Creep rupture strength It is well indicated in that the creep-rupture-life/ gamma-prime-fraction plots are different for each series of alloys but every series is likely to have a maximum in the vicinity or more than 75 vol%. This means that the creep rupture life depends partly on solid solution hardening and partly on gamma-prime precipitation hardening. The maximum solid solution hardening is to be achieved when Cr in gamma-prime is substituted by W and Ta. Additionally gamma-prime fraction is to be obtained for the maximum precipitation hardening. In some Ni base superalloys, gamma-prime fraction in actual alloys at 1000 ºC may be smaller than that designed . 

Tensile properties Tensile properties at 900 ºC were observed for the specimen solution treated in various conditions followed by an aging treatment. Obviously  the variations are  well approximated with linear  functions of gamma-prime  fraction. Results obtained from other series of alloys have indicated that the linearities holds in the range of 50 to 80 vol% of gamma-prime fraction, which differ from the case of creep rupture strength. Effect of solution temperature is linear as well. A higher solution temperature gives a higher yield strength. The lower the solution temperature is, the larger is the tensile elongation, but this tendency ceases to work below a certain temperature; solution treatment below 1080 ºC gave no advantage effect on tensile elongation. For the effects of solid solution hardening and precipitation hardening, clearly W is the most effective in solid solution hardening, while Ta, which is a gamma-prime former is less effective than W as a solid solution hardening element .

    Hot corrosion resistance

 Hot corrosion resistance was evaluated by crucible test , keeping a piece of alloy (6-8 mm in diameter and 3-5 mm in height) in a salt mixture (Na2SO4-25%NaCl) open to air at 900 ºC for 20h. The resistance was quantitatively specified by metal loss after all the scales being removed. Morphologically the hot corrosion was classified into three types; Type I: corrosion layer composed of Cr sulfide, Ni sulfide, and porous oxide, TypeII: corrosion layer of thin tight Cr2O3 with a slight or no amount of sulphide in matrix, Type III: corrosion layer composed of three layers of oxides, Cr2O3, TiO2, and Al2O3 from outside to inside with a little amount of Cr rich sulphide dispersed in matrix. A regression analysis was carried out over 42 alloys giving type I corrosion. The results shown that Hf doped and a high Cr and Ti containing alloy, which is the most preferable in gamma-prime precipitation hardening alloys, while the addition of W, Ta, or Mo, which are essential for increase of high temperature strength, is extremely harmful for the hot corrosion.