CME-260: Chapter 10

Kinetics

Study of time-dependent phase transformations and microstructure evolution.

Heat Treatment

Controlled temperature vs. time process used to modify microstructure and properties.

Diffusional Transformation

Phase transformation involving long-range atomic diffusion.

Diffusionless Transformation

Phase transformation without atomic diffusion; occurs by crystal structure change.

Martensitic Transformation

Diffusionless transformation of FCC austenite → BCT martensite

Martensite

BCT solid solution of Fe and C formed by rapid cooling; hard and brittle.

Metastable

Phase not at equilibrium but persists for long time

Austenite

FCC iron phase stable at high temperatures (>727 °C)

Pearlite

Alternating layers of ferrite and cementite formed during slow cooling.

Bainite

Fine needle-like structure of ferrite and cementite formed at intermediate temperatures.

Tempered Martensite

Mixture of ferrite and cementite formed by reheating martensite.

TTT Diagram

Time-Temperature-Transformation diagram showing % transformation vs. time at constant temperature.

Isothermal Transformation Diagram

Another name for TTT diagram

CCT diagram

Diagram showing phase transformations during continuous cooling.

Nucleation

Initial formation of a stable solid phase cluster from atoms.

Homogeneous Nucleation

Nucleation occurring uniformly within a material.

Heterogeneous Nucleation

Nucleation occurring at defects or surfaces.

Hardness

Resistance to indentation or deformation.

Hardenability

Ability of a material to be hardened by quenching.

Jominy End-Quench Test

Test used to measure hardenability by varying cooling rate along a sample.

Tempering

Reheating martensite to improve ductility and reduce brittleness.

Martempering

Heat treatment with slow cooling through martensite range to reduce stress/cracking.

Austempering

Heat treatment producing bainite by holding above martensitic range.

Precipitation Hardening

Strengthening by forming fine precipitates that block dislocations.

Age Hardening

Time-dependent formation of precipitates that increase hardness.

Overaging

Excess aging causing precipitates to coarsen and reduced hardness.

Guinier-Preston Zone

Small coherent precipitates that strengthen material by blocking dislocations.

Coherent Interface

Interface where crystal structures of precipitate and matrix align.

Solution Treatment

Heating then quenching to form a uniform solid solution before aging.

Annealing

Heat treatment that reduces hardness and restores ductility.

Cold Work

Plastic deformation at low temperature increasing strength and dislocation density.

Recovery

Stage of annealing where dislocations rearrange with little structural change.

Recrystallization

Formation of new strain-free grains replacing deformed structure.

Recrystallization Temperature

Temperature where new grains begin forming and hardness drops.

Grain Growth

Increase in grain size after recrystallization due to boundary reduction.

Sintering

Densification of powder by heating below melting point via diffusion.

Field-Assisted Sintering Technique (FAST)

Sintering using electric current for rapid heating.

Spark Plasma Sintering (SPS)

FAST method using electrical discharge between particles.