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A selective switching device utilizing a phase-change material was investigated. In this work, we present a new concept to realize serially a selective switching and memory operation in a multiple phase-change memory with only phase-change materials without any semiconductor switching device. A phase-change material for selective switching can be expected to have a higher resistance amorphous phase and to show lower melting and crystallization temperatures than a phase-change material for a memory. Here, we present a structural method to obtain the above requirements. In addition, we confirm the switching operation by a selective current pulse for multiple phase-change materials from the experiment. From these results, we expected that one of the multiple phasechange materials can be replaced in a switching device without the need for an additional selective device, and that such a device would be feasible for 3-dimensional PCM architecture.

Amorphous GeCu2Te3 was investigated by X-ray diffraction and extended X-ray absorption fine structure measurements at the Ge, Cu and Te K-edges. Structural models were obtained by fitting the four experimental datasets simultaneously by reverse Monte Carlo simulation. It was found that Ge–Ge and Cu–Cu bonding are both significant. The average coordination numbers of Cu and Te, as well as Ge, are close to four. The high average coordination number of the network contributes to the enhanced thermal stability of amorphous GeCu2Te3.

The electrical and optical properties of Fe­O­N films were investigated in order to find their possibilities for solar cell application. Fe­O­N thin films were deposited on glass substrates by RF magnetron sputtering using an Ar­N2­O2 reactive gas. Under optimum flow rates of nitrogen and oxygen, the Fe­O­N films showed equivalent electrical properties to amorphous Si that has been conventionally used for thin film solar cells. Bandgap narrowing was also observed from 2.0 to 1.9 eV. The observed results were considered to be due to the formation of hematitemagnetite mixed phase, and the introduction of oxygen vacancies and/or nitrogen interstitials. [doi:10.2320/matertrans.M2013170]

Shape-memory alloys (SMAs), which display shape recovery upon heating, as well as superelasticity, offer many technological advantages in various applications. Those distinctive behaviors have been observed in many polycrystalline alloy systems such as nickel titantium (TiNi)–, copper-, iron-, nickel-, cobalt-, and Ti-based alloys but not in lightweight alloys such as magnesium (Mg) and aluminum alloys. Here we present a Mg SMA showing superelasticity of 4.4% at –150°C and shape recovery upon heating. The shape-memory properties are caused by reversible martensitic transformation. This Mg alloy includes lightweight scandium, and its density is about 2 grams per cubic centimeter, which is one-third less than that of practical TiNi SMAs. This finding raises the potential for development and application of lightweight SMAs across a number of industries.

Lead telluride (PbTe) thin films have been deposited on SiO 2 substrate using thermal evaporation method. The structure of the films was found to have a face-centered cubic (fcc) with predominant grain growth in the (200) direction for both as-deposited and annealed samples up to 350 °C in vacuum for 1 h. The in-plain electrical resistivity and Hall measurements via van der Pauw method as a function of annealed temperatures were measured. It was found that increasing the annealing temperature led to increase in grain size, which in turn caused decrease of electrical resistivity and increase of Seebeck coefficient. The high power factor of 141.0 µWm−1K−2 was obtained for the annealed samples at 350 °C in vacuum for 1 h.

This paper reports a new local interconnect structure using Co M0/contact and a Co-Ti amorphous single layer barrier. Good adhesion and low film resistivity were obtained for as-deposited and annealed samples. The metal-oxide-semiconductor samples of Co/Co-Ti/SiO2/p-Si showed a high breakdown voltage and no negative shift of flat band voltage after annealing up to 700 °C for 10 min. The amorphous structure of the Co-Ti layer was maintained throughout annealing and prevented Co diffusion into dielectric. The obtained results indicated that Co-Ti could be a potential candidate to replace conventional local interconnect structure of Ti/TiN.

In this study, we investigated the effect of aging treatment at 573 K on hardness in Mg-Sc alloy with bcc/hcp dual phase prepared by annealing at 873 K. It was found that hardness abruptly increased after some incubation time, and then reached steady state level. Moreover, by further aging, the hardness increased again. It was confirmed by SEM observation and XRD that the initial drastic hardening was due to the precipitation of α phase. Furthermore, XRD results suggested that the secondary hardening was attributed to the formation of MgSc phase with ordered B2 structure.

In this investigation, the fatigue mechanisms of Mg-0.6at%Y dilute alloy were studied by cyclic tensile tests. After annealing at 723 K for 8 h, average grain size of 180 µm was obtained. Yield stress and ultimate tensile strength were 65 and 140 MPa, respectively. Cyclic tensile tests were carried out at room temperature for 10 cycles. Stress was continuously cycled from nil to peak value and subsequently unloading back to nil. Peak stress was controlled according to yield stress: below yield stress, near yield stress and above yield stress. The results showed that basal slip and prismatic slip are the main fatigue mechanisms of Mg-Y alloy. {10–12} tension twinning only occurs when peak stress is above yield stress. Furthermore, compared with previous work on AZ31 Mg alloy, prismatic dislocation slip promotes cyclic strain hardening more obvious than {10–12} tension twinning does.

