Materials exam 3
Describe the changes in the microstructure of a plastically deformed polycrystalline metal specimen?
Alteration of the grain structure of a polycrystalline metal as a result of plastic deformation. Before deformation the grains are equiaxed. The deformation has produced elongated grains. Polycrystalline metals are stronger than their single-crystal equivalents, which means that greater stresses are required to initiate slip and the attendant yielding.
Effect of cold work on the mechanical properties
As cold work increases, Yield strength (σy) increases, Tensile strength (TS) increases, Ductility (%EL or %AR) decreases. Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work (Figures 7.19a and 7.19b); ductility decreases
What methods can be used to avoid creep?
Decrease the parameters of temperature and stress levels so T < 0.4Tm, change in material.
Define creep and specify the conditions under which it occurs.
Deformation under such circumstances is termed creep. Defined as the time-dependent and permanent deformation of materials when subjected to a constant load or stress, creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part.
Understand what fractography represents each failure/fracture mechanisms (dimple, v- marks chevron marks, intergranular, transgranular,...)
Dimple: When the fibrous central region of a cup-and-cone fracture surface is examined with the electron microscope at a high magnification, it is found to consist of numerous spherical "dimples". This structure is characteristic of fracture resulting from uniaxial tensile failure. Each dimple is one half of a microvoid that formed and then separated during the fracture process. Dimples also form on the 45° shear lip of the cup-and-cone fracture. V-marks chevron marks: are arrows that indicate at what point failure originated Transgranular (through-grain) and intergranular (between-grain) crack propagation paths are possible for polycrystalline brittle materials.
What factors affect fatigue fracture?
Fatigue is a common type of catastrophic failure in which the applied stress level fluctuates with time; it occurs when the maximum stress level may be considerably lower than the static tensile or yield strength. Fatigue cracks normally nucleate on the surface of a component at some point of stress concentration. Maximum stress level may be considerably lower than the static tensile or yield strength Measures that may be taken to extend fatigue life include the following: Reducing the mean stress level Eliminating sharp surface discontinuities: Sharp corners may also act as points of stress concentration and should be avoided when designing structures that are subjected to stresses. Improving the surface finish by polishing Imposing surface residual compressive stresses by shot peening Case hardening by using a carburizing or nitriding process
Define fatigue and specify the conditions under which it occurs
Fatigue is a common type of catastrophic failure in which the applied stress level fluctuates with time; it occurs when the maximum stress level may be considerably lower than the static tensile or yield strength. Responsible for 90% of mechanical failures. Cycles to fail decreases as the change in stress increases
Effect of flaws on TS.
Flaws act as stress concentrators that cause failure at stresses lower than theoretical values When the magnitude of a tensile stress at the tip of one of these flaws exceeds the value of this critical stress, a crack forms and then propagates, which results in fracture. more flaws = lower TS flaws are main cause of brittle material failure brittle = more flaws = lower TS
Recrystallization temperature.
For a particular alloy, the minimum temperature at which complete recrystallization occurs within approximately 1 h.
Describe the fracture mechanisms (ductile, moderately ductile, brittle) in detail.
Fracture - in response to tensile loading and at relatively low temperatures may occur by ductile and brittle modes. The maximum stress that may exist at the tip of a crack is dependent on crack length and tip radius, as well as on the applied tensile stress. Ductile (Stable) *Warning before fracture*: Normally preferred. Fracture in which the specimen necks down to a point showing virtually 100% reduction in area. Preventive measures may be taken inasmuch as evidence of plastic deformation indicates that fracture is imminent. More energy is required to induce ductile fracture than for brittle fracture. Cracks in ductile materials are said to be stable (i.e., resist extension without an increase in applied stress) Moderately ductile (Cup and Cone): Moderately ductile fracture after some necking. Brittle (Unstable) *No Warning before fracture *: Brittle fracture takes place without any appreciable deformation and by rapid crack propagation. the fracture surface is relatively flat and perpendicular to the direction of the applied tensile load Cracks are unstable—that is, crack propagation, once started, continues spontaneously without an increase in stress level. Transgranular (through-grain) and intergranular (between-grain) crack propagation paths are possible for polycrystalline brittle materials.
