Metallography

metallographic cut of a bell bronze, polished and etched with Klemm III

Analysis with the reflected light microscope

Metallography is the elucidation as well as qualitative and quantitative description of the structure of metallic materials with the help of microscopic methods. It thus represents a discipline of metal science. The activity is carried out by a metallograph .

In order to see the structure in the microscope, the material has to be prepared. Knowledge of the properties and processing of a metal is important in order to avoid errors in preparation and structure assessment.

Due to the increasing number of composite materials and the emergence of new or further developed materials (ceramic / metal systems, metal / plastic systems, etc.), the term materialography is now preferred.

Metallography / materialography is widely used in quality assurance and damage analysis as well as in research and development.

preparation

Just like non-metallic crystals (e.g. rock crystal ), metals also have a crystalline structure that can be recorded qualitatively and quantitatively. In the metallography This is usually done by the preparation of a Anschliffes, the polished to a high gloss and auflichtmikroskopisch is examined (see, for. Example, the Struers Metalog Guide). A scratch-free section suitable for macro and / or microscopic observation must have a representative, sharp-edged, flat surface of the material to be examined, which clearly shows its structure and which does not show any changes such as deformations , chipping, scratches and smears caused during manufacture . The production of flawless grindings is basically possible for all solid materials , but it is often very expensive.

Immediately after polishing, due to the different reflective properties of metals and non-metals , initial statements about the purity of the material can be made. Impurities usually stand out darkly from the metallic matrix.

Abrasive pads are papers and fabrics on which the grains are applied. Polishing pads are mainly cloths (felt, velvet, synthetic fibers , linen). For better removal and to check the grinding and polishing marks, the sample must be rotated by 90 ° after each grinding or polishing step.

Sample preparation and embedding

The separated samples should first be deburred. We then recommend ultrasonic cleaning in a beaker with ethanol to degrease the samples. The samples are then dried using a grinding dryer. Porous and cracked samples should be dried longer so that the ethanol can dry off completely. The further handling of the samples should only be done with etching tongs or tweezers.

Whoever embeds correctly saves a lot of time in sample preparation. The embedding serves for better handling of the samples as well as for supporting the edge zone or the infiltration of cracks, pores or corrosion coatings. Embedding aids are suitable for fixing thin samples such as sheet metal or CFRP laminates and fibers so that they cannot fall over.

A distinction is made between two different investment methods: hot investment and cold investment.

Warm embedding

clear finished warm embedding with inserted label

In the case of hot mounting, the sample is embedded in an embedding press under temperature and pressure with plastic granules. The advantages of the process are the high hardness of the embedding material (e.g. glass fiber reinforced) and the good absence of gaps. There are phenolic resin-based, also glass-fiber-reinforced, epoxy resin-based embedding agents for gap-free embedding, especially of hard metals, as acrylic resin embedding agents and even graphite-phenolic resin-based as an electrically conductive warm embedding agent for electron microscopy. Warm mounting media are heated to 180 ° C under pressure. Heat-embedded specimens can be removed again relatively easily. For this purpose, the samples must be heated in a heating cabinet at 220 ° C for about 1 hour. The embedding material swells and the sample can simply be pressed out in a parallel vice. If 220 ° C is too high for the samples (embedding temperature is 180 ° C), you can notch the embedding material on the outer surface using a hacksaw and then break it in a vice.

Cold mounting

Cold-embedded samples in the desiccator

The molds are greased before they are filled. Cold mounting requires two to three components, which are mixed in a mixing beaker. Cold mounting materials are available on a methyl methacrylate basis and as transparent epoxy resin mounting materials. Embedding agents based on methyl methacrylate consist of a powder component and a hardening liquid. After stirring, the mixture is poured into the embedding mold. The curing takes place via a chemical reaction (polymerisation). Polymerization is an exothermic reaction, so heat is generated during curing. Depending on the sample mass, temperatures of up to 110 ° C can arise here briefly. Epoxy resin embedding material, on the other hand, consists of two liquid components, the resin and the hardener, and is usually used for vacuum infiltration. With the help of the vacuum in a desiccator, the epoxy resin embedding agent can be infiltrated into the finest cavities, such as crevices, cracks and pores. This prevents the etchant from being sucked into the gap when the sample is etched ( capillary effect ). Porous corrosion deposits can also be infiltrated. In this way, unwanted breakouts from the corrosion deposits can be minimized during preparation. It can be colored for better contrasting of cracks and pores. To reduce the viscosity before pouring the sample, it can be indirectly heated in a warm water bath. The polymerization of both cold mounting media can alternatively take place in a pressure pot. To do this, the pressure pot is brought to 2 bar overpressure 2 to 3 times and completely ventilated again. Adhering air bubbles can rise and burst when the pressure is equalized. Gap formation can be effectively suppressed, especially with embedding materials based on methyl methacrylate. A combination of vacuum infiltration with subsequent curing in the pressure pot is recommended for the epoxy resin embedding agent.

