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There is currently a recognized need to develop novel light-weight, high-strength structural materials of high-temperature capabilities, for use in aerospace and automotive engine and turbine components. Titanium aluminide intermetallics based on the gamma-phase (γ-TiAl) are attractive materials for such applications as they offer the potential for significant component weight savings of up to 50% over conventional Ni-base superalloys while exhibiting good strength retention at intermediate temperatures (600–750 °C).[1-3] Most of the advanced structural applications of γ-TiAl alloys, however, require adequate environmental durability in the 750–1050 °C range. Oxidation rates above 750 °C are extremely high resulting in the formation of an intermixed non-protective TiO2/Al2O3 scale and embrittlement of the alloy by dissolved oxygen, which in turn causes rapid deterioration in component tolerances and premature failure.[4] A prerequisite for good oxidation protection is the presence of a continuous Al2O3 scale through which oxygen permeability is low.[5] Therefore, the applicability of γ-TiAl alloys at temperatures above 750 °C will ultimately depend on their modified oxidation behavior, i.e., on the ability to form a protective alumina scale upon high-temperature oxidation in gases that are an integral part of their operating environments.

It has been recently established that marked improvement in the high-temperature oxidation resistance of γ-TiAl alloys can be achieved by introducing small amounts of halogens, notably fluorine, into the alloy's subsurface region. The introduction of F has been accomplished by either chemical (gas phase, HF-dipping, polymer spraying) or physical [beam-line ion implantation, plasma immersion ion implantation (PIII)] techniques.[6-8] In this case, one takes advantage of the mechanism of the so-called halogen effect, which operates at temperatures in excess of 700 °C and involves selective formation of gaseous aluminum halides followed by oxidation to Al2O3 during their outward diffusion through the incipient oxide scale.[9]

Substantial improvements in oxidation resistance at temperatures in the 900–1050 °C range have been achieved in particular by PIII of fluorine into γ-TiAl alloys.[10-12] Ample information on the PIII technique and extensive coverage of its fundamentals and uses can be found in ref.[13] A major advantage of using PIII of F over the alternative fluorination techniques is the ability to control and specify, via appropriate choice of the process parameters, both the amount of fluorine and the thickness of the F-implanted layer, so that a thin subsurface region is only modified without influencing the properties of the bulk material. An added advantage of the PIII process is the ability to perform fast high-dose implants into components of complex geometry. TiAl alloys surface-engineered in this way are able to form an adherent and highly protective alumina scale, even under conditions of thermal cyclic oxidation in air at temperatures as high as 1050 °C for times as long as 1 year.[11, 14]

In the course of optimization of the PIII process, we have measured by elastic recoil detection analysis (ERDA) anomalously broad, high-concentration (up to 60 at.%) fluorine profiles of either Gaussian or plateau-like shape. Under certain conditions, the implanted F has been found to penetrate several hundred nanometers into the TiAl alloy; far further than one would expect if the mechanism was due to ion bombardment and diffusion only.

In this work, we present analysis data detailing the microstructure of fluorine-implanted γ-TiAl, and elucidate the behavior of the fluorine in the alloy's near-surface. Correlations are done among the shape of the fluorine profiles, the fluorine implantation dose, the substrate temperature, the oxidation resistance of the F-implanted γ-TiAl alloys, and the quality of the resulting oxide scale that forms during subsequent high-temperature exposure to air.

