Metallography of Aluminium alloys

TALAT Lecture 1202
Metallography of Aluminium alloys
20 pages, 7 Figures Basic level
prepared by E. Cerri, E. Evangelista Dipartimento di Meccanica, Università di Ancona-Italy
Objectives This lecture aims at providing a survey of the metallographic techniques available for the examination of aluminium and its alloys. The information must be sufficient to be sure that the students and the users are able to choose the most suitable technique to solve their problems in the examination of samples. The lecture should contain a direct understanding of the main problems in the metallography of the different classes of aluminium materials.
Date of Issue: 1999  EAA - European Aluminium Association

1202 Metallography of Aluminium alloys
Contents
1202 Metallography of Aluminium alloys ________________________________ 2 1202.01. Introduction______________________________________________________3 1202.02 Sample preparation ________________________________________________3
1202.02.01 Optical microscopy: mechanical grinding, mechanical polishing, etching, anodising__ 3 1202.02.02 SEM and TEM : electropolishing, dimpling, ion milling ____________________ 5
1202.03 The techniques used in metallography of aluminium and its alloys _________9
1202.03.01 Polarised light, interference contrast [2]__________________________________ 9 1202.03.02 Electron channelling contrast and electron channelling patterns ______________ 11 1202.03.03 High resolution electron microscopy (HREM) and high voltage electron microscopy (HVEM) _____________________________________________________________________ 13
1202.04 The metallography of the different class of aluminium alloy ___________15
1202.04.01 Commercial purity of aluminium ______________________________________ 15 1202.04.02 Wrought alloys ____________________________________________________ 16 1202.04.03 Foundry alloys ____________________________________________________ 18
1202.05 References _____________________________________________________20 1202.06 List of Figures____________________________________________________20

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1202.01. Introduction
The examination of microstructure is one of the principal means of evaluating alloys and products to determine the effects of various fabrication and thermal treatments and to analyse the cause of failure. Main microstructural changes occur during freezing, homogenisation, hot or cold working, annealing, etc. Good interpretation of the structure relies on having a complete history of the specimen. In general, the metallography of aluminium and its alloys is a hard job in the meaning that aluminium alloys represent a great variety of chemical compositions and thus a wide range of hardnesses and different mechanical properties. Therefore the techniques required for metallographic examination may vary considerably between soft and hard alloys. Moreover, one specific alloy can contain several microstructural features, like matrix, second phases, dispersoids, grains, subgrains and thus grain boundaries or subboundaries according to the type of the alloy and its thermal or thermomechanical history. However, some methods of sample preparation and observation are quite general and apply to all aluminium alloys. In other cases one should refer to specific developed methods.
As a general rule, examination should start at normal eye vision level and proceed to higher magnification. Simplicity and cost make optical examination (macro and micro) the most useful. When the magnification and the depth of focus become too low, the electron microscopies are required.
1202.02 Sample preparation
Aluminium alloys require the same principle of preparation for examination as most metals. Careful visual inspection of the part to be examined should precede cutting or etching. Fracture surfaces must be carefully preserved against abrasion or contamination. If the part is difficult to handle and has to be sectioned, care should be taken to cut the material along directions determined by the working process and by other interesting criteria. As an example, if the alloy has been rolled, it can be interesting to examine the evolution of microstructure along the rolling direction and so the part must be cut in the same direction.
1202.02.01 Optical microscopy: mechanical grinding, mechanical polishing, etching, anodising
The general metallographic sample preparation for optical microscopy includes a series of steps described in the following paragraph. The selected part of the material is cut by a SiC abrasive saw at a certain distance from the plane to be observed, because a thickness of several tens of microns will be then removed by mechanical grinding. A lubricant for cutting is used to avoid temperature increases and structure modification of the specimen. The sectioning is usually followed by mounting of the sample in a plastic