We investigated the microstructure and growth behavior of a manganese oxide layer deposited on SiOC substrates for the purpose of providing a new method to form a thin diffusion barrier layer for advanced LSI interconnections. The Mn oxide layer was formed by chemical vapor deposition (CVD), using bis(ethylcyclopentadienyl)Mn as a precursor and H2 as a carrier gas at 300 °C for 30 min. The Mn oxide layer could be formed on plasma-treated SiOC, but not on as-received SiOC. By using thermal desorption spectroscopy (TDS), moisture absorption in SiOC was evidenced after plasma treatment, using various gases of O2, N2, and Ar. Two adsorbed moisture components, physisorbed (α) and chemisorbed (β, γ), were observed which were responsible for the formation of crystalline MnOx and amorphous MnSixOy, respectively. The position of the Mn oxide layer was also investigated by measuring the variation of the SiOC thickness. The Mn oxide layer was formed within the SiOC substrate. The result indicates that the Mn oxide b...

Niobium was deposited as an electrode material on an n-type SiC wafer for power device application. The reaction microstructure and electrical contact property were investigated after annealing at 700 to 1000 °C and compared with the results for an Ni electrode. Microstructure-related problems of the Ni electrode could be resolved without sacrificing ohmic contact behavior with a low contact resistivity of 1.53 × 10 − 4 Ω cm 2 . Carbon precipitation was completely eliminated with Nb by the formation of carbides, leading to good adhesion upon wire bonding process. At the reaction interface, Nb 5 Si 3 was formed in an epitaxial relationship with SiC, leading to a good interface contact property as well as good interface adhesion.

SUMMARY This paper presents the results on shaking table tests of half-scale brick walls performed to investigate the effectiveness of newly developed Cu–Al–Mn superelastic alloy (SEA) bars in retrofitting of historical masonry constructions. Problems associated with conventional steel reinforcing bars lie in degradation of stiffness and strength, or pinching phenomena, under cyclic loading, and presence of large residual cracks in structures during and after intense earthquakes. This paper attempts to resolve the problems by applying newly developed Cu–Al–Mn SEA bars, characterized by large recovery strain, low material cost, and high machinability, as partial replacements for steel bars. Sets of unreinforced, steel reinforced, and SEA-reinforced specimens are subjected to scaled earthquake excitations in out-of-plane direction. Whereas steel-reinforced specimens showed large residual inclinations, SEA-reinforced specimens resulted in stable rocking response with slight residual inclinations. Corresponding nonlinear finite element (FE) models are developed to simulate the experimental observations. The FE models are further used to examine the sensitivity of the response with respect to the variations in experimental conditions. Both the experimental and numerical results demonstrate the superiority of Cu–Al–Mn SEA bars to conventional steel reinforcing bars in avoiding pinching phenomena. Copyright © 2012 John Wiley & Sons, Ltd.

The thickness-ratio effects of Ni/Nb bi-layer electrodes were studied for power device applications. The reaction microstructure and electrical contact property were investigated after annealing at 1000 °C and compared with the results of an Ni electrode. Microstructure-related problems of the Ni electrode could be successfully resolved without sacrificing ohmic contact behavior by the addition of Nb to a Ni based electrode. Carbon precipitation was reduced with increasing Nb thickness by the formation of carbides, which led to good adhesion between the electrode and a wiring pad. High shear strength of the bonded wire was also obtained by the elimination of the carbon precipitates on the electrode surface.

Phase change random access memory (PCRAM) requires an advanced phase change material to lower its power consumption and to enhance its data retention and endurance abilities. The present work investigated the crystallization behaviors and electrical properties of Ge 1 Cu 2 Te 3 compound films with a low melting point of about 500 °C for PCRAM application. Sputter-deposited Ge 1 Cu 2 Te 3 amorphous films showed a high crystallization temperature of about 250 °C. The Ge 1 Cu 2 Te 3 amorphous film showed an electrical resistance decrease of over 10 2 -fold and exhibited a small increase in thickness of 2.0% upon crystallization. The Ge 1 Cu 2 Te 3 memory devices showed reversible switching behaviors and exhibited a 10% lower power consumption for the reset operation than the conventional Ge 2 Sb 2 Te 5 memory devices. Therefore, the Ge 1 Cu 2 Te 3 compound is a promising phase change material for PCRAM application.

