The microstructure of the oxide layer of a β-solidifying γ-TiAl alloy (Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B) after nitrogen ion implantation was studied after сyclic oxidation tests at 800 ˚C. Oxidation resulted in the formation of the continuous oxide scale, consisting of thin inner and outer TiO2, dense Al₂O₃ and mixed Al₂O₃ + TiO₂ layers. It was revealed that the nitrogen ion implantation considerably inhibited the growth of the oxide scale, internal oxidation and Al-depleted zones. Thick Al-depleted zones and a mixture of Al2O3 and TiO2 were formed along the colony boundarie

Novikov, V. Yu.

Panov, D. O.

Salishchev, G. A.

Sokolovsky, V. S.

Stepanov, N. D.

Zherebtsov, S. V.

DSpace at Belgorod State University

IOP Conference Series: Materials Science and EngineeringPAPER • OPEN ACCESSEffect of microstructure on the oxidation behavior of a β-solidified γ-TiAlbased alloy after nitrogen ion implantationTo cite this article: D O Panov et al 2021 IOP Conf. Ser.: Mater. Sci. Eng. 1014 012054 View the article online for updates and enhancements.This content was downloaded from IP address 188.170.217.49 on 15/04/2021 at 10:26Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing LtdASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012054IOP Publishingdoi:10.1088/1757-899X/1014/1/0120541       Effect of microstructure on the oxidation behavior of a β-solidified γ-TiAl based alloy after nitrogen ion implantation D O Panov1, V S Sokolovsky1*, N D Stepanov1, S V Zherebtsov1, V Y Novikov1, N A Nochovnaya2, P V Panin2, G A Salishchev1 1 Laboratory of Bulk Nanostructured Materials, Belgorod National Research University, Belgorod 308015, Russia 2 Federal State Unitary Enterprise "All-Russian scientific research institute of aviation materials" (FSUE "VIAM"), 17 Radio str., Moscow 105005, Russia  *Corresponding author: sokolovskiy@bsu.edu.ru Abstract. The microstructure of the oxide layer of a β-solidifying γ-TiAl alloy (Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B) after nitrogen ion implantation was studied after сyclic oxidation tests at 800 ˚C. Oxidation resulted in the formation of the continuous oxide scale, consisting of thin inner and outer TiO2, dense Al2O3 and mixed Al2O3 + TiO2 layers. It was revealed that the nitrogen ion implantation considerably inhibited the growth of the oxide scale, internal oxidation and Al-depleted zones. Thick Al-depleted zones and a mixture of Al2O3 and TiO2 were formed along the colony boundaries. Tooth-shaped Al-depleted zones propagated along the lamella boundaries. In addition, the presence of Gd-rich particles in the oxide scale promoted the formation of Al2O3 particles in surrounding areas and increased the depth of the oxide scale around. 1.  Introduction β-solidified gamma titanium aluminide (γ-TiAl) based alloys have attracted tremendous attention as potential high-temperature materials due to their good creep resistance and superior strength-to-weight ratio [1]. These alloys consist mainly of (α2+γ)-lamellae colonies and have the most promising mechanical properties for aerospace engine components. However, the γ-TiAl based alloys are prone to rapid oxidation at temperatures above 600 °C, resulting in the formation of protective Al2O3 layer, TiO2, a mixture of Al2O3 and TiO2, inner oxide and Al-depleted zones on the surface [1-4].  It is well established that plenty of factors affect the oxide scale growth, namely the chemical composition and structure of the base material, surface finish, environment etc. [1, 5-8]. For instance, the oxidation resistance of Ti-45Al-8Nb (at.%) was considerably enhanced by the Y addition of 0.3 at.% that promoted the protective Al2O3 layer formation [9]. On the other hand, coarse Y2O3-particles, found along the grain boundaries in the alloys with high Y content (>0.3 at.%), provided fast path for the inward oxygen diffusion and significantly increased the oxidation rate [9]. Although the beneficial effects of the addition of rare-earth elements on the oxidation resistance of various alloys were known [10], there is no comprehensive understanding of the effect of Gd on the oxidation behavior of a γ-TiAl based alloy. Apart from adjusting the bulk chemical composition, surface modification, for example, by ion implantation of different non-metallic or metallic species (F, I, N, Cl, Nb, Al, Si, Mo) can effectively improve the oxidation resistance of the alloys [11-13].  ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012054IOP Publishingdoi:10.1088/1757-899X/1014/1/0120542       The microstructure of the base material can also affect the oxidation behavior. For example, refinement of α2-phase particles in the Ti–46Al–1Cr–0.2Si alloy inhibits oxidation dramatically [8]. Meanwhile, the orientation of lamellar colonies relative to the surface can influence the oxidation behavior of the γ-TiAl based alloys because lamellar boundaries can serve as fast diffusion pathways. However, this effect has never been properly studied. In this paper, the effect of microstructure on the oxide layer formation of a β-solidifying γ-TiAl alloy is considered.  