Biomedical Engineering Reference
In-Depth Information
Mechanical alloying is conducted through milling of two
elemental metal powders. Longer milling, order of hours, leads to
atomic-level mixing of the metals and produces an alloy consisting
of these metals. Until the advent of this new, non-equilibrium, and
low-temperature solid-state processing method, metal alloys could
only be manufactured by melt casting and metal foundry practices.
The raw materials used for MA, are available commercially high
purity powders that have sizes in the range of 1-100 μm. During the
mechanical alloying process, the powder particles are periodically
trapped between colliding balls and are plastically deformed. Such
a feature occurs by the generation of a wide number of dislocations
as well as other lattice defects. Furthermore, the ball collisions
cause fracturing and cold welding of the elementary particles,
forming clean interfaces at the atomic scale. Further milling lead to
an increase of the interface number and the sizes of the elementary
component area decrease from millimeter to submicrometer
lengths. Concurrently to this decrease of the elementary distribution,
some nanocrystalline intermediate phases are produced inside the
particles or at its surfaces. As the milling duration develops, the
content fraction of such intermediate compounds increases leading
to a inal product whose properties are the function of the milling
conditions.
The sequence of concurrent mechanical and chemical events can
be written as follows [54]: mechanical stressing
→
severe plastic
deformation
→
formation of a submicron lamellar microstructure
→
cold interdiffusion of metal atoms between lamellae or nanograins
(cold welding)
→
fracture
→
formation of nanostructure
→
extended solid solubility
→
mechanical alloying with formation
of thermodynamically stable and /or metastable phases
→
amorphization.
It was shown, that mechanical alloying has produced amorphous
phases in metals. But differentiation between a “truly” amorphous,
extremely ine grained, or a material in which very small crystals
are embedded in an amorphous matrix in so produced materials
has not been easy on the basis of diffraction basis [45]. Only
the supplementary investigations by neutron diffraction can
unambiguously conirmed that the phases produced by MA are truly
amorphous. The milled powder is inally heat treated to obtain the
desired microstructure and properties. Annealing leads to grain
growth and release of microstrain.
