CHE217 LECTURE 1-5

QUIZ 1

LEVEL OF STRUCTURAL ANALYSIS

1. Atomic Structure (Sub-nanometer scale)

2. Microstructure (Nanometer to Millimeter scale)

3. Macrostructure (Millimeter to Meter scale)

ATOMIC STRUCTURE

Deals with the atom, atom bonding, and atom arrangement. This level dictates the fundamental, intrinsic properties of a material

BONDING

The type of bond (metallic, covalent, or ionic) determines the melting temperature, thermal conductivity, and electrical conductivity.

ARRANGEMENT

The type of bond (metallic, covalent, or ionic) determines the melting temperature, thermal conductivity, and electrical conductivity.

MICROSTRUCTURE

Refers to the structure seen with the aid of a microscope. It includes grains, grain boundaries, phases, and phase boundaries. This is the level most actively manipulated by engineers to change a material's behavior.

MECHANICAL PROPERTIES

Microstructure heavily controls yield strength, hardness, and toughness.

GRAIN SIZE EFFECT

According to the Hall-Petch relationship, materials with smaller, more numerous grains are generally stronger because the high number of grain boundaries restricts the movement of defects (dislocations). Heat treatments and varied cooling times during solidification are specifically designed to alter this level.

MACROSTRUCTURE

The structure visible to the naked eye. Involves volume defects (like pores and cracks) and macrograins. This level is critical for overall structural integrity and failure analysis.

SOLIDIFICATION DEFECTS

During the casting and solidification of metals, issues like shrinkage (as the metal cools and contracts) or gas porosity (trapped bubbles) manifest at this scale. These volume defects act as stress concentrators and are frequent starting points for fractures or part failures.

SOLIDIFICATION

process where a liquid metal transforms into a solid upon cooling.

FREEZING POINT

solidification starts when a liquid is cooled below its ____

CRYSTAL STRUCTURE AND ALLOY COMPOSITION

what are decided in the solidification process?

MAIN BRANCH W/ MANY APPENDAGES

solidification of crystals follow a pattern consisting of a ___

SINGLE CRYSTALS

Individual crystals are called grains

POLYCRYSTALLINE

Most materials are ___ meaning they are composed of many crystals.

OVERARCHING PROCESS OF SOLIDIFICATION

Nucleation - Growth - Grains

NUCLEATION

• Formation of “seed” from which a new phase will grow.

• Involves the appearance of very small particles, or nuclei of the new phase (consisting of a few hundred atoms), which are capable of growing.

CRITICAL MASS

Minimum amount of material required to form a stable seed

TYPES OF NUCLEATION

1. Homogeneous Nucleation

2. Heterogeneous Nucleation

HOMOGENEOUS NUCLEATION

• Nucleation in the parent phase (unaided).

• Prominent in pure metals.

• Nuclei of the solid phase form in the interior of the liquid as atoms cluster together

HETERGENEOUS NUCLEATION

• It is easier for nucleation to occur at surfaces and interfaces than at other sites

• Nucleation occurs with the help of impurities or chemical inhomogeneities

• The presence of artificial seeds or inoculants speeds up nucleation by assuming a portion of the critical mass.

• Artificial seeds include: surface or boundaries, dislocations, grain boundaries, and inclusions

GROWTH

• Process of seed enlargement.

• Growth is diffusion based.

• As temperature increases, diffusion increases, leading to increased growth.

DIFFUSION

• Atom movement from a region of high concentration to a region of low concentration.

• Diffusion is vital for processes like nucleation, growth, recrystallization, and phase transformations.

APPLICATIONS OF DIFFUSION

• Carburizing

• Nitriding

CARBURIZING

• A technique of case hardening steel by increasing carbon content at the surface through heat treatment.

NITRIDING

A heat-treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface.

TYPES OF DIFFUSION

1. Self-DIffusion

2. Interdiffusion or Impurity Diffusion

SELF-DIFFUSION

• Atoms of same material moving within lattice

INTERDIFFUSION OR IMPURITY DIFFUSION

• Atoms of different material moving within lattice

PREREQUISITE OF DIFFUSION

1. There must be an empty adjacent site

2. The atoms must have sufficient energy to break bonds with its neighbor and then cause lattice distortion during displacement

VACANCY DIFFUSION

Occurs at high temperature since more vacancies are formed.

INTERSTITIAL DIFFUSION

Involves interdiffusion of impurities such as H, C, N, or O.

FACTORS AFFECTING DIFFUSION

1. TEMPERATURE

2. SIZE OF DIFFUSING SPECIES

3. ATOMIC PACKING FACTOR

4. BONDS OF STRUCTURE

5. PRESENCE OF OTHER DIFFUSION PATHS

TEMPERATURE

Increasing temperature increases diffusion rates

DIFFUSION RATE FORMULA

SIZE OF DIFFUSING SPECIES

The smaller the diffusing atom, the faster diffusion is.

ATOMIC PACKING FACTOR

The lower the APF, the faster the diffusion is.

BONDS OF STRUCTURE

The weaker the bond, the faster diffusion is.

PRESENCE OF OTHER DIFFUSION PATHS

Dislocations and grain boundaries hastens diffusion