The Strengthening Mechanism Of Ceced Mg-Gd Alloys

Mg-1Gd(0.73 wt.%Gd),Mg-4Gd(3.62 wt.%Gd)alloys and pure magnesium were chosen as the research materials to study the microstructure and mechanical properties of magnesium alloys after cyclic extrusion compression(CEC) in this article. The effects of CEC pass and CEC temperature on the grain size and texture of pure Mg and Mg-Gd alloys were investigated by optical microscopy(OM), scanning electron microscope(SEM) and electron backscattered diffraction(EBSD). The mechanical properties at room temperature of CECed pure Mg and Mg-Gd alloys were measured. The emphases were placed on the discussion about how the grain size, texture and the addtion of Gd influence refinement mechanical properties.The microstructure evolution of pure Mg and Mg-Gd alloys after CEC passes of 2~8 at 350℃ was studied. The results show that both pure Mg and the magnesium alloys can be refined effectively after CEC for 2 passes. For the pure Mg, with the addition of passes, the grain sizes of CECed pure magnesium showed no obvious differences, about 50 μm. However,the grain size of Mg-Gd alloys was further reduced as the addtion of CEC passes. And the grain size of Mg-4Gd alloys after CEC for 8 passes is 1.5 μm, which is smaller than Mg-1Gd alloys.Texture evolution of pure Mg and Mg-Gd alloy during CEC processing was studied EBSD. For the pure Mg, it was found that a texture, in which the basal plane inclined 20~40o to extrusion direction. With the addition of CEC passes, the maximum intensity of the texture and Schmid factor of basal slip system increased. For Mg-Gd alloys, the basal plane inclined 40~60o to extrusion direction. The intensity of texture ofMg-Gd alloys is much lower than which in pure Mg, and with the increase of Gd content, the intensity of texture decrease dramatically.The mechanical properties of pure Mg and Mg-Gd alloys after CEC at 350℃for 2 passes were increase markedly. While with the addtion of CEC passes, the yield strength of pure Mg show a gradual decline, but the elongation increases. Different with pure Mg, both of yield strength and the elongation is increase with the addtion of CEC passes. The Mg-4Gd alloys show more higher mechanical properties performance than Mg-1Gd after CEC for 2~8 passes at 350℃. The yield strength of CECed Mg-4Gd for 8 passes is 240 MPa, and the elongation is 20%.The grain size, texture type, texture intensity, and the percentage composition of Gd have a great influence to the mechanical properties of CECed pure Mg and Mg-Gd alloys.The result shows grain size is main factor which make the difference of the mechanical properties of pure Mg CEC at different temperatures(250℃, 350℃, 450℃). The yield stress and grain size of the pure Mg processed by CEC for 2 passes at follows the Hall-Petch relationship, which can be decreased as σs=5.4+338.6d-12. And the yield stress and grain size of Mg-1Gd and Mg-4Gd CEC for 2,4,8 passes at 350 ℃ can be decreased as σs=135.3+61.3d-12 and σs=118.2+144.4d-12.The addition of Gd on magnesium alloy makes an observable increase of mechanical properties because of solid solution strengthening. In addition, Gd significantly refines the grain size of dynamic recrystallization, change the types of texture, reduces the intensity of texture of Mg alloys during CEC process, so after the reciprocating extrusion deformation, significantly improve the strength of the alloy and plastic. These microstructure and texture changes from Gd addtion are the main reasons to improve the performance of CECed Mg-Gd alloys.