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To control the ferrite and martensite microstructure in dual-phase steels, the phase transformation from ferrite (α) to austenite (γ) on heating is essential. For a comprehensive understanding of the γ formation in the early stage of the α → γ phase transformation, the orientation relationships and the boundaries between α and γ grains were investigated using a 3D electron backscatter diffraction analysis. It was found that, when the orientations of the α and γ grains hold the K–S relationship or the N–W relationship, the orientation of the boundary does not correspond to the exact close-packed planes {011}α or {111}γ, but deviates within the vicinity of these crystal planes based on a variety of misorientations of up to 40 deg. Most of the boundary planes with the K–S relationship are parallel to the crystal planes between {011}α and {111}α of the α grain, and {111}γ and {012}γ of the γ grain. Focusing on the crystallographic feature of the boundary planes in austenite phase, the mechanism of the α → γ transformation is discussed.

Different amounts of Cr were added to a metastable β-type Ti–22Nb (at.%) alloy to obtain desirable mechanical properties, including a low Young’s modulus, high strength, and good plasticity. The mechanical properties and microstructural changes were investigated. Cr has a high ability to stabilize the β phase, as well as suppress both α″ martensite and ω phase transformations during quenching and the stress-induced α″ martensite transformation during tension. Solid solution strengthening is scarcely achieved by Cr addition. The changes in mechanical properties can be attributed to the different β stabilities. The Ti–22Nb–(0,1)Cr alloys have metastable β phases and exhibit double yielding phenomena, indicating a stress-induced α″ martensite transformation. The Ti–22Nb–(2,3)Cr alloys with stable β phases exhibit distinct work hardening caused by a {332}β β twinning, which also occurs in the Ti–22Nb–(0,1)Cr alloys, but not in the Ti–22Nb–4Cr alloy. Low Young’s moduli of approximately 60 GPa are obtained for the Ti–22Nb–(1,2)Cr alloys. The Ti–22Nb–2Cr alloy exhibits desirable properties for biomedical applications, including an ultimate tensile strength of approximately 600 MPa and elongation of approximately 20%.

The size effect on the mechanical behaviors of single crystalline Fe-31.2Pd (at.%) micropillars with four different pillar diameters (approximately 2 μm, 1 μm, 500 nm and 400 nm) were studied by compressing in the [001] direction. Both the Young's modulus and yield stress increase with the decrease of pillar diameter. The main defects in the plastically deformed pillars are the {111} deformation twins, which could be covered by high density of dislocations after further plastic deformation in a 400 nm pillar. A repeatable elastic-like strain ~4% was observed for more than 40,000 cycles.

We have investigated the influence of twin boundary properties on the damping behavior of single crystalline Fe-31.2Pd (at%) alloy, which exhibits a weak first-order martensitic transformation. Different static tensile stress was applied by dynamic mechanical analyzer to adjust the twin boundary density and hydrogen was doped to modify the twin boundary mobility. A high damping capacity (tan δ > 0.1) in a wide temperature range from about 140 K to 230 K was observed under a low static stress of about 0.3 MPa. Hydrogen doping further increased the maximum value of the damping capacity by introducing a relaxation tan δ peak. The damping capacity decreased with increasing static stress probably due to the decrease in twin boundary density.

Depending on Pd content, disordered Fe-Pd alloys take various crystal structures: body-centered cubic (BCC), body-centered tetragonal (BCT), face-centered tetragonal (FCT) and face-centered cubic (FCC) structures. We have measured specific heat of Fe-Pd alloys to understand the lattice stability and electronic state of the alloys. At the phase boundary between FCT and FCC phases, electronic specific heat coefficient shows a local maximum and the Debye temperature shows a local minimum. A local maximum of electronic specific heat coefficient also appears in the BCC region with Pd content of near 10 at.%.

