Primary Creep in Nickel-Base Superalloys

Abstract

The nature of primary creep and intermediate temperature creep behavior of nickel base superalloys is reexamined. An extensive analysis of creep data for several 2 nd , 3 rd , and later generation single crystal alloys, with nominal <100> orientation is presented. It is shown that even though primary creep appears to vary in a complex manner with orientation, heat treatment and alloy chemistry, considerable clarity in data analysis can be gained if the associated variation in non-uniform deformation is taken into account. The non-uniform deformation may be quantified by measuring the ratio of major to minor diameters, or ellipticity, of the cylindrical specimens after test. Analysis shows that the logarithm of rupture life linearly decreases and final elongation linearly increases with ellipticity. It is theorized that while the association of high creep elongation with high ellipticity is a natural consequence of the conservation of volume, the decline in rupture life with increasing ellipticity is probably a result of progressively higher average true stress, under a constant load creep test condition. It is further shown that while the nonuniform deformation is a strong function of orientation, the maximum degree of ellipticity attained may be structure sensitive and may vary with alloy chemistry, heat treatment, stress and temperature.