The crystallization kinetics of amorphous Ge 1 Cu 2 Te 3 (GCT) films prepared by sputter deposition were investigated by differential scanning calorimetry under non-isothermal conditions. An exothermic peak due to crystallization was observed in the temperature range 230–270 °C. It was found that the local activation energy for crystallization is almost constant when the crystallization fraction is less than about 0.15 and then monotonically decreases with increasing crystallization fraction, which indicates that the crystallization of amorphous GCT films is a multi-step mechanism. The local Avrami exponent decreased from more than 5 to 1.7 with increasing crystallization fraction. It was demonstrated by the Ozawa method that GCT amorphous films show a higher thermal stability than Ge 2 Sb 2 Te 5 amorphous films, with an estimated failure time of over 70 years at 125 °C, which is well beyond the data retention requirements of the International Technology Roadmap for Semiconductors. In addition, the thickness change in GCT amorphous films accompanying crystallization was measured by atomic force microscopy. The GCT amorphous film was found to show a thickness increase of only 2.0% on crystallization, which is desirable to enhance the endurance of phase change random access memory devices.

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si15Te85 amorphous thin films. The Si15Te85 amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si15Te85 amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si2Te3 crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250–295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.

This paper investigates the applicability of newly developed Cu-Al-Mn shape memory alloy (SMA) bars to retrofitting of historical masonry constructions by performing quasi-static tests of half-scale brick walls subjected to cyclic out-of-plane flexure. Problems associated with conventional steel reinforcing bars lie in pinching, or degradation of stiffness and strength under cyclic loading, and in their inability to restrain residual deformations in structures during and after intense earthquakes. This paper attempts to resolve the problems by applying newly developed Cu-Al-Mn SMA bars, characterized by large recovery strain, low material cost, and high machinability, as partial replacements for steel bars. Three types of brick wall specimens, unreinforced, steel reinforced, and SMA reinforced specimens are prepared. The specimens are subjected to quasi-static cyclic loading up to rotation angle enough to cause yielding of reinforcing bars. Corresponding nonlinear finite element models are developed to simulate the experimental observations. It was found from the experimental and numerical results that both the steel reinforced and SMA reinforced specimens showed substantial increment in strength and ductility as compared to the unreinforced specimen. The steel reinforced specimen showed pinching and significant residual elongation in reinforcing bars while the SMA reinforced specimen did not. Both the experimental and numerical observations demonstrate the superiority of Cu-Al-Mn SMA bars to conventional steel reinforcing bars in retrofitting historical masonry constructions.

Manganese oxide formed by a chemical vapor deposition (CVD) method is a promising candidate for an ultrathin diffusion barrier layer in the interconnect structure of advanced semiconductor devices. This study placed emphasis on the adsorbed moisture in a hydrophilic SiO2 substrate and investigated the effects of the adsorbed moisture on the formation behavior, structure and composition of the CVD Mn oxide layer. The SiO2 substrates were preannealed at 150 to 500 °C for 1 h to control the type and amount of the residual adsorbed moisture. A Mn oxide layer was subsequently deposited by thermal CVD of bis(ethylcyclopentadienyl)Mn at 200 °C for 30 min. The Mn oxide layer deposited on an as-received substrate was composed of the bilayer of crystalline MnOx and amorphous MnSixOy. The Mn oxide layer deposited on a preannealed substrate was composed of the single layer of amorphous MnSixOy. The crystalline MnOx layer was formed by the reaction of the Mn precursor with physisorbed moisture on the as-received SiO2....

Electrical properties and microstructures were investigated between a-IGZO and metal electrodes. We should use Cu-Mn or Ti electrodes as the contact promoter and diffusion barrier. Ohmic behaviors were obtained with Cu-Mn and Ti after annealing at 250 °C. TFT showed high performance with theses. This is due to reduced a-IGZO and oxygen deficient region.

The effect of CVD Mn oxide layer as a barrier layer to Cu diffusion for 3-D TSV was characterized. The impact of oxide substrate on the barrier property of a planar Mn oxide was evaluated by XPS method. Planar Mn oxide layer of 20-nm thickness formed over thermal oxide showed an excellent barrier property to Cu diffusion after annealing at 500°C, whereas the Mn oxide over P-TEOS oxide was good enough up to 400°C annealing. On the other hand, the barrier property of Mn oxide upon O3-TEOS oxide was not as good as thermal and P-TEOS oxides. The effect of a vertical Mn oxide layer as a barrier layer to Cu diffusion from Cu TSV was evaluated by C-t analysis. Vertical Mn oxide layer with 20-nm thickness formed on P-TEOS oxide liner in TSV showed better barrier property, when compared with the sputtered Ta barrier layer, up to 400°C annealing condition. However, the barrier property of CVD Mn oxide layer was degraded after annealing at 500°C.