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medium to form a cylindrical piece that can be handled during grinding and polishing. This stage is not necessary if the sample is large enough. Cold mounting has to be preferred to the hot one if the alloy is sensitive to microstructural modifications (precipitation) also at low temperature (around 100°C).
Mechanical grinding is performed in successive steps using SiC abrasive papers of different grit sizes, usually 180, 220, 320, 600, 800, 1000, 1200, 2400 grit. The starting grit depends on the type of cut surface to be removed. The abrasive particles embed easily into soft aluminium alloys. So a wet grinding (water) to flush away these particles and a small pressure on the specimen are recommended.
The following step is the mechanical polishing. It is important for removing the surface scratches produced during grinding. The mechanical polishing is usually performed in two steps : rough and final polishing. The rough polishing is performed using 3 and 1 µm diamond paste on a short nap cloth disk. The lubricant can be a solution of alcohol and propylene glycol. The final step is made by 0.25 µm diamond paste. It is important to wash the sample after every step in an ultrasonic bath to remove all the abrasive. In cases where polishing with 1 or ¼ µm diamond paste does not produce a sufficiently deformation - free and scratch-free, highly reflective surface, as it is the case of pure aluminium and its soft alloys, the final polishing can be carried out using metal oxides in aqueous suspension. The most commonly used oxides are colloidal silica (SiO2) and alumina (Al2O3). Silica comes as a ready made colloidal suspension OP-S 0.04 µm. The advantages of silica over alumina are the particle size distribution which is much narrower than in alumina and the particles which do not agglomerate and are always in suspension. This means that there is less chance of fine scratches and it is easy to apply because it does not need continuous stirring. The cloths used for final polishing are usually soft compared to the harder cloth for diamond polishing.
The polishing can be also performed in an electrolytic way. This technique is suited for polishing homogeneous structure such as pure aluminium or very soft alloys which are difficult to polish mechanically [1,2]. Ref. 1 reports some of the electropolishing solutions frequently used for aluminium alloys. The principle of electrolytic polishing is described in the section which covers sample preparation for transmission electron microscopy (TEM). After polishing, the sample is ready for the last step before observation; this is called the etching. Etching is basically a controlled corrosion process resulting from electrolytic action between surface areas of different potential. With pure metals and single-phase alloys, a potential is produced between differently oriented grains, between grain boundaries and grain interiors, between impurity phases and the matrix or at concentration gradients in the single-phase alloys. With two phase or multiphase alloys, potential differences are also present between phases of different composition. These potential differences are used to produce controlled dissolution. The quality of the polishing influences the development of the true microstructure. A faulty preparation can lead to misinterpretation of the structure. In general wiping of the surface with moist cotton under running water is adequate, although ultrasonic cleaning especially if cracks or pores are present, is preferable [3].

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The etchants for use in microscopic examination of aluminium alloys are numerous and are illustrated in several books [2,3,4]. The most common for practical use are the followings :
• Keller’s reagent - 2ml HF (48%) + 3ml HCl + 5ml HNO3 + 190ml H2O. This etchant gives the possibility to reveal grain boundary contrast and precipitates in several wrought alloys.
• 1g NaOH + 100ml. H2O - This solution is useful for grain boundary contrast in 6xxx series.
• (HF etch) - 1 ml (HF) (48%) + 200ml H2O. It is used for constituent identification in cast alloys, especially those containing Si.
It is clear for example that the grain structure can not be easily revealed in every alloy. On a specimen with low alloying metal, the etching produces steps at the grain boundaries which do not provide good contrast. In this case anodising is preferable, as described below.
Anodising or anodic oxidation is an electrolytic etching process for depositing an oxide film on the metal surface that is often epitaxial to the underlying grain structure. The resulting interference colours are a function of the anodic film thickness, which depends on the anodising voltage, the solution and the composition and/or structure present in the specimen [2,5]. Anodising was first applied to the study of the grain structure of aluminium. Although aluminium has a cubic crystal structure and is therefore isotropic, the oxide film produced permits the grain structure to be observed under crossedpolarised illumination. If the film is thin, interference colours are produced and physical and chemical inhomogeneities can be observed. Generally thicker films are produced and the optical effect is ascribed to the surface structure of the oxide film or to submicroscopic profile of the interface of the metal and oxide [3]. The solution used for anodising aluminium and its alloys is called Barker’s reagent - 5ml HBF4 (48%) + 200ml. H2O (anodising conditions : J=0.2 A/cm2 for 40-80 s at room temperature). Actually the anodising voltage can be selected by trial and error. It is recommended to electropolish the sample before anodising.
In other materials, grain boundary precipitation can delineate the grain boundary itself if of course the alloy is heat treatable. A similar thinking concerns the examination of precipitates by optical microscopy. Moreover, the identification of precipitates requires other techniques like x-ray analysis or electron microscopy.
1202.02.02 SEM and TEM : electropolishing, dimpling, ion milling
Specimen preparation for electron microscopy has to be differentiated between scanning and transmission electron microscopy. In general, for the scanning electron microscope (SEM) the sample preparation is similar to the one for optical microscopy. The different results in observations are eventually attributed to the technique that is based on the interaction between electrons and metals and not on visible light. If the interesting area is a fracture surface, no special sample preparation is required, only a careful cleaning of the surface to be examined.

