Cobalt-base ferromagnetic shape memory alloys

Single crystalline and polycrystalline CoxNiyGa100-(x+y), 41<xCo<62 and 19.3<xNi<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 xCo≈49±1 at%, the Ni/Ga ratio was shown to be in direct relationship with the transition temperature range, from an Ms of –100 C for a Ni/Ga ≈ (21/29) to a +150 C for a Ni/Ga ratio of about (26/25). For xGa≈26.3±1 at%, the Co/Ni ratio is in indirect relationship with the transition temperature, with an Ms of – 125 C for a (53/19) ratio to a + 175 C for a ratio of about (49/26). Optical and electron microscopy shows that a typical thermoelastic martensitic transformation occurs. The B2 (L21 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.