Materials Science: Dislocations & Strengthening Mechanisms
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Questions and Answers

What is a dislocation in the context of materials science?

  • A linear crystalline defect in the atomic structure (correct)
  • A defect caused by heat treatment
  • A type of grain boundary
  • A type of atom that causes material strength

    • When was the existence of dislocations directly observed?

  • 1950s (correct)
  • 1970s
  • 1920s
  • 1940s

    • What are the two primary types of dislocations?

  • Edge and screw dislocations (correct)
  • Twinning and screw dislocations
  • Point and line dislocations
  • Edge and surface dislocations

    • Which of the following best describes an edge dislocation?

    <p>A dislocation where an extra half-plane of atoms is inserted</p> Signup and view all the answers

    What is the motion of a screw dislocation?

    <p>The dislocation moves perpendicular to the applied stress</p> Signup and view all the answers

    What is the term used to describe permanent deformation of materials by dislocation motion?

    <p>Slip</p> Signup and view all the answers

    Which of the following is true about a slip system?

    <p>The slip system involves the movement of dislocations along a slip plane and direction</p> Signup and view all the answers

    What is a slip plane?

    <p>The plane with the highest density of atoms</p> Signup and view all the answers

    What is slip direction?

    <p>The direction in which dislocations move within the slip plane</p> Signup and view all the answers

    What does grain size reduction do to a material's strength?

    <p>Increases strength by increasing grain boundaries</p> Signup and view all the answers

    According to the Hall-Petch Equation, what happens as grain size decreases?

    <p>Yield strength increases</p> Signup and view all the answers

    Which of the following strengthening mechanisms involves adding alloying elements to a metal?

    <p>Solid solution strengthening</p> Signup and view all the answers

    What type of impurity atom replaces a host atom in solid solution strengthening?

    <p>Substitutional impurity</p> Signup and view all the answers

    What type of impurity atoms fill the gaps between atoms in solid solution strengthening?

    <p>Interstitial impurities</p> Signup and view all the answers

    Which strengthening mechanism involves the metal becoming stronger after plastic deformation?

    <p>Strain hardening</p> Signup and view all the answers

    What is the effect of strain hardening on a metal?

    <p>It makes the metal more brittle</p> Signup and view all the answers

    What is percent cold work (PCW) a measure of?

    <p>The amount of deformation in a material expressed as a percentage</p> Signup and view all the answers

    Which of the following is the formula for percent cold work?

    <p>%CW = (Ao - Aƒ)/ Ao x 100</p> Signup and view all the answers

    What is recovery in the context of metal deformation?

    <p>Mild heating to allow dislocations to rearrange without affecting hardness</p> Signup and view all the answers

    What occurs during recrystallization?

    <p>New, strain-free grains form, reducing dislocation density</p> Signup and view all the answers

    What happens during grain growth?

    <p>The new grains grow larger, reducing the number of grain boundaries and strength</p> Signup and view all the answers

    What is the primary role of grain boundaries in strengthening metals?

    <p>They act as obstacles to dislocation motion</p> Signup and view all the answers

    Which technique is most effective for strengthening polycrystalline materials?

    <p>Grain size reduction</p> Signup and view all the answers

    What happens when metals undergo extensive plastic deformation at low temperatures?

    <p>They experience strain hardening</p> Signup and view all the answers

    Which of the following is NOT a stage in the recovery, recrystallization, and grain growth process?

    <p>Precipitation hardening</p> Signup and view all the answers

    Why is it important to study failure?

    <p>To understand the mechanics of various failure modes - fracture, fatigue, and creep.</p> Signup and view all the answers

    What is one of the three usual causes of failure?

    <p>Improper material selection and processing</p> Signup and view all the answers

    Which type of fracture is usually preferred?

    <p>Ductile fracture</p> Signup and view all the answers

    Why is ductile fracture preferred over brittle fracture?

    <p>It provides a warning before failure due to plastic deformation</p> Signup and view all the answers

    Which type of environment-assisted fatigue failure involves fluctuating thermal stresses?

