MICROSTRUCTURE MECHANICAL PROPERTY RELATIONSHIP IN A DS CAST NI3AL-BASE ALLOY

A Ni-Al-Mo directionally solidified (DS) casting gamma'-base superalloy, with the chemical composition (wt%) 7.5 to 8.5% Ni, 10 to 14% Al, Mo less-than-or-equal-to 0.15% B, has been developed for advanced gas turbine blades and vanes. The mechanical properties of this alloy have been determined by tensile tests at room temperature and in the temperature range 700 to 1000-degrees-C and by stress-rupture tests in the temperature range 760 to 1100-degrees-C. The microstructures of the as-cast and homogenized specimens and of specimens after creep deformation at 1000 to 1100-degrees-C have been examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and optical microscopy techniques. The results show that this alloy has a high yield strength from room temperature up to 1100-degrees-C, excellent creep resistance at temperatures up to 1100-degrees-C, as well as a lower density and higher melting point than currently available nickel superalloys. The microstructural observations and analysis indicate that the superior mechanical properties of this alloy may be attributed to solid solution hardening by the large molybdenum addition, second-phase strengthening by gamma phase and other minor phases that precipitate in various temperature ranges, the formation of a gamma raft structure during creep, and to the existence of high-density misfit dislocation networks at gamma'/gamma interface areas due to a high value of gamma'/gamma misfit.