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Gamma titanium aluminide, TNB
Intermetallics 13 (2005) 959–964 www.elsevier.com/locate/intermet
Gamma titanium aluminide, TNB Wayne E. Voicea,*, Michael Hendersonb, Edward F.J. Sheltonc, Xinhua Wud a Rolls-Royce plc, Elt-38, P.O. Box 31, DE24 8BJ Derby, UK Alstom Power, Cambridge Road, Whetstone, LE8 6LH Leicester, UK c QinetiQ, Cody Technological Park, Farnborough GU14 0LX, UK d IRC in Materials, The University of Birmingham, Edgbaston B15 2TT, UK b
Available online 4 May 2005
Abstract A range of mechanical properties for wrought TNB alloy (Ti–45Al–8Nb–0.2C%) and cast Ti4522XD(Ti–45Al–2Nb–2Mn–XD) have been assessed. The influence of notch sensitivity on fatigue strength has been evaluated and compared in terms of different microstructure and alloy composition. The effects of complex loading on fatigue strength have also been investigated with pre-creep deformations of 0.5, 3 and 5%, pre-exposure to 700 8C for 24 and 150 h or pre-low cycle fatigue testing at 700 8C. The tolerance of these materials to foreign object damage (FOD) has also been compared with the nickel alloy IN718. Under most conditions the TNB alloy has shown superior performance to Ti4522XD and its FOD tolerance appears to be better than IN718. q 2005 Published by Elsevier Ltd. Keywords: A. Titanium aluminides, based on TiAl; B. Fatigue resistance and crack growth; G. Aero-engine components
1. Introduction g-TiAl alloys are a series of intermetallic compounds having a typical composition of titanium plus 45–47 at.% aluminium, with additions such as manganese and niobium for ductility and high temperature capability. The beneficial characteristics of these alloys can be summarised as follows: (1) low density (4 g/cm3); (2) high stiffness (EZ175 GPa at 20 8C to 150 GPa at 700 8C);(3) density normalised strength similar to cast Ni-based alloys; (4) high temperature strength and oxidation resistant to 750 8C; (5) low thermal expansion coefficient and high thermal conductivity. However, the alloys are known to possess a number of limiting properties that have continued to restrict their wider application within the gas turbine engine industry. These are perceived as the following: (1) low ductility at low to intermediate temperatures (!2% at room temperature); (2) pffiffiffiffi pffiffiffiffi low fracture toughness (12 MPa m at 20 8C, 25 MPa m at 650 8C); (3) high fatigue crack growth rates leading to poor damage tolerance. The key benefits offered by g-TiAl technology for the designers of GT engines are: (1) rotating components at * Corresponding author. 0966-9795/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.intermet.2004.12.021
moderate to high temperatures (LPT blades); (2) density reduced centrifugal (CF) loads (reduced disc stresses); (3) high specific stiffness that increases the natural frequencies of compressor and turbine blades. Mechanical property databases have been previously established for a number of g-TiAl alloys together with mechanical models to describe their deformation and fracture behaviour. This has demonstrated that it is necessary to assess the defect sensitivity of these alloys and develop methods of damage summation for complex loading situations. The aim of the study was to address these issues for a range of g-TiAl alloys and the findings of the promising alloy, TNB are presented here.
2. Experimental TNB alloy is a wrought, high niobium-containing alloy (Ti–45Al–8Nb–0.2C, at.%) intended for use in the duplex condition, however, the fully lamellar microstructure has also been assessed for comparison. The extruded material to be investigated was found to have a variable microstructure in the form of bands of larger equiaxed g grains drawn out in the working direction amongst a predominantly fine duplex grain structure, see Fig. 1(a). This would be a worst-case
W.E. Voice et al. / Intermetallics 13 (2005) 959–964
Fig. 1. Secondary electron micrographs showing (a) a variable microstructure with bands of large equiaxed g grains drawn out in the working direction amongst a predominantly fine duplex grain structure and (b) fully lamellar microstructure of the TNB alloy.
