Evaluation of ferromagnetic shape‐memory alloys by the extended Hückel method

Abstract

Ferromagnetic shape-memory alloy (SMA) are powerful candidates as actuators, pressure sensors, magnetic sensors, etc. Magnetic-field-induced strain has been observed in many ferromagnetic SMA. The magnetic-fieldinduced strain is a reversible transformation in the martensite phase with the magnetic field. We have investigated the property of the ferromagnetic shape-memory materials by the extended Hückel method, and estimated the ferromagnetic shape-memory of Fe-Pt and Fe-Pd alloys at high temperatures. We used two physical quantities, i.e. cohesive energy and energy fluctuation, to measure the stability of the materials. On the basis of the cohesive energy and energy fluctuation, we discuss the characteristics of ferromagnetic SMA, in which the energy fluctuation is a measure of thermal stability of the metals and/or alloys. The martensite structure is unstable, which means that the energy fluctuation has to be controlled to a small value to keep the martensite phase. Furthermore, it is estimated that the energy fluctuation is associated with the Curie temperature. The Curie temperature is an essential parameter for ferromagnetic materials. From the discussion presented above, we can propose the following: (i) Alloys possessing a low cohesive energy are associated with a high mobility of atoms and are suitable for ferromagnetic shape-memory materials; (ii) Alloys showing a low energy fluctuation show ferromagnetic shape-memory and are favored for use as memory devices. We found that I (iodine) is the best dopant for Fe-Pt ferromagnetic SMA, and Tc (technetium) is the best dopant for Fe-Pd ferromagnetic SMA.

Morphology and crystallographic phase of V–C particles formed in Fe–Cr–Ni–V–C alloys

“…Besides the experimental examinations described here, we have also carried out the extended Hueckel molecular orbital (EHMO) calculations12,14 with a cluster model comprising 87 atoms and obtained results helpful in understanding the nature of the chemical bonds, which form in VC carbides. Although the EHMO calculation is categorised into an empirical method, this sort of calculations has been successfully applied in various fields and provided useful information on the structures and properties also in alloys 15 – 22. In analysing these calculations, we consider the following three parameters.…”

“…Although the EHMO calculation is categorised into an empirical method, this sort of calculations has been successfully applied in various fields and provided useful information on the structures and properties also in alloys. [15][16][17][18][19][20][21][22] In analysing these calculations, we consider the following three parameters. The first is the cohesive energy, which gives a measure of the strength of the chemical bond.…”

Stability, electronic, magnetic and pressure effect of half-Heusler alloys CNaCa and SiNaCa: A first-principles study

“…It is also an indicator of the mobility of atoms of the alloy, which means that the atoms in the alloys with low cohesive energies are more mobile than those in alloys of high cohesive energies. Therefore, the alloys of low cohesive energy are suitable for shape memory materials [34,35]. For the studied alloys, the estimated low cohesive energy does not only confirm the physical stability but also implies the both alloys are suitable as shape memory alloy (see Table 1).…”