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Microstructure of alumina-forming oxidation resistant Al-Ti-Cr alloys
ScriptaMetallurgicaet Materialia,Vol. 32, No. 10,pp. 1659-1664,199S 1995 ElsevierScienceLtd Printedin the USA. Au rightsmserved 0956-716X/95$9.50+ .OO
0956-716X(95)00251-0
MICROSTRUCTURE
OF ALUMINA-FORMING
OXIDATION
RESISTANT
Al-Ti-Cr ALLOYS
‘M.P. Brady, J.L. SmiaIek, F. Terepka MS 106-1, NASA Lewis Research Center, Cleveland, OH 44135, USA *National Research Council Resident Research Associate (Received September 21, 1994) (Revised December 20,1994) Introduction Gamma-based titanium aluminides are currently being developed for intermediate temperature aircraft engine applications. This interest is due to their low density and to the identification of compositions and microstructures However, despite recent which possess both reasonable mechanical properties and some oxidation resistance. advances, oxidation resistant coatings for y-titanium aluminides are still needed. Alloys in the Al-Ti-Cr system containing approximately 45-55 atom percent (a/o) Al and lo-30 a/o Cr were recently identified by Meier et al. as potential oxidation resistant coatings for y-based titanium aluminides (Fig. 1) [l]. While such alloys are oxidation resistant, forming alumina scales at 1073K in air [2], they are extremely brittle [ 1,3] and may cause premature failure in thermomechanical fatigue. Little phase equilibria information is available in the protective alumina-forming composition range of the Al-Ti-Cr system. Most work on Al-Ti-Cr to date has concentrated on the Ti-rich corner of the phase diagram [4] and the Ll, cubic Al,Ti (T) single phase field [5]. The phase equilibria and mechanical properties of the composition range surrounding the r single phase field, portions of which coincide with the protective alumina forming composition range identified by Perkins et al., were recently investigated by Klansky et al. at 1473K [6] (Fig. 2) and Nakayama et al. at 1423K and (partially) 1273K (Fig. 3) [7]. Phase information in the 1073K-1273K temperature range, where engineering applications are anticipated, is also needed. Therefore, as part of an overall effort to co-optimize the oxidation resistance and mechanical properties of Al-Ti-Cr alloys [8], an investigation of phase equilibria in the protective alumina-forming composition range at 1073K and 1273K was performed. Exnerimental Two alloys were selected for study: 45Al-40Ti-15Cr a/o (alloy A) and 55AI-30Ti-15Cr a/o (alloy B). Alloy A falls on the borderline for protective alumina formation and alloy B falls well within the protective alumina-forming composition range (Fig. 1). The alloys were double arc-melted, cast into 13 X 13 X 50 mm rectangular cross-section buttons, heat treated at 1473K for 100 h in a 95%Ar-5%H, atmosphere, and furnace cooled. Bulk alloy composition and impurity contents after the 1473K heat treatment are reported in Table I. The AI, Ti, Cr contents were determined by inductively coupled plasma, the H content by combustion/extraction, and O/N contents by inert gas fusion. Test coupons 5-7 mm thick were further heat treated in air at 1073K or 1273K for 100 h and then air cooled to room temperature, which required approximately 5 minutes. The samples were sectioned parallel to the coupon face to allow analysis of environmentally unaffected material. The microstructures were characterized by x-ray diffraction, scanning electron microscopy (SEM), wavelength dispersive electron microprobe, and room temperature Vicker’s microhardness. Pure element standards were used for the electron microprobe analysis, and at least 3 locations were measured and averaged to determine each data point. A 100 g load with a 15 second hold time was used for the microhardness testing (average of at least 10 measurements k one standard deviation is reported).
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Vol. 32, No. 10
TABLE I- Bulk Alloy Composition.
