Takuya Ohba

Characteristics and mechanism of martensite ageing effect in Au−Cd alloys

Abstract Characteristics of the martensite ageing effect was extensively studied for the ζ′2 (trigonal) martensite in Au-49.5 and 50.0at.%Cd alloys. Considerable changes in various physical properties were observed with ageing the ζ′2 martensite, such as the increase of the critical stress for the rearrangement of martensite variants and of the reverse transformation temperature etc., in addition to the appearance of the rubber-like behavior, which were shown to originate from the same origin. The ageing effect was further investigated with respect to the temperature dependence, composition dependence and the influence by heat-treatment, from which a close relation with a diffusion mechanism in the martensite was derived. The activation energy for the martensite ageing effect was determined to be 0.44 eV for the ζ′2 martensite in a furnace-cooled Au-49.5at.%Cd alloy, which was found to correspond to activation energy for migration of a vacancy in the martensite. The structural change associated with the ageing effect was examined both for the ζ′2 and γ′2 (orthorhombic) martensites in Au−Cd alloys by careful X-ray diffraction measurements. Only a small and local structural change, which was achieved by some short range diffusion, was found to be responsible for the ageing effect.

A charge density study of the intermetallic compound MgCu2 by the maximum entropy method

The charge density distribution of the intermetallic compound MgCu2 is obtained by the maximum entropy method (MEM) from the synchrotron powder diffraction data. In the MEM charge density map, the overlap of electron densities was clearly observed between the neighbouring Cu atoms. This clearly shows that there is a rather strong covalent bond between Cu-Cu atoms. It is also found that the Cu-Cu bonding is along the very well known kagome net, which characterizes the Laves phase structure. At least, in the case of MgCu2 , the kagome network is the electronic network in reality rather than a geometrical concept to represent structural characteristics of Laves phase compounds.

Synthesis of BaTi2O5 Nanopowders by Sol–Gel Method and the Dielectric Properties of the Ceramics

A sol–gel process using barium diethoxide and titanium tetraisopropoxide as the sources for Ba and Ti, respectively, was employed to synthesize BaTi2O5 nanopowders and ceramics. BaTi2O5 nanopowders in the range of 20–50 nm in size were obtained by calcination of the gel precursors at 650 °C. By a further firing at 1000 °C, some BaTi2O5 nanopowders were grown to 200 nm in size. Conventional sintering at 1150 °C using these BaTi2O5 nanopowders yields BaTi2O5 ceramics of 80–85% relative density. A dense BaTi2O5 ceramic with a relative density of above 95% was fabricated by spark plasma sintering (SPS) at 1000 °C. Furthermore, by SPS combined with the hot-forging technique, we obtained a certain degree of grain orientation along the b-axis in the dense BaTi2O5 ceramic and observed an increase in the dielectric constant reaching 580.

Vacancy migration and long-range ordering due to ageing in AuCd shape memory alloys

Abstract Vacancy concentrations in the martensite phases ζ′ 2 and γ′ 2 of Au 1− c Cd c alloys (0.475≤ c ≤0.500) are measured for samples furnace-cooled (FC) and water-quenched (WQ) from 763 K. There is a tendency for c V to increase with composition. It shows a much higher concentration in WQ samples than in FC samples, indicating the existence of large amount of quenched-in vacancies. Furthermore, the vacancy migration and the long-range ordering during ageing in martensite and parent B2 phases for FC and WQ samples of c =0.495 and 0.475 are examined by measuring the lattice constants and the integrated intensities by the X-ray diffraction method. In both samples, the martensite ageing does not bring about an effective vacancy migration and a detectable change in long-range order (LRO). For the B2 phase of the WQ sample, the quenched-in vacancies are drastically annealed-out by ageing, which accompanies a promotion of the LRO. The difference in ease of the vacancy migration between the martensite and the parent phases is due to the difference in the crystal structure.

Fundamental structure of a Ni2MnGa intermediate phase having an orthorhombic lattice

Ni2MnGa is one of the attractive ferromagnetic shape memory alloys. It is reported that the parent phase is of Heusler type and transforms into an intermediate phase above Ms. As regards the crystal structure of the intermediate phase, it was previously reported that the fcc main reflection remained and additional peaks appeared on cooling. This means that the long range ordered structure was produced with a cubic fundamental lattice. However, we successfully measured peak splitting of the fundamental reflections from the intermediate phase using high energy synchrotron radiation. Careful analyses with the whole pattern decomposition method were carried out and clear evidence of an orthorhombic fundamental lattice with lattice constants a = 5.8285, b = 5.8142 and c = 5.7886 A is presented.

Dielectric and Piezoelectric Properties of 10% KF-Doped BaTiO3 Ceramics

10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ~70 nm in length; the particle size increases to ~200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ~2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature TC = 47 °C, the dielectric constant and loss tan δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.

Landau theory of structural transformations in titanium–nickel and gold–cadmium alloys

Abstract We present a rigorous symmetry-based Landau theory of the two martensitic transformations from the cubic B2 (β2) austenite parent phase to the rhombohedral R (ζ′2) product phase of P3 symmetry and to the orthorhombic B19 (γ′2) product phase of Pmma symmetry. Both are improper ferroelastic transformations; their primary order parameters are expressed in terms of the shuffle displacements that correspond to the two ζζ0 TA2 modes for ζ=1/3 and ζ=1/2, respectively. Numerical agreement with experimental data is exact for the B2–B19 transformation and good/fair for the B2–R transformation in AuCd/TiNi, respectively.

Composition dependence of the rubber-like behavior in ζ2′-martensite of AuCd alloys

Abstract The effect of aging on the rubber-like behavior of martensite in a Au–50.5 at.% Cd alloy was studied, and compared with stoichiometric Au–50% at.% Cd and a Au–49.5 at.% Cd alloy. It is found that for all these alloys the critical stress for domain reversion increases with aging, and gradually saturates at long aging time, following a Chipman function. The magnitude and the rate of the aging effect are strongly dependent on composition. The magnitude decreases monotonically with increasing Cd content, but the aging rate reaches a minimum at the stoichiometric composition. These results suggest that point defects (anti-structure defects and vacancies) play a central role in the martensite aging effect, and anti-structure defects contribute much more to the magnitude of the aging effect than vacancies do.

Effect of Cooling Rate on Morphology and Crystallography of Lath Martensite in Fe-Ni Alloys

The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

Crystallography of β2 → γ2′ martensitic transformation in Au-47.5 at.% Cd and Au-47.5 at.% CdCu alloys

Abstract The crystallography of the β2 → γ2′ stress-induced and thermally-induced martensitic transformations in Au-47.5 at.% Cd and Au-47.at.% CdCu alloys were reexamined by simultaneous measurements was essentially {111}γ2′ Type I twinning, γ2′ Type II twining was also found in one case. No difference was found in the crystallography of stress-induced and thermally-induced transformations. Two non-equivalent solutions of the phenomenological crystallographic theory, Type I (+) and (−) solutions, were in excellent agreement with experimental observations. The results for the Type I (+) solution were essentially the same as those previously reported by Lieberman et al., but the agreement between theory and experiment was much better than theirs. Furthermore, (−) solution for Type I twinning and (+) solution for Type II twinning were also found for the first time. Both results show that the phenomenological crystallographic theory applies well in all respects to the present alloy system.