Examination Tools Commonly Used to Test and Analyze Alloys
It’s extremely difficult to imagine a world without alloys. Alloys can be found in virtually every important structure and gadget today, used in homes, offices, bridges, even airline transportation. They are present in areas with human populations, from museum sculptures and statues to dental amalgam, even ships we send out to outer space. The fact that alloys are used in structures and constructions that impact many aspects of human life makes it very important that alloys undergo some of the most stringent inspection using examination tools. One of these is the metallographic microscope.
A metallographic microscope is designed to offer high power magnification, allowing the tester or observer to view and analyze the prepared metal surface. With a metallographic microscope, the metals and nonmetals that constitute the alloy may be identified, their value and contribution to the product analyzed and their performance given a set of determining factors predicted.
The need for using tools in alloy examination
The nature of alloy makes it important that it should be examined carefully. Unlike elements, alloys are a mixture of metals and/or chemical elements. A cursory examination without the use of any magnifying instrument will not allow thorough inspection, particularly if microstructural quantification is needed.
Some alloys, for example, contain iron, something that is difficult to determine without the high magnification offered by a metallographic microscope. It will also be difficult to determine what types of metals were used in the production of alloy or how they are dispersed throughout the mixture.
Since alloys are produced with a specific use in mind, much thought is given to the metals and non-metals that go into the mixture. Some alloys, for example, are made with titanium which gives them excellent tensile strengths that can reach up to 200,000 lbs. per square inch. This, at only about 60% the density of steel, allowing design and manufacturing engineers to use a tough metal sheet offering excellent coverage but at a lighter weight. Alloys are also capable of withstanding high temperatures, which makes them the choice for the production of jet engines.
Preparing the alloy
Alloys are commonly produced as sheets or blocks, so samples have to be prepared in order to be examined and analyzed. Some common tools used during this pre-examination include a metal polisher, a grinder and a metal etching tool. Even without the use of magnifying instruments, the trained eye will be able to detect any characteristic on the alloy that is considered sub-par or not appropriate.
Although this type of examination will usually suffice, other examination tools are used to analyze the alloy further, especially if certain requirements are in place. This is important if precise combinations are needed in order to produce an alloy of a particular strength or malleability. Detailed examination is also important when research is being done on a new combination of metals, non-metals, composites and chemicals to produce a new alloy.
Alloy examination tools using microscopy
A common examination used for alloys involves non-destructive processes, mostly by using scanning electron microscopy and metallographic microscopy. These tests use observation techniques to identify and analyze alloys for structure. Using either a scanning electron microscope or a metallographic microscope, a sheet of alloy may be identified as solid, inter-metallic, multiphase or ion-implantation alloys.
Generally, light optical microscopy is the examination tool used before an electron metallographic microscope, for reasons of economy. Using light optical metallographic microscopy, certain features and characteristics can be observed, including color, structure and texture.
Magnifications typically range from 50x to about 1000x, although high-end metallographic microscopes offer 2000x or even more. When this happens, it’s often necessary to make adjustments to the objective lenses in order to minimize distortion while offering a better spatial coverage or field of view. Another key consideration is resolution, which affects the visibility of the image, particularly if structure and contrast are a concern. Depending on the observer, bright field, polarized light or dark field illumination may be used.
Although there are many tools used to examine alloys (examinations and tests vary depending on the final use of the product), the metallographic microscope is key to ensuring that observations are accurate and up to standard.
One common test used on alloys is the bend test, which uses a machine that will physically bend the sample and measure its strength per pound of pressure. This tool will determine the alloy’s strength and ductility.
Another alloy examination tool is one that performs impact testing, sometimes referred to as the Charpy test. This test involves testing the strength or toughness of the alloy by using a fixed mass pendulum to strike the sample. The impact is then measured in terms of energy absorption. This is the type of test often used with automobiles, machineries and equipment.
Use of chemicals for alloy examination>
Another important tool in the examination of alloys is chemical testing, which is used to determine the alloy’s weight percentage and for analyzing protective coatings. Alloys are also tested with chemicals for corrosion potential and for determining chemical composition.
When alloy samples are difficult or impossible to cut or remove, field metallography is often used. On-site preparation and examination of the alloy is performed and a replica is made to be used for later for laboratory analysis.
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