Nb Tubes Research Articles

Crack tip stress effect on delayed hydride cracking velocity of Zr–2.5Nb tubes

Using our new delayed hydride cracking (DHC) model, we scrutinized Pan's DHC test results of a cold-worked Zr–2.5Nb tube after neutron irradiation. DHC velocity at 240 °C of the Zr–2.5Nb tube increased gradually at neutron fluences below 5 × 10 25 n/m 2 ( E > 1 MeV) and leveled off to a constant value at higher neutron fluences over it. This neutron fluence dependence of the DHC velocity was found to be similar to that of the Nb concentration in the β-Zr, not that of its tensile stress, which is hard to understand in view of the old DHC models. Normalization of the DHC velocity by hydrogen diffusivity or D H is found to lessen the apparent yield stress effect on the DHC velocity of Zr–2.5Nb tubes. Consequently, it is demonstrated that the DHC velocity of the Zr–2.5Nb tubes is governed not by the crack tip stresses but by hydrogen diffusion, corroborating the validity of the new DHC model.

Effect of Irradiation Damage on the Deformation Properties of Zr-2.5Nb Pressure Tubes

Abstract The diametral expansion, elongation, and sag rates of Zr-2.5Nb pressure tubes in CANDU® (CANada Deuterium Uranium) nuclear reactors are important properties that limit their useful life and the maximum power level for reactor operation. As a result irradiation creep models are needed to predict the deformation behavior of the core components over the reactor life. It is important to know the creep behavior as a function of neutron flux in order to develop creep models over the range of operating conditions in the reactor core. At the edge of the reactor core, the neutron flux is decreasing very rapidly and there is a complex transition in creep behavior from irradiation-dominated creep to thermal-dominated creep. Also, mechanical properties such as tensile strength, fracture toughness, and delayed hydride-cracking are changing in the transition from thermal to irradiation conditions at the edge of the reactor core. Detailed studies have been completed on a Zr-2.5Nb tube irradiated in the NRU materials test reactor at Chalk River Laboratories. Pressure tube 601 was operating for a period of 66 950 h at temperatures ranging from about 547 K at the inlet and 571 K at the outlet. After the tube was removed in 1988 samples were taken for retrospective dosimetry to determine the fast neutron flux along the assembly. It was determined that the tube had been irradiated to a peak fluence of about 6×1025 n⋅m−2 corresponding to a fast neutron flux of about 2×1017 n⋅m−2⋅s−1. The flux profile was mapped and it was clear that the flux dropped rapidly to negligible values at about 0.5 m from the ends of the fueled zone. Samples of pressure tubes were taken for hardness testing and characterization by TEM and XRD analysis at various locations corresponding with different operating conditions (neutron flux and temperature) but at the same time. The creep behavior during operation was obtained by periodic gaging of the pressure tube internal diameter. The results of the microstructure characterization are presented and discussed in relation to the measured mechanical properties (creep and hardness). The microstructure and mechanical properties change significantly in the transition from the unirradiated state up to fluxes of about 1×1017 n⋅m−2⋅s−1.

Threshold stress intensity factor, KIH for delayed hydride cracking of a Zr-2.5Nb tube with loading mode

Abstract The threshold stress intensity factors, KIH's, for delayed hydride cracking of a Zr-2.5Nb tube were determined using either load-increasing or decreasing modes over a temperature range of 160–280 °C with a step-wise load increase or decrease by 0 .5 MPa m , respectively. KIH's in the load-increasing mode were found to be higher than those in the load-decreasing mode. The striation spacing corresponding to the critical hydride length or lc showed a sharp drop between the 1st striation line to the 9th line and then leveled off to a constant value in the load-increasing mode. In the load-decreasing mode, it was rather constant from the 1st line to the 14th line due to a higher stress intensity factor KI with its gradual increase at a reduced KI closed to KIH. Supplementary tests demonstrated that a higher KIH in the load-increasing mode is due to the combined effects of a larger tip radius of the fatigue crack and creep working in the load-increasing mode. KIH had a negative temperature dependency in the load-increasing mode and it had no temperature dependency in the load-decreasing mode. To correlate KIH with twins, tensile tests were conducted on the Zr-2.5Nb tube and its textural changes during the tensile tests were investigated using an X-ray diffractometer. The number of twins increased over a temperature range of 150–300 °C and then decreased rapidly at over 300 °C with no twins at 350 °C. KIH of the Zr-2.5Nb tube with the loading modes was discussed in view of cracking of the hydrides by twining.

