Oxidation-resistant coating for gamma titanium aluminides by pack cementation

doi:10.1016/S1359-6462(99)00182-7

Scripta Materialia

Volume 41, Issue 5, 6 August 1999, Pages 511-516

https://doi.org/10.1016/S1359-6462(99)00182-7 Get rights and content

Introduction

Gamma titanium aluminides (γ-TiAl alloys), having an L1₀-type structure, are candidate materials for use in future gas turbine aero-engines and automotive engines because of their low density, high specific strength and high stiffness 1, 2. In air, however, it is well known that titanium aluminide oxidizes at a more rapid rate at temperatures above 1123 K; therefore, the oxidation resistance becomes a critical factor for TiAl alloys to be used at high temperatures (perhaps above 1073 K) (3). Coatings for the TiAl alloy are essential to high temperature oxidation resistance. The recent approaches toward oxidation resistant coatings for titanium aluminides include the aluminide pack coatings 4, 5, 6, 7, pack coatings with Cr₂O₃, SiO₂ and Cr powders 8, 9, MCrAlY(M = Ni, Fe, Co) alloy coatings 10, 11, 12, silicides/ceramics coatings 13, 14, physical vapor deposition coatings (15), and coatings used a fluidized bed with WO₃ powder (16). Among the coating processes to improve oxidation resistance, pack cementation is a very simple process and remains a widely used technique for gas turbine components. Especially aluminide coatings are the best documented pack processes used to produce high-temperature protective coatings for nickel-based superalloys (17).

On the other hand, the oxidation resistance of Al₃Ti, having a D0₂₂-type structure, appears to be superior owing to its high aluminum content (18). Oxidation resistant coatings based on the Al₃Ti phase have been successfully produced on TiAl alloys by diffusion pack aluminizing 4, 5, 19. But the Al₃Ti is extremely brittle at ambient temperature because of its low-symmetry crystal structure and tetragonal D0₂₂ structure; therefore, its application as a coating material for TiAl alloys has been limited. To improve its ductility, recently, much work on the (Al,X)₃Ti (X = Ni, Fe, Cu, Mn, Cr, Ag and Pd etc.) alloys has been focused on the formation of the cubic L1₂ crystal structure, with the hope that the L1₂ structure is highly symmetrical and may have a sufficient number of slip systems for homogeneous deformation 20, 21, 22, 23, 24. The present authors recently reported that Ti-67Al-8Cr (in mol %) alloy with the L1₂ structure does possess some intrinsic bend ductility at ambient temperature, and that L1₂- Ti-61Al-14Cr alloy (with higher chromium content) was more ductile in bending, with a plastic strain of up to 0.9% being recorded 25, 26.

The Al-Ti-Cr ternary phase diagram at 1423 K, as can be seen in Fig. 1 (26), indicates that the L1₂ phase field is in equilibrium with the TiAl(L1₀) phase field. Furthermore, the L1₀- and L1₂-alloys in this ternary system resemble each other very closely in crystal structure and lattice parameter (TiAl(L1₀); a = 0.4005, c = 0.4070 nm (27), Ti-67Al-8Cr(L1₂); a = 0.3960nm (25)). Therefore, the L1₂ - (Al,Cr)₃Ti alloy can be considered applicable to L1₀ -TiAl alloys as an optimum material for coatings. The purpose of this study is to apply L1₂ alloy coatings to the TiAl alloys by pack cementation techniques, and to evaluate the oxidation resistance of the coated alloys.

