Precipitation and tensile deformation behaviour of a nimonic 105 superalloy

Abstract

Solution treated Nimonic 105 superalloy was subjected to direct ageing at 1123 K and to double ageing treatments involving intermediate ageing at 1333 K and final ageing at 1123 K. The microstructures developed at different stages of these treatments were characterised by optical microscopy and scanning and transmission electron microscopy. The tensile properties of this alloy were evaluated at different temperatures and correlated with the microstructures produced at the end of the ageing treatments. The Kinetics of the growth of the γ' precipitates during ageing at 1123 K obeys the cubic law indicating that the growth is controlled by the diffusion of the γ' forming solutes through the matrix. A transition from spherical to cuboidal morphology of the γ' precipitates was noticed during their growth. Intermediate ageing at 1333 K led to the formation of discrete and globular precipitates of M₂₃C₆ along the grain boundaries. Strength properties evaluated at 300 K for the samples directly aged at 1123 K are found to be superior to those of the samples subjected to double ageing treatments. Tensile tests conducted at elevated temperatures showed that the yield strength of the alloy exhibited a weak temperature dependence while the ultimate tensile strength decreased steadily with an increase in the test temperature. The strengthening mechanisms operating at different stages of ageing are discussed and the experimental measurements of the strengthening contribution due to the γ' precipitates are compared with the theoretical estimates. It is shown that when the mean radius r^- of γ' is below 40 nm, the γ' precipitates are invariably sheared by moving dislocations while precipitates with r^- ≥ 40 nm are bypassed by the Orowan mechanism.