1 Experimental

  1. Top of page
  2. 1 Experimental
  3. 2 Results and Discussion
  4. 3 Conclusions

Samples of the commercial alloys γ-MET [nominal composition in atomic per cent Ti–46.5Al–4(Cr, Nb, Ta, B)] and TNB (Ti–45Al–5Nb–0.2C–0.2B) with dimensions of 10 mm × 10 mm × 1 mm were cut from bar stock or sheet TiAl material, and were wet surface-ground using 1200-grit silicon-carbide paper. Specimens were then cleaned ultrasonically in ethanol for 10 min, rinsed with distilled water, and blown dry. Fluorine was introduced into both sides (front and back) of the γ-TiAl coupons by PIII. The F-containing plasma was generated by an inductively coupled rf discharge. A novel PIII-based process was developed, in line with the requirements for efficient, safe, and environmentally friendly treatment.[11, 12] The process uses a mixture of difluoromethane and argon (CH2F2 + 25% Ar) as the F-containing precursor gas because of the much greater ease with which it can be handled compared with the Ar + 5% F mixture reported in the literature.[10] It should be noted that the difluoromethane is a cheap, non-aggressive, and environmentally friendly (non-ozone depleting) gas which is routinely used in the semiconductor, solar panel, and flat panel display industries as well as in the refrigerant systems as a coolant (Freon 32). The base pressure in the PIII chamber was below 1 × 10−4 Pa, and the operating pressure was about 0.3 Pa. The parameters of the PIII process are listed in Table 1.

Table 1. PIII parameters used for the surface processing of γ-TiAl alloys
Pulse length [µs]RF power [W]Bias voltage [kV]Repetition rate, HzNumber of pulsesImplantation time [min]
10400−30200–7501 × 10622–83

The variable parameter was the repetition rate (frequency). This in turn caused variation in the implantation time and the equilibrium substrate temperature Ts. The pulse number was always 1 × 106 and, depending on the repetition rate, the implantation time ranged from 22 to 83 min. Ts was determined by means of an experimental procedure described in detail elsewhere.[15] This enabled temperature calibration and monitoring with an accuracy of ±5 K. Under the specific conditions used, Ts was reached in the first 4–5 min of implantation.

Information on the concentration/depth profiles of the implanted fluorine was provided by ERDA using an incident beam of 35 MeV Cl7+ ions at a scattering angle of 31°. Characterization of the as-implanted microstructure was undertaken by cross-sectional transmission electron microscopy (XTEM). Specifically, microscopy analyses were carried out in a Zeiss Libra 200 FE scanning/transmission electron microscope (STEM) equipped with an energy dispersive X-ray spectroscopy (EDX) system and an in-column Omega energy filter. Oxidation exposures of F-implanted γ-TiAl specimens were conducted isothermally in a closed-tube furnace using synthetic air at temperatures of 750–1050 °C for times ranging from 5 to 150 days. The oxidation behavior of the F-implanted samples was studied by thermogravimetric analysis (TGA) measurements employing a test rig equipped with a computer-controlled analytical balance. Oxidized samples were characterized by scanning electron microscopy (SEM) and EDX to infer the quality, composition, and thickness of the oxide scales.

2 Results and Discussion

  1. Top of page
  2. 1 Experimental
  3. 2 Results and Discussion
  4. 3 Conclusions

It should be noted from the outset that the two TiAl alloys studied, namely γ-MET and TNB, exhibited practically identical behavior both in the as-implanted state and after high-temperature exposure to air. Therefore, in the description that follows, no specific mention will be made to any of these two materials, and they will be collectively referred to as γ-TiAl alloys.