Section snippets

Experimental

The γ-TiAl alloy as a substrate was prepared by non-consumable electrode arc-melting under an argon atmosphere, using sponge titanium (99.8%), high purity aluminum (99.99%), and high purity chromium (99.99%) as an alloying element. The composition of the alloy was Ti-43Al-5Cr (in mol %). The alloy button was then homogenized at 1423 K in vacuum for two days, resulting in the matrix L1₀-TiAl phase with a small amount of α₂(Ti₃Al)-phase and β(Ti₂AlCr)-phase. On the other hand, the L1₂-type

Results and discussion

Figure 2 shows the cross section of a specimen coated by the pack cementation. Phase identified by the XRD and EPMA techniques are presented in this figure. The optical microstructure indicates that the coated specimen shows a fairly wide layer (order of ∼50 μm in total width), and that the two coating layers develop; outer L1₂ layer and inner L1₀ layer, which have the layer width of ∼20 and ∼30 μm, respectively. Figure 3 shows the layer width as a function of the coating duration. The layer

Summary

To improve the oxidation resistance of γ-TiAl alloys, Ti-43Al-5Cr(L1₀) alloy substrates were coated with the L1₂-(Al,Cr)₃Ti layer by pack cementation. Coatings were formed by immersing the substrate in a mixture consisting typically of 90% Ti-61Al-14Cr(L1₂) alloy powders and 10% Al₂O₃ powders, without activators, and then heating in an alumina boat under a vacuum of ∼10^–4 Pa for 1 ∼ 96 h at 1423 K. The coating layer for γ-TiAl alloys was effectively formed by surface diffusion and bulk

Acknowledgements

This work was supported in part by a Grant-in Aid for Scientific Research on the Priority Area ‘Physics and Chemistry of Functionally Graded Materials’ from the Japanese Ministry of Education, Science, Sports and Culture.

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References (28)

H. Mabuchi et al.
Scripta Metall.
(1989)
J.L. Smialek et al.
Scripta Metall. Mater.
(1990)
S. Taniguchi et al.
Mater. Sci. Eng.
(1995)
T. Shimizu et al.
Mater. Sci. Eng.
(1992)
D.W. McKee et al.
Surface Coatings Technol
(1993)
J.Y. Park et al.
Scripta Mater
(1997)
K.S. Kumar et al.
Scripta Metall.
(1988)
H. Mabuchi et al.
Scripta Metall.
(1989)
S. Zhang et al.
Scripta Metall. Mater.
(1990)
Y. Nakayama et al.
Intermetallics
(1993)

Y-W Kim

JOM

(1994)

D.M. Dimiduk

M.P. Brady et al.

JOM

(1996)

M. Yoshihara et al.

ISIJ Int

(1991)

