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Characterization of interdiffusion growth of aluminized layer on Ti alloys
Journal of Alloys and Compounds 429 (2007) 143–155
Characterization of interdiffusion growth of aluminized layer on Ti alloys S. Romankov a , W. Sha b,∗ , E. Ermakov a , A. Mamaeva a a
b
Institute of Physics and Technology, 050032 Almaty32, Kazakhstan Metals Research Group, School of Planning, Architecture and Civil Engineering, The Queen’s University of Belfast, Belfast BT7 1NN, UK
Received 16 January 2006; received in revised form 11 April 2006; accepted 12 April 2006 Available online 15 May 2006
Abstract During annealing on the surface of the Ti, Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V samples coated with Al films, different aluminide phases were formed successively as the result of reactions between Ti and Al. The structural evolution of the overlayers formed on the substrates during annealing depended on the thickness of the Al film, and the microstructure and chemical composition of the initial substrate. For thicker Al films, thicker overlayers were formed on the surface. Thicker overlayer required more time for its recrystallisation and microstructural transformation. The reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. Chemical composition of substrates had a great effect on the diffusion transformation of overlayers. Reactions occurred faster on the Ti–4%Al–1%Mn samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. After the elimination of the overlayer a fine plate microstructure was formed. © 2006 Elsevier B.V. All rights reserved. Keywords: Thin films; Intermetallics; Phase transitions; Scanning electron microscopy, SEM; X-ray diffraction
1. Introduction Ti and its alloys have excellent resistance against corrosion and desirable specific strengths at ambient temperature. However, due to their poor resistance against adhesive wear and oxidation problems, technical application of titanium alloys is limited to temperatures much lower than those permissible by alloy mechanical properties. Intermetallic compounds of the Ti–Al system have been investigated extensively for high-temperature structural applications because they offer a combination of good oxidation resistance and useful mechanical properties at temperatures higher than those possible with conventional titanium alloys [1]. Moreover, some attempts to prevent the oxidation of Ti3 Al and TiAl-based alloys using Al3 Ti/Al2 Ti compounds have been recently reported [2–5]. The interfacial energies of a thin film system may stabilize phases that are unstable as a bulk solid. Understanding structural formation of aluminide phases as a result of the reaction of Ti and Al can pragmatically help optimizing treatment regime for the fabrication of protective ∗
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coatings based on aluminide phases or metallic–intermetallic laminate composites. In the present work, we have studied how the thickness of Al film and the structure and chemical composition of the initial substrate could have effect on the interdiffusion growth of aluminide phases on the Ti-based substrates.
2. Materials and experimental procedures The samples, cut from a plate with a thickness of 1 mm were mechanically polished and then electropolished. Various microstructures of initial substrates were formed by heat treatment. The polycrystalline commercial titanium was used as the first type of substrates. This is the basis for samples A and B. Sample A consisted of irregular grains (Fig. 1a in Ref. [6]). Sample B had a coarse grain lamellar structure (Fig. 1c in Ref. [6]). The commercial Ti–4%Al–1%Mn alloy was used as the second type of substrates. It is the basis for samples C–E. Fig. 1a shows the structure of sample C, in the as-received condition of the alloy. Sample D had the coarse grain lamellar structure (Fig. 1b). Sample E had a martensite like structure (Fig. 1c), obtained by holding the alloy at 900 ◦ C for 0.5 h and quenching it in water. The commercial Ti–4%Al–3%Mo–1%V alloy was used as the third type of substrates. This is sample F. The Ti–4%Al–3%Mo–1%V alloy consisted of mainly ␣-Ti and a small amount of  phase. Fig. 1d shows the structure of sample F in the as-received condition of the alloy. Sample F had a fine-grained structure like sample C.
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Fig. 1. Optical micrographs of the initial substrates: (a) sample C (Ti–4%Al–1%Mn); (b) sample D (Ti–4%Al–1%Mn); (c) sample E (Ti–4%Al–1%Mn); (d) sample F (Ti–4%Al–3%Mo–1%V).
The Al layer with thickness of about 2, 3.5 and 6 m on the substrates was formed by thermal deposition of 99.999%Al in a vacuum chamber at an operating pressure of 6 × 10−3 Pa. The temperature of the substrates during deposition was fixed at 100 ◦ C. The as-synthesised samples were annealed successively in vacuum of 10−5 Pa at temperatures ranging between 600 and 1000 ◦ C at every 100 ◦ C and were then cooled in the furnace. In all cases, the annealing time was 2 h. The resulting structure was studied by X-ray diffraction (XRD) analysis (Cu K␣ ) and scanning electron microscopy (SEM).
3. Experimental results 3.1. Titanium substrates Figs. 2 and 3 show the X-ray diffraction (XRD) patterns. After annealing treatment up to 900 ◦ C, the microstructure of the overlayer showed contours of
grain boundaries of the substrates. After annealing at 600 ◦ C, degradation was observed (Fig. 4). Particles distributed uniformly on the surface and predominantly had a plate like form (Fig. 5c in Ref. [6]). On the surface of samples with 2 and 3.5 m films, the Al2 Ti, TiAl and Ti3 Al phases were formed after annealing at 700 ◦ C. With increasing of annealing temperature to 800 ◦ C, on the surface of sample A with 6 m film, a slight coarsening of grains took place in comparison with the microstructure after heat treatment at 600 ◦ C. The particle coalescence in some areas was observed (Fig. 5a). In the case of 3.5 m film, coalescence of the particles happened on both samples A and B after annealing at 800 ◦ C (Fig. 5b). By examining the micrographs of samples A and B with 2 m film, it can be seen that annealing at 800 ◦ C resulted in a porous structure (Fig. 5c). The shape of diffraction lines and the intensities of the peaks in the X-ray diffraction patterns of samples A and B with 2 and 3.5 m films indicated the differences in the kinetics of the reaction after annealing at 900 ◦ C (Figs. 2 and 3 here and Figs. 2 and 3 in Ref. [6]).
Fig. 2. X-ray diffraction patterns (Cu K␣ ) of sample A coated with Al film after annealing: (a) 2 m film; (b) 6 m film.
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Fig. 3. X-ray diffraction patterns (Cu K␣ ) of sample B coated with Al film after annealing: (a) 2 m film; (b) 3.5 m film; (c) 6 m film.
