INTERGRANULAR FRACTURE IN AN A1-15 WT. % Zn ALLOY
Final technical rept.
CARNEGIE-MELLON UNIV PITTSBURGH PA DEPT OF METALLURGICAL ENGINEERING AND MATERIALS SCIENCE
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Cracking kinetics involved in the intergranular fracture of an A1- 15wt Zn alloy tested in air, distilled H2O, and 0.5M NaCl have been studied by means of the elastic strain energy released during crack propagation in a double cantilever beam specimen. This technique has been applied to two different types of specimens polycrystals with equi-axed grains and bi-crystals with crack propagation confined to the grain boundary. By varying the microstructure through selective aging treatments, a wide range of cracking susceptibility has been observed. The results show that the fracture toughness of this alloy increases with a decreasing volume fraction of G. P. zones in the matrix. A similar trend in the stress-corrosion cracking tests in 0.5M NaCl is noted, and the susceptibility to fracture increases as the volume fraction of G.P. zones in the matrix increases. The microstructure highly susceptible to fracture exhibits planar slip traces while the more resistant microstructures show a dispersed type surface slip pattern. The dependence of cracking rates on the microstructure is explained in terms of inhomogeneous plastic flow in the matrix being blocked by grain boundaries and leading to severe stress concentrations at the boundaries. The bi-crystal tests substantiate these results and show that orientations favoring partial continuity of slip bands across grain boundaries are less susceptible to stress corrosion cracking than arbitrarily oriented boundaries with no slip continuity.
- Properties of Metals and Alloys