Papers by Keyword: Mg-Gd-Y Alloy

Abstract: Rolling processing has been carried out on samples cut from the extruded Mg-9Gd-3Y-0.5Zr seamless tubes. Effects of rolling reduction, 5%, 20% and 70% per pass, on its microstructure and mechanical properties were investigated. The results showed that the strength and ductility varied with increasing rolling passes at different rolling reduction. The strength of the alloy rolled by 5% reduction per pass gradually improved with increasing rolling passes, and its ductility remained basically constant. However, when 20% reduction per pass was applied, the strength and ductility of the alloy after rolling increased at first and then decreased a little after the accumulative strain higher than 52%. Moreover, as reduction reached 70% per pass, macro-cracks were induced when rolling at 420°C, while the samples were rolled at a high temperature of 500°C and a larger reduction of 70% per pass exhibited the mechanical properties comparable to those fabricated by 5% and 20% reduction. This indicated that a relatively higher productivity via rolling as well adequate mechanical properties can reach for the large scale of industrial products.

Abstract: This paper investigates the evolution of the deformation mechanisms in a homogenized Mg-10Gd-2Y-0.5Zr alloy ingot compressed at 300-500 °C and 0.1-20 s^-1. It can be found that the basal slip and mechanical twinning are the major deformation mechanisms in the alloy compressed at 300 and 0.1-20 s^-1. Increasing the testing temperature to 350 °C, basal slip, non-basal slip and mechanical twinning control the plastic deformation of the alloy compressed at 0.1-20 s^-1. When the testing temperatures increase further to 400-500 °C, the mechanical twinning is replaced gradually by the local shear bands which are formed by dynamic recrystallization (DRX) grains (referred as transformation bands). The transformation bands have the trend to form the typical DRX microstructure with increasing the temperatures (might be caused by increasing testing temperatures or strain rates). Besides, the transformation bands can also be found in the sample compressed at 350 °C and 20 s^-1 when the temperature in the deformation alloy is high enough to activate non-basal slip and form DRX grains at local zone.

Abstract: In this paper, an as-extruded Mg-10Gd-2Y-0.5Zr alloy was compressed at 3265 s^-1 and -20 °C using a split Hopkinson pressure bar (SHPB) machine. Details about the initial texture and the lattice parameters of the as-extruded alloy were obtained by pole figures combined by orientation distribution function (ODF) and X-ray diffraction (XRD) Rietveld refinement method, respectively. It can be obtained that the maximum average orientation factor being 0.297 can be obtained at 45^o angled to the extruded direction that is the force axis, resulting in the fracture of the impact compressed sample along a plane orientated at an angle of ~45^o to the compression axis. Analysis of the fracture reveals that the fracture forms near the periphery of one face of the specimen and then penetrates into the bulk material subsequently meet near the other face. Even though the local shear deformation exists in the compressed sample, the fracture of the as-extruded Mg-10Gd-2Y-0.5Zr alloy compressed at 3265 s^-1 and -20 °C is not caused by forming the adiabatic shear band (ASB).

Abstract: This paper investigated the twinning mode in a homogenized Mg-10Gd-2Y-0.5Zr ingot compressed at 300 oC and 1 s-1 using transmission electron microscopy observation and schematic method. Details about the primary twinning mode was studied based on the minimum sear criterion by comparing the values of the four major twinning modes in the magnesium alloys. The results show that the twins are formed first by {10-11} primary twinning being step characteristics, and then the secondary twins are formed later around the step by {10-12}secondary twinning. The results also show that the XRD Rietveld refinement method is an effective means to obtain the lattice parameter and to calculate the twinning shear that has important effect on the primary twinning mode in a h.c.p alloy.

Abstract: Supersaturated Mg-Gd-Y alloy followed by aging at 225 °C with different times were subjected to quais-static and dynamic strain rates to determine the influence of precipitate phase β′ on the strain rate sensitivity of magnesium alloy. Strain rate sensitivity (SRS) decreases with the increase of the size of β′. SRS decreases from initial condition to peak-aged condition due to the β′ increases the athermal component of flow stress. On the other hand, the influence of precipitate interfaces on dislocation generation and storage mechanisms may be responsible for the decrease of SRS from peak-aged to over-aged condition.

