PSI - Issue 16

Alexander Balitskii / Procedia Structural Integrity 16 (2019) 134–140 Alexander Balitskii / Structural Integrity Procedia 00 (2019) 000 – 000

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Balitskii et al. (2014)). The Ni60Co15Cr8W8Al2Mo3 ( І ) alloy is additionally alloyed by a copper that promotes hydrogen resistance of nickel – ferrous alloys (Symons (2001), Delafosse et al. (2009), Balitskii et al. (2014)). The decreasing maintenance and increasing of carbide forming elements (Mo, Nb) in two of cobalt chemical compositions of Ni62Cr14Co10Mo5Nb3Al3Ti3 ( ІII ), Ni61Cr14Mo6Nb3Al2Ti3 ( ІV ) alloys leads to the formation of multiphase structure with the strong grain boundaries and phases that is provided by the complex multicomponent alloying. Thus the hot resistance of these alloys is the phases the higher, than anymore in the structure is the volume part of strengthening phases and their thermal stability is higher, which means resistance against phase’s dissolution and coagulation at high temperatures. Except the above listed properties alloys for gas turbine aggregates must provides heat tolerance, resistance to oxidation on air or gas, resistance to corrosion in aggressive environments, resistance to the dynamic loading at high temperatures and crack resistance by the ability to maintain the large number of heat exchange cycles without hot crack formation. Also they must be insensitive tothe stress concentration and to own of high erosive resistance, offer the wear resistance in a gas stream during exploitation. For providing of nickel-ferrous alloys high temperature resistance to these alloys has added the Cr up to ~ 20 % and alloying by Ti (1 – 2.8 %), Al (0.55 – 5.5 %). In this case at alloy aging in the structure has appears the intermetallic γ' -phase, carbides and nitrides, which increase alloy durability at high temperatures. The maximum exploitation temperature of nickel-cobalt alloys is equal ~0.8Т liquid . At higher temperatures there are coagulation and dissolution of γ' - phase that leads to decreasing of hot resistance. For the providing of nickel ferrous alloys hot resistance they must have a γ -solution structure with γ' -phase regular distribution. All materials are clean on sulphur and phosphorus. On the base of literature data for almost materials and testing methods the influence of hydrogen increases proportionally to the square root of pressure (Gray (1974), Dadfarnia et al. (2015)). The pressure range for which this dependence is valid is studied insufficiently. In the process of short-term static tension the properties of alloys become worse in the all range of pressures and temperatures. Embrittlement of specimens made of these alloys increases up to 70 MPa (Balitskii et al. (2018a)). In the case of Ni-Co alloy the dependence of plane-stress fracture toughness ( K c ) (Balitskii et al. (2009c) on the hydrogen pressure consists of two regions. In the first region (low pressures), the K c abruptly drops, and in the second, the negative action of hydrogen becomes stable or decreases negligibility. This means that there exists a pressure under which the degradation of this material with hydrogen reaches its limit. The additional effect of preliminary dissolved hydrogen (more than C H = 20 wppm) on the properties of Ni-Co alloy developed at hydrogen environment pressure least of 10 MPa. The strength of Ni-Co alloy significantly decreases in air at temperatures higher than 873 K. This process is accompanied by a decrease in the plasticity of both materials at 873…1073 K and a significant increase in the relative elongation and transverse narrowing for specimens at 1073 K. Hydrogen decreases the stress intensity factor K c and affects on the fracture character. Under condition of maximum hydrogen embrittlement the load-displacement the diagrams become the linear (as functions of the load) and correspond to type I. The fracture surfaces of the specimens are intergranular with cleavage facets typical of brittle fracture, when in helium honeycomb relief with ductile intergranular portions prevailed. The values of K c , i.e., they satisfy the condition: 3. Determination of Ni-Co alloys properties in the hydrogen with high parameters

H 2 σ        c K H 0.2 2.5

, l b

,

(1)

where K c H – yield strength in hydrogen, l , b – specimen parameters (distances from compact specimen hole to the fatigue crack end and distances from the compact specimen hole to but end (Balitskii et al. (2009c, 2018a)). Fracture toughness criterion is very sensitive to structure changing (grain sizes, intermetallic inclusion morphology, etc.). Hydrogen-induced process of H – stress intensity factor under static loading (fracture toughness) in hydrogen,  0.2