Takuro Mimaki

Superplastic deformation of micro-specimens of duplex stainless steel

Abstract Microstructural development in micro tensile test specimens having the gage width and thickness of 0.6 and 0.2 mm, respectively, of a (γ+α) duplex Fe–25Cr–7Ni–3Mo alloy was examined in order to investigate role of interphase boundaries in the superplastic deformation. An elongation of more than 800% was obtained at a strain rate of 5×10 −2 s −1 at 1323 K, and the strain rate sensitivity, m -values, were found to be 0.5 at the same strain rate. It was clearly recognized, by SEM observation on the cross section of the specimen, that several γ-phase grains were arranged in diagonal direction. It was found that cooperative interphase boundary sliding involving a number of grains occurred on the preferential plane composed of interphase boundaries. A model of superplastic deformation of two-phase materials constructed from a stochastic approach, which was reported previously, can be described quantitatively under the premises that (1) several tens of grains are involved in a single sliding event at the interphase boundaries, and (2) the rate of sliding at the γ–α interphase boundary is intrinsically much higher than those of γ–γ and α–α grain boundaries as was demonstrated by the present authors employing bicrystal specimens previously.

Corrosion, stress corrosion cracking and fatigue of ultra-fine grain copper fabricated by severe plastic deformation

Abstract The resistance against environmental degradation of copper with an ultra-fine grain structure created by severe plastic deformation is reviewed in three aspects: corrosion, stress corrosion cracking (SCC) and corrosion fatigue. The results are compared with those for conventional copper. It is shown that a higher overall resistance against SCC and corrosion fatigue can be obtained after grain reduction via equal channel angular pressing although the general liquid corrosion assessed electrochemically by the average dissolution rate at a given potential is somewhat higher for the severely pre-strained material. The experimental findings are discussed in the context of former fatigue studies and results of general corrosion and stress-corrosion tests. A possible scenario of intergranular SCC and corrosion fatigue damage in ultra-fine grain (UFG) copper is proposed.

Intergranular Stress Corrosion Cracking of Pure Copper 〈111〉 Tilt Bicrystals

Behaviour of stress corrosion cracking (SCC) in a series of pure copper bicrystals with a symmetrical 〈111〉-tilt boundary has been investigated. Tests were performed by the slow strain rate technique (SSRT) in 1M NaNO2 solutions. The small-angle tilt bicrystals fractured in both intergranular and transgranular manners accompanied by a large amount of plastic strain to fracture while the large-angle bicrystals fractured in almost intergranular manner with a smaller plastic strain. Susceptibility of SCC increases with increasing misorientation and becomes relatively constant in large-angle grain boundaries. The local minima appeared at the Σ7(321) and Σ3(211) boundaries, suggesting that the susceptibility was partially affected by grain boundary energy. Stress concentration generated by the pile-up of trapped dislocations at the grain boundary could account for the high susceptibility of the intergranular SCC in large-angle grain boundaries.

Tensile orientation dependence of surface-roughness evolution in cyclically deformed Fe–30%Cr alloy single crystals

Abstract In order to investigate orientation dependence of surface roughness evolution, cyclic deformation tests have been carried out in Fe–30%Cr alloy single crystals having the tensile axes of [233], [122], [011] and [001]. Formation of persistent slip bands (PSBs) characterized by the surface roughness was observed in all the orientations. Atomic force microscope (AFM) observations revealed that the maximum heights of the PSB extrusions in the single-slip-oriented specimens ([233] and [122]) were larger than those of the [011] and [001] specimens by a factor of 7.5 and 15, respectively.

Stress corrosion cracking and grain boundaries in copper based alloys.

The effects of grain boundaries on stress corrosion cracking (SCC) of Cu-Zn and Cu-Al alloys have been studied employing various types of orientation-controlled bicrystals. It was found that SCC susceptibility in Cu-Zn alloys decreases in the following order; beta-phase crystal surface, (alpha/alpha)-grain boundary, alpha-phase crystal surface, diffusion-induced twin boundary in alpha phase, (beta/beta)-grain boundary or (alpha/beta)-interphase interface. It was also clarified that, in an alphaCu-Al alloy, the grain boundary shows much higher susceptibility than the grain interior. The susceptibilities of the symmetrical - and -tilt, and the - twist boundaries of the alphaCu-Al alloy to SCC are strongly dependent upon the misorientations. Sharp minima indicating great resistance to SCC exist at some angles, which correspond to relatively small sigma-values. Moreover, the susceptibility of a grain boundary to SCC is much enhanced by a stress field arising from pile-up dislocations in the immediate vicinity of the boundary. It becomes evident that the susceptibility to intergranular stress corrosion cracking is suddenly decreased when the grain boundary misorientation decreases below approximately 15°. This may result from the increased number of dislocations passing through the boundary with decreasing the misorientation angle, by which a stress concentration is avoidable.

Stress corrosion cracking of .ALPHA.-brass bicrystals.

In order to study the effect of grain boundary on stress corrosion cracking (SCC) of α-brass, SCC tests were carried out on Cu-30.6 at%Zn alloy bicrystals in ammoniacal vapor at 303±2K. The susceptibility of the grain boundary to SCC was found to be higher than that of slip step sites in the present bicrystals with an about 15° off-coincidence Σ=13b boundary surface energy although it has been regarded to be as high as that of the so-called general boundary. Tubular pits which are considered to be initiated by the mechanism proposed by Pickering and Swann were mostly observed at the grain boundary and rarely on the crystal surface. Fine cracks propagating along the trace of {110} or {111} in the grain interior were observed not to grow into the main crack responsible to fracture.