Preliminary studies by ERDA of the as-implanted γ-TiAl alloy samples gave intriguing results. These are shown in Figure 1 (depth profiles 1–7). The following features can be noticed. In all ERDA profiles the implanted fluorine exhibits broad distributions of a concentration of about 50–60 at.% centered at larger depths than those predicted by theory (see below). Carbon is co-implanted into the alloy subsurface and, except for the 200 Hz implant (curve 6), forms a shallow surface peak of low concentration, much in accordance with the theory's predictions (not shown). It should be noted that carbon is known to be either slightly beneficial or have little effect on the alumina scale formation t.[16, 17] Hydrogen also is co-implanted in the TiAl samples at low concentrations ranging from a few at.% to almost the detection limit of ERDA (Figure 1, profile 7). Implanting F at 750 Hz for 22 min resulted in a substrate temperature of 490 °C and produced a Gaussian-like F implant profile centered at about 90 nm, with an F dose of 6.0 × 1017 cm−2 (Figure 1, profile 1). As the frequency was reduced from 750 to 500 Hz, the temperature decreased to 430 °C, but overall the implant profile retained its Gaussian-like shape and peak position, with a slight increase in the F dose to about 6.20 × 1017 cm−2 (Figure 1, profile 2). Reducing further the frequency to 400 and then to 300 Hz (respective Ts of 400 and 350 °C) caused a dramatic change in the F implant distribution, which transformed now to a broad, plateau-like profile, with extremely high F doses of 1.55 × 1018 and 1.92 × 1018 cm−2, respectively (Figure 1, profiles 3 and 4). And finally, implanting F at the lowest frequency of 200 Hz (Ts = 280 °C) produced again a Gaussian-like F profile of a somewhat lower F concentration of about 50 at.%, with a pronounced carbon surface peak reaching nearly 90 at.% and arising presumably from both implantation and deposition of carbon.[18]

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Figure 1. ERDA concentration/depth profiles of the implanted elements present in the TiAl samples. (1) 750 Hz, 22 min, 490 °C; (2) 500 Hz, 33 min, 430 °C; (3) 400 Hz, 42 min, 400 °C; (4) 300 Hz, 56 min, 350 °C; (5) 200 Hz, 83 min, 280 °C.

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Theoretical calculations based on the SRIM and T-DYN[19, 20] codes for F ions implanted into TiAl at 30 keV predicted a Gaussian-type F distribution peaking at about 48 nm. The actual F implant profiles, however, appeared to be positioned appreciably deeper. More specifically, the 750 and the 500 Hz Gaussian-like F peaks were centered at ≈90 nm (Figure 1, profiles 1 and 2) while the flat-topped profiles extended down to ≈300 and 400 nm, respectively (Figure 1, profiles 3 and 4).

It should be noted that in ERDA the depth scale values are associated with a certain inaccuracy arising from the inexact knowledge of the actual implanted layer's density necessary to convert the depth in units of at/cm2 to a depth in nm. In our particular case (depth resolution of ERDA ≈20 nm), the density has been taken to be that of bulk TiAl, although the implanted region may have a different (presumably lower) density. The concentration inaccuracy is about 1 at.% for concentrations larger than 10 at.%, and up to 10% (relative) for concentrations below 10 at.%.

The uncertainty involved in the ERDA measurements, however, cannot alone account for the significant difference between theoretical and experimental data. It should also be noted that the simulation (SRIM and T-DYN) results pertain to the case of standard beam-line ion implantation of singly charged atomic F+ species injected into an amorphous TiAl target while in the case of PIII treatment one deals with a complex implantation process occurring in an (initially) polycrystalline TiAl material. Explanation of the reasons for the observed difference between theoretical (simulation) and experimental (ERDA) data is given further in this section.

