N18 Alloy Research Articles

Monitoring the kinetics of the γ’ phase in the N18 superalloy using in situ electrical resistivity measurements

In nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ′ precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ′ precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ’ phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes.

Hydrogen uptake behaviors of Zr-1.0Cr-0.4Fe-(0.2Mo) and N18 alloys during corrosion in 500 °C steam

The hydrogen uptake behaviors of Zr-1.0Cr-0.4Fe, Zr-1.0Cr-0.4Fe-0.2Mo and N18 (Zr-1.0Sn-0.3Nb-0.3Fe-0.1Cr) alloys during corrosion in 500 °C steam are investigated, and the hydrogen uptake fraction (HUF) is used to evaluate the hydrogen absorption ability of these alloys. The results show that the HUF changes are related to the corrosion process. Hydrogen uptake is mainly affected by the precipitated particles in the pre-transition stage; alloys with a larger amount of particles tend to absorb more hydrogen, regardless of the alloying composition. However, oxide morphologies play a dominant role in the hydrogen uptake during a post-transition oxidation. At this stage, the HUF increases sharply to a high level due to the formation of large in size parallel cracks in the oxide layers. The evaluation of the hydrogen uptake behavior of the Zr-1.0Cr-0.4Fe alloy suggests that although the large amount of the fine precipitated particles in this alloy leads to a higher HUF in the initial pre-transition stage, eventually, after long-term exposure, this alloy will have a lower final hydrogen content due to its lower corrosion rate and the effect of the protective oxide film.

Corrosion-related defects in Zircaloys: a preliminary study with slow positron beam

Corrosion-related microstructure and defects in Zircaloy-4 and N18 alloys were investigated by variable energy positron annihilation spectroscopy. The specimens were corroded in 0.01mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa, respectively. Defect profiles were analyzed by measuring the S parameter as a function of incident positron energy from 0.25 to 27 keV. Results indicated that Zircaloys corroded in LiOH aqueous solution contained more defects in the oxide layer than that in superheated steam, which implies that formation of defects in oxide layer may relate to the effects of Li+ ions in corrosion solution.

CORROSION BEHAVIOR OF Zr-0.4Fe-1.0Cr-x Mo ALLOYS IN 500℃ and 10.3 MPa STEAM

4 Mo Zr-0.4Fe-1.0Cr-xMo(x=0, 0.2, 0.4, 0.6, , ABSTRACT The possibility of using Mo as an alloying element in zirconium alloys was considered in terms of its strengthening effect and microstructure refinement effect. However, the impact of Mo addition on the corrosion resistance was not fully understood. In this work, Zr-0.4Fe-1.0Cr-xMo (x=0, 0.2, 0.4, 0.6, mass fraction, %) alloys with addition of different Mo contents were prepared by vacuum arc melting method and their corrosion resistance in 500 , 10.3 MPa steam was investigated. Compared with Zr-4, N18 and M5 alloys, the corrosion rate of Zr-0.4Fe-1.0Cr-xMo alloys was much lower, which was attributed to the large numbers of fine second phase particles in the matrix. Addition of Mo improved the evolution of the oxide film during growth and resulted in the degradation of corrosion resistance. The growth of the oxides remained cubic kinetics in the whole corrosion period (2000 h) for the Mo free alloy, whereas changed from cubic to linear kinetics after a corrosion time of 500—1000 h for the Mo containing alloys.

Optimization of N18 Zirconium Alloy for Fuel Cladding of Water Reactors

In order to optimize the microstructure and composition of N18 zirconium alloy (Zr-1Sn-0.35Nb-0.35Fe-0.1Cr, in mass fraction, %), which was developed in China in 1990s, the effect of microstructure and composition variation on the corrosion resistance of the N18 alloy has been investigated. The autoclave corrosion tests were carried out in super heated steam at 400 C/10.3 MPa, in deionized water or lithiated water with 0.01 mol/L LiOH at 360 C/18.6 MPa. The exposure time lasted for 300-550 days according to the test temperature. The results show that the microstructure with a fine and uniform distribution of second phase particles (SPPs), and the decrease of Sn content from 1% (in mass fraction, the same as follows) to 0.8% are of benefit to improving the corrosion resistance; It is detrimental to the corrosion resistance if no Cr addition. The addition of Nb content with upper limit (0.35%) is beneficial to improving the corrosion resistance. The addition of Cu less than 0.1% shows no remarkable influence upon the corrosion resistance for N18 alloy. Comparing the corrosion resistance of the optimized N18 with other commercial zirconium alloys, such as Zircaloy-4, ZIRLO, E635 and E110, the former shows superior corrosion resistance in all autoclave testing conditions mentioned above. Although the data of the corrosion resistance as fuel cladding for high burn-up has not been obtained yet, it is believed that the optimized N18 alloy is promising for the candidate of fuel cladding materials as high burn-up fuel assemblies. Based on the theory that the microstructural evolution of oxide layer during corrosion process will affect the corrosion resistance of zirconium alloys, the improvement of corrosion resistance of the N18 alloy by obtaining the microstructure with nano-size and uniform distribution of SPPs, and by decreasing the content of Sn and maintaining the content of Cr is discussed.

