Study On The Thermal Stability Of Directionally Solidified Ni-based Superalloy DZ444

Hot-corrosion resistant directionally solidified Ni-based superalloy DZ444,generally used as blades of advanced industrial gas turbines. To figure out the evolutionprocess of microstructure and properties of the alloy during long-term exposure, andthen to make sure its work reliability, microstructure observations were made on γ′phase, primary MC carbide, grain boundary structure, TCP phase(σ-CrNi), and tensiletests(room temperature and950℃) and stress-rupture tests(980℃/180MPaand870℃/370MPa) were undertaken. Microstructure evolution at three temperatures of800℃,850℃and900℃and its influence on properties of the alloys, and optimizationof long-term microstructural thermal stability were investigated in the present paper.The main conclusions are as follows:As long-term exposure proceeds, γ′phase gradually changes with coarsening andcoalescence, the higher long-term exposure temperature is, the faster coarsening rate isand the more serious coarsening phenomenon. Primary MC carbide is TiC-type,containing a small amount of elements of Ta, Cr, W and Mo. Three stages of MCdegeneration process can be described with three different interface(MC/γ) reactions:Firstly, MC reacts with γ matrix and SM-M23C6(and/or SM-M6C) and SM-γ′graduallyarises on the MC/γ interface, slowing forming the “SM structure”, which can bedescribed as MC+γ�'SM-M23C6(and/or SM-M6C)+SM-γ′. At the second stage, thereexists high Ni concentration and Ti/Al value on the SM/MC interface, which promotesthe formation of η phase(Ni3Ti), and the degeneration process can be expressed asMC+γ�'SM-M23C6+SM-M6C+η. When the elements of W, Mo, Cr diffuse difficultly inη phase, the third stage of MC degeneration process happens, which can be describedwith MC+γ�'SM-M23C6+SM-M6C+η+(η-M6C)+(η-M23C6). Grain boundary γ′filmbecomes thicker and thicker, meanwhile, grain boundary M23C6and M6C graduallygrows up and links together, ultimately forming a continuous chain-like microstructurecomposed of coarsened γ′film decorated with basically continuous grain boundarycarbides. Besides, the alloy γ matrix generally does not precipitate needle-like phase,but very a few σ phase appear in the γ matrix around degenerated primary MC carbides when long-term exposure time is beyond5000h at850℃and900℃.As long-term exposure time increases, tensile strengths change a little,stress-rupture life gradually decreases, and the plasticity exhibits with a little volatility.The volatility can be explained by the combined action of the factors that graduallycoarsed γ′enabling deformation, many weak areas which exist at interfaces betweenblocky MC carbide and γ matrix, and continuous carbide chains at Gbs providing thefracture sites along Gbs. Besides, the precipitaion amount of TCP phase is little and ithave little influence on the properties.The PHACOMP method is taken to devise seven designed DZ444alloys withdifferent chemical compositions by adjusting element of C、Cr、Al and Ti. Nv valueclosely related with microstructural stabilty: the higher Nv value is, the more theprecipitation amount of σ phase is and the lower microstructural stabilty is. The alloywith the best thermal stabilty is basically determined and its critical electron holenumberN cVvalue is2.37.