Abstract
The effect of Al on the microstructure and corrosion behavior of low neutron absorption cross-section Ti-Zr-Nb high entropy alloys was investigated. The phase diagram, microstructure, oxidation behavior and corrosion behavior of Ti-Zr-Nb alloys without Al addition and with ~15at% Al were compared. The phase diagram shows that below the melting temperature, Ti-Zr-Nb ternary alloys are bcc phase, and Al is inclined to form intermetallics in Ti-Zr-Nb alloys, and thus reduces the single-phase bcc temperature region in phase diagram. XRD and TEM results show that as-cast Ti-Zr-Nb ternary alloys is simple bcc structure, and Al will transform the crystal structure to ordered B2 structure. The corrosion behavior of Ti-Zr-Nb alloys was studied by thermogravimetric analysis and autoclave exposure. Results show that corrosion oxide layer formed on TiZrNb ternary alloys tends to spall during corrosion process, while Al addition will increase the stability of oxide layer without changing the main type of oxide formed during corrosion process. Oxidation kinetics was evaluated by calculating reaction rate constants and activation energies, and it is found that the high temperature oxidation property of Ti-Zr-Nb alloys with Al addition is comparable to that of Zr alloys.
Science Press
The concept of accident tolerant fuel (ATF) was proposed to achieve higher safety margins under accident scenario
Among HEAs, refractory high-entropy alloys (RHEAs) are one of the most latest developments in structural materials. These alloys have promising high-temperature mechanical properties and improved oxidation resistance compared to conventional refractory alloy
Addition of Al element may have a profound effect on the microstructure and properties of HEAs. First of all, Al may change the phase of HEAs, which may lead to great change of the propertie
In this study, Ti-Zr-Nb HEA alloys with different contents of Al were designed and prepared, and the microstructure, oxidation behavior and corrosion behavior were investigated. The effect of Al on the microstructure and corrosion behavior of Ti-Zr-Nb HEAs was discussed. The results of present study will shed new light on the corrosion behavior of HEAs.
The ability for elemental mixtures to form a single-phase solid solution was assessed by calculating the empirical parameters of HEAs formation. The empirical parameters include the entropy of mixing (ΔSmix), the enthalpy of mixing (ΔHmix), the atomic size difference (δ), parameter Ω, et
| (1) |
| (2) |
Parameter Ω can be defined:
| (3) |
| (4) |
In a multi-component HEA system, for the formation of single phase solution phase, it is indicated that δ≤6.6% and Ω≥1.1 are required to form a solid solutio
The composition of Ti-Zr-Nb HEAs studied in this research was designed and calculated, as shown in
Alloys were prepared by weighing the pure elements according to their target stoichiometries to a total of ~1 kg. A mixture of pure metals (purity≥99.9wt%) was melted and solidified in a magnetic levitation melting furnace at 1
Samples for scanning electron microscope (SEM) observation were prepared by mechanical polishing, and slightly corroded for a few seconds with a diluted Kroll reagent. SEM analysis was conducted on NOVA NANO SEM 400 equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Transmission electron microscopy (TEM) samples were prepared by ion thinning on Leica EM RES102, and analyzed on FEI Tecnai G2 F20. ARL EQUINOX 3000 X with Cu-Kα X-ray source was used to collect reflections on a rotating stage.
Indentation hardness was obtained on INNOVATEST FALCON 500 Vickers hardness tester using a diamond Vickers tip. All indents were made at 4.9 N for 15 s.
An autoclave test was performed in a static autoclave in pure water <45 μg/kg oxygen at 360 °C and a saturation pressure of 18.6 MPa. Mass gain measurements were used to assess the corrosion resistance.
