The Effects of Composition and Aging on the Martensite and Magnetic Transformations in Ni-Fe-Ga Ferromagnetic Shape Memory Alloys

The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X₂YZ) were used to investigate how the martensite and magnetic transitions change with alloy composition and isothermal aging above and below the B2/L2₁ ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 and 35 K, respectively. Aging a Ni₅₃Fe₁₉Ga₂₈ alloy for 3600 seconds at various temperatures shifts the martensite start temperature and the Curie temperature by almost 70 K. Transmission electron microscopy investigations were conducted on the aged Ni₅₃Fe₁₉Ga₂₈ alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and aging treatments. This article is based on a presentation made in the symposium “Phase Transformations in Magnetic Materials: Magnetic Shape Memory Alloys which occurred March 14, 2006, during the TMS Spring Meeting in San Antonio, TX, under the auspices of the ASMI/MPMD-Phase Transformations, EMPMD/SMD-Chemistry & Physics of Materials, and EMPMDNanomaterials Committees."