The relationship between microstructure, deformation behavior, and stress corrosion cracking resistance of an age-harden
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N U investigations M E ofRthe relationship O Ubetween S microstructure and resistance t o hydrogen embrittlement a n d / o r s t r e s s corrosion cracking have been made in iron, aluminum and titanium alloys. Much of this work recently has been reviewed by Thompson and Bernstein. l As t h e s e authors point out, a very common thread running through much of this work is that significant improvements in c r a c k growth resistance can be achieved through control of the slip mode. In general, any changes that reduce the tendency for localized, planar slip and produce more uniform, homogeneous deformation usually are beneficial to the cracking resistance. To data, very little has been done with nickel-base alloys to see if these g e n e r a l principles are applicable. T h e r e have been various studies of hydrogeninduced cracking that appear consistent with the behavior in other materials.' However, to our knowledge, t h e r e have been no s t r e s s corrosion studies of any age-hardened nickel-base alloy that have attempted t o r e l a t e the cracking resistance t o the microstructure and deformation characteristics. The present b r i e f study was therefore undertaken t o examine the p r o p e r t i e s of INCONEL® Alloy 722, which is a wrought ~' hardened nickel-base alloy. A precipitation hardened alloy was chosen because of the ease of altering slip mode by selecting suitable a g i n g treatments. EXPERIMENTAL
solution anneal. Heat Treatment B is an underaging treatment. Heat Treatment C provided m a t e r i a l of peak strength. Heat Treatment D gave slightly o v e r aged material, and Heat Treatment E more fully o v e r aged material. Wedge opening loaded (WOL) fracture toughness samples 12.2 mm thick were machined from the heat treated plate stock. The c r a c k growth orientation in all samples was kept parallel t o the roiling direction. The samples were fatigue precracked at room temperature. The fatigue c r a c k paths were transgranular. S e v e r a l samples were pulled in tension to check the compliance. In all e a s e s the measured compliances a g r e e d quite closely with the Novak and Rolfe2 values. Samples were bolt loaded at room temperature to various c r a c k opening displacements and then placed in the test environments. The loading bolts were made from the same alloy to avoid any galvanic effects, After the completion of the s t r e s s corrosion tests, each sample was unbolted with the c r a c k opening displacement gage in place to m e a s u r e the final displacement. The sample was then pulled in tension with the gage in place t o enable determination of the final load during the t e s t . The final s t r e s s intensity in each sample then was calculated in two ways; from the initial c r a c k opening displacement and final c r a c k length, and from the final load and final c r a c k length.
PROCEDURE
The material used in this study was taken from a commercially melted and hot rolled plate of I N C O N E L Alloy 722. The chemical composition of the heat is given in Table I. The
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