In this work, high-temperature oxidation tests for HEAs were performed using a thermogravimetric analyzer (TGA, SETARAMSETSYS, France) in flowing steam environment at 600, 800, 1000 and 1200 °C for 7200 s to simulate the LOCA condition of nuclear reactor. The test coupon was mounted in Pt hanger inside the furnace and heated from ambient temperature to the setting temperature at a rate of 50 °C/min. Meanwhile, the 99.99% Ar gas was flowed into the chamber at the speed of 20 mL/min to prevent the initial oxidation during the temperature ramp. After the temperature reached to the setting temperature, the mixed gas of Ar and water vapor with the 90% humidity flowed concurrently into the chamber at the same rate of 20 mL/min. The specimen was isothermally oxidized in the steam environment at a setting temperature for 7200 s, and then was cooled to ambient temperature in Ar gas environment.
Predictions from CALPHAD are now presented and discussed.
Fig.1 Predicted equilibrium phase diagrams of TZN1 (a), V0.25-TZN2 (b), Al0.5-TZN1 (c), and Al0.5V0.25-TZN2 (d) at 200~2000 °C
Fig.2 SEM morphologies and EDS results of the dendritic phase and inter-dendritic regions for different Ti-Zr-Nb alloys:
(a) TZN1, (b) Al0.5-TZN1, and (c) Al0.5V0.25-TZN2
;The SEM morphologies (Fig.2) reveal a typical dendritic microstructure for the magnetic levitation melted as-cast samples of Ti-Zr-Nb alloys. The elemental distribution in the dendrite and inter-dendritic regions of Ti-Zr-Nb samples was characterized by EDS. The chemical compositions of the dendritic phase and inter-dendritic regions are close to the designed composition. From EDS analysis, it is clear that the dendritic phase and inter-dendritic regions are predominantly composed of (Nb, Ti) and (Zr, Al), respectively. This is mainly because the rate of atom diffusion cannot keep up with the crystallization process caused by the chemical segregation formed during solidification, with the predominant segregation of the refractory elements (mainly Nb) in the dendrites and the lower melting temperature elements (i.e. Al) in the inter-dendrites. In addition, Zr and Al tend to segregate simultaneously, which is mainly due to the low mixing enthalpy of these two elements (-44 kJ/mol
Fig.3 XRD patterns of different Ti-Zr-Nb alloys
The Al-induced ordering of the microstructure was also confirmed by TEM, as shown in
Fig.4 TEM bright field images and SAED pattern of TZN1 alloy (a) and Al0.5-TZN1 alloy (b)
The microhardness of as-cast samples of different Ti-Zr-Ni alloys was analyzed, as shown in
Fig.5 Microhardness of different Ti-Zr-Nb HEAs
Mass gain measurements of the Ti-Zr-Nb alloys oxidized in steam are shown in
Fig.6 Corrosion behavior of different Ti-Zr-Nb alloys in high temperature steam: (a) TZN1 and TZN2 alloy; (b) Al0.5V0.25- TZN2 and Al0.5-TZN1 alloy
Fig.7 Surface morphologies of TZN1 alloy and Al0.5V0.25-TZN2 alloy after oxidation in steam at 800 °C
The general steady-state rate law equation for the oxide layer thickness (x, or an equivalent mass gain) is represented by
| (5) |
The growth of the oxide layer follows a parabolic rate (n=2.02±0.43 for steam) during steady-state conditions, as determined by linear regression analyses. The parabolic rate is due to the self-shielding of diffusion by the oxide scale as it thickens during growt
| (6) |
where R is the universal gas constant (R=8.314 J·
Taking the natural logarithm for
| (7) |
Because the microstructure and oxidation behavior of Al0.5-TZN1 and Al0.5V0.25-TZN2 alloys are similar, regression analysis was performed using
Fig.8 Parabolic oxidation rate for Al0.5-TZN1, Al0.5V0.25-TZN2 alloys and Zr-4 alloy as a function of the reciprocal temperature in steam
The corrosion kinetics of autoclave tested Ti-Zr-Nb samples is presented in
Fig.9 Corrosion behavior of different Ti-Zr-Nb alloys in water at 360 °C
