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Mechanical Alloying (MA) involves loading the blended elemental powder
particles along with the grinding medium in a vial and subjecting them to heavy
deformation, so that the particles are repeatedly flattened, cold welded, fractured and
re-welded. The severe plastic deformation increases the surface-to-volume ratio of the
particles and ruptures the surface films of adsorbed contaminants, exposing fresh and
nascent surfaces to result in cold welds. Continued cold welding and fracturing leads
to microstructural refinement and decrease of diffusion distances. In addition,
continued cold working results in the formation of a number of crystal defects like
dislocations, vacancies, grain boundaries, etc. The ball-ball, ball-powder, and ball-wall
collisions result in temperature rise, which further facilitates diffusion. Consequently,
true alloy formation takes place. MA is a novel method of materials synthesis (Koch,
1992). It not only overcomes the problem encountered in the melting process of the
systems having high sedimentary tendency and big difference in melting point of
constituents, but also results in formation of non-equilibrium microstructure such as
amorphous, nanocrystalline or supersaturated solid solution in alloys (Zhu et al., 1998 and 2000). The magnitude of improvement in the mechanical properties due to solid
solution formation depends on inter-particle spacing, and size and volume fraction of
the precipitate or dispersoid (Suryanarayana, 2004).
Solid solubility extensions beyond the equilibrium values have been achieved
in many alloy systems by just quenching the alloys, rapid solidification, vapor
deposition, laser processing, sputtering, etc., (Suryanarayana, 1999). In recent years, some
of the investigations reported that extension of equilibrium solid solubility
limits, achieved by MA, have been spectacular. The solid solubility of various solute
elements in Aluminium, achieved by MA and other manufacturing processes are presented
in Figure 1. The relevant data for Figure 1 are noted from
Suryanarayana (2004).
In the recent years, the Al-Pb nanocomposites were made by mechanical
alloying through axisymmetrical compaction and high energy rate forming (Csanady et al., 2006). It has been observed that Pb phase exhibits heterogeneous coarsening
owing to a statistical size distribution of Al grains in the milled powder, and that small
additions of Cu can suppress the grain growth of Lead, in mechanical alloying (Zhu et al., 2009). In this work, Al and Pb powders of various compositions
are manufactured by MA using a laboratory size ball mill, attrition mill and
atmosphere controlled sintering furnace, with a view to analyze the dependence of
densification behavior and mechanical properties of Al-Pb alloys made by MA on the
process parameters, viz., Ball-to-powder Charge Ratio (BCR) and mixing route. |