Because of aggressive downscaling of the dimensions of future semiconductor devices, they will suffer from increased line resistivity and resistance-capacitance delay. In this work, NiAl thin films are investigated as a potential liner- and barrier-free interconnect material. The results show that NiAl has strong adhesion, does not undergo interdiffusion with SiO2, and has a favorable resistivity size effect. These features suggest that NiAl is a good candidate for replacing Cu as a liner- and barrier-free interconnect for linewidths below 7 nm.Because of aggressive downscaling of the dimensions of future semiconductor devices, they will suffer from increased line resistivity and resistance-capacitance delay. In this work, NiAl thin films are investigated as a potential liner- and barrier-free interconnect material. The results show that NiAl has strong adhesion, does not undergo interdiffusion with SiO2, and has a favorable resistivity size effect. These features suggest that NiAl is a good candidate for replacing Cu as a liner- and barrier-free interconnect for linewidths below 7 nm.

Fatigue tests were performed on AZ31 Mg alloys at room temperature in tension cycles. Deformation twinning of the { 1 0 1 ¯ 2 } type was observed both below and above the fatigue limit, indicating that the { 1 0 1 ¯ 2 } twins do not contribute directly to fatigue failure. However, prismatic slip and substantial cyclic hardening were observed above the fatigue limit. Progressive hardening gave rise to { 1 0 1 1 } – { 1 0 1 ¯ 2 } double twinning that led to the formation of large surface steps, cracks and eventual failure.

We present the results of the quasi-static cyclic tensile tests of Cu–Al–Mn shape memory alloy (SMA) bars of 4 and 8 mm diameters to examine their superelasticity and other mechanical properties closely related to seismic applications. The present Cu–Al–Mn SMA bars have achieved the recovery strains of over 8% and the fracture strains of over 17%. Low-cycle fatigue was observed in neither of the bars. The mechanical properties obtained from the test, along with the lower material cost and higher machinability than Ni–Ti SMAs, demonstrate the high potential of the present Cu–Al–Mn SMA bars to be used in seismic applications. Copyright © 2010 John Wiley & Sons, Ltd.

Rolled sheets of AZ31 Mg alloys were deformed at room temperature under tension along the rolling direction. A number of large surface steps were observed in the region near the fractured edge. Transmission electron microscopy showed the presence of twins under these steps. Crystallographic analysis indicated that the twins are of the { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin type with their basal planes tilted by 37° with respect to the basal planes of the matrix, which is considered to make basal dislocation slip highly active within the twin. The localized deformation within the twins may lead to the formation of crack-like faults and to failure in a macroscopically brittle manner.

Phase-change random access memory (PCRAM) is enabled by a large resistance contrast between amorphous and crystalline phases upon reversible switching between the two states. Thus, great efforts have been devoted to identifying potential phase-change materials (PCMs) with large electrical contrast to realize a more accurate reading operation. In contrast, although the truly dominant resistance in a scaled PCRAM cell is contact resistance, less attention has been paid toward the investigation of the contact property between PCMs and electrode metals. This study aims to propose a non-bulk-resistance-dominant PCRAM whose resistance is modulated only by contact. The contact-resistance-dominated PCM exploited here is N-doped Cr2Ge2Te6 (NCrGT), which exhibits almost no electrical resistivity difference between the two phases but exhibits a typical switching behavior involving a three-order-of-magnitude SET/RESET resistance ratio owing to its large contact resistance contrast. The conduction mechanism was discussed on the basis of current–voltage characteristics of the interface between the NCrGT and the W electrode.

The effects of heat treatment on the microstructure and hardness of Ti-Mo-N films were investigated for various N contents. Ti-Mo-N films were deposited onto a AISI304 stainless steel substrate by reactive RF magnetron sputtering in a mixture of argon (7.5 ccm) and nitrogen (0-2.0 ccm) gases using a Ti 50 Mo 50 target. X-ray analysis of the as-deposited films indicated that the main phases of the Ti-Mo-N film produced at nitrogen gas flow rates of (f N2 ) ≤ 0.2ccm and >0.3 ccm were bcc-(Ti,Mo) and δ-(Ti,Mo)N phase, respectively. As-deposited films were heat treated in an argon atmosphere at 300-1100°C for 30 min. Hardness was measured using a nanoindentation system. There was almost no change in the hardness of the Ti-Mo-N films deposited at f N 2 = 2.0 ccm after heat treatment. In contrast, the hardness of the films deposited at f N2 = 0.2 and 0.3 ccm was significantly increased by heat treatment at temperatures higher than 900°C. In particular, the film deposited at f N2 = 0.3 ccm showed a maximum hardness of approximately 35 GPa by heat treatment at 1000°C for 30 min. X-ray measurements and transmission electron microscopy (TEM) observations indicated that the increment of hardness in the Ti-Mo-N film deposited at f N2 = 0.3 ccm was due to the formation of a bcc-(Ti,Mo) phase in a δ-(Ti,Mo)N phase.