3.2. Ti–4%Al–1%Mn substrates Table 1 summarizes the phase composition of the overlayer formed on the Ti–4%Al–1%Mn samples during heat treatment, as a function of the thickness of the Al film and the grain structure of the initial substrate. After annealing at 600 ◦ C, the diffraction lines of the Al3 Ti and Al24 Ti8 compounds were detected in the X-ray diffraction patterns of all three samples with 2 m film (Fig. 6). The Al24 Ti8 phase was not observed in the X-ray diffraction patterns of these samples with 3.5 and 6 m films (Figs. 7 and 8). Furthermore, additional diffraction peaks at the high-angle side of the main ␣-Ti ones were observed. These peaks could be attributed to the Ti3 Al phase. The result means that interdiffusion layers could be formed from both sides of the Ti/Al interface. Ti3 Al could be formed from the side of the Ti-alloy and Al3 Ti on the Al film. It can be assumed that during annealing at 600 ◦ C, diffusion transformation of the ␣-Ti phase to the Ti3 Al concentration occurred faster in the Ti–4%Al–1%Mn alloy than in pure Ti (cf. [6]). After annealing at 600 ◦ C, contours of grain boundaries of the initial substrates are particularly apparent in the case of samples C and D with 3.5 m film (Fig. 9a and b). On the surface of these two samples with 2 m film, similar
to what was observed in the micrograph of the Ti sample (Fig. 4), degradation of microstructure happened. In the case of quenched sample E with 6 m film, a fine lamellar morphology was observed after annealing at 600 ◦ C (Fig. 9c). For sample E with 3.5 m film, a smoother overlayer was formed (Fig. 9d). In either case, the microstructure of overlayer did not show any sign of grain boundaries of the initial substrate (Fig. 1c). The variation and the development of the overlayer morphology of sample E as function of the thickness of the Al film was likely related with the changes in the microstructure of the quenched substrates during heat treatment with its varying temperature and time. Fig. 10 shows the detailed structure of the overlayers after annealing at 600 ◦ C. For the sample with 2 m film, coalescence of the particles happened as observed on the Ti samples with 2 m film (Fig. 5a in Ref. [6]). The structure of the overlayer of sample C with 3.5 m film consisted of equiaxed particles with the size of about 0.6 m (Fig. 10a). For sample E with 3.5 m film, a smoother overlayer was formed (Fig. 10b). The distribution of particles on the surface did not show clearly the microstructure of the initial substrate. The micrograph of sample C with 6 m film shows that the formation of particles had not completed yet (Fig. 10c). The grain boundaries of the substrate were not seen clearly. It can be assumed that the particles were in the process of growth. The similar stage of
Table 1 The phase composition of overlayer on the Ti–4%Al–1%Mn samples as a function of annealing temperature, the thickness of the Al film and the grain structure of the initial substrate ◦C
2 m film
3.5 m film
6 m film
600 700 800 900 1000
Al3 Ti, Al24 Ti8 , Ti3 Al Al2 Ti, TiAl, Ti3 Ala Ti3 Al, X-phaseb Ti3 Al, X-phaseb ␣-Ti (C), Ti3 Al (D), Ti3 Al and X-phase (E)
Al3 Ti, Ti3 Al Al2 Ti, TiAl, Ti3 Ala TiAl, Ti3 Al Ti3 Al, X-phase ␣-Ti (C), Ti3 Al and X-phase (D and E)
Al3 Ti, Ti3 Al Al3 Ti, Al2 Ti, Ti3 Al Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al ␣-Ti (C), Ti3 Al and X-phase (D and E)
a b
Sample D also contained Al3 Ti. Sample C only contained trace amount of the X-phase after annealing at 800 ◦ C and contained no X-phase after 900 ◦ C.
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the evolution was observed on sample E with 6 m film. The distinction between them was that the form of particles was better seen on sample E than on sample C. Furthermore, the particles on sample E formed lamellar like morphology (Fig. 10d). The form of particles, their distribution on the surface and their evolution during heating depended on the structure of initial substrate as well as the thickness of the Al film. After annealing at 600 ◦ C, the samples were annealed at 700 ◦ C. After second annealing, the Al24 Ti8 compounds completely decomposed on the surface of all three samples with 2 m film (Table 1 and Fig. 6). The shape and intensity of the diffraction lines indicated that Al2 Ti, TiAl and Ti3 Al on the three samples differed from one another in structural imperfections and volume fraction. The weak diffraction peaks of Al3 Ti were only detected in the X-ray diffraction patterns of sample D with 2 and 3.5 m films (Figs. 6 and 7). So, this compound did not decompose fully only on the surface of sample D with the initial lamellar microstructure. It is necessary to note that Al3 Ti and Al24 Ti8 compounds did not decompose completely on the surface of all pure Ti samples with 2 and 3.5 m films. It could indicate that the process of diffusion transformation occurred faster on the surface of the Ti–4%Al–1%Mn alloy than on the one of Ti. After subsequent annealing at 800 ◦ C, in the X-ray diffraction patterns of samples with 2 m films, the Ti3 Al diffraction lines and very weak peaks of the X-phase [6] were observed (Fig. 6). With 3.5 m film, judging by the intensity of the diffraction lines, the maximum volume fraction of the TiAl phase occurred on the surface of sample D, and the Ti3 Al compound on sample E (Fig. 7). After annealing at 800 ◦ C, on the surface of sample D, the lamellar like morphology started to decompose (Fig. 11a). Coalescence of the particles happened. It was observed clearly on the sample with 2 m film, and less so on the sample with 6 m one (Fig. 11b and c). It is worth making a remark that for the pure Ti sample with the initial lamellar microstructure, decomposition of the lamellar like morphology started at 900 ◦ C. The change in the temperature range indicates that diffusion transformation occurred faster on the Ti–4%Al–1%Mn samples than on the pure Ti ones. Furthermore, the particles on sample B and on sample D differed from one another in their morphology, after annealing at 800 ◦ C (Fig. 6 in Ref. [6] and Fig. 11 here). In the case of sample E, with 6 m film, the lamellar like morphology completely disappeared after annealing at 800 ◦ C. Morphology similar to grain boundary precipitation in matrix was formed (Fig. 11d). Microcracks were observed (Fig. 11e). Annealing at 800 ◦ C gave the continuous and homogeneous microstructure on the surface of sample E with 3.5 m film (Fig. 11f). By examining the intensity of the diffraction lines, the maximum volume fraction of the X-phase occurred on the surface of sample D with 3.5 m film after annealing at 900 ◦ C (Fig. 7). Furthermore, the diffraction lines with ˚ and d = 2.27 A ˚ appeared. When these peaks were strong, the TiAl d = 2.63 A ˚ was diffraction lines were weak (Fig. 8). A very weak peak with d = 2.27 A
Fig. 5. Detailed structure on the Ti sample A substrates after annealing at 800 ◦ C as a function of the thickness of the Al film: (a) 6 m film; (b) 3.5 m film; (c) 2 m film. Fig. 4. Microstructure of the overlayer on the Ti substrate after annealing: sample B, 2 m film, 600 ◦ C.
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Fig. 6. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 2 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E. detected in the diffraction pattern of sample E with 3.5 m film after annealing ˚ was observed in the at 800 ◦ C as well. The similar weak peak with d = 2.27 A X-ray diffraction patterns of the pure Ti samples especially with 6 m film after ˚ was not attributed to the X-phase annealing at 900 ◦ C. The peak with d = 2.27 A because it completely disappeared when all typical ones of the X-phase were detected in the X-ray diffraction patterns (Figs. 2 and 3). At 900 ◦ C, the Ti–4%Al–1%Mn alloy was in the ␣ +  region. This led to the virtually complete decomposition of the lamellar like morphology on the surface of sample D with 6 m film (Fig. 12a). After coalescing particles, a continuous overlayer was formed (Fig. 12a). On the surface of sample D with 2 m film, a porous structure was observed (Fig. 12b). Similar changes in the structure happened on samples C and E. There is a spectrum of microstructures observed after annealing at 900 ◦ C. For comparison, Fig. 12c shows the structure of sample C with 3.5 m film. Annealing at 1000 ◦ C led to the elimination of the overlayer on sample C. The diffraction lines of ␣-Ti only were observed in the X-ray diffraction patterns of sample C (Figs. 6–8). The typical microstructure after the elimination of the overlayer consisted of very fine plates (Fig. 13b). This microstructure was similar to the one observed on the pure Ti samples (Fig. 13a). With increasing of annealing temperature from 900 to 1000 ◦ C, the intensity of the X-phase diffraction lines decreased noticeably (Fig. 7). Fig. 13c and d show two types of microstructures formed, likely showing the different stages of interdiffusion transformation and elimination of overlayers.
