Creep tests at 700°C/500 MPa for the 3 heat treatments at 730°C, 760°C or 790°C (L. Thébaud, PhD Thesis ENSMA-PPrime, Poitiers). Precipitates/matrix concentration profiles and TEM observations of the deformation mechanisms in the samples aged at 760°C (a) and 790°C (b)

These superalloys mainly contain nickel but also a very precise proportion of other elements such as chromium, cobalt or titanium. When analyzed at the microscopic scale, they appear to consist of a matrix and hardening precipitates. Research to date has correlated the characteristics of these precipitates with microscopic strain mechanisms and mechanical properties. However, the different chemical species are distributed between the two phases and their distribution can evolve under solicitation. The influence of the distribution of chemical elements at a very small scale remains scarcely studied, as well as the evolution of this distribution when the alloy is subjected to thermal and/or mechanical stresses, while it can produce tremendous changes.

To answer this question, experiments were carried out on the new AD730 superalloy developed by Aubert & Duval, using different transmission electron microscopes, in order to explain the totally different lifetimes that result from the different heat treatments. Following the analysis of local chemical compositions of the various phases (matrix and precipitates), an evolution has been evidenced when the alloy is heat-treated at 790°C. This evolution of chemical composition has been related to the physical parameters that control the micromechanisms of deformation and thus allowed to explain the decrease of lifetime of the alloy when heat-treated at such temperature.

This work proves the need to carry out fine analyzes of local chemistry coupled with identification of the deformation micromechanisms to improve the understanding of the mechanical behavior of these efficient materials.
This work take part in the cross study INCA INfluence of local Chemistry on the mechanical behavior of metallic Alloys

References

Creep behavior in the new AD730TM nickel-based disk superalloy – Influence of aging heat treatment and local chemical fluctuations
F. Pettinari-Sturmel, W. Vultos, M. Hantcherli, B. Warot-Fonrose, C. Marcelot, Joël Douin, J. Cormier, P. Villechaise, A. Devaux
Materials Science and Engineering: A , Elsevier, 2019.
DOI : 10.1016/j.msea.2019.02.088 .

Contact

Dr. Florence PETTINARI , CEMES (CNRS)