Abstract: Tensile and compressive tests were performed on extruded Mg-10Gd-2Y-0.5Zr (mass fraction, %) alloy specimens with different tilt angles relative to extrusion direction. The microstructures were examined by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Calculations of the orientation factors for basal slip were done for that existed texture. The results show that the alloy doesn’t show tensile-compressive yield strength asymmetry, and the highest flow stress is appeared along the extrusion direction. Meanwhile the extruded textures are much more randomized. The reason may be that the addition of rare-earth is benefit to activate the non-basal slips, especially pyramidal <c+a>slip. The reduction of c/a ratio is helpful to enhance the symmetry of the Mg crystal, which decreases the critical resolved shear stress of <c+a> slip.

Abstract: The Mg-12Gd-2Y-0.5Sm-0.5Sb-0.5Zr alloy was prepared under flux protection. The morphology and crystal structure of β′ precipitate phases in aged alloy has been studied using transmission electron microscopy and X-ray diffraction. The orientation relationship between β′ precipitate and matrix could existed as [ 010]B_αB// [ 00]Bβ′B, (01 0) BαB// (020)B_β′B and (0001)B_αB was coherent with (001)B_β′B; [0001]B_αB// [001]B_β′B, (1 00)B_αB// (240)B_β′B and ( 010)B_αB was coherent with (0A^_,8EEA0)B_β′B. The HREM images indicated that the β' precipitates have a long-period ordered structure at the same time.

Abstract: In this paper, a new phenomenological and empirically based constitutive model was proposed to change the temperature term in the original Johnson-Cook constitutive model. The new model can be used to describe or predict the stress-strain relation of the metals deformed over a wide range of temperatures even though the current temperatures are lower than the reference temperature. Based on the impact compression data obtained by split Hopkins pressure bar (SHPB) apparatus about one hot-extrudedMg-10Gd-2Y-0.5Zr alloy, the material constants in the new model can be experimentally determined using isothermal and adiabatic stress-strain curves at different strain rates and temperatures. Good agreement is obtained between the predicted and the experimental flowcurves for the hot-extrudedMg-10Gd-2Y-0.5Zr alloy at both quasi-static and dynamic loadings under a wide range of temperatures ever though the current temperatures are lower than the reference temperature.

Abstract: Deformation mechanism maps at 0-883 K and shear strain rate of 10-10-10+6 s-1 were built from available rate equations for deformation mechanisms in pure magnesium or magnesium alloys. It can be found that the grain size has little effect on the fields of plasticity and phonon or electron drag, though it has important influence on the fields of power-law creep, diffusion creep, and Harper-Dorn creep in the maps within the present range of temperature, strain rate, and grain size. A larger grain size is helpful to increase the field range of power-law creep but decrease that of diffusion creep when the grain size is smaller than ~204 μm. Harper-Dorn creep dominates the deformation competed to diffusion creep in the grain size range of ~204-255 μm. The maps include only plasticity, phonon or electron drag, and power-law creep when the grain size is higher than ~255 μm, then the grain size has little influence on the maps. Comparison between the reported data for the Mg-Gd-Y alloys and the maps built from available rate equations, it can be conclude that the maps are an effective tool to predict or achieve a comprehensive understanding of the deformation behavior of the Mg-Gd-Y alloys and to classify systematically their discrepancies in the deformation mechanism. However, differences exist in the deformation mechanisms of the alloys observed by the reported data and that predicted by the maps. Therefore, refinement of the maps from the viewpoint of mechanical twining, DRX, and adiabatic shear are necessary.

Abstract: Mg-10wt%Gd-3wt%Y alloy was cast in a step-like mould, which provided five different cooling rates. The dentrite morphology and hardness of the as-cast alloy from the surface to the center was investigated and the influence of the cooling rates on these was analyzed. It was indicated that there were two different trends for the hardness distribution: in the section of the step castings with the two slowest cooling rates, the hardness decreased with the increase of depth; while in the other three step castings the hardness increased with the increase of depth. Also it was founded that the hardness could be influenced by the grain boundary, dentrite morphology and dentrite arm spacing (DAS) in the alloy. At last, according to the standard deviation curves of the hardness, the chilled depths were about 5mm in 1st step and 10mm in 2nd step. And the other steps were fully chilled.