Mane, Rahul B and Panigrahi, Bharat Bhooshan
(2017)
GRAIN BOUNDARY SLIDING ASSISTED DENSIFICATION DURING INITIAL SINTERING STAGE OF MECHANICALLY ALLOYED CoFeNi POWDERS.
TRANSACTIONS OF POWDER METALLURGY ASSOCIATION OF INDIA, 43 (1).
pp. 13-19.
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Abstract
A multi-principal-element alloy with simple solid solution gives the unique combination of
properties, which has led to the development of medium and high entropy alloys (HEA). Because of
its elevated temperature phase stability and good oxidation resistance; they have potential for various
applications such as aerospace, nuclear, automobile etc. The mechanical alloying method is known
for its capability for extending the solid solubility limits and making the alloys of those metals which
are difficult to form through conventional method. Sintering can be done to get high density products
from as-milled powders. Densification is dependent on many things, out of which diffusivity is one
of the important parameter. However, till date there is no clarity about densification behavior of HEA
powders; and hardly there has been any study on their sintering kinetics. The present investigation
aims to study the densification behavior of mechanically alloyed CoFeNi medium entropy powders
through the help of dilatometer. It was observed that when the green compact was heated, sample
expanded slowly, as result of thermal expansion behaviour of the material. While continuous heating,
in the temperature range of about 250 to 450 ºC, sample exhibited noticeable shrinkage. After this
temperature, on further heating, sample started expanding again till about 800ºC. Above this, sample
shows continuous shrinkage, attributed to the diffusion based sintering. However, the shrinkage
anomaly observed during 250 to 450ºC range, could not be attributed to the diffusional process. It
appears that this densification could be related to grain sliding phenomenon. To understand in details,
the data was analyzed through the available diffusional-creep based models;which states that the
sliding of grains can occur due to the movement of dislocations, i.e., dislocation gliding, climbing, and
annihilation. Many of the school believe that such process may occur during sintering of particles.
Activation energy of densification shows extremely low value, much smaller than activation energies
of any diffusional mechanisms; indicating possibility of grain sliding process. This process lasts until
the permanent and rigid neck form between the grains.
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