Based on the deforming technique of severe plastic deformation (SPD), the grain refinement of a Mg-9Gd-3Y-2Zn-0.5Zr alloy treated with decreasing temperature reciprocating upsetting-extrusion (RUE) ...and its influence on the mechanical properties and wear behavior of the alloy were studied. The RUE process was carried out for 4 passes in total, starting at 0 °C and decreasing by 10 °C for each pass. The results showed that as the number of RUE passes increased, the grain refinement effect was obvious, and the second phase in the alloy was evenly distributed. Room temperature tensile properties of the alloy and the deepening of the RUE degree showed a positive correlation trend, which was due to the grain refinement, uniform distribution of the second phase and texture weakening. And the microhardness of the alloy showed that the microhardness of RUE is the largest in 2 passes. The change in microhardness was the result of dynamic competition between the softening effect of DRX and the work hardening effect. In addition, the wear resistance of the alloy showed a positive correlation with the degree of RUE under low load conditions. When the applied load was higher, the wear resistance of the alloy treated with RUE decreased compared to the initial state alloy. This phenomenon was mainly due to the presence of oxidative wear on the surface of the alloy, which could balance the positive contribution of severe plastic deformation to wear resistance to a certain extent.
Reciprocating Upsetting-Extrusion (RUE) deformation process can significantly refine the grains size and weaken the basal plane texture by applying a large cumulative strain to the alloy, which is of ...great significance to weaken the anisotropy of magnesium (Mg) alloys and increase the application range. In this paper, the Mg-8.27Gd-3.18Y-0.43Zr (wt %) alloy was subjected to isothermal multi-passes RUE. The microstructure and texture evolution, crystal orientation-dependent deformation mechanism of the alloy after deformation were investigated. The results clearly show that with the increase of RUE process, the grains are significantly refined through continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms, the uniformity of the microstructure is improved, and the texture intensity is reduced. At the same time, a large number of particle phases are dynamically precipitated during the deformation process, promoting grain refinement by the particle-stimulated nucleation (PSN) mechanism. The typical 10-10 fiber texture is produced after one pass due to the basal plane of the deformed grains with a relatively high proportion is gradually parallel to the ED during extrusion process. However, the texture concentration is reduced compared with the traditional extrusion deformation, indicating that the upsetting deformation has a certain delay effect on the subsequent extrusion texture generation. After three or four passes deformation, the grain orientation is randomized due to the continuous progress of the dynamic recrystallization process.
The thermal deformation behaviour of Mg-9Gd-4Y-2Zn-0.5Zr alloy at temperatures of 360-480 °C, strain rates of 0.001-1 s
and a maximum deformation degree of 60% was investigated in uniaxial hot ...compression experiments on a Gleeble 3800 thermomechanical simulator. A constitutive equation suitable for plastic deformation was constructed from the Arrhenius equation. The experimental results indicate that due to work hardening, the flow stress of the alloy rapidly reached peak stress with increased strain in the initial deformation stage and then began to decrease and stabilize, indicating that the deformation behaviour of the alloy conformed to steady-state rheological characteristics. The average deformation activation energy of this alloy was Q = 223.334 kJ·mol
. Moreover, a processing map based on material dynamic modelling was established, and the law describing the influence of the machining parameters on deformation was obtained. The experimental results indicate that the effects of deformation temperature, strain rate and strain magnitude on the peak dissipation efficiency factor and instability range were highly significant. With the increase in the strain variable, the flow instability range increased gradually, but the coefficient of the peak power dissipation rate decreased gradually. The optimum deformation temperature and strain rate of this alloy during hot working were 400-480 °C and 0.001-0.01 s
, respectively.
The Mg–Gd–Y–Zn–Zr alloy containing a long period stacking ordered (LPSO) phase was subjected to multi-pass deformation by means of a multi-directional forging process, and the microstructure ...evolution and the influence of the LPSO phase on its dynamic recrystallization (DRX) were studied. The results showed that multi-directional forging can effectively refine the grain with the DRX fraction increased, and DRXed grains lead to the decrease of the texture intensity, which can significantly improve the mechanical properties of the alloy. The different morphologies of the LPSO phase have different degrees of promotion relative to DRX behavior. The lamellar LPSO phase with kinks promoted dislocation plugging, where there could be a potential nucleation site for DRX grains. The fragmented lamellar LPSO phase promoted the DRX process through the particle-stimulated nucleation mechanism, and the block-shaped phase was more prone to stress concentration, which promoted DRX. These effects resulted in continuous grain refinement and a more uniform microstructure.
The strain-controlled cyclic deformation behaviour of Mg-9Gd-4Y-2Zn-0.5Zr with different structures was investigated. Alloys were prepared by solution, extrusion and pre-ageing extrusion, and the ...microstructures before and after the fatigue tests were characterized. Experimental results indicated that the bimodal structure owned the better performance in fatigue test, which was attributed to the higher yield strength. For the equiaxed structure, cyclic hardening induced stress concentration until the failure. Stable cyclic deformation and persistent cyclic softening played an important role at the low and high strain amplitudes, respectively. This was attributed to the formation of fine grains relieving the stress concentration during cyclic loading. In addition, residual twins were observed in equiaxed structure to induce crack, and the bimodal structure effectively restrain it.
