A study on the effect of stacking fault energy on fatigue crack propagation as deduced from dislocation patterns
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T H E R E are a number of evidences that alloys with low stacking fault e n e r g y are superior in fatigue p r o p e r t i e s (long fatigue lives and low c r a c k growth rates) to the solvent metals with high stacking fault energy. Such an effect of the stacking fault energy (SFE or ~) has been discussed by many authors.1-12 Initially it was uncertain whether the good fatigue properties were due t o the d e c r e a s e d stacking fault energy or due t o the inc r e a s e d yield s t r e s s of t h e s e alloys. However, it is now established by the experiments of Miller, Avery and Backofen using copper-base alloys that these p r o p e r t i e s are influenced by the f o r m e r r a t h e r than the latter. 5 Two i d e a s have been proposed concerning the effect of the stacking fault energy on the fatigue properties. One of Avery e t al considers the cell structure developed in high 7 metals to influence the c r a c k growth rate on the assumption that cell boundaries offer the pref e r r e d route of c r a c k .1 The other of L a i r d r e g a r d s g r e a t work-hardening capacity of low ~, alloys t o be a controlling factor t o r e s i s t the c r a c k tip deformation concerned with the growth rate (assuming the plastic blunting proce s s ) .1° T o prove these ideas, however, more work s e e m s to be necessary because of a lack of conclusive evidence for t h e m . Since a promising clue t o elucidate this problem was considered to be found by examining closely dislocation structures very near the t i p s of the c r a c k s formed in two materials with high and low stacking fault energy, respectively, transmission electron microscopy was performed on fatigued specimens of polycrystalline copper and 70//30 b r a s s . EXPERIMENTAL
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PROCEDURE
The chemical composition was Cu 99.98 pct for copper specimens, and Cu 70.34 pct, Z n 29.62 pet, Pb 0.02 pct, other impurities less than 0.01 pct for b r a s s specimens. The plates of these materials of 50 x 20 mm2 in c r o s s section were ammaled at 1023 K (750°C) and 923 K (650°C) for 1 h, respectively, and then machined to test pieces as shown in Fig. l(a). T h e i r tensile p r o p e r t i e s a v e r a g e g r a i n sizes are given in Table I. JOBU AWATANI, KAZUMUNEK A T A G I R I and KAZUO KOYANAGI are Professor, Assistant Professor, and Research Fellow, respectively, Osaka University, Suita, Osaka, Japan. Manuscript submitted May 1, 1 9 7 8 . METALLURGICAL
Constant moment fatigue tests were c a r r i e d out in completely r e v e r s e d plane bending with a Shimadzu UF-500 fatigue machine, operating at 1000 cycles per min. The length of the c r a c k s developed at notch roots were measured by surface observations u s i n g a travelling microscope; the s i d e s of the test pieces between two notches having been electropolished to facilitate measurements. When a c r a c k grew t o 1.0 t o ~2.5 mm in length from the notch root, the test piece was r e m
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