l
Alloy
Al (a/o)
Ti (a/o)
Cr (a/o)
‘H (wppm)
‘0 (wppm)
w (wppm)
A
45
40
15
18
1369
105
B
55
29
16
5
885
34
weight parts per million Results
1273K Exnosure The microstructure of alloy A after exposure at 1273K for 100 h (Fig. 4, Table II) consisted of the Ll, y TiAl phase (dark) and the TiCrAl laves phase (light gray) (refer to [6]). The y phase had a Vicker’s hardness (VHN) of 345, while the mixed y/TiCrAl laves regions had a WIN of 473 (Table III). Three phases were observed in alloy B (Fig. 5, Table II): the Ll, t phase (dark), the TiCrAl laves phase (light gray), and a Cr-Al rich phase (white). The volume fraction of the Cr-Al rich phase in alloy B was too low to produce sufficient x-ray diffraction peak intensity for phase identification. Based on electron microprobe analysis (Table II) and the 1473K Al-Ti-Cr isotherm of [6] (Fig. 2), the Cr-Al rich phase is tentatively identified as Ti-modified Cr,Al. The VI-IN of the r phase, containing a small volume fraction of coarse TiCrAl laves phase precipitates (Fig. 5), was 342 (Table III). The WIN of the mixed TiCrAl laves/Cr,Al phase regions was over 700, and microcracks were observed around the indentations (Table III). The low cracking resistance of TiCrAl laves/Cr,Al mixtures has been reported previously [6]. 1073K Exuosure The microstructure, including phase composition and WIN, of alloy A after the 1073K exposure was the same as after the 1273K exposure (Tables II and III). However, the microstructure of alloy B was markedly different after the 1073K exposure (Fig. 6) than after the 1273K exposure (Fig. 5). The regions which were single phase r in alloy B after the 1273K exposure now contained a tine mixture of a large volume fraction of an Al-Ti rich phase (dark) and a small volume fraction of a Cr-Al rich phase (light in dark) (Fig. 6). The x-ray diffraction data indicated the presence of major quantities of the r-Al,Ti phase (refer to [9,10]). &o r nhase was detected. Based on the x-ray diffraction data, the Al-Ti rich phase present in the former r regions is identified as r-Al,Ti. The Cr-Al rich phase in this region was too tine for quantitative composition analysis by microprobe and again the low volume fraction precluded identification by x-ray diffraction. The mixed TiCrAl laves/Cr,Al regions (light gray/white), similar to those observed in alloy B after the 1273K exposure (Fig. 5) were also observed after the 1073K exposure (Fig. 6). However, dark Al-Ti rich precipitates were These precipitates were too tine for quantitative now present at the TiCrAl laves/Cr,Al interphase boundaries. composition analysis by microprobe, but are speculated to be r-Al,Ti. To further study the decomposition of the T phase in alloy B which occurred between 1273K and. 1073K, samples based on the 1273K r phase composition (60Al-29Ti-1lCr a/o, Table II) were double arc-melted, cast, and heat treated at 1273K or 1073K for 100 hours. After the 1273K exposure, the microstructure was predominantly single phase r. Several small peaks of less than 1% relative intensity were also present in the x-ray diffraction data that could not be identified, and a small volume fraction of a second phase was observed using SEM techniques. After the 1073K exposure, the structure consisted of a mixture of a large volume fraction of an Al-Ti rich phase and a small volume fraction of a Cr-Al rich phase. X-ray diffraction indicated the presence of large amounts of the r-Al,Ti phase (the Al-Ti rich phase) and a small amount of a second phase with diffraction peaks consistent with those of Cr,Al (the Cr-Al rich phase). The Cr-Al rich phase observed in the former ‘Tregions of 1073K exposed alloy B is therefore identitled as Cr,Al. The t nhase was not detected after the 1073K exnosure.
Vol. 32, No. 10
OXIDATION RESISTANT ALLOYS
TABLE II- Phase Composition
Data (a/o).
1073K Exposure
1273K Exposure
Y
53Al-42Ti-5Cr
51Al-45Ti-4Cr
TiCrAl laves (alloy A)
36Al-34Ti-30Cr
38Al-33Ti-29Cr
r-Al,Ti (alloy B)
too fine for probe
phase not detected
r-Ll,
phase not detected
‘60Al-29Ti-11 Cr
42Al-30Ti-28Cr
42Al-3 lTi-27Cr
II WOY 4 (alloy B)
TiCrAl laves (alloy B)
II
“35Al-14Ti-51Cr
’ Composition
may not represent single phase r-see Discussion for further details. ‘* The sampling volume may have overlapped slightly with the surrounding phases.
TABLE III- Microhardness Phase
1073K Exposure
1273K Exposure
Y (alloy A)
349 f 22
345 * 34
mixed y/TiCrAl laves (alloy A)
508 + 46
473 f 83
mixed r-Al,Ti/Cr,Al
557 + 25
phase not detected
phase not detected
l342 + 28
l*708 + 62
“728 -I 62 (average of only 8 measurements)
r-Ll,
(alloy B)
(alloy B)
mixed TiCrAl laves/Cr,Al B) l
l
Data (VI-IN).
