Galfenol: A Disruptive Technology

Galfenol power generator

By Marilyn Wun-Fogle and Dr. James B. Restorff

In late 1999, researchers at the Naval Surface Warfare Center Carderock Division invented an exciting new smart material: Galfenol, an iron-gallium (Fe-Ga) alloy system. Smart materials are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, or electric or magnetic fields. Galfenol is a “magnetostrictive” material with a combination of properties unmatched by any existing smart material. Magnetostriction is a process whereby some materials alter their physical dimensions when a magnetic field is applied to them. It is important because it offers an avenue to convert electrical energy into mechanical energy, similar to the more familiar electric motor. These materials also can be used in the inverse manner and act as sensors.

Magnetostriction is not new. It was discovered in 1847 when James Prescott Joule measured the change in length of an iron rod when it was placed in a magnetic field. In some applications such as magnetic recording or transformers, magnetostriction is considered to be highly undesirable because it leads to audible noise and energy loss. One person’s problem, however, is another person’s opportunity; materials with significant magnetostrictions have proven to be technologically valuable. In World War II, the United States and its allies used nickel in sonar transducers. Magnetostrictive materials were so crucial to the war effort that the Japanese, for whom nickel was unavailable, developed a new iron-aluminum (Fe-Al) magnetostrictive alloy to perform the same function.

Funded by the In-House Laboratory Independent Research program and the Office of Naval Research (ONR), the magnetic materials group at Carderock began a search for materials with room-temperature magnetostrictions larger than that provided by nickel and Fe-Al in the early 1970s. This culminated in 1979 with the discovery of the technologically useful giant magnetostrictive alloy Terfenol-D (Tb0.3Dy0.7Fe1.98), which has a useful strain of about 1,800 parts per million. Magnetostrictive materials can produce very high forces, but the range of motion is small. A 2.5-inchdiameter rod of Terfenol-D that is 10 inches long, for example, can lift an 80,000-pound object (such as a fully loaded tractor trailer truck) 0.018 inches with the application of a modest magnetic field. These materials use low voltages and are generally rugged, impervious to adverse environmental conditions, and highly reliable. Terfenol-D is now commercially available for transducer applications.

In 1998, Carderock began a search for another magnetostrictive material that was less expensive, had a high strain, and could support substantial amounts of both compressive and tensile force. The search was prompted by the brittleness of Terfenol-D and its ceramic counterparts. These materials cannot be exposed to tensile stresses during either operation or handling. This search ended successfully with the discovery of Galfenol in collaboration with Ames Laboratory. Galfenol is the second room-temperature magnetostrictive material with large magnetostriction discovered by the Carderock group. A U.S. patent for Galfenol was issued in November 2012.

The Galfenol alloys fill a void in the spectrum of smart materials. They are mechanically tough and have saturation magnetostrictions as high as 400 parts per million in single crystal form and 300 parts per million in the more easily produced, highly textured polycrystalline form. In addition, these alloys can sustain approximately 350 megapascals of tensile stress and are stable over a wide temperature range of minus 450 degrees Fahrenheit to above 300 degrees Fahrenheit. Galfenol alloys can be machined and welded with conventional metalworking technology. The full magnetostriction can be accomplished with an easily obtainable magnetic field. Galfenol alloys also can be stress-annealed by heating with a compressive force. This eliminates the need to apply an external compressive stress when the material is used in a device and it simplifies device design. Stress-annealed Galfenol is also magnetostrictively active under a tensile load that is not attainable with either Terfenol-D or piezoelectric ceramic materials.

“The magnetic materials group at Carderock has once again come up with an amazing new material,” said Jan Lindberg, an ONR science officer. “Galfenol is a new active transduction material that both answers many current needs and challenges transducer designers to discover new mechanisms that, before the advent of the material, could not even be imagined. It is truly a disruptive technology because it is simple yet complex.”

