Nickel-Based Single-Crystal Superalloys (Ni, Co)-Al-(Ta, Ti)-(Cr, Mo, W) Designed By Cluster-Plus-Glue-Atom Model And Their 1000 H Long-Term Ageing Behavior At 900 Degrees C

It has been pointed out recently that the compositions of industrial alloys are originated from cluster-plus-glue-atom structure units in solid solutions. Specifically for nickel-based superalloys, after properly grouping the alloying elements into Al, Ni-like ((Ni) over bar, including Ni, Co, Fe, Re, Ru and Ir), gamma '- forming Cr-like ((Cr) over bar (gamma '), including Ta, Ti, V, Nb), and gamma-forming Cr-like ((Cr) over bar;(gamma), including Cr, Mo and W), the optimal formula for single-crystal superalloys has been established [Al-(Ni) over bar (12)] (Al-1(Cr) over bar (gamma ')(0.5)(Cr) over bar (gamma)(1.5)). In this work, the first generation single-crystal superalloys were investigated on the basis of the proposed formula, by using (Ni) over bar=(Ni and Co), (Cr) over bar (gamma ')=(Ta and Ti), and (Cr) over bar (gamma)=(Cr, Mo and W). Two series of alloys were designed, formulated respectively as group A: [Al-Ni11Co1] (Al1TaxTi0.5-xCr1W0.25Mo0.25), with x=0, 0.25 and 0.5 (the corresponding mass fractions of Ta and Ti are respectively 0Ta-2.65Ti, 4.82Ta-1.26Ti and 9,32Ta-0Ti), and group B: [Al-Ni12-yCo gamma](Al1Ta0.25Ti0.25Cr1W0.25MO0.25), with y=1.5, 1.75, 2 and 2.5 (the corresponding mass fractions of Co are respectively 9.43Co, 11Co, 12.57Co and 15.71Co). The single-crystal superalloys were prepared using selector technique. And then they underwent the following tests of incipient melting, standard heat treatment and 1000 h long term ageing at 900 degrees C. It is found that: (1) In group A, with increasing Ta content (decreasing Ti), all the incipient melting temperatures are increased to above 1330 degrees C, and to the highest value is between 1335 degrees C and 1340 degrees C for alloy 9.32Ta-0Ti; the gamma(')gamma(') lattice negative misfits after standard heat treatment are reduced from -0.262% (0Ta-2.65Ti) to -0.247% (9.32Ta-0Ti); the gamma(') coarsening tendency after long-term ageing is deduced, and alloy 9.32Ta-0Ti has the lowest coarsening rate (K=5.6x10(-5) mu(3)/h). (2) In group B, the Co content does not influence the incipient melting temperature (always above 1330 degrees C) and the coarsening rate of gamma(') after long-term ageing. The major role of Co is to increase the mean size of the gamma(') precipitates to about 0.55 pm and the gamma(') volume fraction to about 69% after the standard heat treatment. These two groups of alloys have their gamma(') coarsening rates approaching the level of third-generation single-crystal superalloys (K approximate to(2.08 similar to 3.82)*10(-5) mu m(3)/h).