3.3. Ti–4%Al–3%Mo–1%V substrates Table 2 summarizes the phase composition on the Ti–4%Al–3%Mo–1%V samples. After annealing at 600 ◦ C, the diffraction lines of the Al3 Ti compound
and -Ti were observed in the X-ray diffraction patterns of sample F with 2 and 6 m films (Table 2 and Fig. 14). Annealing at 600 ◦ C led to the formation of the equiaxed fine-grained structure on sample F with 2 m film. On the surface, there were areas where the structure of the particles was not seen clearly (Fig. 15a). The microstructure on sample F with 2 m film was more homogenous and it was degraded less severally than the ones on the pure Ti (Fig. 4 in this paper and Fig. 5a in Ref. [6]) and Ti–4%Al–1%Mn samples after annealing at 600 ◦ C. By examining the micrograph of sample F with 6 m film, it can be assumed that the particles were in the process of growth (Fig. 15b). The structure was inhomogeneous. The fine-grained particles with irregular forms were observed. It is necessary to note that the density of particles on the surface of sample F was lower than on the one of sample C from Ti–4%Al–1%Mn alloy (Fig. 10c). It can point to some differences in the growth kinetics of particles. After annealing at the higher temperature of 700 ◦ C, the Al3 Ti compound did not decompose completely on sample F with 2 m film as it was observed Table 2 The phase composition on the Ti–4%Al–3%Mo–1%V samples as a function of annealing temperature and the thickness of the Al film ◦C
2 m film
6 m film
600 700 800 900 1000
Al3 Ti, -Ti Al3 Ti, Al2 Ti, TiAl, Ti3 Al, -Ti Ti3 Al, X-phase, -Ti Ti3 Al, X-phase, -Ti ␣-Ti, -Ti
Al3 Ti, -Ti Al3 Ti, Al2 Ti, -Ti Al3 Ti, Al2 Ti, TiAl, Ti3 Al, -Ti TiAl, Ti3 Al, -Ti Ti3 Al, X-phase, ␣-Ti, -Ti
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Fig. 7. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 3.5 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E.
Fig. 8. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 6 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E.
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Fig. 9. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 600 ◦ C: (a) sample C, 3.5 m film; (b) sample D, 3.5 m film; (c) sample E, 6 m film; (d) sample E, 3.5 m film.
Fig. 10. Detailed structure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 600 ◦ C as a function of the thickness of the Al film: (a) sample C, 3.5 m film; (b) sample E, 3.5 m film; (c) sample C, 6 m film; (d) sample E, 6 m film.
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Fig. 11. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 800 ◦ C: (a and b) sample D, 6 m film; (c) sample D, 2 m film; (d) and (e) sample E, 6 m film; (f) sample E, 3.5 m film.
on samples C and E from the Ti–4%Al–1%Mn alloy with 2 and 3.5 m films (Tables 1 and 2). Annealing at 700 ◦ C gave the homogeneous structure on sample F with 6 m film, which consisted of the plate like particles with the size of about 0.5 m (Fig. 15c). Thus, with increasing of annealing temperature from 600 to 700 ◦ C, the particles developed. A structure similar to the ones on the pure Ti (Fig. 5) and Ti–4%Al–1%Mn (Fig. 10) samples was formed. The distinctions between them were the detailed forms and sizes of particles, the temperature range of their formation, and morphology of their distribution on the surface. After subsequent annealing at 800 ◦ C, coalescence of the particles was observed. The structure of the overlayer on sample F with 2 m film became inhomogeneous and porous (Fig. 15d). In contrast to the Ti–4%Al–1%Mn samples, the Al3 Ti compound did not decompose completely on the Ti–4%Al–3%Mo–1%V sample with 6 m film after annealing at 800 ◦ C (Tables 1 and 2). After subsequent annealing at 900 ◦ C, asymmetry of the ␣-Ti peaks at the high-angle side in the X-ray diffraction pattern might indicate the presence of the Ti3 Al phase. However, the superstructure line (1 0 1) at 2θ = 26◦ was not detected (Fig. 14). In general, the development of the overlayer microstructure followed these stages: nucleation and growth of particles, coalescence of
the particles, formation of continuous structure and finally porous structure. Fig. 15 shows all stages of the structural formation by the example of sample F. The thickness of the Al film only modified the temperature ranges of recrystallisation. Annealing at 1000 ◦ C led to the elimination of the overlayer on sample F with 2 m film. A fine lamellar microstructure was formed similar to the ones observed on the pure Ti and Ti–4%Al–1%Mn samples (Fig. 13a and b). On the surface of sample F with 6 m film, the remains of the overlayer and formation of the fine plates were observed (Fig. 13e). It is necessary to note that, when the substrate with coarse grain structure from the Ti–4%Al–3%Mo–1%V alloy was used, the elimination of the overlayer with 6 m film did not happen. Fig. 13 shows all types of microstructure observed after annealing at 1000 ◦ C. The final structure formed after the elimination of the overlayer consisted of the very fine plates. Consequently, it can be assumed that the succession of the micrographs Fig. 8b in Ref. [6], and Fig. 13c–e and a showed the development of the microstructure during annealing at 1000 ◦ C. It demonstrates that the stage of the microstructural evolution on heating to 1000 ◦ C depended on the thickness of the Al film, and the structure and chemical composition of the initial substrate.
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Fig. 12. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 900 ◦ C: (a) sample D, 6 m film; (b) sample D, 2 m film; (c) sample C, 3.5 m film.
4. Discussion That Ti3 Al could be formed from the Ti side and Al3 Ti from the Al one was observed more significantly on the Ti–4%Al–1%Mn samples (first paragraph under Section 3.2) than on the Ti ones because Al and Mn could increase atom mobility [7,8], accelerating the diffusion transformation of the ␣-Ti phase to the Ti3 Al concentration. Formation of the Ti3 Al phase could not be observed on the Ti–4%Al–3%Mo–1%V sample because Mo and V could strongly decrease atom mobility [8]. To our knowledge, Ti diffusion in pure Al or Al alloyed with Ti has never been thoroughly studied. Beginning of the formation of the Al3 Ti interface layer could decrease Al diffusion into Ti, since, according to [8,9], Al diffuses in titanium aluminides slower, and with a higher activation energy than Ti. Furthermore, Al3 Ti has a lower free energy of formation than Ti3 Al [10,11]. Consequently, it can be assumed that mainly at the Ti/Al interface, the Al3 Ti phase was formed on heating to 600 ◦ C. This phase grew in the direction of the aluminum film as Ti diffused faster to the front of the reaction through Al3 Ti layer than Al. The whole aluminum film was gradually used on the formation of this overlayer. Then, Fig. 5d in Ref. [6] shows the remains of the Al film and overlayer growing under the film. Fig. 15b and c shows the evolution of the particles with increasing of annealing temperature. Formation and stability of the Al24 Ti8 phase depended on the thickness of the Al film and the chemical composition of
the substrates. The mechanism of the Al24 Ti8 formation is still not well known. In our case, it can be assumed that the Al24 Ti8 compound was formed at the determined concentration ratio of the elements and the stresses between substrate and the Al film at the Ti/Al interface had a great effect on its formation and stability. Table 3 summarises the principal temperature ranges of the aluminide phase formation as a function of the thickness of the Al film independently from structure and chemical composition of the substrates. It can be assumed that with increasing of annealing temperature, the changes in the concentration ratio of elements in the overlayers resulted in the sequential formation of aluminide phases according to the Ti–Al phase diagram. Since Ti diffused in the aluminide phases faster than Al and the consequently aluminide phase formation was directed towards the Ti-rich compounds, the primary site of the reactions was the interface of the Al-rich ones.
Table 3 The principal temperature ranges of the aluminide phase formation on Ti and its alloys as a function of thickness of the Al film ◦C
2 m film
3.5 m film
6 m film
600 700 800 900
Al3 Ti Al2 Ti, TiAl, Ti3 Al Ti3 Al Ti3 Al
Al3 Ti Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al Ti3 Al
Al3 Ti Al3 Ti, Al2 Ti Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al
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Fig. 13. Microstructure of the samples with 6 m Al film after annealing at 1000 ◦ C: (a) sample A; (b) sample C; (c) sample D; (d) sample E; (e) sample F.