Comment on ‘Influence of metallurgical variables on delayed hydride cracking in Zr–Nb pressure tubes’ by P. Cirimello et al., J. Nucl. Mater. 350 (2006) 135–146

Reanalysis of the DHC initiation temperature, T c and the crack propagation rate, V p of CANDU and RBMK Zr–2.5Nb tubes, recently published by Cirimello et al., was conducted, using Kim’s DHC model. It is shown that a negative temperature dependency of V p at above T(max V p) for both Zr–2.5Nb tubes is due to a decreased hydrogen supersaturation below Δ C min over which V p becomes independent of hydrogen concentrations. When compared to more than 200 data of V p for the same Zr–2.5Nb tubes collected through the IAEA round robin tests, it is concluded that the temperature dependencies of their V p which the authors determined using a single specimens are faulty.

Development of Experimental Procedure Based on the Load Separation Method to Measure the Fracture Toughness of Zr-2.5Nb Tubes

The load separation method was employed to measure the pl η -factor, the growing crack length and the applied J-integral during the course of the test of small curved CT specimen of Z-2.5Nb pressure tube material. The effect of the notch tip radius of the notched (reference) CT specimen on the separation parameter was analyzed to predict the crack growing length in the precracked specimen. To avoid the effect of load relaxation in the reference specimen on the separation parameter and the crack growing length estimation, the load was assumed to be maximum and constant value behind the peak load in the reference curve.

Substitution Effects in Zintl Phases: Synthesis and Crystal Structure of the Novel Phases Ae3Sn4−xBi1+x (x ≤ 1; Ae = Sr, Ba) Containing Shubnikov‐Type Nets $^{2}_{\infty}\rm [Sn_{4-x}Bi_{x}

AbstractThe compounds Ae3Sn4−xBi1+x (Ae = Sr, Ba) with x < 1 have been synthesized by solid‐state reactions in welded Nb tubes at high temperature. Their structures were determined by single crystal X‐ray diffraction studies to be tetragonal; space group I4/mcm (No. 140); Z = 4, with a = 8.968(1) Å, c = 12.859(1) Å for Sr3Sn3.36Bi1.64(3) (1) and a = 9.248(2), c = 13.323(3) Å for Ba3Sn3.16Bi1.84(3) (2). The structure consists of two interpenetrating networks formed by a 3D Ae6/2Bi substructure (anti‐ReO3 type) forming the host, and layers of interconnected four‐member units [Sn4−xBix] with “butterfly”‐like shape as the guest. According to the Zintl‐Klemm concept, the compounds are slightly electron deficient and will be charge balanced for x = 1. The electronic structures of Ae3Sn4−xBi1+x calculated by the TB‐LMTO‐ASA method indicate that the compounds correspond to ideal semiconducting Zintl phases with a narrow band gap for x = 1 (zero‐gap semiconductor). The origin of the slight deviation from the optimal electron count for a valance compound is discussed.

Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

Delayed hydride cracking (DHC) tests were conducted on CANDU 1 CANadian Deuterium Uranium. 1 Zr–2.5Nb tubes containing hydrogen with a microstructure of elongated α-Zr and semi-continuous β-Zr at temperatures ranging from 100 to 300 °C. DHC velocity (DHCV) was found to be around two times higher in the axial direction than that in the radial direction even on the same cracking plane. According to Kim’s DHC model that DHCV at temperatures lower than 300 °C is governed mainly by hydrogen diffusion and to a lesser extent by the hydrogen concentration gradient at the crack tip, we suggest that the enhanced DHCV in the axial direction arises from a faster hydrogen diffusion in the axial direction with a semi-continuous distribution of β-Zr. Evidence to this suggestion is provided by Levi’s experiment that Zr–2.5Nb tubes or plates with a fully discontinuous β-Zr due to annealing at 400 °C for 1000 h or 650 °C for 9 h has no anisotropic DHCV with the orientation. Hence, we conclude that Kim’s DHC model is plausible. An anisotropic DHCV of irradiated CANDU Zr–2.5Nb tubes is discussed based on their microstructural evolution with the neutron fluence and operating temperatures.