For the Ti-Zr-Nb alloys with Al addition about 15at%, such as Al0.5-TZN1, Al0.5-TZN2 and Al0.5V0.25-TZN2, corrosion kinetics are almost the same before kinetic transition, which happens after 150 d. The pre-transition kinetics can be fitted to a simple power-law expression, with an exponent of 0.53 for these three kinds of alloys. For Al0.5-TZN1 and Al0.5V0.25-TZN2, corrosion rates are accelerated after transition, while for Al0.5-TZN2, after transition the oxide surface begins to spall, which decreases the mass with increasing the exposure time. Before and after kinetic transition, the oxide is black in these three kinds of samples. The difference between the post-transition corrosion behaviors of these three kinds of alloys may be because of the different microstructures of the alloys. In this study, autoclave samples are prepared from as-cast ingot, so grain size and microstructure can differ a lot in each ingot. Grain size and microstructure will affect the microstructure of the oxide layer formed during corrosion, so alloys with similar composition can behave differently during the corrosion proces
Furthermore, the phase structures of corroded TZN1 alloy and Al0.5-TZN1 alloy were analyzed by GIXRD, as shown in
Fig.10 GIXRD patterns of Ti-Zr-Nb alloys after the autoclave testing under 360 °C/18.7 MPa for 50 d
Although XRD results show that the oxides formed on TZN1 alloy and Al0.5-TZN1 alloy are basically the same type, the mass gain and corrosion behavior of the alloys are totally different after 50 d exposure. For TZN1 alloy, the mass gain is about 50.17 mg/d
The oxide layer formed on the surface of Ti-Zr-Nb ternary alloys will spall, both during the oxidation in steam and autoclave testing process, which indicates that the oxide layers of Ti-Zr-Nb ternary alloys are unstable. The spallation of oxide layer mainly occurs on the surface of metals or alloys with high P-B ratio, which induces high stress field in the oxide layer. The oxide behavior of Ti-Zr-Nb alloys in air was investigated by different research. It is shown that the oxides formed on Ti-Zr-Nb alloys are mainly TiNb2O7, TiO2, Nb2Zr6O17 and Ti3O5, etc, which can hardly protect the alloys from further oxidatio
In this study, it is found that addition of about 15at% Al can improve the stability of oxide layer formed in steam and 360 °C pure water, and no spallation occurs on the surface. In previous studies, Al2O3 was found in the oxide layer formed on oxidation surface, which can protect the matrix from further oxidation, and promote the oxidation resistanc
1) The crystal structure of Ti-Zr-Nb ternary alloys is basically simple bcc structure, while after addition of Al, crystal structure of Ti-Zi-Nb allays will transform to ordered B2 structure. Phase diagram shows that Al is inclined to form intermetallics with different stotchinmetries in Ti-Zr-Nb alloys, which reduces the single-phase bcc temperature region.
2) During the corrosion process in 360 °C water and high temperature (600~1200 °C) steam, the corrosion oxide layer on TiZrNb ternary alloys tends to spall during the corrosion process. An appropriate amount of Al addition (about 15at%), will increase the stability of oxide layer formed on Ti-Zr-Nb alloys in 360 °C water and high temperature steam.
3) The oxide layer growth of Ti-Zr-Nb alloys with Al addition (about 15at%) follows a parabolic rate in high temperature steam. The activation energy of the steady-state oxidation reaction is around 163 kJ·mo
4) With increasing the Al addition from 0at% to 15at%, the corrosion resistance of Ti-Zr-Nb alloys is improved for exposure in 360 °C pure water environment. With Al content about 15at%, pre-transition kinetics follows a simple para-bolic law, and the corrosion transition happens after 150 d of exposure. With or without Al addition, the oxides formed on Ti-Zr-Nb alloys are mainly Srilankite type (Ti, Zr)O2.
