Effect of Elastic Driving Force on the Evolution of Microstructures in the Secondary Creep Stage

Abstract

Abstract. The effect of elastic driving force on the microstructural change of superalloys in the secondary creep stage is evaluated by elastic energy calculations with the concept of effective eigen strain where both lattice mismatch and creep strain are taken into account The elastic energy calculations indicates that the elastic state in the secondary creep stage is totally different to that in the initial one where the lattice misfit between γ and γ' phases is over accommodated along the [100] and [010] directions by creep deformation in the γ phase. The excess creep dislocations for the over accommodation are required so as to develop an internal stress field to prevent further creep deformations. The planer raft structure with the plane normal oriented to the [001] direction is unstable in the over accommodated state. The γ/γ' lamellar interfaces will be inclined to make a wavy raft structure of which elastic energy is lower than the ideal 001 planer raft structure.

Micromechanics of primary creep in Ni base superalloys

“…These phenomena minimize the elastic strain energy initially stored in optimally heat treated superalloys (Ratel et al, 2010) and ensure a marked minimum creep rate for alloys with a large negative misfit (Zhang et al, 2005;Pyczak et al, 2009). Recently, Tanaka et al (2011) pointed out that the initial elastically strained state of the  matrix channels along the tensile direction 33, was drastically altered during the first stages of a uniaxial creep test. By use of the concept of eigenstrain of the  matrix submitted to the stress resulting from the ' misfit and to the plastic strain provided by creep, they show that the optimal resistance of the rafting ' microstructure is obtained when the density of interfacial dislocations barely reaches the optimal value for which the misfit is globally balanced in the horizontal channels.…”