Microstructure And Ageing Hardening Behavior Of Mg-Dy-Ni Alloy Enhanced By LPSO

Duing to high strength,heat resistant and good ductility of magnesium alloys with rear earth,it has became an important part of commercialized magnesium alloys.Morever,the forming of long period stacking oedered structure(LPSO)in magnesium alloys with rear earth would further improve the obdurability and high resistance of magnesium alloys with rear earth and reduce the cost of the alloy with the amount of rear earth elements decreasing.In this thesis,the microstructure,mechanical properties and ageing hardening behavior of Mg-2Dy-0.5Ni alloy under as-cast state,solid solution state and ageing state were researched.Firstly,the microstructure and mechanical properties of rear earth magnesium alloys with long period stacking oedered structure which formed in Mg-2Dy alloy with 0.5at.%Ni element were analyzed.Furthermore,the microstructure evolution and precipitation hardening behavior of different morphology LPSO phases of the Mg-2Dy-0.5Ni alloy under different temperature solution treatment and different ways cooling treatment were investigated.Also,microstructure and ageing hardening behavior of the extrude Mg-2Dy-0.5Ni alloy under ageing process were investigated.The specific conclusions as follows:Microstructures of the as-cast Mg-2Dy-0.5Ni alloy mainly consisted of ?-Mg phase,lamellar Mg12 Dy Ni phase with LPSO structure distributed between dendrites and a small amount of cubic Dy phases.Comparing the microstructure and mechanical properties of Mg-2Dy-0.5Zn alloy,the addition Ni into Mg-2Dy alloy was easier to form the LPSO phases,more effective ro reduce the second dendrite arm spacing and more obvious to improve mechanical properties of Mg-2Dy alloy than Zn.In the different temperature(560-570?)solution process,lamellar 18 R LPSO phases of the Mg-2Dy-0.5Ni alloy distributing between dendrites transformed into block-shaped 18 R LPSO phases which randomly distributed in the grain interior and web-likely distributed in the boundary.18 R LPSO phases did not completely dissolved in solution process,so 18 R LPSO phases forming by Ni had very high heat resistant.Addition,in solution process of the alloy,dot-shaped 14 H LPSO phases precipitated in the grain interior.At 565? solution 12 h,the second phases of the alloy dissolved relatively sufficiently and microhardness of matrix reached peak(65 HV).The alloy was cooled in different ways after 565? solution 12 h.For continuous cooling conditions(furnace and air cooling),the fine lamellar LPSO phase generally forms in grain interior and block LPSO phase coarsens with increasing cooling time.For discontinuous cooling conditions(air cooling after furnace cooling to 415? and air cooling afte furnace cooling to 265?),block LPSO phase appears from continuous to discontinuous distribution in the Mg matrix and rod-shaped LPSO phase forms.The microhardness of magnesium matrix of the alloy under air cooling after furnace cooling to 415? exhibits the highest value of 62 HV.The high microhardness of magnesium matrix was mainly attributed to block LPSO phase strengthening,while the lowest mirohardness(56 HV)of magnesium matrix under air cooling after furnace cooling to 265? arised from the precipitation of rod-shaped LPSO phase.Grains were obviously refined,18 R LPSO phases were crushed and formed a banded structure along the extrusion direction after the alloy heat extrusion at 360?.Addition,the dynamic recrystallization zone formed in the alloy after heat extrusion and fine lamellar 14 H LPSO phases precipitated in the dynamic recrystallization grains.The extruded Mg-2Dy-0.5Ni exhibited a double-peak ageing behavior in 250? ageing process,which was mainly attributed to the grain refinement strengthening and the LPSO strengthening.The first ageing peak of the alloy was in 54 h,which was mainly attributed to founction together of the fine grain strengthening and the LPSO strengthening.The second ageing peak of the alloy was in 108 h,which was mainly due to the 14 H LPSO phase strengthing with a high volume fraction.