(alloy
The t phase regions contained a low volume fraction of coarse TiCrAl laves phase precipitates. * Cracks were formed around the microhardness indents. Discussion
The ternary tie triangle determined from the phases observed in alloy B after exposure at 1273K (Fig. 7) is in reasonably good agreement with the 1473K isotherm of Klansky et al. (Fig. 2) and the 1423K isotherm of Nakayama et al. [7] (not shown). However, the agreement with the estimated 1273K isotherm of Nakayama et al. is poor (Fig. 3). According to the estimated Nakayama et al. diagram, alloy B falls within a r-y-Cr,Al three phase field. However, alloy B was observed to fall near the r-TiCrAl edge of a r-TiCrAl-Cr,Al three phase field. The same 1273K r-TiCrAl-Cr,Al three phase tie triangle observed in alloy B was confirmed in an alloy of composition 50Al30Ti-20Cr a/o following the procedures described for alloys A and B. The extent of the single phase r field observed by Nakayama et al. after exposure at 1273K consisted of a 1 a/o radius centered at 67Al-25Ti-8Cr a/o. The r composition determined from 1273K alloy B (60Al-29Ti-11Cr a/o) is much richer in Ti and Cr and does not fall within the Nakayama et al. single phase r field. It has been suggested that the composition range of the ‘c single phase field is overestimated by electron microprobe analysis because of the presence of precipitates not resolvable at the SEM level [6,11]. Because electron microprobe analysis was used to determine the composition of the r phase in this study, it is likely that 60Al-29Ti-11Cr a/o is an overestimate of the actual Ti and Cr content in r. However, the r phase composition of alloy B must fall on the Ti and Cr rich periphery of the r single phase field because it is in equilibrium with phases richer in Ti and Cr (TiCrAl laves and Cr,Al).
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Vol. 32, No. 10
Precipitation of Al,Ti in Al-Ti-Fe r alloys is associated with Ti rich r compositions and with decreasing temperature below 1273K [12]. In Al-Ti-Cr r alloys, shrinkage of the t single phase field with decreasing temperature to a point centered around 67Al-25Ti-8Cr a/o [7] and precipitation of Al,Ti in 64SAl-27STi-8Cr a/o on exposure at 973K1173K (approximately) [13] have also been observed. Therefore, it is not surprising that the r phase in alloy B, which falls at the Ti and Cr rich periphery of the r single phase field at 1273K, decomposed on exposure at 1073K to r-Al,Ti and Cr,Al. Titanium and Cr rich t phase and, to a lesser extent, Al rich TiCrAl laves phase dominate much of the protective alumina-forming Al-Ti-Cr composition range identified by Perkins et al. (45.55 a/o Al and lo-30 a/o Cr) from 1273K (Fig 7.) to 1473K (Fig. 2). Both single phase T [14,15] and single phase TiCrAl laves [ 161 alloys exhibit protective alumina formation, which accounts for the excellent oxidation resistance of the Al-Ti-Cr alloys. However, the TiCrAl laves phase exhibits a low resistance to cracking [6] and the r phase exhibits only compressive ductility [5] and is brittle in tension at ambient temperatures [l 11. Several Cr rich r compositions have been identified which exhibit an ambient bend ductility of nearly 1% [7] and riTiCrA1 laves mixtures exhibit enhanced cracking resistance [6]. The results of this study indicate, however, that Ti and Cr rich r phase, which is the basis for much of the AlTi-Cr alumina forming composition range, decomposes to r-Al,Ti and Cr,Al on exposure at 1073K. Both A1,Ti and Cr,Al are brittle [6]. Any beneficial effects of the r phase on mechanical properties are therefore lost after exposure in the temperature range where engineering application of these alloys is expected. The borderline protective alumina-forming Al-Ti-Cr alloys fall within the ylTiCrA1 laves two phase field at 1273K and 1073K. This result is consistent with the 1473K isotherm of [6] (Fig. 2), the estimated 1273K isotherm of [7] (Fig. 3), and the microstructural data reported by Meier et al [17]. Mixed ylTiCrA1 laves microstructures have also been reported to exhibit enhanced cracking resistance [6], and appear to be stable from room temperature up to 1473K. Correlation of the oxidation map of [2] (Fig. 1) with the 1473K isotherm of [6] (Fig. 2) indicates that the ylTiCrA1 laves two phase field extends (slightly) into the alumina forming composition range. Therefore, ylTiCrA1 laves alloys offer the possibility for both oxidation resistance and adequate mechanical properties. Future coating alloy development in the Al-Ti-Cr system should therefore focus on the y/TiCrAl laves two phase field.
