Tomoyuki Kakeshita

Effects of hydrostatic pressure and magnetic field on martensitic transformations

Abstract Martensitic transformations are extensively influenced by external fields, such as temperature and uniaxial stress, modifying transformation temperatures, crystallography and amount and morphology of the product martensites. Therefore, in order to clarify the effect of external fields on martensitic transformations it is very important to understand the essential problems of the transformation, such as thermodynamics, kinetics and the origin of the transformation, whose information is naturally useful for those technological application fields in which the transformation is used. Hydrostatic pressure and magnetic fields are also important external fields because there exist some significant differences in atomic volume and magnetic moment between the parent and martensitic states. In the present paper, therefore, we summarize the effects of hydrostatic pressure and magnetic field on martensitic transformations in some ferrous and non-ferrous alloys by referring to past and recent works made by our group and many other researchers. Especially, we discuss the following six topics: (i) the effect of hydrostatic pressure on the martensitic transformation start temperature and the validity of a new equation proposed by our group to evaluate the relation between M s and hydrostatic pressure; (ii) the morphology of martensite induced by a hydrostatic pressure; (iii) the effect of a magnetic field on the martensitic transformation start temperature, M s , and the validity of another equation proposed by our group to evaluate the relation between M s and the critical magnetic field, H c , for inducing the martensitic transformation; (iv) the effect of a magnetic field on the magnetoelastic martensitic transformation in an ausaged Fe–Ni–Co–Ti shape memory alloy, which occurs only while a magnetic field is applied and disappears when the magnetic field is removed; (v) the effect of a magnetic field on the morphology and arrangement of martensite plates in Fe–Ni alloy single crystals; (vi) the effects of hydrostatic pressure and magnetic field on the martensitic transformation process.

Negative Temperature Coefficient of Electrical Resistivity in B2-Type Ti-Ni Alloys.

A negative temperature coefficient of electrical resistivity (TCR) has been observed in Ti50-XNi50+X (at.%; X=1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5) in the temperature range between 20 and 350 K. The resistivity vs temperature curve has no hysteresis and the relative resistivity at 20 K (ρ20 K/ρ350 K) decreases with increasing Ni content. X-ray diffraction and magnetic susceptibility measurements show that a negative TCR is not caused by structural phase transition nor by magnetic transition. The Debye temperature of Ti48Ni52, obtained from the analysis of specific heat measurements, is quite low (217 K) and its spin relaxation process has a time duration between 400 µs and 20 ms, suggesting that some lattice instability exists. Based on these results, the origin of a negative TCR is discussed.

Martensitic transformations in some Ferrous and non-ferrous alloys under magnetic field and hydrostatic pressure

Abstract Martensitic transformations are extensively influenced by external fields, such as temperature and uniaxial stress, in transformation temperatures, crystallography and amount and morphology of the product martensites. Therefore, to clarify the effect of external fields on martensitic transformations it is very important to understand the essential problems of the transformation, such as thermodynamics, kinetics and the origin of the transformation, whose information is naturally useful in technological applications using the transformation. Magnetic field and hydrostatic pressure are important in such external fields because there exist some significant differences in magnetic moment and atomic volume between the parent and martensitic states. In the present paper, therefore, we summarizz the effects of magnetic field and hydrostatic pressure on martensitic transfonnations in some ferrous and non-ferrous alloys by referring to past and recent works made by our group and many other researchers. Th...

Electronic structure and stability of intermetallic compounds in the Ti–Ni System

Abstract For the systematical understanding of the stability of martensitic phases and precipitates which appear in Ti–Ni shape memory alloys, we made a first principle electronic structure calculation of them by using the tight-binding linear muffin-tin orbital method in the atomic sphere approximation (TB-LMTO-ASA). The obtained results are the following: (1)the total electronic density of state (DOS) at the Fermi energy D ( e F ) of TiNi decreases as the successive B2→R→B19′ transformation proceeds; (2) when the number of valence electrons increases, D ( e F ) of the R-phase increases but that of the B19-phase decreases; (3) D ( e F ) of Ti 3 Ni 4 decreases as the number of valence electrons decreases and that of TiNi 2 decreases as the number of valence electrons increases. By comparing these results with experimentally obtained results, we derived a criterion that phases appearing in Ti–Ni system tend to become stable at 0 K as D( e F ) decreases.

Time-dependent nature and origin of displacive transformation

Abstract We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni and Fe–Ni–Cr alloys under pulsed and static magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. Also, we have calculated electronic structures of B2 and ζ′2 phases in AuCd by FLAPW and/or LAPW methods in order to understand the origin of B2–ζ′2 transformation. The following results were obtained. (i) The two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process under a hydrostatic pressure and the isothermal process changes to the athermal one under a magnetic field. (ii) These findings of (i) can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group. (iii) The calculation of the generalized susceptibility, x(q), for the B2 phase of AuCd shows that there exists a nesting vector of near 1/3<110>2Π/a as in the B2 phase of TiNi calculated previously. The density of states at the Fermi energy of the ζ′2 phase is lower than that of the B2 phase, which is similar to the case of B2–R transformation in TiNi previously calculated.

Kinetics of martensitic transformations in some ferrous and non-ferrous alloys

Abstract The influence of hydrostatic pressure and magnetic fields on the athermal and the isothermal martensitic transformations in Fe-29.9at.% Ni, Fe-31.7at.% Ni, Fe-32.3 at.% Ni, Cu-29.1 at.% Al-3.6 at.% Ni and Fe-24.0 at.% Ni-4.0 at.% Mn alloys have been examined in order to clarify the difference between the athermal and isothermal transformation processes. The following results were obtained. Firstly, a martensitic transformation occurs after some incubation time during isothermal holding at a temperature higher than the transformation start temperature Ms in zero external field and no hydrostatic pressure for Fe-Ni and Cu-Al-Ni alloys which originally exhibit athermal martensitic transformations. Secondly, athermal martensitic transformations in Fe-Ni alloys change to isothermal transformations under hydrostatic pressure. Thirdly, in the isothermal martensitic transformation in an Fe-Ni-Mn alloy, a static magnetic field lowers the nose temperature and increases the incubation time reauired for the formation of martensite and a hydrostatic pressure raises the nose temperature and reduces the incubation time. These results suggest that the two transformation processes are closely related to each other as has been discussed on the basis of a phenomenological theory previously presented.

Effects of magnetic field on martensitic transformations

Magnetic field is one of intensive variables which influence phase transformations in solids. In the present study we will show how the characteristics of martensitic transformations are influenced by magnetic field, such as transformation temperature, morphology and kinetics.