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For transmission electron microscopy (TEM), sample preparation is more difficult compared with SEM and optical microscopy, especially because of the very thin and small pieces that have to be handled. The procedure for TEM preparation of aluminium and its alloys is standard, and like that used for other metallic materials [1]. Extreme care must be taken to avoid the introduction of artificial defects into the microstructure, especially during cutting and mechanical grinding, that can substantially modify its evaluation. All the preparation steps are listed in the following section but more emphasis will be given to the final thinning techniques. First the sample is cut in slice 1 mm thick by SiC abrasive saws and then mechanically ground and polished down to ≈ 100 µm using the same papers and clothes as for optical microscopy preparation. In this case the foil has to be polished on both sides. When the thickness of the slice has reached 100µm, other techniques are used to thin the sheet or disc specimen down to its final electron-transparent thickness. These are the electropolishing or the dimpling and the ion milling. These three techniques are described hereafter.
The principle of electropolishing is quite simple. An electrolytic cell is established with the specimen as the anode and an appropriate potential is applied so that the sample is dissolved in a controlled way (Figure 1202.02.01). Electropolishing is performed until a hole has formed : this means that the region around the hole should be thin enough for TEM. Experimentally there are many variables to control like cell geometry, applied potential, temperature and stirring velocity of the solution. The electropolishing must remove very fine surface irregularities and must thin the specimen uniformly.

The electrolyte must contain an oxidising agent together with reagent that will form a thin but stable viscous film. The fine electropolishing action is achieved by dissolution controlled by the length of the diffusion path through the viscous film to the electrolyte (Figure 1202.02.02).

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In order to start polishing, the essential components are the electrolyte, the cell and a power supply (usually d.c.) to provide 5-50 V (see Figure 1202.02.03).

Some of the most used electrolytic solutions are listed in ref. 1.
In practice, after the slice is uniformly 50µm in thickness, it is punched into several disc of three mm in diameter and then every disc is again electropolished in an automatic jet polisher. In a jet polisher a jet of electrolyte is directed at the centre of the disc on both sides of the specimen until it is appreciably thinned. This produces an ideal specimen for mounting in the TEM with strong edges that support the thin areas in the centre. Usually the electrolyte and the other parameters are the same as for electropolishing.

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If it is desirable to have a disc with a depression in the centre for the final thinning (for example when using the ion milling technique) it is necessary to mechanically dimple the disc. This is a means by which a disc is created with a depression in its centre only on one side. Usually a 1mm thickness metal wheel tool is all that required to dimple discs cut by spark machining or ultrasonic drilling. During dimpling, rough abrasive diamond paste is used for grinding and fine diamond paste for polishing with felt wheels. Care must be taken in all cases that the thickness remaining in the centre of the disc is not so small that damage has occurred through the final thin area. If the starting
disc thickness is around 100µm, the depression can reach 2/3 of its initial thickness (Figure 1202.02.04).

