Chap.1. Phase and Phase Diagram

• Phase( 상 ): Any material system usually contains regions which exhibit same properties such as specific volume, composition and structure, and distinguished from other regions by an identifiable interface, called “phase”

* Even one component system can have many phases, and multi component system can have a fewer phases.

• Materials Properties: depend much on the nature, number, amount and forms of various possible phases present and can be changed by alterations in these quantities.

⇒ vital to know the conditions under which any given system will exist in its various possible forms.

• (Equilibrium) Phase Diagrams( 평형 상태도 ) : plot of most stable phases that occur under equilibrium state of a system as a function of temperature, pressure and composition for various material systems.

* Equilibrium state( 평형 상태 ): properties of the system will not change with time unless alteration in state variables such as temperature, pressure and/or composition occurs.

* mostly empirical but based firmly on thermodynamic principles.

Fe-C(or Fe-Fe3C) Equilibrium phase Diagram

• Guide for heat treatment of steels. - Modification due to alloying and deviation from equilibrium

state. - Graphite is more stable form than Fe3C but graphitization

rarely occur in steels → Fe-Fe3C diagram is more pertinent. - In cast irons(>2%C), high carbon and high Si addition promote

graphite formation →Fe-C diagram is more valid - Valid at 1 atm. - wt%⁄at% conversion: at.%C =C atoms/(C atoms+Fe atoms)x100 =(wt%C/at.wtC)/(wt%C/at.wtC+wt%Fe/at.wtFe)x100 • Phases present - Ferrite ( α- iron): BCC ; a =2.86A0 ,    - Austenite (γ-iron) : FCC: a = 3.56A0 - Delta Ferrite: BCC formed near to  melting  temperature due to

inter-dendritic segregation of  alloying or impurity - Cementite(Fe3C):orthorhombic;a=4.52A0, b=5.09A0,c=6.74A0, Contains 12 Fe-atoms and 4 C-atoms

Effects of Carbon• Interstitial solid solution element in steels - stabilizes austenite. - max. solubility of carbon in austenite is 2.11% at 1148 0C and in ferrite

is only 0.02% at 7270C - When solubility limit for carbon in austenite and ferrite is exceeded,

iron carbide or cementite(Fe3C) forms. • Interstitial sites - Two types in bcc and fcc : Octahedral and Tetrahedral  sites : Fig.1.5 &

1.6 - In austenite, an octahedral site can accommodate an atom of 0.052nm

in radius, tetrahedral site  0.028nm. - In Ferrite a tetrahedral site can accommodate an interstitial atom of

0.035nm in radius and the octahedral site 0.019nm in radius. - carbon atoms have radii of 0.07nm and are therefore more readily

accommodated in the octahedral site of austenite and are less soluble in ferrite.

- The octahedral site in ferrite are not symmetrical and carbon atoms prefer the octahedral sites by displacing two atoms at a distance of a/2.

- Rapid cooling from austenite to ferrite produce a severe distortion of the lattice by carbon atoms, altering the cube structure to tetragonsl structure (Martensitic transformation)

Effects of alloying Elements

• Alloying elements and impurities: Mn, Cr, Mo, W, Cr, P, C, N  etc.

- Each alloying element, itself or collaborating with other element exert different effects.

- Austenite stabilizer: widens the temperature range for stable austenite by depressing the α-γ transformation and raising the γ-δ transformation.

* Type 1: Forms no iron-rich compounds: Mn, Ni, Co, etc. * Type 2: Forms iron-rich compounds: Cu, Zn, Au, N and

C, etc. - Ferrite stabilizer: Narrows temperature range  for

stable austenite * Type 1:Austenite zone is completely  surrounded by a

two phase (α+γ or γ+δ) field : Si, Cr, W, Mo, P, V, T, Be, Sn, Sb, As, Al

* Type 2:Intermetallic compounds or other   constituents appear: Ta, Zr, B, S, Ce, Cb

Critical (Transformation) Temperatures

• Critical (Transformation) Temperatures - Al(or Ael): Boundary between α+ Fe3C 와 α+γ or γ+ Fe3C, - A3 : α+γ 와 γ - Acm (Aecm): γ 와 γ+Fe3C - The transformations are diffusion controlled → sensitive to

composition and heating/cooling rate. - Ac : on heating,  Ar : on cooling• Experimentally determined by measuring  changes in heat

transfer(DTA) or dimension(Dilatometer) - On heating, heat absorption and shrinkage occur as ferrite

transforms into close packed austenite. - On cooling, the reverse occur. - Can be calculated using thermodynamic data • Alloying effects on transformation can  be formulated. - e.g. Ac1= 723-10.7Mn-16.9Ni+29.1Si+16.9Cr+290As+6.38W * austenite stabilizer : negative sign, ferrite stabilizer : positive

sign