Tomoyuki Kakeshita

Giant magnetostriction in an ordered Fe3Pt single crystal exhibiting a martensitic transformation

Magnetostriction measurements have been made in an ordered Fe3Pt single crystal with degree of order of about 0.8, which exhibits a cubic-tetragonal martensitic transformation at 97 K. The specimen was cooled down to 4.2 K without magnetic field, and then a magnetic field of 4 T is applied to the specimen along 〈001〉 at 4.2 K and removed. As a result, a reversible giant magnetostriction of about 0.5% is observed. This reversible magnetostriction will be caused by the rearrangement of crystallographic domains, being three times as large as that of Terfenol-D (Fe2DyxTb1−x: typical magnetostrictive materials).

Composition dependence of magnetic field-induced martensitic transformations in FeNi alloys☆

Abstract The composition dependence of magnetic field-induced martensitic transformations in FeNi alloys has been examined by carrying out magnetization measurements and optical microscopy for Fe-29.9 and −32.5 at.% Ni alloys and the results are compared with those for a previously examined Fe-31.7at.% Ni alloy. Temperature dependence of critical strength of magnetic field to induce the martensitic transformation in the present two alloys shows the same tendency as in the previous alloy, exhibiting two straight lines bent at a temperature characteristic of each alloy. However, the slope of the temperature dependence of critical strength becomes smaller in order of the 31.7, 29.9 and 32.5 at.% Ni compositions. That is, the influence of magnetic field on the increase of M s temperature is most effective in the Fe-31.7 at.% Ni alloy. The amount of magnetic field-induced martensite in the Fe-29.9 at.% Ni alloy is almost constant without regard to maximum strength of the applied magnetic field if ΔT = ( T − M s ) is kept constant, as in the previous Fe-31.7 at.% Ni alloy, but that in the 32.5 at.% Ni alloy increases with maximum strength of the applied magnetic field for any ΔT . The morphology of magnetic field-induced martensite (including internal structures) is the same as that of thermally-induced martensite in each alloy irrespective of ΔT and strength of the magnetic field, and therefore the morphology is independent of the formation temperature. It is proposed from a thermodynamical consideration that the effect of a magnetic field on the martensitic transformations in FeNi alloys is due to not only the Zeeman effect but also to high field susceptibility and forced volume magnetostriction effects.

Magnetic field-induced strain in iron-based ferromagnetic shape memory alloys

Magnetic field-induced strain (MFIS) along the [001]P (the symbol “P” represents parent phase) direction due to the conversion of martensite variants has been investigated for a disordered Fe–31.2Pd(at. %) single crystal and an ordered Fe3Pt single crystal exhibiting martensitic transformations from cubic phases to tetragonal phases at 85 and 230 K, respectively. The tetragonality c/a of Fe–31.2Pd at 77 K is 0.940 and that of Fe3Pt at 14 K is 0.945. When magnetic field is applied to the martensite phase along the [001]P direction, the specimen expands along the field direction for Fe–31.2Pd and contracts for Fe3Pt, suggesting that the c axis is the hard axis of magnetizaion for Fe–31.2Pd and is the easy axis for Fe3Pt. The conversion of variants by magnetic field is almost perfect for Fe–31.2Pd and is not perfect for Fe3Pt. The recoverable strain in the field removing process is small for Fe–31.2Pd and is about 0.6% for Fe3Pt. In the cooling process under magnetic field of 3.2 MA/m, the fraction of prefer...

Grain size effect of electro-plated tin coatings on whisker growth

Tin and tin-lead coatings electro-plated in various solutions have been observed by means of high voltage electron microscopy, and the grain size effect of the coatings on whisker growth has been examined. As a result, it was found that the tin and tin-lead coatings from which whiskers hardly grew consisted of well-polygonized grains which were a few micrometre in size, and that the tin coatings from which whiskers easily grew consisted of irregular-shaped grains which were a few tenths of a micrometre in size. The irregularshaped grains contained dislocation rings which might be formed by clustering of vacancies or interstitial atoms upon electro-plating.

