Friction stress measurements during creep of Nimonic 105

Abstract

With Nimonic 105, the dependence of the secondary creep rate ε_s on stress σ and temperature T can be described as

ε_s = Aσⁿ exp - Q _c/RT

with n ≃ 7 and Q _c ≃ 515 kJ mol⁻¹. Using a technique which involves making consecutive small stress reductions during creep, definitive values of the friction stress σ₀ can be determined such that these large values of n and Q _c can be rationalized as

ε_s = A*(σ-σ₀)⁴ exp - Q _c ^*/RT

where Q _c ^*, derived from the temperature dependence of the creep rate at constant (σ-σ₀), is equal to the activation energy for creep of single phase nickel–chromium alloys (∼ 310 kJ mol⁻¹). This suggests that processes occurring in the matrix are rate-controlling during creep of nickel-base superalloys. Differences in the stress and temperature-dependence of creep resulting from variations in creep testing procedure are shown to be attributable to micro-structural instability. Furthermore, measurements of σ₀ during conventional creep tests and during tests in which the specimens were subjected to creep/heat-treatment cycles demonstrate that, under the conditions studied, tertiary creep commences as a result of precipitate coarsening rather than as a consequence of grain boundary cavity and crack development. Improved creep rupture lives can then be achieved by periodically reheat-treating the material to restore the original particle distribution.