Mechanical properties of Fe-20Mn-(10-14)Al-(0-1.8)C (mass%) quaternary and Fe-20Mn-(10-14)Al-(0.75-1.8)C-5Cr (mass%) quinary alloys were investigated by hardness, cold-workability and tensile tests at room temperature. The γ(fcc) alloys in both quaternary and quinary systems with a low density of less than 7.0 g/cm 3 showed an excellent ductility and their hardness and tensile strength increased with increasing Al and C contents. The γ+α(bcc) duplex alloys also exhibited a high tensile strength by controlling the α volume fraction. TEM observation confirmed that high hardness and tensile strength of the alloys with high Al and C contents are caused by the precipitation of nano-size κ-carbide with perovskite structure during cooling from the annealing temperature. Fe-20Mn-11Al-1.8C-5Cr alloy with a density of 6.51 g/cm 3 showed a high specific strength of more than 180 MPa · cm 3 /g with a good tensile elongation of 40 %. The present Fe-20Mn-Al-C(-5Cr) alloys showed a higher specific strength than conventional steels.

The effects of ageing at 473–573 K on the hardness, microstructure and thermoelastic martensitic transformation in Cu–Al–Mn-based shape memory alloys were investigated. It was found that hardness was dramatically increased by ageing due to the formation of fine bainitic plates and that the volume fraction of the bainite phase with ageing time can be described by the Austin–Rickett equation. The martensitic transformation temperatures decreased with the formation of bainite plates, mainly due to the composition change of the β-matrix. Moreover, the growth of thermally induced martensite plates was disturbed by the existence of bainite plates. Consequently, the transformation intervals ( M s – M f and A f – A s ) and transformation hystereses ( A f – M s and A s – M f ) increased with the progress of bainitic transformation.

Low-frequency elastic modulus, internal friction, tensile stress-strain loops, and thermally-induced strain recovery of single-crystal fibers obtained by pulling-down method from the melts of NiFeGaCo alloys were studied. A minimum of the elastic modulus and the maximum peak of the internal friction were obtained at the martensitic transition temperature. A large super-elastic strain of about 10% was obtained in the fiber of the Ni(49)Fe(18)Ga(27)Co(6) alloy. Thermodynamic estimation by the Clausius-Clapeyron equation was consistent with the experimental results.

Polycrystalline Cu–Al–Mn shape memory alloys (SMAs) with a low degree of order of the β (L21) phase show excellent ductility and exhibit shape memory (SM) properties such as superelasticity, the one way memory effect and the two way memory effect based on martensitic transformation. These SM properties can be greatly enhanced by controlling microstructural factors such as grain size and texture by thermomechanical treatments. In the present paper, the SM properties of ductile Cu–Al–Mn based SMAs and microstructure control to obtain excellent SM characteristics are reviewed. Furthermore, an example of the application of Cu–Al–Mn based SMAs to a guidewire for medical use is also presented.

A novel Bi-base solder for mounting semiconductor power devices on inverters used in hybrid and fuel-cell vehicles was studied. The solder consists of Bi-base matrix (m.p. (melting point) =270°C), which facilitates high-temperature soldering, and dispersion particles of a Cu-Al-Mn alloy, whose thermoelastic martensitic microstructure is expected to relax the thermal stress. The tensile strength of the developed solders was found to be remarkably improved compared with that of pure Bi. Results of tensile tests at elevated temperatures showed greater elongation and lower strength than those at room temperature. However, sufficient tensile strength was still remained at 195°C. Effects of the CTE (coefficient of thermal expansion) and the surface substance of chips and substrates on the IMC (intermetallic compound) formation and thermal cycling properties were investigated. Even after 1983 thermal cycles between -40°C and 200°C, neither intermetallic compounds nor cracks were observed at the Cu/Bi-base solder interfaces in the CTE matched samples.

Effects of pre-strain and heat treatment on shape recovery stress and phase transformation temperature in Ti-Ni-Nb shape memory alloys with various Nb content (6, 9, and 12 mol%) were investigated by tensile testing. The recovery stress of Ti-Ni-Nb alloys increased with increasing the pre-strain and then decreased after reaching the maximum recovery stress of around 450–500 MPa at about 9% pre-strain. Martensitic transformation temperatures (Ms, As, Af) also increased with increasing the applied pre-strain, while the relation between As and prestrain did not depend on the Nb contents and Ni/Ti ratio. Moreover, the recovery stress in the Ti-Ni-Nb alloy heat treated at 673 and 773 K after solution treatment was slightly higher than that in the solution-treated alloy. These behaviors were examined relative to the increased defect density due to pre-straining and the elimination of defects by heat treatment. [doi:10.2320/matertrans.MRA2007266]

Phase equilibria and martensitic and magnetic transitions of the β (B2 and L21) phase in the Ni–Fe–Ga system were investigated. The b phase was found to be in equilibrium with the γ (A1 structure) or γ′ (L12 structure) phase. The Curie temperature, T c , equilibrium temperature, T o 5 (Ms + Af)/2, martensitic transition starting temperature, M s , and reverse transition finishing temperature, Af , of the β single–phase alloys were sensitive to the Fe and Ga compositions. The Fe substitution for Ni decreased and increased the T o and T c , respectively. The Ga substitution for Ni or Fe decreased both the T o and T c . The entropy change accompanying the reverse martensitic transition showed compositional dependence due to the magnetic contribution. The saturation magnetization I s of the Ni–Fe–Ga system showed a strong dependence on the magnetic valence Z M . The Is values of the Ni–Fe–Ga alloys annealed at 1023 K showed the same Z m dependence as other ferromagnetic shape memory alloy (FSMA) systems.

