Principal Investigator :

Magali Brunet ()
CEMES (M3)

Type of project : Collaborative Project

Partner team(s) :

CEMES (PPM (Joël Douin))

Date (start/end) : 2016 - 2018

     

       

Abstract

Precise understanding of the slow, while significant, evolution of the microstructure of metallic alloys is crucial in at least two fields: industry and cultural heritage. In this project, we want to address the question of the long-term aging of the Al-Cu alloys, based on the examination of materials coming from old airplanes. The aim is to determine the structural evolution of light alloys over very long periods of time thanks to an approach coupling archival information and laboratory analyses. Various laboratory investigations will be used to characterize the composition and the present nanostructure of the selected naturally-aged alloys, and their correlation with their mechanical behavior.

The knowledge of materials behavior and aging is a key element in industry, in particular for aeronautics where structures and materials are exposed to severe environment and mechanical stress for a long period of time. Typical alloys used in aeronautics are age-hardened aluminum alloys (ex: 2xxx series which contain copper about 4 wt.% and magnesium). Hardening is obtained thanks to a fine copper precipitation hindering the motion of dislocations, formed from the super-saturated solid solution upon quenching. This precipitation, in the form of Guinier-Preston zones, evolves at low temperatures (150-200°C) into semi-coherent phases of different shapes and composition (dispersoïds).

However, this nanostructure is unstable: it continues to evolve slowly with the variations of temperature and mechanical stress undergone by airplanes. The mechanical properties of hardening alloys directly result from their structure: a small change of the latter can have strong effects on their tensile and yield strength or toughness.

Because of the difficulty to define the evolution of nanostructure over long period of time, most of the studies try and deduce the evolution of mechanical and chemical properties both from phenomenological and extrapolation based approaches: accelerated aging (cycling, thermal aging or in severe saline environment) is currently widely used to predict the behaviour of aluminum alloys. Real-condition tests are very rare and complex to be realized.

We propose in this project, a  different approach which consists in studying naturally aged alloys , over long period of times (more than 50 years), directly taken on old airplanes. These materials have undergone accumulated constraints (change of T°C, humidity, vibrations, polluted atmospheres). But, although no direct obvious link could be drawn between the precise exposure conditions and the current macroscopic properties, a lot of crucial information about ageing could be obtained from the study of the present metallurgic state, and in particular from the nanostructure of aluminum alloys. The nanoscale analysis is central: reorganisation of precipitation, grain boundaries evolution, dislocation movement can explain macroscopic properties: mechanical behavior or sensitivity to corrosion.

From archival research and study of original pieces we hope to be able to remodel the metallurgic initial state. Metallurgic investigation on different scales (nm to mm) should allow us to have a precise description of current structure.

The objectives of the project is a better understanding of ageing behavior mechanisms of aluminum alloys thanks to parallel study of microstructure and fatigue/corrosion of old alloys and in particular to identify the physical processes spanning at very different scales of both space (nanometers to millimeter) and time (nanosecond to tenths of years).

Methodology:

At the association “les Ailes anciennes” in Blagnac, a large collection of old aircrafts mainly post-World War II are stored and repaired. A Breguet 765 double-decker which flew from 1958 till 1972 is under renovation and a lot of materials are collected on this aircraft. Typical alloys employed for the structures are Duralumin A-U2G, A-U4G, A-U3G. Various qualities of alloys were used depending on the role of the element in the aircraft.

A stock of spare parts dedicated exists for the studied plane: these parts are the original parts from the 1950’s, not yet used and properly stored. They were not exposed to mechanical stress and large temperature variations: this state could be considered as a natural aging state and it is most probably very close to original state (out of factory). It is thus possible to have access to two states for the same part: the original state and the aged state (see Fig. 1). Furthermore, recent alloys of the same composition (2024-T251 equivalent to A-U4G1: 4% Cu; 1,5% Mg) are also available and will provide a third reference in our study.

The first phase of the project will be a thorough study at various scales of the aluminum alloys thanks to X-ray Diffraction (XRD) and Raman spectroscopy for structural information and thanks to electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and associated spectroscopy (EDX, EELS) for structural and chemical information. The mechanical properties of the as-received pieces will be measured with our partner at CIRIMAT-ENSIACET (outside NEXT).

The second phase of the project will be the understanding of material’s microstructure behavior upon aging: artificial aging of recent alloys will be compared to naturally aged alloys.

In summary, these alloys (collected, spare or recent) are  model materials  for carrying out the proposed study: they will help in determining key parameters in the mechanisms involved at various scales in long-term aging.