Energy absorption in Ni-Mn-Ga/ polymer composites

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 139-143).In recent years Ni-Mn-Ga has attracted considerable attention as a new kind of actuator material. Off-stoichiometric single crystals of Ni2MnGa can regularly exhibit 6% strain in tetragonal martensites and orthorhombic martensites have shown up to 10% strain when subjected to a magnetic field. These crystals are brittle and the production of single crystals can be quite costly. Terfenol-D, a commercially available giant-magnetostrictive material, suffers from some of the same drawbacks. It was found that composite materials made from Terfenol-D particles in a polymeric matrix could solve the issue of the brittleness while retaining a large fraction of the strain output of the alloy. At first glance a similar approach could be used to solve the brittleness issue of Ni-Mn-Ga, but the low blocking force of these alloys reduces the chances of achieving a Ni-Mn-Ga/polymer composite actuator. However, the stress-strain loops for Ni-Mn-Ga show a large mechanical hysteresis. This ability to dissipate energy makes this alloys very desirable for damping applications, and by putting particles of Ni-Mn-Ga in a composite, their brittleness becomes less of an issue.(cont.) It is shown that by curing Ni-Mn-Ga/polymer composites under a magnetic field it is possible to align the particles in chains and to orient them so they will respond to a uniaxial load. The magnetic measurements show that there are twin boundaries in the particles that can be moved by an external stress. Stress-induced twin boundary motion in the particles is confirmed more directly by x-ray diffraction measurements, transmission electron microscope micrographs, and scanning electron microscope micrographs. Finally we demonstrate the ability of the Ni-Mn-Ga/polymercomposites to dissipate mechanical energy when subjected to cyclic loads. The Ni-Mn-Ga/polymercomposites can dissipate more than 70% of the energy they are given in every cycle, while pure polymer, Fe-filled and Terfenol-filled control samples dissipate less than 50% of the input energy in every cycle. The additional loss in these composites is shown to be due to the motion of twin boundaries. Simple numerical models reproduce the cyclic stress-strain behavior of the composites and explain non-conventional features observed in the Ni-Mn-Ga composites.by Jorge Feuchtwanger.Ph.D