Research on Fabrication,Toughening,Oxidation Behavior of Laves Phase TaCr2 Alloy

Author:Yao Jing

Supervisor:lu shi qiang


Degree Year:2018





Laves phase TaCr2 has potential application as high temperature structural materials because of its excellent mechanical properties at elevated temperature.However,its wide application is limited by the ambient brittleness and poor high temperature oxidation resistance.Traditional casting method cannot avoid the problems of coarse structure and composition segregation of the alloy.In this thesis,the Laves phase TaCr2 alloy was prepared by powder metallurgy method of mechanical alloying(MA)+hot pressing(HP).The process parameters were optimized.The effects of different process parameters on the microstructure and properties of TaCr2 alloy were studied.The effects of adding alloying elements on the microstructure and mechanical properties at room temperature of TaCr2 alloy were discussed.The mechanism of action of alloy element to improve the fracture toughness was studied.Finally,the effect of microstructure on the high temperature oxidation resistance of TaCr2alloy was studied.The effects of Mo,Si and Nd2O3 on the high temperature oxidation of TaCr2 alloy were analyzed.The results indicate that the MA can produce an effect of mechanically activated synthesis of TaCr2,making synthesizing temperature fall to 1000℃.The temperature above 1350℃is required for fully synthesizing of Laves phase TaCr2 from elemental chromiu and tantalum.The model from Miedema theory was used to calculate the molar formation energy of Cr-33.3%Ta powder in the process of MA+HP,in which disordered,amorphous and ordered compounds were formed.The result showed the molar formation energy was consistent with the actual experimental process.MA50h only formed supersaturated solid solutions of Cr(Ta)and Ta(Cr).The amorphous and Laves TaCr2intermetallic compounds were formed during the heating process.The microstructure and mechanical properties of Laves phase TaCr2 alloy produced by MA+HP were studied.The researches revealed the influence law of the ball milling time,hot-pressing temperature,hot-pressing time and other process parameters on alloy phase,grain size,thermal density,Vickers hardness,fracture toughness,and microstructure of Laves phase TaCr2.Grain size increased as hot pressing temperature increased.The increase of hot pressing temperature can increase the density of TaCr2 alloy and the effect of hot-pressing time on density is limited.The 1350℃×1 h hot-pressing process is the optimized process parameter.It regulated the microstructure and composition of TaCr2 alloy by doping Mo,Si and Nd2O3,to improve the fracture toughness and high temperature oxidation resistance of TaCr2 alloy.The results showed that the addition of the Mo was beneficial to the in-situ formation of the TaCr2 alloy,without changing the TaCr2 crystal type,and increased the alloy density,and the tissue is more uniform and fine.When the content of Mo increased from 0 to 5%,the position of Cr in TaCr2 was mainly replaced by Mo.When the content of Mo increased to 7.6%and 10%,Mo replaced Ta more dominantly.The hardness of TaCr2 alloy decreased slightly after adding Mo.The addition of Si elements facilitates the formation of a hexagonal structure of TaCr2(C14 and C36 types).The main phase of TaCr2 was hexagonal structure.After adding Si,the density of sintered samples increased greatly.Si preferentially occupies Cr position in TaCr2-x Si alloy.Ta position substitution also occurs when Si addition exceeds a certain amount(7.6%Si in this experiment).The addition of Si made the alloy sacrifice some hardness but obtained a small increase in fracture toughness.7.6 Si alloy fracture toughness reached the maximum value of 3.13 MPa·m1/2.The addition of Nd2O3 resulted in Cr-rich aggregation in TaCr2 alloy and decreased density.However,Nd2O3 can refine TaCr2 grain and promote TaCr2 to form a cubic structure(C15 type).The results showed that the Mo can improve TaCr2 oxidation resistance at 900℃,the oxide layer is composed of outer Cr2O3+TaCrO4 internal oxidation layer.The weight gain rate of Mo10alloy was only one quarter of that of TaCr2 alloy at 100h.Adding Mo made TaCr2 alloy at 900℃of oxidation resistance increased.The Si improved the oxidation resistance of alloy.Its mechanism is that the increase of Si content in matrix reduced the CrO3 volatilization,promoted the formation of Cr2O3 oxide film,and the enrichment of Si in the Laves phase and generate silicide has increased the stability of the intermediate phase,and reduced the oxygen diffusion velocity to the inside metal.900℃oxide,TaCr2-x Si alloy surface was covered with single Cr2O3 oxide layer.During 1200℃oxide,pesting oxidation occurred in the TaCr2-3.0 Si alloy.The Cr2O3 outer oxidation layer+the TaCrO4 inner oxidation layer were formed on the surface of TaCr2 alloy with an additive amount of over 3.0%of Si.At 900℃oxidation,adding Nd2O3 reduced the oxygen diffusion to the matrix lead the oxide film thinner.It has little effect on the resistance to oxidation.At 1200℃oxidation,Nd2O3can inhibit the reduction between Ta and Cr2O3,help to improve the content of Cr2O3 membrane with better antioxidant properties and stability.But the Cr2O3 oxide film is interrupted and layered so it can not block the oxygen diffusion to the matrix.The oxidation resistance of the alloy is not improved.