The ability to change overall shapes to a much greater degree than conventional polycrystalline alloys appears to be the breakthrough that will allow single-crystal, shape memory alloys to realise their full potential in mechanics.
The alloy elements, which can deliver large amounts of force in small spaces are expected to have applications in robotics and, while speeds of movements are limited by the times required to transfer heat into metal wires and rods, there are a great many other mechanical movement uses to which these devices can be put.
Shape-memory alloys, as their name implies, change their shape back to their original shape when heated. They can be bent, extended or compressed, and then recover and remember their original shape upon the application of heat above a certain transition temperature.
They were originally developed for applications in space, because they allow actuators to be made without fluids, seals, or rotating parts, all potential areas of poor reliability. They were then promoted for wired bras, which could be bent during washing and recover their shape when worn. Their most important application, however, is as stents, which are welded assemblages of wires, which are inserted into blocked human arteries, and then opened out to expand the available passages. They save thousands of lives every year.
Despite much development work, some of the most recent of which was described in a feature article in Eureka's July 2000 edition, other practical uses have, until now, been limited. This is because all practical shape memory alloys consist of a multitude of crystals, all in different orientations joined together at their faces. The result is that if the most popular nickel titanium alloy elements are deformed with strains of much more than about 4%, they will be plastically deformed, and not return fully to their original shape. Practical shape memory actuators have therefore been limited to long lengths of wires and helical spring shapes. However, thanks to recent developments, this is no longer the case.
Dr Ivan Vahhi, who lectures on the principles of designing shape memory actuators at St. Petersburg State Technical University, is one of the leaders of a team, which has found a way of mass producing copper-aluminium-nickel shape memory alloy rods as single crystals. The crystals can be deformed with strains of up to 10%. For reasons which should be obvious, he is reluctant to reveal details of his mass production technique, but it is possible to make an intelligent guess. See box story
Having the means to make shape memory alloy elements as single crystal rods, it is possible to bend them by a significant amount, and then extract forces of up to 450 - 600MPa when they are warmed. When a metal change its shape, it does so with a force limited only by the yield stress of the material. The limiting stress available from transforming polycrystalline nickel-titanium is about 250 - 400MPa. A further advantage of single-crystalline elements over polycrystals is that they are much less prone to fatigue failure.
Much less force is required to bend a rod of single crystal than polycrystalline alloy, so that the deforming or bending stress is only 20 to 40MPa. The net result is that the overall size of an actuator made using single crystalline elements is about 10% of that required by polycrystalline elements.
The team in St. Petersburg has come up with a number of designs which they have constructed and evaluated. The simplest, single-acting linear actuator uses a single bent rod of alloy, which straightens when heated; the return stroke being effected by a spring. A slightly more sophisticated type of linear actuator is the double-acting variant. The stroke in one direction is achieved by heating one bent rod, which straightens, bending the other rod as it does so. The reverse stroke is achieved by heating the second, bent rod.
Single acting linear actuator (without heater on wire)
Double
acting linear actuator