situation and, in practice, the additional forging operation would break up the larger grains to give a completely fine grain structure. The fully lamellar microstructure of TNB is shown in Fig. 1(b). TNB will be compared to the Ti–45Al–2Nb–2Mn–XD (at.%) alloy, cast and HIP’ed by Howmet Corporation (XD copyright). Ti–4522XD generally contains a near-lamellar grain structure and blocky boride particles. However, this can vary across the cast section with equiaxed gamma in central regions (HIP closed porosity) and long ribbon, blocky then needle boride particles depending on distance from the surface (cooling rate dependent), Fig. 2. Defect sensitivity testing has been carried out to establish and validate a series of fatigue failure limit diagrams, commonly known as Kitigawa plots, shown schematically in Fig. 3 [1] that show fatigue strength as a function of defect or crack size. Baseline fatigue data was generated using a specifically designed defect sensitivity test specimen, Fig. 4 which contains eight, machined ‘cracks’ chosen to sample across the whole range of grain size and lamella colony
Fig. 2. Back scatter SEM micrographs showing variable Microstructure in Ti–4522XD: (a) near fully lamellar with long boride particles at mid-radius of cylindrical bar, (b) equiaxed microstructure at centre of cylindrical bar due to closing of porosity by HIP’ing.
orientation with respect to the crack plane. Tests conducted on Ti–4522XD and TNB alloy used specimens having a range of small crack sizes: from 0.05 to 0.25 mm (produced by grinding) and 0.01 to 0.04 mm (produced by electro-discharge machining).
Real materials, Process zone corrected analysis LOG STRESS, σ
960
Endurance limit stress, σ
FATIGUE REGIME
Threshold stress intensity, K
Growth below long crack threshold Growth below endurance limit LOG CRACK LENGTH, a
Fig. 3. Schematic representation of the Kitagawa diagram to model fatigue failures in pristine and damaged materials.
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3. Results and discussion 3.1. Notch sensitivity of the materials
Fig. 4. g-TiAl defect sensitivity specimen for Kitigawa short crack testing (5 mm gauge dia).
A series of load-controlled fatigue tests were conducted on Ti–4522XD and TNB defect specimens at 20 8C and elevated temperature at RZ0.1. Incremental fatigue stresses were determined by applying an initial, relatively low stress level and if the specimen remained intact after accumulating 107 cycles the load was automatically adjusted to provide a stress increase of 25 MPa. Ultimately, the fatigue strength was defined as the peak stress level to induce complete rupture of the specimen within a block of 107 cycles. This can be justified for g-TiAl due to its characteristically flat S–N curve. Conventional high cycle fatigue tests (at various mean loads) are often employed to characterise endurance and provide data for life prediction. These tests are invariably performed on virgin specimens that have seen no prior strain history and provide data for the ‘first loading’. However, in subsequent major cycles the fatigue strength of the material may be reduced by prior creep, which can provide sites for preferential crack initiation (particularly in materials that are prone to creep cavitation damage). Sequential creep plus HCF tests were performed on TNB alloy specimens. Creep exposure was conducted at 700 8C over 100 h to different degrees of creep strain, i.e. 0.5, 3 and 5%, followed by incremental fatigue testing. HCF vibration (flap) testing has been conducted to provide an in-depth evaluation of the fatigue performance of sub-element specimens of Ti–4522XD, TNB and the nickel alloy IN718 and a method of assessing the susceptibility of these materials to foreign object damage (FOD) impact. The ballistic impacts were carried out by QinetiQ at impact energies of between 4 and 13 J, i.e. velocities of up to 300 msK1. Impacts were based around 0.75 mm deep edge damage as this represents the limit of detection during routine boroscope inspection. Nominal 0.4 and 2.5 mm damage levels were also selected to give a wider range for study.
Fig. 5 is a combination of the best fit lines through Kitagawa type data (fatigue strength vs defect size) for duplex and fully lamellar microstructures of TNB. It can be seen that the influence of temperature on fatigue strength varies with microstructure. The difference in fatigue strength between 20 and 700 8C is evident in duplex material but negligible in fully lamellar microstructure. At 700 8C the fatigue strength of duplex is lower than that at room temperature. Fully lamellar microstructure appears to be insensitive when the defect size is smaller than 10 mm but becomes very sensitive at any defect larger than this at either 20 or 700 8C. Fully lamellar has slightly better fatigue performance in general for short defects (!10 mm) whereas the duplex has slightly better performance than fully lamellar microstructure at room temperature for large defects (O10 mm). A comparison of the Kitigawa data for Ti–4522XD and fully lamellar microstructure of TNB is shown in Fig. 6. The fatigue strength at elevated temperature is similar to that at room temperature for both alloys, however, the fatigue strength of TNB is much greater than Ti–4522XD by about 150 MPa. Apart from the Ti–4522XD tests at room temperature, there is an apparent defect threshold of w10 microns across the temperature range tested. The effect of defect size is greater on the fatigue strength at room temperature compared to that at 650 8C in the Ti–4522XD. Notch sensitivity is normally influenced by microstructure or grain size and ductility of the material. The critical crack length for a material under static stress can be described by KIC sf Z pffiffiffiffiffiffi pa where sf is the fracture stress and KIC is the fracture toughness and a is the critical crack length. The relationship
Fig. 5. Summary of fatigue strength vs defect size of duplex and fully lamellar TNB microstructure.
