吉林大学材料科学基础10.ppt

Equilibrium Cooling Fig. Schematic representation of the development of microstructure during the equilibrium solidification of a 35 wt% Ni–65 wt% Cu alloy. composition C4 10.17 The Gibbs Phase Rule P + F = C + N P is the number of phases present F is termed the number of degrees of freedom C represents the number of components in the system. N is the number of noncompositional variables (e.g., temperature and pressure). Applying it to copper–silver system: pressure is constant (1 atm), So, N= 1 temperature is the only noncompositional variable. P+F =C+1 And C=2 Fig.10.26 The iron-iron carbon phase diagram. composition (wt% C) composition (at% C) L ?+L 2.14 ? 1147?C 4.30 ?+Fe3C 727?C ? ?, Austenite 1394?C ? ?+Fe3C Cementite (Fe3C) ?, ferrite ? 0.76 0.022 ? 912?C ? ?+? 1493?C ? 1538?C L+Fe3C Phase sequence of pure Fe ?(ferrite, BCC) ?(austenite, FCC) ?(ferrite, BCC) L (melt) 912?C 1394?C 1538?C Fe3C — intermediate compound iron carbite, or cementite Being formed at concentration of 6.70 wt% C 2. Steel: ★ Contain between 0.008 and 2.14wt% C in practice, carbon concentration rarely exceed 1.0 wt% C. ★ In most steels the microstructure consists of both ? and Fe3C phases Classification scheme of ferrous alloys 1. Iron: ? ?0.008 wt% C ? Be composed of the ferrite(?) phase at room temperature 3. Cast iron: ? Contain between 2.14 and 6.7 wt% C Commercial cast irons normally contain less than 4.5 wt% C The reason is that: all steels and cast irons have carbon contents less than 6.7 wt% C. We consider only the Fe-Fe3C system 6.7 wt% C corresponds to 100 wt% Fe3C In ?, ?, and ? solid solution: Fe: Prime component C: interstitial impurity in Fe Fig.10.27 Photomicrographs of (a) a ferrite (90?) and (b) austenite (325?). (a) (b) Photomicrograph showing the microstructure of a lead-tin alloy of eutectic composition.This microstructu