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72 Cards in this Set
- Front
- Back
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What are the 2 fundamental dislocation types?
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Edge and Screw
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What is Slip?
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Plastic deformation produced by a dislocation motion
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What is Dislocation Density?
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The number of dislocations or the total dislocation length over unit length
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When a material is deformed, does the material retain energy internally? If yes, how much?
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YES! About 5%
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When a material deforms, what type of energy is typically retained internally?
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Strain energy
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What type of material typically has high dislocation density?
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Metals!
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What are Lattice Strains?
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Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms.
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In screw dislocations, lattice strains are what in terms of shear?
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Lattice strains are pure shear that radiate from the dislocation line.
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Does the magnitude of lattice strain increase or decrease with radial distance from the dislocation?
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Decreases with radial distance
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What is a Slip System?
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The combination of a crystallographic plane and within that plane, a crystallographic direction along which slip occurs
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What is Slip Direction?
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The direction of movement of the slip
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Which crystal structures have the largest number of slip systems?
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FCC and BCC!
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What makes FCC and BCC ductile?
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Their high number of slip systems!
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Which crystal structure has few slip systems? What does this mean for this material?
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HCP have few slip systems and tend to be more brittle
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Which plane tends to be the Slip Plane?
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The plane with the greater planar density
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What direction tends to be the Slip Direction?
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The direction that is most closely packed with atoms
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During plastic deformation, the number of dislocations increase due to what 4 sources?
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1. Existing dislocations
2. Grain boundaries 3. Internal defects 4. Surface irregularities (scratches or nicks) |
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When strain fields interact, what happens if the force is attractive?
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Attractive force results in dislocation and annihilation when they meet
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What is Resolved Shear Stress?
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An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane.
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In resolved shear stress, shear components exist at all alignments except what two alignments?
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Parallel and perpendicular!
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In resolved shear stress, magnitude depends on what 3 things?
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1. Applied stress
2. Orientation of slip plane 3. Direction |
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What is Critical Resolved Shear Stress?
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Minimum shear stress required to initiate slip!
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In regards to metals, what does Critical Resolved Shear Stress determines?
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Property that determines when yielding occurs
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In Plastic Deformation for Polycrystalline Metals, dislocations occur along what type of slip systems?
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Slip systems that have the most favorable orientation for it
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For polycrystalline metals, during deformation, do grain boundaries come apart? What does this mean?
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NO therefore each individual grain distorts and is a result of gross plastic deformation (becomes elongated)
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Are polycrystalline metals stronger or weaker than single-crystal equivalents?
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Stronger!
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Do polycrystalline metals require a higher or lower stress to initiate slip in comparison to single-crystal equivalents?
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HIGHER stress to initiate slip in polycrystalline metals
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Virtually all strengthening techniques rely on what principle?
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Restricting or hindering dislocation motion renders a material harder and stronger
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The ability of a metal to plastically deform depends on what?
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On the ability of dislocations to move
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If you decrease the mobility of dislocations, does mechanical strength increase or decrease?
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Increases!
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What 2 reasons allow grain boundaries to act as barriers to dislocation motion?
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1. Since 2 grains are @ different orientations, a dislocation passing into grain B will have to change its direction of motion
2. Atomic disorder within grain boundary will result in discontinuity of slip planes from one grain to another. |
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Why is fine grained metals harder?
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Because they have greater total grain boundary area to impede dislocation motion
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As grain size decreases, what happens to toughness?
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It increases!
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In Solid Solution Strengthening, if you increase the concentration of an impurity, what happens to tensile and yield strength?
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They increase as well!
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Why are alloys stronger than pure metals?
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Because impurity atoms that go into solid solution typically impose lattice strains on the surrounding host atoms.
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What is Solid Solution Strengthening?
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Alloying with impurities that go either into substitutional or interstitial solid solution
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What is Strain Hardening?
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The phenomenon by which a ductile metal becomes harder and stronger as it is plastically deformed.
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What is Strain Hardening also called?
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Work Hardening
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What is Cold Working?
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The plastic deformation of a metal at a temperature below that at which it recrystallizes.