References
Zinkle S J, Terrani K A, Gehin J C et al. Journal of Nuclear Materials[J], 2014, 448: 374 [Baidu Scholar]
Yeh J W, Chen S K, Lin S J et al. Advanced Engineering Materials[J], 2004, 6: 299 [Baidu Scholar]
Senkov O N, Senkova S V, Woodward C. Acta Materialia[J], 2014, 68: 214 [Baidu Scholar]
Lu C Y, Niu L L, Chen N J et al. Nature Communications[J], 2016, 7: 13 564 [Baidu Scholar]
Qiu X W, Zhang Y P, He L et al. Journal of Alloys and Compounds[J], 2013, 549: 195 [Baidu Scholar]
Shi Y, Collins L, Feng R et al. Corrosion Science[J], 2018, 133: 120 [Baidu Scholar]
Miracle D B, Senkov O N. Acta Materialia[J], 2017, 122: 448 [Baidu Scholar]
Senkov O N, Miracle D B, Chaput K J et al. Journal of Material Research[J], 2018, 33: 3092 [Baidu Scholar]
Komarov F F, Pogrebnyak A D, Konstantinov S V. Technical Physics[J], 2015, 60: 1519 [Baidu Scholar]
Wang J, Li X, Wang J et al. Corrosion Science[J], 2015, 95: 125 [Baidu Scholar]
Liu Y, Chen M, Li Y X et al. Rare Metal Materials and Engineering[J], 2009, 38(9): 1602 (in Chinese) [Baidu Scholar]
Zhou Y J, Zhang Y, Wang Y L et al. Rare Metal Materials and Engineering[J], 2007, 36(12): 2136 [Baidu Scholar]
Kao Y F, Lee T D, Chen S K et al. Corrosion Science[J], 2010, 52: 1026 [Baidu Scholar]
Liu Y X, Cheng C Q, Shang J L et al. Transactions of Nonferrous Metals Society of China[J], 2015, 25: 1341 [Baidu Scholar]
Zhang W, Tang R, Yang Z B et al. Surface and Coatings Technology[J], 2018, 347: 13 [Baidu Scholar]
Zhang Y, Zuo T T, Tang Z et al. Progress in Materials Science[J], 2014, 61: 1 [Baidu Scholar]
Miracle D B, Miller J D, Senkov O N et al. Entropy[J], 2014, 16: 494 [Baidu Scholar]
Takeuchi A, Inoue A. Materials Transactions[J], 2005, 46(12): 2817 [Baidu Scholar]
Li W D, Xie D, Li D Y et al. Progress in Materials Science[J], 2021, 118: 100 777 [Baidu Scholar]
Ding Q Q, Zhang Y, Chen X et al. Nature[J], 2019, 574: 223 [Baidu Scholar]
Pieraggi B. Oxidation of Metals[J], 1987, 27(3-4): 177 [Baidu Scholar]
Atkinson A. Materials Science and Technology[J], 1988, 4(12): 1046 [Baidu Scholar]
Zhang X Y, Shi M H, Li C et al. Materials Science and Engineering A[J], 2007, 448: 259 [Baidu Scholar]
Ralston K D, Birbilis N. Corrosion[J], 2010, 66: 75 005 [Baidu Scholar]
Kraleva E, Spojakina A, Saladino M L et al. Journal of Nanoscience and Nanotechnology[J], 2010, 10: 8417 [Baidu Scholar]
Kong L B, Ma J, Zhu W et al. Journal of Alloys and Compounds[J], 2002, 335: 290 [Baidu Scholar]
Butler T M, Chaput K J, Dietrich J R et al. Journal of Alloys and Compounds[J], 2017, 729: 1004 [Baidu Scholar]
Cao Y K, Liu Y, Liu B et al. Transactions of Nonferrous Metals Society of China[J], 2019, 29: 1476 [Baidu Scholar]
Chen J, Zhou X Y, Wang W L et al. Journal of Alloys and Compounds[J], 2018, 760: 15 [Baidu Scholar]