Martensitic and magnetic transformation behaviors of Ni50MnIn, Ni45Co5MnIn, and Ni42.5Co7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M s ) decreases with increasing In composition, while the Curie temperatures (T c ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M s and an increase of the T c , and the degree of the decline of M s was accelerated by magnetic transformation of the parent phase. The M s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T c -M s . These behaviors can be qualitatively explained on the basis of thermodynamic considerations.

This paper describes the effect of heat aging on thermal and mechanical properties of shape memory alloy. The materials used in this study are Ti-Ni-(6-12 at%)Nb shape memory alloy. The tensile tests at 173 K and the heating-cooling tests under constrained strain condition were carried out using the specimens aged at 561 K for between 20 h and 1000 h in normal water chemistry. The martensite start temperature rapidly decreases in the initial short aging time, but there is little further decrease of the martensite start temperature with aging time more than 100 h. The recovery stress gradually increases with aging time, but on the contrary, the aging time of more than 500 h causes reduction of the recovery stress. (doi:10.2320/matertrans.48.439) In order to develop an SMA joint with the prescribed performance, it is necessary to optimize the manufacturing conditions such as the chemical composition, the processing condition and the heat treatment condition of shape memory alloys, that is, the SMA joint should keep up the prescribed function at the storage temperature and reproduce the prescribed function in the normal usage environment. The effects of the Ti/Ni ratio, the Nb concentration, the heat treatment temperature and the processing rate on the mechanical and functional properties were discussed, and the manufacturing conditions of the SMA joint were clarified. The thermal and mechanical properties such as the pre-strain, the recovery strain, the transformation temperature and the recovery stress were also clarified so as to determine the amount of tube expansion of the joint ring. The SMA joint rings are normally used in high-temper- ature water for long-time. In such cases, there is a possibility of the functional property change due to aging. Therefore, in order to maintain the long-term stability of the function of the SMA joint ring and secure the function of the SMA joint ring at lowered environmental temperature, it is necessary to investigate the shape memory property, such as the marten- site start temperature and the recovery stress, of the SMA joint ring exposed to high-temperature water for a long term. In this study, we investigated the thermal and mechanical properties of the joint ring material exposed to high-temper- ature water for a long term, and clarified the effect of the aging time on the function property of the joint ring material, in order to evaluate the performance and the stability of the joint ring in the usage environment of the actual equipment.

The microstructure and superplastic behavior at 450°C and 500°C in Cu-Al-Mn-based shape memory alloys were investigated using optical microscopy and a tensile test. It was found that a fine α (fcc) + β (bcc) two-phase structure with a grain size of 3 μm in diameter can be obtained in Cu-Al-Mn-Ni alloy by annealing at 600°C. The flow stress of Cu-Al-Mn-Ni alloy depends on the strain rate and the strain rate sensitivity is over 0.3 with an elongation of over several hundred percent, which shows that the Cu-Al-Mn-Ni shape memory alloy exhibits superplasticity. For the test temperature at 500°C and a strain rate of 5 × 10 -4 s -1 , a maximal elongation of 1150% was obtained. The formation of cavity stringers lying parallel to the tensile axis was observed and the size of the cavity was larger as the specimen was more highly deformed.

Several Ti-Ni-Nb shape memory alloys (SMAs) with various Nb contents (0� 15 mol%) or Ni/Ti atomic ratios (1:0� 1:1) were tested to investigate the martensitic transformation temperatures and mechanical properties. The martensitic transformation temperatures (Ms, Mf, As and Af ) of the solution-treated Ti-Ni-Nb alloys at 1173 K for 300 seconds with Ni/Ti ratio of 1.0 decrease with increasing the Nb content. These transformation temperatures also depend on the Ni/Ti ratio, heat treatment condition and cold working rate. Furthermore, in the alloy with Nb content of 6 mol% and Ni/Ti atomic ratio being less than 1.05, the fracture stress and fracture strain reached about 1,000 MPa and over 20%, respectively. Based on the obtained results, several processing conditions such as the chemical composition, fabrication process and heat treatment are optimized for the Ti-Ni-Nb SMAs to develop SMA pipe joints. [doi:10.2320/matertrans.48.445]

