Materials Science: Properties, Degradation, and Shaping Processes
Materials Science
Atomic Structure and Bonding
Atomic Number: Represents the number of protons in an element’s nucleus.
Alloy: A combination of two or more materials, often metals, designed to improve specific characteristics.
Polymer: Materials formed by combining organic molecules into long chains.
Interatomic Spacing: In solids, this refers to the apparent diameter of an atom.
Binding Energy: The energy needed to separate two atoms, reflecting the strength of their bond.
Avogadro’s Number: The number of molecules in one mole, approximately 6.023 * 1023.
Pauli Exclusion Principle: States that no more than two electrons in a material can possess the same energy level.
Valence Electrons: Electrons in the outermost shell of an atom that participate in chemical reactions.
Unit Cell: The smallest repeating unit of a crystal lattice that retains the overall characteristics of the network.
Mechanical Properties and Testing
Engineering Strain: The amount of deformation a material undergoes per unit length when subjected to a tensile test.
Engineering Stress: The applied load or force divided by the original cross-sectional area of the material.
Effect of Hardening on Conductivity: Hardening generally reduces electrical conductivity.
Annealing Process: A heat treatment used to remove some or all of the effects of cold working.
True Stress: The load divided by the actual cross-sectional area of the specimen at that load.
True Strain: The change in length divided by the instantaneous length, calculated as E = ln(I/Io).
Mechanical Properties: Include tension, compression, hardness, impact, shear, fatigue, and creep.
Brinell Hardness Test: Measures a material’s resistance to indentation.
Plastic Deformation: Permanent deformation that remains after the applied load is removed.
Recrystallization Temperature: The temperature at which new, strain-free grains form during annealing, eliminating the effects of work hardening.
Cold Work: Deformation of a material below its recrystallization temperature, leading to increased dislocation density and hardening.
Elastic Deformation: Temporary deformation that is fully recovered when the applied force is removed.
Metal Shaping and Alloying
Manufacturing Methods
Six Core Manufacturing Methods for Shaping Metals:
- Forging
- Drawing
- Extruding
- Deep Drawing
- Stretching
- Bending
Response of a Metallic Material to Cold Work: Characterized by the coefficient of deformation.
Unlimited Solubility: Occurs when one material can dissolve an unlimited amount of another without forming a second phase. Example: Copper-Nickel alloy.
Limited Solubility: Occurs when only a maximum amount of solute can dissolve in a solvent. Example: Lead-Tin alloy.
Aluminum Can Shaping Technique: Deep drawing (drawing).
Extrusion Technique: Material is pushed through a die to create products with a specific cross-section, such as rods and tubes.
Drawing Technique: Metal is pulled through a die to produce wire or other elongated shapes.
Annealing Stages
- Recovery: Occurs at low temperatures, relieving internal stresses.
- Recrystallization: Occurs at higher temperatures, forming new, strain-free grains.
- Grain Growth: Occurs at even higher temperatures, leading to larger grain sizes.
Solidification
Nucleation: The initial stage of solidification where embryos form in the liquid phase below the melting point and grow into nuclei.
- Homogeneous Nucleation: Occurs spontaneously within the liquid upon cooling.
- Heterogeneous Nucleation: Occurs on existing surfaces, such as impurities or mold walls, promoting faster nucleation.
Growth: The stage after nucleation where the solid phase expands by adding atoms from the liquid.
- Planar Growth: Occurs when the solid-liquid interface is relatively flat, typically with slow cooling rates.
- Dendritic Growth: Occurs with undercooled liquids, resulting in a branched, tree-like structure.
Solidification Time: The time required for complete solidification, influenced by the cooling rate.
Solidification Curves: Graphical representations of the cooling and solidification behavior of pure materials and alloys.
Disadvantages of Solidification
- Volume Reduction (Shrinkage): Can lead to voids or porosity in the solidified material.
- Deformation: Stresses during solidification can cause warping or distortion.
Corrosion and Degradation
Chemical Corrosion
Dissolution: The process where a solid material dissolves into a corrosive liquid. Smaller ions and similar solvent structures accelerate dissolution. Higher temperatures also increase corrosion rates.
Liquid Metal Deterioration: Attack of grain boundaries by liquid metals.
Selective Solution: Corrosion that preferentially attacks one component of an alloy.
Electrochemical Corrosion
Electrochemical Cell: A system where two materials (anode and cathode) are connected in an electrolyte solution, allowing electron flow and corrosion.
Electrochemical Cell Components
- Anode: The material that loses electrons (oxidizes) and corrodes.
- Cathode: The material that gains electrons (reduces).
- Physical Contact: Connection between anode and cathode for electron flow.
- Electrolyte: Solution that conducts ions, facilitating the corrosion process.
Types of Electrochemical Corrosion
- Uniform Corrosion: Occurs evenly across the material’s surface.
- Galvanic Corrosion: Occurs when dissimilar metals are in contact, with one acting as the anode and the other as the cathode.
- Composition Cells: Corrosion due to variations in alloy composition.
- Stress Cells: Corrosion concentrated at areas of high stress.
- Concentration Cells: Corrosion caused by differences in electrolyte concentration.
Anodic Reaction (Oxidation): Occurs at the anode, releasing electrons.
Polarization: Change in potential at the anode or cathode due to reaction kinetics or concentration gradients.
- Activation Polarization: Related to the energy barrier for the electrochemical reaction.
- Concentration Polarization: Caused by a buildup of reaction products near the electrode surface.
Resistance Polarization: Caused by the resistance of the electrolyte to ion flow.
Corrosion Control Methods
- Coatings: Isolate the metal from the corrosive environment.
- Inhibitors: Chemicals added to the electrolyte to slow down the corrosion reaction.
- Cathodic Protection: Applying a negative potential to the metal, forcing it to become the cathode and preventing corrosion.
- Anodic Passivation: Forming a protective oxide layer on the metal surface to reduce corrosion.
Oxidation
Oxidation: Corrosion that occurs when a material reacts with oxygen, often forming an oxide layer.
- Linear Oxidation: Occurs when the oxide layer is porous and does not fully protect the metal (e.g., Magnesium).
- Parabolic Oxidation: Occurs in metals like iron, nickel, and copper, where the oxide layer grows more slowly over time.
- Logarithmic Oxidation: Occurs when the oxide layer is dense and protective, limiting further oxidation.
Oxidation of Ceramics: Ceramics are generally resistant to oxidation, except at high temperatures.
Microbial Corrosion: Corrosion caused by microorganisms that produce corrosive substances, such as acids.
Selective Oxidation: In an alloy, the element with the higher oxidation potential will preferentially oxidize.
Wear
Abrasive Wear: Material removal due to contact and friction between two surfaces.
Liquid Erosion: Wear caused by the impact of high-pressure liquids.
Cavitation: Wear caused by the collapse of vapor bubbles in a liquid near a material’s surface.
Grain Boundaries and Defects
Edge Angle: The angle between two grains at a grain boundary. Small displacements can cause disorientation and weaken the material.
Twin Boundary: A special type of grain boundary where the lattice on either side is a mirror image. It can act as a barrier to dislocation movement.
Edge Dislocation: A line defect in a crystal lattice where an extra half-plane of atoms is inserted.
Twist Boundary: A grain boundary formed by an array of screw dislocations.
Screw Dislocation: A line defect where the lattice is sheared along a spiral path.