1) The protective alumina-forming and TiCrAl laves phases.
Al-Ti-Cr alloys identified by Perkins et al. [2] are based primarily on the r (Ll,)
2) The r phase in the multiphase protective alumina-forming to r-Al,Ti and Cr,Al. 3) Future coating alloy development
Al-Ti-Cr alloys is not stable at 1073K and decomposes
in the Al-Ti-Cr system should focus on the y/TiCrAl laves two phase field.
1. G.H. Meier, R.A. Perkins, J.C. Schaeffer, and R. L. McCarron, GE Aircraft Engines Interim Report No. 1, Naval Air Development Center Contract N62269-90-C-0287 (March 1991). 2. R.A. Perkins and G.H. Meier, in Proceedings of the Industry-University Advanced Materials Conference II, Smith, F. ed., Advanced Materials Institute, p. 92 (1989). 3. D.W. McKee and SC. Huang, Cor. Sci., 33, p. 1899 (1992). 4. F.H. Hayes, J. Phase Equil., 13, p. 79 (1992). 5. H. Mabuchi, K. Hirukawa, H. Tsuda, and Y. Nakayama, Scripta Metall. Mater., 24, p. 505 (1990). 6. J.L. Klansky, J.P. Nit, and D.E. Mikkola, J. Mater. Res., 9, p. 255 (1994). 7. Y. Nakayama and H. Mabuchi, Intermetallics, 1, p. 41 (1993). 8. M.P. Brady, J.L. Smialek, and D.L. Humphrey, to be published in High-Temperature Ordered Intermetallic Alloys-VI, J. Horton, I. Baker, S. Hanada, R. Noebe, and D. Schwartz, MRS, Pittsburgh, PA (1994). 9. H. Mabuchi, T. Asai, and Y. Nakayama, Scripta Metall. Mater., 23, p. 685 (1989). 10. J.C. Schuster and H. Ipser, Z. Metallk, 81, p. 389 (1990).
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OXIDATION RESISTANT ALLOYS
11. KS. Kumar, in Structural Intermetallics, R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, and M.V. Nathal, TMS, Warrendale, PA, p. 87 (1993). 12. Z.L. Wu and D.P. Pope, Acta Metall. Mater., 42, p. 509 (1994). 13. R.N. Wright, A.E. Erickson, M.H. Obrien, and B.H. Rabin, Scripta Metall. Mater., 28, p. 1293 (1993). 14. J.L. Smialek, Cor. Sci., 35, p. 1199 (1993). 15. L.J. Pa&t, J.L. Smialek, J.P. Nit, and D.E. Mikkola, Scripta Metall. Mater., 25, p. 727 (1991). 16. J.C. Schaeffer, G.H. Meier, and R.L. McCarron, GE Aircraft Engines Interim Report No. 4, Naval Air Development Center Contract N62269-90-C-0287 (October 1992). 17. G.H. Meier, N. Birks, F.S. Pettit, R.A. Perkins, and H.J. Grabke, in Structural Intermetallics, R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, and M.V. Nathal, TMS, Warrendale, PA, p. 869 (1993).
One author (MPB) wishes to acknowledge the financial support of a National Research Research Associateship. This research was funded under the NASA HITEMP program.
Council Post-Doctoral
1073K Alumina 50 Ti Boundary PI
m.
Cr
Al
FIG. 1. Schematic of the 1073K oxidation map of Perkins et al. [2] (a/o).
1073K Alumina 50 Ti
80 Cr
FIG. 2. Schematic of the partial 1473K isotherm et al. for Al-Ti-Cr alloys hot isostatically pressed 1473Wl72 MPa/2 h [6] (a/o). Alloys A and B of work and the 1073K alumina boundary of Perkins also plotted.
of Klansky (HIPed) at the present et al.[2] are
Boundary ,_ [21
are speculative phase boundaries [7]). Alloys A and B of the present work and the 1073K alumina boundary of Perkins et al. [2] are also plotted.
FIG. 4. SEM (backscatter mode) micrograph of alloy A (45Al-40Ti-15Cr) exposed at 1273K for 100 h. The microstructure of alloy A exposed at 1073K for 100 h was similar.
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OXIDATION RESISTANT ALLOYS
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tau IiCrAl Cr,AI
FIG. 5. SEM (backscatter mode) micrograph of alloy B (55Al-30Ti-15Cr a/o) exposed at 1273K for 100 h.
