Quiz 3

Lecture 5-1: Phase Diagrams

Definitions:

Eutectic Point - the lowest temperature an alloy melts at, occurs at a specific concentration of each element in the alloy

Concepts: 

Microstructures of varying tin compositions:

• C < 2 wt% Sn - at room temperature, polycrystalline with grains of alpha phase having a composition of C

• 2 wt% Sn < C < 18.3 wt% Sn - at temperatures in alpha + beta range, polycrystalline with alpha grains and small beta phase particles

• 18.3 wt% Sn < C < 61.9 wt% Sn - alpha phase particles and a eutectic microconstituent

• C = Ceutectic - eutectic microstructure (lamellar), alternating layers (lamellae) of alpha and beta phases

Thermal Process During Solidification:

• Cooling curve near equilibrium for a pure metal or eutectic alloy - supercooling may occur is nucleation is difficult, Tm is the solidification or melting temperature

• Cooling curve of a binary alloy where solidification takes place over a range of temperatures - Tb is the temperature where solidification begins and Tc is the temperature where solidification is complete

Equations:

Phase compositions: 

• Use tie lines if in a two phase region

Relative amount of each phase:

Conversion between volume fraction and mass or weight fraction:

• If the densities are the same:

volume fraction = weight fraction

• If the densities are different:

Lecture 5-2: Weight Fraction Pb-Sn

Equations:

Average density of eutectic mixtures:

Weight fraction:

Volume fraction:

Lab Report 5: Phase Diagrams

Definitions:

Hyper-eutectic - to the left of the eutectic point, eutectic structure with beta regions

Hypo-eutectic - to the right of the eutectic point, eutectic structure with alpha regions

Superheating - heating above a material’s boiling point

Supercooling - heating below a material’s boiling point

Non-equilibrium solidification - when the rate of cooling is limited

Concepts:

Advantages of eutectic alloys: significantly low melting point

Properties of eutectic alloys: significantly low melting point

Effect of diffusion on microstructure development: causes lamellar microstructures

Appearance of alpha phase microconstituent:

Appearance of beta phase microconstituent:

Appearance of eutectic microconstituent:

Lecture 6: Phase Transformations

Definitions:

Hardenability - the capacity of a steel to be hardened in depth when quenched from austenitizing temperature, higher hardenability means it needs a slower cooling to form martensite

Bainite - elongated Fe3C particles in alpha ferrite

Upper - forms at higher temperatures, lathes of ferrite separated by cementite particles

Lower - forms at lower temperatures, fine lathes of ferrite separated by carbides

Pearlite - lamillae of ferrite and cementite

Martensite - forms when austenite is rapidly cooled (quenched)  

Concepts:

Gamma to martensite (M) transformation: forms when austenite is quenched

Pearlite transformation:

• Slows as austenitic grain size increases

Grain size effect on hardenability: an increase in grain size increases hardenability

Lab Report 6: Phase Transformations

Definitions:

Proeutectoid - a phase that forms before the eutectoid austenite decomposes when a material is cooled

Fine pearlite - formed when there are more nuclei and the cooling is fast

Coarse pearlite - formed when it is slow cooled and there are fewer nuclei

Quench media - the substance in which a heat treated part is submerged to cool down

Concepts:

How to evaluate the hardenability of steel:

Steel hardenability and nuclear structure manufacturing:

Proeutectoid pearlite:

Properties of fine vs coarse pearlite:

• Fine pearlite is stronger than coarse pearlite, but is more ductile than fine pearlite, both have good toughness

Quench media:

• Oil - moderate to fast cool time, helps prevent cracking and distortion

• Water - rapid cooling, risk of cracking and distortion