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FP7

MICROMECH Report Summary

Project reference: 620078
Funded under: FP7-JTI

Final Report Summary - MICROMECH (Microstructure Based Material Mechanical Models for Superalloys)

Executive Summary:
A microstructure-based simulation tool has been developed to predict the mechanical behavior of the polycrystalline Ni-based superalloy Inconel 718 under monotonic loading, creep and fatigue. The tool is based in numerical homogenization, linking polycrystalline microstructure (grain size, shape and orientation distributions) and crystal behavior with the macroscopic alloy response. The numerical homogenization is performed by finite element simulations of periodic representative volume elements (RVE) of the microstructure, subjected to a macroscopic strain or stress history: monotonic loading, cyclic loading or creep. The grain behavior is model by crystal plasticity (CP) models developed to represent the monotonic and cyclic response of In718 crystals. The CP models include both physically based and phenomenological assumptions and include the effect of both temperature and grain size. The parameters of the CP model are directly measured from micro-testing on grains (pillar compression) or, if not possible, adjusted by inverse fitting from macroscopic experimental data.

The microstructure based simulation tool is able to predict the mechanical behavior of an In718 wrought alloy as function of its actual microstructure and operation temperature. For monotonic loading, the tool predicts the stress-strain curve for any temperature, strain rate and microstructure, providing also some design parameters as yield stress and ultimate tensile stress. For an applied constant stress (creep), the models can provide the strain versus time response for a given microstructure and temperature. In the case of cyclic loading, the model is able to predict the material response cycle by cycle including all cyclic characteristics of the alloy: hardening, Bauschinger effect, cyclic softening, ratcheting and mean stress relaxation. Finally, fatigue life models are developed, based on the tool to simulate cyclic behavior and that predict the number of cycles for crack initiation or sample fracture for a given cyclic loading conditions. The tools and models have been tested and adjusted for In718 with different microstructures and at different temperatures. The tool was able to accurately predict the material behavior in all the cases.

In addition to this deterministic tool, the effect of random distributions of defects (hard particles, voids and surface roughness) has been accounted in the method to provide a stochastical simulation tool, specially for fatigue life prediction. This tool provides a dispersion of fatigue life results for a given level and type of defects and the inherent experimental scatter of fatigue results can be obtained in addition to the average fatigue life

Project Context and Objectives:
A microstructure-based simulation tool will be developed to predict the mechanical behavior of the polycrystalline Ni-based superalloy Inconel 718 under monotonic loading, creep and fatigue. As first assumption it will be considered that the effect of the subgrain nanostructure can be embedded in the behavior of the In718 crystal that can be treated as a monolithic material at the grain level. The tool will be based in numerical homogenization that links polycrystalline microstructure (grain size, shape and orientation distributions) and crystal behavior with the macroscopic alloy response. The numerical homogenization is performed by finite element simulations of periodic representative volume elements (RVE) of the microstructure, subjected to a macroscopic (farfield) strain or stress history: monotonic loading, cyclic loading or creep.

The microstructure of the alloy enters in the models through the RVE that will be synthetically generated to contain the same grain size, shape and orientation distributions experimentally measured for the particular alloy microstructure considered. In addition to the grain distributions, RVEs

Crystal plasticity (CP) models will be developed to simulate the In718 crystal behavior. Some model parameters will be obtained from micromechanical tests on single crystals milled from the polycrystalline specimens by focus ion beam in both cast and forged materials. A few parameters, non accessible by microtesting, will be obtained by inverse fitting from macroscopic response. Three CP models have been developed: A fist model for monotonic loading, a second model for creep and finally a third CP model was developed for cyclic plastic behavior. The three models will include the effect of temperature and grain size in the formulations.

The tool will be tested and adjusted for different microstructures from ultrafine grained to coarse grain materials in which experimental data is available. The resulting tool will be able then to deterministically predict the mechanical behavior of the alloy using microstructure, temperature and loading conditions as input data. The prediction of the mechanical behavior will include full monotonic stress strain curves, stress-strain cyclic response until stable cycle, creep strain-time curves under applied stress and the estimation of full fatigue life. The fatigue life models will be based on the extraction of Fatigue Indicator Parameters (FIP) from microfields results of the cyclic simulations. The models will relate the FIPs obtained as result of a cyclic simulation with the number of cycles to break the specimen (life) under the conditions considered.

In addition to this deterministic tool, a system to include typical defectology in the RVEs will be developed. The defects considered will be surface roughness, hard particles (carbides) and porosity. First, it will be analyzed the general effects that those defect populations have in the alloy behvior respect to the perfect polycrystals. Then , the new RVEs will be used as a stochastic tool to estimate the range of possible responses for the type and level of defectology considered.

Project Results:
As a summary, the main results of the Project are:
-The mechanical behavior of In718 alloy has been deeply analyzed at micro and macro levels. The main deformation mechanisms operating, the effect of temperature, precipitates, grain size, defects, etc, have been quantified.
-A database has been built comprising the date of the mechanical behavior of In718.
-A tool has been developed to generate synthetic Representative Volume Elements (and their corresponding Finite Element meshes) based on the statistical description of the microstructure that is obtained from metallographic data.
-A tool based on crystal plasticity finite element (CPFE) simulations has been developed to predict the stress-strain curve of an In718 alloy as function of its actual microstructure, temperature and strain rate.
- A tool based on CPFE has been developed to predict the cyclic stress-strain curves until the stable cycle of an In718 alloy as function of microstructure, temperature and cyclic loading conditions (R index, strain or stress range...). The model reproduces all the phenomena experimentally observed: bausching effect, cyclic softening, ratcheting and mean stress relaxation
-A model has been developed to predict fatigue life for a given microstructure under any test conditions.
-A model has been developed to predict the creep strain-time curves of fatigue experiments of an In718 alloy as function of its actual microstructure, temperature and applied stress.
-A stochastic tool has been developed to provide probabilistic results of the alloy performance (specially fatigue life) for several types of defectology

Potential Impact:
The potential impact of the project, described in terms of potential exploitable results, is explained in the previous section and in section 4.2 below. The potential results of the MICROMECH project are interesting both from pure scientific and technological / commercial points of view. The former is already shown by the number of communications in congresses, scientific papers and events performed along the project. Some academic institutions have already express their interest on some of the exploitable results. Stimulation of further research is already a result of the MICROMECH project and potential licensing of the codes to relevant industrial targets is still to be achieved. This second, more ambitious goal will be accomplished after the end of the MICROMECH project.

Dissemination actions are listed under point 4.2 below. It shlould be noted that due to the heavy work load along the project, particularly during the few last months, relative few papers (3) have been published. Various papers are currently under elaboration and will be most likely published along year 2016. However, results of the MICROMECH project were presented in quite a few (19) conferences, workshops, technical sessions and other types of events.
List of Websites:
Public website can be found in the following URL: http://www.micromech.eu/

For more information on this document or MICROMECH, please contact:

Dr. Javier Segurado – MICROMECH Project Manager
javier.segurado@imdea.org

Miguel Ángel Rodiel – MICROMECH Financial and Quality Responsible
miguel.angel.rodiel@imdea.org

Fundación IMDEA Materiales
Eric Kandel, 2
Parque Científico y Tecnológico - Tecnogetafe
28906 Getafe (Madrid) SPAIN
Tel: +34 91 549 3422 / Fax: +34 91 550 3047

Contact

Rodiel, Miguel Ángel (Technology Manager)
Tel.: +34 915493422
E-mail
Record Number: 187144 / Last updated on: 2016-07-20