FIG. 6. SEM (backscatter mode) micrograph exposed at 1073K for 100 h.
of alloy B
1073K Alumina
Al
80 Cr
FIG. 7. Partial 1273K Al-Ti-Cr isotherm of the present work (a/o). The 1073K alumina boundary of Perkins et al. [2] is also plotted.
0956-716X(95)00251-0
MICROSTRUCTURE
OF ALUMINA-FORMING
OXIDATION
RESISTANT
Al-Ti-Cr ALLOYS
‘M.P. Brady, J.L. SmiaIek, F. Terepka MS 106-1, NASA Lewis Research Center, Cleveland, OH 44135, USA *National Research Council Resident Research Associate (Received September 21, 1994) (Revised December 20,1994) Introduction Gamma-based titanium aluminides are currently being developed for intermediate temperature aircraft engine applications. This interest is due to their low density and to the identification of compositions and microstructures However, despite recent which possess both reasonable mechanical properties and some oxidation resistance. advances, oxidation resistant coatings for y-titanium aluminides are still needed. Alloys in the Al-Ti-Cr system containing approximately 45-55 atom percent (a/o) Al and lo-30 a/o Cr were recently identified by Meier et al. as potential oxidation resistant coatings for y-based titanium aluminides (Fig. 1) [l]. While such alloys are oxidation resistant, forming alumina scales at 1073K in air [2], they are extremely brittle [ 1,3] and may cause premature failure in thermomechanical fatigue. Little phase equilibria information is available in the protective alumina-forming composition range of the Al-Ti-Cr system. Most work on Al-Ti-Cr to date has concentrated on the Ti-rich corner of the phase diagram [4] and the Ll, cubic Al,Ti (T) single phase field [5]. The phase equilibria and mechanical properties of the composition range surrounding the r single phase field, portions of which coincide with the protective alumina forming composition range identified by Perkins et al., were recently investigated by Klansky et al. at 1473K [6] (Fig. 2) and Nakayama et al. at 1423K and (partially) 1273K (Fig. 3) [7]. Phase information in the 1073K-1273K temperature range, where engineering applications are anticipated, is also needed. Therefore, as part of an overall effort to co-optimize the oxidation resistance and mechanical properties of Al-Ti-Cr alloys [8], an investigation of phase equilibria in the protective alumina-forming composition range at 1073K and 1273K was performed. Exnerimental Two alloys were selected for study: 45Al-40Ti-15Cr a/o (alloy A) and 55AI-30Ti-15Cr a/o (alloy B). Alloy A falls on the borderline for protective alumina formation and alloy B falls well within the protective alumina-forming composition range (Fig. 1). The alloys were double arc-melted, cast into 13 X 13 X 50 mm rectangular cross-section buttons, heat treated at 1473K for 100 h in a 95%Ar-5%H, atmosphere, and furnace cooled. Bulk alloy composition and impurity contents after the 1473K heat treatment are reported in Table I. The AI, Ti, Cr contents were determined by inductively coupled plasma, the H content by combustion/extraction, and O/N contents by inert gas fusion. Test coupons 5-7 mm thick were further heat treated in air at 1073K or 1273K for 100 h and then air cooled to room temperature, which required approximately 5 minutes. The samples were sectioned parallel to the coupon face to allow analysis of environmentally unaffected material. The microstructures were characterized by x-ray diffraction, scanning electron microscopy (SEM), wavelength dispersive electron microprobe, and room temperature Vicker’s microhardness. Pure element standards were used for the electron microprobe analysis, and at least 3 locations were measured and averaged to determine each data point. A 100 g load with a 15 second hold time was used for the microhardness testing (average of at least 10 measurements k one standard deviation is reported).
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Vol. 32, No. 10
TABLE I- Bulk Alloy Composition.