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the entire colony and form a crack about 150 mm long, which is very close to 200 mm estimated critical crack length for this microstructure under static stress conditions. This could be one of the reasons why the fully lamellar microstructure is very sensitive even to a small crack about 15 mm long. The fatigue limit of Ti45XD is markedly lower that that of TNB material (Fig. 6) and the fatigue limit at room temperature also decreases monotonically with the increase of the notch length. These are probably attributed to lower strength, low fracture toughness and heterogeneous microstructure of cast Ti45XD. Fig. 6. Comparison of fatigue strengths against defect Size for Ti–4522XD and lamellar TNB at ambient and elevated temperatures, RZ0.1 (failure !107 cycles).
is normally applied to through (thickness) cracks rather than shallow surface cracks. For duplex microstructure of TNB material, sf is about 1000 MPa between room temperature and 700 8C, and KIC pffiffiffiffi pffiffiffiffi is about 12 MPa m at room temperature and 25 MPa m at 700 8C. According to the above equation the critical crack length for causing catastrophic failure of the duplex microstructure is about 40 mm at room temperature and 200 mm at 700 8C. For fully lamellar microstructure of the TNB alloy sf is about 1000 MPa and KIC is about 25 MPa pffiffiffiffi m at room temperature, which gives a critical crack length of about 200 mm. From Fig. 5 it can be seen that the critical defect length for TNB with a duplex microstructure under fatigue loading is also about 40 mm at room temperature. However, it is noted that at 700 8C, where the fracture toughness is doubled, whilst the fracture strength remains almost unchanged, a much longer critical fracture length (200 mm) ought to be expected from the above simple estimation. The experimental results show that at 700 8C the critical crack length remains at about 40 mm (Fig. 5), which means that the material is more sensitive to notches under cyclic loading at a high temperature than estimated. Note that the crack introduced in the samples was in fact a surface crack and it is surprising that the surface crack is as damaging as a through crack for the duplex microstructure at room temperature. Repeating the above calculation for the fully lamellar microstructure the result shows that the effect of cracks is significantly underestimated. For samples with a fully lamellar microstructure the actual critical length under fatigue is about 15 mm rather than 200 mm as is estimated for static stress conditions. This indicates that under fatigue condition the fully lamellar microstructure is significantly more sensitive to notches. This could be attributed to the large colony size in a fully lamellar microstructure. The colony size of the TNB fully lamellar microstructure is about 150 mm. If a crack initiated parallel to lamellae, it is very easy for the crack to extend across
3.2. Influence of pre-creep or exposure on fatigue limit The Creep-HCF test results in Fig. 7 show little influence of the pre-creep level on subsequent fatigue properties to a level of 3% inelastic strain. Beyond this there was a reduction in creep resistance that probably corresponds to the formation of internal voids. It was also found that pre-LCF exposure at 700 8C did not affect HCF relative to the low strain creep. However, there does appear to be an underlying effect of thermal exposure at 700 8C as the limiting HCF stress was reduced after 24 h and still further after 150 h. The decrease of fatigue limit after pre-creep at 5% is attributed to crack formation in the material under high creep strain. However, it is surprising to see that thermal exposure at 700 8C would have a distinct effect on fatigue limit since this material has very good oxidation resistance up to 750 8C. This reduction in HCF may be related to the embrittlement found in tensile samples tested at room temperature after short term (2 h) exposure at 700 8C [2,3]. The loss of ductility on short term exposure may reflect the influence of subsequent plastic strain on the surfacedegraded region, whereas the longer times required to degrade the fatigue properties would reflect the influence of exposure on the ease of crack nucleation.
Fig. 7. Pre-load testing: incremental HCF test results for wrought TNB at 700 8C shown as a function of pre-creep strain level. Also shown on axis are pre-LCF and thermally exposed results.