Depending on the structure and chemical composition of the substrate, deviations from the principal temperature ranges of the structural formation shown in Table 3 were observed. These deviations could indicate that the rate of reactions on the surface depended on the structure and the chemical composition of the substrates. By examining the intensity of the diffraction lines and data in Table 1 in Ref. [6] and Tables 1 and 2 in the present paper, it was concluded that on heating up to 900 ◦ C, the reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. It could depend on the density of grain boundaries. Furthermore, the diffusion parameters for Ti are anisotropic [7,8]. It means that the crystallographic orientation of the initial grains could have a great effect on the diffusion transformation as well. Depending on the chemical composition of the substrates, the following tendency was observed. Reactions occurred faster on the Ti–4%Al–1%Mn
samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. By examining the structural evolution of the overlayer on the Ti–4%Al–3%Mo–1%V samples, it was concluded that reaction on the Ti–4%Al–3%Mo–1%V ones occurred slower than on the Ti samples with irregular grains. As above-mentioned, it could be related with the different alloying elements having different effect on atom mobility. The structural degradation was observed dominantly on the Ti sample with 2 m film, and to lesser extents on the Ti–4%Al–3%Mo–1%V one. The degradation of the thin overlayer formed during annealing was likely related with the distribution of stresses between substrate and the Al film, which in turn had a great effect on the diffusion fluxes during subsequent annealing. The general tendency in the structural evolution was observed on heating up to 900 ◦ C. A thinner Al film resulted in faster coa-
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Fig. 14. X-ray diffraction patterns (Cu K␣ ) of sample F coated with Al film after annealing: (a) 2 m film; (b) 6 m film.
lescence of the particles (Figs. 5, 11 and 15). For the Al film of 2 m, coalescence of the particles was observed after annealing at 600 ◦ C, but for the Al film of 6 m, the particles kept their form even after 800 ◦ C annealing. Coalescence of the particles was likely to be related with recrystallisation of the overlayers. Nucleation and growth of new phases led to the changes in microstructure. Thicker overlayer required more time for its recrystallisation and microstructural and diffusion transformation. The faster reactions occurred on the surface, the faster morphology decomposed. Furthermore, the overlayers changed their morphologies according to changes in the microstructure of the substrate as observed on the quenched samples from Ti–4%Al–1%Mn (Figs. 9c and d, 11d and f). After annealing at 900 ◦ C, the continuous structure was typical for the two-phase TiAl + Ti3 Al overlayer formed from the 6 m films. The porous one was typical for the Ti3 Al + X-phase overlayer formed from the film with thickness less than 6 m (Fig. 7 in Ref. [6] and Figs. 12 and 15 here). The samples differed from one another in phase ratio in the overlayer. Annealing at 1000 ◦ C did not lead to elimination of the overlayer on the substrates with the initial lamellar microstructure. The distinction in the structure and phase composition on the different substrates after annealing above 900 ◦ C could be explained in the following way. Annealing at 900 ◦ C caused Ti to enter the  region, and the Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V alloys to enter the ␣ +  one. Consequently, during annealing at 900 ◦ C, growth of -grains took place. On heating to 900 ◦ C, diffusion coefficients changed and mobility of atoms increased dramatically [7,8]. Formation of porous morphology after annealing at 900 ◦ C, and presence of
Al after elimination of the overlayer in Ti could indicate that during heat treatment in  or ␣ +  regions the atoms started diffusing into the substrates. The initial substrates had the different structure and different thickness of the Al films represents the different strength of the Al source. Interdiffusion growth could result in the different concentration ratio of Al from the side Ti and its alloys depending on the structure of the initial substrate. Al increased temperature of ␣ →  transformation and in that way it stabilized ␣-Ti in near surface region. It could lead to the non-uniform growth of the -grains in near surface region and have a great effect on diffusion transformation. Accordingly, the distinctions in the kinetics of the reactions and in the stability of overlayers were observed. In the process of the microstructural evolution of overlayer, in the X-ray diffraction patterns, peaks from an unknown phase were detected. The diffraction lines of the X-phase were detected only along with the Ti3 Al ones (Table 1 in Ref. [6] and Tables 1 and 2 here). The ranges of the existence of the oxide and carbide phases are still not well known for the ternary Ti–Al–O or Ti–Al–C systems. Some of them are not stable at high temperatures. They could form upon cooling and decompose ˚ upon heating. For instance, the diffraction lines with d = 2.63 A ˚ and d = 2.27 A (Fig. 8) could be attributed to the Ti2 AlC or Ti3 AlC2 compounds. These lines disappeared when the typical ones of the X-phase were detected. However, the Ti2 AlC or Ti3 AlC2 phases are normally synthesized at special conditions at high temperatures [12,13]. Formation of the X-phase depended on the thickness of the Al film, and the structure and chemical composition of the initial substrate. It deserves further study.
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Fig. 15. Evolution of overlayer microstructure on sample F after annealing: (a) 2 m film, 600 ◦ C; (b) 6 m film, 600 ◦ C; (c) 6 m film, 700 ◦ C; (d) 2 m film, 800 ◦ C; (e) 6 m film, 900 ◦ C.
5. Conclusions During annealing, on the surface of the Ti, Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V samples coated with Al film, different aluminide phases were formed successively as the result of reactions between Ti and Al. The thickness of the Al film had a great effect on the structure of the overlayer formed during heat treatment. The temperature ranges of phase existence depended on the thickness of the initial Al film. Thicker overlayers were formed on the surface with thicker Al film, which required more time for its recrystallisation and microstructural transformation. After annealing treatment up to 900 ◦ C, the microstructure of the overlayer clearly showed contours of grain boundaries of the initial substrates. The shapes, sizes and distribution of the particles in the overlayer depended on the microstructure of the
initial substrates as well as on the thickness of the Al film. The particles formed colonies along the grain boundaries of the initial substrate in the way that they showed their microstructure. The overlayer changed its morphology according to changes in the microstructure of the substrate. The kinetics of reactions on the surface depended on the microstructure of the initial substrates. The reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. Annealing at the temperatures above 900 ◦ C resulted in the elimination of the overlayer on the substrates in the favor of releasing Al into Ti. After the elimination of the overlayer, a fine plate microstructure was formed. Chemical composition of substrates had a great effect on the diffusion transformation of overlayers. Reactions occurred faster on the Ti–4%Al–1%Mn samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. Reaction on the
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Ti–4%Al–3%Mo–1%V ones occurred slower than on the Ti samples with irregular grains. When reactions occurred fast on the surface, the morphology decomposed fast, too. References [1] D.M. Dimiduk, in: Y.-W. Kim, R. Wagner, M. Yamaguchi (Eds.), Gamma Titanium Aluminides, TMS, Warrendale, PA, 1995, p. 3. [2] X.Y. Li, Y.-C. Zhu, K. Fujita, N. Iwamoto, Y. Matsunaga, K. Nakagawa, S. Taniguchi, Surf. Coat. Technol. 136 (2001) 276–280. [3] M.S. Chu, S.K. Wu, Acta Mater. 51 (2003) 3109–3120. [4] M.S. Chu, S.K. Wu, Surf. Coat. Technol. 179 (2004) 257–264. [5] Z. Liu, G. Wang, Mater. Sci. Eng. A 397 (2005) 50–57.
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[6] S. Romankov, W. Sha, E. Ermakov, A. Mamaeva, J. Alloy. Compd. 420 (2006) 63–70. [7] M. K¨oppers, Ch. Herzig, M. Friesel, Y. Mishin, Acta Mater. 45 (1997) 4181–4191. [8] Y. Mishin, Ch. Herzig, Acta Mater. 48 (2000) 589–623. [9] R. Kainuma, J. Sato, I. Ohnuma, K. Ishida, Intermetallics 13 (2005) 784–791. [10] U.R. Kattner, J.C. Lin, Y.A. Chang, Metall. Mater. Trans. 23A (1992) 2081–2090. [11] L.M. Peng, J.H. Wang, H. Li, J.H. Zhao, L.H. He, Scripta Mater. 52 (2005) 243–248. [12] A. Zhou, C. Wang, Y. Huang, Mater. Sci. Eng. A 352 (2003) 333–339. [13] W.B. Zhou, B.C. Mei, J.Q. Zhu, X.L. Hong, Mater. Lett. 59 (2005) 131– 134.
