The effect of pressurization on the pseudo-brittleness in an iron oxygen alloy

Abstract

Free grained iron containing 600 ppm oxygen is fairly brittle below room temperature. The brittleness essentially comes from the difficulty for the Lfiders band to initiate and propagate in fine grained specimens at the reqmred velocity and fracture occurs m the severely strained region wathin and near the band front, before deformation propagates through a specimen.Pressurization at several thousand atmospheres, which introduces free dislocations into a specimen, removes the brittleness and increases the elongation at low temperatures. It is shown that the pressure-reduced dislocations modify the stress dependence of the band velocity and increase the band velocity at a given stress. In~oducfionIn an impact tensile test coarse grained iron and steel show sufficient elongation while fine grained specimens fracture with very tittle elongations [1]. This is quite different from the case of a static tensile test in which finer gained specimens show larger elongation. It seems that the brittleness of fine grained specimens in an impact tensile test is not explained by the well known effects of grain size, strain rate and temperature on the fracture of iron and steel. It was confirmed that the brittleness in these cases essentially comes from the difficulty for the Liiders band to propagate in fine grained specimens at the required velocity, and fracture occurs in the severely strained region within and near the band front before deformation propagates throughout a specimen. It was also confirmed that these specimens having little elongation always fractured in ductile manner by drawing down with high reduction in area. In this sense Sakui and Moil [1] called such a phenomenon in an impact tensile test "pseudo-brittleness fracture".The velocity of propagation of the Lfiders bands is known to be sensitively dependent on either the test conditions [2] or material variables of grain size and matrix [3]. Recently, the present authors [4,5] have shown that pressurization at several thousand atmospheres, which is assumed to introduce free dislocations into material, makes it easy for the Lfiders band to initiate and propagate in iron, making the yield more homogeneous. This observation leads us to the expectation that pressurization or the pressure-induced free dislocations may remarkably affect the appearance of pseudo-brittleness in iron and steel. In this paper it will be shown that pseudo-brittleness generally appears just below room temperature in a fine grained Fe-O2 alloy even in static tensile test, and that pressurization removes the phenomenon and increases the elongation at low temperatures. The qualitative expression of the velocity of the Lfiders bands as a function of initial free dislocation density will also be given.