W.E. Voice et al. / Intermetallics 13 (2005) 959–964
Fig. 8. Corrosion pitting on surface of TNB elevated temperature test-pieces (650 8C).
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originated as a result of prior machining or handling operations. It is interesting to note, however, that the ultimate fatigue strength of similar specimens was not significantly affected by this coincident corrosion damage. Clearly, further investigation is needed to ascertain the influence of surface degradation. With reference to the Kitagawa model, the retention of fatigue strength for specimens subjected to 3% or less prior creep appears to indicate that any creep damage must remain relatively limited at these magnitudes of bulk strain and other inherent defects still control the subsequent fatigue performance. Clearly, once entered into service, aerofoil components will experience a complex duty comprising the combined effects of creep and fatigue damage accumulation. More detailed evaluations of creepfatigue damage are the subject of ongoing investigations. 3.3. HCF vibration testing
Fig. 8 illustrates that crack initiation can also be influenced by high temperature corrosion pitting in these intermetallics. This particular specimen was deliberately tested without being subjected to the normal final cleaning procedure once gripped within the load train of the mechanical test rig. Hence, this salt deposit may have
The simulated blade shape specimen, shown in Fig. 9, has been specifically designed to give a direct performance comparison between the different materials. It is designed to fail half way along the leading edge when excited in the second flap mode. The central highest stress region is wide
Fig. 9. Specifically designed flap-HCF simulated blade test-piece and stress distribution during second order flap mode.
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characteristics under near service conditions. A deeper interpretation of the significance of the AF results is currently underway. Further work will look to analyse the magnitude of the peak stresses for the different materials and the influence of specific stiffness on the natural frequencies and bending stresses. For example, for the same amplitude the peak stress in the IN718 specimens is 50% greater than for g-TiAl, however, the TNB sustains a higher stress at the failure AF and tolerates a much larger displacement.
4. Conclusions Fig. 10. HCF AF strength of undamaged and FOD impacted Ti4522XD, duplex TNB and IN718 specimens as a function of impact site damage depth. Bars show the range of results.
enough for easy targeting with the edge of an impacting 3 mm cube to simulate in-service foreign object damage (FOD). The incremental AF (amplitude!frequency) strength levels (for failure in 107 cycles) for Ti–4522XD, duplex TNB and IN718 are shown in Fig. 10 where a AF value above 1.1 is generally considered to be a good result. Undamaged g-TiAl specimens show a large scatter in AF strength compared with those for IN718 (AF ranges of 1 compared to 0.2) such that IN718 and the Ti–4522XD have a similar average AF of 1.35, but the scatter of Ti–4522XD makes it unacceptable for use as a compressor blade. This is attributed to the variable microstructure across the HIP’ed cast section particularly in terms of differences in boride particle distribution and proportion of lamellar to equi-axed gamma grains, Fig. 2. The TNB alloy gives an average AF strength above 2 with no results below 1.5 so that it actually out performs the currently used IN718. In addition, the scatter in results for TNB should be significantly reduced in the forged product as the banded large grain structure would be refined to increase the usable minimum AF strength. Impact damage was generally found to reduce AF strength, however, the overlap of results between the plain and damaged Ti–4522XD specimens suggests that there were already inherent ‘defects’ in the cast material. It was surprising to observe that damaged Ti–4522XD performs better than the damaged IN718, particularly above depths of 2 mm, where the average IN718 AF was just 0.3. The TNB AF results for damage up to 0.75 mm are better than the critical level and even the heavily impacted 2 mm deep TNB specimens gave AF strengths of around 1. Completion of these test results represents a significant step forward in establishing g-TiAl blading technology and developing a better understanding of the performance
1. The susceptibility and sensitivity of the TNB g-TiAl alloy to defects, impact damage and pre-loading has been assessed for plain and sub-element aerofoil-shaped specimen geometries. 2. Defect sensitivity testing and modelling have established a series of tensile and fatigue failure limit diagrams (Kitigawa plots) for Ti–4522XD and TNB that show the alloy strengths as a function of defect size. A fatigue strength reduction was observed as a consequence of defects greater than w10 mm. 3. Thermal exposure at 700 8C appears to reduce the HCF stress limit at that temperature. 4. It was found that HCF is not greatly affected by preexposure to creep strains of up to 3%. Pre-exposure to low-cycle fatigue was also found to have little effect on the HCF properties of TNB. 5. Sub-component tests on undamaged and ballistic impacted aerofoil test-pieces have found that the AF fatigue strength of TNB to be better and more defecttolerant than both Ti–4522XD and the existing blade alloy IN718. The cast Ti–4522XD was shown to be unsuitable because of excessive scatter in properties, probably due to variable microstructural features.