Characterization of interdiffusion growth of aluminized layer on Ti alloys S. Romankov a , W. Sha b,∗ , E. Ermakov a , A. Mamaeva a a
b
Institute of Physics and Technology, 050032 Almaty32, Kazakhstan Metals Research Group, School of Planning, Architecture and Civil Engineering, The Queen’s University of Belfast, Belfast BT7 1NN, UK
Received 16 January 2006; received in revised form 11 April 2006; accepted 12 April 2006 Available online 15 May 2006
Abstract During annealing on the surface of the Ti, Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V samples coated with Al films, different aluminide phases were formed successively as the result of reactions between Ti and Al. The structural evolution of the overlayers formed on the substrates during annealing depended on the thickness of the Al film, and the microstructure and chemical composition of the initial substrate. For thicker Al films, thicker overlayers were formed on the surface. Thicker overlayer required more time for its recrystallisation and microstructural transformation. The reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. Chemical composition of substrates had a great effect on the diffusion transformation of overlayers. Reactions occurred faster on the Ti–4%Al–1%Mn samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. After the elimination of the overlayer a fine plate microstructure was formed. © 2006 Elsevier B.V. All rights reserved. Keywords: Thin films; Intermetallics; Phase transitions; Scanning electron microscopy, SEM; X-ray diffraction
1. Introduction Ti and its alloys have excellent resistance against corrosion and desirable specific strengths at ambient temperature. However, due to their poor resistance against adhesive wear and oxidation problems, technical application of titanium alloys is limited to temperatures much lower than those permissible by alloy mechanical properties. Intermetallic compounds of the Ti–Al system have been investigated extensively for high-temperature structural applications because they offer a combination of good oxidation resistance and useful mechanical properties at temperatures higher than those possible with conventional titanium alloys [1]. Moreover, some attempts to prevent the oxidation of Ti3 Al and TiAl-based alloys using Al3 Ti/Al2 Ti compounds have been recently reported [2–5]. The interfacial energies of a thin film system may stabilize phases that are unstable as a bulk solid. Understanding structural formation of aluminide phases as a result of the reaction of Ti and Al can pragmatically help optimizing treatment regime for the fabrication of protective ∗
Corresponding author. Tel.: +44 28 90974017; fax: +44 28 90663754. E-mail address: [email protected] (W. Sha).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.017
coatings based on aluminide phases or metallic–intermetallic laminate composites. In the present work, we have studied how the thickness of Al film and the structure and chemical composition of the initial substrate could have effect on the interdiffusion growth of aluminide phases on the Ti-based substrates.
2. Materials and experimental procedures The samples, cut from a plate with a thickness of 1 mm were mechanically polished and then electropolished. Various microstructures of initial substrates were formed by heat treatment. The polycrystalline commercial titanium was used as the first type of substrates. This is the basis for samples A and B. Sample A consisted of irregular grains (Fig. 1a in Ref. [6]). Sample B had a coarse grain lamellar structure (Fig. 1c in Ref. [6]). The commercial Ti–4%Al–1%Mn alloy was used as the second type of substrates. It is the basis for samples C–E. Fig. 1a shows the structure of sample C, in the as-received condition of the alloy. Sample D had the coarse grain lamellar structure (Fig. 1b). Sample E had a martensite like structure (Fig. 1c), obtained by holding the alloy at 900 ◦ C for 0.5 h and quenching it in water. The commercial Ti–4%Al–3%Mo–1%V alloy was used as the third type of substrates. This is sample F. The Ti–4%Al–3%Mo–1%V alloy consisted of mainly ␣-Ti and a small amount of  phase. Fig. 1d shows the structure of sample F in the as-received condition of the alloy. Sample F had a fine-grained structure like sample C.
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Fig. 1. Optical micrographs of the initial substrates: (a) sample C (Ti–4%Al–1%Mn); (b) sample D (Ti–4%Al–1%Mn); (c) sample E (Ti–4%Al–1%Mn); (d) sample F (Ti–4%Al–3%Mo–1%V).
The Al layer with thickness of about 2, 3.5 and 6 m on the substrates was formed by thermal deposition of 99.999%Al in a vacuum chamber at an operating pressure of 6 × 10−3 Pa. The temperature of the substrates during deposition was fixed at 100 ◦ C. The as-synthesised samples were annealed successively in vacuum of 10−5 Pa at temperatures ranging between 600 and 1000 ◦ C at every 100 ◦ C and were then cooled in the furnace. In all cases, the annealing time was 2 h. The resulting structure was studied by X-ray diffraction (XRD) analysis (Cu K␣ ) and scanning electron microscopy (SEM).
3. Experimental results 3.1. Titanium substrates Figs. 2 and 3 show the X-ray diffraction (XRD) patterns. After annealing treatment up to 900 ◦ C, the microstructure of the overlayer showed contours of
grain boundaries of the substrates. After annealing at 600 ◦ C, degradation was observed (Fig. 4). Particles distributed uniformly on the surface and predominantly had a plate like form (Fig. 5c in Ref. [6]). On the surface of samples with 2 and 3.5 m films, the Al2 Ti, TiAl and Ti3 Al phases were formed after annealing at 700 ◦ C. With increasing of annealing temperature to 800 ◦ C, on the surface of sample A with 6 m film, a slight coarsening of grains took place in comparison with the microstructure after heat treatment at 600 ◦ C. The particle coalescence in some areas was observed (Fig. 5a). In the case of 3.5 m film, coalescence of the particles happened on both samples A and B after annealing at 800 ◦ C (Fig. 5b). By examining the micrographs of samples A and B with 2 m film, it can be seen that annealing at 800 ◦ C resulted in a porous structure (Fig. 5c). The shape of diffraction lines and the intensities of the peaks in the X-ray diffraction patterns of samples A and B with 2 and 3.5 m films indicated the differences in the kinetics of the reaction after annealing at 900 ◦ C (Figs. 2 and 3 here and Figs. 2 and 3 in Ref. [6]).
Fig. 2. X-ray diffraction patterns (Cu K␣ ) of sample A coated with Al film after annealing: (a) 2 m film; (b) 6 m film.
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Fig. 3. X-ray diffraction patterns (Cu K␣ ) of sample B coated with Al film after annealing: (a) 2 m film; (b) 3.5 m film; (c) 6 m film.
