Modelling superalloy behaviour to improve aircraft engine design

The MICROMECH project, part of the Sustainable and Green Engines (SAGE) Integrated Technology Demonstrators (ITD) of Clean Sky, has successfully developed a multi-scale computational tool to predict the mechanical behaviour of the Ni-based superalloys used in the hottest parts of aircraft engines. This model will help optimising the performance of new component designs.

Superalloys are a special type of metallic alloys with excellent mechanical performance and oxidation resistance at elevated temperatures, which make them suitable for structures and components undergoing high mechanical stresses at high temperatures. In particular, nickel (Ni)-based superalloys have become essential for turbine blades and discs in the hottest part of aircraft engines. They also find critical applications in space vehicles and nuclear reactors. A better understanding of the behaviour of these alloys and the development of computational models able to relate their actual performance (allowables) with the microstructure resulting from the processing route are needed by the industry to improve aircraft engine design (emissions reduction and improved efficiency).

The MICROMECH project (Microstructure based material mechanical models for superalloys, http://www.micromech.eu/index.php), carried out by IMDEA Materials Institute in collaboration with ITP, has had the objective of developing a multi-scale computational tool to predict the mechanical performance of polycrystalline Ni-based superalloys as function of their real microstructure (i.e. grain size, shape and orientation distributions). The widely used alloy Inconel 718 has been used as benchmark in this study.

The approach used by IMDEA Materials Institute in the MICROMECH project is based on computational homogenisation of polycrystals, that link the microstructure and the crystal behaviour with the material response through virtual testing (see Figure 1). The microscopic strain and stress fields and the overall macroscopic response are obtained by the Finite Element simulation of the deformation of a Representative Volume Element (RVE) of the alloy. The microstructure enters in the models through the RVE, where a set of grains are represented containing the actual grain size, shape and orientation distributions measured. The behaviour of the grains is considered by a Crystal Plasticity (CP) model, developed to account for all the micro-deformation mechanisms at the crystal level. Most of the crystal properties are obtained by direct microtesting on single crystalline samples built from grains being the rest of the CP parameters identified by inverse fitting from standard macroscopic tests on polycrystalline samples.

Figure 1: The approach employed in the MICROMECH project for the virtual testing of the mechanical properties of superalloys.

The extensive micromechanical testing and the physical motivation of the mechanical models developed led to a realistic and accurate multi-scale model able to predict the mechanical properties of specimens used in the design of components. The resulting models are able to predict the monotonic behaviour (stress-strain curve), creep behaviour (creep strain vs time as function of applied stress) and fatigue life (number of cycles for a given load range) as function of temperature and actual microstructure. Moreover, a stochastic model has been also generated to predict the effect of some defects (surface conditions, presence of carbides) in the performance. As Koldo Ostolaza (Materials & Processes Engineer at ITP) explains: “the tools developed under the MICROMECH project are providing very valuable information for our engineers, particularly on fatigue life prediction. We are working with IMDEA Materials to expand the current capabilities of the model to tackle additional phenomena on different metallic materials“.

Source: http://cordis.europa.eu/news/rcn/139455_en.html?isPermaLink=true?WT.mc_i...