Materials Science and Engineering Ch 6

What material properties are found in tensile testing?

Yield strength, Young's Modulus, and Ultimate Tensile Strength

What material properties are found in compression testing?

Compressive strength and compressive modulus

What material properties are found in shear testing?

Shear strength and shear modulus

What is elastic deformation?

Bonds stretch but return to their initial state after the load is removed

What is plastic deformation?

Bonds stretch and planes shear, when load is removed planes still sheared and object is permanently changed. (bonds change)

What is the linear elastic region?

The first part of the stress vs strain curve where the curve is linear. The slop of the region is called the elastic modulus or Young's modulus.

What is Young's/Elastic Modulus? (E)

The slope of the linear region of the stress vs strain curve. Think of it as a material's resistance to stretching/elastic deformation (high E - Stiffer material).

What if there is no linear region?

Use secant or tangent modulus below the proportionality limit.

secant: slope from zero to a point.

tangent: derivative of the curve at a point.

What is ductility?

Used to measure a material's degree of plastic deformation. Expressed as either % elongation or % reduction in area. (Brittle if <5%)

Why does temperature matter?

Young's modulus, yield strength, and tensile strength are inversely related to temperature. Ductility increases with increased temperature.

What is resilience (Ur)?

The material's ability to absorb energy when elastically deformed.

How do you calculate resilience (Ur)?

Area under the curve up to the yield strength. Ur = (1/2)(sigma y)(epsilon y)

What is toughness?

A material's ability to resist fracture once a crack is formed.

How can you measure toughness?

in dynamic loading (high strain) until a notch is present [impact testing]. For static loads, we use the area under the stress vs strain curve up to fracture.

What is the difference between true and engineering stress and strain?

When we look at the engineering stress-strain curve after necking begins, the material becomes stronger.

What is hardness?

A material's resistance to localized deformation (small dents/scratches). High hardness leads to better wear properties.

Why do we measure hardness?

Compared to other methods, it is simple and inexpensive, Non-destructive, and can be used to estimate other material properties

What is the Rockwell hardness test?

Most common hardness method. Uses a hardened steel ball to apply a minor and major load and measure size of indentation.

Safety factor formula

sigma d = N' * sigma c

sigma d: design stress

sigma c: calculated stress

Why not just choose a large factor of safety?

Too much material, higher-than-necessary strength, expensive

How are dislocations formed in part formation?

During solidification, plastic deformation, or thermal stresses from rapid cooling.

What are strain fields?

Dislocations or imperfections cause distortions of the crystal lattice. Strain fields radiate out from the dislocation line, losing magnitude with distance.

How do dislocations form?

Grain boundaries, internal defects, and surface irregularities.

How do we predict the movement of dislocations?

The preferred plane is the slip plane, and the preferred direction is the slip direction. Together, they are the slip system.

What are slip systems?

The number of slip systems in a material correlates to its ductility. They are made up of a slip plane and a slip direction

What is a slip plane?

The plane that has the highest planer density. This is the plane on which the easiest slip occurs.

What are slip directions?

Directions that have the highest linear densities. This is the direction of dislocation motion.

What is twinning?

Planes that are mirror images on either side of a dislocation line.

What are the three main mechanisms for single-phase materials to manipulate properties?

Grain size reduction, solid-solution strengthening, and strain hardening.

What are grain boundaries?

Neighboring grains typically have differing crystallographic orientations, and where these meet is the grain boundary.

What are the two ways grain boundaries impede slip?

Differing orientations mean dislocations must change direction at the grain boundary to continue moving. Atomic disorder of the grain boundary means discontinuities in the slip planes from one grain to the next.

What are grain boundary angles?

The angle between the slip planes of one grain to another.

What is the relation between grain size and the strength of a material?

The smaller the average grain diameter, the stronger and harder the material is. The total grain boundary area increases, thereby impeding slip.

How do we control grain size?

Solidification from the liquid state. Plastic deformation followed by heat treatment. Twin boundaries effectively block sli and increase strength. Multiple phases impede slip.

What is solid solution strengthening?

Alloying a material creates impurities (interstitial or substitutional). With increasing concentrations of impurities, a material can be strengthened.

What is dislocation diffusion?

As impurities diffuse throughout the bulk material, they tend to segregate and concentrate along dislocations.

What is impeding slip?

The resistance to slip comes from the increased required force to move the dislocations from around the impurity. You need to increase lattice strain to get them to move.

What is strain hardening?

Ductile metals become harder and stronger with plastic deformation. Often called work hardening or cold working.

What are the drawbacks of strain hardening?

You sacrifice ductile strength and hardness.

What are the three stages of cold working?

Recovery, Recrystallization, and Grain Growth.

What is recovery?

The stored strain energy is relieved by dislocation motion. Reduction in dislocations. Electrical and thermal conductivity is recovered.

What is recrystallization?

Formation of strain-free equiaxed grains. Low dislocation density. small nuclei form and grow and consume the mulf material to form grains. Mechanical properties are recovered.

What is recrystallization temperature?

The temperature at which it takes 1 hour to fully recrystallize a material.

How does recrystallization differ in alloys?

Alloys take longer to recrystallize than pure metals. Impurities interact with recrystallized grain and impede motion. Recrystallization rate is lowered in recrystallization temp increases.

What is grain growth?

Once a material is fully recrystallized, the equiaxed grains begin to grow when left at elevated temperatures. Grain boundaries migrate scarificing smaller grains. Overall grain size increases. (Time-dependent)