3.2. Ti–4%Al–1%Mn substrates Table 1 summarizes the phase composition of the overlayer formed on the Ti–4%Al–1%Mn samples during heat treatment, as a function of the thickness of the Al film and the grain structure of the initial substrate. After annealing at 600 ◦ C, the diffraction lines of the Al3 Ti and Al24 Ti8 compounds were detected in the X-ray diffraction patterns of all three samples with 2 m film (Fig. 6). The Al24 Ti8 phase was not observed in the X-ray diffraction patterns of these samples with 3.5 and 6 m films (Figs. 7 and 8). Furthermore, additional diffraction peaks at the high-angle side of the main ␣-Ti ones were observed. These peaks could be attributed to the Ti3 Al phase. The result means that interdiffusion layers could be formed from both sides of the Ti/Al interface. Ti3 Al could be formed from the side of the Ti-alloy and Al3 Ti on the Al film. It can be assumed that during annealing at 600 ◦ C, diffusion transformation of the ␣-Ti phase to the Ti3 Al concentration occurred faster in the Ti–4%Al–1%Mn alloy than in pure Ti (cf. [6]). After annealing at 600 ◦ C, contours of grain boundaries of the initial substrates are particularly apparent in the case of samples C and D with 3.5 m film (Fig. 9a and b). On the surface of these two samples with 2 m film, similar
to what was observed in the micrograph of the Ti sample (Fig. 4), degradation of microstructure happened. In the case of quenched sample E with 6 m film, a fine lamellar morphology was observed after annealing at 600 ◦ C (Fig. 9c). For sample E with 3.5 m film, a smoother overlayer was formed (Fig. 9d). In either case, the microstructure of overlayer did not show any sign of grain boundaries of the initial substrate (Fig. 1c). The variation and the development of the overlayer morphology of sample E as function of the thickness of the Al film was likely related with the changes in the microstructure of the quenched substrates during heat treatment with its varying temperature and time. Fig. 10 shows the detailed structure of the overlayers after annealing at 600 ◦ C. For the sample with 2 m film, coalescence of the particles happened as observed on the Ti samples with 2 m film (Fig. 5a in Ref. [6]). The structure of the overlayer of sample C with 3.5 m film consisted of equiaxed particles with the size of about 0.6 m (Fig. 10a). For sample E with 3.5 m film, a smoother overlayer was formed (Fig. 10b). The distribution of particles on the surface did not show clearly the microstructure of the initial substrate. The micrograph of sample C with 6 m film shows that the formation of particles had not completed yet (Fig. 10c). The grain boundaries of the substrate were not seen clearly. It can be assumed that the particles were in the process of growth. The similar stage of
Table 1 The phase composition of overlayer on the Ti–4%Al–1%Mn samples as a function of annealing temperature, the thickness of the Al film and the grain structure of the initial substrate ◦C
2 m film
3.5 m film
6 m film
600 700 800 900 1000
Al3 Ti, Al24 Ti8 , Ti3 Al Al2 Ti, TiAl, Ti3 Ala Ti3 Al, X-phaseb Ti3 Al, X-phaseb ␣-Ti (C), Ti3 Al (D), Ti3 Al and X-phase (E)
Al3 Ti, Ti3 Al Al2 Ti, TiAl, Ti3 Ala TiAl, Ti3 Al Ti3 Al, X-phase ␣-Ti (C), Ti3 Al and X-phase (D and E)
Al3 Ti, Ti3 Al Al3 Ti, Al2 Ti, Ti3 Al Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al ␣-Ti (C), Ti3 Al and X-phase (D and E)
a b
Sample D also contained Al3 Ti. Sample C only contained trace amount of the X-phase after annealing at 800 ◦ C and contained no X-phase after 900 ◦ C.
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the evolution was observed on sample E with 6 m film. The distinction between them was that the form of particles was better seen on sample E than on sample C. Furthermore, the particles on sample E formed lamellar like morphology (Fig. 10d). The form of particles, their distribution on the surface and their evolution during heating depended on the structure of initial substrate as well as the thickness of the Al film. After annealing at 600 ◦ C, the samples were annealed at 700 ◦ C. After second annealing, the Al24 Ti8 compounds completely decomposed on the surface of all three samples with 2 m film (Table 1 and Fig. 6). The shape and intensity of the diffraction lines indicated that Al2 Ti, TiAl and Ti3 Al on the three samples differed from one another in structural imperfections and volume fraction. The weak diffraction peaks of Al3 Ti were only detected in the X-ray diffraction patterns of sample D with 2 and 3.5 m films (Figs. 6 and 7). So, this compound did not decompose fully only on the surface of sample D with the initial lamellar microstructure. It is necessary to note that Al3 Ti and Al24 Ti8 compounds did not decompose completely on the surface of all pure Ti samples with 2 and 3.5 m films. It could indicate that the process of diffusion transformation occurred faster on the surface of the Ti–4%Al–1%Mn alloy than on the one of Ti. After subsequent annealing at 800 ◦ C, in the X-ray diffraction patterns of samples with 2 m films, the Ti3 Al diffraction lines and very weak peaks of the X-phase [6] were observed (Fig. 6). With 3.5 m film, judging by the intensity of the diffraction lines, the maximum volume fraction of the TiAl phase occurred on the surface of sample D, and the Ti3 Al compound on sample E (Fig. 7). After annealing at 800 ◦ C, on the surface of sample D, the lamellar like morphology started to decompose (Fig. 11a). Coalescence of the particles happened. It was observed clearly on the sample with 2 m film, and less so on the sample with 6 m one (Fig. 11b and c). It is worth making a remark that for the pure Ti sample with the initial lamellar microstructure, decomposition of the lamellar like morphology started at 900 ◦ C. The change in the temperature range indicates that diffusion transformation occurred faster on the Ti–4%Al–1%Mn samples than on the pure Ti ones. Furthermore, the particles on sample B and on sample D differed from one another in their morphology, after annealing at 800 ◦ C (Fig. 6 in Ref. [6] and Fig. 11 here). In the case of sample E, with 6 m film, the lamellar like morphology completely disappeared after annealing at 800 ◦ C. Morphology similar to grain boundary precipitation in matrix was formed (Fig. 11d). Microcracks were observed (Fig. 11e). Annealing at 800 ◦ C gave the continuous and homogeneous microstructure on the surface of sample E with 3.5 m film (Fig. 11f). By examining the intensity of the diffraction lines, the maximum volume fraction of the X-phase occurred on the surface of sample D with 3.5 m film after annealing at 900 ◦ C (Fig. 7). Furthermore, the diffraction lines with ˚ and d = 2.27 A ˚ appeared. When these peaks were strong, the TiAl d = 2.63 A ˚ was diffraction lines were weak (Fig. 8). A very weak peak with d = 2.27 A
Fig. 5. Detailed structure on the Ti sample A substrates after annealing at 800 ◦ C as a function of the thickness of the Al film: (a) 6 m film; (b) 3.5 m film; (c) 2 m film. Fig. 4. Microstructure of the overlayer on the Ti substrate after annealing: sample B, 2 m film, 600 ◦ C.
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Fig. 6. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 2 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E. detected in the diffraction pattern of sample E with 3.5 m film after annealing ˚ was observed in the at 800 ◦ C as well. The similar weak peak with d = 2.27 A X-ray diffraction patterns of the pure Ti samples especially with 6 m film after ˚ was not attributed to the X-phase annealing at 900 ◦ C. The peak with d = 2.27 A because it completely disappeared when all typical ones of the X-phase were detected in the X-ray diffraction patterns (Figs. 2 and 3). At 900 ◦ C, the Ti–4%Al–1%Mn alloy was in the ␣ +  region. This led to the virtually complete decomposition of the lamellar like morphology on the surface of sample D with 6 m film (Fig. 12a). After coalescing particles, a continuous overlayer was formed (Fig. 12a). On the surface of sample D with 2 m film, a porous structure was observed (Fig. 12b). Similar changes in the structure happened on samples C and E. There is a spectrum of microstructures observed after annealing at 900 ◦ C. For comparison, Fig. 12c shows the structure of sample C with 3.5 m film. Annealing at 1000 ◦ C led to the elimination of the overlayer on sample C. The diffraction lines of ␣-Ti only were observed in the X-ray diffraction patterns of sample C (Figs. 6–8). The typical microstructure after the elimination of the overlayer consisted of very fine plates (Fig. 13b). This microstructure was similar to the one observed on the pure Ti samples (Fig. 13a). With increasing of annealing temperature from 900 to 1000 ◦ C, the intensity of the X-phase diffraction lines decreased noticeably (Fig. 7). Fig. 13c and d show two types of microstructures formed, likely showing the different stages of interdiffusion transformation and elimination of overlayers.
