The Effect Of Replacing W By Mo On The Microstructure And Properties Of Waspaloy Ni-based Alloy

The ultra-supercritical power plants with temperature of 600-620?is becoming more and more mature.Materials-development projects for advanced ultra-supercritical?A-USC?power plants with steam temperatures of700°C and above have been performed in order to achievehigh efficiency ofpower plants.When the steam temperature increase to 700?and above,the performance requirements of materials for building the key components?such as blade,main steam pipe etc.?of power plant is far exceed the bearing capacity of heat-resistant steels,thus some Ni-based superalloys are developed to replace the 9-12%Cr heat resistant steels.Waspaloy,Nimonic 105 and USC141 are the main candidate materials for making blades.In this paper,a series of new nickel-based alloys with different W and Mo contents were designed by using W replace some Mo in Waspaloy.The hot-working properties of the alloysareresearched by hot compression test.The mechanical properties of tensile,toughness,hardness and creep rupture strength at 700?are obtained by testing at associated equipment.The microstructuresevolution of the new alloys were characterized by OM,SEM,HR-TEM,chemical phase analysis and XRD.In the study,the hot deformation equations and hot working maps of alloys were established.The alloys of 2Mo2W and 2Mo4W have stable deformation temperature range of more than150?,and can be deformed in a range of 1s^-1 and larger deformation rate.The substitution of W with equal mass for Mo in the alloy will reduce the thermal activation energy,and the alloy has lower dynamic recrystallization temperature and better hot working performance.The effect of W substitution for Mo on the evolution of structure and properties of the alloys during long-term aging at 700/750?was investigated,and the ratio of W to Mo in the alloy was optimized.When the W addition quality of the new alloy is controlled within 4wt.%and Mo is controlled around 2wt.%,the alloy can maintain a good match of plasticity and toughness,further increase the content of W and Mo,the plasticity and toughness will decrease.The evolution of precipitations in the alloys with W replaces Mo.The content of W and Mo has no obvious effect on the structure,amount and coarsening rate of the precipitation of??phase.With the addition of W in the range of 8%,1.6-3.2%W enter the??phase,and the W atoms entering the??phase tend to increase with increasing aging time.Thermodynamic software calculation shows that the interfacial energy between?and??phase is about 0.05J/m2.M₆C precipitated in 3.5Mo8W alloy,and the amount of it increased sharply after 3000 h aging at 750?,and part of M₆C developed into M₁₂C after 5000 h aging.?phases began to precipitate after aging at 750?for 1000 h,the shapes of them were long strip and square,and there were a lot of stacking faults in them.After aging for 5000 h,the amount of?phase increased sharply.The composition elements of M₆C and?phase are the same,but the content of W and Mo in M₆C is higher and the content of Ni in M₆C is lower than that of?phase.The creep fracture strength of 2Mo2W at 700?is better than that of Waspaloy,The main reason is that the addition of W leads to more??phase precipitated in the alloy.Thus,the mismatch of ?-??phases and the coarsening rate of ??phase is reduced,and the stability of ??phase is increased during long-term service.