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Title:

To predict the mechanical behavior/damage of bi-phased materials by coupling

between a microstructure-sensitive model and a phase-field model: Application to

Ni-based single crystal superalloy

Research abstract:

A phase-field and a microstructure-sensitive model will be coupled together to

predict both the microstructural evolutions (coarsening, dissolution/precipitation,

porosity, etc.) and the mechanical behavior/damage of a bi-phased material at high

temperature (a Ni-based single crystal superalloy, for instance).

A phase-field model is a mathematical model for solving interfacial problems. The

aim of the phase-field modeling is to describe the microstructure of the material atthe mesoscopic scale by means of continuous fields. The evolution of these fields

is governed by both elastic and chemical driving forces. More recently, a plastic

driving force was added to phase-field modeling. This driving force was revealed

to be necessary to simulate the directional coarsening occurring in Ni-base single

crystal superalloys during long high-temperature mechanical tests, such as creep

tests. This framework is able to predict many microstructural evolutions such as

the directional coarsening of the strengthening phase, known as rafting, and the

void nucleation and growth.

On the other hand, microstructure-sensitive constitutive models were developed to

predict the mechanical behavior and damage during complex thermomechanical

loadings. This type of model explicitly takes into account microstructure

evolutions (volume fractions, coarsening, dissolution/precipitation, etc.) to modify

the mechanical behavior of materials by introducing internal variables.

A coupling between these two kinds of model was never done before, especially

for thermomechanical loadings. Thus, softening and hardening will be introduced

in the viscoplastic constitutive model by microstructure changes given by the

phase-field model and vice versa. The ultimate goal of this research is then to do

Finite Element Simulations to predict the entire microstructure (precipitation,

coarsening, porosity, etc.) during thermomechanical loadings performed on

specimens whose geometries produce multiaxial stresses.

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Real life synopsis:

Today, many components operate at high temperature and have to keep extremely

good mechanical properties with time. It is in this context that nickel-based

superalloys have been developed to fulfill the mechanical requirements needed forhigh pressure (HP) turbine blades in turboshafts due to the high temperatures

(about 1920F/1050C) and stresses (about 120 MPa) applied. However, a new

problem has appeared with the emergence of extreme operating conditions for the

turbine blades of twin engine helicopters known by the acronym O.E.I. (One

Engine Inoperative). These conditions are met in emergency service when one of

the two engines stops. Additional power must be supplied by the engine still

running leading to both a rapid increase in temperature at the turbine inlet

(temperature rise from 1920F/1050C to 2190F/1200C in 5 seconds at the

blades) and an increase in rotation speed of the blades. These O.E.I. regimes can be

introduced at various times during certification tests (early, mid-life, end life) and

repeated.

Work plan:

1. Study phase

2.

Modeling phase

3.

FE Simulation phase

4.

Publication of the results

Group formulation:

Preferred background Aerospace, Materials and metallurgy, Mechanical

engineering