3.3. Ti–4%Al–3%Mo–1%V substrates Table 2 summarizes the phase composition on the Ti–4%Al–3%Mo–1%V samples. After annealing at 600 ◦ C, the diffraction lines of the Al3 Ti compound
and -Ti were observed in the X-ray diffraction patterns of sample F with 2 and 6 m films (Table 2 and Fig. 14). Annealing at 600 ◦ C led to the formation of the equiaxed fine-grained structure on sample F with 2 m film. On the surface, there were areas where the structure of the particles was not seen clearly (Fig. 15a). The microstructure on sample F with 2 m film was more homogenous and it was degraded less severally than the ones on the pure Ti (Fig. 4 in this paper and Fig. 5a in Ref. [6]) and Ti–4%Al–1%Mn samples after annealing at 600 ◦ C. By examining the micrograph of sample F with 6 m film, it can be assumed that the particles were in the process of growth (Fig. 15b). The structure was inhomogeneous. The fine-grained particles with irregular forms were observed. It is necessary to note that the density of particles on the surface of sample F was lower than on the one of sample C from Ti–4%Al–1%Mn alloy (Fig. 10c). It can point to some differences in the growth kinetics of particles. After annealing at the higher temperature of 700 ◦ C, the Al3 Ti compound did not decompose completely on sample F with 2 m film as it was observed Table 2 The phase composition on the Ti–4%Al–3%Mo–1%V samples as a function of annealing temperature and the thickness of the Al film ◦C
2 m film
6 m film
600 700 800 900 1000
Al3 Ti, -Ti Al3 Ti, Al2 Ti, TiAl, Ti3 Al, -Ti Ti3 Al, X-phase, -Ti Ti3 Al, X-phase, -Ti ␣-Ti, -Ti
Al3 Ti, -Ti Al3 Ti, Al2 Ti, -Ti Al3 Ti, Al2 Ti, TiAl, Ti3 Al, -Ti TiAl, Ti3 Al, -Ti Ti3 Al, X-phase, ␣-Ti, -Ti
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Fig. 7. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 3.5 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E.
Fig. 8. X-ray diffraction patterns (Cu K␣ ) of the Ti–4%Al–1%Mn substrates coated with 6 m Al film after annealing: (a) sample C; (b) sample D; (c) sample E.
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Fig. 9. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 600 ◦ C: (a) sample C, 3.5 m film; (b) sample D, 3.5 m film; (c) sample E, 6 m film; (d) sample E, 3.5 m film.
Fig. 10. Detailed structure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 600 ◦ C as a function of the thickness of the Al film: (a) sample C, 3.5 m film; (b) sample E, 3.5 m film; (c) sample C, 6 m film; (d) sample E, 6 m film.
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Fig. 11. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 800 ◦ C: (a and b) sample D, 6 m film; (c) sample D, 2 m film; (d) and (e) sample E, 6 m film; (f) sample E, 3.5 m film.
on samples C and E from the Ti–4%Al–1%Mn alloy with 2 and 3.5 m films (Tables 1 and 2). Annealing at 700 ◦ C gave the homogeneous structure on sample F with 6 m film, which consisted of the plate like particles with the size of about 0.5 m (Fig. 15c). Thus, with increasing of annealing temperature from 600 to 700 ◦ C, the particles developed. A structure similar to the ones on the pure Ti (Fig. 5) and Ti–4%Al–1%Mn (Fig. 10) samples was formed. The distinctions between them were the detailed forms and sizes of particles, the temperature range of their formation, and morphology of their distribution on the surface. After subsequent annealing at 800 ◦ C, coalescence of the particles was observed. The structure of the overlayer on sample F with 2 m film became inhomogeneous and porous (Fig. 15d). In contrast to the Ti–4%Al–1%Mn samples, the Al3 Ti compound did not decompose completely on the Ti–4%Al–3%Mo–1%V sample with 6 m film after annealing at 800 ◦ C (Tables 1 and 2). After subsequent annealing at 900 ◦ C, asymmetry of the ␣-Ti peaks at the high-angle side in the X-ray diffraction pattern might indicate the presence of the Ti3 Al phase. However, the superstructure line (1 0 1) at 2θ = 26◦ was not detected (Fig. 14). In general, the development of the overlayer microstructure followed these stages: nucleation and growth of particles, coalescence of
the particles, formation of continuous structure and finally porous structure. Fig. 15 shows all stages of the structural formation by the example of sample F. The thickness of the Al film only modified the temperature ranges of recrystallisation. Annealing at 1000 ◦ C led to the elimination of the overlayer on sample F with 2 m film. A fine lamellar microstructure was formed similar to the ones observed on the pure Ti and Ti–4%Al–1%Mn samples (Fig. 13a and b). On the surface of sample F with 6 m film, the remains of the overlayer and formation of the fine plates were observed (Fig. 13e). It is necessary to note that, when the substrate with coarse grain structure from the Ti–4%Al–3%Mo–1%V alloy was used, the elimination of the overlayer with 6 m film did not happen. Fig. 13 shows all types of microstructure observed after annealing at 1000 ◦ C. The final structure formed after the elimination of the overlayer consisted of the very fine plates. Consequently, it can be assumed that the succession of the micrographs Fig. 8b in Ref. [6], and Fig. 13c–e and a showed the development of the microstructure during annealing at 1000 ◦ C. It demonstrates that the stage of the microstructural evolution on heating to 1000 ◦ C depended on the thickness of the Al film, and the structure and chemical composition of the initial substrate.
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Fig. 12. Microstructure of the overlayer on the Ti–4%Al–1%Mn substrates after annealing at 900 ◦ C: (a) sample D, 6 m film; (b) sample D, 2 m film; (c) sample C, 3.5 m film.
4. Discussion That Ti3 Al could be formed from the Ti side and Al3 Ti from the Al one was observed more significantly on the Ti–4%Al–1%Mn samples (first paragraph under Section 3.2) than on the Ti ones because Al and Mn could increase atom mobility [7,8], accelerating the diffusion transformation of the ␣-Ti phase to the Ti3 Al concentration. Formation of the Ti3 Al phase could not be observed on the Ti–4%Al–3%Mo–1%V sample because Mo and V could strongly decrease atom mobility [8]. To our knowledge, Ti diffusion in pure Al or Al alloyed with Ti has never been thoroughly studied. Beginning of the formation of the Al3 Ti interface layer could decrease Al diffusion into Ti, since, according to [8,9], Al diffuses in titanium aluminides slower, and with a higher activation energy than Ti. Furthermore, Al3 Ti has a lower free energy of formation than Ti3 Al [10,11]. Consequently, it can be assumed that mainly at the Ti/Al interface, the Al3 Ti phase was formed on heating to 600 ◦ C. This phase grew in the direction of the aluminum film as Ti diffused faster to the front of the reaction through Al3 Ti layer than Al. The whole aluminum film was gradually used on the formation of this overlayer. Then, Fig. 5d in Ref. [6] shows the remains of the Al film and overlayer growing under the film. Fig. 15b and c shows the evolution of the particles with increasing of annealing temperature. Formation and stability of the Al24 Ti8 phase depended on the thickness of the Al film and the chemical composition of
the substrates. The mechanism of the Al24 Ti8 formation is still not well known. In our case, it can be assumed that the Al24 Ti8 compound was formed at the determined concentration ratio of the elements and the stresses between substrate and the Al film at the Ti/Al interface had a great effect on its formation and stability. Table 3 summarises the principal temperature ranges of the aluminide phase formation as a function of the thickness of the Al film independently from structure and chemical composition of the substrates. It can be assumed that with increasing of annealing temperature, the changes in the concentration ratio of elements in the overlayers resulted in the sequential formation of aluminide phases according to the Ti–Al phase diagram. Since Ti diffused in the aluminide phases faster than Al and the consequently aluminide phase formation was directed towards the Ti-rich compounds, the primary site of the reactions was the interface of the Al-rich ones.
