Microstructure and mechanical properties of polycrystalline nickel-base superalloys of the aerojet engines of the future – TOPAZE

To meet this multi-faceted challenge, the ANR TOPAZE industrial chair is implementing a research program based on two teams with recognized expertise in their respective fields: microstructure evolution during thermomechanical processing at CEMEF (MINES ParisTech CNRS, Sophia Antipolis) and relationships between microstructure, properties and durability at the Pprime Institute (ISAE-ENSMA CNRS, Poitiers).
The overall approach combines state-of-the-art experimental methods with advanced physics-based numerical simulation. The work program of the TOPAZE chair is based on four CIFRE PhD theses closely related to industrial needs and four PhD theses for upstream developments (experimental or numerical).
The complexity of the studied metallurgical mechanisms and their multi-scale character calls indeed for the development of adapted experimental methods for a complete description of the microstructure and of the mechanisms governing the mechanical behavior. This original know-how is a key for the identification of the relevant microstructural parameters for the development of predictive models. From an academic point of view, these same skills also allow to address from a new standpoint the physical mechanisms that are at work but remained poorly understood (e.g. thermal maclage), or even to discover new mechanisms.

The results of the first 18 months of the TOPAZE chair have not yet been published (July 2021). These results are added to the overall production of the cooperation between Safran, Cemef and the Pprime Institute since 2015 when the Opale chair started, followed by TOPAZE in 2020.Several examples of upstream work have made it possible to remove obstacles in applied studies, which in turn have provided useful information for the conduct of industrial processes.
In addition to an extensive scientific production: (57 articles, 51 oral communications, 12 theses defended over the period 2015-2021), the results are also expressed in terms of:
- visibility and recognition (9 awards and distinctions since 2015),
- increasing the skills of academic and industrial teams and developing a cross-culture,
- training of young PhDs with a broad scientific and technical culture, with a profile that is appreciated on the job market in the academic or industrial sector,
- definition and implementation of original technical means,
- targeted pedagogical contents,
- reinforcement or initiation of academic and industrial cooperation around the Chair, in France and internationally.

Polycrystalline nickel-based superalloys have other possible applications than those targeted in the TOPAZE Chair's work.
For example, they meet the specifications for high temperature mechanical strength and corrosion resistance for heat transfer tubes in solar power plants. The mastery of their shaping behavior will allow to extend the application of the knowledge acquired in the TOPAZE chair for turbine disks to many other applications like this one and other manufacturing processes.
The know-how and physics-based models developed could also be applied to other types of alloys used in aeronautics and more broadly in all major sectors such as energy or ground transportation.

The list of published articles (in international journals or conference proceedings) is available online at :
chaire-opale.cemef.mines-paristech.fr/publications-et-communications/

The references of the articles arising specifically from the work of the TOPAZE chair will be posted when available on the TOPAZE chair website :
chaire-topaze.cemef.mines-paristech.fr

The articles are also referenced on the HAL open archive :
hal.archives-ouvertes.fr

The French group Safran is among the world leaders for the manufacture of aircraft and helicopter engines, and therefore an advanced expertise in the field of superalloys metallurgy and processing. This position, shared with General Electric, Pratt & Whitney or Rolls-Royce must be defended, especially with regards to the American supremacy. Within this highly technological field, the industrial competitiveness is closely related to the scientific research progresses. Therefore Safran has decided few years ago to team with academics through mid-term research programs and has co-funded the OPALE ANR industrial chair, selected by the ANR after the 2014 call. This first very fruitful program comes now to its end and one of the outcomes is the definition of a road map for the future.
The new- and next- generation turbomachines must work at higher temperatures to gain better efficiency and in turn reduce the air transport ecological impact, in line with the objectives “Vision 2020” defined by the ACARE. The gamma-gamma' nickel-based superalloys are nowadays the only materials fulfilling the requirements for turbine disks working in the hottest part of the engines; those are among the most critical parts of an engine. The OPALE chair (ANR-Safran 2014-2019) demonstrated that the metallurgical behavior of gamma-gamma' superalloys and in turn the mastery of forging processes are by far much more complex than for formerly used alloys. Those materials will be used in aircraft engines for the next 30 years, at least. Being able to control their microstructure evolution upon forging operations and gain in the knowledge of their in-service behavior appears to be both an absolute need from the industrial perspective, and a great scientific challenge. The ANR industrial chair project TOPAZE aims at facing this wide challenge, with a research program involving two academic teams of renowned expertise in microstructure evolution upon thermomechanical processing at Cemef (MINES ParisTech, Sophia Antipolis) and in microstructure - mechanical behavior and durability relationships at Institut P' (ISAE-ENSMA CNRS, Poitiers). The proposed research, made of both applied and fundamental topics, will lay the groundwork of predictive models for microstructures and properties, which are the indispensable tools of the metallurgy of the future.
The person proposed to be the chair holder, Nathalie Bozzolo, is an experienced scientist with strong connection with industry. Expert in physical metallurgy, and especially in the analysis of microstructural mechanisms, she is at the junction between the two branches of the project: process microstructure and microstructure-properties relationships. The other involved researchers complete the large competency spectrum needed to achieve that ambitious project: from fine material and properties analysis to process-microstructure and microstructure-properties relationship modelling, including the development of new numerical simulation tools and original experimental setups.
Fundamental works (experimental and numerical developments, metallurgical mechanism analysis) will help solving efficiently and originally the industrial issues treated in the works in direct connection with industrial concerns (support for processing new alloys or optimizing new processes). In the long run, the output of the project will be applicable to other types of alloys (titanium-based, aluminum-based, steels) used in aeronautics, but also in other fields like ground transportation or energy. Another important aim of the TOPAZE chair is the education of a new generation of metallurgists to fulfill the recruitment needs of the industry.