Fine
particle strengthening is a two phase strengthening mechanism. Only a
relatively small number of alloy systems permit extensive solid solubility
between two or more elements, and only a relatively small hardening effect can
be produced in most alloy systems by solid solution additions. Therefore, most
commercial alloys contain a heterogeneous micro structure consisting of
two or
more metallurgical phases. The two phases may be ductile and present in the micro structure in relatively massive form. The strengthening produced by
second-phase particles is usually additive to the solid-solution strengthening
produced in the matrix. For two phase alloys produced by equilibrium methods,
the existence of a second phase ensures maximum solid-solution hardening
because its presence resulted from super saturation of the continuous phase.
Moreover, the presence of second phase particles in the continuous matrix phase
results in localized internal stresses which modify the plastic properties of
the continuous phase. Many factors must be considered for a complete
understanding of strengthening from second phase particles. These factors
include the size, shape, number, and distribution of the second-phase
particles, the strength, ductility, and strain-hardening behaviour of the
matrix and second phase, the crystallographic fit between the phases, and the
interfacial energy and interfacial bonding between the phases.
Dispersion Hardening:
The
strengthening produced by a finely dispersed insoluble second phase in a
metallic matrix is known as dispersion hardening. The second phase in
dispersion-hardening systems has very little solubility in the matrix, even at
elevated temperatures. In dispersion hardened systems there generally is no
coherency between the second-phase particles and the matrix. Dispersion hardened
systems are produced by mixing finely divided metallic powders and second phase
particles (oxides, carbides, nitrides, borides, etc.) and consolidating them by
powder metallurgy techniques. Dispersion hardened systems which are thermally
stable at very high temperatures. The degree of strengthening resulting from
second-phase dispersions depends upon the distribution of particles in the soft
matrix. In addition to shape, the second-phase dispersion can be described by
specifying the volume fraction, average particle diameter, and mean inter particle
spacing. These factors are all interrelated so that one factor cannot be
changed without affecting the others.
Precipitation Hardening:
Precipitation
hardening, or age hardening, is produced by solution treating and quenching an
alloy in which a second phase is in solid solution at the elevated temperature
but precipitates upon quenching and aging at a lower temperature. For
precipitation hardening to occur, the second phase must be soluble at an
elevated temperature but must exhibit decreasing solubility with decreasing
temperature. Usually there is atomic matching, or coherency, between the
lattices of the precipitate and the matrix in precipitation hardened systems. The
requirement of a decreasing solubility with temperature places a limitation on
the number of useful precipitation hardening alloy systems. The formation of a
coherent precipitate in a precipitation-hardening system, such as Al-Cu, occurs
in a number of steps. After quenching from solid solution the alloy contains
regions of solute segregation, or clustering. The clustering may produce local
strain, so that the hardness of GP is higher than for the solid solution. With additional
aging the hardness is increased further by the ordering of larger clumps of copper
atoms on the {100} planes of the matrix. This structure is known as GP or θ”.
Next, definite precipitate platelets of CUAI2, or θ, which are
coherent with the matrix, form on the {100} planes of the matrix. The coherent
precipitate produces an increased strain field in the matrix and a further
increase in hardness. With still further aging the equilibrium phase CUAI2,
or θ, is formed from the transition lattice θ’. These particles are no longer
coherent with the matrix, and therefore the hardness is lower than at the stage
when coherent θ' was present.
The
properties that that usually affect the strengthening mechanisms are coherency
strain, stacking fault, ordered structure, modulus effect, surface energy and
lattice friction. The particle displaces in two methods.
- Particle dislocation through slip system.
- Dislocation will move around or pass by.