Elastocaloric effects in shape memory alloys are usually caused by latent heat associated with the stress-induced martensitic transformation. We report here that the rubber-like behavior of the R-phase in nanocrystalline Ti-50.8Ni (at.%) wires, realized by an aging heat treatment under tensile stress, also contributes a significant elastocaloric effect. The efficiency of the elastocaloric effect caused by the rubber-like behavior is higher than that caused by the stress-induced B2 ↔ B19′ transformations because of the small energy dissipation. The usage of the rubber-like behavior is a method to extend the temperature window of the elastocaloric effect in shape memory alloys.

We have investigated the influence of thermal cycles on martensitic transformation of a Co2Cr(Ga,Si) ferromagnetic Heusler alloy. The as-quenched specimen exhibits successive L21(L)–D022–L21(H) martensitic transformation in the cooling process, which is known as reentrant martensitic transformation. However, heating to 800 K (527 °C) for reverse D022–L21 transformation with a rate of 10 K/min (10 °C/min) stabilizes the parent phase, meaning that the martensitic transformation is suppressed by the thermal cycles. We found precipitate after thermal cycles, and it will be the reason for the stabilization of parent phase.

We have detected an inverse elastocaloric effect in a Ti-51Ni (at%) alloy which contains aligned coherent particles of Ti3Ni4 formed by aging under tensile stress. In this alloy, the temperature of the specimen decreases when stress is applied due to a stress-induced reverse martensitic transformation (from the R-phase to the B2-phase) The temperature decrease caused by this effect is about 1.1 K at 300 K. The internal stress field around the Ti3Ni4 precipitate is responsible for this effect. A further increase in the tensile stress causes a temperature rise due to a conventional elastocaloric effect.

We have determined the easy axis of magnetization of Fe3C (cementite) using a powder specimen prepared by an electrolytic extraction method. The easy axis is determined to be the c-axis (the shortest axis) of the orthorhombic structure with the space group Pnma. The result is different from the one previously reported by neutron diffraction (b-axis). The magnetic energy difference between the easy axis and the average of other two axes is approximately 400 kJ/m3 at 5 K.

We have constructed a time-temperature-transformation (TTT) diagram for the martensitic transformation in a Ti-51.3Ni (at%) alloy. The transformation proceeds when the specimen is held at fixed temperatures between 110 K and 140 K, but not at other temperatures. The time required for the formation of 0.1% of the martensite phase is plotted as a TTT diagram. It exhibits a C-curve with a nose temperature near 130 K. Application of a thermal activation model to the TTT diagram suggests that the size of a nucleus of the martensite phase is about (2.2 nm)3.

The elastocaloric effect under tensile stress along the [001] direction has been investigated in an Fe-31.2Pd (at.%) alloy, which exhibits a weak first-order martensitic transformation at 230 K from a face-centered cubic structure to a face-centered tetragonal (c/a

The solid-state amorphization (SSA) behavior of the intermetallic compound Ti50Ni44Fe6 was investigated using high-voltage electron microscopy. In situ transmission electron microscopy clarified a particular SSA process consisting of the appearance of diffuse scattering during the initial stage of SSA and the subsequent disappearance of the diffuse scattering during irradiation. Irradiation-induced commensurate–incommensurate (C–IC) and IC–B2 transformations occurred before SSA. Both the martensitic transformation (MT) and SSA behaviors were significantly affected by the replacement of Ni by Fe in Ti50Ni50−xFex.

A disordered Fe-31.2Pd (at.%) alloy exhibits a weak first-order martensitic transformation (MT) from a face-centered cubic structure to a face-centered tetragonal structure. We employed a Dynamic Mechanical Analysis (DMA) to understand the transformation behavior. As the temperature decreases, the storage modulus E ′ decreases in the parent phase and increases in the martensite phase. The loss tangent (tan δ ) shows a slight increase when approaching M s (=225 K), suggesting the formation of movable interfaces even above M s . The value of tan δ is nearly proportional to (1 − c/a ) 2 below M s , meaning that highly mobile twinned interfaces are essential for internal friction below M s . The so-called transitory term of tan δ is negligibly small for the MT of this alloy.

