that the grains of as-cast AZ91 alloy are refined by hot extrusion due to dynamic recrystallization, and the mechanical properties are improved obviously. The ductility is significantly enhanced after solution treatment of the as-extruded AZ91 alloy, tensile strength is almost the same as before, and hardness is significantly reduced after solution treatment and artificial aging treatment. The tensile strength reduced and the ductility is significantly enhanced of as-extruded after annealing processes. The fracture surface of as-extruded AZ91 magnesium alloy has the mixed modes of ductile and brittle characteristics. But after T6 or annealing treatment, the dimple number increases evidently.