Material science chapter 6

Mechanical Properties of Metals - I

• Metal ingots are formed into functional shapes via metal forming operations, including casting, forging, rolling, stamping, drawing, and extrusion.
• Functional shapes like sheets, plates, tubes, cylinders, rods, disks, and wires are used in various industries, including the automotive industry.
• Metals are first melted in a furnace during the casting process.
• Alloying elements are added and mixed, and impurities and gases are removed before pouring into a mold to solidify.
• Simple shapes like ingots and complex ones can be cast.
• Metal sheets and plates are produced using the rolling process, which utilizes strong rollers to reduce thickness.
• Rolling is performed in multiple passes, reducing thickness and increasing length.
• Cold rolling is done at room temperature, and hot rolling is done below the melting temperature.
• During hot rolling, the work material is heated to high temperatures called recrystallization temperature.
• This allows for easier deformation and greater reduction of thickness in a single pass.
• For steels, ingots are preheated to ~1200°C before rolling.
• Reheating may be needed between passes if the ingot cools and becomes hard to roll.
• Usually, a series of 4 high rolling mills are used for continuous thickness reduction.
• In cold rolling, the temperature of the work is significantly below recrystallization temperature.
• Cold rolling makes the material stronger and more brittle due to strain hardening.
• Reheating enhances ductility.

Processing of Metals - Extrusion

• In extrusion, a metal workpiece (billet) is placed under high pressure and forced through an opening in a die.
• Normally performed at high temperatures.
• There are direct and indirect extrusion processes.
• Indirect extrusion has lower friction on the billet, requiring less power, but has limitations on the applied load.
• Extrusion produces cylindrical rods, tubes, and sometimes irregular shapes in some metals.
• Lubricants like molten glass may be required for the extrusion of strong metals.

Processing of Metals - Forging

• Forging involves hammering or pressing metal into a desired shape, mostly in a hot condition.
• There are two types based on die geometry:
  • Open die: Dies are flat or simple. Steel shafts are an example.
  • Closed die: Dies have upper and lower impressions (more complex). Engine connecting rods are an example.
• Cold forging enhances structural properties, removes porosity, and increases homogeneity.

Processing of Metals - Drawing

• In wire drawing, a rod or wire is drawn through several drawing dies to reduce its diameter.
• Flat sheets of metal can be deformed into cup-like components.

Stress and Strain in Metals

• Metals deform under applied force.
• Elastic deformation: Atoms elongate, causing overall elongation in the specimen, but return to original dimensions after the force is removed.
• Plastic deformation: Atoms break bonds, slip past each other, and do not return to original positions, resulting in permanent deformation.

Engineering Stress and Strain - Normal Stress and Strain

• Engineering normal stress, σ, is the average uniaxial tensile force divided by the original cross-sectional area.
• Engineering normal strain, ε, is the change in length divided by the original length.

Engineering Stress and Strain - Shear Stress and Strain

• Engineering shear stress, τ, is the shear force divided by the area of application.
• Engineering shear strain, γ, is the amount of shear displacement divided by the distance over which the shear acts (for small angles, γ ≈ tan θ).

The Tensile Test

• Important material properties are determined by performing a tensile test and plotting the engineering stress-strain diagram.
• Force data is obtained from a load cell.
• Elongation data is obtained from an extensometer.

The Tensile Test, Continued

• Standard specimens are cylindrical or flat (dog-bone).
• A tensile tester applies displacement to the specimen, causing stress and strain development.
• The engineering stress (σ) is calculated by dividing the load (F) by the original cross-sectional area (A).
• The engineering strain (ε) is calculated by dividing the change in length (Δl) by the initial gage length (l).
• Plots of σ vs. ε yield the engineering stress-strain curve.

The Engineering Stress-Strain Diagram

• The σ-ε diagram exhibits key features:
  • Linear-elastic range: σ and ε are linearly related, and σ=Εε. Elastic deformation is recoverable. E is the modulus of elasticity.
  • Yield point: The point where the elastic range ends.
  • Plastic range: Deformation beyond the yield point is permanent.
  • Ultimate tensile strength (UTS): Occurs at the peak stress.
  • Fracture point: The point on the curve where the specimen fails.

Mechanical Properties Obtained from the Tensile Test - Modulus of Elasticity, E

• Modulus of Elasticity (E): Stress and strain are linearly related in the elastic region.
• Higher bonding strength correlates to a higher modulus of elasticity.
• Examples include: Steel (207 GPa), Aluminum (76 GPa).

Mechanical Properties Obtained from the Tensile Test - Poisson's Ratio, v

• Poisson's ratio (v) is the ratio of lateral strain to longitudinal strain experienced by an elastic material subjected to an axial load.
• v is always positive for isotropic materials. It typically ranges from 0.25 to 0.4.

Mechanical Properties Obtained from the Tensile Test - Yield Strength, oy

• Yield strength (σy) is the stress at which a material begins permanent or plastic deformation.
• The 0.2% offset yield strength standard is used to identify the yield point.
• A construction line is used to determine the 0.2% offset yield strength on the stress-strain curve. It starts at 0.2% strain (0.002) and is parallel to the elastic region.

Mechanical Properties Obtained from the Tensile Test - Tensile Strength, ou

• Tensile strength (UTS), σu, is the maximum stress reached in the engineering stress-strain curve.
• Indicates the highest stress the material can withstand before fracture.
• Necking becomes more prominent near the UTS.

