The pair potentials based on the linear screening theory were adopted to carry out molecular dynamics simulations in the molten K0.8 (KCl)0.2 system at 1073 K. The Ashcroft potential for the electron-cation interaction was employed. As for the electron-anion interaction, both of the Shaw and the Ashcroft potential were assumed. In both systems abrupt increases of the partial structure factors between Cl− ions at low momentum transfer k were observed, which implied a local structural ordering of Cl− ions. The estimated number density fluctuation of Cl− ion in the real space was shown to be much larger than that of K+ ion. The Shaw potential for the electron-anion interaction gave silimar behavior of Cl− ion in the molten K0.8 (KCl)0.2 system to that in the pure KCl melt rather than the Ashcroft one.
The effect of cold work on the hydrogen permeation in iron specimens with two levels of carbon content (30 and 110 ppm) has been studied in the temperature range between 273 and 333 K by means of the electrochemical permeation technique. The effective hydrogen diffusivity (Deff) in the fully annealed specimens are independent of carbon content, and expressed as D=7.69×10−8exp(−5800J/mol·RT) m2s−1. The decrease of hydrogen diffusivity by cold deformation, however, is much larger in the specimen with higher carbon content. The trap density estimated from the permeation tests exhibits a linear relationship with the inverse square of the average cell size. This indicated that the reduction of hydrogen diffusivity in cold worked iron specimens depends on the dislocation density and that the difference in carbon content affects the diffusivity indirectly through the dislocation multiplication. The binding energies for the traps obtained are 23–27 kJ·mol−1, and are in good agreement with that for the elastic fieled of a dislocation.
Magnetic filed-induced martensitic transformations in Fe–Ni–C Invar (Fe-30.2 Ni-0.4C and Fe-30.4 Ni-0.3C (mass%)) and non-Invar (Fe-26.0 Ni-0.4C (mass%)) alloys were studied to clarify the Invar effect on the transformations by means of magnetization measurement and optical microscopy, applying a pulsed ultra-high magnetic field. The morphology of the magnetic field-induced martensites was compared with that of deformation-induced martensites to examine the formation temperature effect on the martensite morphology. As a result, the following are found. A magnetic filed higher than a critical one is needed to induce the martensitic transformations above Ms irrespective as to whether the alloy is Invar or non-Invar. The critical field increases with increasing temperature, and when plotted against the temperature difference (ΔT) from Ms, it lies on a line consisting of two straight parts for the Invar alloys, but on a straight line for the non-Invar alloy. This result and a thermodynamical analysis suggest that the influence of magnetic field on the martensitic transformations in the Invar alloys comprises the Zeeman, high field susceptibility and forced volume magnetostriction effects, while in the non-Invar alloys it comprises the former two effects only. The amount of the magnetic field-induced martensite increases with the maximum strength of magnetic field for all the three alloys, but a little difference is observed among the three alloys in the manner of the increase at the critical magnetic field. The morphology of the magnetic field-induced martensite is the same as that of thermally-induced one in each alloy, irrespective of ΔT and the strength of the magnetic filed. However, the deformation-induced martensites in the Fe-30.2 Ni-0.4C and Fe-26.0 Ni-0.4C alloys are lenticular and butterfly, whereas the magnetic field-induced martensites formed at the same temperature are thin plate-like and lenticular, respectively. This fact indicates that the martensite morphology is not decided by the formation temperature alone.
A detailed observation on the successive stress-induced martensitic transformations in Cu–Al–Ni alloy single crystals has been carried out to clarify ambiguous problems left in a previous work (Acta Met., 27 (1979), 585). As a result, the structural change associated with two stage transformation pseudoelasticity above Af has been revealed to differ depending upon the temperature range. In the temperature range between Af and Tc (a critical temperature), distinct β1→β1′ and β1′→α1′ transformations successively take place on loading, and distinct α1′→β1′ and β1′→β1 reverse transformations do on unloading. In the temperature range above Tc, the transformation behavior of loading is the same as that in the temperature range between Af and Tc. However, on unloading, when the loading has been stopped on the halfway of the second stage, the β1′→β1 and continuous α1′→β1′→β1 transformations successively take place. On the other hand, when the loading has been continued until the β1′→α1′ transformation is completed, the associated stress-strain curve on unloading has clearly showed one stage of the continuous α1′→β1′→β1 reverse transformations. On the basis of these observations, the previous interpretation for the unloading process of stress-strain curves above Tc has been corrected, and the critical stress vs temperature diagram has been replotted.
Thermal cycling effects in a Cu-13.7 Al-4.0 Ni (mass%) alloy have been studied by optical and electron microscopy observations and X-ray diffraction. Transformation temperatures, Ms and Af, decrease gradually with increasing thermal cycles, the magnitude of the decrease reaching about 40 K after 10000 cycles. Martensite plates tend to become smaller with increasing thermal cycles, but their morphology is not so different even after 10000 cycles. The reproducibility of martensite plate formation becomes to be recognized after 10000 cycles, although it is not so perfect as in Cu–Zn and Fe–Pt alloys. Dislocations in the parent phase increase in density with increasing thermal cycles, and a characteristic array of the dislocations is seen after 10000 cycles. The degree of DO3 order of the parent phase decreases with thermal cycling, although that of B2 order remains almost constant, and this results in the decrease of transformation temperatures.
Low frequency internal friction (f∼1 Hz) has been measured on titanium specimens containing 0.5–7 at% H or D over the temperature range from 10 to 650 K. A large internal friction peak appears as the dissolution or precipitation of hydrides occurs, being accompanied by self-twist of a wire specimen. The temperature of the peak is very different between heating and cooling runs, indicating a large hysteresis in precipitation and dissolution. The hysteresis and the twist behaviour are interpreted in terms of elastic and plastic accommodation of the misfit strain between the hydride and matrix. The magnitude of the hysteresis is discussed quantitatively, following Puls’s approach. Below room temperature three relaxation peaks are observed. One located at the lowest temperature (P1) is identified as a dislocation relaxation peak, and the other two (P2 and P3) are considered to be related to hydride precipitates. The relaxation parameters of the peak P2 are dependent on different isotopes of hydrogen.
Improvements of high-field properties of in situ Nb3Sn superconductors were studied by additions of 2%Zr, 2%Hf and 2%Ta to Cu-20 mass%Nb melts. The eutectic structures around Nb dendrites were observed in all as-cast materials, and some difficulties arose from the structures in drawing the ingots with Zr and Hf into wires. The critical current densities were measured up to 15 T after various heat treatments. Additions of Zr and Hf affected a little Jc in the high magnetic field region and HC2* of in situ Cu-20%Nb–Sn superconductors. On the contrary, additions of Ta improved appreciably the high-field performance of in situ Cu-20%Nb–Sn superconductors in spite of the fact that Ta was not soluble in the Nb dendrites in the as-cast state. The highest Jc of (1–2)×108 A/m2 at 12 T and HC2* of 17.5 T were obtained on the in situ Cu-20%Nb–2Ta–Sn superconductors.