Application of phase shifting interferometric microscopy to studies of the behaviour of coarse intermetallic particles in 6056 aluminium alloy
Introduction
The 6056 T6 alloy has recently been developed by Pechiney to replace the 2024 T3 alloy in aircraft structures. However, it seems to be susceptible to different forms of corrosion, such as intergranular and pitting corrosion. Very few works have studied the corrosion properties of this alloy or that of its American equivalent, the 6013 T6 alloy. We showed in a previous work, [1]that the 6056 T6 alloy was susceptible to pitting corrosion; the coarse intermetallic particles did not seem to have a significant influence on the behaviour of the material but this study was based on scanning electron microscopy (SEM) observations. It is interesting to use another technique to provide further explanation of the role of the intermetallic particles in order to understand the corrosion properties of the material.
Since pitting corrosion can be initiated on very small sites and\or sites of different types (intermetallic particles), it was necessary to use a method which allows each particle to be studied individually. Moreover, this method has to be very accurate since the pit initiation stage concerns very small volumes. Such localised attack cannot be detected by electrochemical techniques because the material volume involved is very low. SEM, which we used in earlier works, did not provide quantitative information on attack depth. Mechanical profilometers may modify the surface under investigation and often the depth sensitivity is not very great.
Atomic Force Microscopy and Scanning Tunneling Microscopy are as well adapted as the phase shifting interferometric microscopy (PSIM) that we used. Several authors, for example Warner et al., [2]have determined the pit initiation sites in the 2024 T3 alloy using atomic force microscopy. But, Escalona et al. [3]used phase shifting interferometric microscopy to study pitting corrosion of ion-implanted stainless steel. This method provides sufficient lateral resolution considering the dimensions of the intermetallic particles and a very high resolution in depth. The different types of intermetallic particles are rapidly identified because their reflection properties are different from those of the matrix. The quantitative analysis of attack is very quick.
The behaviour of the intermetallic particles present in the 6056 T6 alloy after immersion in a sulphate solution was studied using phase shifting interferometric microscopy. The influence of heat treatment was investigated. The results were related to the composition of the particles.
Section snippets
Profile measurement technique: phase shifting interferometric microscopy
The technique is based on classical interferometry (Fig. 1). A light beam emerging from a visible light source (wavelength λ) is split into two beams. One is reflected by a reference mirror and the other by the sample surface. After reflection the two beams are recombined and they generate an interference pattern which is an image of the sample point by point. The intensity of the light at each point depends on the path difference between the two beams. If the sample is flat and in a plane
Materials and experimental conditions
The material used was a 3.2 mm thick sheet of 6056 T6 alloy. The T6 state consists of solution heat treatment at 550°C, air quenching and tempering at 175°C for 8 hours. The overall thermomechanical treatment confers on the alloy an average grain size of 30–40 μm. This alloy was further heat treated at a temperature higher than 175°C for a time t; this overaged material is referred to as 6056 T78. Two intermediate heat treatments were performed in order to understand the influence of overaging
Observation of the materials by PSIM before immersion in the electrolyte
Whatever the heat treatment, two types of coarse intermetallic particle referred to as type a and type b particles were observed in the 6056 alloy. 3D profiles were plotted for the two types of particle and for the different heat treatments in Fig. 3Fig. 4 On these profiles, the z axis is very expanded compared to the x and y axes. Even if the particles appear as a high relief on the profile, the mean height of the particles is only equal to a few hundred nanometers. The relief observed may
Conclusions
This work showed the following.
- 1.
PSIM is a technique which allows quantitative results to be obtained when the reactivity of intermetallic particles is studied.
- 2.
EDS analyses showed that, when the 6056 T6 alloy was overaged, the composition of the intermetallic particles was modified. This induced a variation in the reactivity of these particles.
- 3.
The Al-Mg-Si-containing particles largely dissolved in the overaged alloy whereas their dissolution was very low for the 6056 T6 alloy.
- 4.
The
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