Mechanical Properties Obtained from the Tensile Test - % Elongation at Fracture, ɛf

• % elongation at fracture (εf) measures material ductility.
• It is the elongation of a material before fracture, expressed as a percentage of its original length.
• % reduction in area provides the same information as % elongation at fracture, about ductility.

Mechanical Properties Obtained from the Tensile Test - Modulus of Resilience, Ur

• Modulus of resilience (Ur) is a measure of energy needed to cause yielding in a material.
• Equal to the area under the linear elastic region of the stress-strain curve.
• Toughness is calculated by finding the area under the full curve.

Comparison of Properties Among Different Metals

• Data compares engineering stress-strain curves for various metals (steel alloys, titanium, aluminum, etc.).

True Stress – True Strain Curve

• True stress and true strain are based on instantaneous cross-sectional area and length.
• They are different from engineering stress and strain, as they consider material's changing area.
• This difference is not significant in the elastic range.

Hardness and Hardness Testing of Materials

• Hardness measures material's resistance to localized plastic deformation or indentation
• Harder materials wear less. Diamonds are the hardest.
• Hardness testing is important where components articulate against each other.

Hardness Tests and Scales

• Defines various hardness tests and their formulas (Brinell, Vickers, Knoop, Rockwell). The table shows different indenters, loads, etc.

Plastic Deformation in Single Crystals

• Plastic deformation in single crystals results in slip bands.
• Atoms on specific crystallographic planes (slip planes) and in specific directions slip to form these bands.
• Slip bands are uniform in ductile metals and occur on multiple slip planes, being approximately 200A thick and offsetting about 2000A.

Slip Mechanism

• Slip is caused by applying shear stress to the grain.
• Slip occurs due to the movement of dislocations, which resemble ripples in a rug.
• Once a dislocation reaches the end of the grain, a unit step of slip occurs.

Slip Preference

• Slip occurs more readily on close-packed planes and along close-packed directions.
• A smaller shear stress is required for slip in densely packed planes.
• Less energy is needed to move atoms along denser planes.

Slip Systems

• Metals have a preferred slip system, a combination of a densely packed plane and a densely packed direction.
• Each crystal structure has a number of characteristic slip systems.
• Examples include FCC (e.g., copper, aluminum), BCC (e.g., iron, tungsten), and HCP (e.g., magnesium, zinc).

Schmid's Law

• Describes the relationship between uniaxial stress applied to a single crystal and the resulting resolved shear stress on a slip system.

Critical Resolved Shear Stress, Tc

• Critical resolved shear stress (τc): Shear stress needed to initiate slip in a single crystal.
• Depends on crystal structure, atomic bonding character, temperature, and orientation of slip plane.

Twinning

• A twin is a region where a portion of the atomic lattice is deformed to create a mirror image of the original lattice.
• Crucial in the deformation of HCP crystals because it involves a lower number of slip systems.
• Distance moved by atoms is related to their twinning plane distance.
• Deformation from twinning is small.
• Twins in pure titanium are presented in a figure.

Effects of Grain Boundaries on the Strength of a Metal

• Grain boundaries hinder dislocation movement, strengthening the metal.
• Fine grain size is desired for high strength at room temperature.
• In polycrystalline metals, slip bands are parallel inside grains but the direction changes from grain to grain.

Hall-Petch Equation

• Empirically relates strength to grain size within a material.
• Strength of the material increases with decreasing average grain size.

Effects of Plastic Deformation

• Plastic deformation causes shearing of grains relative to each other and elongate the grains in the rolling direction, resulting in stronger materials.

Effect of Cold Work on Tensile Strength

• Increased cold work increases dislocation numbers but hinders dislocation movement due to the presence of grain boundaries and other dislocations..

Solid Solution Strengthening

• Strengthening of a metal through the introduction of solute atoms.
• Solute atoms disrupt lattice structure, preventing slip and increasing strength.
• Examples include solid solutions like cartridge brass (mostly copper and zinc).

Annealing: Recovery and Recrystallization and Grain Growth

• Heat treatment to remove residual stresses, causing new equiaxed grains and softening.
• Three stages of annealing: recovery, recrystallization, and grain growth.
• Recrystallization occurs above the recrystallization temperature (Tr) for about one hour.

Facts About Recrystallization

• A minimum amount of deformation is necessary to initiate recrystallization.
• Smaller deformation requires a higher recrystallization temperature.
• Higher temperatures result in lower recrystallization time.
• Larger original grain size requires greater deformation to produce equivalent temperature.
• Recrystallization temperature increases with metal purity.

Superplasticity in Metals

• At elevated temperature and slow loading, some alloys exhibit enormous deformation (2000% or more).
• Conditions favorable for superplasticity include very fine grain size, high strain sensitivity, and a temperature above 0.5 melting temperature.
• Slow strain rate is also important.

Nanocrystalline Metals

• Nanocrystalline metals have an average grain diameter below 100 nm.
• High strength and hardness, and superplasticity are characteristics.
• Reducing grain size from microns to nanometers significantly increases the yield strength of materials like copper.
• Production of nanocrystalline metals is challenging.
• Hall-Petch equation does not apply to lower nanocrystalline ranges. (Negative Hall-Petch effect is possible).

Explore the fundamental mechanical properties of metals and the various metal forming operations involved in creating functional shapes. This quiz delves into processes such as casting, forging, rolling, and the role of alloying elements. Understand how these techniques are applied across industries, especially in automotive manufacturing.