Table 3 The principal temperature ranges of the aluminide phase formation on Ti and its alloys as a function of thickness of the Al film ◦C
2 m film
3.5 m film
6 m film
600 700 800 900
Al3 Ti Al2 Ti, TiAl, Ti3 Al Ti3 Al Ti3 Al
Al3 Ti Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al Ti3 Al
Al3 Ti Al3 Ti, Al2 Ti Al2 Ti, TiAl, Ti3 Al TiAl, Ti3 Al
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Fig. 13. Microstructure of the samples with 6 m Al film after annealing at 1000 ◦ C: (a) sample A; (b) sample C; (c) sample D; (d) sample E; (e) sample F.
Depending on the structure and chemical composition of the substrate, deviations from the principal temperature ranges of the structural formation shown in Table 3 were observed. These deviations could indicate that the rate of reactions on the surface depended on the structure and the chemical composition of the substrates. By examining the intensity of the diffraction lines and data in Table 1 in Ref. [6] and Tables 1 and 2 in the present paper, it was concluded that on heating up to 900 ◦ C, the reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. It could depend on the density of grain boundaries. Furthermore, the diffusion parameters for Ti are anisotropic [7,8]. It means that the crystallographic orientation of the initial grains could have a great effect on the diffusion transformation as well. Depending on the chemical composition of the substrates, the following tendency was observed. Reactions occurred faster on the Ti–4%Al–1%Mn
samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. By examining the structural evolution of the overlayer on the Ti–4%Al–3%Mo–1%V samples, it was concluded that reaction on the Ti–4%Al–3%Mo–1%V ones occurred slower than on the Ti samples with irregular grains. As above-mentioned, it could be related with the different alloying elements having different effect on atom mobility. The structural degradation was observed dominantly on the Ti sample with 2 m film, and to lesser extents on the Ti–4%Al–3%Mo–1%V one. The degradation of the thin overlayer formed during annealing was likely related with the distribution of stresses between substrate and the Al film, which in turn had a great effect on the diffusion fluxes during subsequent annealing. The general tendency in the structural evolution was observed on heating up to 900 ◦ C. A thinner Al film resulted in faster coa-
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Fig. 14. X-ray diffraction patterns (Cu K␣ ) of sample F coated with Al film after annealing: (a) 2 m film; (b) 6 m film.
lescence of the particles (Figs. 5, 11 and 15). For the Al film of 2 m, coalescence of the particles was observed after annealing at 600 ◦ C, but for the Al film of 6 m, the particles kept their form even after 800 ◦ C annealing. Coalescence of the particles was likely to be related with recrystallisation of the overlayers. Nucleation and growth of new phases led to the changes in microstructure. Thicker overlayer required more time for its recrystallisation and microstructural and diffusion transformation. The faster reactions occurred on the surface, the faster morphology decomposed. Furthermore, the overlayers changed their morphologies according to changes in the microstructure of the substrate as observed on the quenched samples from Ti–4%Al–1%Mn (Figs. 9c and d, 11d and f). After annealing at 900 ◦ C, the continuous structure was typical for the two-phase TiAl + Ti3 Al overlayer formed from the 6 m films. The porous one was typical for the Ti3 Al + X-phase overlayer formed from the film with thickness less than 6 m (Fig. 7 in Ref. [6] and Figs. 12 and 15 here). The samples differed from one another in phase ratio in the overlayer. Annealing at 1000 ◦ C did not lead to elimination of the overlayer on the substrates with the initial lamellar microstructure. The distinction in the structure and phase composition on the different substrates after annealing above 900 ◦ C could be explained in the following way. Annealing at 900 ◦ C caused Ti to enter the  region, and the Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V alloys to enter the ␣ +  one. Consequently, during annealing at 900 ◦ C, growth of -grains took place. On heating to 900 ◦ C, diffusion coefficients changed and mobility of atoms increased dramatically [7,8]. Formation of porous morphology after annealing at 900 ◦ C, and presence of
Al after elimination of the overlayer in Ti could indicate that during heat treatment in  or ␣ +  regions the atoms started diffusing into the substrates. The initial substrates had the different structure and different thickness of the Al films represents the different strength of the Al source. Interdiffusion growth could result in the different concentration ratio of Al from the side Ti and its alloys depending on the structure of the initial substrate. Al increased temperature of ␣ →  transformation and in that way it stabilized ␣-Ti in near surface region. It could lead to the non-uniform growth of the -grains in near surface region and have a great effect on diffusion transformation. Accordingly, the distinctions in the kinetics of the reactions and in the stability of overlayers were observed. In the process of the microstructural evolution of overlayer, in the X-ray diffraction patterns, peaks from an unknown phase were detected. The diffraction lines of the X-phase were detected only along with the Ti3 Al ones (Table 1 in Ref. [6] and Tables 1 and 2 here). The ranges of the existence of the oxide and carbide phases are still not well known for the ternary Ti–Al–O or Ti–Al–C systems. Some of them are not stable at high temperatures. They could form upon cooling and decompose ˚ upon heating. For instance, the diffraction lines with d = 2.63 A ˚ and d = 2.27 A (Fig. 8) could be attributed to the Ti2 AlC or Ti3 AlC2 compounds. These lines disappeared when the typical ones of the X-phase were detected. However, the Ti2 AlC or Ti3 AlC2 phases are normally synthesized at special conditions at high temperatures [12,13]. Formation of the X-phase depended on the thickness of the Al film, and the structure and chemical composition of the initial substrate. It deserves further study.
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Fig. 15. Evolution of overlayer microstructure on sample F after annealing: (a) 2 m film, 600 ◦ C; (b) 6 m film, 600 ◦ C; (c) 6 m film, 700 ◦ C; (d) 2 m film, 800 ◦ C; (e) 6 m film, 900 ◦ C.
5. Conclusions During annealing, on the surface of the Ti, Ti–4%Al–1%Mn and Ti–4%Al–3%Mo–1%V samples coated with Al film, different aluminide phases were formed successively as the result of reactions between Ti and Al. The thickness of the Al film had a great effect on the structure of the overlayer formed during heat treatment. The temperature ranges of phase existence depended on the thickness of the initial Al film. Thicker overlayers were formed on the surface with thicker Al film, which required more time for its recrystallisation and microstructural transformation. After annealing treatment up to 900 ◦ C, the microstructure of the overlayer clearly showed contours of grain boundaries of the initial substrates. The shapes, sizes and distribution of the particles in the overlayer depended on the microstructure of the
initial substrates as well as on the thickness of the Al film. The particles formed colonies along the grain boundaries of the initial substrate in the way that they showed their microstructure. The overlayer changed its morphology according to changes in the microstructure of the substrate. The kinetics of reactions on the surface depended on the microstructure of the initial substrates. The reactions occurred faster on the substrates with fine-grained microstructure than on the ones with lamellar structure. Annealing at the temperatures above 900 ◦ C resulted in the elimination of the overlayer on the substrates in the favor of releasing Al into Ti. After the elimination of the overlayer, a fine plate microstructure was formed. Chemical composition of substrates had a great effect on the diffusion transformation of overlayers. Reactions occurred faster on the Ti–4%Al–1%Mn samples than on the Ti and Ti–4%Al–3%Mo–1%V ones. Reaction on the
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Ti–4%Al–3%Mo–1%V ones occurred slower than on the Ti samples with irregular grains. When reactions occurred fast on the surface, the morphology decomposed fast, too. References [1] D.M. Dimiduk, in: Y.-W. Kim, R. Wagner, M. Yamaguchi (Eds.), Gamma Titanium Aluminides, TMS, Warrendale, PA, 1995, p. 3. [2] X.Y. Li, Y.-C. Zhu, K. Fujita, N. Iwamoto, Y. Matsunaga, K. Nakagawa, S. Taniguchi, Surf. Coat. Technol. 136 (2001) 276–280. [3] M.S. Chu, S.K. Wu, Acta Mater. 51 (2003) 3109–3120. [4] M.S. Chu, S.K. Wu, Surf. Coat. Technol. 179 (2004) 257–264. [5] Z. Liu, G. Wang, Mater. Sci. Eng. A 397 (2005) 50–57.
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