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Cards In This Set
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T/F Higher the Ductile-Brittle-Transition-Temperature of a material, better it is for use in engineering applications.
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False. The higher DBTT means the material will change it behavior from ductile to brittle at a higher temp, meaning the possible range of temp over which it behaves or fails in ductile manner, a desirable type of failure will be narrowed. This is no good for any material or engineering component.
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T/F Increased loading rate increases the chances for ductile fracture to occur
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False. The increased rate gives less tiem for dislocations to move past obstacles and slip to occur for the plastic deformation to take place, which is necessary for any material to fail in ductile manner.
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T/F Rockwell Hardness test constitutes the most common method used to measure hardness where hardness number is determined by measuring the diameter of indentation.
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False. Rockwell is simple to perform, and doesn't need special skills. The combination of different loads and indenters, a wide range of scales are available for testing. The hardness number is determined by the measurement of the depth of penetration.
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T/F Hardness of a material is the deformation of the material under a load.
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False. Hardness is the resistance to deformation.
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T/F The fracture strength of a ductile material is less than its tensile strength.
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True. Because of necking in ductile materials a lower stress is sufficient to continue plastically deforming the specimen, so the F.S. is lower than the T.S.
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T/F Single crystal materials are 'isotropic'.
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False. A single crystal is anisotropic, but when the material is present in a polycrystalline form, due to the random orientation of the grains, the anisotropy gets nullified, and the polycrystalline material becomes isotropic.
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T/F Pulling atoms apart from their equilibrium positions, results in the development of a repulsive force between them.
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False. Pulling atoms apart results in the development of an attractive restoring force between the atoms that tend to pull them back together.
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T/F Al alloys are ductile, but Mg alloys are brittle.
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True. The al possesses F.C.C. structure which makes it more ductile creating 12 slip systems (4 slip planes, 3 slip directions); whereas Mg possesses H.C.P structure making it brittle with 3 slip systems (1 slip plane, 3 slip directions).
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T/F Many polymeric materials are used in the form of thin films or fine fibres.
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True. Polymers are used in fiber formation because the moles entangle and inhibit significant movement of each other. (2) polymers are confined to relatively low temps where the thermal energy is low. The diffusivity is relatively low. In order to counter act this effect polymers are used as films to help reduce 1 or 2 dimensions, and reduce the diffusion path, which in turn increases the rate of diffusion.
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T/F Cu diffuses more easily in Cu, than in Al
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False. Cu diffuses more readily in Al, because the Cu-Cu bonds are stronger than the Al-Al bonds, as evidenced by their melting points. So, the movement of atoms (diffusion) is slower in Cu-Cu than Cu-Al.
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Essay. Factors affecting DBTT
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(1) Crystal structure: FCC metals and alloys remain ductile to extermely low temps, BCC and HCP experience DBTT.
(2) Alloy compositions and microstructure (3) Grain size: decrease grain in lowering of DBTT (4) Carbon content: increase C raises DBTT (5) Increase Mn content: decreases DBTT (6) High rate of loading, irradiation of samples, surface cracks, notches, and notch sharpness... increase DBTT. |
