A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at. %) Co–(27–32 at. %) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.

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Magnetic properties and magnetic-field-induced strains (MFIS) have been investigated for off-stoichiometric Ni–Mn–Al Heusler alloys with an ordered L21 structure. A clear martensitic transformation in Ni53Mn25Al22 alloy was revealed below the Curie temperature. In the polycrystalline specimen, an irreversible relative change due to the MFIS was confirmed between the martensite start and finish temperatures Ms and Mf, and a maximum relative length change ΔL/L|7T of about −100 ppm was observed at just above Mf. On the other hand, a large irreversible relative length change of about 1000 ppm has been demonstrated in the magnetic field of 7 T for a single crystal cut from the polycrystalline specimen. A delay of the response of strains against the magnetic field was also confirmed.

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It has recently been reported that the movement of twin (or variant) boundaries, induced by a magnetic field during the martensitic transformation, affects the magnetic properties and shape change in the ferromagnetic NiGaMn shape memory (SM) alloys. [1][2][3][4] These observations suggest that the NiGaMn SM alloys with an L2 1 (Ni 2 GaMn: Heusler) structure have the potential for use as a new type of smart materials whose SM properties can be controlled not only by temperature and stress, but also by a magnetic field. [4] Fig. 1-DSC curves of Ni-22 at. pct Al-28 at. pct Mn specimen aged at This gives rise to the possibility that such effects may be 400 ЊC for 3 days.present in the related Ni-Al-Mn system of alloys, which also exhibit thermoelastic martensitic transformations. While there have been reports of some investigations relating to the martensitic transformations in this system, [5,6,7] very little has been published on the magnetic properties associated temperatures of the as-aged 50Ni-Al-Mn alloys determined by the DSC curves are plotted in Figure 2(a), along with with the martensitic transformations. [8] Very recently, the present authors have detected that in the alloys with composisome results obtained on as-quenched specimens shown in Figure 2(b). The present data obtained on M s temperatures tions near the stoichiometric Ni 2 AlMn, the ferromagnetic L2 1 phase appears on low-temperature aging and it transof the as-quenched specimens is in agreement with previous data reported for the same alloys. [5,8] The M s temperature forms martensitically from B2 to 2M. [9] This suggests that the NiAlMn Heusler alloys could also exhibit unique magof the as-quenched specimens decreases on aging at 400 ЊC by ordering from B2 to L2 1 , and the difference in M s netic and SM properties similar to those of the NiGaMn alloys. It is the purpose of this article to report the results between the as-quenched and as-aged specimens increases with increasing Al content. This effect may be explained of investigations on the magnetic properties of the NiAlMn Heusler alloys.as due to the stabilization of the parent phase on ordering.It is interesting to note that the Curie temperatures of the Single-phase ␤ alloys of the NiAlMn ternary system were prepared in an induction furnace under an argon atmoas-aged specimens are located near the Neel temperatures of the as-quenched specimens. Chernenko [10] has reported sphere by melting pure metals of Ni (99.9 pct), Al (99.7 pct), and Mn (99.9 pct) in appropriate quantities. All the that in the NiGaMn system, the T c and M s temperatures can be normalized as a function of electron concentration cast alloys were solution treated at 1000 ЊC for 72 hours and quenched into ice water. Some alloys were aged at 350 e/a, assuming that the numbers per atom are Ni ϭ 10, Mn ϭ 7, and Ga ϭ 3. This type of normalization on the ЊC or 400 ЊC to obtain an ordered L2 1 structure. The I-H curve for the as-aged Ni-22.5Al-25Mn alloy was deterbasis of the e/a for T c and M s can be extended to the NiAlMn...