Stress-corrosion cracking in equiaxed 7075
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INTRODUCTION
IT has been shown lq for stress-corrosion cracking (SCC) of a commercially produced 7075 aluminum alloy that even in a very aggressive A1CI3 solution, which leads to very high corrosion rates, t2] the main operating SCC mechanism under tension loading in the underaged tempers (UT) and peak-aged tempers is likely to be hydrogen embrittlement. This is consistent with several reviews t3-1~j which have identified a major or dominant role of hydrogen embrittlement in SCC of high-strength AI alloys. The SCC properties m in AIC13 were obtained for longitudinally oriented specimens. It is well established that the specimen orientation relative to the rolling or extruding direction has a great influence on SCC behavior of high-strength A1 alloys; 18,~2'j31 SCC rates and susceptibility determined in smooth-bar tests t8,~2,j4'jS1 are the highest in the short transverse direction. The rationale given for this orientation dependence is the magnitude of resolved normal stress on grain boundaries. 18,~3-t51 In a "pancake" grain structure, as found in commercially produced alloys, 13j the major area of the grain faces is not subjected to normal stresses if the material is stressed in the longitudinal direction. Thus, crack growth is virtually "forced" onto a transgranular path, leading to an increased SCC resistance. Thus, it is possible that the SCC crack-path dependence on orientation in commercial aluminum alloys results from grain structure. The same appears to be true for hydrogen embrittlement of high-strength A1 alloys such as 7075, t8'~61 7 0 5 0 , [17'181 and 2124.1191 In equiaxed high-strength A1 alloys, SCC is predominately intergranular (IG) and the resulting SCC resistance is often even poorer than in the short transverse ANTHONY W. THOMPSON, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. MATTHIAS P. MUELLER, formerly with Carnegie Mellon U n i v e r s i t y , is with M a s c h i n e n s c h a d e n Verhiitungsdienst, Zurich, Switzerland. I.M. BERNSTEIN, formerly with Carnegie Mellon University, is Vice President with Tufts University, Medford, MA 02155. Manuscript submitted March 19, 1993. METALLURGICAL TRANSACTIONS A
direction of commercially produced alloys, p'12'13'2~ Moreover, cracking rates are considerably higher in equiaxed alloys and the cracking behavior is qualitatively different from commercial alloys, t31 The main purpose of the present study was to investigate the extent to which an equiaxed grain structure can influence the SCC properties (and perhaps the type of SCC mechanism) of a high-strength A1 alloy, such as 7075. The approach was to compare the SCC behavior of 7075 in the commercial composition and microstructure (pancake grains) tu with that of a high-purity or HP AI-Zn-Mg-Cu version of 7075 having an equiaxed grain structure, under loading mode I (tension) and mode III (torsion). lj,5,8,1~.22,231In comparing modes I and III, it is essential to take into account the possibility that mode I forces may ar
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