2.  Materials and methods The β-solidifying γ-TiAl alloy with the actual composition Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B (at. %) was used as the program material. The samples with a size of 20×20×2 mm3 were cut from the ingots using a Sodick AQ300L wire electrical discharge machine. The samples surface was ground with emery paper up to № 2000, and then mechanically polished with an OP-S colloidal silicon suspension. The nitrogen ion implantation was produced at temperature of 400 °С in a vacuum chamber with a pressure in the range of 10-4 - 10-5 Pa. The radiation dose and ion energy were 1017 cm-2 and 40 keV, respectively. Cyclic oxidation tests was performed in open air using a Nabertherm furnace. The samples were soaked for 24 hours at 800 °C with subsequent air cooling. After that the samples were weighed and then could be returned to the furnace again for the next test cycle. Up to 5 cycles of oxidation tests were performed. The structure of the alloy was studied mostly using scanning electron microscopy (SEM). The following tapes of samples were tested by SEM: a beveled sample with a polished surface inclined ~5° – for the surface and material structure after nitrogen ion implantation; a cross-section of samples - for structure and element distribution of oxidation scale. The samples were examined using a FEI Quanta 600 electron microscope equipped with a back-scattered electron (BSE) and an energy-dispersive X-ray spectroscopy (EDS) detector with an accelerating voltage of 30 kV.   3.  Results and discussion The microstructure of the β-solidifying Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy consisted of (α2+γ) lamellae colonies surrounded by γ- and (β+ω)-phase particles (Figure 1a, b). Particles of γ- and (β+ω)- phases were mainly found between the colonies. The volume fraction of γ- and (β+ω)- phase’s particles was 1%. Some Gd-rich phases and a small amount of borides were found in the microstructure (Figure 1a). The volume fraction of Gd-rich particles was ≈2%. A more detailed investigation of the as-cast microstructure of the program material was presented in the previous paper [14]. The microstructure after ion implantation was presented in figure 1c. The modified layer had light grey color and depth of 1 µm; the boundary between the base material and modified layer was curved. No difference in the microstructure between the base material and modified layer was detected by SEM-BSE.     Figure 1. Microstructure of the Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy in the as-cast condition (a, b) and after ion implantation (c): (a, c) SEM-BSE images; (b) TEM image.  ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012054IOP Publishingdoi:10.1088/1757-899X/1014/1/0120543       Figure 2 shows the growth of oxide scale and the internal oxidation and Al-depleted zones (inner zones) depth for the as-cast and nitrogen ion implanted alloy depending on the number of oxidation cycles. In this case, the internal oxidation zone referred to the oxygen-enriched aria under the oxide scale, which might also include some oxide particles. A typical example of an oxidized microstructure with identification of all layers is shown below in Figure 3. Two stages were distinguished in both conditions. At the first stage, parabolic kinetics was observed up to 3 cycles which is associated with the formation of a protective oxide layer. The formation of a protective layer led to slowing the oxide scale and inner oxide and Al-depleted zones growth at the second stage (3-5 cycles) in case of the nitrogen ion implanted alloy. After 5 cycles, the depth of the oxide scale and inner zones of the ion implanted alloy was 14 μm and 2 μm, respectively. Note that these values were considerably larger than the depth of the modified layer (1 µm). However, almost linear growth of the oxide scale and inner oxide and Al-depleted zones for the as-cast alloy was detected at the second stage.    Figure 2. Dependence the depth of the oxide scale and inner zones (internal oxidation area and Al-depleted zones) of the Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy in the as-cast condition (a) and after ion implantation (b) on number of oxidation cycles at 800 °C.  