High temperature deformation & fracture of nickel based superalloys: Influence of type Ⅰ and type Ⅱ hot corrosion
(0) Accelerated oxidation and corrosion of aero engine components due to the presence of hot salt mixture in the operating condition is known as hot corrosion.Hot corrosion degrades life of aero engine components and is broadly divided as type Ⅰ and type Ⅱ based on the prevailing temperature and type of salts present in the salt mixture and accordingly the mechanisms of degradation are different.Though type Ⅰ and type Ⅱ hot corrosion mechanisms are established but simultaneous interaction of cyclic and steady loads and hot corrosive media on aero engine materials is relatively less investigated.The different micro-mechanisms of interaction of types Ⅰ and Ⅱ hot-corrosion and low-cycle fatigue are studied in the present investigation on Nimonic 263 by conducting fatigue tests at 700, 800℃ on bare and salt-coated specimens; studying specimen surface degradation and fracture surface through extensive microscopy.Type-Ⅰ hot corrosion is induced at 800℃ by depositing 90% Na2SO4 + 10% NaCl and type-Ⅱ HC is induced at 700℃ by depositing 88% Na2SO4 + 7% NaCl + 5% NaVO3 on fatigue test specimens.Fatigue life is significantly reduced in both the hot corrosive atmospheres; however, the operative mechanisms are significantly different.Type-Ⅰ atmosphere leads to sporadic fluxing of protective Cr2O3 layer exposing the substrate to hot corrosive media leading to grain-boundary oxidation and inter-granular failure whereas;type-Ⅱ atmosphere leads to surface cracking and trans-granular failure.Creep tests have been carried out on Nimonic 263 with two salt compositions 90% Na2SO4 + 10% NaCl and 75 Na2SO4 + 25% NaCl at temperatures of 800 and 850℃.Results revealed that for short duration tests the rupture life is not much affected by the corrosive action of salt mixture at both the temperatures whereas for long term creep tests, the salt mixture has pronounced adverse effect on creep rupture properties compared to bare samples.Variation in the composition of the salt mixture did not have any significant effect on rupture life.Low cycle fatigue behaviour of superalloy IN 718 was studied with 75 Na2SO4 + 25% NaCl and 90% Na2SO4 + 5% NaCl + 5% V2O5 salt coatings at 650℃.Like in Nimonic 263, significant reduction in fatigue life was observed in salt coated specimens and corrosion effects were more pronounced at lower values of strain amplitudes.The reduction in fatigue life from the salt coating was found to be associated with the early crack initiation from the roots of the corrosion pits on the surface and faster crack propagation typically associated with type Ⅱ hot corrosion.As some engine components experience only sodium chloride salt environment at relatively lower temperatures, LCF behaviour of alloy IN 718 was also studied with NaCl salt coating at 550℃.Fatigue life of the salt coated sample was found to be significantly reduced at the lowest strain amplitude of±0.4% similar to the findings in type Ⅱ hot corrosion.
High temperature Nickel-based superalloy Deformation Fracture Hot corrosion
S.L.Mannan
Consultant,Gas Turbine Research Establishment,Bengaluru,India
国际会议
2015 International Symposium on Structural Integrity(2015年国际结构完整性学术会议)
沈阳
英文
11-12
2015-05-16（万方平台首次上网日期，不代表论文的发表时间）