We have examined the transformation strain, Δe, in the [0 0 1] direction of an Fe-31.2Pd (at.%) shape memory alloy under compressive stress applied in the same direction. When the stress is absent, the alloy exhibits a cubic to tetragonal martensitic transformation at 230 K with |Δe| of 1.4%. As the stress increases, the transformation temperature increases linearly and |Δe| decreases linearly and vanishes at 40 MPa (280 K). This point is the critical point of this transformation at which the first-order nature disappears, and the critical exponent β is evaluated to be 0.47 ± 0.04.

In typical shape memory alloys, significant elastocaloric effects appear in association with stress-induced martensitic transformations. However, we report here that in an Fe-31.2Pd (at.%) alloy, a significant elastocaloric effect appears due to lattice softening, which causes a weak first-order martensitic transformation at T0 (∼230 K). We found that in a wide temperature range of between 175 K ( T0), the alloy exhibits an adiabatic temperature decrease of more than 1.5 K when a compressive stress of 200 MPa applied in the [0 0 1] direction is removed. The refrigeration capacity calculated in this temperature range is 5 MJ/m3.

Fe 3 Pt with a degree of order of S = 0.75 shows a second-order-like martensitic transformation from a cubic structure to a tetragonal one at approximately 90 K. We found by neutron diffraction that this alloy exhibits a large elastic strain of more than 6% by the application of compressive stress in the [001] direction. A large elastic anisotropy caused by lattice softening in elastic constant C’ , which occurs on approaching the transformation temperature, was found to be responsible for the large elastic strain of Fe 3 Pt.

We have investigated local structures around Ni and Fe atoms by X-ray fluorescence holography (XFH) method in incommensurate (IC) phase of Ti-42Ni-8Fe(at.%) alloy and obtained the results as the followings; (i) The atomic distances between Ni atom and Fe atom and those 2nd nearest neighbor atoms are 0.27 nm and 0.29 nm, respectively. (ii) The atomic image intensities of 1st nearest neighbor atoms around Fe atoms are much weaker than those around Ni atoms. These results indicate that nanometer-sized heterogeneous strain exists and it is the cause of nanometer-sized domain in IC phase.

In this Rapid Communication we report a phenomenon in a disordered interacting electron system. The measurements of structural, magnetic, and transport properties of $\mathrm{FeA}{\mathrm{l}}_{2\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}$ ($0\ensuremath{\le}x\ensuremath{\le}0.5$) show that antiferromagnetic transition in these intermetallic compounds occurs concomitantly with a disorder-induced weak localization of electrons; the temperatures ${T}_{N}$ and ${T}_{m}$, at which antiferromagnetic transition and the weak localization respectively occur, closely track each other as the Ga concentration is varied. The antiferromagnetic transition is confirmed from the magnetic and specific heat measurements, and the occurrence of weak localization is confirmed from the temperature variation of resistivity and magnetoresistance measurements. With increasing Ga concentration, substitutional disorder in the system increases, and the consequent disorder-enhanced magnetic exchange interaction and disorder-induced fluctuations simultaneously drive antiferromagnetic transition and weak localization, respectively, to higher temperatures.

Diffuse satellites appearing in electron diffraction pattern of shape memory Ti–(50 − x )Pd– x Fe (14, 16, 18, 19 and 20, in at.%) alloys have been investigated. The satellites appear in each alloy below T min , where its electrical resistivity shows a local minimum. The positions of satellites are g B2 + 〈 ζ ζ ¯ 0〉 * , where g B2 is a reciprocal lattice vector of the B2-phase. The value of ζ is smaller than 1/5 at T min for all the alloys; it increases with decreasing temperature and decreases with increasing iron content. The value of ζ at T min agrees with the length of the nesting vector previously calculated by the present authors. This result implies that Fermi surface nesting is the origin of diffuse satellites in Ti–(50 − x )Pd– x Fe alloys.