Grinding and polishing

Wet sanding on SiC paper with grain size P320 / P500 / P800 / P1000 / P1200
Pre-polishing with diamond suspension with grain sizes 6 µm, 3 µm, 1 µm on rayon or cotton cloth with alcoholic lubricant
Fine polishing with aluminum oxide suspension on a wool or velvet cloth.

alternatively

Fine polishing with silicon dioxide (grain size 0.04 µm) + various chemicals such as ammonia or hydrogen peroxide for materials that otherwise tend to deform, such as copper, titanium, nickel on a chemical-resistant polishing cloth.

A very good and quick alternative:

Wet grinding on a 75 µm diamond disc
Polishing with a 9 µm diamond suspension on a honeycomb disc
Polishing with a 3 µm diamond suspension on a honeycomb disc
Polish with a 3 µm diamond suspension on a hard, smooth disk
Polish with oxidic polishing suspension on a soft cloth

Careful cleaning between the individual grinding and polishing stages is necessary in order to avoid dragging along coarser cutting grains and abrasion. For drying, the liquids that evaporate hard (e.g. water, oil) are first rinsed off the sample with alcohol . In the hot air stream (hair dryer) this liquid dries quickly and without spots.

etching

macro-etched pure copper disk

macro-etched pure aluminum piece

The microscopic examination is initially carried out at a small magnification, which allows a good overview. In general, a polished ground surface does not show any structure because the incident light is reflected almost evenly. Lenses with higher magnification are used as required.

The cut should be viewed in the unetched as well as in the etched state. The following can be observed on the unetched section:

Coatings and anodic layers
Soldering and welding seams
Pores and gas bubbles
Micropipes
Sintered pores
Material separations
non-metallic inclusions , slags
Layers of scale on the surface or in cracks
Poorly reflective phases such as graphite , silicon and their compounds.

If statements are to be made about the structure, it is necessary to etch the polished section - a wide variety of solutions ( acids , alkalis , neutral solutions, molten salts , etc.) are used. The actual structure is examined on the etched section . The etching takes place “in time” to the finished preparation (polishing). If the structure is correctly contrasted, statements can be made about the heat treatment condition and quality, and in many cases conclusions can be drawn about the manufacturing process and the causes of defects in the event of damage.

Internal processes in etching

The etching of a metallic ground joint is a chemical or electrochemical process. Due to local differences in the chemical composition, local currents are formed as a result of potential differences. The individual phases / grains are removed to different degrees or contrasted by a fine layer of precipitation. An etching attack caused by the crystal orientation also takes place. A relief is created due to the different removal rates of the individual structural components. The incident light experiences a change in reflection at the phase boundaries.

A basic distinction is made between macro and micro etching. Etchings shown up to approx. 30 times magnification are called macroetching. For this, a macro-sanding with a finely sanded (wet sanding) or slightly polished surface is required. The primary cast structure, forged and rolled textures as well as segregation or the contour of welded joints are made visible by means of macro etching . Further variants of the macro-etching process are, in addition to the general macro-etching, deep etching and the impression process (Baumann impression).

Microetch

To make the (secondary) structure visible by means of microscopic imaging methods from a magnification of approx. 50 times. A distinction is made between two methods: wet chemical and electrolytic. In addition, a distinction is made between grain boundary and grain surface etching. Micro-etchings can be carried out using various techniques.

The most common method of wet chemical etching is the so-called dip etching. The microsection is completely immersed in the etching solution. The cut should be moved in the etching solution with the etching tongs in order to destroy the hydrogen bubbles and to compensate for any differences in the concentration of the etchant. Some immersion etchings require the etchant to be heated (e.g. V2A stain).

