Hiroyuki Anada

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti−34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al2O3 barrier being formed. This is probably due to the doping of those elements in TiO2. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al2O3 layer as the second layer from the outer surface and the relatively massive precipitation of Al2O3 in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.

The influence of ternary element addition on the oxidation behaviour of TiAl intermetallic compound in high temperature air

Abstract A variety of ternary element additions were made to a binary TiAl (Ti-34.5 wt% Al) intermetallic compound and the oxidation behaviour was studied with particular interest in the influences of the ternary elements. As a result of the oxidation tests in air at temperatures between 1073 and 1273 K, the effect of various elements was classified into three groups, i.e. (a) detrimental—Cu, Y, V, Cr and Mn; (b) neutral—Sn, Zr, Hf, Ta, Ni and Co; (c) beneficial—Si, Nb, Mo and W. Their influence was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. Particularly, the W- and the Mn-modified alloys were approximately 200 and 100 K more resistant than the binary alloy. The mechanism for the improvement by the W and Mo additions may be explained as follows: the elements are enriched on the metal side of the scale/metal interface and may cause the formation of β and/or δ phases in which Al diffusion may be fast and oxygen solubility may be small. This leads to Al-enriched scale formation and/or Al 2 O 3 layer formation along the interface.

Thermal and mechanical properties of zirconium hydride

The physico–chemical properties of zirconium hydride such as mechanical and thermal properties have been studied in the present study. The zirconium hydride specimens in the form of pellets (6 mm φ×10 mm l) had the hydrogen contents with 1.5–1.7 H/Zr, which were fabricated directly from zirconium metal in a modified UHV Sieverts apparatus. All the zirconium hydrides prepared in the present study showed CaF2 type δ ZrH2−x. The lattice parameter slightly increased with the hydrogen content. The thermal expansion coefficients of the zirconium hydrides evaluated from high-temperature X-ray diffraction data were larger than that of zirconium metal and increased with the hydrogen content. The longitudinal and shear sound velocities of the zirconium hydride were slightly different from those of zirconium metal, which enabled us to estimate the elastic properties. The zirconium hydride had higher elastic moduli than zirconium metal and the elastic moduli slightly depended on the hydrogen content. The microhardness of the zirconium hydride was much higher than that of zirconium metal and decreased with increasing hydrogen content. The Debye temperature of the zirconium hydride estimated from the sound velocities was larger than that of pure zirconium metal. The heat capacity was also estimated from the sound velocities and the thermal expansion data.

Thermal properties of zirconium hydride

Abstract Zirconium hydride specimens with hydrogen contents of 1.45–1.70 H/Zr were fabricated directly from zirconium metal in a modified UHV Sieverts apparatus. All the zirconium hydride fabricated in the present study were found from X-ray diffraction analysis to be CaF2-type δ-ZrH2−x. At temperatures of 350–700 K, the heat capacity of the zirconium hydride was measured using an enthalpy method by means of a differential scanning calorimeter (DSC), and the experimental results were consistent with the values estimated from the sound velocities and the thermal expansion coefficient. In the temperature range of 300–700 K, the thermal diffusivity of the zirconium hydride was examined by a laser flash method. The thermal diffusivity decreased with increasing temperature and was not markedly influenced by the hydrogen content. The thermal conductivity of the zirconium hydride was calculated from thermal diffusivity and heat capacity and found to have slightly lower thermal conductivity than pure zirconium metal.

Characteristics of zirconium hydride and deuteride

The electrical and thermal properties of zirconium hydride and deuteride have been measured. The lattice parameter of δZrD2−x was smaller than that of δZrH2−x, and the deuteride had higher elastic moduli than the hydride. The electrical and thermal conductivities of δZrH2−x were slightly different from pure Zr metal. The electronic structure of the zirconium hydride was found from XPS measurements to differ from those in the pure Zr metal, and there was a peak due to the Zr–H bond at 6.4 eV below Fermi energy in the XPS spectra. The density of states was estimated by a molecular orbital calculation, and the agreement between the XPS valence band measurements and the calculations was quite satisfactory. Some of the mechanical and thermal properties of zirconium hydride and deuteride were interpreted in terms of the results of the molecular orbital calculation.