Galfenol is scientifically as well as technologically interesting. Unlike previous active materials, the physical mechanism that generates the magnetostriction is not well understood. In 2006, a five-year, multi-university research initiative led by the University of Maryland was awarded by ONR to investigate the entire class of alloys related to Galfenol, to advance strategies for processing of structural magnetostrictive alloys, and to demonstrate heretofore unachievable actuation and sensing capabilities for these alloys in critical Navy applications. The research goals focused on understanding the fundamental mechanisms of magnetostriction in alloys such as Galfenol, learning how to optimize alloy fabrication and processing at the nano- and macroscale, and developing proof-of-concept devices and systems that demonstrate novel sensing and actuation capabilities. The systems investigated were compact, highly sensitive, and shock-tolerant sonar sensors, load-bearing active elements for shock and vibration mitigation, energy harvesting devices, and nanowire-based artificial cilia sensors for underwater acoustic sensors and communication systems.

The initiative’s objective was to accelerate the development of the next generation of structural iron-based magnetostrictive alloys through fundamental science and engineering studies that focused on three key research areas:

-The modeling needed to understand where the magnetostriction in Fe-Ga alloys comes from, how mechanical and/or magnetostrictive performance might be improved through the addition of ternary elements to these alloys, and to use this understanding to identify alloys of different compositions that exhibit similar or even more desirable attributes

-Investigations into the processes for making alloys with desired magnetomechanical performance at the nanoscale, microscale, and macroscale

-The development and use of models for design, building, and testing of prototype hardware to demonstrate and take advantage of the novel capabilities of these unique alloys in sensor, actuator, and energy harvesting systems.

The initiative investigated a broad range of fundamental issues focused on the development of the nextgeneration, structural, iron-based magnetostrictive alloys. Eight partner institutions (University of Maryland, University of Minnesota, Ohio State University, Pennsylvania State University, Iowa State University, Virginia Polytechnic Institute and State University, University of California at Irvine, and Rutgers University) participated. More than 125 journal publications, 16 doctoral degrees, and eight master of science degrees resulted from this project.

Carderock also has been engaged in a cooperative research and development agreement with industry partner Etrema Products for further development and widespread use of Galfenol. For more than a decade, Carderock, Ames Laboratory, and Etrema Products have collaborated to design the alloy, develop production processes, and perfect methods of producing the material in large solid form, rolled sheets, and wires in an effort to shorten the usual 20-year time frame between the discovery and commercial/military use of a new material. The diversity of forms allows Galfenol-based parts to be used in a variety of new applications, both commercial and military.

ONR supported the magnetic materials group’s efforts in two initiatives: the Naval International Cooperative Opportunities Program (NICOP) and the Technical Cooperation Program (TTCP). Under NICOP, the Carderock group worked with Mechano Transformer Corporation in Japan and the University of Tokyo on Galfenol uses as a microactuator in a microspeaker, microinjector, and other components. NICOP involvement was extended to the University of Kanazawa in Japan, where they are researching the use of Galfenol for energy harvesting applications. Under the TTCP, Carderock collaborated on an operating assignment with the defense laboratories in Canada, Australia, and the United Kingdom, with an eye toward using these materials in defense applications. The success of this assignment resulted in a TTCP achievement award for significant advances in the development and exploitation of novel magnetostrictive and magnetic shape memory alloy technologies for defense applications—laying the groundwork for developing international acceptance standards and contributing to future substantial enhancement of military capabilities of the TTCP member nations.

Since the discovery of Galfenol in 1999, it has advanced from being a curiosity in a laboratory to a material being investigated worldwide by a variety of university and government laboratories. In the United States, efforts to commercialize the material are well under way and showing substantial progress. Continued development of magnetostrictive materials will result in their optimization for as-yet-untapped potential in structural applications: vibration sensors, vibration control actuators, and energy-harvesting devices and systems. We believe this material has a bright future and will give designers a material with previously unavailable performance.

About the authors:

Marilyn Wun-Fogle and Dr. Restorff are scientists in the metallurgy and fasteners branch at the Naval Surface Warfare Center Carderock Division.