







Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
These are the Lecture Slides of Material Science for Engineers which includes Structure of Wood, Moisture Content, Density of Wood, Mechanical Properties of Wood, Expansion and Contraction of Wood, Concrete Materials, Properties of Concrete etc. Key important points are: Phase Transformations in Metals, Kinetics of Phase Transformations, Multiphase Transformations, Isothermal Transformation Diagrams, Mechanical Behavior, Tempered Martensite, Superheating
Typology: Slides
1 / 13
This page cannot be seen from the preview
Don't miss anything!
1
Heat Treatment (time and temperature) ⇒ ⇒ Microstructure ⇒ Mechanical Properties
¾ Kinetics of phase transformations
¾ Multiphase Transformations
¾ Phase transformations in Fe-C alloys
¾ Isothermal Transformation Diagrams
¾ Mechanical Behavior
¾ Tempered Martensite
Not tested: 10.6 Continuous Cooling Transformation Diagrams
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
2
Phase transformations (change of the microstructure) can be divided into three categories:
¾ Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.)
¾ Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations)
¾ Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure (e.g. martensitic transformation discussed in later in this chapter)
Phase transformations do not occur instantaneously. Diffusion-dependent phase transformations can be rather slow and the final structure often depend on the rate of cooling/heating.
University of Tennessee, Dept. of Materials Science and Engineering (^) 3
Most phase transformations involve change in composition ⇒ redistribution of atoms via diffusion is required.
The process of phase transformation involves:
¾ Nucleation of of the new phase - formation of stable small particles (nuclei) of the new phase. Nuclei are often formed at grain boundaries and other defects.
S-shape curve: percent of material transformed vs. the logarithm of time.
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
4
University of Tennessee, Dept. of Materials Science and Engineering (^) 7
The S-shaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation (nucleation rate increases with supercooling) and not by diffusion (which occurs faster at higher T).
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 8
9
The thickness of the ferrite and cementite layers in pearlite is ~ 8:1. The absolute layer thickness depends on the temperature of the transformation. The higher the temperature, the thicker the layers.
Fine pearlite
Austenite → pearlite transformation
α ferrite Coarse pearlite
Fe 3 C
Austenite (stable)
Denotes that a transformation is occurring
Eutectoid temperature
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
10
¾ The family of S-shaped curves at different T are used to construct the TTT diagrams.
¾ The TTT diagrams are for the isothermal (constant T) transformations (material is cooled quickly to a given temperature before the transformation occurs, and then keep it at that temperature).
¾ At low temperatures, the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.
¾ Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite ( fine pearlite ).
¾ At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite ( coarse pearlite ).
¾ At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite. Additional curves for proeutectoid transformation must be included on TTT diagrams.
University of Tennessee, Dept. of Materials Science and Engineering (^) 13
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
14
a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit cell axis is longer than the other two).
15
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 16
Austenite-to-martensite is diffusionless and very fast. The amount of martensite formed depends on temperature only.
A: Austenite P: Pearlite B: Bainite M: Martensite
19
The strength and hardness of the different microstructures is inversely related to the size of the microstructures (fine
Mechanical properties of bainite, pearlite, spheroidite
Considering microstructure we can predict that
¾ Spheroidite is the softest
¾ Fine pearlite is harder and stronger than coarse pearlite
¾ Bainite is harder and stronger than pearlite
Mechanical properties of martensite
Of the various microstructures in steel alloys
¾ Martensite is the hardest, strongest and the most brittle
The strength of martensite is not related to microstructure. Rather, it is related to the interstitial C atoms hindering
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 20
21
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 22
Higher temperature & time: spheroidite (soft)
25
Chapter 11: Thermal Processing of Metal Alloys
¾ Process Annealing, Stress Relief
¾ Heat Treatment of Steels
¾ Precipitation Hardening