Many materials
such as iron, tin, zinc, nickel, cobalt, etc. undergo crystalline
transformation in the solid state when there is a change in their temperature.
They exist in one lattice form over a certain range of temperature, but at a
somewhat higher or lower temperature, the lattice form changes to another
lattice form, which is stable in that temperature range.
Allotropy
or Polymorphy
An element that
occurs in more than one crystallographic or lattice form is called allotropy or
polymorphy, and the material in which such changes occur are known as
allotropic. The process in which the crystal lattice is changed in accordance
with the temperature is called allotropic or polymorphic transformations of the
material. Diamond and graphite are two allotropic forms of the element carbon.
The allotropic forms in which a metal exists are called its modifications. The
different modifications of the same metal are designated by the Greek letter
alpha (α), beta (β), gamma (γ), delta (δ), etc. Pure iron (Fe) is body centred
cubic in structure at all temperature up to 910˚ C. It is then called α iron.
Between 910˚c and 1390˚C the structure is face centred cubic and this form is
called γ iron. And from1390˚C to 1537˚C it
returns to body centred cubic in structure and called delta (δ) iron.
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Allotropic
Forms of Iron at Different Temperature
Another example is elemental tin (Sn), which is malleable near ambient temperatures but is brittle when cooled. This change in mechanical properties due to existence of its two major allotropes, α- and β-tin. The two allotropes that are available at normal pressure and temperature, α-tin and β-tin, are more commonly known as gray tin and white tin respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several GPa (Giga Pascal).
Anisotropy and Isotropy
In a single
crystal, the physical and mechanical properties often differ with orientation.
If a crystalline solid has a lattice structure whose atoms are arranged or
spaced differently when viewed in any or all of the 3 planes, that crystal is
called anisotropic. Also it can be defined as a
difference, when measured along different axis, in a material's physical
property (absorbance, refractive index, density, strength, etc.). Some
materials, such as wood and fiber-reinforced composites are very anisotropic, being much
stronger along the grain/fiber than across it. Wood is a naturally anisotropic
material. Its properties vary widely when measured with the growth grain or
against it. Wood's strength and hardness will be different for the same sample
if measured in different orientation.
Alternately,
when the properties of a material are same in all directions, the material is
said to be isotropic. For many polycrystalline materials the grain
orientations are random before any working (deformation) of the material is
done. Therefore, even if the individual grains are anisotropic, the property
differences tend to average out and, overall, the material is isotropic. The examples of
isotropic materials are aluminium, steel etc, in standard wrought forms. They
typically have the same stiffness regardless of the directional orientation of
the applied forces.
