Cobalt-base Ferromagnetic Shape Memory Alloys

Single crystalline and polycrystalline CoxNiyGa100-(x+y), 41<x(Co)<62 and 19.3<y(Ni)<32.7 [at%], Ferromagnetic Shape. Memory Alloys (FMSMAs) have been produced in the range of the Heusler-type composition, Elasto-mechanical properties have been analyzed for the annealed and quenched states, respectively. The mechanical spectroscopy data show the occurrence of martensitic phase transformations with the transition range and characteristics depending on the state and the composition of the alloys. For x(Co)approximate to49+/-1 at%, the Ni/Ga ratio was shown to be in direct relationship with the transition temperature range, from an Ms of -100degreesC for a Ni/Ga approximate to (21/29) to a +150degreesC for a Ni/Ga ratio of about (26/25). For Gaapproximate to27+/-0.4 at%, the Co/Ni ratio is in indirect relationship with the transition temperature, with an Ms of 125 degreesC for a (53/19) ratio to a + 175degreesC for a ratio of about (49/26). Optical and electron microscopy shows that a typical thermoelastic martensitic transformation occurs. The L2(1) Structurbericht parent phase transforms into monoclinic or orthorhombic martensite upon cooling. The formation of a Co-rich phase has been observed for alloys with lower Ga content and is considered to be one of the reasons for the difference in the transformation range for annealed and quenched alloys.