Slides-Lab_4 - BCC single crystals is markedly temperature...

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Lab #4 Impact Performance: Brittle-ductile transitions
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In metals, plastic deformation at room temperature occurs by the motion of dislocation. The stress required to move a dislocation depends on the atomic bonding, crystal structure, and obstacles such as solute atoms, grain boundaries, precipitate particles and other dislocations. If the stress required to move the dislocation is too high, the metal will fail instead by the propagation of cracks and the failure will be brittle. Thus, either plastic flow (ductile failure) or crack propagation (brittle failure) will occur, depending on which process requires the smaller applied stress. In FCC metals, the flow stress, i.e. the force required to move dislocations, is not strongly temperature dependent . Therefore, dislocation movement remains high even at low temperatures. the material remains relatively ductile. In contrast to FCC metal crystals, the yield stress or critical resolved shear stress of
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Unformatted text preview: BCC single crystals is markedly temperature dependent, in particular at low temperatures. The temperature sensitivity of the yield stress of BCC crystals has been attributed to the presence of interstitial impurities on the one hand, and to a temperature dependent Peierls-Nabarro force on the other. EFFECT of CRYSTAL STRUCTURE Ductile-to-Brittle Transition The ductile-brittle transition is exhibited in BCC metals, such as low carbon steel, which become brittle at low temperature or at very high strain rates. FCC metals, however, generally remain ductile at low temperatures. Charpy Impact Tester Materials 1018 steel (0.18wt%C) 1045 steel (0.45wt%C)l 70/30 Brass 2024 Aluminum 1. Impact the test specimens that were conditioned at temperature ranged from (-190 o C to 100 o C). 2. Record the impact energy vs. the conditioned temperature to examine the impact performance of materials at different temperature...
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Slides-Lab_4 - BCC single crystals is markedly temperature...

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