Benefits of Improving Nitinol Hardness with Copper Alloy
The mechanical and thermal properties of pure nitinol are extremely sensitive to the Ni:Ti ratio. So it should not be surprising that the addition of a small amount of a third element will have an effect also. For this reason, controlled amounts of iron, niobium, copper, and other elements can be used as alloying elements for nitinol to tailor its properties for certain applications .
In particular, the addition of copper can cause a number of changes to nitinol, including changing its hardness, shape memory behavior, and superelastic durability. These will be discussed in detail below. It should be noted that, while copper has benefits in terms of mechanical properties, it can have a negative effect on the corrosion resistance of nitinol in some cases .
When Cu is added to nitinol, it generally takes the place of Ni, while the Ti content remains constant at around 50% .
Adding copper to nitinol has been shown to increase its hardness, from roughly 221-235 Vickers units to 305-319 Vickers units. In these cases, 2.5-2.59 wt% copper was added as an alloying element before Ni-Ti sintering .
In any application where nitinol will be in mechanical contact with other solid materials, increasing the hardness of the nitinol part will increase its durability and lifetime, though it may do this at the expense of damaging surrounding structures.
Even before adding copper, nitinol is a relatively hard material, with a hardness similar to steel. In fact, certain types of machining are avoided due to the hardness of nitinol . Adding copper has the potential of increasing its hardness by roughly 30-40%. For this reason, specially designed cutters with sharp cutting edges made of hard materials are recommended for high quality, precise cuts of nitinol and copper-modified nitinol wires, tubes, and sheets . These cutters should be cleaned and inspected regularly, and should be replaced if they become damaged or dull.
Phase Transition Temperature and Hysteresis
The key phenomenon behind the shape memory of nitinol is a transition between a high temperature (austenite) phase and a low temperature (martensite) phase. If a nitinol object is formed into a certain shape at high temperature, then cooled and deformed at low temperature, the object will “remember” and revert to its high temperature shape when it is heated.
The transition between the low and high temperature phase takes place at a different temperature when nitinol is being heated versus when it is being cooled. The difference in temperature, referred to as hysteresis (see Figure 1), can be in the range of 30 - 60 °C depending on the specific Ni:Ti ratio.
Figure 1: Schematic of hysteresis in the phase transformation between martensite and austenite in nitinol. From .