Composition effect on elastic properties of model NiCo-based superalloys

NiCo-based superalloys exhibit higher strength and creep resistance over conventional superalloys.Compositional effects on elastic properties of the γ and γ' phases in newly-developed NiCo-based superalloys were investigated by first-principles calculation combined with special quasi-random structures.The lattice constant,bulk modulus,and elastic constants vary linearly with the Co concentration in the NiCo solution.In the selected(Ni,Co)3(Al,W)and(Ni,Co)3(Al,Ti)model γ' phase,the lattice constant,and bulk modulus show a linear trend with alloying element concentrations.The addition of Co,Ti,and W can regulate lattice mismatch and increase the bulk modulus,simultaneously.W-addition shows excellent performance in strengthening the elastic properties in the γ' phase.Systems become unstable with higher W and Ni contents,e.g.,(Ni0.75Co0.25)3(Al0.25 W0.75),and become brittle with higher W and Co addition,e.g.,Co3(Al0.25 W0.75).Furthermore,Co,Ti,and W can increase the elastic constants on the whole,and such high elastic constants always correspond to a high elastic modulus.The anisotropy index always corresponds to the nature of Young's modulus in a specific direction.

Doping effects on the stacking fault energies of the γ' phase in Ni-based superalloys

The doping effects on the stacking fault energies(SFEs),including the superlattice intrinsic stacking fault and superlattice extrinsic stacking fault,were studied by first principles calculation of the γ' phase in the Ni-based superalloys.The formation energy results show that the main alloying elements in Ni-based superalloys,such as Re,Cr,Mo,Ta,and W,prefer to occupy the Al-site in Ni3Al,Co shows a weak tendency to occupy the Ni-site,and Ru shows a weak tendency to occupy the Al-site.The SFE results show that Co and Ru could decrease the SFEs when added to fault planes,while other main elements increase SFEs.The double-packed superlattice intrinsic stacking fault energies are lower than superlattice extrinsic stacking fault energies when elements(except Co)occupy an Al-site.Furthermore,the SFEs show a symmetrical distribution with the location of the elements in the ternary model.A detailed electronic structure analysis of the Ru effects shows that SFEs correlated with not only the symmetry reduction of the charge accumulation but also the changes in structural energy.

First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter.Here,we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia,which is one of the major constituents of Uranus and Neptune.The nitrogen K-shell x-ray absorption spectrum(XAS)is determined along the Hugoniot curve,and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions.The atomic structures at these conditions are ascertained.Results indicate that the ammonia could dissociate to NH_(x)(x=0,1,or 2)fragments and form nitrogen clusters,and the ratios of these products change with varying conditions.The contributions to the XAS from these products show quite different characteristics,inducing the significant change of XAS along the Hugoniot curve.Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.