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Therefore, there is a requirement for coatings which exhibit stability and are chemically compatible with that of the substrate so that the mechanical properties of the coated alloy are not affected by the formation of complex brittle intermetallic phases at the coating/substrate interface. The diffusion aluminide coatings comprising the Al3Ti phase provide good oxidation resistance to the Ti alloys [30–32] and can also serve as potential oxidation barriers for the γ-TiAl alloys [6,33–38]. Since the coating and substrate are constituted of Ti and Al, greater stability of phases and less interdiffusion of elements across the coating/substrate interface can be expected.
The use of γ-TiAl alloys at higher temperatures exceeding 700–800 °C is restricted due to their inadequate oxidation resistance. Their durability and high temperature capability can be enhanced by the application of oxidation resistant coatings. The present study examines the cyclic oxidation behavior of a diffusion Al₃Ti coated lamellar Ti–45Al–5Nb-0.2B-0.2C (in at.%) γ-TiAl alloy during exposure at 800 and 950 °C in air for 500 h. The effect of oxidation on the coating microstructure and the compression properties at room temperature, 800 and 950 °C are assessed. The coating forms a protective Al₂O₃-rich oxide scale, which lowers the overall oxidation rate and induces steady parabolic weight gain in the coated alloy; unlike the extensive weight loss occurring in the bare/uncoated alloy during oxidation at higher temperatures. The parabolic oxidation rate constant (K_p) for the coated alloy at 6–7 x 10⁻⁷ mg² cm⁻⁴ s⁻² is two orders of magnitude lower than that of the bare alloy. Surface embrittlement during oxidation causes deterioration in the compression yield strength of the bare alloy. On the other hand, the coating prevents oxidation-induced surface degradation of the alloy, which manifests in improved compression strength of the oxidized coated alloy as compared to that of the oxidized bare alloy. Though the coating is constituted of the brittle intermetallic Al₃Ti phase, the cracks in the coating do not penetrate the substrate and are innocuous to the compression properties of the underlying intermetallic γ-TiAl alloy. The coating is beneficial in providing oxidation resistance and prevents significant oxidation-induced deterioration in compression strength of the alloy during high temperature exposure. Besides, the Al₃Ti coating does not induce significant deterioration in the tensile strength of the γ-TiAl alloy.
Microstructurally inhomogeneous composites: Is a homogeneous reinforcement distribution optimal?
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The above two factors result in no spallation and detachment of oxidation scales, not only at 973 K but also at 873 K and 1073 K [71], which can effectively obstruct the oxidation progress to protect the titanium matrix. Although little oxygen crosses the fastened oxidation scale, the self-assembled TiCp wall can arrest its progress, which in effect is similar to continuous ceramic coatings reducing oxygen ingress at high temperatures [245–247]. Therefore, not only the mechanical properties but also the chemical properties of the TiCp/Ti6Al4V composites are improved by designing the interesting network reinforcement distribution.
Since the 1960s, it has been a common practice worldwide to pursue a homogeneous distribution of reinforcements within a matrix material, discontinuous metal matrix composites (DMMCs) in particular. Taking an overview of the worldwide activities in DMMC research, despite many favourable attributes such as improved specific strength, stiffness and superior wear resistance, DMMCs with a homogeneous microstructure tend to exhibit a very low room temperature damage tolerance even with a highly ductile matrix material such as aluminium. In this review, a range of uniquely multi-scale hierarchical structures have been successfully designed and fabricated by tailoring reinforcement distribution for DMMCs in order to obtain superior performance. A variety of specific microstructures that were developed in Al, Mg, Cu, Fe, Co and TiAl matrices indicate that there must be adequate plastic regions among the reinforcements to blunt or deflect cracks if one wants to toughen DMMCs. Following this path, aided by theoretical analyses, the most recent success is the design and fabrication of a network distribution of in situ reinforcing TiB whiskers (TiBw) in titanium matrix composites (TMCs), where a tailored three-dimensional (3D) quasi-continuous network microstructure displays significant improvements in mechanical properties. This resolves the brittleness surrounding TMCs fabricated by powder metallurgy. It is the large reinforcement-lean regions that remarkably improve the composite’s ductility by bearing strain, blunting the crack and decreasing the crack-propagation rate. The fracture, strengthening and toughening mechanisms are comprehensively elucidated in order to further understand the advantages of such an inhomogeneous microstructure, and to justify the development of novel techniques to produce such inhomogeneous microstructures. This approach opens up a new horizon of research and applications of DMMCs and can be easily extended to general multi-phase composites with enhanced physical and mechanical properties.
Oxidation behavior of in situ TiCp/Ti6Al4V composite with self-assembled network microstructure fabricated by reaction hot pressing