Effect of fast neutron fluence on the creep anisotropy of Zr–2.5Nb tubes

The in-reactor behaviour of internally pressurised capsules of Zr–2.5Nb tubes is analysed in detail to separate the stress dependent component of deformation (creep). It is found by a rigorous statistical analysis that the creep rate varies with fast neutron fluence. At 555 K the axial creep rate increases while the transverse creep rate decreases with fluence. At 588 K the creep rate in both the axial and transverse directions increases with fluence. It is also shown that the creep anisotropy ratio R, i.e., the ratio of axial to transverse creep rate for a pressurised tube, varies with fluence, stress and irradiation temperature. These findings are discussed in terms of the irradiation-induced evolution in microstructure. The possible impact of the evolution of the dislocation substructure is discussed with reference to a self-consistent polycrystalline model that takes into account the crystallographic texture and the grain interaction strains present in zirconium alloys. The lower temperature creep behaviour is consistent with an increase with fast fluence of the single crystal creep compliance related to prismatic dislocation climb and glide, or a decrease in the single crystal creep compliances relating to basal and pyramidal slip. The creep behaviour at the higher irradiation temperature is more complicated, and there may be an influence of phase changes as well as dislocation structure. It appears that all three eigenvalues describing the single crystal creep behaviour depend on fast fluence.

Jc enhancement of high density MgB2 bulk made by Premix-PICT-Diffusion method

A high Jc of 1.05MA/cm2 at 20K in self-field was achieved by a MgB2 bulk synthesized by newly developed “Premix-Powder-In-Closed-Tube-Diffusion” (Premix-PICT-Diffusion) method starting from Mg and powder mixture of B and MgB2, which were separately packed into Nb tubes. MgB2 bulks prepared by this method are highly dense without voids and cracks, resulting in two or three times larger effective grain connecting area contributing dramatically high Jc compared with the conventional PICT-processed porous bulks. This method will be easily extended for development of superconducting current leads with large capacities.

Effect of Texture on Anisotropic Creep of Pressurized Zr-2.5Nb Tubes

In this paper we report the stress dependence of the diametral and axial creep rates of cold-worked Zr-2.5Nb tubes with three different combinations of texture and microstructure in biaxial tests at 623K. One type of tube has a predominantly radial texture and it exhibits a high diametral creep rate and negative long term axial creep rate. A second type, with a predominantly transverse texture and a very fine elongated grain structure exhibits an intermediate diametral strain rate and a positive axial strain rate. The third type, with predominantly transverse texture and coarser elongated grain structure exhibits the lowest diametral creep and a positive axial strain rate. The stress exponents for the diametral creep are in the range 5-7, but those for axial creep vary from 0.7 to 7.3. The creep anisotropies were calculated with the polycrystalline model SELFPOLY-7, assuming that slip is the strain producing mechanism. The calculations give qualitative agreement with the experimental results.

Temperature dependency of delayed hydride cracking velocity in Zr–2.5Nb tubes

Delayed hydride cracking (DHC) tests were conducted on Zr–2.5Nb tubes with different distributions of the β-Zr at temperatures ranging from 398 to 573 K. Compact tension specimens charged to 27–100 ppm hydrogen were used to determine the temperature dependencies of their DHC velocity (DHCV) and their striation spacing. The CANDU Zr–2.5Nb tube with a higher yield strength and a semi-continuous β-Zr had a higher DHCV and a smaller striation spacing than the RBMK Zr–2.5Nb tube with a fully discontinuous β-Zr and a lower yield strength. It is found that the activation energy for the DHCV is the sum of the activation energies for hydrogen diffusion and the striation spacing representing the hydrogen concentration gradient at the crack tip. Quantitative contribution of hydrogen diffusion and the hydrogen concentration gradient to the DHCV is discussed. This study provides supportive evidence for the plausibility of Kim's DHC model.

Gallium–tin mixing in BaGa4−xSnx[x= 0.89 (2)] with the BaAl4structure type

Single crystals of the barium trigallium tin, BaGa3.11 (2)Sn0.89 (2), have been obtained by a reaction of the elements in a sealed Nb tube. The title compound crystallizes with the BaAl4-type (Pearson's code tI10), and some of the Ga atoms are replaced by Sn atoms. All atoms occupy special positions: Ba (4/mmm), Ga1/Sn1 (\overline{4}m2) and Ga2/Sn2 (4mm). The refined composition has also been confirmed by means of elemental microanalysis.

