This section displays photo micrographs of commercially important and/or metallurgically interesting copper and copper alloys and processings.
To view each alloy group, simply click on the links below:
|Copper & High Copper Alloys||Brasses & Bronzes||Other Copper Alloys||Special|
|Grain Size Comparison|
Microstructures - An Overview
What is a microstructure?
Metals are crystalline when in the solid form. The crystal structure of a solid metal refers to the internal structure or arrangement of the atoms in an ordered, repeating, three dimensional pattern. Normal metallic objects are polycrystalline, which means they consist of an aggregate of many very small crystals. These crystals are called grains. Some metallic objects, such as castings, have very large grains that can be resolved with the naked eye and these structures are referred to as macrostructures. Typically, the grains of a metal object are very small, and can not be viewed with the naked eye. The structural features of the small grains are observed using an optical microscope or metallograph, or an electron microscope, at magnifications greater than 100 times. Structures requiring this range of magnification for their examination are called microstructures.
Why is the microstructure of a material important?
The most important aspect of any engineering material is its structure. The structure of a material is related to its composition, properties, processing history and performance. And therefore, studying the microstructure of a material provides information linking its composition and processing to its properties and performance. Interpretation of microstructures requires an understanding of the processes by which various structures are formed. Physical Metallurgy is the science which provides meaningful explanations of the microstructures, through understanding what is happening is inside a metal during the various processing steps. Metallography is the science of preparing specimens, examining the structures with a microscope and interpreting the microstructures.
The structural features present in a material are a function of the composition and form of the starting material, and any subsequent heat treatments and or processing treatments the material receives. Microstructural analysis is used to gain information on how the material was produced and the quality of the resulting material. Microstructural features, such as grain size, inclusions, impurities, second phases, porosity, segregation or surface effects, are a function of the starting material and subsequent processing treatments. The microstructural features of metals are well defined and documented, and understood to be the result of specific treatments. These microstructural features affect the properties of a material, and certain microstructural features are associated with superior properties.
What is Microstructural Analysis used for?
Macrostructural and microstructural examination techniques are employed in areas such as routine quality control, failure analysis and research studies. In quality control, microstructural analysis is used to determine if the structural parameters are within certain specifications. It is used as a criterion for acceptance or rejection. The microstructural features sometimes considered are grain size, amount of impurities, second phases, porosity, segregation or defects present. The amount or size of these features can be measured and quantified, and compared to the acceptance criterion. Various techniques for quantifying microstructural features, such as grain size, particle or pore size, volume fraction of a constituent, and inclusion rating, are available for comparative analysis.
Microstructural analysis is used in failure analysis to determine the cause of failure. Failures can occur due to improper material selection and poor quality control. Microstructural examination of a failed component is used to identify the material and the condition of the material of the component. Through microstructural examination one can determine if the component was made from specified material and if the material received the proper processing treatments. Failure analysis, examining the fracture surface of the failed component , provides information about the cause of failure. Failure surfaces have been well documented over the years and certain features are associated with certain types of failures. Using failure analysis it is possible to determine the type of stress that caused the component to fail and often times determine the origin of the fracture.
Microstructural analysis is used in research studies to determine the microstructural changes that occur as a result of varying parameters such as composition, heat treatment or processing steps. Typical research studies include microstructural analysis and materials property testing. Through these research programs the processing - structure - property relationships are developed.
What is Metallography?
Metallography is the study of the structure of metals. It includes the techniques used to prepare specimens for examination, examining the specimen and interpreting the structures. Specimen preparation is an important part of metallography. A specimen must be appropriately prepared to ensure correct observation and interpretation of the microstructure. Specimen preparation consists of sample selection, sectioning, grinding, polishing, and etching. Adequate sample selection provides a statistically reliable description of the material quality. The number, location and orientation of the samples examined are important parameters in sample selection. Sectioning, grinding and polishing are used to prepare a flat specimen with a mirror like finish. Care must be taken during sample preparation not to introduce artifacts which lead to invalid microstructure interpretations. Sometimes it is beneficial to examine the specimen in the as polished condition. The as polished condition is useful for examining the microstructures of materials whose constituents exhibit large differences in light reflectivity after polishing. Porosity and inclusions are examples of features that are easily observed in the as polished condition. But most materials are etched to reveal the microstructure. Etching is a controlled corrosion process resulting form electrolytic action between surface areas of different potential. Etching reveals the microstructure of a material by selective dissolution of the structure. Specimens are then examined using optical and electron microscopes. There are also many other techniques used to characterize the structure of metals, but this article will concentrate on microstructural characterization.