Fundamental study of plasticity of advanced recyclable Al/Al-Cu-Fe composites – FutureAlCo

The mechanical properties are tightly dependent of the material microstructure, which by itself depends on the synthesis process. Three interconnected research directions are planed to achieve the proposed project, which starts from composite material production up to the complete microstructure characterisations and plastic behaviour modelling. These research lines are:
- Production of model composite materials with controlled microstructural parameters such as volume fraction, size and spatial distribution of reinforcement particles, porosity, … ,
- Full characterisation of the material microstructure before and after deformation, and identification of the pertinent parameters: chemical and structural nature of particles/matrix interfaces, structure and spatial distribution of particles, dislocation densities and configurations in the matrix and at particles/matrix interfaces, elastic strain field around the reinforcement particles, … ,
- Complete characterisation of mechanical properties of the model composite materials and modelling of the deformation mechanisms.

The major challenges of the proposed project are:
• Fundamental points:
- Determination of the exact role of particle/matrix interfaces at the microscopic scale,
- Relations between the complex microstructure and mechanical properties, in particular determination of the different strengthening contributions,
- Analysis of recently in-situ deformation methods.
• Technical bottleneck: elaboration of model composite materials with homogenous distribution of size controlled particles.

1- Production of Al/omega-Al70Cu20Fe10 composite by spark plasma sintering (SPS)

The first step was to determine the experimental conditions (time, temperature, pressure) allowing the production of a final Al/omega-Al70Cu20Fe10 composite by SPS. The starting powder is a mixing of Al and i-Al63,5Cu24Fe12,5 (icosaedral phase) powder. The analysis of the morphology and composition of samples obtained with different experimental conditions allows to
- determine that using 550°C/100MPa/2min allows for production of the optimized al/omega phase
- analyse the intermediate phases of the complex phase transformation icosaedral to crystalline omega phase.

2- Production of Al/Al70Cu20Fe10 with controlled particles size :
In order to produce a dense composites with uniform spatial distribution of Al-Cu-Fe particles in the Al matrix, the powder processing (milling, sieving and mixing) has been optimized. A complete microstructural characterisation at different scales has been performed by X-Ray diffraction, scanning electron microscopy.

3- Microstructural analysis of Al/Al70Cu20Fe10 composite by EDXS analysis and transmission electron microscopy (TEM)
The SEM observations coupled with EDXS analysis gives evidence of copper diffusion from the Al-Cu-Fe particles into the Al matrix. Observations by TEM reveal precipitation into the matrix. These observations will be correlated to the composite mechanical properties.

Future prospect :
- Analysis of the phase transformation icosaedral phase to omega crystalline phase by producing model samples.

- Microstructural analysis of composites by TEM :
- determination of the residual strain field around the observed precipitates using the geometric phase analysis (GPA).
- analysis of the complete dislocations configurations

- Mechanical analysis at different scales :
- conventional testing by compression tests at constant strain rate.
- nano-mechanical testing by nanoindentation tests for characterizing the plasticity of the matrix at small scale.

First results have been presented in an international workshop :
CMAC DAY 2014 – Zagreb - Mechanical properties of Al/Al-Cu-Fe composites newly elaborated by Spark Plasma Sintering – Aurélie JOSEPH – Pprime- CEMES

Most of material applications require combinations of properties that cannot be reached using one class of material only. The composite approach to design new materials consists of combining at least two materials, each one owning one of the final required properties. For structural applications, the key parameters are high elastic modulus, high strength and high wear resistance. Metal Matrix Composites (MMCs) combine a light and ductile matrix with hard reinforcement particles. Reinforcement of the matrix results from load transfer between the matrix itself and the reinforcement particles and by modification of its microstructure (dislocation density, grain size,…). Two categories of particle-reinforced composites must be distinguished. In a first category, in-situ composites are obtained by heat treatments involving chemical reactions that usually lead to a fine and stable reinforcement phase homogeneously distributed in the matrix. The second category deals with composites where pre-existing reinforcement particles are embedded in a metallic matrix, such as for instance those produced by powder metallurgy processes. In this case, a large variety of reinforcement particles can be used with an accurate control of particle content.
Thanks to their mechanical properties (high hardness together with high elastic modulus and yield stress) and tribological properties, Al-Cu-Fe particles appear as good candidates for reinforcement particles in Al matrix composites. Moreover, Al-Cu-Fe alloys are currently produced by conventional casting, which is an incontestable advantage compared to the expensive ceramic particles. In addition, according to the current and future environmental concerns, it is of prime interest to develop materials with high potential for recyclability. Al based MMCs reinforced by metallic particles are recyclable by a simple annealing above the melting point of the composite material.
In this context, the proposed project aims at optimizing the mechanical properties of promising Al/Al-Cu-Fe composites. This first implies a deep understanding of the mechanical properties of the composite material based on a detailed microstructure analysis in connection with the related deformation mechanisms. From a fundamental point of view, this project questions different length scales that include the macroscopic load transfer, the microscopic particle bypassing by dislocations and at the atomic level the matrix-particle interfaces. It goes beyond the interpretation of deformation mechanisms of composite materials and intends to answer to one of the main existing objectives in materials science that consists to describe the mechanical properties of composite materials in terms of their microstructural defects by relating various length scales of observations, starting from nano- to macro-scale.
We would like here to emphasize the choice of a pure Al matrix. Because studies of Al/Al-Cu-Fe composites are only at their very beginning, our goal is to use simple model composite materials to study the actual role of the reinforcements Al-Cu-Fe particles only. It is therefore of prime importance to avoid possible secondary phases that could be produced by precipitation using for the matrix, for example, an AlCuMg alloy.
In a second step, based on the above understanding, part of the project will be dedicated to the tuning of the processes for composite fabrication by determining the best compromise between spatial distribution and volume fraction of the reinforcement particles.