Transcript phases
Phase Diagrams melting / production process / alloying (strength, Tm...) heat treatment material properties microstructure system (e.g. Cu-Ni) components (pure substances or compounds) phases (uniform physical/chemical characteristic) solid solution – solvent + solute solubility limit=> precipitation pure materials – 2 phases: liquid + solid Phase Equilibria G=min Gibbs free energy: G=H-TS =f(T,c,p) =min internal energy entropy (bonding enthalpy) (disorder of molecules entropy of mixing) metastable stable Phase Equilibrium melting point Tm G solid liquid T phase composition <=> G minimization Cu-x%Ni Phase Diagram (p=const) isomorphous phase composition =f (T,c) liquid phase Gibbs phase rule F=K-P+2 (F degrees of freedom depends on number of components K and number of phases P) liquid+ solid phase solid phase Cu Ni Equilibrium Cooling first solid nuclei two-phase solidification polycrystalline solid Cu %Ni Amounts W Phase Composition WSolid=Wa: length of tie line from CL to Co divided by length of tie line tie line 100% liquid %Ni Non-Equilibrium Cooling first solid nuclei concentration changes in solid AND liquid phase required!! by diffusion Ni concentration in solid nuclei remains high: low Dsolid segregation Diffusion flux 1 dN J A dt c cs t3 t1 t2 x A steady state diffusion dc J D dx c x Non-Steady-State Diffusion Ficks‘s 2nd Law c 2c D 2 t x solution (c(0)=0): c c x x 1 erf cs 2 Dt cs diffusion coefficient: QD D D0 exp RT t1 t2 erf ( z ) t3 x 2 z exp zdz 0 Binary Eutectic Phase Diagrams (Pb-Sn) liquid phase a and b solid solution solid phase limited solubility Pb %Sn eutectic reaction a(18.3%Sn) +b (97.8%Sn ) Lamellar Eutectic Microstructure Tm, similar to pure components, but: two phases!! Sub-Eutectic Microstructure lead-rich a phase eutectic: white b dark a Microstructural Examination Metallography preparation of cross section -cutting -grinding (SiC paper) -polishing (e.g. 1µm diamond suspension) -etching (selective chemical attack of grains/grain boundaries) optical microscopy electron microscopy (transmission (TEM) or scanning (SEM)) Metallography (SEM) detector secondary electrons electron beam specimen Binary Eutectic Phase Diagrams a phase solvus line: precipitation of b phase Pb %Sn Intermetallic Phases/Compounds (e.g. ceramics) CaZr4O9 The Iron-Carbon Phase Diagram peritectic subsystem bcc d ferrite fcc g austenite eutectic system Fe3C precipitation eutectoid subsystem ferrite precipitation bcc a ferrite Steel Cast Ironcementite Fe3C Hypoeutectoid Steels (CC<0.76%) ( hypereutectoid) Ck45 Cc=0.45% dark pearlite: lamellae of Fe3C and ferrite light a ferrite Hypoeutectoid Steels (CC<0.76%) ( hypereutectoid) Ck15 Cc=0.15% dark pearlite: light a ferrite