Lec 6 - Diffusion
What is diffusion and what direction does it go in?
Diffusion is the transport of material through atomic motion. It goes from areas of high concentration to low concentration.
What is the difference between true stress/strain and engineering stress/strain?
True stress/strain take into account the instantaneous dimensions of the material.
Engineering stress/strain only focus on the original dimensions.
How are strength and dislocation motion related?
Dislocations facilitate plastic deformation by enabling the slip of atomic planes. The interaction between dislocations and other crystal defects influences the material's strength. Strengthening mechanisms hinder dislocation motion, increasing resistance to deformation. Yield and ultimate strength are affected by dislocation density and obstacles.
Briefly describe the differences between ductile and brittle fracture. Also, explain the difference between intergranular and intragranular fracture.
Ductile: Occurs with plastic deformation. High energy absorption due to plastic deformation. Warning signs before failure. Cup and Cone features after failure.
Brittle: Little or no plastic deformation. Catastrophic. Low energy absorption. Chevron Patterns. Unstable crack propagation
Intergranular: Between grains. Always Brittle.
Intragranular: Within Grains. Either Ductile or Brittle.
(Trick to remember which is which: Intergranular has an "e" in it and so does "between")
What is fatigue? Describe some of its properties. What are the three stages of fatigue failure?
Fatigue is the lowering of strength/failure of a material below its yield strength due to repetitive stress/cyclic loading. There is little plastic deformation. Catastrophic. Crack perpendicular to direction of applied stress.
1) Crack initiation 2) Crack Propagation 3) Sudden/fast fracture
Briefly describe Fick's First Law.
Fick's First Law describes the diffusion of a substance through a homogeneous medium. It states that the rate of diffusion (flux) of a substance is directly proportional to the concentration gradient (change in concentration per unit distance) and inversely proportional to the distance over which diffusion occurs.
What is hardness? What are the five hardness tests? Describe the tips that each test uses.
Hardness is the resistance to permanent deformation of the surface of a material.
Brinell: Steel sphere or tungsten carbides (HB)
Vickers microhardness: Diamond Pyramid (HV)
Knoop microhardness: Diamond Pyramid (HK)
Rockwell B: Steel sphere
Rockwell C: Diamond Cone
What causes dislocation movement? What is a Slip Plane? What is the Slip Direction?
Dislocation movement is caused by resolved shear stress.
A slip plane is the plane in which the easiest slippage can occur. The slip plane has the highest planar density and has wider interplanar spacing.
The slip direction is the direction of the movement of slip. It has the highest linear density and the shortest distance for each displacement step.
What is the ductile-to-brittle transition temperature?
The ductile-to-brittle transition temperature (DBTT) marks the point at which a material shifts from ductile to brittle behavior. At higher temperatures, dislocations within the material can move more freely, allowing for plastic deformation and ductility. However, at lower temperatures, dislocation movement becomes restricted due to factors like decreased atomic mobility and increased pinning by impurities or grain boundaries. This restriction leads to decreased ductility and increased susceptibility to brittle fracture.
Explain how you would improve fatigue life and give some methods as to how.
To improve fatigue life, use materials with high fatigue strength, avoid sharp corners or notches that act as stress concentrators, and apply surface treatments like shot peening to induce compressive stresses, which resist crack initiation and propagation. Additionally, careful design, maintenance, and operational practices that minimize cyclic loading and stress concentrations can help enhance fatigue life. Carburizing can also improve fatigue life as a carbon rich gas is absorbed into the metal and induces a compressive stress.
What is Diffusion Flux? Give an example.
The time-dependent movement of atoms within a material. It's like watching a group of ants moving around in search of food, but in this case, it's atoms shifting within a solid. For example, when you heat a metal rod, the atoms start to diffuse towards the cooler end, creating a temperature gradient.
What is plastic deformation and how does it relate to slip planes?
It is permanent deformation caused by the motion of dislocations along a slip plane.
What is a very important aspect of Cold working that must be considered when using it as a strengthening mechanism? (Hint: Think about recrystallization)
Cold-working cannot be used as a strengthening mechanism for components subjected to high temperature because the effects of cold working may revert back due to the recrystallization of the grain boundaries.
Briefly discuss the concept of fracture toughness and its significance in material design. Provide examples of materials with high fracture toughness and explain how it impacts their mechanical properties.
