Lab7-ColdWorkAnnealingF06-1

Lab7-ColdWorkAnnealingF06-1 - Laboratory 7 Metals &...

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MatE 215 Lab 7: Cold Work & Annealing (Fall 06) Page 1 of 11 Laboratory 7 Metals & Alloys: Effects of Cold Work and Annealing Goal: To investigate the change in mechanical properties because of various amounts of cold working and annealing in metals & alloys, and to relate the properties to the microstructure. Learning Objectives: 1. Discuss how metals are able to plastically deform because of dislocations. 2. Explain how and why cold working strengthens a metal. 3. Explain how and why annealing can restore an alloy’s ductility. 4. Describe the microstructural changes and property (e.g., hardness) changes that take place during recrystallization . 5. Perform rolling operations to reduce the thickness of metal pieces and calculate the % cold work . 6. Relate the trend of amount of cold working with hardness measurements from experiments. 7. Estimate the recrystallization temperature (T rex ) by observing hardness changes during annealing. 8. Recognize instances of natural variation in samples and measurements, and the need to take averages.
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MatE 215 Lab 7: Cold Work & Annealing (Fall 06) Page 2 of 11 Why Study Deformation And Annealing In Metals? It’s no surprise to you that in order to get metals into the form of useful products, be they bicycle crankshafts or stents for angioplasty, they must be deformed (or “worked”) in some way. This deformation is often done at temperatures less than 40% of the metals’ melting temperature and thus categorized as “cold work.” A lot of it is actually done at room temperature. Cold work can dramatically change the properties of a metal…sometimes to our advantage (you can often increase a material’s strength), sometimes to our disadvantage (you can lose a material’s inherent ability to withstand further deformation). For example, did you ever notice that when you bend a metal paper clip back and forth, it gets harder and harder to bend before it eventually breaks? This is an example of cold work and it actually summarizes some important aspects of this lab. What Does Cold Working Do To The Microstructure? When you permanently bend a metal—take bending a metal coat hanger, for instance—you have disturbed the arrangement of the atoms in the metal’s crystal lattice. In fact, you’ve dislocated the atoms from their original position. For obvious reasons, these tiny crystal disturbances are called dislocations . They represent a region in the crystal where the arrangement of atoms departs from the regular arrangement, be it face-centered cubic, body-centered cubic, hexagonal close-packed or what-have-you. As you can imagine, the distortion in the crystal causes strain all along the length of the dislocation. You can think of a dislocation as a “tube” of strain, about 50 atoms in diameter and running from one edge of a grain to the other. A single grain is the same thing as a single crystal. Metals consist of countless grains that fit together like jigsaw puzzle pieces. Dislocations are much too
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Lab7-ColdWorkAnnealingF06-1 - Laboratory 7 Metals &...

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