References [1] Kitagawa H, Takahashi S. Applicability of fracture mechanics to very small cracks or cracks in the early stage. In: Proceedings of the second international conference on mechanical behaviour of metals. Boston, MA: American Society of Metals; 1976. p. 627–31. [2] Draper SL, Lerch BA, Locci IE, Shazly M, Prakash V. Effect of exposure on the mechanical properties of gamma MET PX. Proceedings of the second international conference on mechanical behaviour of metals. [3] Pather R, Mitten WA, Holdway P, Ubhi HS, Wisbey A, Brooks JW. The effects of high temperature exposure on the tensile properties of r TiAl alloys. Intermetallics 2003;11:1015–27.
Gamma titanium aluminide, TNB Wayne E. Voicea,*, Michael Hendersonb, Edward F.J. Sheltonc, Xinhua Wud a Rolls-Royce plc, Elt-38, P.O. Box 31, DE24 8BJ Derby, UK Alstom Power, Cambridge Road, Whetstone, LE8 6LH Leicester, UK c QinetiQ, Cody Technological Park, Farnborough GU14 0LX, UK d IRC in Materials, The University of Birmingham, Edgbaston B15 2TT, UK b
Available online 4 May 2005
Abstract A range of mechanical properties for wrought TNB alloy (Ti–45Al–8Nb–0.2C%) and cast Ti4522XD(Ti–45Al–2Nb–2Mn–XD) have been assessed. The influence of notch sensitivity on fatigue strength has been evaluated and compared in terms of different microstructure and alloy composition. The effects of complex loading on fatigue strength have also been investigated with pre-creep deformations of 0.5, 3 and 5%, pre-exposure to 700 8C for 24 and 150 h or pre-low cycle fatigue testing at 700 8C. The tolerance of these materials to foreign object damage (FOD) has also been compared with the nickel alloy IN718. Under most conditions the TNB alloy has shown superior performance to Ti4522XD and its FOD tolerance appears to be better than IN718. q 2005 Published by Elsevier Ltd. Keywords: A. Titanium aluminides, based on TiAl; B. Fatigue resistance and crack growth; G. Aero-engine components
1. Introduction g-TiAl alloys are a series of intermetallic compounds having a typical composition of titanium plus 45–47 at.% aluminium, with additions such as manganese and niobium for ductility and high temperature capability. The beneficial characteristics of these alloys can be summarised as follows: (1) low density (4 g/cm3); (2) high stiffness (EZ175 GPa at 20 8C to 150 GPa at 700 8C);(3) density normalised strength similar to cast Ni-based alloys; (4) high temperature strength and oxidation resistant to 750 8C; (5) low thermal expansion coefficient and high thermal conductivity. However, the alloys are known to possess a number of limiting properties that have continued to restrict their wider application within the gas turbine engine industry. These are perceived as the following: (1) low ductility at low to intermediate temperatures (!2% at room temperature); (2) pffiffiffiffi pffiffiffiffi low fracture toughness (12 MPa m at 20 8C, 25 MPa m at 650 8C); (3) high fatigue crack growth rates leading to poor damage tolerance. The key benefits offered by g-TiAl technology for the designers of GT engines are: (1) rotating components at * Corresponding author. 0966-9795/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.intermet.2004.12.021
moderate to high temperatures (LPT blades); (2) density reduced centrifugal (CF) loads (reduced disc stresses); (3) high specific stiffness that increases the natural frequencies of compressor and turbine blades. Mechanical property databases have been previously established for a number of g-TiAl alloys together with mechanical models to describe their deformation and fracture behaviour. This has demonstrated that it is necessary to assess the defect sensitivity of these alloys and develop methods of damage summation for complex loading situations. The aim of the study was to address these issues for a range of g-TiAl alloys and the findings of the promising alloy, TNB are presented here.
