Materials Science Forum Vols. 783-786

Abstract: An experimental setup that was utilized to evaluate the constrained thermal fatigue (CTF) behavior of several cast ferrous alloys is described. The tests performed allowed the assessment of the relative performance of different materials in CTF loading. The stable hysteresis loop of the individual CTF tests further allowed the development of a unified parameter that determined the level of inelastic (plastic and creep) deformation and the CTF life under those conditions. A CTF life prediction methodology for cast ferrous alloys is outlined.

Abstract: Charpy impact toughness values show large statistical scatter, particularly in the ductile-to-brittle transition temperature (DBTT) range. Although the statistical distribution of Charpy absorbed energy has not been clarified, critical values of the stress intensity factor, J-integral and crack-tip opening displacement (CTOD) at brittle fracture generally show the Weibull distribution with two or three parameters. This study proposes a brittle fracture model, based on the weakest link theory, for evaluating the scatter in Charpy absorbed energy KV. The numerical results show that the amplitude of the opening stress fields ahead of the V-notch at varying amounts of KV are uniquely characterized as the square of the applied load. With these numerical results, the Weibull shape parameter of the statistical distribution of KV is almost equal to 2. The proposed statistical model is verified through experimental results. It is found that the statistical distribution of KV is characterized by a two-parameter Weibull distribution with the shape parameter of 2 under the condition of pure brittle fracture.

Abstract: The relaxor state has been found in the mixed ferroelectric system Ba (Ti_1-xZr_x)O₃ around x = 0.35. To understand the nature of the relaxor state, the crystallographic features of the paraelectric (PC), ferroelectric, and relaxor states for 0.15 ≤ x ≤ 0.40 have been investigated mainly by transmission electron microscopy. It was found that a microstructure of the ferroelectric state for 0.15 ≤ x ≤ 0.28 consisted of banded structures with boundaries parallel to the {110}_PC and {100}_PC planes. Based on the Sapriel theory concerning ferroelastic transitions, it was understood that the banded structures were consistent with domain structures in the ferroelectric rhombohedral (FR) state having a polarization vector parallel to one of the <111>_PC directions. With the help of the failure of Friedel’s law in diffraction, furthermore, polar regions having <001>_PC and <110>_PC components of a <111>_PC polarization vector were also found to be separately observed in the PC and relaxor states as well as the FR state. Then, in-situ observation for 0.29 ≤ x ≤ 0.40 made in this study indicated that the PC and relaxor states consisted of polar nanometer-sized regions having these two components. Based on this, the relaxor state in BTZ can be identified as an assembly of polar nanometer-sized regions, which were produced by the suppression of the (PC→FR) ferroelectric transition on cooling.

Abstract: In this work, the orientation of the β variants within a single parent α grain on the α→β up-transformation induced by electric current pulses in a Cu-Zn alloy was investigated. Electron backscatter diffraction (EBSD) was used to determine the relationship between the α phase and the β variants. By EBSD analysis, it was found that crystallographic variant selection was observed not only across those prior α/α grain boundaries, but also within the α grain interior. Results revealed that the orientation relationship between the α phase and the β associated with nucleation from α phase was close to the Kurdjumov-Sachs (K-S) orientation relationship, which better described the orientation relationship for α nucleation within β grains.

Abstract: Experimental results on the fatigue damage of quasi defect-free materials in the VHCF range are presented. For nickel-based superalloys and pure nickel the likelihood of crack initiation at favorable grain morphologies is studied. Slip band and microcrack formation at the surface was observed even in run-out samples. Hence, microcracks were evaluated regarding their propagation capabilities according to grain orientation and barrier function of grain boundaries. In the VHCF regime crack initiation can shift from surface to subsurface, consequently early crack growth has to be studied by means of optical methods and indirect detection techniques or tomographic methods. In the study presented crack initiation and crack growth was monitored through optical observation and quasi 3-D observation by means of synchrotron radiation. For an as-received and a coarse-grained condition of pure nickel Ni201 fatigue crack growth in the VHCF regime occurs at deltaK as low as 3.54 MPam^1/2 for a crack growth rate da/dN = 10E^-12 m/cycle. The grain size had no effect on the threshold limit but crack growth retardation at grain boundaries and crack path deflection lead to lower crack growth rates for the coarse-grained condition In the nickel-based alloy Nimonic 80A the influence of microstructure on the intercrystalline crack initiation and propagation was confirmed. Here, the combination of the misorientation angle between two adjacent grains and the orientation of their boundary with respect to the external load defines the magnitude of stress concentration at grain boundaries.

Abstract: The magnetic properties of the off-stoichiometric Ni₂XIn (X=Mn, Fe, Co) are systematically investigated by means of the first–principles calculations within the framework of the density functional theory (DFT) using the Vienna ab initio software package (VASP). The magnitude of the variation in the Ni moments is much larger than that of Mn in the defective Ni₂XIn. The value of the Ni magnetic moment sensitively depends on the distance between Ni and X.

