Apparent and effective creep parameters in single crystals of a nickel base superalloy—I Incubation period

• Published in: Acta metallurgica 1977-07, Vol.25 (7), p.767-777
• Main Authors: Carry, C, Strudel, J.L
• Format: Article
• Language: English
• ISSN: 0001-6160
• DOI: 10.1016/0001-6160(77)90092-X

During steady state creep, the effective stress σ e , acting directly on the dislocations is defined as the difference between σ a the stress applied to the sample and σ i the average internal stress resulting from the presence of all other dislocations and of a second phase in the matrix ( σ e = σ a − σ i ). The effective activation energy ( Q e = −R[∂ In ϵ ∂ ( 1 T )]σ e ) and the effective stress exponent ( n e = [∂ In ϵ ∂ Inσ s] T where ϵ is the creep rate) are the only parameters relevant to the elementary deformation processes. These effective parameters are related to their apparent counterparts ( Q a = −R[∂ In ϵ ∂ ( 1 T )] σa and n a = [∂ In ϵ t6 Inσ a] T by the following equations: Q e = Q a + RT 2n a σ ∗ i σ a(1−σ' i) and n e = n 2 σ e σ a(1−σ' i) where σ ∗ i = (∂ σ i ∂T ) σ a and σ i = (∂ σ i ∂σ a) T can be estimated experimentally. In two-phase materials, the average internal stress may be described as σ i ( σ a T) = α( σ a , T) σ a + σ c ( T) where σ c is a threshold stress resulting from the presence of the precipitates. At intermediate temperatures (700–900°C) the first stage creep of nickel base superalloy single crystals exhibits an incubation period characterized by a very low and steady creep rate (10 −6h