Influence of precipitated hydride on the fracture behavior of Zircaloy fuel cladding tube

In order to clarify the influence of precipitated hydride on the fracture behavior of Zircaloy cladding tubes, the stress-strain distribution of the cladding was estimated by finite element method (FEM) analysis. The mechanical properties of α-phase of zirconium and zirconium hydride required for the analysis were examined by means of an ultrasonic pulse-echo method and a tensile test. It was found from the analysis that the non-hydrided cladding has the highest equivalent plastic strain at the inner surface of the cladding, suggesting that the fracture initiated at the inner surface of the cladding. Since the hydride accumulated layer located in the outer surface of the hydrided cladding fails at a lower internal pressure, the crack appears to initiate at the outer surface of the cladding. The fracture behavior estimated from the stress states of the hydrided cladding was in good agreement with the experimental results obtained by pulse irradiation tests of the Nuclear Safety Research Reactor (NSRR) and high-pressurization-rate burst tests in the Japan Atomic Energy Research Institute (JAERI).

Analysis of the fracture behavior of a hydrided cladding tube at elevated temperatures by fracture mechanics

An analysis, based on fracture mechanics at elevated temperatures, has been carried out for several types of hydrided Zircaloy cladding tubes to elucidate the fracture behavior of high burn up light water reactor fuel cladding during reactivity initiated accidents. Fracture mechanics parameters such as stress intensity factor, J-integral and plastic yield load were estimated by a finite element method analysis, and the material properties of α-phase of zirconium required for the analysis were obtained by tensile tests at elevated temperatures. The failure assessment diagram (FAD) was constructed using the fracture mechanics parameters to estimate the failure stress of the cladding. It was found from the FAD that the predicted failure stress of the cladding qualitatively agreed with the experimental results obtained by burst tests at elevated temperatures for the hydrided Zircaloy cladding tube at Japan Atomic Energy Research Institute.

Isotope effects on the physicochemical properties of zirconium hydride

Abstract The isotope effects on the physicochemical properties of zirconium hydride have been studied. The zirconium deuteride specimens in the form of pellets (6-mm diameter×10 mm length) were loaded with deuterium contents with 1.5–1.7 D/Zr, and were fabricated directly from zirconium metal in a modified UHV Sieverts apparatus. The X-ray diffraction analysis showed that all the zirconium deuterides prepared exhibited CaF2-type structure(δZrD2−x), and the lattice parameter of δZrD2−x was smaller than that of δZrH The zirconium deuteride had higher elastic moduli than the hydride. The microhardness of the zirconium deuteride was slightly higher than that of hydride. The physicochemical properties of the zirconium deuteride such as Debye temperature and heat capacity were estimated from the sound velocities and the thermal expansion data. The Debye temperature and heat capacity of zirconium deuteride were higher than that of zirconium hydride.

Analysis of the electronic structure of zirconium hydride

Abstract The electronic structure of zirconium hydride was analyzed by a discrete-variational (DV)-Xα molecular orbital method. The density of states of zirconium hydride estimated by DV-Xα calculation agreed with X-ray photoemission spectroscopy spectra of zirconium hydride. The net charges of Zr atom and H atom were found to be small and independent of the hydrogen content. While the Zr–Zr bond order decreased with the hydrogen content, the Zr–H bond order was hardly affected by the hydrogen content. The mechanical properties such as elastic modulus were qualitatively discussed on the basis of the results for the molecular orbital calculation.

Tensile test of hydrided Zircaloy

In order to examine the influence of precipitated zirconium hydride on the failure behavior and fracture strength of light water reactor (LWR) cladding tubes, tensile tests were performed at room temperature for non-hydrided and hydrided Zircaloy sheet-type specimens with gauge section of 10.0×5.0 mm and thicknesses of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm. For specimens with thickness more than 2.5 mm, the ultimate tensile strength of the specimens appeared to be independent of thickness, which implied that plane strain condition was attained. For the specimen with 2.5 mm thickness, the ultimate tensile strength increased slightly with increasing average hydrogen concentration. Through microscopic observation of the hydrided specimen surface by scanning electron microscopy (SEM), it was found that matrix/hydride debonding was not generated but that micro-cracks perpendicular to the axial direction were produced at the hydride layer.