The Cu precipitation 780 MPa grade steel has higher resistance to cold cracking than the conventional steels. To study this, we have investigated the hydrogen evolution characteristics of the Cu steel heat-affected zone (HAZ) by the thermal desorption spectroscopy method and compared with those of conventional 780 MPa steel HAZ. The thermal cycle tests were conducted to obtain the HAZ microstructures, and the peak temperatures were selected from 500 to 1400 °C at intervals of 100 °C. Hydrogen was then charged, and the hydrogen evolution curves were measured at a heating rate of 100 °C/h. The results show that the amounts of hydrogen released from the specimens whose peak temperatures of the thermal cycle tests were 1000 °C or higher are lower, while the results of the conventional steels have the opposite tendency. We numerically calculated the hydrogen evolution curves to determine physical constants, and using these constants, we also calculated the hydrogen concentration profiles of the y-groove weld cracking test and found that the hydrogen concentration of the Cu steel HAZ near the fusion line tends to become lower than those of the conventional steel HAZ. This phenomenon is considered one of the reasons for the higher cold cracking resistance of the Cu steel.

Magnetic behavior of equiatomic FeAl alloy is still not satisfactorily understood. In this work, we studied the magnetic properties of disordered FeAl alloy both experimentally and using first-principles theories and revisited the alloy system in perspective of the inhomogeneity present in the system. After obtaining magnetic exchange interactions from first-principles theories, we carried out Monte-Carlo simulations on special quasi-random structures (SQS) and compared the results with experimental measurements. We tried to understand the plethora of often differing results and explain them in terms of possible inhomogeneities in the system.

We have studied the structure change of an ordered Fe3Pt (degree of order ∼0.75) under a compressive stress applied in the [001] direction by neutron diffraction. In the absence of the stress, the alloy exhibits a weak first order martensitic transformation at 90 K from the L12-type cubic structure to the L60-type tetragonal structure. Under the compressive stress of 100 MPa, the first order nature of the thermally-induced martensitic transformation was undetectable in the temperature range of between 70 K and 270 K. The first order nature of the stress-induced martensitic transformation was also undetectable in the stress range of between 6 MPa and 300 MPa when tested at 120 K and higher temperatures. Under these conditions, the lattice parameters change continuously both in the cooling process and in the stress-applying process. Despite the disappearance of the first order nature of martensitic transformation, a significant stress-induced softening of lattice, which is regarded as a precursor phenomenon of martensitic transformation, was observed between 120 K and 265 K but not at 93 K and 295 K.We have studied the structure change of an ordered Fe3Pt (degree of order ∼0.75) under a compressive stress applied in the [001] direction by neutron diffraction. In the absence of the stress, the alloy exhibits a weak first order martensitic transformation at 90 K from the L12-type cubic structure to the L60-type tetragonal structure. Under the compressive stress of 100 MPa, the first order nature of the thermally-induced martensitic transformation was undetectable in the temperature range of between 70 K and 270 K. The first order nature of the stress-induced martensitic transformation was also undetectable in the stress range of between 6 MPa and 300 MPa when tested at 120 K and higher temperatures. Under these conditions, the lattice parameters change continuously both in the cooling process and in the stress-applying process. Despite the disappearance of the first order nature of martensitic transformation, a significant stress-induced softening of lattice, which is regarded as a precursor phenomenon...

We have investigated the martensite transformation phenomena of a Cr-Ni type weld metal, especially the effects of stress and plastic strain that exist before the martensite transformation start. Tensile specimens machined from the weld metal were firstly heated up to 800 °C to austenitize the specimens and then cooled down. During the cooling, we applied stress and plastic strain before the martensite transformation starts. The results show that the stress increases the M s temperature, while the plastic strain decreases the M s temperature. In addition, the transformation-induced plasticity was also investigated. Finite element method (FEM) analysis was conducted considering these effects, and the results show that these effects on the residual stress and the weld distortion are considerably large. The FEM analysis shows that the compressive residual stress in the weld metal is likely to be decreased by the transformation-induced plasticity. We also conducted hydrogen diffusion analysis for the case that the retained austenite exists in the weld metal by varying the M s temperature in order to consider the appropriate time period to keep a cold cracking test specimen before the crack observation.