2. Experimental TNB alloy is a wrought, high niobium-containing alloy (Ti–45Al–8Nb–0.2C, at.%) intended for use in the duplex condition, however, the fully lamellar microstructure has also been assessed for comparison. The extruded material to be investigated was found to have a variable microstructure in the form of bands of larger equiaxed g grains drawn out in the working direction amongst a predominantly fine duplex grain structure, see Fig. 1(a). This would be a worst-case
W.E. Voice et al. / Intermetallics 13 (2005) 959–964
Fig. 1. Secondary electron micrographs showing (a) a variable microstructure with bands of large equiaxed g grains drawn out in the working direction amongst a predominantly fine duplex grain structure and (b) fully lamellar microstructure of the TNB alloy.
situation and, in practice, the additional forging operation would break up the larger grains to give a completely fine grain structure. The fully lamellar microstructure of TNB is shown in Fig. 1(b). TNB will be compared to the Ti–45Al–2Nb–2Mn–XD (at.%) alloy, cast and HIP’ed by Howmet Corporation (XD copyright). Ti–4522XD generally contains a near-lamellar grain structure and blocky boride particles. However, this can vary across the cast section with equiaxed gamma in central regions (HIP closed porosity) and long ribbon, blocky then needle boride particles depending on distance from the surface (cooling rate dependent), Fig. 2. Defect sensitivity testing has been carried out to establish and validate a series of fatigue failure limit diagrams, commonly known as Kitigawa plots, shown schematically in Fig. 3 [1] that show fatigue strength as a function of defect or crack size. Baseline fatigue data was generated using a specifically designed defect sensitivity test specimen, Fig. 4 which contains eight, machined ‘cracks’ chosen to sample across the whole range of grain size and lamella colony
Fig. 2. Back scatter SEM micrographs showing variable Microstructure in Ti–4522XD: (a) near fully lamellar with long boride particles at mid-radius of cylindrical bar, (b) equiaxed microstructure at centre of cylindrical bar due to closing of porosity by HIP’ing.
orientation with respect to the crack plane. Tests conducted on Ti–4522XD and TNB alloy used specimens having a range of small crack sizes: from 0.05 to 0.25 mm (produced by grinding) and 0.01 to 0.04 mm (produced by electro-discharge machining).
Real materials, Process zone corrected analysis LOG STRESS, σ
960
Endurance limit stress, σ
FATIGUE REGIME
Threshold stress intensity, K
Growth below long crack threshold Growth below endurance limit LOG CRACK LENGTH, a
Fig. 3. Schematic representation of the Kitagawa diagram to model fatigue failures in pristine and damaged materials.
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3. Results and discussion 3.1. Notch sensitivity of the materials
Fig. 4. g-TiAl defect sensitivity specimen for Kitigawa short crack testing (5 mm gauge dia).
A series of load-controlled fatigue tests were conducted on Ti–4522XD and TNB defect specimens at 20 8C and elevated temperature at RZ0.1. Incremental fatigue stresses were determined by applying an initial, relatively low stress level and if the specimen remained intact after accumulating 107 cycles the load was automatically adjusted to provide a stress increase of 25 MPa. Ultimately, the fatigue strength was defined as the peak stress level to induce complete rupture of the specimen within a block of 107 cycles. This can be justified for g-TiAl due to its characteristically flat S–N curve. Conventional high cycle fatigue tests (at various mean loads) are often employed to characterise endurance and provide data for life prediction. These tests are invariably performed on virgin specimens that have seen no prior strain history and provide data for the ‘first loading’. However, in subsequent major cycles the fatigue strength of the material may be reduced by prior creep, which can provide sites for preferential crack initiation (particularly in materials that are prone to creep cavitation damage). Sequential creep plus HCF tests were performed on TNB alloy specimens. Creep exposure was conducted at 700 8C over 100 h to different degrees of creep strain, i.e. 0.5, 3 and 5%, followed by incremental fatigue testing. HCF vibration (flap) testing has been conducted to provide an in-depth evaluation of the fatigue performance of sub-element specimens of Ti–4522XD, TNB and the nickel alloy IN718 and a method of assessing the susceptibility of these materials to foreign object damage (FOD) impact. The ballistic impacts were carried out by QinetiQ at impact energies of between 4 and 13 J, i.e. velocities of up to 300 msK1. Impacts were based around 0.75 mm deep edge damage as this represents the limit of detection during routine boroscope inspection. Nominal 0.4 and 2.5 mm damage levels were also selected to give a wider range for study.