加工工艺对N18锆合金在360 ℃/18.6 MPa LiOH水溶液中腐蚀行为的影响

The effect of thermal processing on corrosion behavior of N18 alloy has been inves- tigated by autoclave tests in 0.01 mol/L LiOH aqueous solution at 360 and l8.6 MPa, and the microstructures of these specimens were examined by TEM and SEM. The results show that the cor- rosion resistance of specimens is improved obviously by β phase quenching before cold rolling and annealing due to nano second phase particles (SPPs) precipitated dispersively, but when the inter- mediate annealing temperature increased to 740 before cold rolling and annealing, the corrosion resistance is lowered greatly due to SPPs coarsened to several hundred nanometers, where the cor- rosion rate accelerates markedly after the corrosion transition at 150 d exposure. The existence of β-Zr phase in the microstructures of specimens treated at 780 and 800 for 2 h in dual phase region is harmful to the corrosion resistance, but the corrosion resistance returns to a better level after the decomposition of β-Zr phase during the following processes of cold rolling and annealing at 580 to obtain fine SPPs.

Threshold stress intensity factor for delayed hydride cracking of a recrystallized N18 alloy plate along the rolling direction

The objective of this study is to obtain the threshold stress intensity factor, K IH, for an initiation of delayed hydride cracking in a recrystallized N18 (Zr–Sn–Nb–Fe–Cr) alloy plate which was manufactured in China, gaseously charged with 60 ppm of hydrogen by weight. By using both the load increasing method and load drop method, the K IH’s along the rolling direction were investigated over a temperature range of 150–255 °C. The results showed that K IH along the rolling direction was found to be higher in the load increasing method than that in the load drop method. In the load increasing method, K IH’s of the N18 alloy plate appeared to be in the range of 31 – 32.5 MPa m , and K IH in the load drop method appeared to be in the range of 27.5 – 28.6 MPa m . This means that the N18 alloy plate has high tolerance for DHC initiation along the rolling direction. The texture of a N18 alloy plate was investigated using an X-ray diffraction and the K IH was discussed based on texture and analytically as a function of the tilting angle of hydride habit planes to the cracking plane.

PREDICTION OF CRITICAL TEMPERATURE FOR DELAYED HYDRIDE CRACKING IN IRRADIATED N18 ZIRCONIUM ALLOY

Delayed hydride cracking (DHC) has been recognized as a potential threat to the structural integrity of zirconium alloy components in water–cooled nuclear reactors since the early 1970 s. Although DHC has been studied for a long time, most of the investigations were focused on unirradiated Zr–Nb and Zr–Sn alloys, no information was found on Zr–Sn–Nb alloy. Currently, several new Zr–based alloys have been developed from the point of view of enhancement of corrosion resistance and hydrogen pickup properties. In China, N18 (Zr–Sn–Nb) alloy has been developed based on Zr–Sn and Zr–Nb systems to meet the requirements of higher fuel burn–up. Therefore, it is necessary to investigate the DHC behavior of N18 alloy systematically. The objective of this study is to predict the critical temperatures for initiating and arresting DHC in irradiated N18 alloy. A model used to forecast DHC critical temperature of irradiated zirconium alloys was established based on the terminal solid solubility of hydrogen in irradiated zirconium alloys and stress–induced hydrogen diffusion. By using the model, the critical temperatures for irradiated N18 alloy were predicted. The result showed that the irradiated N18 alloy has high sensitivity for DHC initiation, because neutron irradiation increased the solubility of hydrogen and yield strength of N18 alloy. Compared with unirradiated N18 alloy, the critical temperature * $! AE# JJXM–QNJJ–09–04–04 % : 2009–12–29, &% : 2010–03–31 $%' : ! , !, 1981 , # , DOI: 10.3724/SP.J.1037.2009.00868

