Mitsuru Ishii’s research while affiliated with Hitachi, Ltd. and other places

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 σfr 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 σfr(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 σn. The variation of transition pressure Pc with grain size comes from the dependence of σfr(0) on grain diameter d of specimens, i.e. σfr(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 σn also depends on grain size. However, within a narrow grain size range in which the variation of σn 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”.

It was found that the strain rate dependence of the flow stress of Nb-26 at%Zr alloy was larger than that of pure Nb at all temperatures from 201° to 573°K. Above room temperature the strain rate dependence of pure Nb disappears, but that of the alloy is still significant. At 77°K the alloy deforms by twinning and the flow stress becomes insensitive to the strain rate. The stress dependence of activation volume of this alloy was almost the same as that of Nb. The activation volume increases sharply with decreasing effective stress at low stress level, but it becomes nearly constant at the stress larger than 5 kg/mm², taking 10-20b³. The activation energy however was always larger than that of pure Nb. It is suggested that the rate controlling mechanism for the flow of the alloy is the same as for Nb. Assuming that double kink formation is the rate controlling process, the kink energy of the alloy was determined as about 1.25 eV, which is nearly three times larger than that of Nb.

Observations by transmission electron microscopy on the pressurized iron thin foils have confirmed that the density of dislocations increases in the region near the particles of second phase upon hydrostatic pressure exposure at 10 or 14 kb. These pressure-induced dislocations are observed around FeO particles of several microns in diameter but not around the small carbides (size effect). These observations are in agreement with the following experimental results. (1) The most significant pressure effect, i.e. the lowering of yield stress and the decrease of the Lüders elongation are found for iron containing 600 ppm O2 but no effects are found for a zone refined iron the oxygen content of which is 68 ppm. (2) The pressure effect on iron containing 200 ppm O2 is not prominent, although the iron contains a great number of fine carbide particles. The shear stress arising from the surface of inclusions by the volume indentation effect under a pressure of several thousand atmospheres is about one hundredth of the stress to nucleate dislocations, which leads to the conclusion that pressurization unpins old dislocations and multiplicates them around large inclusions.

Tensile properties of Nb-Zr solid solution alloys composed of the retained high-temperature β phase were investigated at temperatures from −196°C to +800°C. The room-temperature yield stress increases linearly with the zirconium content up to 15 at% and it reaches a maximum value of about 100 kg/mm² at 42 at%Zr. The mechanical behavior of Nb-15 at%Zr alloy was found to be similar to that of pure niobium, but the alloys with higher zirconium content, e.g. 26 and 42 at%, showed a different behavior from pure niobium. Nb-26 or 42 at%Zr alloy does not show a sharp yield drop. The work hardening rate of the same alloy is smaller than that of niobium, being insensitive to the test temperature. Nb-26 or 42 at%Zr undergoes twin deformation below −72°C and a distinct serration appears in its load vs. elongation curves. The low-temperature flow stress of these alloys increases significantly with decreasing temperature. The temperature dependence of flow stress of Nb-15 at%Zr is similar to that of niobium whereas that of Nb-26 or 42 at%Zr alloy is small compared with that of Nb and Nb-15 at%Zr alloy. At temperatures above room temperature, the flow stress of niobium is insensitive to the test temperature and keeps constant up to 600°C, while the flow stress of alloys decreases with temperature and becomes constant at some higher temperatures. The higher the alloy content, the greater is the decreasing rate. The temperature at which the flow stress becomes insinsitive to temperature also rises with alloy content. Elongation at room temperature decreases with zirconium content. But at high temperature, the effect of zirconium content disappears due to the large decrease of elongation of niobium. At temperatures above 600°C the elongation of niobium increases with temperature, while that of alloys continues to decrease with temperature.