Caustic

The choice of etchant depends on the type of material to be etched and the size of the structures to be examined.

material	Macro etchant		Micro-etchants
cast iron			Nital: Alcoholic nitric acid: 95 - 98 ml of ethanol (until the 1960s, often methanol for cost reasons ) + 2 - 5 ml of concentrated nitric acid
Unalloyed and low-alloy steels	90 ml of ethanol + 10 ml of concentrated nitric acid	Etching agents according to Adler: Solution A: 3 g ammonium chlorocuprate + 25 ml distilled water Solution B: 15 g iron (III) chloride + 50 ml concentrated hydrochloric acid After everything has completely dissolved, add solution B to A.
High alloy steels, nickel
Copper and copper alloys			120 ml distilled water + 10 g ammonium chlorocuprate Add ammonia dropwise to the solution until the precipitate that forms disappears
Aluminum and aluminum alloys	10 ml distilled water + 10 ml concentrated hydrochloric acid + 10 ml concentrated nitric acid + 2.5 ml hydrofluoric acid (38-40 percent)		according to Kroll : 100 ml dist. Water + 2–6 ml conc. Nitric acid + 1–3 ml conc. Hydrofluoric acid Hydrofluoric acid : 99.5 ml of distilled water + 0.5 ml of hydrofluoric acid Caustic soda : 90 ml of distilled water + 10 g of sodium hydroxide 10% H 3 PO 4 : → Very good for secondary excretions 90 ml dist. Water + 10 ml of phosphoric acid
High-alloy chrome-nickel steels			V2A stain according to Goerens: 100 ml distilled water + 100 ml concentrated hydrochloric acid + 10 ml concentrated nitric acid + 0.3 ml Vogels Sparbeize

In addition to reflected light microscopy, electron microscopy is also used , in which not only polished sections but also fractured surfaces can be examined and the chemical composition of the structural components can be analyzed using spectroscopy .

Individual evidence

↑ ^a ^b Egon Kauczor: metal under the microscope. 5th edition. Springer, Berlin / Heidelberg / New York / Tokyo 1985, ISBN 3-540-15611-9 (Berlin ...), ISBN 0-387-15611-9 (New York ...)

literature

Heinrich Hanemann, Angelica Schrader : Atlas Metallographicus: A collection of photographs for technical metallography . Borntraeger , Berlin: Volume 1: Carbon steels, slowly cooled and annealed (1927–1933), Volume 2: Cast iron: Part 1. Gray cast iron (and other parts) (1936–1939), Volume 3: Aluminum, Part 1. Binary alloys des aluminum (1941 (1943 edition)), Volume 3: Part 2. Ternary alloys of aluminum (1952).
Angelica Schrader : Etching book: Process for the production of the cut and structure development for metallography . 4th edition (revised with the assistance of Werner Schaarwächter): Borntraeger, Berlin-Nikolassee 1957.
L. Habraken, J.-L. de Brouwer: De ferri metallographia (Alta auctoritas Communitatis Europaeae carbonis ferrique), Part 1: Basics of metallography . Verlag Stahleisen, Düsseldorf 1966.
Angelica Schrader , Adolf Rose : De ferri metallographia (Alta auctoritas Communitatis Europaeae carbonis ferrique), Part 2: Structure of the steels . Verlag Stahleisen, Düsseldorf 1966.
Annik Pokorny, Jean Pokorny: De ferri metallographia (Alta auctoritas Communitatis Europaeae carbonis ferrique), Part 3: Solidification and deformation of steels . Verlag Stahleisen, Düsseldorf 1967.
Sarkis A. Saltykov : Stereometric Metallography . VEB German publishing house for basic industry, Leipzig 1974.
Günter Petzow : Metallographic, ceramographic, plastographic etching (material science-technical series; Vol. 1). Borntraeger, Berlin / Stuttgart 2006, ISBN 978-3-443-23016-6 (unchanged reprint of the Berlin 1994 edition).
Hans-Jürgen Bargel, Günter Schulze (Ed.): Material science. 10th edition. Springer, Berlin / Heidelberg 2008, ISBN 978-3-540-79296-3 .
Hermann Schumann, Heinrich Oettel (ed.): Metallography. With an introduction to ceramography . 15th edition. Wiley-VCH, Weinheim 2011, ISBN 978-3-527-32257-2 .
Wolfgang Weißbach: Materials science and material testing. , 17th edition. Vieweg + Teubner 2010, ISBN 978-3-8348-0739-7 .
Kay Geels, Daniel B. Fowler, Wolf-Ulrich Kopp, Michael Rückert: Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing . ASTM International, West Conshohocken, Pa. 2006, ISBN 978-0-8031-4265-7 .
Heinz-Hubert Cloeren: "Materialographic preparation techniques" 1st edition. CTV 2014, ISBN 978-3-9816824-0-3 .

Web links

Commons : Metallography - collection of images, videos and audio files

Wiktionary: Metallography - explanations of meanings, word origins, synonyms, translations

[Kauczor-1] Egon Kauczor: metal under the microscope. 5th edition. Springer, Berlin / Heidelberg / New York / Tokyo 1985, ISBN 3-540-15611-9 (Berlin ...), ISBN 0-387-15611-9 (New York ...)