    <p>Thermal fatigue</p> Signup and view all the answers

    What are the three usual causes of failure?

    <p>Improper material selection and processing, inadequate component design, component misuse</p> Signup and view all the answers

    Which type of fracture occurs with very little accompanying plastic deformation?

    <p>Brittle fracture</p> Signup and view all the answers

    What is the main characteristic of brittle fracture?

    <p>It spreads rapidly and is perpendicular to the tensile load</p> Signup and view all the answers

    What type of failure is the single largest cause of failure in metals?

    <p>Fatigue</p> Signup and view all the answers

    Which testing techniques are used to measure impact energy?

    <p>Charpy and Izod tests</p> Signup and view all the answers

    What is fatigue?

    <p>Failure that occurs after repeated stress or strain cycling</p> Signup and view all the answers

    What is the definition of creep?

    <p>Time dependent and permanent deformation of materials when subjected to a constant load or stress</p> Signup and view all the answers

    Which phase of creep involves a constant creep rate?

    <p>Secondary creep</p> Signup and view all the answers

    What type of creep occurs at elevated temperatures due to fluctuating stresses?

    <p>Thermal creep</p> Signup and view all the answers

    Which type of crack is more stable, requiring an increase in applied stress to propagate?

    <p>Ductile crack</p> Signup and view all the answers

    What does the stress concentration factor depend on?

    <p>Crack tip radius and crack length</p> Signup and view all the answers

    In which type of fracture do cracks spread extremely rapidly?

    <p>Brittle fracture</p> Signup and view all the answers

    What is the primary cause of thermal fatigue?

    <p>Fluctuating thermal stresses</p> Signup and view all the answers

    What is the main focus of fracture mechanics?

    <p>Understanding the formation and propagation of cracks</p> Signup and view all the answers

    Which phase of creep occurs first and involves a continuously decreasing creep rate?

    <p>Primary creep</p> Signup and view all the answers

    What does the Charpy test measure?

    <p>Impact energy</p> Signup and view all the answers

    Which factor can significantly increase the maximum stress at the crack tip?

    <p>Small tip radius of curvature</p> Signup and view all the answers

    What type of fracture is characterized by a cup-and-cone profile?

    <p>Ductile fracture</p> Signup and view all the answers

    Which environment-assisted fatigue failure involves chemical attack during cyclic stress?

    <p>Corrosion fatigue</p> Signup and view all the answers

    In the context of creep, what does tertiary creep signify?

    <p>Acceleration of the creep rate leading to failure</p> Signup and view all the answers

    What are polymers?

    <p>Large molecules made up of repeating units</p> Signup and view all the answers

    What are the repeating units in polymers called?

    <p>Monomers</p> Signup and view all the answers

    Which of the following is a natural polymer?

    <p>Rubber</p> Signup and view all the answers

    Which polymer is used to make plastic bags?

    <p>Polyethylene</p> Signup and view all the answers

    Which polymer is known for its elasticity?

    <p>Rubber</p> Signup and view all the answers

    Which polymer is commonly used to make pipes?

    <p>PVC</p> Signup and view all the answers

    Which polymer is lightweight, strong, and used in packaging?

    <p>Polyethylene</p> Signup and view all the answers

    Which of the following is a synthetic polymer?

    <p>Teflon</p> Signup and view all the answers

    What is the main monomer used to make polystyrene?

    <p>Styrene</p> Signup and view all the answers

    What is cross-linking in polymers?

    <p>Formation of covalent bonds between polymer chains</p> Signup and view all the answers

    What is the main feature of biodegradable polymers?

    <p>Break down into harmless substances in the environment</p> Signup and view all the answers

    Which polymer is used for making bottles?

    <p>PET</p> Signup and view all the answers

    What is Bakelite used for?

    <p>Electrical insulators</p> Signup and view all the answers

    What is the process called when monomers join without producing any byproduct?

    <p>Addition polymerization</p> Signup and view all the answers

    What property allows polymers to stretch and return to their original shape?

