Microstructure Evolution in Precipitation Hardening Nickel Base Alloys (Alloy 263) Under Long Term Creep Loading

Projektnummer 339

Alloy 263 is a candidate material for the future 700°C power plant. Due to the precipitation hardening mechanisms it shows a higher strength in comparison to the Alloy 617.

The appearance of the microstructure after heat treatment in the initial state and its possible subsequent changes under service loading could be directly related to the creep strength and thus to life time consumption under creep load. In case of creep resistant steels a considerable knowledge base already exists and is applicable to practical purposes. For nickel alloys such knowledge exists only in the special case of gas turbines designed for shorter lifetimes. In comparison to steels, nickel alloys exhibit a complex microstructure. High amounts of γ' phases are present in precipitation hardening nickel alloys, which are applicable at high temperatures due to their high strengths. In nickel alloys a wide range of precipitations is found. Besides the already mentioned γ' or γ'' phases, there are borides present in the interior of the grain. On grain boundaries carbides as well as δ and η phases can be found. The coarsening of γ' particles is increasing with temperature and time.

The project aims to characterize the microstructure of Alloy 263 and its development with time under influence of stress or temperature by investigating ruptured creep specimens. For this purpose SEM and TEM will be used. The changes of the initial microstructure are to be determined as functions of the parameters time, temperature and mechanical load by comparing the as received state, the shaft of the specimens (thermal and creep load) and the specimen's grip (thermal load). The specimens will be delivered by DONG from the VGB research project 290 "Creep Tests from AD700-1".

In parallel, the evolution of precipitate phases will be modelled by thermodynamic and kinetic microstructure modelling. Calculations will include prediction of amount and composition equilibrium phases as a function of temperature by means of the Thermocalc software. Furthermore, predictions of the evolution of precipitate number densities and sizes as a function of time and temperature during quality heat treatment and subsequent creep exposure of the material will be made with the MatCalc software. Predictions will be compared with results of the microstructure characterization.

The calculation matched to the Alloy 263 by means of the TEM results could be used for the theoretical determination of the microstructure of other melts.

The research project is carried out from October 2010 to March 2011 by the following Institutes:

The project is supported by the VGB-Technical Committee Materials and Quality Supervision.