In situ grown composites: growth and morphology
The term composite applies in general to materials made up of at least
two chemically distinct phases with well define separation interfaces and
having characteristics properties which are not found in the individual
components.
This lecture deals with the fibrous composite materials in which one of
the phases is present in the form of fibers (whiskers,filamints,lamellae)
aligned in the same direction. The thickness of the fibers varies widely
from fractions of a micron (e.g. whiskers) up to several hundreds of
micron while the volume concentration of the fibers lies between a few
percent and 70%.
The varies teqniques used for the production of fibers composite materials
may be grouped in two main categories:
1. The first includes those methods in which two basic operations are
involved, the fabrication of fibers and the sub sequent insertion into the
matrix.
2.the second include the method in which the fibers are directly produced
in the matrix(in situ growth in single operation by unidirectional
solidification of eutectic alloys.
This teqnique has been used first for the solidification of binary eutectics
and later also eutectic sub sequent for with three or more components. The
addition of the third elements enables to adjust either the composition of
the matrix or the volume percent of the fibers phase.
As well as the eutectic reaction, a number of other reactions have been
exploited for the production of fibrous composite materials, namely:
1. Liquid –solid (paratactic reactions).
2. solid-solid reaction (continuous and discontinues precipitation and
eutectoid transformations).
Situ grown' composite is the realization of coherent or semi coherent
matrix –fiber interfaces with a strong relative bonding which is necessary
condition for smooth stress transformation from the matrix to the fiber.in
addition the forming conditions for the two phase material (fiber –matrix)
approach those of thermodynamic equilibrium. This allows a good thermal
stability which is important in the case of application in high temperature.
The excellent strength of the fiber composites has been recognized such as
the strength of tin whisker is higher than the massive tin. Filaments
possess a diameter of the order of micron are characterized by a high
length/ diameter ratio and by the absence of the defects such as micro
cracks, dislocations, cavities and surface imperfections which are present
in crystalline matter. The tensile strength of whiskers reach the value of
0.1 E (E being Young modulus) while poly crystalline metallic filaments
and for commercial metals in the massive state respectively values of
0.02E and 0.001 E are reached.
In situ Production Methods
Particular attention has been paid to nonmetallic substances like
Al2O3,BeO,B4C,SiC,Si3N4 because of their optimal heat characteristics and
low density and the high young modulus which are the principle
requirement for the wide range of applications.
The in situ production route of aluminum matrix composites is highly
interesting because it avoids the need for intermediate formation of the
reinforcement. Indeed, in this process the reinforcements are formed by
reaction in situ in the metal matrix in a single step. A further advantage is
that the interfaces between the reinforcement and the matrix are very
clean, enabling better wetting and bonding between them and the matrix
(no gas adsorption, no oxidation, no other detrimental interface reactions).
Also costs and safety hazards are reduced, as the handling of the fine
particulate reinforcement phases are eliminated Lanxide Process (Figure
1)the process is developed by the Lanxide Corporation and involves the
controlled outward oxidation or nitridation of molten aluminum alloys to
form metal-ceramic mixtures (Al-Al2O3 or Al-AlN combinations). Other
reinforcements (e.g. Al2O3 fibres, TiB, B4C) may also be added to the
liquid aluminum alloy.
XD-Process: The XD-process is patented by the Martin Marietta
Corporation. Blends of ceramic and metallic powders are heated up to a
reaction temperature which is usually above the metal melting point. The
components react with each other preferably exothermally) and form a
dispersion of a new ceramic phase in the matrix.
Examples are the production of aluminum composites with a dispersion
of borides or nitrides
Unidirectional solidification: the unidirectional solidification of eutectics
was studied for the production of composite materials as in the whiskercharacter of Cr-fiber extracted from the unidirectional solidification.
The eutectic reaction can form during cooling of two solid faces from one
liquid phase of eutectic composition.
The reaction can be schematized as followed.
Eutectic liquid ⟶α solid solution ⟶𝞫 solid solution
By using sand casting, mould casting, the phases α and 𝞫 consisting the
eutectic which present different morphologies –lamellar, needle shaped,
and a circular.
The solidification is controlled in such a way as to extract the heat in one
direction.