Toshio Saburi

Crystal structure and morphology of the metastable X phase in shape memory Ti-Ni alloys

Abstract The crystal structure and the morphology of the metastable X phase which appears in shape memory Ti-Ni alloys have been investigated by transmission electron microscopy and X-ray diffraction. The crystal structure has been determined to be rhombohedral and to belong to the space group R 3 , where the rhombohedral unit cell contains six titanium atoms and eight nickel atoms. The atom positions and the lattice parameters have been determined. The orientation relationship between the X phase and the matrix, and the habit plane have also been determined.

Temperature and Orientation Dependence of the Yield Strength of Ni3(Al, W)

The temperature and orientation dependence of the mechanical behavior of Ni3(Al, W) was investigated by compression tests on single crystal specimens. It was found that at low temperatures {111} slips operate, whereas at elevated temperatures {100} slips operate, and also that the positive temperature dependence of the yield strength occurs only in the temperature range of the {111} slips. The peak-strength temperature (transition temperature of the slip system) changes largely depending on the orientation of the compression axis. The critical resolved shear stress of the primary (111)[10] slip is also orientation dependent in a similar way to that of Ni3Ga and Ni3Ge; it increases as the stress component of the (010)[10] cross-slip increases.

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.

Electron microscope observation of the early stages of thermoelastic martensitic transformation in a TiNiCu alloy

Abstract In situ observations by electron microscopy were made on nucleation and growth of a thermoelastic martensite in a Ti39.5Ni10.0Cu(at.%) alloy. It has been found that a stress field near defect aggregates plays an important role in the martensite nucleation. The habit plane of the martensite plates in the early stages of formation was determined to be the (334) plane of the parent cubic lattice by trace analysis. The evperimentally determined habit plane was in good agreement with the theoretically predicted one. The atomic configuration along the habit plane was considered on the basis of the evperimentally determined lattice deformation. It has been found that the macroscopic (334) habit plane is composed of (112) steps.

Influence of thermal annealing on the martensitic transitions in Ni–Ti shape memory alloys

We report mainly on resistance R(T) measurements between 4.2 and 1300 K on NixTi100−x alloys, with 51<x<54.5at%, quenched from different annealing temperatures TA to room temperature. When quenched from the B2-phase stability range (TA=1273 K), alloys with 51<x<54 show an increase in R(T) with decreasing T below 300 K. Subsequent annealing at TA=653 K (1 h) and quenching leads to a reduction of the R(T) anomaly below 320 K and the occurence of a martensitic transition (MT) from the B2- to the R-phase, with TR=310 K, rather independent of x. After annealing at 723 K (1 h) and 823 K (1 h), respectively, two-step MTs occur from B2 to R and subsequently to B19′, with MS(B19′) depending on x and TA. After annealing at 923 K or higher TA, MTs cannot be found anymore, and the R(T) behaviour is similar to that after quenching from 1273 K. Studies of R(T) at high T on samples quenched from 1273 K reveal the occurence of mainly two annealing stages. The first one at around 500 K marks structural changes inducing the martensitic phases at lower temperatures. The second one at about 900 K marks the formation of the B2-phase and the disappearance of other phases triggering the MT. The R(T) results are compared with the thermal expansion a(T) and X-ray investigations. The structural phase diagram of Ni–Ti around NiTi is discussed.

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.

Effect of Magnetic Field and Hydrostatic Pressure on Martensitic Transformation and Its Kinetics

We performed recent studies on the effects of magnetic field and hydrostatic pressure on martensitic transformations in some ferrous and nonferrous alloys. The studies clarified the effects of magnetic field and hydrostatic pressure on martensitic transformation start temperature, the nature of magnetoelastic martensitic transformation and the morphology of martensites and transformation kinetics of athermal and isothermal transformations. Transformation start temperatures of all ferrous alloys examined increase with increasing magnetic field, but those of nonferrous alloys, such as Ti–Ni and Cu–Al–Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu–Al–Ni alloys. The magnetic field and hydrostatic pressure dependences of the martensitic start temperature are in good agreement with those calculated by our proposed equations. While investigations in the work on the ferrous Fe–Ni–Co–Ti shape memory alloy, we found that magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in the specimen of Fe–Ni alloy single crystal. We further found that the isothermal process in Fe–Ni–Mn alloy changes to the athermal one under magnetic field and the athermal process changes to the isothermal one under hydrostatic pressure. Based on these facts, a phenomenological theory was constructed, which unifies the two transformation processes.

Anti-phase domains and dislocation configurations in the Fe-13 at. % Si alloy

Abstract Transmission electron microscopic observations have been made on the thermally produced anti-phase domain structures and the dislocation configurations associated with ordering in the Fe-13 at. % Si alloy. Dislocations have been found to move as pairs of ordinary dislocations bound together by an anti-phase boundary of the (¼)a 0′ 〈111〉 type on {110}, {112} and {123} planes. The anti-phase boundaries of the (1/2)a 0′ 〈100〉 type very likely left behind dislocation pairs have also been observed.