| Magnesium(Mg)rare-earth(RE)alloys containing the long-period stacking order(LPSO)phases,have elicited great interests due to their potential properties for lightweight deformed materials.Now,the research of the alloys is mainly about the improvement of mechanical properties and the investigation of microstructure.The excellent mechanical properties of the Mg-RE alloys are due to the LPSO phases.The LPSO phases are very important in Mg-RE alloys and have many properties,including refining the grains,providing additional slip systems,hindering the movement of the dislocations and storing hydrogen.In this thesis,based on the crystallographic theory the stacking sequences of four kinds of LPSO structures in hexagonal close-packed(HCP)system have been derived in detail.In a heat-treated as-cast Mg-1.0 at.%Zn-2.5 at.%Gd alloy and an as-cast Mg-1.6 at.%Zn-0.5 at.%Dy-2.3 at.%Ni alloy,the stacking sequences of the LPSO phases have been investigatied by using a Cs-corrected scanning transmission electron microscopy(STEM).In a hot-extruded Mg-1.5 at.%Zn-2.0 at.%Y-x at.%Sn(x=0,0.5,1.0,2.0)alloys,the effects of Sn on the microstructure and mechanical properties have been investigatied by using X-ray diffraction(XRD)technology,scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Based on crystallographic theory there are 63 kinds of polytypes of 13H LPSO structure,126 kinds of polytypes of 14H LPSO structure,120 kinds of polytypes of 39R LPSO structure and 223 kinds of polytypes of 42R LPSO structure in HCP system.Besides,their stacking sequences and space groups have been derived in detail.By using a STEM,the stacking sequences of the LPSO phases in the Mg-1.0 at.%Zn-2.5 at.%Gd alloy and Mg-1.6 at.%Zn-0.5 at.%Dy-2.3 at.%Ni alloy have been investigatied.Several atomic resolution high-angle annular dark-field(HAADF)photos have been taken,and it is found that there is a kind of 14H LPSO phase in the as-cast Mg-1.6 at.%Zn-0.5 at.%Dy-2.3 at.%Ni alloy.Meanwhile,a kind of stacking fault is found in the as-cast Mg-1.6 at.%Zn-0.5 at.%Dy-2.3 at.%Ni alloy,and the stacking fault is formed by two atomic layers,which is different from the stacking fault found in Mg-1.0 at.%Zn-2.5 at.%Gd alloy.The stacking fault in Mg-1.0 at.%Zn-2.5 at.%Gd alloy is formed by one atomic layer.0.5,1.0,2.0 at.%Sn have been added to the as-cast Mg-1.5 at.%Zn-2.0 at.%Y alloy and these alloys including the as-cast Mg-1.5 at.%Zn-2.0 at.%Y alloy are extruded at 723 K in order to decrease the cast defects and refine the grain.Tensile test and hardness test are carried out on the alloys.It is found that with the addition of 0.5 at.%Sn the ultimate tensile strength(UTS)increased from 343 MPa to 379 MPa,but with the addition of more Sn the UTS decreased continuously,especially with the addition of 2.0 at.%Sn the UTS decreased from 340 MPa to 239 MPa.The yield strength(YS)tended to change in the same trend of the UTS.With the addition of 0.5 at.%Sn the YS increased from 199 MPa to 230 MPa.And with the addition of 1.0 at.%Sn the YS decreased to 203 MPa.With the addition of 2.0 at.%Sn the YS decreased to 147 MPa.The elongation decreased from 15.9%to 11.3%with the addition of 0.5 at.%Sn,but increased to 17.6%with the addition of 1.0 at.%Sn.With the addition 2.0 at.%Sn the elongation decreased to 3.43%.With the addition of 0.5 at.%Sn the micro-hardness increased from 82.9 HV to 89.1 HV,and then decreased to 81.0 HV with the addition of 1.0 at.%Sn.With the addition of 2.0 at.%Sn the micro-hardness decreased to 51.0 HV.Through the analysis of the XRD,SEM and TEM,it is found that with the increasing content of Sn there is a continuous phase transformation in the hot-extruded Mg-1.5 at.%Zn-2.0at.%Y-xat.%Sn(x=0,0.5,1.0,2.0)alloys.With the 0.5 at.%Sn,some MgZnY particles precipitate and distribute randomly in the LPSO phase.The size of the newborn MgZnY particles is at nanoscale.With the 1.0 at.%Sn,the amount and size of the MgZnY particles increase and those of the LPSO phase decrease obviously.With the increasing content of Sn,the Zn atoms in the MgZnY particles dissolve in the a-Mg matrix.The amount of the MgZnY particles decreases and the amount of the MgSnY particles increases continously.With the 2.0 at.%Sn,the Zn atoms in the a-Mg matrix precipitate as MgZn particles.The grain size of 405 randomly selected particles are measured.It is found that there are 169 MgSnY particles and 236 MgZn particles.The average size of the MgSnY particles is 1.214 ?m and that of the MgZn particles is 1.007 ?m.Besides,the phase Sn3Y5 is observed.In the hot-extruded Mg-1.5 at.%Zn-2.0 at.%Y-xat.%Sn(x=0,0.5,1.0,2.0)alloys,with different concentration of Sn the microstructure evolution is presented by the four qualitative formulas:a)MgZnY(LPSO)+ Sn ? MgZnY(particle)+ Sn3Y5;b)MgZnY(particle)+ Sn ?(MgZnY)Sn(particle);c)(MgZnY)Sn(particle)+ Sn?(MgSnY)Zn(particle)+ MgZn(particle);d)(MgSnY)Zn(particle)+ Sn? MgSnY(particle)+ MgZn(particle).It is found that the enhanced mechanical properties in the hot-extruded Mg-1.5 at.%Zn-2.0 at.%Y-0.5 at.%Sn alloy are attributed to the precipitated MgZnY particles with nanoscale size and disordered distribution in the LPSO phase.Besides,it is found that in the hot-extruded Mg-1.5 at.%Zn-2.0 at.%Y-1.0 at.%Sn alloy,the decreasing amount and size of the LPSO phase lead to an increase of the velocity of the dislocation motion,which resulted in the increase of the ductility.It is found that in the hot-extruded Mg-1.5 at.%Zn-2.0 at.%Y-2.0 at.%Sn alloy the sharp decline of the strength and ductility is assumed as the absence of the strengthening of the LPSO phase. |
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