In this study, a Mg-9Gd-4Y-2Zn-0.5Zr (wt.%) alloy was subjected, after solution treatment, to hot compression deformation at different temperatures (350 °C, 400 °C and 450 °C) and different strain ...rates (0.001 s
, 0.01 s
, 0.1 s
and 0.5 s
) on a Gleeble-3800 thermal simulator. The evolution of the stress-strain curves under different conditions was compared. The changes in microstructure caused by the different deformation parameters and the change law of the long-period stacking-ordered (LPSO) phase during compression were observed and analyzed by optical microscope (OM) and scanning electron microscope (SEM). The results show that with the increase in the deformation temperature and the decrease in the strain rate, the degree of dynamic recrystallization (DRX) gradually increased, and the morphology of the phase also changed through, for example, twist fracture. The continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms activated during the thermal deformation process can effectively refine the grains and weaken the texture in the alloy.
This paper illustrated the effects of pre-aging on the microstructure, texture evolution, and deformation behavior of Mg–9Gd–4Y–2Zn–0.5Zr alloy during hot compression. Dynamic recrystallization (DRX) ...behavior, texture evolution, stress-strain curve, and activation energy of the solid solution and pre-aged alloy were evaluated. The results showed that the second-phase particles of the pre-aged samples exhibit combination of two types particles structure, which is composed of coarse dynamic precipitation particles and fine nanoparticles. The combination of these two types particles showed the best grain refinement effect by accelerating the DRX process, as the coarse dynamic precipitation particles promoted DRX through the PSN mechanism and the fine granular phase pinned dislocations and inhibited grain growth. As a result, the pre-aged samples had a higher DRX grain volume fractions and smaller average grain sizes than the solid solution alloys. In addition, the pre-aged sample had higher deformation activation energy (Q = 294.8 kJ/mol) than solid solution sample (Q = 223 kJ/mol) because β-phase pin dislocations formed during deformation restrained the movement of grain boundaries. The pre-aged alloys also showed a more concentrated texture than the solid solution alloys, with the c-axis of the coarse grains distributed from the compression direction (CD) at 30° inclination to the normal direction (ND). The c-axis distribution of the coarse grains was more scattered in the solid solution alloys. A hot processing map was constructed for the pre-aged alloy and used for evaluation. The optimal deformation parameters for the pre-aged Mg–9Gd–4Y–2Zn–0.5Zr alloy were between 0.001 and 0.1 s−1 at 400 °C–480 °C. These significant observations not only provide theoretical guidance but also practical application for setting plastic forming deformation parameters in industrial production.
In order to further explore the effect of pre-aging treatment on large-scale alloys, a pre-aging extruded Mg-9.32Gd-4.02Y-2.16Zn-0.48Zr (wt.%) alloy with large extrusion ratio (ER=19.5) and extrusion ...temperature of 450°C in large-scale (Φ308 mm×800 mm) was investigated in this paper. It exhibited good properties and a more pronounced anisotropy. The comprehensive performance in the 45° direction was the best. The extrusion direction (ED) presented a high strength, while the transverse direction (TD) showed poor strength and ductility contemporaneously. The main results of strength anisotropy were the alignment orientation of the long-period stacking ordered (LPSO), fine grain strengthening, and texture strengthening. The c-axis of most of the grains in TD samples was perpendicular to the direction of applied stress because of the abnormal //ED texture, which reduced the ductility. In addition, the formation mechanism of abnormal texture has also been studied. The higher extrusion temperature reduced the critical shear stress (CRSS) of the non-basal slip system, and the higher strain promoted the rotation of the base plane along the principal stress axis under the operation of continuous non-basal slip, resulting in the abnormal //ED texture due to lattice rotation. On the other hand, dynamic recrystallization (DRX) also greatly affected the generation of the abnormal texture. Firstly, DRX produced a proportion of grains with a nucleation orientation of , then the subsequent preferential growth of DRX grains led to the vast majority of grains with an orientation tending towards . However, the former has less effect on abnormal texture than the latter.
In this study, the effect of upsetting-extrusion-upsetting (UEU) on the microstructure of an Mg-9Gd-4Y-2Zn-0.5Zr alloy and on dynamic recrystallization (DRX) was investigated. The results showed that ...after UEU, DRX occurred in the Mg-9Gd-4Y-2Zn-0.5Zr alloy. Namely, the grain refinement at the cross-sectional edges of UEU specimens was significant, with the grain size being refined from 8.27 µm in the necking region to 6.57 µm. Grain refinement is mainly caused by DRX where the long-period stacking order (LPSO) phase stimulates DRX around the grain boundaries through a particle-stimulated nucleation mechanism. DRX grains play an important role in weakening the texture by offsetting the severely deformed structure of deformed grains.
The microstructural evolution law and the α variant selection relationship, as well as the deformation mechanism of the HIPed Ti–6Al–4V alloy, were investigated under the conditions of thermal ...compression and subsequent heat treatment. The results showed that lamellar α phase bending/kinking and fragmentation were the main spheroidizing mechanisms in the α+β phase region (850 °C and 920 °C), DDRX occurred as the restoration mechanism in the near-β region (940 °C), and the heterogeneous microstructure originated from the temperature gradient distribution. The strain rate clearly affected the dynamically recrystallized grain fraction and diameter, e.g … Both the fraction and diameter increased at a lower strain rate. The acicular α precipitated from β phase at 940 °C obeyed the Burgers relationship with β, and their misorientation angle intensified at approximately 60°. GBα formed by reducing the adjacent lamellar α misorientation angle; therefore, the GBα nucleation energy was lowest. The microhardness variation was relatively stable along the X direction at α+β phase region deformation, while oscillation appeared at near-β deformation, which can be explained by microstructure heterogeneity after thermal compression. For heat treatment, the trimodal microstructure appeared at 940 °C/0.001 s−1/60% thermal compression and subsequent 950 °C/1 h/WQ heat treatment, and the result might guide practical production to optimize mechanical properties.
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