The cross-section structure after 2 cycles of oxidation test is shown in Figure 3. The oxide scale mostly consisted of two oxides phases (Ti and O-rich light gray areas - TiO2; Al and O-rich dark gray areas - Al2O3) [9] (Figure 2). The following layers were found: I – outward-growing TiO2; II – Al2O3; III – inner TiO2; IV – a mixture of Al2O3 + TiO2; V – internal oxidation zone; VI – Al-depleted zone. The structure of the oxide scale was typical for γ-TiAl based alloys [9]. The sequence of layers depended on the ratio of outward-diffusion of Al and Ti and inward-diffusion of O on the one hand, and the ratio of the growth rate of TiO2 and Al2O3, on the other hand. Obviously, TiO2 and Al2O3 formed and grew simultaneously because of the similar free energy of these formation, but the growth rate of TiO2 was drastically higher in comparison with A12O3 [15]. Thus, the outward-growing TiO2 layer quickly covered the surface of the material. The A12O3 layer was found underneath the TiO2 layer because of the diffusion of Ti toward the surface. The inner TiO2 layer was likely formed because of the subsequent depletion of Al beneath the A12O3 layer. The mixture of Al2O3+TiO2 grew by parallel outward- and inward-diffusion [16]. As a result of the higher diffusion coefficient of Al in comparison with Ti [4], the Al-depleted zone appeared.  Thick Al-depleted zones and a mixture of Al2O3 and TiO2 oxides formed along the colony boundaries. Closer look has revealed that tooth-shaped Al-depleted zones propagated along the lamella boundaries (see high resolution insert in figure 3). The presence of Gd-rich particles enhanced the formation of Al2O3 in surrounding areas and increased the depth of the oxide scale. Rapid diffusion of oxygen anions along colony boundaries, γ-lamellas, and Gd-rich particles can therefore increase the effective oxidation rate. It should be suggested that the noticeable positive effect of the nitrogen implantation on suppressing the oxide scale growth can be probably associated with enhancing the formation of the protective Al2O3 layer and, as a result, slowing the oxygen propagation. However, further studies are required to establish the oxidation mechanisms in detail.  ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012054IOP Publishingdoi:10.1088/1757-899X/1014/1/0120544        Figure 3. Cross-section SEM-BSE images and element distribution maps of the program material after nitrogen ion implantation after 2 cycles of oxidation tests. I – outward-growing TiO2 layer; II – Al2O3 layer; III – inner TiO2 layer; IV – a mixture of Al2O3 + TiO2; V – internal oxidation zone; VI – Al-depleted zone; red lines indicate lamellae direction. 4. Conclusions The dependence of the oxide scale formation on the microstructure of the base layer was studied using the example of a β-solidifying γ-TiAl alloy (Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B) after nitrogen ion implantation. The nitrogen ion implantation improved the oxidation resistance of the alloy considerably, as was evidenced by the dependence of oxide scale thickness on number of oxidation cycles. Thick Al-depleted zones and a mixture of Al2O3 and TiO2 were formed along the colony boundaries. Tooth-shaped Al-depleted zones propagated along the lamella boundaries. In addition, the presence of Gd-rich particles in the oxide scale promoted the formation of Al2O3 particles in surrounding areas and increased the depth of the oxide scale around.   Acknowledgment  The work was supported by the Russian Science Foundation under Grant № 19-79-30066. The authors are also grateful to Joint Research Center “Materials and Technologies”, Belgorod State University, for the assistance with instrumental analysis.  References [1] Leyens C, Peters M 2003 Titanium and Titanium Alloys: Fundamentals and Applications (Weinheim: Wiley-VCH) [2] Fergus J W 2002 Mater. Sci. and Eng. A 338 108-125 [3] Tetsui T, Ono S 1999 Intermetallics 7 689–697 [4] Rahmel A, Quadakkers W J, Schutze M 1995 Mater. Corros. 46 271–285 [5] Zheng N, Quadakkers W J, Gil A and Nicke H 1995 Oxidation of Metals 44 477-499 [6] Taniguchi S, Hongawara N, Shibata T 2001 Materials Science and Engineering A 307 107–112 [7] Rakowski J M, Meier G H and Pettit F S 1996 Scripte Materialia Vol. 35 1417-1422 [8] Perez P, Jimenez J A, Frommeyer G and Adeva P 2000 Oxidation of Metals Vol. 53 138-147 [9] Zhao L L, Li G Y , Zhang L Q, Lin J P, Song X P, Ye F, Chen G L 2010 Intermetallics 18 1586–1596 [10] Cueff R, Buscail H, Caudron E, Issartel C, Riffard F 2003 Corros Sci 45 1815-1831 [11] Taniguchi S, Uesaki K, Zhu Y C, Fujita K, Iwamoto N, Matsunaga Y, Nakagawa K 1999 Mater. Sci. Eng. A 266 267-275 [12] Hui S, Zhang Z, Xiao J, Yuan G 1999 Rare Met. 18 (3) 162-167 [13] Donchev A, Schütze M 2008 Mater Corros 59 489-493 [14] Sokolovsky V S, Stepanov N D, Zherebtsov S V, Nochovnaya N A, Panin P V, Zhilyakova M A, Popov A A, Salishchev G A 2018 Intermetallics 94 138–151 [15] Dai J, Zhu J, Chen C, Weng F 2016 J. Alloys Compd 685 784–798 [16] Becker S, Rahmel A, Schorr M, Schütze M 1992 Oxid Met 38 425–464  

Effect of microstructure on the oxidation behavior of a β-solidified γ-TiAl based alloy after nitrogen ion implantation

http://dspace.bsu.edu.ru/bitstream/123456789/41444/1/Panov_Effect%20of%20microstructure.pdf

Effect of microstructure on the oxidation behavior of a β-solidified γ-TiAl based alloy after nitrogen ion implantation

Abstract

Similar works

Full text

Available Versions

DSpace at Belgorod State University