Microstructures And Mechanical Properties Of Mg-8Li-3Al-lZn Alloys Containing Dilute Content Of Ag, Y, Gd

Magnesium alloys are the lightest of all practical metallic alloys for structures. They possess excellent mechanical properties including high specific strength and high specific rigidity. They are remarkable as materials for future generation aircraft. Notwithstanding, the specific strength of practical magnesium alloys has not attained that of high-strength aluminum alloy. It is important for expansion of magnesium usage that the specific strength of magnesium alloy exceeds the value of high-strength aluminum alloys. In Mg alloys, the (0002)<1120> basal slip takes place preferentially because the critical resolved shear stress (CRSS) for a basal slip is lower than those for prismatic and pyramidal slips. This gives rise to the poor ductility for a polycrystalline Mg and its alloy. Two main ways exist to increase the specific strength of magnesium alloys. One way is the pursuit of high strength through addition of hardening elements such as yttrium(Y) and zinc(Zn), which usually increases the alloy's density. The other way is addition of an element lighter than magnesium such as Lithium (Li). According to this paper, Mg-8Li-3Al-1Zn alloy is prepared and the effects of Gd, Y, Gd on microstructure and mechanical properties of Mg-8Li-3Al-1Zn alloy are studied.In order to obtain Mg-Li-Al-Zn samples with various grain sizes and textures, the as-cast ingots are extruded into thick plates in this paper. Phase analysis and crystal lattice parameters(c/a) are determined by X-ray diffractometer (XRD). Phase composition and concentration of element are determined by EDS. Grain morphology and microstructure along the direction of the ED are determined by optical microscope. Tensile experiments are conducted on the MTS (858.01 M) testing machine with the constant strain rate of 1×10-3 s-1 at room temperature and 423K, respectivly. Crystal-plane slip of Mg-Li-Al-Zn alloys with dilute content of Ag, Y, Gd is determined by (0002) pole figure and ODF. Combinating of above mentioned testing methods, this paper mainly on the following three points:(1) The influence of addition of element Ag on Mg-Li-Al-Zn alloy is studied in section 3.1. There is a new phase MgLi2Ag in the grain boundary of Mg-Li-Al-Zn-xAg alloy(x=0,0.2, 0.5,1.0 wt%). MgLi2Ag phase can effectively weaken main phase, refine grain, improve grain boundary, slack off (0002) texture. The addition of element Ag can make <a> and <a+c> silps easily to carry out by recede composition of texture as {0001}<1010>, {0001}<1011>,{1210}<0001>,{1103}<1211>,{0110}<0001> and{1105}<1120>. The ductility of the alloy at room temperature and 423K are both increased. However, excessive addition of Ag will weaken the elongation of alloy.(2) The influence of addition of element Y on Mg-Li-Al-Zn alloy is also studied in section 3.2. There is a new phase Mg3Al3Y2 in the grain boundary of Mg-Li-Al-Zn-xY alloy(x=0,0.5,1.0 wt%). Mg3Al3Y2 phase can effectively strengthen main phase, refine grain, improve grain boundary, slack off (0002) texture. On the other hand, the basal plane texture of Mg-Li-Al-Zn-xY alloy exhibites the unique basal texture, where the position of basal poles is tilted by about 35°from the ED toward the TD. The ultimate strength, yield strength and elongation of the alloy at room temperature and 423 K are both increased. When the concentration of element Y in 0-1.0 wt%, the more mechanical properties of the alloy are impoved by addition the more amount of Y.(3) The influence of addition of element Gd on Mg-Li-Al-Zn alloy is also studied in section 3.3. There is a new phases MgAlGd2 in the grain boundary of Mg-Li-Al-Zn-xGd(x=0, 1.0wt%). MgAlGd2 phase could effectively strengthen main phase, refine grain, improve grain boundary, slack off (0002) texture. One the other hand, the basal plane texture of Mg-Li-Al-Zn-xY alloy exhibited the unique basal texture, where the position of basal poles is tilted by about 35°from the ED toward the TD. It is a typical feature of extruded Mg alloys with RE. With the addition of Gd,{1210}<1010>,{1102}<1101> and {1101}<1543> plane appeares. The elongation, ultimate strength, yield strength and elongation of the alloy at 423K are also increased. The elongation of alloy at normal temperature is improved, but not obviously.The result shows that the elements optimize the mechanical properties of Mg-Li-Al-Zn alloys with incorporation of Ag, Y and Gd.