Effects Of Ni On The Microstructure,high Temperature Oxidation And Hot Corrosion Behaviour In Co-Al-W-based Superalloys

Superalloys are widely used in aeroengines,space rocket engines and gas turbine engines for decades due to the strengthening of ?' phase.In conventional Co-based superalloys,there is no ?' phase that can be used for strengthening,so that the application is limited.The discovery of Co3(Al,W)-?' phase provides Co-based superalloys with a new strengthening method.The high temperature strength of Co-based superalloys could be significantly increased.Moreover,the melting points of Co-based superalloys are higher than that of Ni-based superalloys.The service temperature of superalloys could be increased.So it is necessary to develop Co-Al-W-based superalloys for high temperature applications.It was found that the solvus temperature of ?' phase could be increased by adding of Ni.It is possible to increase the service temperature of Co-Al-W-based superalloys by increasing Ni addition.But the relationship between Ni addition and the microstructure,high temperature oxidation behaviour as well as hot corrosion behaviour in multicomponent Co-Al-W-based superalloys are unknown.So in the present research,Co-Al-W-based superalloys with various Ni additions were designed.The superalloys also contain Ta,Ti and Cr etc.Microstructures,high temperature oxidation tests and hot corrosion tests were studied.The aim is to clarify the effect of Ni addition on ? phase dissolution and precipitation behaviour,oxidation properties and corrosion properties in Co-Al-W-based superalloys.And it also helps to provide a fundamental understanding for developing the Co-Al-W-based superalloys with excellent properties.Microstructures of Co-Al-W-based superalloys with 16-28 at.%Ni after different heat treatments were studied.The results show that the as-cast superalloys consist of ? phase,? phase,?' phase and ? matrix.When the superalloys are treated at 1000?1200?,? phase dissolves and secondary lath-like ? phase precipitates.When the heat treatment temperature is 1300?,? phase dissolves.Increasing Ni addition promotes the dissolution and hinders the precipitation of ? phase.With the increasing of Ni addition,both the solvus and precipitation temperatures of ? phase are decreased.? phase exhibits an orientation relationship with ?' phase:{0001}?//{111}?'.? phase is easy to nucleate on the {111} planes in ?' phase.Increasing Ni addition decreases the amount of W in {111} planes,which hinders the nucleation of ? phase.Therefore,the precipitation of ? phase can be hindered by increasing Ni addition in Co-Al-W-based superalloys.High temperature oxidation behaviours of Co-Al-W-based superalloys with 16?24 at.%Ni were investigated at both 800? and 900?.The results show that the superalloys show excellent oxidation resistance due to the formation of compact spinel oxide scales.Increasing Ni addition increases the Cr3+ flux while decreases the Co2+flux to the reaction front.It promotes the growth of Cr2O3 and hinder CoO nucleation.Therefore,increasing Ni addition helps the formation of protective spinel oxide scales and delays nodular oxidation,which favours to be better oxidation resistance.Hot corrosion behaviours of Co-Al-W-based superalloys with 16?24 at.%Ni exposed to 75 wt.%Na2SO4+25 wt.%NaCl were investigated at both 800? and 900?.The results show that the corrosion layer is composed of an outer scale with mainly(Co,Ni)O and an inner scale with spinel oxides and CoWO4.Increasing Ni addition increases the Ni2+flux to the outer scale,which leads to more porous(Co,Ni)O formed.Consequently,the pores are increased.Moreover,Co3W could reduce the diffusivity of corrosive medium.Therefore,Ni addition should be increased to hinder the precipitation of ? phase.And the formation of continuous Co3W in the scale/alloy interface is promoted,which helps to increase the corrosion resistance.Based on the studies above,a new Co-Al-W-based superalloy named as CoC-1 is developed.CoC-1 superalloy consists of mainly y' phase and ? matrix.When oxidised at 900?,protective spinel oxide scales forms.CoC-1 superalloy shows excellent oxidation resistance.Pre-oxidation treatment was used in CoC-1 superalloy.When corroded at 800? and 850?,the pre-oxide layer can hinder the diffusion of corrosive medium.Therefore,the mass gains are decreased by over 80%,and the pre-oxidation treated CoC-1 superalloy shows excellent hot corrosion resistance.At 600?900?,the high temperature strength of CoC-1 superalloy is at the range from 570 MPa to 722 MPa,Compared with conventional Co-based superalloy DZ40M,the high temperature strength of CoC-1 superalloy is higher.