Effects Of Cr On The Weldability And Oxidation Properties Of Novel ?'-strengthened Co-based Superalloys

Co-Al-W based superalloys,which have strengthened ?'-Co3(Al,W)phase of L12 structure,own superior mechanical properties and oxidation and corrosion resistance.Meantime,the melting temperature of cobalt-base superalloys is higher than that of nickel-base superalloys,which makes cobalt-base superalloys a potential high-temperature material.Welding is one of the most important process to connect components and repair defects.Cr element is an indispensable solid solution element and anti-oxidation element for cobalt-base superalloys.It's of great significance to study the effect of Cr on the weldability,mechanical properties and high-temperature oxidation resistance of ?'-strengthened cobalt-base superalloys.In this paper,2.5 at%,5 at%and 7.5 at%contents of Cr elements are separately added into the Co-Al-W-Cr-C superalloys,which are referred as 2.5Cr,5Cr and 7.5Cr.There are ? matrix,?' phases,M6C carbides and ?-CoAl phases existed in all ageing alloys.With Cr content increasing,the content of carbides increases significantly,and the morphology of the ?' phases changes from cubic shape into round-cornered shape and the solvus temperature of ?' decreases.The weldments of 2.5Cr,5Cr and 7.5Cr,which are welded by argon arc welding,have the same phases with base metals.With the increase of Cr content,the volume fraction of carbides inner the grains and along the grain boundaries increases significantly.In addition,the welding solidification cracks length declines significantly with increasing Cr content.The relative low solidification cracking susceptibility of 7.5Cr is mainly attributed to the narrow solidification temperature range,sufficient liquid phases backfilling the grain boundary in partially melted zone,serrated grain boundaries and large numbers of solidification sub-grain boundaries.To study the mechanical properties of aged samples and welded samples of 2.5Cr,5Cr and 7.5Cr,the Vickers microhardness and tensile test of room temperature,760?,850?,1000? were carried out.Results show that with Cr content increasing,the Vickers microhardness in BM of weldments gradually reduces,and the difference of microhardness between FZ and HAZ is small.In 7.5Cr weldment,the Vickers microhardness in BM is less than microhardness in FZ and HAZ,and it's opposite in 2.5Cr and 5Cr weldments.The ultimate strength of all samples shows a downward trend at elevated temperature.Among all the samples,the ultimate tensile strength of 5Cr aged sample is the highest which is 975MPa.The ultimate tensile strength of 7.5Cr welded sample is larger than 5Cr welded sample,and ?b value of 7.5Cr welded sample accounts for more than 75%of the 7.5Cr aged sample.It's found that most welded tensile samples fracture in the FZ and intergranular fracture occur in all tensile samples,where there are dendrites,cleavage steps and dimples.It is due to the fewer precipitations and weaker adhesion of grain boundaries.The cracks preferentially form in lateral grain boundaries and propagate to fracture during the tensile test.At room temperature,the cracks form inside the carbides along the grain boundaries.At high temperature,cracks preferentially form in the transverse grain boundaries and the interfaces between carbides and matrix,then easily oxide and propagate to fracture.The thermogravimetric oxidation curves of 2.5Cr,5Cr and 7.5Cr at 900? are shown to obey the parabolic law.With Cr content increasing,the rate of oxidation weight gain kp significantly reduces and the increasing rate of thickness of oxide layers slows down,which shows that the oxidation resistance improves.In 2.5Cr and 5Cr alloys,Co3O4/CoO outer layer,a mixture of Co,Al,W,Cr oxide layer,y/Co3W zone are established on the substrate alloys.The oxide layer of 2.5Cr happens to crack during the oxidation.In the oxide layer of 7.5Cr alloys,there are Co3O4/CoO outer layer,CoWO4 layer,a mixture of Co,Al,W,Cr oxide layer,continuous Cr2O3/Al2O3 layer and y/Co3W zone.Continuous and dense Cr2O3/Al2O3 layer effectively hinders the atomic diffusion and enhances the oxidation resistance of 7.5Cr.