Investigation On The Plastic Characteristics Of A Mg-Gd-Y Alloy

Commonly, the comprehensive mechanical properties of the wrought magnesium alloys are higher than those of the cast magnesium alloys. For the Mg-Gd-Y alloys, they present higher strength at room and high temperatures and better creep resistance. Therefore, they are attractive for potential application in aerospace, weapons and high performance automobiles. Nevertheless, the greatest obstacle to deform the magnesium alloys from their low plasticity caused by the limited slip systems due to their close-packed hexagonal structure.One of the effective means to improve the ductility of the magnesium alloys is grain refinement by plastic deformation at a certain temperature (commonly at high temperature) and strain rate. It is very sensitive to the temperatures and strain rates of the magnesium alloys to deform. Under a certain deformation condition, the plastic deformation of the magnesium alloys might be controlled by dislocation slip, cross slip and climbing, vacancy diffusion, grain boundary slide, and mechanical twinning. Previous works have shown that it was synergistic, competitive and transformational among the different plastic deformation mechanisms of the polycrystalline magnesium alloys. Besides, the dynamic recrystallization (DRX) or local shear deformation might have important influence on their plastic deformation. Though a considerable amount of researchs have been conducted on the plastic deformation characteristics and the mechanisms of the Mg-Gd-Y alloys with high rare earth contents, there still exist many problems to be solved urgently, as shown following:(1) The strain rate constitutive equations for the magnesium alloys have been built based on the recovery creep theory of the dislocations thermal activation. Therefore, the deformation mechanisms of the alloys under a certain experimental condition can be proposed by the obtained apparent active energy. However, the detailed studies about the range of convenience of the constitutive equations which were built based on the recovery creep theory of the dislocations thermal activation had not been conducted on the Mg-Gd-Y alloys. (2) It is an important mechanism of the mechanical twinning to coordinate the plastic deformation of the Mg-Gd-Y alloys. There is a strong relationship between the contribution of the mechanical twinning on the plastic deformation and the twinning modes. Previous works on the twinning modes in the Mg-Gd-Y alloys applied mainly the data for pure magnesium or the other magnesium alloys, especially the Mg-Al-Zn alloys. Due to the fact that the twinning shear depends on the lattice parameters which were affected by the variety and contents of the alloy elements. This might lead different results by using the obtained results for the pure magnesium or the other magnesium alloys to analyze the twinning modes in the Mg-Gd-Y alloys with high rare earth contents because of the major difference for the lattice parameters existing between those Mg-Gd-Y alloys and the pure magnesium or the other magnesium alloys. (3) Previous works has found that the adiabatic shear bands existed in the Mg-Gd-Y alloys deformed at high rate. Nevertheless, the criterion to form the adiabatic shear bands for these alloys has been not proposed.Present work focuses on the above three aspects about the necessary to investigate the plastic deformation characteristics and mechanisms in the Mg-Gd-Y alloys. The plastic deformation mechanism maps for the magnesium alloys with different grain size were built by existing strain rate constitutive equations about dislocation slip, cross-slip and climb (recovery creep theory of the dislocations thermal activation), and vacancy diffusion, etc. The plastic deformation mechanisms in the Mg-Gd-Y alloys deformed at different conditions were investigated using the obtained deformation mechanism maps. The compressive deformation experiments at different temperatures and strain rates were conducted on the Mg-10Gd-2Y-0.5Zr alloy using Gleeble thermal analog compression and the split Hopkinson pressure bar (SHPB) techniques. The building of the strain rate constitutive equation using the recovery creep theory of the dislocations thermal activation for the Mg-10Gd-2Y-0.5Zr alloy was tried. The lattice parameters and the axis ratio c/a of the pre-deformed Mg-10Gd-2Y-0.5Zr alloy were obtained