Single solid phase binary alloy -1 Phase rule predicts that solid lines on one component systems become areas on a binary. Results in region where solid and liquid are in equilibrium, instead of a line.

Single solid phase binary alloy - 2 As cool from the melt and cross the liquidus line, crystals begin to form and then grow as cooling proceeds. The composition of the crystal follows the solidus line, ideally, and the composition of all crystals adjusts continuously, not just simply the crystal being formed at a particular temperature.

Single solid phase binary alloy - 3 In reality the diffusion rate of atoms inside a crystal is very slow. The composition of the crystals is unable to change at the rate indicated by the diagram. The composition of each crystal varies from the center outwards - known as coring

BINARY ALLOY WITH EUTECTIC Solubility limits  and  are the solid solutions Solubility limits are given by triple points Eutectic is given by central triple point

Line that crosses into solid solution phase FIRST EXAMPLE Line that crosses into solid solution phase Cross liquidus - a crystals begin to form Cross into single phase region - a crystals fill material Cross into two phase region - b crystals from inside a crystals Precipitation hardening

SECOND EXAMPLE Eutectic Alloy Alloy has the eutectic composition Converts directly from liquid to two solid phases

THIRD EXAMPLE Complex Phases Cross liquidus a crystals begin Grow as T drops Cross eutectic line Remaining liquid converts into a + b forming eutectic grains Final state consists of a grains and eutectic grains

Cu - Ag System Same general shape as Pb-Sn Melting points different Solubility limits different Eutectic different General features of behavior are the same

COMPOUND FORMATION Forms Mg2Pb Treat as two phase diagrams fused together One is phase diagram of Mg with Mg2Pb Other is phase diagram of Pb with Mg2Pb Vertical line means Mg2Pb does not form solid solutions

Phase diagram of Fe and Fe3C IRON CARBON DIAGRAM Phase diagram of Fe and Fe3C Would be a simple eutectic if not for fcc - bcc change Left hand axis shows changes in Fe crystal structure At high T bcc - fcc transition At lower T fcc - bcc transition Generates two additional horizontal lines Higher one is called peritectic Lower one is called eutectoid

THE EUTECTOID REGION

HYPO-EUTECTOID STEEL Carbon content less than eutectoid composition Soft steel used for plates and pressing operations

HYPER – EUTECTOID STEEL Carbon content more than eutectoid composition Used for machine tools, knives, etc

TIME-TEMPERATURE-TRANSFORMATION DIAGRAM FOR EUTECTOID STEEL

MARTENSITE – not on phase diagram Bct crystal structure “stretched” bcc Hardest of the phases Hardness increases with carbon content

HARDNESS OF PEARLITE DEPENDS ON MICROSTRUCTURE

HEAT TREATMENTS Conventional Martempering austempering