Dissolution and precipitation behaviors of hydrides in N18, Zry-4 and M5 alloys

The terminal solid solubilities of hydrides dissolution (TSSD) during heatup and precipitation (TSSP) during cooldown for N18, Zry-4 and M5 alloys with hydrogen concentrations of 20–240 ppm were measured by differential scanning calorimetry (DSC) in the range of 50–500 °C. There is little difference in either TSSD or TSSP solvi among these alloys, and best-fit equations were derived from the two solvi. A significant hysteresis between the solvi of TSSD and TSSP occurred, resulting from the hydride–matrix volumetric misfit strain. Based on the widths of the DSC peaks obtained during cooldown, the average precipitation rates of zirconium hydrides from the super-saturated state were evaluated by best-fit equations. The hydride precipitation is rate-controlled by hydrogen diffusion, and the average rates increased with increasing temperature.

Effect of Temperature on Cyclic Deformation Behavior of Zr-Sn-Nb Alloy

Effect of temperature on the cyclic deformation behaviors of Zr-Sn-Nb alloy (N18 alloy) at temperatures of 200, 300, 400 °C and the low cycle fatigue (LCF) behaviors at room temperature (RT) and 400 °C were investigated. The results indicate that LCF lifetime of N18 alloy decreases with increasing of the plastic strain range ?ep and obeys Coffin-Manson relation: NfβΔɛp=C; At 400 °C, there appear sawtooth shaped hysteresis loops; Such phenomenon belongs to “Portevin-LeChatelier effect” (PLC effect). At 200, 300 and 400 °C, there are different cyclic characteristics. At elevated temperature, the fatigue fracture is characterized by dimples and secondary cracks, while mainly by secondary cracks at 400 °C.

A superior corrosion behavior of Zircaloy-4 in lithiated water at 360 °C/18.6 MPa by β-quenching

To further understand the mechanism on the heat-treatment effect on corrosion behavior of Zircaloy-4, β-quenched and recrystallized specimens are corroded in an autoclave with 0.01 M LiOH aqueous solution at 360 °C/18.6 MPa. Results show that the β-quenched specimen possesses excellent corrosion resistance, the weight gain of which is consistently comparable to that of ZIRLO and N18 alloys during 529 days exposure, while the recrystallized specimen exhibits accelerated corrosion rate after 100 days exposure. The increase of the supersaturated solid solution contents of Fe and Cr in α-Zr matrix is responsible for the improvement of corrosion resistance by β-quenching treatment. The fracture surface and inner surface morphology of oxide films indicates that β-quenching treatment significantly slow down the development of pores to cracks in oxide films and reduces the uneven growing tendency of the oxide, which is related to the corrosion behavior.

Effect of heat treatment and Nb and H contents on the phase transformation of N18 and N36 zirconium alloys

Abstract The effects of thermal processing and alloying elements on the microstructure and phase transformation of N18 and N36 zirconium alloys were investigated using Optical Microscopy (OM), Transmission Electron Microscopy (TEM), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC). The results showed that the difference of alloying element niobium among the two alloys was significant, and the α → (α + β) phase transformation temperature decreases with an increase of niobium content. Niobium is an element extending β phase region. Tα→α+β of N18 and N36 measured by DTA and DSC were higher than those of the results from TEM. It was shown that the variation of absorbing heat could be recorded with possessing certain β-Zr fraction at heating and it maybe related to the rate of heating when measured by the thermal analysis methods. Hydrogen in the alloys extends the β phase region and accelerates the nucleation and growth of β-Zr in the process of heating. The hydrogen was also mainly soluble in the β phase of α + β temperature region and resulted in a different microstructure. A consequence of hydrogen ingress into Zr alloy is not only to affect its mechanical behavior, but also decrease the α → (α + β) phase transformation temperature.

N18, Powder metallurgy superalloy for disks: Development and applications

The preliminary industrial development of a powder metallurgy (PM) superalloy, designated N18, for disk applications has been completed. This alloy exhibits good overall mechanical properties after appro-priate processing of the material. These properties have been measured on both isothermally forged and extruded billets, as well as on specimens cut from actual parts. The temperature capability of the alloy is about 700 °C for long-term applications and approximately 750 °C for short-term use because of micro-structural instability. Further improvements in creep and crack propagation properties, without signifi-cant reduction in tensile strength, are possible through appropriate thermomechanical processing, which results in a large controlled grain size. Spin pit tests on subscale disks have confirmed that the N18 alloy has a higher resistance than PM Astroloy and is therefore an excellent alloy for modern turbine disk ap-plications.