The influence of heat treatment conditions on γ′ characteristics in Udimet® 720

Abstract

The influence of various heat treatment conditions on the kinetics of γ′ dissolution, re-precipitation and growth in nickel base superalloy Udimet 720 (U720) was studied. Cubical shape samples were extracted from a forged blade and solution treated at 1110, 1140, 1150, 1160 and 1170 °C for holding times ranging from 2 to 24 h. Samples were quenched at the end of each heat treatment process and the dissolution kinetics of γ′ precipitates was studied using optical and scanning electron microscopy. The γ′ solvus temperature was determined to be approximately of 1150 °C for the investigated alloy. Samples were aged at 760 and 850 °C from 2 to 90 h to study precipitation and growth kinetics of γ′ precipitates. A Lifshitz–Slyozov–Wagner (LSW) type relationship was found, leading to an activation energy value of 250 kJ/mol for the growth of cuboidal shape γ′ precipitates. A methodology was developed to take into account the temperature dependence of the growth coefficient, k, in the LSW equation. This results in more accurate evaluation of the activation energy value, compared to the general practice that assumes a temperature independent value for k. A heat treatment map for the Udimet 720 alloy studied in this investigation is proposed. The reference graph could be used to estimate γ′ volume fraction and size at different times and temperatures allowing the development of heat treatment schedules. The methodology employed in this investigation could be applied to other types of nickel base superalloys.