Fig. 5 is a combination of the best fit lines through Kitagawa type data (fatigue strength vs defect size) for duplex and fully lamellar microstructures of TNB. It can be seen that the influence of temperature on fatigue strength varies with microstructure. The difference in fatigue strength between 20 and 700 8C is evident in duplex material but negligible in fully lamellar microstructure. At 700 8C the fatigue strength of duplex is lower than that at room temperature. Fully lamellar microstructure appears to be insensitive when the defect size is smaller than 10 mm but becomes very sensitive at any defect larger than this at either 20 or 700 8C. Fully lamellar has slightly better fatigue performance in general for short defects (!10 mm) whereas the duplex has slightly better performance than fully lamellar microstructure at room temperature for large defects (O10 mm). A comparison of the Kitigawa data for Ti–4522XD and fully lamellar microstructure of TNB is shown in Fig. 6. The fatigue strength at elevated temperature is similar to that at room temperature for both alloys, however, the fatigue strength of TNB is much greater than Ti–4522XD by about 150 MPa. Apart from the Ti–4522XD tests at room temperature, there is an apparent defect threshold of w10 microns across the temperature range tested. The effect of defect size is greater on the fatigue strength at room temperature compared to that at 650 8C in the Ti–4522XD. Notch sensitivity is normally influenced by microstructure or grain size and ductility of the material. The critical crack length for a material under static stress can be described by KIC sf Z pffiffiffiffiffiffi pa where sf is the fracture stress and KIC is the fracture toughness and a is the critical crack length. The relationship
Fig. 5. Summary of fatigue strength vs defect size of duplex and fully lamellar TNB microstructure.
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the entire colony and form a crack about 150 mm long, which is very close to 200 mm estimated critical crack length for this microstructure under static stress conditions. This could be one of the reasons why the fully lamellar microstructure is very sensitive even to a small crack about 15 mm long. The fatigue limit of Ti45XD is markedly lower that that of TNB material (Fig. 6) and the fatigue limit at room temperature also decreases monotonically with the increase of the notch length. These are probably attributed to lower strength, low fracture toughness and heterogeneous microstructure of cast Ti45XD. Fig. 6. Comparison of fatigue strengths against defect Size for Ti–4522XD and lamellar TNB at ambient and elevated temperatures, RZ0.1 (failure !107 cycles).
is normally applied to through (thickness) cracks rather than shallow surface cracks. For duplex microstructure of TNB material, sf is about 1000 MPa between room temperature and 700 8C, and KIC pffiffiffiffi pffiffiffiffi is about 12 MPa m at room temperature and 25 MPa m at 700 8C. According to the above equation the critical crack length for causing catastrophic failure of the duplex microstructure is about 40 mm at room temperature and 200 mm at 700 8C. For fully lamellar microstructure of the TNB alloy sf is about 1000 MPa and KIC is about 25 MPa pffiffiffiffi m at room temperature, which gives a critical crack length of about 200 mm. From Fig. 5 it can be seen that the critical defect length for TNB with a duplex microstructure under fatigue loading is also about 40 mm at room temperature. However, it is noted that at 700 8C, where the fracture toughness is doubled, whilst the fracture strength remains almost unchanged, a much longer critical fracture length (200 mm) ought to be expected from the above simple estimation. The experimental results show that at 700 8C the critical crack length remains at about 40 mm (Fig. 5), which means that the material is more sensitive to notches under cyclic loading at a high temperature than estimated. Note that the crack introduced in the samples was in fact a surface crack and it is surprising that the surface crack is as damaging as a through crack for the duplex microstructure at room temperature. Repeating the above calculation for the fully lamellar microstructure the result shows that the effect of cracks is significantly underestimated. For samples with a fully lamellar microstructure the actual critical length under fatigue is about 15 mm rather than 200 mm as is estimated for static stress conditions. This indicates that under fatigue condition the fully lamellar microstructure is significantly more sensitive to notches. This could be attributed to the large colony size in a fully lamellar microstructure. The colony size of the TNB fully lamellar microstructure is about 150 mm. If a crack initiated parallel to lamellae, it is very easy for the crack to extend across
3.2. Influence of pre-creep or exposure on fatigue limit The Creep-HCF test results in Fig. 7 show little influence of the pre-creep level on subsequent fatigue properties to a level of 3% inelastic strain. Beyond this there was a reduction in creep resistance that probably corresponds to the formation of internal voids. It was also found that pre-LCF exposure at 700 8C did not affect HCF relative to the low strain creep. However, there does appear to be an underlying effect of thermal exposure at 700 8C as the limiting HCF stress was reduced after 24 h and still further after 150 h. The decrease of fatigue limit after pre-creep at 5% is attributed to crack formation in the material under high creep strain. However, it is surprising to see that thermal exposure at 700 8C would have a distinct effect on fatigue limit since this material has very good oxidation resistance up to 750 8C. This reduction in HCF may be related to the embrittlement found in tensile samples tested at room temperature after short term (2 h) exposure at 700 8C [2,3]. The loss of ductility on short term exposure may reflect the influence of subsequent plastic strain on the surfacedegraded region, whereas the longer times required to degrade the fatigue properties would reflect the influence of exposure on the ease of crack nucleation.