The F-implanted samples were studied by TGA to infer their oxidation kinetics. It is commonly agreed that a weight gain lower than 1 g cm−2 is indicative of adequate oxidation resistance. Figure 2 shows the results of weight gain measurements for selected F-implanted samples during isothermal oxidation at 900 °C in air. The rapid weight gain exhibited by an as-received (unimplanted) TiAl sample is shown as a reference for comparison. TGA curve labeled (a) results from the oxidation of the 500 Hz (cf. Figure 1, ERDA profile 2). After a short initial time of scale growth the TGA curve levels off and runs parallel to the time scale, with the mass gain remaining always below 1 g cm−1, which is indicative of a high degree of oxidation protection. Practically no difference in high-temperature oxidation behavior was observed between the 750 and the 500 Hz samples (see ERDA profiles 1 and 2 in Figure 1, respectively). It is evident that PIII of F under these particular conditions provided excellent protection against oxidation. Nevertheless, implanting F at 750 Hz is clearly preferable as it requires a shorter processing time (22 min vs. 33 min). Oxidation curves (b) and (c) pertain to two additional samples implanted at 750 Hz (ERDA profiles not shown in Figure 1), but for shorter times of 15 and 12 min (respective F doses of 4.1 and 3.2 × 1017 cm−2. As can be seen, weight gains are still above 1 g cm−2, with the lower-dose sample showing a higher weight gain. Curve (d) corresponds to the 200 Hz sample, i.e., the one that exhibits a pronounced carbon surface peak (cf. profile 5 in Figure 1). This sample, while containing a relatively high F dose (4.8 × 1017 cm−2), showed insufficient oxidation resistance, presumably because of the extremely high surface concentration of C influencing unfavorably the mechanism of protective scale formation. And finally, curve (e) results from the oxidation of the highest F-dose sample (1.92 × 1018 cm−2; see ERDA profile 4 in Figure 1) showing practically insignificant oxidation resistance. The 400 Hz sample (F dose of 1.55 × 1018 cm−2) exhibited almost identical oxidation behavior as the 300 Hz one (not shown). Thus, the TGA runs done on specimens containing either an excess or deficiency of ion implanted fluorine confirmed large weight gains indicative of poor oxidation protection.

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Figure 2. Weight gain versus time data for unimplanted TiAl and fluorine-implanted TiAl specimens during isothermal oxidation at 900 °C in air. Oxidation curve (a): 500 Hz sample, 6.2 × 1017 cm−2 (cf. profile 2 in Figure 1); (b): 750 Hz, 4.1 × 1017 cm−2; (c): 750 Hz, 3.2 × 1017 cm−2; (d): 200 Hz, 4.8 × 1017 cm−2 + a pronounced C peak (cf. profile 5 in Figure 1); (e): 400 Hz, 1.92 × 1018 cm−2 (profile 4 in Figure 1).

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Following oxidation exposure, selected F-implanted specimens were further examined by SEM. Figure 3a shows a SEM image of an oxidized 750 Hz TiAl sample. After oxidation at 900 °C for 150 days the oxide scale consisted of a continuous protective alumina film overlying the TiAl substrate. Isolated F-containing microvoids were occasionally found at the Al2O3 scale/substrate interface. These are interpreted as fluorine reservoirs in which the fluorine is chemically bonded to aluminum in the form of solid AlFx. This compound is then oxidized to Al2O3 during elevated-temperature exposure to air.

image

Figure 3. SEM images of F-implanted TiAl samples after oxidation in air at 900 °C. (a) Implantation conditions: 750 Hz, 22 min, 490 °C, 6.0 × 1017 cm−2; oxidation for 150 days. An alumina protective layer has formed on the TiAl surface. (b) Implantation conditions: 300 Hz, 56 min, 350 °C, 1.92 × 1018 cm−2; oxidation for 25 days. The extremely high F dose results in structural degradation of the specimen.

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Figure 3b shows a SEM image of an oxidized 300 Hz TiAl sample. As can be seen, the sample was partly destroyed after oxidation in air at 900 °C for 25 days. Specifically, the upper end of the sample was totally oxidized exhibiting a thick mixed-oxide scale. The same type of scale is also present on the two sample faces while the metallic portion of the cross-section is largely reduced. This finding shows that implanting too much fluorine is detrimental to the oxidation resistance in a way similar to the halogen-induced corrosion, e.g., in steels.[21]

Another important inference from the above-described results is that a high degree of oxidation protection appears to be always associated with Gaussian-like F implant profiles whereas plateau-like profiles result in poor oxidation resistance. The Gaussian-like shape of the implanted F distribution alone, however, is not a sufficient condition for achieving enhanced oxidation resistance. Our experiments revealed that implanting at an optimum frequency of 750 Hz for times shorter than 22 min, equivalent to introducing F doses lower than ≈6.0 × 1017 cm−2, resulted in Gaussian-like F profiles, but nevertheless did not provide an adequately good oxidation-resistant surface (see Figure 2). Similarly, implanting F to doses higher than ≈9.0 × 1017 cm−2 produced a plateau-like F distribution and led to deterioration of the high-temperature oxidation behavior. Therefore, efficient oxidation protection can only be achieved under PIII conditions that produce Gaussian-like F profiles, within a narrow window of optimal F doses of about 5.0–9.0 × 1017 cm−2.

