Study On Cobalt Based Novel Co-Al-W Superalloys Design, Preparation And Properties

Co-Al-W superalloy is strengthened by a ternary compoundγ′-Co₃(Al,W) phase with the precipitation strengthening onγ-Co matrix novel Co-base superalloys. Conventional Co-base superalloys lack effective precipitation strengthening by intermetallic compounds, and depend on alloying elements and precipitation of low volume fraction of carbides for their strength. The microstructures,carbides typology and distribution have effect on characteristics and properties of Stellite 6 alloy by combustion synthesis (CS) and vacuum induction melting(VIM), and on top of this we study on microstructures and properties of tungsten content and alloying elements have effect onγ′strengthening phase of novel Co-Al-W superalloy by combustion synthesis (CS) and vacuum induction melting(VIM).In addition, tungsten content and alloying elements, namely Tantalum,Niobium,Titanium,Molybdenum, have effect on electrochemistry characteristic, hot corrosion and high temperature oxidation behavior of Co-Al-W superalloy by vacuum induction melting in this paper. Finally, Tungsten Inert Gas (TIG) welding was used to deposit Co-8.8Al-9.8W (at.%) superalloy on 304 austenite stainless steel plate and cladding layer shape, dilution, Vickers hardness, microstructure and distribution of alloying elements were investigated. At the same time, the microstructure and corrosion behavior of cobalt-base Stellite 6 alloy by combustion synthesis and vacuum induction melting.The following results can be obtained:(1) The effect of alloying elements,such as Tantalum, Niobium, Titanium, Molybdenum onγ′precipitation strengthen phase of Co-Al-W superalloys by combustion synthesis and vacuum induction melting. The microstructures of Co-Al-W superalloy are composed of richγ-Co matrix,γ′-Co₃(Al,W) phase and few carbides. Tungsten stabilizeγ′phase with precipitation strengthening, the melting temperatures of novel Co-Al-W superalloy gradually increasd with tungsten content increasing. Positive effects of alloying elements Tantalum, Niobium, Titanium and Molybdenum are made of A3B-Co₃(Ta,Nb,Mo,Ti) strengthen phase withγ-Co matrix stabilizeγ′-Co₃(Al,W) phase, promoted the crystallization ofγ′-Co₃(Al,W) phase andγ-Co matrix, increased solvus temperature of Co-Al-W superalloy.(2) Corrosion behaviors of Co-Al-W superalloy additional alloying elements are investigated in different pH value NaCl solutions at room temperature by electrochemical techniques. Different Tungsten contents and alloying Co-Al-W superalloys completely suffered from serious pitting corrosion and pits located at grain boundaries. The pitting corrosion resulted from the electro-migration of Cl- into the pits from the buck solution by Occluded Corrosion Cell(OCC). The pitting corrosion resistance of Co-Al-W superalloy gradually increased with addition to alloying elements, such as Tantalum, Niobium, Titanium, Molybdenum.(3) The kinetic of hot corrosion at 800℃in 75%Na₂SO₄+25%NaCl molten salt of Co-Al-W superalloy with tungsten content and alloying elements, namely, Molybdenum, Niobium,Tantalum and Titanium. The results show the hot corrosion oxide scale of Co-Al-W superalloy are made up of three layers, that is the external layer consists of Co oxide Co₃O₄, the intermediate mixed oxides layer is composed of complex oxide and nonuniform-barren oxide layer of Co, Al, W and alloying elements.The internal attacked layer with different compounds of Co, Al and O. When added to alloying elements, Molybdenum, Niobium, Tantalum and Titanium the hot corrosion resistance of Co-Al-W superalloys provides added protection against corrosion and the film oxidation compactness is gradually increased in NaCl solutions.(4) The kinetic data of weight gain and the cyclic oxidation behavior of Co-Al-W superalloy were measured and investigated at 800 and 900℃in air.The rate constants at different temperatures were determined by the relevant linear treatment. According to Arrhenius relation of rate constants of oxidation, the activation energy of oxidation was derived for Co-Al-W superalloy and the specified expression for the rate constant was further obtained in the case of oxidation at different temperatures. The results show that the oxide scale at different temperature exhibited a multi-layered structure including an outer layer of Co₃O₄ oxide, a layer is composed of complex oxide and non-uniform Co-barren oxide layer, an intermediate mixed oxides layer and an internal attacked layer with complicated oxides of Co and Al. The oxidation film of Co-Al-W superalloy surface appears agglomeration, oxide film crack propagation and oxide layer deterioration phenomena at different temperatures. Adding to alloying elements Molybdenum, Niobium, Tantalum and Titanium of Co-Al-W superalloy can reduce weight gain and increase activation energy of oxidation. Positive effects of alloying Co-Al-W superalloy provide added protection against hot corrosion at different temperatures.(5) Tungsten Inert Gas (TIG) welding was used to for deposit Co-8.8Al-9.8W(at.%) superalloy on 304 austenite stainless steel plate and cladding layer shape, dilution, Vickers hardness, microstructure and distribution of alloying elements were investigated. It was found that TIG cladding layer has the characteristics of large dilution rate, fine microstructure, narrow heat-affected zone (HAZ), narrow alloying elements segregation, high Vickers hardness, high contents and low contents of Fe in the cladding layers.TIG cladding layer dilution of Co-8.8Al-9.8W(at.%) superalloy on 304 stainless steel is about 12% when current and Voltage are 100A and 12V,respectively. High Vickers microhardness can be available in cladding layer, which arrives at 1050 (Hv50). The average Rockwell hardness value of cladding layer arrives at HRC53.1.(6) This investigation is undertaken to microstructures and corrosion behavior of Stellite 6 alloy by combustion synthesis and vacuum induction melting. The microstructures of Stellite 6 alloy by combustion synthesis (CSed Stellite 6) and vacuum induction melting (VIMed Stellite 6) are composed ofγ-Co matrix and primarily carbides at grain boundaries. Carbides existγ-Co matrix in the form of carbide mixture for the primary carbide and secondary eutectoid carbide of CSed Stellite 6, yet the carbides of VIMed Stellite 6 are single form at grain boundary for secondary eutectoid carbide. Comparing corrosion resistance of CSed Stellite 6 with VIMed Stellite 6 in NaCl neutral solutions, the former corrosion resistance is superior to the later.