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MSE_442_Lecture_20_Nov 8 2013

19 hot pressing equations 20 hot pressing equations

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Unformatted text preview: is that of inadequate densificaKon. One soluKon to this difficulty is the applicaKon of an external stress or pressure to the powder system during heaKng, giving rise to the techniques of hot pressing, hot isostaKc pressing, and hot forging 17 Hot Pressing Mechanisms q༇  The mechanisms discussed earlier for sintering also operate during hot pressing, but the nondensifying mechanisms can be neglected because they are not enhanced by the applied stress, whereas the densifying mechanisms are significantly enhanced. q༇  Because of this enhancement of the densifying mechanisms relaKve to the nondensifying mechanisms, hot pressing provides an effecKve technique for determining the mechanism of densificaKon. q༇  New mechanisms can also be acKvated by the applied stress and may provide complicaKons, so their occurrence needs to be recognized. 18 Hot Pressing Mechanisms q༇  Since the diameter of the hot pressing die is fixed, powder compacKon occurs predominantly in the direcKon of the applied pressure, so the grains will be flajened. q༇  This shape change must be accommodated by sliding of the grains over one another. q༇  The major mechanisms that commonly operate during hot pressing are la[ce diffusion, grain boundary diffusion, plasKc deformaKon by dislocaKon moKon, viscous flow, rearrangement, and grain boundary sliding. 19 Hot Pressing Equations 20 Hot Pressing Equations – Large Pressure Densifica*on rate during hot pressing (when the applied stress is much greater than the driving force due to curvature) Numerical constant diffusion coefficient of the rate- controlling species stress intensificaKo n...
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