Know and be able to discuss the types of strengthening mechanisms in detail.
Grain Size Reduction: Grain boundaries are barriers to dislocation motion for two reasons: When crossing a grain boundary, a dislocation's direction of motion must change. There is a discontinuity of slip planes within the vicinity of a grain boundary. A metal that has small grains is stronger than one with large grains because the former has more grain boundary area and, thus, more barriers to dislocation motion. For most metals, yield strength depends on average grain diameter according to the Hall-Petch equation, Equation 7.7. Solid Solution Alloying: The strength and hardness of a metal increases with increase of concentration of impurity atoms that go into solid solution (both substitutional and interstitial). Small atoms - Tension in neighbors, it segregates just above the perpendicular. Large atoms - Compression in neighbors, it segregates just below the perpendicular. Solid-solution strengthening results from lattice strain interactions between impurity atoms and dislocations; these interactions produce a decrease in dislocation mobility. Strain Hardening (Cold Work): Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is deformed plastically. Degree of plastic deformation may be expressed as percent cold work, which depends on original and deformed cross-sectional areas as described by Equation 7.8. Yield strength, tensile strength, and hardness of a metal increase with increasing percent cold work (Figures 7.19a and 7.19b); ductility decreases (Figure 7.19c). During plastic deformation, dislocation density increases, the average distance between adjacent dislocations decreases, and—because dislocation-dislocation strain field interactions, are, on average, repulsive—dislocation mobility becomes more restricted; thus, the metal becomes harder and stronger. Precipitation Hardening: Large shear stress needed to move dislocation toward precipitate and shear it Dislocation "advances" but precipitates act as "pinning" sites with spacing S. σy ineversly 1/S
What are the common methods used in the industry to prevent fatigue fracture? How do they prevent fatigue fracture?
Impose compressive surface stresses Shot pinning, remove stress concentrators, and carbonizing.
Describe failure stages for moderately ductile failure.
Initial necking: mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material. The resulting prominent decrease in local cross-sectional area provides the basis for the name "neck" Void nucleation: small cavities, or microvoids, form in the interior of the cross section Void growth and coalescence: these microvoids enlarge, come together, and coalesce to form an elliptical crack, which has its long axis perpendicular to the stress direction. The crack continues to grow in a direction parallel to its major axis by this microvoid coalescence process Shearing at surface: fracture ensues by the rapid propagation of a crack around the outer perimeter of the neck Fracture: shear deformation at an angle of about 45° with the tensile axis—the angle at which the shear stress is a maximum.
Describe how plastic deformation occurs.
Plastic deformation corresponds to the motion of large numbers of dislocations. An edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line.
Describe the annealing process and the three stages of the annealing process in detail.
Recovery: some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature. Recrystallization: A new set of strain-free and equiaxed grains form that have relatively low dislocation densities. The metal becomes softer, weaker, and more ductile. Are small in size. Consume and replace the parent cold worked grain. For pure metals, the recrystallization temperature is normally 0.4Tm, where Tm is the absolute melting temperature; for some commercial alloys it may run as high as 0.7Tm. Grain Growth: After recrystallization is complete, the strain-free grains will continue to grow if the metal specimen is left at the elevated temperature
Define resolved shear stress and critical resolved shear stress. When does plastic deformation take place?
Resolved Shear Stress - is the shear stress resulting from an applied tensile stress that is resolved onto a plane that is neither parallel nor perpendicular to the stress direction. Its value is dependent on the applied stress and orientations of plane and direction according to Equation 7.2 Critical Resolved Shear Stress - is the minimum resolved shear stress required to initiate dislocation motion (or slip) and depends on yield strength and orientation of slip components per Equation 7.4. For a single crystal that is pulled in tension, small steps form on the surface that are parallel and loop around the circumference of the specimen. The single crystal plastically deforms or yields when τR(max) = τcrss, Conditions for dislocation motion yield τR>τcrss. For deformation to occur σ ≥ σy
Slip motion in single crystal materials vs slip motion in polycrystalline materials.