by application of the XRD Rietveld analysis. The twinning characteristics, twinning modes and the twinning modes activated first in the alloy compressed at 300～500℃and 0.01～20 s-1 were investigated using the TEM observation accompanied by the minimum shear hypothesis. The deformation characteristics and deformation mechanisms in the Mg-10Gd-2Y-0.5Zr alloy were studied by optical microscopy, TEM and XRD. The constitutive equation at high rate deformation for the Mg-10Gd-2Y-0.5Zr alloy with grain size of 10μm was built based on the modified Johnson-Cook (J-C) constitutive model. Therefore, the criterion to form the adiabatic shear bands for the alloy was studied. The major conclusions of this work are shown following:(1) The plastic deformation mechanism maps of the magnesium alloys were made up of phonon or electron drag, dislocation slip/mechanical twinning, dislocation climbing/cross slip (power-law creep), vacancy diffusion (diffusion creep) and Harper-Dorn (H-D) creep regions when these alloys were deformed at shear strain rates of 10-6-102 s-1 and temperatures homogenized by the melting point of 0-1. With decreasing the grain size, the region for power-law creep decreases but that for diffusion creep or H-D creep increases. Decreasing the grain size to～0.1μm further, the region for power-law creep might be replaced by diffusion creep controlled by grain boundary diffusion. The region for H-D creep can only be found in the grain size of～204-255μm when the region for diffusion creep disappears. The region for H-D creep and diffusion creep would disappear when the grain size is higher than～255μm, and the grain size has little effect on the maps. The predominant plastic deformation mechanisms in the Mg-Gd-Y alloys deformed at a certain condition can be investigated using the obtained maps for magnesium alloys. Nevertheless, if the non-thermal activation mechanisms, such as mechanical twinning, have important or predominant influence on these alloys, the effect of the non-thermal activation mechanisms on the maps should be considered.(2) The plastic deformation of the texture-free Mg-10Gd-2Y-0.5Zr alloy compressed at 300-350℃and 0.01-20 s-1 was controlled by basal dislocation slip and mechanical twinning. Besides, the local shear bands which were consisted of dynamic recrystallization (DRX) grains (transformation bands) can be found in the sample compressed at 350℃and 20 s-1 The twinning process was obtained by forming the primary twins with step characteristic first by means of the {10-11} twinning mode and then forming the secondary twins in the primary twins near the step through the{10 12} twinning mode. The area fraction of the twinning increases first then decreases with increasing the strain rate. The strain rate to obtain the maximum area fraction in the compressed samples decreases from 10 s-1 at 300℃to 5 s-1 at 350℃. When the compressive temperatures increase to 400-500℃, the microstructures of the observed alloy compressed at 0.01-20 s-1 was made up of transformation bands. The transformation bands characteristics become weak with increasing the strain rate. And the deformation microstructure has a change trend from transformation bands to DRX. It is cooperation, competition and transformation among the dislocation slip, mechanical twinning, transformation band and DRX in the compressive process of the observed alloy.(3) The plastic deformation behavior of the Mg-10Gd-2Y-0.5Zr alloy under lower temperatures cannot be described by the strain rate constitutive equation built based on the dislocation thermal activation.(4) The modified J-C constitutive model can be used to show the dynamic mechanical response of the Mg-10Gd-2Y-0.5Zr alloy with 10μm compressed at -100-460℃and～103 s-1. The modified J-C model which was characterized by stressσ, strainε, strain rateε,ε0 for the quasi-static experiment, temperature T, melting point Tm, and the Tr for the quasi-static experiment for the Mg-10Gd-2Y-0.5Zr alloy with grain size of 10μm can be expressed by:(5) The relation among the minimum strainεc, the minimum strain rateεA when the adiabatic shear bands can be formed in the Mg-10Gd-2Y-0.5Zr alloy with grain size of 10μm, and the heat capacity Cp, thermal conductivity k, melting point Tm, stressσ, the characteristic dimension of the sample R, the surface area and the volume of the sample, and a constant ofα0 with order 2 were: where:...