    <p>Elasticity</p> Signup and view all the answers

    Study Notes

    Materials Science Multiple Choice Questions and Answers

    • Dislocations and Strengthening Mechanisms

      • Dislocation is a linear crystalline defect in the atomic structure.
      • Dislocations were directly observed in the 1950s.
      • Two primary types of dislocations are edge and screw dislocations.
      • Edge dislocations have an extra half-plane of atoms inserted.
      • Screw dislocations move in a direction perpendicular to the applied stress.
      • Permanent deformation of materials by dislocation motion is called slip.
      • Increasing grain size reduces material strength by reducing dislocation movement.
      • Grain size reduction increases material strength
      • Solid solution strengthening involves adding alloying elements.
      • Interstitial impurities fill the gaps between atoms.
      • Substitutional impurities replace host atoms.
      • Strain hardening occurs after plastic deformation, increasing material strength.
      • Slip systems involve dislocation motion along a slip plane and direction, primarily in cubic crystal structures.
      • Slip plane is the plane with highest atom density.
      • Slip direction involves movement along a dislocation within the slip plane within the direction of applied stress.
      • Grain size reduction increases strength by increasing the number of grain boundaries.
      • Recovery is mild heating to rearrange dislocations without affecting hardness.
      • Recrystallization involves high-temperature heating to create new grain structures.
      • Grain growth increases grain size, reducing grain boundaries and strength.
      • Percent cold work (PCW) is the amount of deformation in a material expressed as a percentage.
      • Formula for percent cold work calculation is %CW = [(Ao-Ad)/Ao] *100 where, Ao is the original area and Ad is the deformed area.

    • Failure

      • Common failure modes are fracture, fatigue, and creep.
      • Typical causes of failure include improper material selection, and processing, inadequate component design, component misuse.
      • Brittle fracture is usually less preferred to ductile fracture.
      • Ductile fracture offers a warning before failure.
      • Environment-assisted fatigue failure, such as thermal fatigue, is caused by fluctuating thermal stresses.

    • Polymers

      • Polymers are large molecules made of repeating units (monomers).
      • Examples of polymers include nylon, Teflon, rubber and polystyrene.
      • Natural polymers include rubber.
      • Monomers are the repeating units in polymers.
      • Examples of polymers used in packaging include polyethylene and polystyrene.
      • Elasticity is a polymer property, meaning the ability to stretch and return to a previous shape.
      • An example of a polymer reinforced to resist impacts is Kevlar.
      • An example of a polymer for electrical insulation is PVC.
      • Cross-linking forms covalent bonds between polymer chains, enhancing thermal and chemical resistance.
      • Biodegradable polymers break down into harmless substances in the environment after use.

    • Composite Materials

      • Composite materials combine multiple chemically distinct phases separated by an interface.
      • This characteristic is often advantageous in specific applications, like aerospace, due to increased properties like high strength-to-weight ratio and high temperature conductivity.
      • The matrix material in a composite protects and transfers stress to the reinforcing phase.
      • The dispersed reinforcing phase in composites is responsible for providing strength and stiffness.
      • Examples of composite materials include reinforced concrete, carbon fiber-reinforced polymers, and metal-matrix composites.

    • Recycling of Materials

      • Freecycling involves recycling waste products by giving them to others for free.
      • Deconstruction breaks down materials into their raw materials.
      • Recycling is the reuse of discarded materials.
      • Recovery involves using waste materials.
      • Reuse involves the reuse of existing products.
      • Reducing waste involves reduction in total use of materials.
      • Examples of recyclable materials are glass bottles, aluminum cans, and steel scrap.
      • Examples of the major sources of waste are plastics, rubber, styrofoam, and cellophane.
      • Processes in recycling metals involve collection, transportation, and processing.
      • Adaptive reuse involves repurposing existing buildings.
      • Major sources of waste are from packaging, junked autos, and other domestic durable goods.

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    Dislocations and Strengthening Mechanisms PDF

    Description

    Test your knowledge on dislocations and strengthening mechanisms in materials science! This quiz covers various concepts including types of dislocations, grain size effects, and strengthening methods such as solid solution strengthening and strain hardening.

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