Single Crystal - A 1 slip system (plane and direction). For a single crystal that is pulled in tension, small steps form on the surface that are parallel and loop around the circumference of the specimen. Polycrystals - A 3 slip system. Stronger than single crystals because grain boundaries serves as an obstacle to dislocation motion. The highest resolved stress will yield first. Random crystallographic orientations of the numerous grains, the direction of slip varies from one grain to another. For each, dislocation motion occurs along the slip system that has the most favorable orientation.
Understand lattice strain around edge dislocations.
Some atomic lattice distortion exists around the dislocation line because of the presence of the extra half-plane of atoms. As a consequence, there are regions in which compressive, tensile, and shear strains exist in the vicinity of the dislocation line. Shear lattice strains only are found for pure screw dislocations. Note that for an edge, the dislocation line moves in the direction of the applied shear stress t; for a screw, the dislocation line motion is perpendicular to the stress direction.
Describe and explain the phenomenon of strain hardening in terms of dislocations and strain field interactions.
Strain hardening is the enhancement in strength (and decrease of ductility) of a metal as it is deformed plastically. During plastic deformation, dislocation density increases, the average distance between adjacent dislocations decreases, and—because dislocation-dislocation strain field interactions, are, on average, repulsive—dislocation mobility becomes more restricted; thus, the metal becomes harder and stronger.
What is the DBTT and explain it using a graph.
The ductile-to-brittle transition is related to the temperature dependence of the measured impact energy absorption.
Define fracture toughness (statement and equation).
The fracture toughness of a material is indicative of its resistance to brittle fracture when a crack is present Fracture toughness— dependence on critical stress for crack propagation and crack length Three factors that may cause a metal to experience a ductile-to-brittle transition are exposure to stresses at relatively low temperatures, high strain rates, and the presence of a sharp notch
Describe recrystallization in terms of both the alteration of microstructure and mechanical properties of metals.
The metal becomes softer, weaker, and more ductile The driving force for recrystallization is the difference in internal energy between strained and recrystallized material. during recrystallization, the mechanical properties that were changed as a result of cold working are restored to their precold-worked values- that is, the metal becomes softer and weaker, yet more ductile
Define slip system and cite one example.
The motion of dislocations in response to an externally applied shear stress is termed slip. Slip occurs on specific crystallographic planes, and within these planes only in certain directions. A slip system represents a slip plane-slip direction combination. FCC crystal structure, there is a set of planes, the {111} family, all of which are closely packed and it occurs in <110> directions
Lattice strain interactions. When does repulsion and attraction occurs? What is annihilation?
Two edge dislocations of the same sign and lying on the same slip plane exert a repulsive force on each other; C and T denote compression and tensile regions, respectively. Edge dislocations of opposite sign and lying on the same slip plane exert an attractive force on each other. Upon meeting, they annihilate each other and leave a region of perfect crystal. Annihilation - That is, the two extra half-planes of atoms align and become a complete plane
Explain how grain boundaries impede dislocation motion and why a metal having small grains is stronger than one having large grains.
When crossing a grain boundary, a dislocation's direction of motion must change. There is a discontinuity of slip planes within the vicinity of a grain boundary. A metal that has small grains is stronger than one with large grains because the former has more grain boundary area and, thus, more barriers to dislocation motion.
Know what a slip system is, and know what is solid solution hardening, strain hardening, precipitation strengthening, and grain boundary strengthening are
done
Define Stress Concentration Factor (statement and equation)
is a dimensionless factor that is used to quantify how concentrated the stress is in a material. It is defined as the ratio of the highest stress in the element to the reference stress
