Is Nitinol Armor Feasible

Armor provides necessary protection for soldiers and it increases operational capability in environments where hostile fire is likely.  However, the huge weight of the armor dramatically limits the operational capability of both soldiers and vehicles. [insert table demonstrating consumption of operational capacity].  The most expensive vehicles in the fleet—aircraft—usually go unarmored because the weight represents such a strong limiter on the operational capacity.

What is needed is an armor that provides the same protection as traditional materials, but at significant weight reduction.  At Kellogg’s Research Labs, it seemed that nitinol would be a good substitute material for armor.  With a density of 6.45 g/cm³, nitinol represents a 20% weight reduction over steel for the same thickness of armor.  However, with the superior superelasticity permitting recoverable deformations up to 100x greater than steel, it seemed likely that the nitinol armor would also be thinner than traditional armor.

Test Description: To test the concept of ballistic nitinol, the company prepared 1mm thick sheets of nitinol, each 153mm square and fired a 7.62mm round at a distance of 10m at each sample.  The initial spread of test samples included superelastic, standard temperature shape memory (45°C transition temperature), and high temperature shape memory (80°C transition temperature) sheets.  All sheets were binary NiTi, cold rolled to 1mm thickness and mechanically polished.  Various heat treatment profiles were applied to the nitinol to explore the effects of heat treatment.

Test Results: It was expected that the superelastic samples would have the best performance in this ballistic test.  However, the 80°C sheets dramatically out performed the superelastic sheets.  The results from the highest performing sample can be viewed in Figures 1 and 2 above. 

Conclusions: This test demonstrates the possibility of using nitinol sheets as protection against small arms fire.  With a mass of just 6.45kg/m², even aircraft can be protected against small arms fire, allowing them to take on more dangerous missions without compromising operational capability.  The small size of this sample limited its ability to respond to impacts.  This means that the larger sheet sizes found on vehicles and aircraft would be able to protect against larger threats.

Areas of Continued Research: There are four areas that could benefit from continued research: chemistry, cold work, lamination, and monocrystalline nitinol.  A well funded project is likely to find out that the ideal solution is a mixture of all four variables.

1. Chemistry: The samples prepared in this experiment were strictly binary NiTi.  However, we have found that most applications of nitinol perform better with a ternary alloy than they do with standard binary.  Some suggestions include:
   1. Aluminum
   1. Copper
   1. Cobalt
   1. Hafnium
   1. Iron
   1. Niobium
2. Cold Work: The samples prepared in this experiment were taken from inventory.  This means that they may not have had the ideal cold work done to them.  There also exists the possibility that the grain orientation may have a significant effect on the ballistic performance of the nitinol sheets.  To evaluate cold working, in continued research, the lab’s cold roll would be used to control the amount of cold work done in the final processing step.  Since having grain oriented in the <1,0,0> direction is functionally identical to the <0,1,0>, the only way to study the effect of grain orientation is to prepare a 150mm square bar and cut off 1mm thick sheets for testing.
3. Lamination: The samples that were prepared were a single sheet, 1mm thick.  Using two sheets 0.5mm thick or three sheets 0.33mm thick allows for interfacial slip, permitting greater deformation.  Continued research in lamination would study the benefits of utilizing multilayer nitinol.  It is likely that further weight reductions and/or performance enhancements will be found.  The tradeoff is that, below 1mm thickness, nitinol begins to increase in price on a per square centimeter basis.
4. Monocrystalline: Nitinol is highly anisotropic, meaning that the properties of the nitinol are highly dependent on the crystal orientation.  In polycrystalline nitinol, each grain has its own orientation and the distribution of the angle of these orientations can be quite large.  This represents an internal inefficiency.  Growing monocrystalline nitinol can keep each grain closely aligned to minimize the effect of this inefficiency.  It is likely that this will result in dramatically higher ballistic performance of the nitinol.

Read more on Nitinol FAQ here.