We further centered our attention on studying 750 Hz as-implanted TiAl samples because of their good oxidation resistance, and 300 Hz samples because of the anomalous behavior of the implanted fluorine, which exhibited a flat-topped profile and had penetrated far deeper into the TiAl material than one would expect if the driving mechanism was due to standard ion–solid interactions and diffusion.

An XTEM image of an as-implanted 300 Hz TiAl sample is shown in Figure 4a. Two well-defined regions can be seen in the micrograph, namely the F-implanted layer on top of the TiAl substrate. The layer thickness is about 350 nm, which correlates with the ERDA measurements (see Figure 1, curve 4). Furthermore, one can clearly distinguish between the left (amorphous) and the right (polycrystalline) part of the F-implanted layer. This micrograph is actually an interesting momentary image capture of a short-lasting transitory process. The analyzing electron beam had initially struck the right-hand side of the fluorinated layer, and had interacted with it for a certain time (the layer itself must have been in a highly metastable amorphous state). The energy deposited in the layer by the electron beam during the time of interaction had presumably brought about an amorphous-to-crystalline transition, thereby establishing a steady-state (nanocrystalline) morphology. Then the sample was moved quickly to the right, so that the left-hand side was exposed to the electron beam for a short time during which the deposited energy was not sufficient to induce such a transition. Thus, the left side is in fact an image “snapshot” showing the microstructure of the “pristine” as-implanted state shortly before the start of beam-induced crystallization.

Figure 4b is a high-resolution TEM image of the same sample after the occurrence of beam-induced crystallization. One can see numerous nanocrystallites of various orientations whose lattice plains generate Moire fringe patterns.

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Figure 4. (a) Bright-field XTEM micrograph of as-implanted TiAl (300 Hz, 56 min, 350 °C, 1.92 × 1018 cm−2). The fluorinated layer on the left side of the image is still amorphous (the actual as-implanted layer morphology) whereas on the right side a beam-induced amorphous-to-crystalline transition has occurred during analysis (steady-state morphology). (b) High-resolution TEM image of the nanocrystalline region shown in Figure 4a.

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Figure 5 shows a Fourier transform (FT) of the two distinct regions in the F-implanted layer shown in Figure 4a. The FT in Figure 5a exhibits a typical amorphous pattern while the FT in Figure 5b reveals numerous single diffraction spots resulting from the nanocrystallites present in the crystallized region.

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Figure 5. Fourier transform of the amorphous (a) and the nanocrystalline (b) regions of the fluorinated layer shown in Figure 4a.

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Figure 6a shows an EDX line scan across the F-implanted layer of the 300 Hz specimen. The line scan gives a quantitative approximation to the F, Ti, and Al depth distributions. The results of the EDX measurements are superimposed on the STEM image of the same sample (Figure 6b). As can be seen in Figure 6, the TiAl substrate contains no fluorine. Within the region encompassed by the F implant, both the Ti and the Al concentrations decrease to about 20 at.% while the F concentration increases abruptly to ≈60 at.% and remains constant across the layer, consistent with the ERDA results (cf. Figure 1, profile 4). The contrast in the STEM image (Figure 6b) follows the abrupt change in density, i.e., from higher within the TiAl matrix (appearing bright in the micrograph) to lower within the F-implanted layer (appearing dark). It should be pointed out that the morphology of the immediate near-surface region was strongly affected by the analyzing electron beam, and for this reason calculations of the element concentrations therein were somewhat inexact. Because of the small thickness of this region, even the low beam intensity used during the STEM-image acquisition was sufficient to produce detectable morphology changes.

