The pressure-induced brittle ductile transition in polycrystalline zinc was examined, with special reference to the variation of transition pressure with the grain size of specimens. Fracture stress σ
fr in the brittle pressure range increases linearly with pressure
P applied and is expressed as
(This article is not displayable. Please see full text pdf.)
where a constant η is nearly 0.35 and σ
fr(0) is the fracture stress at ambient pressure.
The pressure-induced transition has been found to occur when the fracture stress reaches the necking stress σ
n. The variation of transition pressure
Pc with grain size comes from the dependence of σ
fr(0) on grain diameter
d of specimens, i.e. σ
fr(0)=
Kfrd−1⁄2, where
Kfr is a constant, and the following relation is established:
(This article is not displayable. Please see full text pdf.)
Generally speaking,
Pc does not vary linearly with
d−1⁄2, since the necking stress σ
n also depends on grain size. However, within a narrow grain size range in which the variation of σ
n is rather small,
Pc satisfies the relation
Pc∝
d−1⁄2.
Finally, the rapid or discontineous increase of ductility of zinc at the transition pressure has been explained on the basis of the previously proposed criterion of ductile fracture, viz. “the constancy of hydrostatic tensile stress”.
View full abstract