| Good high-temperature strength and creep resistance make rare-earth magnesium alloys widely used in transportation and aerospace parts such as power systems,transmission housing and structural skeletons,even at high prices.However,due to the hexagonal close-packed(HCP)has poor crystal symmetry and limited slip system in room temperature.Magnesium alloy usually needs the addition of a large number of rare earth elements for solution aging to obtain high melting point precipitation phase.It is built on the sacrifice of alloy ductility to improve the strength.Furthermore,a strong basal texture induced by the preferred orientation of grains during deformation,which is highly unfavorable to the subsequent processing.Based on the considerable solid solubility of Gd,Y and Sn elements in magnesium,a kind of plasticization method by microalloying and high temperature solid-solution has been proposed.It provides a low cost and short process for the alloying element with larger solid solubility in magnesium alloy.The main research contents and results are as follows:(1)The solid solution,which similars to a single phase solid-solution,was obtained by means of microalloying and high-temperature solution treatment.The efficient solution of large ingot requiring the content of Gd and Y separately below 0.6 wt.% and 0.9 wt.%.As for the content of Sn,it belows 0.3 wt.% on the condition of mixed additions,and belows 3wt.% when added alone.After high temperature solution,the maximum area percentage of the second phase in Mg-Y,Mg-Gd and Mg-Sn binary alloy was 0.315%,0.606% and 0.037% respectively.In ternary alloys,Mg-Gd-Y has the best solution effect,and the area percentage of the secondary phase was between 0.246% and 0.421%.The area percentage of the secondary phase in Mg-Sn-Y was between 0.665% and 0.896%.In Mg-Sn-Gd alloys,the increase of Gd content has little influence on the secondary phase content of the alloy;all area percentage of the secondary phase was above 1%.(2)The micro-alloying and solution treatment of ingot results in the fine microstructure of as-rxtruded sheets.Large ingots with high alloying elements have poor solid-solution effect at elevated temperature.Their hot extrusion deformation is influenced by both solid-solution atoms and second phase.At this condition,a uniform fine microstructure could be obtained only by further increasing the alloying element content.(3)Addition Gd or Y alone produced a bimodal texture along the extrusion direction(ED).With the increase of element content,tilted angle increased along ED direction,and the texture strength increased first and then decreased.Mg-Gd alloy shows higher ductility than Mg-Y alloy.The maximum elongation-to-failure(?)of Mg-Gd was 35.2%.There is no obvious difference in the ultimate tensile strength.The high-temperature solid solution makes the sheet show good formability at room temperature.The Erichsen value of Mg-0.14 wt.% Y and Mg-0.55 wt.% Gd was high up to 5.4mm and 4.6mm respectively.Trace Sn elements make a significant stretch along transverse direction(TD)in basal texture,and leaded to good isotropy of the alloy.In the Mg-0.2Sn-xY and Mg-0.2Sn-xGd(x=0.4,0.8,1.2,all in wt.%)sheets,the increase of Gd and Y content remain unchanged the texture characteristics.The two major types of alloys showed similar maximum polar density value in the range of 7 M.R.D-9 M.R.D,and distributed in the range of 25o-37o along ED direction.The Mg-0.2Sn-0.4Y alloy exhibits good isotropic.Its difference between the maximum ? and the minimum ? was 2.2%,and the average ? was above 20%.The difference between the maximum ? and the minimum ? of Mg-0.2Sn-0.8Gd alloy was only 0.6%,and the average ? was above 23%.(4)In mixed additions of Gd and Y,microalloying has better grain refinement and texture weaken.The average grain size of Mg-0.4Gd-xY(x=0.4,0.8,1.2,all in wt.%)alloys were in the range 8?m-10?m,and showed high ? above 30% along ED direction.However,with the increase of Y content,the anisotropy of alloy increases.The maximum ? and the minimum ? of Mg-0.4Gd-1.2Y were 33% and 17.5% respectively.In the Mg-0.2Gd-0.2Y alloy,the average grain size was only 6.42?m,which was refined the microstructure of Mg-0.4Gd-0.8Y by 38%.The bimodal texture of Mg-0.2Gd-0.2Y alloy was stretched about 35.7o along TD,but still showed strong anisotropy.The maximum ? and the minimum ? were 32.3% and 21.2%,respectively.(5)The ductility of the traditional alloy was improved after microalloying and high-temperature solid solution.The addition of Sn element reduced the grain refinement brought by solution of Gd and Y in magnesium matrix,but increased the isotropy.The average grain size of Mg-0.2Gd-0.2Y-0.2Sn alloy was 11.16?m,which was 74% larger than the grain size of the Mg-0.2Gd-0.2Y alloy,but still relatively small.The maximum ? and the minimum ? of the alloy separately appeared at the direction of 45o and 90o,were 21.2% and 17.8%,while the yield strength was increased from 110 MPa to 119 MPa.The average recrystallization grain size of Mg-0.2Gd-0.2Y-0.2Zr was only 1.01?m,and a large number of small grain sizes were distributed between 300 nm and 500 nm.The addition of Zr elements has changed the bimodal texture of the rare-earth alloy to a strong basal texture.However,the ductility of the alloy is relatively high and the anisotropy is effectively controlled.The maximum ? was 26.9%,and the ? at the direction of 0o and 90o were separately 25.7% and 24.6%.The ultimate tensile strength ranges from 250 MPa and 270 MPa,which was slightly weaker than the Mg-0.2Gd-0.2Y alloy.(6)Good formability and room temperature plasticity was obtained by micro-alloying and solution treatment.The trace addition of Gd,Y and Zr can promote the start of pyramidal <c+a> and <c> slip.And the Sn element is mainly to promote the prismatic <a> slip,which has little effect on the non-basal slip system of alloy.(7)The fracture surface of alloys with high ductility composed of deep dimples,torn edges and small crystal face.With the reduction of ductility,the dimples gradually became shallow and the quantity decreased,while the torn edges became bigger.Further decrease given an obvious cleavage fracture with a large number of cleavage steps and surface.In addition,there were some secondary cracks and secondary phase particles. |
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