An Fe–55Pd (at.%) alloy exhibits a disorder–order transformation from the A1-type disordered cubic phase to the L10-type ordered tetragonal phase. We have investigated the influence of the magnetocrystalline anisotropy energy on the selective formation of a variant of the ordered phase during heat treatment. We found that the strength of the magnetic field required to produce a single variant decreases from approximately 4 T to approximately 0.4 T when the ordering heat-treatment temperature is decreased from 673 to 603 K. The magnetocrystalline anisotropy energy was evaluated at these temperatures as a function of magnetic field. The nucleation ratio between different variants was evaluated through the magnetocrystalline anisotropy. It is revealed that a single variant state is obtained during the growth process when the nucleation ratio of the dominant variant exceeds approximately 1.1.

Magnetocrystalline anisotropy and magnetic field-induced strain were investigated for three tetragonal martensite phases formed in Fe 3 Pt ferromagnetic shape memory alloys. The phases examined are the body-centered tetragonal (bct, c / a S = 0.57, the face-centered tetragonal (fct, c / a S = 0.75 and the fct ( c / a > 1) martensite with S = 0.88. It was found that the c -axis is the easy axis of magnetization for the two fct martensites and the hard axis of magnetization for the bct martensite. The uniaxial magnetocrystalline anisotropy constant, K u , is nearly proportional to |1 − c / a | for the two fct martensites, but the relation is not satisfied for the bct phase. When a magnetic field is applied in the [0 0 1] direction, a large magnetic field-induced strain appears in the two fct martensites but not in the bct martensite. The strain in the field direction is a contraction in the fct martensite with c / a 1, whereas it is an expansion in the fct martensite with c / a > 1. These behaviors are explained by evaluating the magnetic shear stress, which is given by K u divided by the twinning shear.

The tetragonality c / a of FCT martensite phase in an Fe 3 Pt depends on the degree of order S of the L1 2 -type parent phase. As the degree of order increases, the tetragonality c / a changes from being less than 1 ( S = 0.75) to being larger than 1 ( S = 0.88). In this study, we have investigated a martensitic transformation behavior in a single-crystalline Fe 3 Pt with a degree of order which is in between 0.75 and 0.88, and have found the existence of orthorohombic martensite phase in the Fe 3 Pt.

We have investigated martensitic transformation in micrometer-sized single and poly crystalline particles of an Fe–30.5 at.%Ni alloy by their temperature dependences of magnetic susceptibility measurement and X-ray diffraction. Following results are obtained: (1) poly crystalline particles show a “so-called” athermal martensitic transformation and M s temperature decreases from 215 K to 151 K with decreasing particle size from 1000 μm (average grain size is 250 μm) to 18 μm (average grain size is 3.7 μm). The single crystalline particle with its size of 7.6 μm also shows a “so-called” athermal martensitic transformation as in the poly crystalline particles, but its M s temperature decreases drastically ( M s = 77 K). (2) The single crystalline particle with its size of 4.6 μm has no M s temperature and shows a “so-called” isothermal martensitic transformation. However, M s temperature appears again in this particle when we introduce grain boundaries and dislocation in the particle by sintering and/or deforming. Considering these results, we conclude that the suppression of martensitic transformation in the micrometer-sized particles of an Fe–Ni alloy is due to the lack of lattice defects for the nucleation, such as grain boundary and dislocation. Furthermore, the single crystalline particle with its size of 4.6 μm shows the athermal martensitic transformation again under the magnetic field and its chemical driving force of the martensitic transformation is estimated to be about 2.0 kJ/mol.

We have investigated the relation between the negative temperature coefficient (NTC) in electrical resistivity and the formation of the athermal ω phase in Ti–xNb (26 ≤ x ≤ 29 at.%) alloys. The alloys with 26 ≤ x ≤ 28 exhibit NTC in electrical resistivity and a temperature hysteresis appears in the temperature region of NTC. In these alloys, athermal ω phase forms in the temperature range of NTC in resistivity. The alloy with x = 29 also shows NTC but no detectable temperature hysteresis appears. In this alloy, the athermal ω phase does not form, but diffuse satellites appear at g β + 1 / 2 ζ ζ ¯ 0 ∗ , where gβ is a reciprocal lattice vector of the β phase in the temperature range of NTC. These results suggest that NTC in the electrical resistivity in Ti–xNb alloys is related not only to the formation of the athermal ω phase but also to the appearance of diffuse satellites at g β + 1 / 2 ζ ζ ¯ 0 ∗ .