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In Strain Hardening, an increase in the # of dislocations causes what?
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The motion of dislocation to be hindered because of the presence of another dislocation
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What is Recovery?
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The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.
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What is Recrystallization?
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The formation of a new set of strain-free and equiaxed grains that have low dislocation densities and are characteristic of pre-cold-worked conditions.
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What is the driving force to produce new grain structure in recrystallization?
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The difference in internal energy between the strained and unstrained material.
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What is Annealing?
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A heat treatment in which the microstructure and therefore the properties of a material are altered.
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Annealing typically refers to what?
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A heat treatment whereby a previously cold-worked metal is softened by allowing it to recrystallize.
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What is Recrystallization Temperature?
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The temperature at which recrystallization just reaches completion in 1 hour
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What is the principle behind recrystallization?
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New grains form as a small nuclei and grow until they consume the parent material
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When Percent Cold Work increases, what happens to Recrystallization Temperature?
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It decreases
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If percent cold work goes below 2-20, what happens?
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Recrystallization cannot occur
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Is recrystallization faster or slower in pure metals or in alloys?
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Recrystallization is faster in PURE METALS than in alloys.
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Do grains grow in size or decrease in size when kept at elevated temperatures?
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They grow in size!
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As grain size increases, what happens to the total boundary area, and the total energy?
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They decrease!
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The migration of grain boundaries causes what?
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Growth at the expense of smaller grains
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In Crystalline Ceramics, plastic deformation is caused by what?
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Motion of dislocations
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The ionic bonds in crystalline ceramics means what? Why?
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There are few slip systems along which dislocations can move. This is because of the electrically charged nature of ions
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Covalent bonds in crystalline ceramics means what 4 things?
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1. Slippage is difficult
2. Strong 3. Limited # of slip systems 4. Dislocation structures are complex |
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In non-crystalline ceramics, plastic deformation is caused by what?
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Viscous flow!
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In non-crystalline ceramics, the rate of deformation is proportional to what?
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Applied stress!
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In non-crystalline ceramics, atoms/ions do what?
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Slide past each other and reform bonds
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What is Viscosity?
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A measure of a noncrystalline material's resistance to deformation
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In semicrystalline polymers, what is Elastic Deformation?
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Elongation of chain molecules in direction of applied stress by bending + stretching of covalent bonds in amorphous region.
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In Semicrystalline Polymers, what is Plastic Deformation?
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Lamellar ribbons slide past each other as tie chains in amorphous region become extended
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In semicrystalline polymer plastic deformation, crystalline block segments from separating from what?
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Lamellae
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In semicrystalline polymer plastic deformation, the blocks and tie chains become what?
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They become oriented in the direction of tensile axis
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What is Drawing in terms of Metals?
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A forming technique used to fabricate metal wire and tubing. Deformation is accomplished by pulling the material through a die by means of a tensile force applied on the exit side.
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What is Drawing in terms of Polymers?
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A deformation technique in which polymer fibers are strengthened by elongation.
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What 5 factors influence the mechanical properties of semicrystalline polymers?
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1. Temperature + Strain rate
2. Molecular weight 3. Degree of Crystallinity 4. Predeformation by Drawing 5. Heat treating |
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Regarding factors that influence the mechanical properties of semicrystalline polymers, if molecular weight increases, what happens to tensile strength?
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It also increases!
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Regarding factors that influence the mechanical properties of semicrystalline polymers, if temperature increases and strain rate decreases, what happens to tensile modulus, tensile strength and ductility?
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Tensile modulus and tensile strength decreases and ductility increases
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Regarding factors that influence the mechanical properties of semicrystalline polymers, if crystallinity increases, what happens to tensile strength? Why?
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Tensile strength increases because of secondary bonds in crystalline region. This also increases brittleness.
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Regarding factors that influence the mechanical properties of semicrystalline polymers, if the annealing temperature increases, what happens to yield strength and ductility?
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Yield strength increases and ductility decreases which is the opposite in metals.
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Regarding factors that influence the mechanical properties of semicrystalline polymers, predeforming by drawing causes what to happen?
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Causes stiffness and strength to increase by permanently deforming the polymer in tension
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