Effect Of Rare Earth On The Microstructure, Properties And Texture Of α-Mg-Li Alloy

As the lightest metallic structural materials with low density, high specific stiffness andstrength, weaken anisotropy, Mg-Li alloy is very attractive in defense, aerospace, automotiveand electronics industry. However, the poor strength limits the application of Mg-Li alloy.Adding alloying elements is an effective and simple method to improve the strength of thealloy among the method of strength improving. Rare earth elements are the main elementsused in magnesium alloys as alloying elements.Y and Nd were added into α phase Mg-Li alloy in the study to optimizing the content of thetwo rare earth elements, researching the effects of Y and Nd on the microstructure, phasescomposition, mechanical properties of the alloy and the differences of strengthening andtoughening mechanism of Nd and Y and confirming that Nd and Y have the compositestrengthening and toughening in Mg-5Li-3Al-2Zn alloy, studying the characteristics throughanalyze the deformation textures, realizing the effects of changing plastic deformationmechanism with Y and Nd’s addition in alloy.The as-cast Mg-5Li-3Al-2Zn alloy is composed of α-Mg substrate and filamentous AlLiphase. After adding Y, Al2Y generates in the alloy, and the amount of AlLi reduces. When thecontent of Y is0.8%, the grain refining effect of as-cast alloy reaches the best, and the tensilestrength reaches the highest level of202MPa. The dynamic recrystallization arising afterextrusion, as the content of Y is1.2%, the alloy has the highest tensile strength and elongation,325MPa and23.3%, respectively.With the increase of Nd content, needle-like Al11Nd3and massive Al2Nd intermetalliccompounds exist in the alloy. When the content of Nd is1.2%, the alloy achieves the bestmechanical properties,195.2MPa of tensile strength and17.2%of elongation. When the Ndcontent is2.0%, the average grain size of the alloy reduces to68μm, smaller than that of alloywithout Nd addition by44%. For the extruded alloys, the alloy with0.8%Nd possesses thehighest tensile strength of281.9MPa. When the Nd is1.6%, the elongation of the extrudedalloy reaches the maximum value of27.2%.The α-Mg, AlLi, Al11Nd3and Al2Y exist in Mg-5Li-3Al-2Zn alloy with the addition of Yand Nd. When the content of Y and Nd is1.2%and0.8%, respectively, the effect of grainrefining is the best, while the average grain size is of30μm, and the tensile strength achieves the highest value of231MPa and the elongation also reaches a maximum value of16%.The dynamic recrystallization arises in the Mg-5Li-3Al-2Zn-1.2Y-0.8Nd alloy during hotextrusion, resulting in the equiaxed fine grains. The tensile strength at room temperature is224MPa and the elongation is24.6%. At the temperature of100℃,the tensile strength andelongation are166MPa and37.9%, respectively. When the strain rate is10-3s-1, thecompressive strength is397.7MPa, and the compressive strength is406.7MPa when the strainrate is10-2s-1. The yield strength under the corresponding strain rates also has a similar law.The plastic instability phenomenon appears in the alloy during the quasi-static tensile processat room temperature. The compression test along ED at room temperature shows that the yieldstrength and the maximum compressive strength are both insensitive to the strain rate.Comparing the tensile strength and elongation of Mg-5Li-3Al-2Zn-1.2Y,Mg-5Li-3Al-2Zn-0.8Nd and Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, it shows that the increasing ratiofor the1.2Y+0.8Nd is greater than that for the sum of adding of1.2Y and0.8Nd separately,confirming the effect of the composite strengthening of Y and Nd in Mg-5Li-3Al-2Zn.There is no specific type of texture arising in as-cast alloys, the texture of the alloy with Yand Nd shows more scatter. In the compressive strain of0.17, the C-axis of most of the grainsis toward ND about75°in Mg-5Li-3Al-2Zn. For Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, a strongcylindrical texture appear when the compressive strain is0.10, most of the grains distributing_with the orientation of <1010>//ND.The texture components analysis between Mg-5Li-3Al-2Zn-1.2Y-0.8Nd andMg-5Li-3Al-2Zn under different compressive strain showed that when the strain is less than0.10, the texture components of the two alloys distribute both evenly. When the strain is0.17,_the texture components of the two alloys distribute both unevenly.(1120)<0001> texture ofMg-5Li-3Al-2Zn-1.2Y-0.8Nd increases with the strain increasing. The two alloys show goodplasticity under compression process at room temperature, exceeding20%. The highestcompressive strength of Mg-5Li-3Al-2Zn is311MPa, after adding Y and Nd, it can reach403MPa, about increasing by30%.In as-extruded Mg-5Li-3Al-2Zn-0.4Nd alloy, the (0002) pole figure exhibites the texturealong TD, appearing in ND deviating leftwards TD about5°. When the content of Nd is2.0%,the texture is non-basal texture (ND deviating rightwards TD about30°).Obvious non-basal texture appears in as-extruded Mg-5Li-3Al-2Zn-1.2Y-0.8Nd, the maximum density area appears from C-axis toward ED60~70°of the (0002) pole figure.EBSD shows that the grain orientation of the non-basal texture is about85°. The grainboundary analysis of the result show that the small-angle grain boundaries take a smallproportion, while the large-angle grain boundaries distribute more evenly. The grain of thealloy after deformation becomes small, the proportion of grains smaller than5μm is50.1%.After annealing treatment at400℃for30minutes,60minutes and90minutes, the non-basaltexture weakens, while the average grain size of the alloy also significantly increases, thestrength and ductility of the alloy decline. The plastic instability phenomenon appears in thetensile test at room temperature.The significant non-basal texture appears in Mg-5Li-3Al-2Zn-1.2Y-0.8Nd afterwarm-rolled and hot-rolled with a reduction percentage of larger than50%. Symmetricaldouble-peak on both sides of RD appears in (0002) pole figure. When the reductionpercentage reaches28%under warm rolling, the C-axis of most of grains is toward RD of25°.The texture is strengthened when the reduction percentage reaches52%.The studies of the textures of compressed, extruded, warm-rolled and hot-rolled ofMg-5Li-3Al-2Zn-1.2Y-0.8Nd alloy shows that the prismatic slip systems is easier to beactivated than the basal slip system, and the prismatic slip and pyramidal twinning are themain deformation mechanisms of this alloy.