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Figure 6. (a) EDX line scan across the as-implanted layer (300 Hz, 56 min, 350 °C, 1.92 × 1018 cm−2) showing the distribution of the fluorine, titanium, and aluminum. (b) EDX-STEM overlay of the same specimen. The darker contrast of the F-implanted layer is indicative of its lower density compared with that of the TiAl substrate.

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A 750 Hz samples was similarly analyzed by XTEM as shown in Figure 7a. The thickness of the F-implanted layer was found to be smaller, about 90 nm, compared with that of the 300 Hz sample. Since part of the fluorinated layer had been removed during sample preparation, the actual thickness must have been larger as confirmed by STEM analysis (see below). For this particular sample a different EDX analysis approach was applied. Instead of an EDX line scan as in Figure 6a, the F-implanted layer was divided into adjacent “box-like” regions through which a line scan was done. Specifically, four rectangular areas were chosen in the STEM image (Figure 7b) over which integrated signals were measured. The actual layer thickness was found to be about 150 nm, in good agreement with the results of the ERDA measurements (cf. Figure 1a). The respective EDX spectra are shown in Figure 8 (area 1–4). In all four areas signals of Ti, Al, and F were detected.

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Figure 7. (a) Bright-field XTEM micrograph of as-implanted TiAl (750 Hz, 22 min, 490 °C, 6.0 × 1018 cm−2). (b) STEM image including four line scans over the areas shown by the rectangles 1–4.

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Figure 8. EDX spectra for each of the rectangular areas 1 to 4 indicated in Figure 7b.

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Let us now make an attempt to explain the behavior of the implanted fluorine taking into account the above-described findings. With the aid of the fluorine diffusion model at elevated temperatures reported in ref.,[22] one can estimate that even at a much higher temperature of 900 °C the fluorine concentration will decrease to one-half of its initial value at a diffusion length inline image for a time of 56 min assuming a diffusion coefficient of D = 2.50 × 1015 cm−2 s−1. Calculations using the SRIM code[19] for a TiAl target and F ions at 30 keV show that the peak in the implant profile will be centered at about 48 nm. However, the experimentally measured penetration depths of the implanted fluorine exceed the theoretical values by a factor of about 2–7, and the fluorine concentration profiles do not exhibit the shape typical of a bulk diffusion distribution. Moreover, the flat-topped profiles cannot be accounted for by standard diffusion and implantation processes, and point toward the operation of a specific mechanism responsible for the fluorine behavior in the TiAl material during implantation.

