Dalhousie University engineers use neutron beams to develop inexpensive ways to process lightweight materials for actuators that fold airplane wings during flight—just one of many possible energy-saving aerospace and automotive applications for shape memory alloys.
Source: Canadian Neutron Beam Centre (CNBC)
Image: Shape memory alloys can be used to fold the outer portions of airplane wings during flight, which saves energy by reducing air resistance. (NASA)
Although car and airplane manufacturers have been implementing many improvements in energy efficiency over the past few decades, there’s still a lot of room for advancement when it comes to reducing energy usage in the transportation industry.
Titanium alloys can be used to make parts lighter, enhance engine efficiency, and create more efficient actuators.
Titanium alloys can play a role in conserving energy for several reasons. First, since titanium parts weigh less than steel parts, they require less energy to propel. Second, titanium alloys retain their strength at high temperatures where many other metals lose strength. This temperature resistance is valuable because the higher the temperature, the more efficiently engines run. It is this combination of lightness and temperature resistance that has made titanium alloys very desirable for many aerospace applications, especially engine parts.
A third reason why these alloys can help the transportation industry to conserve energy is because one titanium alloy in particular (i.e., nickel-titanium alloy) has an unusual property that enables parts to be moved more energy-efficiently. Specifically, when titanium is alloyed evenly with nickel, it becomes a ‘shape memory alloy,’ meaning that it ‘remembers’ its original shape after it is deformed—and can return to that shape when activated by heat or a mechanical force. This remarkable ability can be harnessed to create a push or pull effect.
For this reason, nickel-titanium shape memory alloys can be used to replace conventional devices that move parts, known as actuators. Conventional actuators often rely on heavy and energy-intensive systems, such as electric motors or hydraulics. Therefore, replacing conventional actuators with lighter shape memory alloy systems will conserve energy and simplify designs.
The potential applications for shape memory alloy actuators are already being demonstrated. For example, in January 2018, NASA announced the successful demo flight of an aircraft that used a nickel-titanium alloy actuator to fold the outer portions of the wings up and down during flight. Such adjustment to the angle of a plane’s wings, in response to flight conditions such as wind direction and turbulence, leads to more energy savings by reducing air resistance.
Titanium alloys are attractive to the auto industry for the same reasons they are appealing to the aerospace sector: they can be used to make parts lighter, enhance engine efficiency, and create more efficient actuators. But titanium is also expensive—which has hindered its widespread adoption, particularly by the price-sensitive auto industry.
The prospect of making greater use of parts made of titanium alloys is one reason why both the automotive and the aerospace industries have funded the research of Stephen Corbin, a professor of materials engineering at Dalhousie University. Corbin’s research chair is sponsored by Pratt and Whitney Canada, and several of his recent studies on titanium alloys were supported by Automotive Partnership Canada.
One of Corbin’s research goals is to lower the cost of parts made of nickel-titanium shape memory alloys by developing simpler methods of processing them. “Traditionally, these alloys are melted, cast, and rolled or drawn into a desired shape, and then machined as needed to make further adjustments,” explains Corbin. “But these metals are hard to melt because of their excellent temperature resistance. And their ability to change shape is a problem for machining, because [the metal] can move while it is being machined. These factors complicate processing and ultimately raise cost.”
Corbin sees powder metallurgy as a solution for simplifying the processing of nickel-titanium alloys. In a manner similar to that proposed by his colleague Paul Bishop for using aluminum powder metallurgy to process car parts (described in a previous article), Corbin proposes compressing metal powders in a mold and then heating them just enough to cause the powders to join (a process called ‘sintering’). At the end of this process, a component close to its final state is created, greatly reducing the number of processing steps involved.
[Corbin’s set of experimental results] is an important contribution to the field because it provides the fundamental knowledge required to optimize the sintering process for inexpensive nickel-titanium shape memory alloys, which are needed for lightweight actuators in cars and airplanes.
Other researchers have experimented with sintering powders composed of pure nickel-titanium alloy (NiTi), but this alloy powder is expensive. Therefore, to meet the goal of reducing manufacturing costs, Corbin’s experiments use a starting mixture of readily available and less expensive titanium (Ti) powder and nickel (Ni) powder, with the aim of creating a component composed of pure NiTi through sintering. As Corbin explains, “The goal is to determine what conditions of sintering give 100 percent of the NiTi shape memory compound.”
Creating a component of pure nickel-titanium shape memory alloy through sintering is a complex task. Indeed, while the sintering process can produce the desired pure NiTi alloy, in which the nickel and titanium atoms are evenly dispersed throughout, it can also result in other unwanted variations. These variations (e.g., Ti2Ni and Ni3Ti) are referred to as ‘phases’ and do not exhibit the sought-after shape memory effect. Adding to the challenge is the fact that there has been some confusion in the research community as to the exact nature (e.g., the molecular composition) of these unwanted phases.
Years of previous research led Corbin to the hypothesis that unwanted phases might be forming during the sintering of the mixture of titanium and nickel powders, and that the culprit could be the presence of oxygen in these starting powders. To test this hypothesis, Corbin and graduate student Daniel Cluff accessed the Canadian Neutron Beam Centre (CNBC) in Chalk River to perform a series of neutron diffraction experiments. Their goal was to determine exactly what phases were forming—and at what temperatures they were forming—during the sintering process.
Neutron diffraction was preferred over x‑ray diffraction for this research because neutrons can penetrate deeply into a material, whereas x‑rays provide information about surface effects only. This penetrating power of neutrons also enabled measurements to be taken during the sintering process, since the neutrons could go through the many pieces of equipment surrounding the metals being sintered—equipment that was necessary for precise control of factors such as temperature and the composition of the surrounding atmosphere during the measurements.
Aided by CNBC scientist Michael Gharghouri, Corbin and Cluff first investigated the impact of differing particle sizes in the starting titanium and nickel powders. Results reported in 2014 showed that nickel powders with large particles significantly slowed the formation of pure NiTi. They also showed that one of the unwanted phases—Ti2Ni—persisted throughout the sintering process, and the researchers hypothesized that oxygen was responsible for stabilizing this non-desirable variant.
Corbin and Cluff conducted further investigations at the CNBC to more thoroughly explore the changes that were occurring during the sintering process at temperatures ranging from 500 °C to 1200 °C. One set of experiments compared two grades of titanium powder, each with a different purity level, to observe how varying amounts of oxygen contaminants in the original powder might impact the phases that form during sintering.
The results, published in 2017, definitively showed that oxygen causes the undesirable Ti2Ni phase to persist all the way up to 1200 °C, implying that it is necessary to start with contaminant-free powders in order to achieve pure NiTi alloy via sintering (doi:10.1016/j.intermet.2016.12.001). A second set of experiments, published in January 2018, led to a better understanding of why even small amounts of oxygen are so detrimental to the production of nickel-titanium shape memory alloy (doi:10.1016/j.jallcom.2017.10.272).
Importantly, when taken together with observations from other experimental approaches, these experiments clearly show what phases occur—and at what temperatures they develop or disappear—during sintering. “We’ve cleared up much of the confusion about what’s going on in these materials during sintering,” concludes Corbin. “This is an important contribution to the field because it provides the fundamental knowledge required to optimize the sintering process for inexpensive nickel-titanium shape memory alloys, which are needed for lightweight actuators in cars and airplanes.”