Fabrication and characterization of Ni-Mn-Ga ferromagnetic shape-memory alloy composites

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002.Includes bibliographical references (leaves 46-48).Ferromagnetic shape-memory alloys (FSMAs) are a recently-developed class of active materials which show large extensional strains when a magnetic field is applied. Shear strains of 6% have been observed at room temperature in martensitic Ni-Mn-Ga single crystals. The strain effect in Ni-Mn-Ga FSMAs is a result of twin boundary motion in the martensite phase, and can be induced by either field or stress. Most Ni-Mn-Ga FSMA research so far has focused on actuation in single crystals. However, the mechanical loss inherent to twin boundary motion also makes this material attractive for energy absorption/vibration damping applications. This thesis describes the preliminary investigation of FSMA/polymer composites for eventual use in vibration damping, and should serve as a stepping stone toward further studies. The research moved through four stages: suction-casting polycrystalline Ni-Mn-Ga alloys with suitable target compositions, processing the polycrystalline material into powder, fabricating FSMA/polymer composites, and preliminary characterization of the stress-strain behavior of these composites. Suction casting produced three polycrystalline alloys which all showed significant variance from the target compositions as well as a high degree of compositional inhomogeneity. From the composition, structural analysis, and magnetic characterization, it was determined that Alloy 1 was martensite at room temperature, while Alloys 2 and 3 were austenitic. Alloy 3 showed a martensite transition temperature around 10° C. While only one of the three alloys shows a majority of martensite at room temperature, it is likely that the powders made from the polycrystalline material cover a wide range of compositions, so results concerning the structure of the powders reflect only the major phase present. The powders were mixed with urethane polymer at powder volume fractions of 10%, 20%, and 30%, followed by curing in a 4.2 kOe field to align the particles. Scanning electron microscopy and magnetic characterization confirmed the alignment of particles into chains within the polymer matrix. The dynamic stress-strain behavior of composites was characterized for low stresses at various frequencies. The static stress-strain behavior of the composites under compressive loading to stresses of 10 MPa was also characterized. In the FSMA/polymer composite samples, the first compressive loading test gave a stress/strain curve which is linear with one modulus up to a threshold stress. Beyond this threshold stress, the curve is linear but with a smaller modulus. Successive compressive stress-strain curves exhibited a linear stress/strain relationship with a modulus value between those of the two regions in the first test, with some hysteresis in the stress-strain response present between the first and second tests. A urethane sample and a urethane/20% volume fraction Al powder composite, by comparison, showed only linear stress-strain behavior with no significant changes between the first and second compressive loading tests. It is likely that the observed stress-strain behavior of the FSMA composites derives from stress-induced twin boundary motion in the martensite phase present.by Robin H.C. Ivester.S.B