If the material is composed of two phases and one phase electropolishes faster than the other, it is more suitable to use ion milling for the final thinning. In aluminium alloys, ion milling is useful for Al-Si system and their metal matrix composites.
The principle of ion beam thinning is rather simple. A beam of inert gas ions or atoms is directed at the disc specimen from which it removes surface atoms in a process known as sputtering. If this can be achieved without the creation of artefactual damage then the ion beam thinning is an ideal method for the preparation of foils from both conducting and non-conducting materials. However it is necessary to control several undesirable effects like ion implantation, the development of a rough surface and the heating of the specimen that for aluminium alloys is a very important problem to avoid. For these reasons it is necessary to control the nature of ions, their energy and direction of incidence and their frequency of arrival (ion beam current). Sputtering occurs when any ion carrying more than about 100eV of energy hits a surface. The number of atoms ejected by each incident ion is called the sputtering yield Y. In general Y increases with ion energy and ion mass, but decreases with increasing specimen atom mass. The argon is the inert gas mostly used for ion beam thinning because of its reasonable cost and good sputtering yield. The optimal ion energy is easy to select according to Figure

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1202.02.05 (a), where the Y at high ion energy decreases because the ion start to implant in the surface instead of ejecting atoms.

The value of energy corresponds to an accelerating voltage of 4-6 kV. The sputtering yields depend also on incident angle as shown in Figure 1202.02.05 (b). Usually the starting incident angle is around 14° to increase the yield, but after a while it is decreased to 12° to a have a better control on the thinning. For aluminium alloys it is recommended to have a cooled specimen holder because the sample temperature can increase also to 100°C or more due to ion interactions with the material and this can affect the structure. The ion beam technique has become useful also for ceramics and non-conducting materials. It is less useful for light ductile conducting metals where the microstructural damage introduced by ion implanting in the surface may lead to confusing artefacts.
1202.03 The techniques used in metallography of aluminium and its alloys
This section will deal with the main techniques used in optical and electron microscopy for observation of aluminium and its alloys. As the techniques are common to other metallic materials, tables will be reported containing a list of the most common observation techniques with emphasis on the visible structure and the sample preparation required. Information about specific techniques can be found in the indicated references.
1202.03.01 Polarised light, interference contrast [2]

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Light microscopy is the major tool for microstructural examination of aluminium alloys and is recommended for use before electron microscopy. It can reach magnification of 1500X and resolves features as small as 0.1µm. Usually, optical microscopy can show the size and distribution of resolvable particles, and shows the state of the crystal structure. It does not reveal precipitate particles responsible for precipitation hardening or dislocation arrangement. Generally, analysis of these conditions is the domain of electron microscopy.
Table I reports some of the most common techniques used in optical microscopy for observation of metallic materials. In the following a description of polarised light and interference contrast mode will be reported with reference to their use for aluminium alloys.

Table I : optical microscopy observation techniques

technique

sample preparation

structure

bright field dark field polarised light interference contrast

normal normal anodisation normal

g, gb, p p, gb g g, gb, p

g : grain, gb : grain boundary, p : phases and precipitates, fs : fracture surface, d : defects and dislocations, i : interfaces, e : etched, u : unetched

The polarised light. An isotropic metal (cubic or amorphous crystal structure) transmits or reflects light of the same velocity in all directions. If a plane polariser light beam strikes normal to the surface of an isotropic metal, it will be reflected as a planepolarised beam with the same azimuth of polarisation. The amplitude of the beam of course will be reduced by an amount that varies with the reflectivity of the particular metal. If a plane-polarised light beam is passed through a second polarising filter (the analyser) placed at 90°C to the polariser, the light will be extinguished. This position of the polariser and analyser is referred as crossed. If an anisotropic metal is substituted for an isotropic metal in the previous experiment, an image of the microstructure will be observed. The rotation of the sample under the beam of 360° produces four positions of maximum and minimum light intensity in each grain. The use of monochromatic light is recommended, but white light also produces colour contrast effects [2]. A strain free, clean surface is required for best results. So electropolishing is the best preparation method. The technique of anodising, frequently used with aluminium, produces a thick oxide film on the metal surface electrolytically : irregularities in the film lead to double

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