Time dependence of the first order magnetostructural transition in Pd doped B2-type FeRh (Fe0.45Rh0.45Pd0.1 alloy) has been investigated. This alloy exhibits a first order ferro–antiferro magnetostructural transition at 170 K in the cooling process under a low magnetic field (0.1 T). The transition also proceeds by holding at temperatures near 170 K. In addition, we also found that the transition temperature decreases with increasing magnetic field, and the transition is suppressed under a magnetic field of 5 T or higher. Removal of the magnetic field at 4.2 K induces the magnetostructural transition, but a part of the ferromagnetic phase remains at 4.2 K. In the subsequent heating process, the remaining ferromagnetic phase (high temperature phase) transforms to the antiferromagnetic phase (low temperature phase). Such an abnormal transition implies that the first order magnetostructural transition in Fe0.45Rh0.45Pd0.1 alloy is caused by a thermal activation process.

Effect of magnetic field on isothermal martensitic transformation of a sensitized SUS304 stainless steel has been investigated. The TTT diagram of 0.5 vol.% α′ martensite shows a double C-curve with two noses located at about 100 and 200 K. As the strength of magnetic field increases, the nose temperature decreases and the incubation time shortens for the upper part of the double C-curve, that is due to the γ → α′ martensitic transformation. Meanwhile, the nose temperature of the lower part of the C-curve, that is due to the γ → ɛ′ → α′ transformation, does not change under the magnetic field although the incubation time shortens by the application of magnetic field. Morphology of the α′ martensite is almost independent of the strength of magnetic field when holding is made at upper nose (200 K). On the other hand, when holding is made at the lower nose (100 K), the width of the ɛ′ martensite including α′ martensite becomes thinner as the strength of the applied magnetic field increases.

We have investigated twin-like microstructure of the martensite phase in a Ni 55.5 Mn 20.0 Ga 24.5 (at.%) alloy by using transmission electron microscope (TEM). The bright field image of the martensite phase showed striped microstructure composed of two crystals. Assuming that both crystals have orthorhombic structures, we calculated the axial ratios c / a and b / a from the 1 1 1 diffraction patterns by using angles between reflections. As a result, we found that the axial ratios of one crystal are slightly different from those of the other crystal. This result implies that the twin-like (striped) microstructure of Ni 55.5 Mn 20.0 Ga 24.5 is not true twin but is pseudo-twin.

We have investigated the magnetic field direction and temperature dependences of rearrangement of crystallographic domains in an antiferromagnetic pseudo-tetragonal phase in CoO (Neel temperature: 293 K). As a result, we found that the rearrangement occurs at temperatures between 170 and 293 K when a magnetic field is applied along [0 0 1] p (p represents the parent phase) direction, but not when a magnetic field is applied along [1 1 0] p or [1 1 1] p directions. We have calculated the shear stress driven by a magnetic field, τ mag , and compared it with the shear stress required for twinning plane movement, τ req and found that when τ mag is equal to or larger than τ req , the rearrangement of crystallographic domains occurs, and vice versa. This observation is the same as in many ferromagnetic shape memory alloys.

Recently, we reported single variant formation in an Fe–55at.%Pd is certainly realized from a disordered fcc-phase to an ordered L1 0 -phase by heat-treatment under magnetic field. In the present study, we have investigated microstructure evolution during the process of the single variant formation by an X-ray diffraction and an electron microscopy observation. As a result, followings are obtained: size of the ordered particles at the early stage of ordering is about 2 nm and the nucleation ratio of preferable variant, whose easy axis lies in the field direction, is higher than that of other variants. Each of the ordered preferable variant grows by consuming the order variants and finally come together to become a single variant. Based on the observation, a model is proposed for the single variant formation of the ordered L1 0 -phase under magnetic field.