Significant elastocaloric effect in a Fe-31.2Pd (at. %) single crystal

A significant elastocaloric effect was detected in a Fe-31.2Pd (at. %) single crystal when a compressive stress was applied in the [001]P direction of the parent phase at temperatures of 240-290 K. The temperature decrease caused by the removal of adiabatic stress was approximately 2 K and the refrigeration capacity was calculated to be 2 MJ/m3 under a stress of 100 MPa. This significant elastocaloric effect over a wide temperature range was caused by the remarkable temperature dependence of the Youngs modulus in the [001]P direction.

Magnetic and martensitic transformations in Ni50AlxMn50−x alloys

Abstract The Ni–Al shape memory alloy system is well known by the unique characteristics of phonon-softening and tweed, which are closely related to the B 2-14 M [(5 2 ) 2 ] martensitic transformation. If Mn is added to the alloy, additional martensite appears such as 10 M [(3 2 ) 2 ] long period stacking order structure. The magnetic and martensitic transformations in the Ni 50 Al x Mn 50− x alloys were investigated in order to understand their relations. This was accomplished by extensively utilizing various techniques such as magnetic susceptibility and magnetization measurements, transmission electron microscopy, differential scanning calorimetry (DSC) and electrical resistivity measurements. These investigations uncovered strange successive magnetic transformations (antiferromagnetic to spin-glass state), which were examined further in relation to their martensitic counterparts.

Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti–(50 − x)Ni–xFe (x = 2, 4, 6, 8 in at.%) alloys. We found that a Ti–44Ni–6Fe alloy exhibits a second-order-like incommensurate–commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3⟨ 110⟩ as the temperature decreases and lock into the commensurate position at 180 K. The commensurate phase is not expanded along one of the ⟨ 111⟩ directions, unlike the R-phase formed by a first-order transformation in Ti–48Ni–2Fe and Ti–46Ni–4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti–44Ni–6Fe alloy. In the case of a Ti–42Ni–8Fe alloy, the incommensurate state remains even at 19 K.

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.

Rearrangement of variants in Ni2MnGa under magnetic field

abstract Magnetic field-induced strain which appears in association with rearrangement of variants in a stoichiometric Ni2MnGa single crystal exhibiting a martensitic transformation at 202 K has been investigated. The tetragonality of the martensite phase decreases slightly as temperature decreases and its value at 77 K is about 0.940. When the field is applied along the [001]P (‘P’ stands for the parent phase) direction after cooling down to 77 K under zero magnetic field, the specimen contracts along this direction. In association with this contraction, the fraction of the variant whose c axis (easy axis of magnetization) lies along the field direction increases and reaches about 100%. When the specimen is cooled under magnetic field of 3.2 MA/m applied along [001]P, this fraction reaches 100% at the martensitic transformation temperature of 202 K. The energy dissipated due to rearrangement of variants by magnetic field is nearly the same as that obtained from its Stress–strain curve. The maximum shear stress by the magnetic field acting on the twinning plane is evaluated to be about 2.5 MPa from its uniaxial magnetocrystalline anisotropy constant (310 kJ/m3 at 77 K). This is an adequate value because it is larger than the shear stress required for the rearrangement of variants (1.2–2.2 MPa) obtained by compressive tests.

Disorder and strain-induced complexity in functional materials

Ferroics and multiferroics.- Principles of complexity in materials physics.- Understanding glassy phenomena in materials.- Soft electronic matter.- Hysteresis and avalanches.- High resolution visualization techniques: Structural aspects.- High resolution visualization techniques: Magnetic aspects.- Strain glasses and martensites.- Defects in ferroelectrics.- First principle calculations.- Nanoscale strain textures and interfaces: Magnetic Martensites.- Neutron scattering and shuffle based transitions: Precursor phenomena in magnetic materials.- Magnetostructural coupling and magnetocaloric properties in Heusler shape-memory alloys.- Ni-Mn-X Heusler materials.- Precusor nanoscale textures in martensites and magnetic materials.- Conclusion and future directions.