Martensitic transformation from the face-centered cubic to the hexagonal close-packed structure, shape memory (SM) effect and magnetic properties in the Co–14 at.% Al alloy were investigated and the microstructures of the martensite were examined using transmission electron microscopy. It was found that the SM effect occurs at temperatures between 200 and 300 °C corresponding to the reverse transformation temperatures, and that a metastable L1 2 -structured phase is formed in Co–14 at.% Al aged at 600 °C. The magnetic properties were measured by a vibrating sample magnetometer, and the Curie temperature and saturation magnetization were determined to be 689 °C and 120 emu/g at 25 °C. All these results indicate that Co–Al alloys can be listed as a new class of ferromagnetic SM alloys.

The effect of Co addition on the microstructure, hardness, martensitic transformation and shape memory (SM) effect in Fe–Ni–Si alloys was investigated by means of optical and transmission electron microscopy, Vickers micro-hardness testing, differential scanning calorimetry and bending tests. The addition of Co was found to increase the hardness of the austenite phase and to facilitate the formation of thin-plate martensite. Furthermore, the SM effect was improved by the addition of Co.

The martensitic and magnetic phase transitions of Co–Ni–Al β single and β + γ two-phase alloys have been systematically investigated. The Curie temperature T C and the martensitic transition temperature of the β single-phase alloys are sensitive to the Ni and Al compositions. The value of T C increases with decreasing both the Ni and Al contents, while the martensitic transition starting temperature M s decreases with decreasing the Ni content and with increasing the Al content. T C and M s of the β + γ two-phase alloy depend on the annealing temperature due to the variation of the chemical composition of β phase equilibrated with γ phase, increasing by the increase of annealing temperature. In addition, the superelasticity (SE) in tensile loading of the Co–Ni–Al polycrystal β + γ alloys have been investigated. It was found that the SE properties in the β + γ two-phase alloys strongly depend on the average β grain size as well as the volume fraction of γ phase. Consequently, the maximum recovery SE strain of about 6.3% was achieved in a sheet specimen with the large β grains.

Effects of grain size and texture on the damping properties in the Cu–Al–Mn-based shape memory alloys were investigated mainly by dynamic mechanical spectrometry. Dynamic damping tan ϕ obtained from the martensite single-phase specimens increases with increasing relative grain size d / t , where d and t indicate the grain size and the specimen sheet thickness, respectively. Moreover, tan ϕ increases due to the development of texture formation, a textured sheet with d / t = 1.34 shows a large tan ϕ of about 0.09 in the martensite phase. Damping alloys with tensile strength over 850 MPa and tan ϕ of about 0.04 can be obtained by controlling the relative grain size and texture.

Effects of heat treatment conditions on mechanical and superelastic properties of Ti–6 at.% Mo–4 at.% Sn alloy were investigated mainly by cyclic tensile testing. The cold-drawn wires were heat-treated in the temperature range of 773–1273 K for 300 s. Young's modulus and ultimate tensile strength of the wires heat-treated at over 823 K decreased with increasing heat treatment temperature, while premature failure was observed in the wire heat-treated at 773 K. A maximum recovery strain of about 4%, including both genuine elastic and superelastic strain, was obtained in the wires heat-treated in the temperature range of 1073–1273 K. On the other hand, the wire heat-treated at 823 K showed a high Young's modulus of 77 GPa and a high tensile strength of 1698 MPa. Given these characteristics, it is expected that a new class of Ni-free Ti-based guidewire possessing mechanical properties graded from the tip to the end can be developed.

Effects of Co addition on the phase equilibria between the β (B2) and γ (A1) phases at 800 and 1000 °C and aging temperature dependence on the martensitic and magnetic transformation temperatures in Ni–Ga–Fe–Co alloys were examined with energy dispersion X-ray spectroscopy (EDX), differential scanning calorimetry (DSC) and vibrating sample magnetometry (VSM). The equilibrium compositions of Fe and Ga are little changed by the addition of Co, and Co tends to be distributed in the γ phase rather than in the β phase. The B2/L21 order–disorder and magnetic transformation temperatures increase with increasing Co content, while the martensitic transformation temperatures decrease. While the martensitic transformation temperatures monotonously decrease with decreasing aging temperature, the Curie temperature increases with the decrease of aging temperature in the high temperature region over 300 °C and then starts to decrease at low temperatures.

Reliabilities of joints for power semiconductor devices using a Bi-based high temperature solder has been studied. The Bi-based solder whose melting point is 270 °C were prepared by mixing of the CuAlMn particles and molten Bi to overcome the brittleness of Bi. Then, joined samples using the solder were fabricated and thermal cycling tests were examined. After almost 2000 test cycles of −40/200 °C test, neither intermetallic compounds nor cracks were observed for CTE (Coefficient of Thermal Expansion) matched sample with Cu interface. On the other hand, certain amount of intermetallic compound such as Bi 3 Ni was found for a sample with Ni interface. In addition, higher reliability of this solder than Sn-Cu solder was obtained after −40/250 °C test. Furthermore, an example power module structure using double high temperature solder layers was proposed.

