Deformation behavior of long-range ordered AgMg
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MITCHELL, OSAMA ABO-EL-FOTOH, AND J . E. DORN
The deformation behavior of long-range o r d e r e d AgMg single crystals was investigated in tension and compression for s e v e r a l orientations and temperatures. The study revealed that AgMg exhibits a complex and asymmetric deformation behavior corresponding t o the o p e r a tion of two different primary slip vectors, (111) and (001>, whose activation depends on the crystallographic orientation and s e n s e of the applied uniaxial s t r e s s and on temperature. Crystals of various orientations deformed between 20° and 298°K exhibited six different o p e r ative slip systems, {112} (111>, {123} (111>, {110} (111>, {120} (001), {100} (001>, and {110} (001>. It is shown that the high strain-hardening rate exhibited by polycrystalline AgMg does not a r i s e from the Vidoz-Brown5 mechanism of formation of antiphase boundary (APB) tubes on jogged superdislocations, but is associated w i t h the operation of the {hko} primary slip system is approximately 3.5 × 108 dyne per sq cm for specimens in orientations A through J. The tensile specimen in orientations K through T yielded by slip on the {hko} (001) slip systems at a higher CRSS of approximately 5.3 x 108 dyne per sq cm, even though the resolved s h e a r s t r e s s on the {112} (111) slip system was generally e q u a l and in some c a s e s g r e a t e r than on the operating {hkl} slip systems in AgMg, {110} (111> systems in CuZn and {110} (001> systems in AuZn, from slip-line observations on cylindrical gage sections of crystals deformed in tension (apparently at room temperature) to strains l a r g e enough to give well-defined slip t r a c e s b e f o r e fracture. They did not report the crystallographic orientation of the tensile axis of the specimens. Our results on tensile specimens in orientations E and G in Fig. 1 deformed at 298°K a r e in agreement with those of Rachinger and Cottrell,2° in that the {123 } system is an operative slip mode during d e f o r mation at room temperature. Our observations indicate that at and below 298°K the (123} (111> is not an active p r i m a r y slip system in t e r m s of extensive slip b e i n g initiated by the operation of existing dislocation sources on these planes. Slip lines corresponding to (123 } planes in tensile specimens of orientation E and G deformed at room temperature t o strains below 1 pct were generally observed to be wavy, with evidence of extensive cross-slip from the (112} (111) and {110} (111> slip systems, as illustrated in Fig. 11. In our view the ~123 } slip system operates principally as a cross-slip mode for dislocations initially a c t i vated on p r i m a r y {112} (111> and {110} slip systems. The absence of any evidence of slip on {123 } planes on specimens of orientation E deformed below 298°K, except at l a r g e strains, suggests that the c r o s s slip r e q u i r e s t h e r m a l activation, which is not adequately provided at the lower temperatures. During the compressive deformation of crystals in orientation A at 237°K, the
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