l
Alloy
Al (a/o)
Ti (a/o)
Cr (a/o)
‘H (wppm)
‘0 (wppm)
w (wppm)
A
45
40
15
18
1369
105
B
55
29
16
5
885
34
weight parts per million Results
1273K Exnosure The microstructure of alloy A after exposure at 1273K for 100 h (Fig. 4, Table II) consisted of the Ll, y TiAl phase (dark) and the TiCrAl laves phase (light gray) (refer to [6]). The y phase had a Vicker’s hardness (VHN) of 345, while the mixed y/TiCrAl laves regions had a WIN of 473 (Table III). Three phases were observed in alloy B (Fig. 5, Table II): the Ll, t phase (dark), the TiCrAl laves phase (light gray), and a Cr-Al rich phase (white). The volume fraction of the Cr-Al rich phase in alloy B was too low to produce sufficient x-ray diffraction peak intensity for phase identification. Based on electron microprobe analysis (Table II) and the 1473K Al-Ti-Cr isotherm of [6] (Fig. 2), the Cr-Al rich phase is tentatively identified as Ti-modified Cr,Al. The VI-IN of the r phase, containing a small volume fraction of coarse TiCrAl laves phase precipitates (Fig. 5), was 342 (Table III). The WIN of the mixed TiCrAl laves/Cr,Al phase regions was over 700, and microcracks were observed around the indentations (Table III). The low cracking resistance of TiCrAl laves/Cr,Al mixtures has been reported previously [6]. 1073K Exuosure The microstructure, including phase composition and WIN, of alloy A after the 1073K exposure was the same as after the 1273K exposure (Tables II and III). However, the microstructure of alloy B was markedly different after the 1073K exposure (Fig. 6) than after the 1273K exposure (Fig. 5). The regions which were single phase r in alloy B after the 1273K exposure now contained a tine mixture of a large volume fraction of an Al-Ti rich phase (dark) and a small volume fraction of a Cr-Al rich phase (light in dark) (Fig. 6). The x-ray diffraction data indicated the presence of major quantities of the r-Al,Ti phase (refer to [9,10]). &o r nhase was detected. Based on the x-ray diffraction data, the Al-Ti rich phase present in the former r regions is identified as r-Al,Ti. The Cr-Al rich phase in this region was too tine for quantitative composition analysis by microprobe and again the low volume fraction precluded identification by x-ray diffraction. The mixed TiCrAl laves/Cr,Al regions (light gray/white), similar to those observed in alloy B after the 1273K exposure (Fig. 5) were also observed after the 1073K exposure (Fig. 6). However, dark Al-Ti rich precipitates were These precipitates were too tine for quantitative now present at the TiCrAl laves/Cr,Al interphase boundaries. composition analysis by microprobe, but are speculated to be r-Al,Ti. To further study the decomposition of the T phase in alloy B which occurred between 1273K and. 1073K, samples based on the 1273K r phase composition (60Al-29Ti-1lCr a/o, Table II) were double arc-melted, cast, and heat treated at 1273K or 1073K for 100 hours. After the 1273K exposure, the microstructure was predominantly single phase r. Several small peaks of less than 1% relative intensity were also present in the x-ray diffraction data that could not be identified, and a small volume fraction of a second phase was observed using SEM techniques. After the 1073K exposure, the structure consisted of a mixture of a large volume fraction of an Al-Ti rich phase and a small volume fraction of a Cr-Al rich phase. X-ray diffraction indicated the presence of large amounts of the r-Al,Ti phase (the Al-Ti rich phase) and a small amount of a second phase with diffraction peaks consistent with those of Cr,Al (the Cr-Al rich phase). The Cr-Al rich phase observed in the former ‘Tregions of 1073K exposed alloy B is therefore identitled as Cr,Al. The t nhase was not detected after the 1073K exnosure.
Vol. 32, No. 10
OXIDATION RESISTANT ALLOYS
TABLE II- Phase Composition
Data (a/o).
1073K Exposure
1273K Exposure
Y
53Al-42Ti-5Cr
51Al-45Ti-4Cr
TiCrAl laves (alloy A)
36Al-34Ti-30Cr
38Al-33Ti-29Cr
r-Al,Ti (alloy B)
too fine for probe
phase not detected
r-Ll,
phase not detected
‘60Al-29Ti-11 Cr
42Al-30Ti-28Cr
42Al-3 lTi-27Cr
II WOY 4 (alloy B)
TiCrAl laves (alloy B)
II
“35Al-14Ti-51Cr
’ Composition
may not represent single phase r-see Discussion for further details. ‘* The sampling volume may have overlapped slightly with the surrounding phases.
TABLE III- Microhardness Phase
1073K Exposure
1273K Exposure
Y (alloy A)
349 f 22
345 * 34
mixed y/TiCrAl laves (alloy A)
508 + 46
473 f 83
mixed r-Al,Ti/Cr,Al
557 + 25
phase not detected
phase not detected
l342 + 28
l*708 + 62
“728 -I 62 (average of only 8 measurements)
r-Ll,
(alloy B)
(alloy B)
mixed TiCrAl laves/Cr,Al B) l
l
Data (VI-IN).
