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when the size and spacing of the dispersed particles are of nanodimensions, the increase in strength is dramatic. Finally, ductile
materials become harder when deformed because of work hardening, shown in Figure 4.41c. Dislocations accumulate on many slip
planes during plastic deformation; those on one set of slip planes
intersect the slip planes of others, obstructing their motion.
4.4 therMal Behavior
Heat is atoms or molecules in motion. In gases, the molecules fly
between occasional collisions with each other. In solids, by contrast,
they vibrate about their mean positions; the higher the temperature, the greater the amplitude of vibrations. From this perception
emerges all our understanding of the intrinsic thermal properties of
solids: their heat capacity, expansion coefficient, conductivity, even
melting.
Heat affects mechanical, electrical, and optical properties, too. As
temperature rises, materials expand, the elastic modulus decreases,
the strength falls, and the material starts to creep, deforming slowly
with time at a rate that increases as the melting point is approached
until, on melting, the solid loses all stiffness and strength. The electrical resistivity rises with temperature, the refractive index falls,
color may change—all effects that can be exploited in design.
intrinsic thermal properties
Two temperatures, the melting temperature, T m , and the glass temperature, T g (units for both: Kelvin, K, or Centigrade, C), are fundamental points of reference because they relate directly to the strength
of the bonds in the solid. Crystalline solids have a sharp melting
point, T m . Noncrystalline solids do not; the glass temperature T g
characterizes the transition from true solid to very viscous liquid. It
is helpful in engineering design to define two further temperatures:
the maximum and minimum service temperatures T max and T min (units
for both: K or C). The first tells us the highest temperature at which
the material can be used continuously without oxidation, chemical
change, or excessive distortion becoming a problem. The second is
the temperature below which the material becomes brittle or
otherwise unsafe to use.
It costs energy to heat a material. The energy to heat 1 kg of a material by 1°K is called the heat capacity or specific heat, and since the
measurement is usually made at constant pressure (atmospheric
Thermal Behavior
when the size and spacing of the dispersed particles are of nanodimensions, the increase in strength is dramatic. Finally, ductile
materials become harder when deformed because of work hardening, shown in Figure 4.41c. Dislocations accumulate on many slip
planes during plastic deformation; those on one set of slip planes
intersect the slip planes of others, obstructing their motion.
4.4 therMal Behavior
Heat is atoms or molecules in motion. In gases, the molecules fly
between occasional collisions with each other. In solids, by contrast,
they vibrate about their mean positions; the higher the temperature, the greater the amplitude of vibrations. From this perception
emerges all our understanding of the intrinsic thermal properties of
solids: their heat capacity, expansion coefficient, conductivity, even
melting.
Heat affects mechanical, electrical, and optical properties, too. As
temperature rises, materials expand, the elastic modulus decreases,
the strength falls, and the material starts to creep, deforming slowly
with time at a rate that increases as the melting point is approached
until, on melting, the solid loses all stiffness and strength. The electrical resistivity rises with temperature, the refractive index falls,
color may change—all effects that can be exploited in design.
intrinsic thermal properties
Two temperatures, the melting temperature, T m , and the glass temperature, T g (units for both: Kelvin, K, or Centigrade, C), are fundamental points of reference because they relate directly to the strength
of the bonds in the solid. Crystalline solids have a sharp melting
point, T m . Noncrystalline solids do not; the glass temperature T g
characterizes the transition from true solid to very viscous liquid. It
is helpful in engineering design to define two further temperatures:
the maximum and minimum service temperatures T max and T min (units
for both: K or C). The first tells us the highest temperature at which
the material can be used continuously without oxidation, chemical
change, or excessive distortion becoming a problem. The second is
the temperature below which the material becomes brittle or
otherwise unsafe to use.
It costs energy to heat a material. The energy to heat 1 kg of a material by 1°K is called the heat capacity or specific heat, and since the
measurement is usually made at constant pressure (atmospheric
Thermal Behavior