Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59

2nd Revised Pages

438 • Chapter 11 / Phase Transformations

Austenite

+ a proeutectoid phasePearlite Bainite Martensite

Tempered martensite

Slow

cooling

Moderate

cooling

Rapid

quench

Reheat

Figure 11.37

Possible

transformations

involving the

decomposition of

austenite. Solid

arrows,

transformations

involving diffusion;

dashed arrow,

diffusionless

transformation.

Precipitation Hardening

The strength and hardness of some metal alloys may be enhanced by the formation of

extremely small uniformly dispersed particles of a second phase within the original

phase matrix; this must be accomplished by phase transformations that are induced

precipitation by appropriate heat treatments. The process is calledprecipitation hardeningbecause

hardening the small particles of the new phase are termed “precipitates.” “Age hardening” is also

used to designate this procedure because the strength develops with time, or as the

alloy ages. Examples of alloys that are hardened by precipitation treatments include

aluminum–copper, copper–beryllium, copper–tin, and magnesium–aluminum; some

ferrous alloys are also precipitation hardenable.

Table 11.2 Summary of Microstructures and Mechanical Properties for Iron–Carbon Alloys

Mechanical Properties

Microconstituent Phases Present Arrangement of Phases (Relative)

Spheroidite αFerrite+Fe 3 C Relatively small Fe 3 C sphere-like

particles in anα-ferrite matrix

Soft and ductile

Coarse pearlite αFerrite+Fe 3 C Alternating layers ofαferrite

and Fe 3 C that are relatively

thick

Harder and stronger than

spheroidite, but not as ductile

as spheroidite

Fine pearlite αFerrite+Fe 3 C Alternating layers ofαferrite

and Fe 3 C that are relatively

thin

Harder and stronger than coarse

pearlite, but not as ductile as

coarse pearlite

Bainite αFerrite+Fe 3 C Very fine and elongated particles

of Fe 3 Cinanα-ferrite matrix

Hardness and strength greater

than fine pearlite; hardness

less than martensite; ductility

greater than martensite

Tempered

martensite

αFerrite+Fe 3 C Very small Fe 3 C sphere-like

particles in anα-ferrite matrix

Strong; not as hard as martensite,

but much more ductile than

martensite

Martensite Body-centered

tetragonal, single

phase

Needle-shaped grains Very hard and very brittle