Temperature Dependence of Threshold Stress Intensity Factor, K<sub>IH</sub> in Zr-2.5Nb Alloy and Its Effect on Temperature Limit for Delayed Hydride Cracking

Threshold stress intensity factor or KIH required to initiate a delayed hydride crack in the axial direction of a Zr-2.5Nb tube was determined at temperatures of 160 oC to 280 oC. KIH remained constant at temperatures from 160 to 250 oC, increased with an increasing temperature in excess of 280 oC using a load decreasing method. To correlate KIH and dislocation twins, tensile tests were conducted on the Zr-2.5Nb tube and textural changes during tensile tests were investigated using an X-ray diffractometer. The extent of the twins increased from 150 to 300 oC and then decreased at temperatures in excess of 300 oC with no twins occurring at 350 oC. Temperature dependencies of KIH and a temperature limit for delayed hydride cracking were discussed with cracking of hydrides by the twins. This study provides supportive evidence to the feasibility of Kim’s DHC model.

Artificial pinning centre Nb–Ti superconducting wire for AC applications

The present work presents the steps for the manufacture of an Nb–Ti artificialpinning centre (APC) multifilament wire for AC applications. The wire productionstarts from a Ti bar inserted in a Nb tube. The superconductor Nb–Ti alloywas obtained using heat treatments during the wire production steps throughthe growth of the diffusion layer between the pure Nb and Ti regions. The finalwire is composed of many filaments dispersed in a Cu–Ni matrix. The transportcritical current density was measured in samples for different conditions of heattreatment, followed by wire diameter reduction. Magnetization hysteresis loops weremeasured to extract the hysteretic losses in wires with filament diameters of 76 and92 µm and compared to the results in commercial wires with comparable filamentdiameters. We obtained an Nb–Ti APC wire with hysteretic losses of the sameorder of magnitude as the commercial wires. However, critical current densitiesJc are higher for fields lower than 2 T, where AC applications are most important. TheJc results confirmed a previous result that the decrease of normal region sizes is a lessimportant pinning source in this material than the pinning from variations in thecomposition of the diffusion layer.

Development of local microstructure and crystallographic texture in extruded Zr–2.5Nb tubes

We report the bulk and local crystallographic texture determined by neutron diffraction in Zr–2.5Nb tubes extruded in the α + β and the β-phase fields. Bulk textures and through-wall variations were determined using neutron diffraction. We correlate these with microstructures determined by transmission electron microscopy. With a preheat temperature of 925 K where the material is ∼90% α-phase, the basal plane are distributed between radial (R) and transverse (T) directions with a small bias towards the T direction. End-to-end, and through-wall variation depends upon the prior microstructure of the billet and the balance between adiabatic heating and chilling. With a preheat temperature of 1090 K, where the material is ∼60% α, the basal plane normals are predominantly transverse, more so on the internal diameter, because of the extrusion geometry. With a preheat temperature of 975 °C, there is a strong axial component of texture at the front end of the tube, associated with the β- to α-phase transformation. This component is more pronounced at the core because of chilling of the billet. At the back end of the extrusion the texture is very similar to that produced after preheating at 815 °C indicating that the back of the billet was substantially transformed by the time that it entered the die.

Synthesis and crystal structure of CaAgBi and BaAg 1.837Bi 2

Two ternary alkali earth silver bismuthides, CaAgBi and BaAg 1.837Bi 2, have been synthesized by solid-state reactions of the corresponding metals in welded Nb tubes at high temperature. Their structures have been established by single-crystal X-ray diffraction studies. CaAgBi crystallizes in the hexagonal space group P6 3 mc (No.186) with cell parameters of a = b = 4.8113(4) Å, c = 7.8273(9) Å, V = 156.92(3) Å 3, and Z = 2. BaAg 1.837Bi 2 belongs to tetragonal space group P4/ nmm (No.129) with cell parameters of a = b = 4.9202(2) Å, c = 11.628(1) Å, V = 281.50(3) Å 3, and Z = 2. The structure of CaAgBi is of the LiGaGe type, and features a three-dimensional four-connected (3D4C) anionic network with Ca 2+ encapsulated in the channels formed by [Ag 3Bi 3] six-membered rings. BaAg 1.837Bi 2 is isostructural with CaBe 2Ge 2, a variant of the tetragonal ThCr 2Si 2-type structure. Its structure exhibits a three-dimensional anionic network built of (0 0 1) and (0 0 2) puckered [Ag 2Bi 2] layers interconnected via additional Ag–Bi bonds along the c-axis. BaAg 1.837Bi 2 is metallic based on band structure calculations.

