The unidirectional shape-shape-memory effect was first observed by Chand and Read in 1951 in a Gold-Cadmium alloy and in 1963 Buehler described this effect for nitinol, which is an equiatomic Nickel-Titanium alloy.
This effect in metals and ceramics is based on a change in the crystal structure, called martensitic phase transition. The disadvantage of these materials is that it is an equiatomic alloy and deviations of 1% in the composition modify the transition temperature by approximately 100 K.
Some metals and ceramics present the effect bidirectionally, which means that at a certain temperature there is a shape and this can be changed by changing the temperature, but if the first temperature is recovered, also the first shape is recovered. This is achieved by training the material for each shape at each temperature.
Metals and ceramics with thermally induced bidirectional shape-memory effect have had great application in medical implants, sensors, transducers, etc. Many present a risk however due to their high toxicity.