(alloy
The t phase regions contained a low volume fraction of coarse TiCrAl laves phase precipitates. * Cracks were formed around the microhardness indents. Discussion
The ternary tie triangle determined from the phases observed in alloy B after exposure at 1273K (Fig. 7) is in reasonably good agreement with the 1473K isotherm of Klansky et al. (Fig. 2) and the 1423K isotherm of Nakayama et al. [7] (not shown). However, the agreement with the estimated 1273K isotherm of Nakayama et al. is poor (Fig. 3). According to the estimated Nakayama et al. diagram, alloy B falls within a r-y-Cr,Al three phase field. However, alloy B was observed to fall near the r-TiCrAl edge of a r-TiCrAl-Cr,Al three phase field. The same 1273K r-TiCrAl-Cr,Al three phase tie triangle observed in alloy B was confirmed in an alloy of composition 50Al30Ti-20Cr a/o following the procedures described for alloys A and B. The extent of the single phase r field observed by Nakayama et al. after exposure at 1273K consisted of a 1 a/o radius centered at 67Al-25Ti-8Cr a/o. The r composition determined from 1273K alloy B (60Al-29Ti-11Cr a/o) is much richer in Ti and Cr and does not fall within the Nakayama et al. single phase r field. It has been suggested that the composition range of the ‘c single phase field is overestimated by electron microprobe analysis because of the presence of precipitates not resolvable at the SEM level [6,11]. Because electron microprobe analysis was used to determine the composition of the r phase in this study, it is likely that 60Al-29Ti-11Cr a/o is an overestimate of the actual Ti and Cr content in r. However, the r phase composition of alloy B must fall on the Ti and Cr rich periphery of the r single phase field because it is in equilibrium with phases richer in Ti and Cr (TiCrAl laves and Cr,Al).
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Precipitation of Al,Ti in Al-Ti-Fe r alloys is associated with Ti rich r compositions and with decreasing temperature below 1273K [12]. In Al-Ti-Cr r alloys, shrinkage of the t single phase field with decreasing temperature to a point centered around 67Al-25Ti-8Cr a/o [7] and precipitation of Al,Ti in 64SAl-27STi-8Cr a/o on exposure at 973K1173K (approximately) [13] have also been observed. Therefore, it is not surprising that the r phase in alloy B, which falls at the Ti and Cr rich periphery of the r single phase field at 1273K, decomposed on exposure at 1073K to r-Al,Ti and Cr,Al. Titanium and Cr rich t phase and, to a lesser extent, Al rich TiCrAl laves phase dominate much of the protective alumina-forming Al-Ti-Cr composition range identified by Perkins et al. (45.55 a/o Al and lo-30 a/o Cr) from 1273K (Fig 7.) to 1473K (Fig. 2). Both single phase T [14,15] and single phase TiCrAl laves [ 161 alloys exhibit protective alumina formation, which accounts for the excellent oxidation resistance of the Al-Ti-Cr alloys. However, the TiCrAl laves phase exhibits a low resistance to cracking [6] and the r phase exhibits only compressive ductility [5] and is brittle in tension at ambient temperatures [l 11. Several Cr rich r compositions have been identified which exhibit an ambient bend ductility of nearly 1% [7] and riTiCrA1 laves mixtures exhibit enhanced cracking resistance [6]. The results of this study indicate, however, that Ti and Cr rich r phase, which is the basis for much of the AlTi-Cr alumina forming composition range, decomposes to r-Al,Ti and Cr,Al on exposure at 1073K. Both A1,Ti and Cr,Al are brittle [6]. Any beneficial effects of the r phase on mechanical properties are therefore lost after exposure in the temperature range where engineering application of these alloys is expected. The borderline protective alumina-forming Al-Ti-Cr alloys fall within the ylTiCrA1 laves two phase field at 1273K and 1073K. This result is consistent with the 1473K isotherm of [6] (Fig. 2), the estimated 1273K isotherm of [7] (Fig. 3), and the microstructural data reported by Meier et al [17]. Mixed ylTiCrA1 laves microstructures have also been reported to exhibit enhanced cracking resistance [6], and appear to be stable from room temperature up to 1473K. Correlation of the oxidation map of [2] (Fig. 1) with the 1473K isotherm of [6] (Fig. 2) indicates that the ylTiCrA1 laves two phase field extends (slightly) into the alumina forming composition range. Therefore, ylTiCrA1 laves alloys offer the possibility for both oxidation resistance and adequate mechanical properties. Future coating alloy development in the Al-Ti-Cr system should therefore focus on the y/TiCrAl laves two phase field.
1) The protective alumina-forming and TiCrAl laves phases.