Effects of tube fabrication variables on the oxidation of experimental Zr–2.5Nb tubes

Experimental Zr–2.5Nb tubes, manufactured with various fabrication routes, were oxidized in air at 400 and 500 °C, and their oxide–metal interfaces are characterized by scanning electron microscopy. Various tube microstructures are formed depending to a large extent on the extrusion temperatures in the range of 650–975 °C. The second phase distribution of the tube has a great impact on the oxidation resistance. The tubes that have fine and discrete second phase structures show a superior oxidation resistance to those of coarse and continuous second phase structures. The oxide interface structure evolves as the oxide grows by mirroring the metal microstructure at the beginning of the oxidation, and then develops either a rough structure at higher oxidation rate or a flat structure at lower oxidation rate. The oxide granules in the flat interfaces grow with increases both in oxide thickness and oxidation temperature, but the granule size is held constant in the rough interfaces. Therefore, the oxidation resistance of Zr–2.5Nb tube may be improved when the tube has fine distribution of second phases with the formation of a flat structure at the metal–oxide interface.

MgB2 composite wires with Fe, Nb and Ta sheaths

Single-core MgB2 composite wires have been made by the powder-in-tube method using commercial Mg, B andMgB2 powders (Alfa Aesar) in Fe, Nb and Ta tubes and both in situ and ex situ processes.Prepared wires were subjected to annealing at temperatures ranging from600 °C up to950 °C for 30 min in argonatmosphere. Resistive (R(T)) and transport current (Ic(μ0H)) measurements have shown how the sheath material that was used influences the criticaltemperature and critical current density. Inter-diffusion and reaction has been observedonly for the iron sheath. Niobium has appeared as the best sheath material forMgB2 wires made by the in situ process but as the worst sheath material for the ex situ method.The reason is mainly due to the large transversal cracks generated in the fully Nb sheathedex situ wires during the deformation, which are not healed by the subsequent heattreatment.

Thin walled Nb tubes for suspending test masses in interferometric gravitational wave detectors

In a previous Letter, we have shown that the use of orthogonal ribbons could provide a better mirror suspension technique in interferometric gravitational wave antennas. One of the key improvements presented by the orthogonal ribbon is the reduction in the number of violin string modes in the direction of the laser. We have considered more elaborate geometries in recent simulations and obtained a suspension that provides further reduction in the number of violin string modes in the direction of the laser, as well as in the direction orthogonal to the laser. This thin walled niobium tube suspension exhibits a reduction in the number of violin modes to 5 in each direction up to a frequency of 5 kHz. Furthermore, the violin mode thermal noise peaks can be reduced in amplitude by 30 dB.

Effect of microstructural evolution on in-reactor creep of Zr–2.5Nb tubes

Dislocation density, decomposition of the β-Zr phase and diametral creep were examined as a function of the location of the Zr–2.5Nb tube irradiated in the Wolsong Unit 1 for 9.3 effective full power years (EFPYs). The maximum a-dislocation density occurred at the inlet part of the irradiated Zr–2.5Nb tube exposed to the lowest temperature while the outlet part of the tube exposed to the higher temperature had the higher extent of decomposition of the β-Zr phase and the maximum diametral creep. Thus, it is concluded that in-reactor creep of the Zr–2.5Nb tube is not related to the dislocation density but governed by the Nb concentration of the α-Zr grains caused by thermal decomposition of the β-Zr phase. Supplementary creep tests on the Zr–2.5Nb sheets with different Nb contents in the β-Zr phase provide supportive evidence to this conclusion. The acceleration of the in-reactor creep of the Zr–2.5Nb tubes is suggested after a long-term operation.