Basing ourselves on the TEM data, we propose the following interpretation of the processes that occur when a γ-TiAl alloy is implanted with high doses of fluorine. Let us first consider the 400 and the 300 Hz samples (Ts = 400 and 350 °C, respectively). We believe that during implantation under these particular conditions, a metastable amorphous layer forms in the near-surface region of the TiAl substrate (cf. Figure 4a, left). The amorphization of the initially polycrystalline near-surface zone is a product of the accumulation of implantation-induced damage resulting from the extremely high-dose rates and the relatively low Ts. Between the amorphized layer and the polycrystalline substrate an amorphous/crystalline (a/c) front forms, which advances gradually into the TiAl bulk driven by the impact of the energetic ions and the favorable defect environment. The experimental results show that the a/c front progresses further into the bulk at 400 and 350 °C (Figure 1, profile 4), but stops at 280 °C (Figure 1, profile 5). At this temperature both implantation and deposition occur leading to a pile-up of carbon at the surface, and formation of a pronounced carbon peak, thereby hindering the penetration of fluorine. Significantly, the finding that the a/c front progresses no further at 280 °C is indicative of the existence of a threshold temperature (presumably ≈300 °C) for diffusion at the a/c front. The amorphized layer is highly disordered and of lower density (cf. Figure 6a and b). The fluorine diffusion within this layer is enhanced and is faster compared with the diffusion in the polycrystalline bulk material. Accordingly, the fluorine diffusion rate will be faster than the a/c front velocity, which in turn will lead to an increased concentration of fluorine atoms at the a/c front. Diffusion of fluorine takes place at the a/c front once the required concentrations have built up, and the activation temperature for this process has been reached. Increasing the Ts to 430 °C and then to 490 °C (frequencies of 500 and 750 Hz, respectively), however, hinders the process of complete amorphization by dynamic annealing and partial recovery of the polycrystalline substrate material. This in turn leads to a decrease in the overall penetration depth of the fluorine, with an attendant transformation from a flat-topped to a Gausian-like F distribution. In other words, the concurrent process of dynamic recovery counteracts the amorphization process during implantation, and reduces the thickness of the amorphized layer in which a plateau-like concentration profile could form. In effect, much less fluorine is incorporated into the implanted layer, which in turn is manifested by a reduction in the retained fluorine dose from about 2.0 × 1018 to ≈6.0 × 1017 cm−2 and a shrinkage in the implant profile.

The existence of an optimal F dose range of ≈5.0 × 1017 to 9.0 × 1017 cm−2 (equivalent to the existence of a fluorine-effect window) may be explained on the basis of thermodynamic considerations and calculations.[23] The flux of Al atoms necessary to form an alumina-containing scale can be calculated via the parabolic rate constants at the defined temperature. The amount of Al can also be computed through the partial pressure of the respective gaseous fluorides. These fluorides incorporate a single Al atom at temperatures above 750 °C (AlF(g), AlF2(g), AlF3(g)). The partial pressures of the fluorides can in turn be calculated using the well-known thermo-chemical software package called FactSage (formerly ChemSage). The minimum partial pressure of AlFx(g) necessary to form alumina via the kp-value is fixed, and this sets a lower limit to the F concentration. Conversely, the maximum fluorine pressure is established when the titanium fluorides' pressure reaches such a high value that it exceed this minimum, thereby favoring the formation of TiO2 concurrently with Al2O3, which is undesirable. This in turn sets an upper limit to the F concentration. As a result, an optimum window of F concentrations is established over which the mechanism of the F effect is operational. Outside this window the F concentrations (or the F doses in terms of PIII of F) are such that the preferential formation of alumina is not favored.

3 Conclusions

  1. Top of page
  2. 1 Experimental
  3. 2 Results and Discussion
  4. 3 Conclusions

The present work has dealt with the effects of high-dose PIII of fluorine into γ-TiAl alloys. Under optimum implant conditions, this surface treatment technique confers marked improvement in environmental durability of the alloys for long periods of time at high temperatures. TGA runs done on specimens containing either an excess or deficiency of ion-implanted fluorine confirm large weight gains indicative of poor oxidation resistance. A model, which assumes the operation of a combined temperature-dependent mechanism of amorphization, crystallization, and enhanced diffusion, has been put forward to account for the behavior of the fluorine in the as-implanted state. The final F distribution is implantation-temperature-dependent, with higher temperatures causing partial dynamic recovery of the amorphized TiAl material and attendant profile shrinkage. Long-term post-implantation oxidation tests have indicated that enhanced oxidation resistance is always associated with Gaussian-type as-implanted fluorine profiles coupled with optimal fluorine doses over a narrow window of about 5.0–9.0 × 1017 cm−2. Implanting fluorine to doses outside this optimum process window provides little or no oxidation protection to TiAl at temperatures above 750 °C. The results obtained have been helpful in understanding the behavior of the implanted fluorine from both a scientific and a technological standpoint, thereby providing a basis for future development of oxidation-resistant γ-TiAl alloys.