Torsional Creep of Polycrystalline Metallic Materials under Hydrostatic Pressure at Room Temperature

Abstract

It has been confirmed by the present authors, as the result of the experimental studies they performed of the creep of pure aluminum and pure iron in the normal temperature range below 0, 5T_m, 1T_m being the absolute melting temperature, that the creep rate of polycrystalline metals tends to decrease with the increase in hydrostatic pressure regardless of such a normal temperature range in which its diffusion mechanism is not to be expected theoretically³⁾¹³⁾. It has also been confirmed that the effect of hydrostatic pressure on the flow stress of metals can be described satisfactorily by assuming the formulation of yield condition including the first invariant of stress
J₁=σ_kk,
(a) J₂'=k(k²+CkJ₁+DJ₁²)^1/2,
and further that the qualitative explanation of creep rate decreasing with hydrostatic pressure of metals subjected to constant load is given uniformly by this yield condition from a side view of continuum mechanics.
Secondly, however, in order to understand better the relationship of hydrostatic pressure to the deformation mechanism in plasticity and creep of metals it is necessary to evaluate quantitatively the difference in the intensity of the influence of hydrostatic pressure between these deformations.
In the present study, therefore, it has been aimed at to evaluate numerically the change in the structure of metallic materials, i.e. the effects of hydrostatic pressure on the magnitude both of their plastic and of their creep deformation, and with these ends in view, various hydrostatic tests, creep tests and simple torsion tests, of polycrystalline materials, e.g. pure aluminum, pure iron and pure zinc, have been carried out under several levels of hydrostatic pressure at room temperature. It is to be noticed that the torsion test is found effective in the investigation because its stress condition does not cause the change in the form of the test specimens, nor does it enter into the hydrostatic stress component.
The results are summarized as follows:
(1) There has been little influence of hydrostatic pressure on the plastic flow stress of pure aluminum but on pure iron there has been effect to some extent after leaving plastic defomation, while remarkable influence appears on pure zinc in an early stage of deformtion. On the whole, such effects grows gradually with advanced deformation.
(2) The shape of the yield surface that varies with plastic deformation is determined by replacing the parameters both C and D in the Eq. (a) with numerical values. The calculated values of C and D, without regard to the kind of metal, are nearly zero during the shearing strain between zero and 30% but increases rapidly with the advanced shearing strain.
(3) On the creep of metals at room temperature, the confining pressure has a distinctive effect of decreasing the creep rate. The numerical values of C and D in the creep process of pure zinc agree with those in their static plastic flow respectively. Therefore, it is clear that the intensity of the influence of hydrostatic pressure on the creep rate is of the same magnitude as that on the static plastic deformation.