Recrystallization-thermal etching of local plastic strain in heat resistant alloys
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order to thoroughly understand the mechanism of fracture for engineering materials, it is necessary to analyse the local plastic strain and plastic zone around notches or cracks at the various temperatures where the material is used. Strain measuring techniques employing electric-resistance strain gauges and photoelastic coatings are not adequate in this case, because they can not measure the steep strain gradient near crack. The Moire technique can be used to detect local strain and can be applied to elevated temperatures but is confined only to the surface. The chemical etching technique has been used to reveal plastic zones at discontinuities not only on the surface but in the interior. Using this technique, observations of plastic zone have been made of mild steel, ~,2 Fe-3Si steel, 3-6 Armco iron, 7 SAE 4340-steel, s-j~ low alloy high strength steel tl and nickel base superalloysJ 2,t3 However, the temperature range covered by the technique is limited to a low temperature range. It would be difficult, therefore, to apply these techniques in cases of elevated temperatures where heat resistant alloys are used. Recently a recrystallization technique has been used successfully to observe plastic deformation around notches and cracks, i.e. 1) monotonic plastic zones in carbon steel, ~4low alloy high strength steel, ~5unalloyed steel, t6 mild steep 7 and Type 304 stainless steel, ~8,~9 2) fatigue plastic zones in mild steel, ~7.20.2~low alloy high strength steel, 22 aluminum 23 and T y p e 304 steel 24 and 3) creep plastic zone in Type 304 steelJ 9 The technique seems to be very effective for the analysis of local plastic strain, since a) an engineering material itself can be used, b) plastic deformation both on surface and in the interior is revealed and c) the temperature range is wide. (Recent study has shown that the technique is applicable in the range from 77 to 1223 K in Type 304 stainless steel25). The technique should be, however, examined in more detail with regards to its capability in cases of high temperature applications, because 1) there are both ferritic and austenitic heat resistant alloys that have been practically used and 2) the failure analysis of such Y. IINO is Associate Professor, Department of Engineering Science, Faculty of Engineering, Tohoku University, Sendai/980, Japan. Manuscript submitted February 22, 1980.
fractures in the alloys as caused by either creep, fatigue or creep-fatigue interaction requires the quantitative measurement of deformation and strain near the crack. Furthermore, the recrystallization technique used up to date, as well as the chemical etching technique, need chemical or electrical etching which is, in general, complex and difficult for heat resistant alloys. In order to develop a more accessible recrystallization technique for elevated temperature applications of heat resistant alloys, the present author examined one ferritic and two austenitic alloys by use of thermal etching. The thermal etching was adapted to visualize the recrystallized zone i.e. plastically
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