The effect of γ′ size and alloy chemistry on dynamic strain ageing in advanced polycrystalline nickel base superalloys

In-situ stair case loading experiments at 500 °C using neutron diffraction have been carried out on two γ′ strengthened polycrystalline nickel base superalloys, RR1000 and Udimet 720LI. In order to study the effect of γ′ particle size, uni-modal γ′ size distributions were generated in both alloys. Both alloys showed signs of dynamic strain aging (DSA) although the strain bursts were far more significant in RR1000 than Udimet 720LI. The neutron diffraction experiment revealed load partitioning between γ and γ′ in the case of the coarse γ′ microstructure but not in the fine γ′ microstructure. When load partitioning occurred, γ′ picked up significantly larger elastic strains during a DSA related strain burst than the γ matrix, indicating that mainly the matrix phase deforms plastically during such an event. The peak broadening data confirmed this. Subsequent to the strain burst, a reduction in the elastic strain partitioning was observed indicating preferred yielding of the γ′ phase during this stage. The differences in DSA detected between RR1000 and Udimet 720LI are discussed in terms of possible contribution of the various alloying elements.