Abstract: In order to develop new β (bcc) Ti alloys, the Ti-Fe-Sn system was focused and phase constitution, microstructure, mechanical properties of Ti-5mol%Fe-6mol%Sn and Ti-6mol%Fe-3mol%Sn were clarified in addition to aging effect. It was estimated by differential scanning calorimetry (DSC) that α phase is formed at temperature from 773-779K and that β transus temperature is 1019K in both the alloys. X-ray diffraction analysis revealed that, in both alloys, β single phase is formed after the solution treatment (ST) at 1273K followed by water quenching, while α phase is formed after the aging at 773K and 873K for 3.6ks. The formation of α phase is also confirmed by optical microscopy. The volume fraction of α phase reaches to 90% in Ti-5Fe-6Sn and 80% in Ti-6Fe-3Sn after the aging at 873K for 3.6ks. The 0.2% proof stress was increased by aging at 873K from 550MPa to 650MPa in Ti-5Fe-6Sn and 500MPa to 690MPa in Ti-6Fe-3Sn. Besides, apparent Young’s modulus measured by dynamic mechanical analysis was raised by the aging treatment. These changes in the mechanical properties were discussed in connection with α phase precipitation.

Abstract: Although their experimental errors can be observed, pure polyurethane (PU) elastomers are one of the most important class of polymers due to some remarkable electromechanical characteristics such as large electric field induced strain, high specific energy and fast speed of response. In order to obtain the large strain at low electric field, a dependence of the solidification condition on strain was investigated for pure polyurethane films. Optimum solidification condition to get thin film with 19 μm thickness remarkably enhanced the strain at high electric field at high electric filed, although they show the low strain at low electric field at low electric filed. The starting point of the convergence occurred at a lower electric field for the solidification condition to get thick film with 150 μm thickness as opposed to for the optimum condition to obtain the thin film with 19 μm thickness. Based on results of crystalline volume fraction and crystalline periodicity, strongly attributed to not only polarization, but also electrostriction, the strain was controlled by the solidification condition. The optimum solidified samples do not have convergence until 20 MV/m. Based on the prediction and experimental results, the electrostriction of PU films depended on its solidification condition.

Abstract: To enhance the mechanical properties of Mg alloys, 0-30vol% Al₂O₃/Mg powders were formed by ball milling powder mixtures of pure Mg and Al₂O₃ particles, and then Spark plasma sintering (SPS) compacts (Al₂O₃/Mg discs) were made by the Al₂O₃/Mg powders. The effect of the cooling conditions in the SPS process on the mechanical properties of the Al₂O₃/Mg discs was investigated. From the results of SEM, XRD and TEM-EDS, the microstructures of the Al₂O₃/Mg discs were identified to consist of α-Mg solid solution, Al₂O₃ particles, refined MgO particles and refined needle-like Mg₁₇Al₁₂ ( more than 20vol% Al₂O₃ content). The mechanical properties of the discs were able to control by the regulation of the cooling conditions (cooling rate (v_c) and applied pressure in the cooling (p_c)) in SPS process, and as a result, the SPS discs possessing the mechanical properties beyond HP compacts were obtained under the cooling conditions of “v_c = 0.83K/s and p_c = 20MPa”. Main factor that the cooling conditions in SPS process effect on the mechanical properties of the Al₂O₃/Mg discs are considered to be the compresive residual stress generated in the α - Mg solid solution by the thermal stress associated with deference of the coefficients of thermal expansion between the α - Mg and ceramics particles (Al₂O₃ and MgO) in the discs.

Abstract: Through computer aided design, manufacturing and evaluation, various ceramics dendrites with spatially ordered micro cavities were successfully fabricated by utilizing stereolithography. Micrometer order ceramic lattices were propagated spatially in computer graphic space. Ceramics nanoparticles were dispersed in to photo sensitive liquid resins to obtain thixotropic slurries. The paste material was spread on a grass substrate by using a mechanical knife edge, and an ultra violet micro pattern was exposed to create cross sectional solid layer. After the layer stacking process, the obtained composite precursor was dewaxed and sintered in an air atmosphere. By the micro patterning stereolithography, solid electrolyte dendrites of yttria stabilized zirconia with spatially ordered porous structures were fabricated for fuel cell miniaturizations. Gaseous fluid profiles and pressure distributions in the formed ceramic lattices with various porosity percent were visualized and analyzed by a finite element method. Subsequently, alumina micro photonic crystals with a diamond lattice structure were fabricated. Electromagnetic wave properties were measured by using a terahertz time domain spectroscopy. A complete photonic band gap was exhibited, and a localized mode to select the wavelength was obtained by introducing a defect cavity.