Shape-memory alloy transformations can be tuned to achieve desired characteristics

Shape-memory alloys are materials that undergo property transformations with changes in temperature or stress and can transition back to their initial state. Specifically, the highly symmetric austenitic phase transitions into the low-symmetry martensitic phase as the material cools down. This change in material properties is useful in applications like tires, which need to be able to withstand high stress in rough terrain.

With the goal of increasing the temperature of the start of the phase transformation, Desai et al. simulated how the characteristics changed when a second phase is added to a shape-memory alloy – the additional phase that leads to a change in properties of the base material – to observe its effects on the martensitic phase.

“Recently discovered magnesium scandium alloys have pretty low start temperatures,” said author Saaketh Desai. “To use these alloys, you want to push that up. That’s why we’re interested in ideally increasing that temperature.”

The group found a softer set of second phases leads to an easier transformation, but there is a lower boundary to how much the transformation temperature can be reduced. This limit depends on the lattice mismatch between the second phase and the martensitic phase.

“Putting this all together is what gives the complete picture,” Desai said. “You want to change the martensitic start temperature? One way you can do that is second phases.”

Now that the group has an idea of what second phases to pick, they are working on fabricating shape-memory alloys with a desired set of characteristics. Desai noted this work can help inform materials decisions for applications ranging from Mars rovers to actuators.

Source: “Tuning martensitic transformations via coherent second phases in nanolaminates using free energy landscape engineering,” by Saaketh Desai, Samuel Temple Reeve, Karthik Guda Vishnu, and Alejandro Strachan, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5145008 .