Al-Ti-Cr alloys identified by Perkins et al. [2] are based primarily on the r (Ll,)
2) The r phase in the multiphase protective alumina-forming to r-Al,Ti and Cr,Al. 3) Future coating alloy development
Al-Ti-Cr alloys is not stable at 1073K and decomposes
in the Al-Ti-Cr system should focus on the y/TiCrAl laves two phase field.
1. G.H. Meier, R.A. Perkins, J.C. Schaeffer, and R. L. McCarron, GE Aircraft Engines Interim Report No. 1, Naval Air Development Center Contract N62269-90-C-0287 (March 1991). 2. R.A. Perkins and G.H. Meier, in Proceedings of the Industry-University Advanced Materials Conference II, Smith, F. ed., Advanced Materials Institute, p. 92 (1989). 3. D.W. McKee and SC. Huang, Cor. Sci., 33, p. 1899 (1992). 4. F.H. Hayes, J. Phase Equil., 13, p. 79 (1992). 5. H. Mabuchi, K. Hirukawa, H. Tsuda, and Y. Nakayama, Scripta Metall. Mater., 24, p. 505 (1990). 6. J.L. Klansky, J.P. Nit, and D.E. Mikkola, J. Mater. Res., 9, p. 255 (1994). 7. Y. Nakayama and H. Mabuchi, Intermetallics, 1, p. 41 (1993). 8. M.P. Brady, J.L. Smialek, and D.L. Humphrey, to be published in High-Temperature Ordered Intermetallic Alloys-VI, J. Horton, I. Baker, S. Hanada, R. Noebe, and D. Schwartz, MRS, Pittsburgh, PA (1994). 9. H. Mabuchi, T. Asai, and Y. Nakayama, Scripta Metall. Mater., 23, p. 685 (1989). 10. J.C. Schuster and H. Ipser, Z. Metallk, 81, p. 389 (1990).
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11. KS. Kumar, in Structural Intermetallics, R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, and M.V. Nathal, TMS, Warrendale, PA, p. 87 (1993). 12. Z.L. Wu and D.P. Pope, Acta Metall. Mater., 42, p. 509 (1994). 13. R.N. Wright, A.E. Erickson, M.H. Obrien, and B.H. Rabin, Scripta Metall. Mater., 28, p. 1293 (1993). 14. J.L. Smialek, Cor. Sci., 35, p. 1199 (1993). 15. L.J. Pa&t, J.L. Smialek, J.P. Nit, and D.E. Mikkola, Scripta Metall. Mater., 25, p. 727 (1991). 16. J.C. Schaeffer, G.H. Meier, and R.L. McCarron, GE Aircraft Engines Interim Report No. 4, Naval Air Development Center Contract N62269-90-C-0287 (October 1992). 17. G.H. Meier, N. Birks, F.S. Pettit, R.A. Perkins, and H.J. Grabke, in Structural Intermetallics, R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, and M.V. Nathal, TMS, Warrendale, PA, p. 869 (1993).
One author (MPB) wishes to acknowledge the financial support of a National Research Research Associateship. This research was funded under the NASA HITEMP program.
Council Post-Doctoral
1073K Alumina 50 Ti Boundary PI
m.
Cr
Al
FIG. 1. Schematic of the 1073K oxidation map of Perkins et al. [2] (a/o).
1073K Alumina 50 Ti
80 Cr
FIG. 2. Schematic of the partial 1473K isotherm et al. for Al-Ti-Cr alloys hot isostatically pressed 1473Wl72 MPa/2 h [6] (a/o). Alloys A and B of work and the 1073K alumina boundary of Perkins also plotted.
of Klansky (HIPed) at the present et al.[2] are
Boundary ,_ [21
are speculative phase boundaries [7]). Alloys A and B of the present work and the 1073K alumina boundary of Perkins et al. [2] are also plotted.
FIG. 4. SEM (backscatter mode) micrograph of alloy A (45Al-40Ti-15Cr) exposed at 1273K for 100 h. The microstructure of alloy A exposed at 1073K for 100 h was similar.
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OXIDATION RESISTANT ALLOYS
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tau IiCrAl Cr,AI
FIG. 5. SEM (backscatter mode) micrograph of alloy B (55Al-30Ti-15Cr a/o) exposed at 1273K for 100 h.
FIG. 6. SEM (backscatter mode) micrograph exposed at 1073K for 100 h.
of alloy B
1073K Alumina
Al
80 Cr
FIG. 7. Partial 1273K Al-Ti-Cr isotherm of the present work (a/o). The 1073K alumina boundary of Perkins et al. [2] is also plotted.