John K. Tien

On the creep rate stress dependence of particle strengthened alloys

Abstract 1. 1. The magnitude of the apparent stress exponent for creep is dependent on the ratio of σ R σ and the change of σR with σ through the following equation: n = [ n o (1− σ R σ ) ](1− d σ R d σ ) . 2. 2. The comparatively larger values for n of ODS alloys relative to precipitation strengthened alloys is due to d σ R d σ being minimal for the former and appreciable for the latter. Both alloy systems possess large values for the ratio σ R σ . 3. 3. The gradual lessening of the apparent stress exponent for creep in ODS with decreasing temperature and increasing values of σ E occurs because the value of σ R σ is gradually decreasing.

Environmental effects on the creep behavior of a nickel-base superalloy

Environmental effects on the 760 °C creep behavior of a nickel-base superalloy are isolated by testing at varying stress levels, in laboratory air and vacuum, specimens of different gage diameters and grain sizes, and in a few cases, different grain boundary microstructures. For all specimens receiving a standard heat treatment that results in grain boundaries free of discrete carbides, the minimum creep rate is greater in air than in vacuum for a given specimen and grain size. In contrast, for specimens aged to precipitate carbides along the grain boundaries, the creep rate is lower in air than in vacuum. The minimum creep rate and the sensitivity of this rate to applied stress also are found to be functions of grain size, specimen size, and grain boundary microstructure to extents dependent on the test environments. Rationalizations of these environmental creep behaviors are suggested in terms of the apparently competing phenomena of the relative weakening of the alloy grain boundaries in the air environment, and the alloy creep strengthening in air due to the external surface oxide scale.

Recrystallization and Texture Development in Dispersion-Strengthened Nickel-Base Superalloys

The recrystallization heat treating cycle is shown to be a critical factor in determining the final texture of an extruded and hot-rolled yttria-dispersed superalloy. It appears that recrystallization in such systems is not a dynamic process,i.e., no recrystallization occurs during hot-rolling operations. Further, pole figures obtained from fully recrystallized specimens demonstrate that the temperature from which the recrystallization treatment is started determines the final crystallographic orientation of the system. (110){001} and (110){110} texture will be developed only if the cycle is started within a specific temperature range; initial temperatures either too low or too high promote other orientations.

Cryogenic Creep of Metals

That significant creep can occur at cryogenic temperatures has been recognized since 1930, when it was first found in cadmium by Meissner, Polanyi, and Schmid.1 From the engineering viewpoint, cryogenic creep is particularly releveant to the design of superconducting generators and magnets, structures for deep space, Josephson Diode type computers, and other components for cryogenic applications. For example, the performance of superconductive windings (NbTi/Cu and/or Nb3Sn/Cu) composites) is affected by the matrix material behavior under applied stresses. The working life of cryogen storage vessels under high pressure also depends on the cryogenic creep behavior of the structural materials.

The effect of the lower stress level on the cyclic creep behavior of an ods superalloy

Cyclic creep and stress rupture results are reported and discussed as a function of the lower applied load for INCONEL MA6000, which is a mechanically alloyed oxide dispersion strengthened (ODS) and γ’ precipitation strengthened nickel-base superalloy. In prior work, very large decreases in the effective minimum cyclic creep rate, based on time at maximum load, were found to occur for cyclic creep testing using a lower load near zero. The present results show that this decrease in creep rate depends on the value of the lower load. As the lower load increases, the cyclic creep rate increases proportionally, but remains less than the corresponding static creep rate. The values of the cyclic creep rates are shown to be related to the magnitude of the anelastic strain recovery occurring during the lower load portion of the cyclic creep test. The results are also used to improve on the anelastic relaxation model for cyclic creep of materials with strong pinning points.

Effects of Cobalt in Nickel-Base Superalloys

The role of cobalt in a representative wrought nickel-base superalloy was recently determined. The results show cobalt affecting the solubility of elements in the γ matrix, resulting in enhanced γ’ volume fraction, in the stabilization of MC-type carbides, and in the stabilization of sigma phase. In the particular alloy studied, these microstructural and microchemistry changes are insufficient in extent to impact on tensile strength, yield strength, and in the ductilities. Depending on the heat treatment, creep and stress rupture resistance can be cobalt sensitive. In the coarse grain, fully solutioned and aged condition, all of the alloy’s 17% cobalt can be replaced by nickel without deleteriously affecting this resistance. In the fine grain, partially solutioned and aged condition, this resistance is deleteriously affected only when one-half or more of the initial cobalt content is removed. The structure and property results are discussed with respect to existing theories and with respect to other recent and earlier findings on the impact of cobalt, if any, on the performance of nickel-base superalloys.