lec.2

?Upon cooling from liquid state, the temperature of the pure
metal (A or B) drops continuously till melting point at which
solidification starts. Solidification happens at a constant
temperature (line PQ) as F =0 (F = 1 – 2 +1 = 0). The
temperature drops again on completion of solidification.
?For any alloy (1, 2, 3 etc.) temp. drops till the liquidus (L1, L2,
L3). However, in this case, solidification proceeds over a range
of temperature as F = 1 (2 – 2 + 1 = 1). Once solidification
completes at the solidus (S1, S2, S3) the temp. drops again.?Three phases (L+?+?) coexist at point E. This point is called
eutectic point or composition. Left of E is called hypoeutectic
whereas right of E is called hypereutectic.
?A eutectic composition solidifies as a eutectic mixture of ?
and ? phases. The microstructure at room temperature (RT)
may consist of alternate layers or lamellae of ? and ?.
?In hypoeutectic alloys the ? phase solidifies first and the
microstructure at RT consists of this ? phase (called
proeutectic ?) and the eutectic (?+?) mixture. Similarly
hypereutectic alloys consist of proeutectic ? and the eutectic
mixture.
?The melting point at the eutectic point is minimum. That’s
why Pb-Sn eutectic alloys are used as solders. Other eutectic