Fracture toughness measures a material's ability to resist crack propagation under mechanical loading. It's crucial in material design for ensuring structural integrity and safety. Examples of materials with high fracture toughness include certain metals like steel and titanium alloys. These materials can withstand mechanical stresses without fracturing, making them essential for various engineering applications.
What is Creep? How many failure modes are there for creep and what are they?
Name the 3 different creep rates and what they relate to.
A time dependent process in which permanent deformation occurs due to exposure to high temperatures and static mechanical stresses.
2 -> Rupture and excessive deformation.
1) Primary Creep: Creep rate decreases with time 2) Secondary Creep: Steady-state creep rate 3) Tertiary Creep: Acceleration of creep rate till rupture.
What is the process of Carbon/Nitrogen diffusion into steel called and what effect does this process have on the steel?
Case Hardening.
Increase in surface hardness and wear resistance.
Increase in fatigue resistance by imparting compressive stress.
At an atomic level, discuss the differences between when elastic deformation occurs and when plastic deformation occurs.
For elastic deformation, the bonds between the atoms stretch. This is why a material that has experienced elastic deformation can return to its original state.
For plastic deformation, the bonds stretch AND there is shear in the planes. It is because of the slippage that plastic deformation is permanent.
What are the four strengthening mechanisms? Describe each of them.
Grain size reduction: Because grain boundaries act as barriers to dislocation motion, when grain size decreases, the dislocation density increases. As a result, dislocations encounter more resistance as they move through the material.
Solid Solution: The injection of impurity atoms that tend to concentrate at dislocations and reduce the mobility
Precipitation hardening: Dislocation loops around precipitates requiring additional energy to advance because precipitates are harder to shear.
Cold Working/Strain hardening: Increases dislocations/unit area. Increase in strength and hardness, decrease in ductility.
Briefly discuss the design considerations for preventing brittle fracture in materials. Explain how the relationship between stress intensity factor and design stress influences material performance under loading conditions.
To prevent brittle fracture, minimize flaws, control stress concentrations, and select materials with high fracture toughness. The relationship between stress intensity factor and design stress determines material performance; keeping design stress below fracture toughness ensures stress intensity factor remains manageable, preventing brittle fracture under loading conditions.
Explain the effect that temperature and stress have on creep.
Higher temperatures increase atomic mobility, accelerating creep, while higher stress levels promote more rapid deformation.
Why is the diffusion coefficient for interstitial atoms higher than the diffusion coefficient of substitutional atoms.
Interstitial atoms are smaller than the atoms in the host lattice, making it easier for them to diffuse through the lattice. Additionally, interstitial atoms do not disrupt the regular arrangement of atoms in the lattice as much as substitutional atoms do when they migrate, leading to fewer obstacles for their movement. Consequently, interstitial atoms typically have higher diffusion coefficients compared to substitutional atoms.
Discuss the difference between the proportional limit of a material and the elastic limit of a material.
Proportional limit: the level of stress above which the relationship stress and strain is linear
Elastic limit: The critical stress value needed to initiate plastic deformation
They are not equivalent
What are the 3 steps of heat treatment? Describe each stage.
Recovery: Reduction of dislocation density by annihilation.
Recrystallization: Formation of equiaxed grains with low dislocation density; the mechanical properties have fully recovered. Must have seen cold work for this to happen.
Grain growth: The driving force behind this is the reduction of grain boundary energy. Happens above recrystallization temperature.
Briefly explain the role of flaws in material failure and how stress concentration factors impact crack propagation. Discuss the relationship between stress intensity factor and fracture toughness in determining material failure.
Flaws in materials act as stress concentrators, where stress tends to concentrate around the flaw, making it more susceptible to failure. Stress concentration factors exacerbate this effect by amplifying the stress at the flaw compared to the surrounding material. As a result, cracks are more likely to initiate and propagate from these flaws, leading to material failure. The stress intensity factor (K) quantifies the severity of stress at the tip of a crack. It is directly related to the applied stress and the size and shape of the crack. Fracture toughness (Kc) is a material property that represents its resistance to crack propagation. A material with high fracture toughness can withstand higher stress intensity factors before failure.
Describe the process of creep failure.
How would you prevent creep failure?
1) Internal cavities form (mostly along grain boundaries)
2) Cavities accumulate
3) Damage occurs at the START of tertiary phase
To prevent creep failure, choose materials with a high melting temperature, an alloy, a reduced number of grain boundaries/no grain boundaries.