Investigation Of The Preparation Process And Property Of Lanthanum Oxide Doped Molybdenum Alloys By Liquid-Liquid Doping

In the paper,the Mo-La₂O₃ alloy powders with La₂O₃ doping content of 0.3wt.%were prepared by liquid-liquid doping method.Mo-La₂O₃ alloys were prepared by high energy ball milling+spark plasma sintering?spark plasma sintering and hot pressing sintering.It was studied that the effect of liquid-liquid process on the phase,morphology and grain size of rare earth molybdate.The microstructure and mechanical properties of sintered Mo-La₂O₃ alloys were studied,and the mechanism of strengthening and toughening of Mo-La₂O₃ alloys was discussed.The liquid-liquid doping process result showed that the pH of lanthanum nitrate has a great influence on the crystal procduct phase.Lanthanum nitrate with pH of 3.5?4.4 would produce single rare earth dimolybdate phase as crystal product.However,the crystal products would turn to mixture of molybdic acid and rare earth ammonium molybdenum oxide when the pH of lanthanum nitrate was 4.6.Furthermore,the rare earth dimolybdate grain sizes of crystal products were also increased with the increasing of the stirring rate of the mixed solutions and the increasing of the pH of lanthanum nitrate.The rare earth dimolybdate grain sizes of crystal products were decreased with the increasing of heating power.The uniform grain sizes?surface smooth and regular shape of crystal products when the heating power was 0.18kW,the stirring was 600r/min and the pH of lanthanum nitrate was 4.0.The average grain size of spark plasma sintered Mo-La₂O₃ alloy is 3.93 ?m,and the average grain size of hot pressing sintered Mo-La₂O₃ alloy is about 40.43 ?m.Molybdenum alloy was sintered by high-energy ball mill+spark plasma sintering which the alloy grain size can reach nanometer scale,the average grain size is about 42 1.7nm.The molybdenum alloys had a higher density after sintered.Hardness values of alloys were increased with the increasing of grain size.The result of compression experiments at room temperature showed that the yield strength of the Mo-La₂O₃ alloys is improved from 420Mpa to 2000Mpa with the decreasing grain size from dozens of microns to hundreds of nanometers.The result of three-point bending test showed that the bending strength of the hot pressing sintered Mo-La₂O₃ alloy reaches the least is 848Mpa,the sintered molybdenum alloys by spark plasma sintering had the highest bending strength value which could reach to 1377Mpa.The result of fracture toughness test showed that the fracture toughness values of sintered Mo-La₂O₃alloys were increased with the increasing of grain size.The mainly reason is that the coarse lanthanum oxide particles make the direction of crack expand change,increase the crack expand path.Through the observation of fracture morphology,Fracture morphology of Mo-La₂O₃ alloy by high-energy ball mill+spark plasma sintering is intergranular fracture,and Mo-La₂O₃ alloys by hot press sintering and spark plasma sintering are intergranular fracture and transgranular fracture.