The phase equilibria among the face-centered cubic (fcc), body-centered cubic (bcc), and βMn phases at 800, 900, 1000, 1100, and 1200 °C were examined by electron probe microanalysis (EPMA), and the A2/B2 and B2/D03 ordering temperatures were also determined using the diffusion couple method and differential scanning calorimetry (DSC). The critical temperatures for the A2/B2 and B2/D03 ordering were found to increase with increasing Mn content. Thermodynamic assessment of the Fe−Mn−Al system was also undertaken with use of experimental data for the phase equilibria and order-disorder transition temperatures using the CALPHAD (Calculation of Phase Diagrams) method. The Gibbs energies of the liquid, αMn, βMn, fcc, and e phases were described by the subregular solution model and that of the bcc phase was represented by the two-sublattice model. The thermodynamic parameters for describing the phase equilibria and the ordering of the bcc phase were optimized with good agreement between the calculated and experimental results.

Effects of low temperature aging and the addition of Co on the martensitic and magnetic transitions in the Ni–Al–Fe β (B2-type structure) alloy system were investigated using differential scanning calorimetry, transmission electron microscopy and vibrating sample magnetometry. The martensitic transition temperatures of a Ni57Al25Fe18 specimen homogenized at 1 473 K for 1 h were decreased by aging after homogenization, the maximum decrease by aging being observed at 573 K. On the other hand, the substitution of Co or Fe for Ni induced a decrease of the martensitic transition temperatures and an increase of the Curie temperature, while the substitution of Co for Fe resulted in an increase of the martensitic transition and Curie temperatures. Co was thus shown to be an effective element by which both the martensitic and magnetic transition temperatures can be controlled.

Effects of alloying elements (Si, Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, Ta and W) on � (fcc)/" (hcp) martensitic transformation, ductility and shape memory (SM) properties of Co90Al10 alloy were investigated by means of differential scanning calorimetry, X-ray diffraction method, cold-rolling and an SM test. The addition of Ti, V, Mn, Fe, Ni, Nb, Mo, Ta or W decreased the volume fraction of the " martensite phase (Vm)

Nitinol shape memory alloys (SMAs) are attracting considerable attention as core materials for medical guidewires because of their excellent flexibility and shape retention. However, since Nitinol guidewires possess low rigidity, the pushability and torquability of the guidewires are insufficient. On the other hand, although guidewires made of stainless steel have high pushability, plastic deformation occurs easily. We have developed a new class of superelastic guidewires with functionally graded properties from the tip to the end by using new SMA core materials such as Cu-Al-Mn-based or Ni-free Ti-Mo-Sn SMAs. The tip portion of the guidewire shows excellent superelasticity (SE), while the body portion possesses high rigidity. These functionally graded characteristics can be realized by microstructural control. These guidewires with functionally graded properties show excellent pushability and torquability and are considerably easier to handle than conventional guidewires with Nitinol or stainless steel cores. Moreover, a metallic catheter using a Ni-free Ti-based SMA with high biocompatibility is introduced.

The effect of the relative grain size d/D (d: grain size, D: wire diameter) on stress–strain characteristics was investigated in Cu–Al–Mn-based shape memory alloy (SMA) wires. The yield stress (σy), the work-hardening rate after yielding (dσPE/de) and the stress hysteresis (Δσ) in the wires with a random texture decrease with increasing d/D. The transformation strain (eTS) and the maximum pseudoelastic strain ( e PE MAX ) increase with increasing d/D. The effect of grain size on pseudoelastic behaviors can be clarified from the volume fraction of three-dimensionally constrained grains and the σy, dσPE/de and Δσ increase proportionally with increasing (1 − (d/D))2 while the eTS decreases proportionally with increasing (1 − (d/D))2. Consequently, the effect of grain size on the pseudoelastic behaviors can be expressed using the Taylor and inverse Schmid factors. The σy and the eTS for wires with a 〈1 1 0〉 fiber texture are larger and smaller than those for wires with a random texture, respectively.

The transformation behavior of Cu–Al–Ga–Mn alloy was investigated by differential scanning calorimetry, X-ray diffraction and vibrating sample magnetometer techniques. It was found that the Al substitution for Ga suppresses the formation of the ω phase which occurs to a great extent in the Cu–Ga–Mn ternary alloys.

The microstructure and the shape memory effect in aged Co–14Al alloy were investigated by means of optical and electron microscopy and a bending test. It was found that a good shape memory effect can be obtained in aged alloys through β (B2) phase precipitation by aging at 500–900 °C.