Shape Memory Alloy Technique

Another deployment design concept considered the abilities of shapememory effect alloys. Shape memory effect, or SME, is the ability of metal alloys to change from one shape to another. The transformation of the alloy's crystal structure is typically a result of temperature changes. An SME alloy can be deformed, then recover and permanently maintain its original shape when heated. This is known as one-way memory. A two-way SME retains its original shape at one temperature and takes on another at a different temperature.

Shape Memory Alloys were first discovered in the 1960's when it was noted that nickel titanium alloy, named NiTinOl, had a shape memory effect when heated. When the alloy was cool it could be deformed, but returned to its trained shaped when heated. The alloy obtains this ability through the alignment of its crystals. The trained shape, or parent shape, is the setting of the austenite crystal phase. In the Austenite phase the nickel and titanium atoms are aligned in an ordered cubic structure and the alloy becomes very rigid. When the alloy cools the atoms become cubic. The structure appears "squashed". When deformed the alloy does not act like other metals. The crystal bands bend and align themselves instead of dislocating when deformed. The deformable phase is the Martensite crystal phase.

Previous experiments have shown the feasibility of SMA's on a small scale. Using a 1.5 oz piece of Nitinol, a 5 ft wing was deployed. Torques up to 100 in-lbs through more than 180 deg of travel were present. The disadvantage of using SMA's is their lockup stiffness. The range of temperatures in space will bring the SMA into the Martensite phase where the alloy can be easily deformed. This temperature fluctuation may be overcome by sending an occasional current through the alloy to raise it above the Austenite temperature.

SMA's can be implemented in the deployment of large solar arrays in a variety of ways. The accordion method uses SMA's to unfold a solar array from the fan pattern. A backbone design can incorporate SMA's as smart hinges. Using flexible solar cells, an SMA can unroll or untwist a tightly packed array.