Microstructure, Properties And Creep Behavior Of Mg-Y-Gd-Zn-Zr Alloys

Magnesium alloys are the lightest structural materials with high specific strength, good electric conduction, thermal conduction, damping capacity, electromagnetic shielding, formability, as well as easy recycled. Recently, Mg-Y-Nd-Zr and Mg-Gd-Y-Zr alloys were development. Many researchers found adding some cheaper zinc into the Mg-RE alloys brought great changes. Zinc can control the precipitate phase and induced the LPSO (Long Period Stacking Ordered Structure). It can exhibit excellent room and high temperature mechanical properties and creep properties. Based on the results of Mg-Gd-Y-Zr alloys, Zn (0.5wt.%-3wt.%) were added into the alloys. The microstructure and mechanical properties of Mg-10Y-5Gd-2Zn-0.5Zr alloy were researched.Several Mg-(5-14)Y-5Gd-(0-3)Zn-0.5Zr alloys were prepared. Effects of variant content of Y and Zn, heat treatment and thermal-mechanical process on the microstructure, mechanical properties and creep resistance were mainly investigated, by computer data collection system, optical microscopy (OM), image analysis apparatus with a image analysis software, X–ray diffract meter (XRD), inductively coupled plasma analyzer (ICP), Differential Thermal Analysis (DTA), Differential Scanning Calorimeter (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive X-ray analyses (EDAX) and micro-diffraction etc.. The strengthening mechanism of the alloys was analyzed and discussed, and micro-structural evolution during aging, including the morphology, structure, size and distribution of the precipitates, was studied in detail. The purpose of the present work is to provide theoretical and practical results for the development of high-performance magnesium- rare earth alloys. The main conclusions can be summarized as follows:1.The as-cast Mg-10Y-5Gd-0.5Zr alloy contains the majorαphase which is supersaturated Gd + Y solid solution in Mg matrix; Mg24(GdY)5 eutectic phase which looks like narrow island morphology and has higher Gd + Y content than the matrix; and the intra-crystalline zirconium rich cores. Adding 2wt.% Zn to Mg-10Y-5Gd-0.5Zr alloy leads to form a long-period stacking-ordered structure via a conventional casting method. The structure is a 6H′-type (ABCBCB′) which is a distorted stacking order from an ideal hexagonal lattice of 6H-type. The angle between the c- and a-axis is estimate to be approximately 88°.2. At 535℃for 16h, the Mg24(GdY)5 net-work of second phase was completely dissolved, and only remained some Mg-Y-Gd cuboid-shaped compound. The optimum solid-solution condition of Mg-10Y-5Gd-0.5Zr is 535℃/16 h. And the optimum solid-solution of Mg-10Y-5Gd-2Zn-0.5Zr is also 535℃/16 h. The peak hardness was obtained at about 225℃for 24h.3. The strengths of cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr decline very slowly from room temperature to 250℃. However, at the During the room temperature to 250℃or more, the strengths steeply decrease. The instant tensile strengths of cast-T6 Mg-10Y-5Gd-xZn-0.5Zr alloys are remarkable superior to those of WE54. A very high strength of cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr alloy with UTS=326MPa, and UTS=261MPa which is at 250℃and 300℃are remarkable superior to those of WE54.4.The present work has investigated theβ′precipitates are formed within grains when the alloy is aged at 225℃for 24h. The globular shapeβ′precipitates are formed in the under aged state, which is corresponding to the highest strength of the alloy. The precipitates coalesce, and the plate shapeβ′precipitates are formed lying in the {2110 } habit planes with increasing ageing time. This intermediate phaseβ′has a base-centered orthorhombic structure (a=0.640nm, b=2.223nm,c=0.521nm), the orientation relationship betweenβ′and the matrix phase is: (100)β′∥( 2110 )a, (001)β′∥(0001)a, [010]β′∥[1010]a. Some Mg12ZnY phases was remain on the grain boundary and the 6H′phase was not changed in the grain.5. The research was focused on the creep properties of Mg-10Y-5Gd-xZn-0.5Zr alloys at high temperature. It is found that the creep resistance was very good when temperature is lower 250℃and the stress is lower 80MPa; but when the temperature is higher than 300℃and the stress is higher than 80MP, the creep properties was serious deteriorated. So the cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr can not be used on above 300℃. In all the Mg-Y-Gd-Zn-Zr alloys, the content of 10-12 wt.% Y alloys have excellent higher temperature creep properties.6. Influences of Zn addition on microstructure and mechanical properties at room and elevated temperatures up to 300℃have been investigated. It can be seen that: 2% element Zn had remarkable improved the creep properties of the Mg-10Y-5Gd-0.5Zr alloy at room and elevated temperatures. For the cast-T6 Mg-10Y-5Gd-2Zn-0.5Zr alloy, at 300℃/50MPa, the steady-state creep rate is 6.60×10-8s-1 and the creep strain after the creep life of 100 hours is 1.76%. The stress exponent at 250℃and 300℃is 2.3 and 5.1, and the apparent activation energy value is 191.9KJ/mol and 216.4KJ/mol when stress was 30MPa and 50MPa.7. Furthermore, the LPSO phase increased after crept at 300℃/50MPa for 100 hours , the quantity and density also increased which is the first time discovered . The purpose of the present work is to provide theoretical and practical results for the development of high-performance magnesium-heavy rare earth alloys. The Mg12ZnY phase was still at the grain boundary, its structure was not changed. Also it can be seen that the large numbers of plate shape equilibriumβphases with bcc crystal structure precipitate along the {1010 } habit planes in the grain.8. Through the analysis of creep data and the TEM results, it is found that zinc element can induce the stacking fault energy debased, and the 6H'-LPSO structure within grains prevent the dislocation movement. During the creep test, some basal dislocation changed non-basal dislocation. The 6H'-LPSO structure brought out serious crystal lattice aberration which hampered the dislocation movement, So the creep properties were increased.9. It is found that the large numbers of plate shape equilibriumβphases hampered the dislocation movement. The inhibition of the basal ship in the Mg-Y-Gd-Zn-Zr alloy is due to the formation of Mg12ZnY phase and the LPSO structure. The deformation twin is deflected in the formation of Mg12ZnY phase and the LPSO structure. The base plane trace of LPSO phase is inclined with constant angle about 4.1°. The stacking fault energy of Mg-Y-Gd-Zn-Zr alloy is quite low due to high Y and Zn additions. During the deformation, the stacking faults can be easily introduced in and the dislocation would be accumulated at the front of stacking fault region.10. The creep deformation mechanisms of Mg-Y-Gd-Zn-Zr alloy were investigated systematically by TEM. According to the results, dislocation slip and the grain boundary slip were the main deformation mechanisms. The Mg12ZnY phase pins up the grain boundary and barrage the grain boundary movement. The LPSO structure andβphases contribute to the strengthening of the alloys and hampered the dislocation movement.