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Once the oxidation scale is formed, the scale is fastened by the preformed oxide thereby inhibiting the oxidation process. Even though slight oxygen crosses the oxidation scale, its progress is arrested by the TiCp wall (Fig. 9), which in effect is similar to continuous ceramic coatings reducing oxygen ingress at high temperatures [7,18,19]. Therefore, the network structured TiCp/Ti64 composite exhibits a superior oxidation resistant performance due to tailoring of the novel network microstructure and the self-assemble TiCp wall.
This study focuses on an attempt to improve the oxidation resistance of titanium matrix composites (TMCs) by tailoring an in situ TiC particles reinforced Ti6Al4V (TiCp/Ti64) composites with a novel three-dimensional (3D) network microstructure. The oxidation behavior of the novel composite was investigated in air, between a temperature range of 873–1073 K for 100 h. Oxidation kinetics and microstructural observation reveal that the novel composite exhibits a superior oxidation resistance due to the 3D network microstructure and a self-assembled TiCp wall. The networked microstructure not only effectively blocks oxygen diffusion but also decreases growth and spallation of the oxidation scales.
Oxidation of double-glow plasma chromising coating on TC4 titanium alloys
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By contrast, applying oxidation protective coatings is an effective and economical method [8]. Pack cementation, low-pressure oxidation, and coating deposition by magnetron sputtering are various methods used to establish an oxidation protective coating on titanium alloys [9,10]. Many coatings act as barriers that reduce the ingress of oxygen at elevated temperatures [11].
Double-glow plasma surface chromising was performed on TC4 (Ti–6Al–4V) alloys, and the isothermal oxidation behaviour of TC4 was investigated at 650, 750, and 850 °C. The surface treatment caused the formation of a Ti–Cr mutual diffusion layer beneath the alloy surface. The chromising coating consisted of Cr, Cr_1.97Ti_1.07, and CrTi₄. During oxidation, Cr, Ti, and Al diffused outward to form multi-layer oxide films, which prevented the diffusion of oxygen.
Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium
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However, titanium readily absorbs oxygen in air, and forming a brittle layer on its surface, which limits the maximum use temperature to about 500 °C [4,5]. A large amount of technologies are used to improve the oxidation resistance of titanium and titanium alloy, which involves sputtering Al films [6], pre-oxidizing [7], pack cementation [8,9], plasma electrolytic oxidation (PEO) [10], magnetron sputtering [11–13], laser surface alloying (LSA) [14,15], nanocrystallization of aluminized surface [16], halogen effect methods [17,18], etc. Although some good results have been achieved in terms of oxidation resistance, a simpler, inexpensive and less pollution technology is still desirable [2,4].
Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.
Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti-48Al-2Cr-2Nb alloy
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Citation Excerpt :
Further improvement of their oxidation resistance is still necessary for widespread applications towards higher temperatures. Many studies have already been carried out to successfully improve the oxidation resistance of TiAl [6–11]. To a lesser extent, some studies about corrosion by fused salts mainly composed of Na2SO4 are described in the literature [12–15].
48-2-2 specimens were coated with Au using electrochemically process and vacuum heat treatment. Au diffused and the coating was composed of TiAlAu₂ and TiAlAu layers from the surface to the bulk alloy. A transition zone between the coating and the sound alloy composed of Au-enriched TiAl is also formed. The oxidation and NaCl salt corrosion behaviours of the coated specimen were investigated through cyclic tests and the effects of coating on mechanical properties of TiAl were assessed by means of creep properties. Au coating was effective in improving the oxidation and NaCl salt corrosion resistances of the coated specimens at 800 °C and 600 °C respectively. This good resistance is attributable to the formation of an Al₂O₃ scale on the surface of the coated specimen. During oxidation, or NaCl salt corrosion, the upper TiAlAu₂ layer of the coating transformed in an Al₂O₃ layer on a TiAu₂ layer. The slightly lower creep properties exhibited by the coated specimens are presumably linked to Cr-enriched phases that segregate at grains boundaries during the vacuum heat treatment. The degradation of creep properties after coating and corrosion is believed to be brought about either by the formation of the brittle TiAu₂ phase or by corrosion or oxygen diffusion through the upper scale.

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Original ArticlesOxidation-resistant coating for gamma titanium aluminides by pack cementation

Introduction

Section snippets

Experimental

Results and discussion

Summary

Acknowledgements

First page preview

Scripta Metall.

Scripta Metall. Mater.

Mater. Sci. Eng.

Mater. Sci. Eng.

Surface Coatings Technol

Scripta Mater

Scripta Metall.

Scripta Metall.

Scripta Metall. Mater.

Intermetallics

JOM

JOM

ISIJ Int

Original Articles
Oxidation-resistant coating for gamma titanium aluminides by pack cementation