Fig. 7. Pre-load testing: incremental HCF test results for wrought TNB at 700 8C shown as a function of pre-creep strain level. Also shown on axis are pre-LCF and thermally exposed results.
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Fig. 8. Corrosion pitting on surface of TNB elevated temperature test-pieces (650 8C).
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originated as a result of prior machining or handling operations. It is interesting to note, however, that the ultimate fatigue strength of similar specimens was not significantly affected by this coincident corrosion damage. Clearly, further investigation is needed to ascertain the influence of surface degradation. With reference to the Kitagawa model, the retention of fatigue strength for specimens subjected to 3% or less prior creep appears to indicate that any creep damage must remain relatively limited at these magnitudes of bulk strain and other inherent defects still control the subsequent fatigue performance. Clearly, once entered into service, aerofoil components will experience a complex duty comprising the combined effects of creep and fatigue damage accumulation. More detailed evaluations of creepfatigue damage are the subject of ongoing investigations. 3.3. HCF vibration testing
Fig. 8 illustrates that crack initiation can also be influenced by high temperature corrosion pitting in these intermetallics. This particular specimen was deliberately tested without being subjected to the normal final cleaning procedure once gripped within the load train of the mechanical test rig. Hence, this salt deposit may have
The simulated blade shape specimen, shown in Fig. 9, has been specifically designed to give a direct performance comparison between the different materials. It is designed to fail half way along the leading edge when excited in the second flap mode. The central highest stress region is wide
Fig. 9. Specifically designed flap-HCF simulated blade test-piece and stress distribution during second order flap mode.
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characteristics under near service conditions. A deeper interpretation of the significance of the AF results is currently underway. Further work will look to analyse the magnitude of the peak stresses for the different materials and the influence of specific stiffness on the natural frequencies and bending stresses. For example, for the same amplitude the peak stress in the IN718 specimens is 50% greater than for g-TiAl, however, the TNB sustains a higher stress at the failure AF and tolerates a much larger displacement.
4. Conclusions Fig. 10. HCF AF strength of undamaged and FOD impacted Ti4522XD, duplex TNB and IN718 specimens as a function of impact site damage depth. Bars show the range of results.
enough for easy targeting with the edge of an impacting 3 mm cube to simulate in-service foreign object damage (FOD). The incremental AF (amplitude!frequency) strength levels (for failure in 107 cycles) for Ti–4522XD, duplex TNB and IN718 are shown in Fig. 10 where a AF value above 1.1 is generally considered to be a good result. Undamaged g-TiAl specimens show a large scatter in AF strength compared with those for IN718 (AF ranges of 1 compared to 0.2) such that IN718 and the Ti–4522XD have a similar average AF of 1.35, but the scatter of Ti–4522XD makes it unacceptable for use as a compressor blade. This is attributed to the variable microstructure across the HIP’ed cast section particularly in terms of differences in boride particle distribution and proportion of lamellar to equi-axed gamma grains, Fig. 2. The TNB alloy gives an average AF strength above 2 with no results below 1.5 so that it actually out performs the currently used IN718. In addition, the scatter in results for TNB should be significantly reduced in the forged product as the banded large grain structure would be refined to increase the usable minimum AF strength. Impact damage was generally found to reduce AF strength, however, the overlap of results between the plain and damaged Ti–4522XD specimens suggests that there were already inherent ‘defects’ in the cast material. It was surprising to observe that damaged Ti–4522XD performs better than the damaged IN718, particularly above depths of 2 mm, where the average IN718 AF was just 0.3. The TNB AF results for damage up to 0.75 mm are better than the critical level and even the heavily impacted 2 mm deep TNB specimens gave AF strengths of around 1. Completion of these test results represents a significant step forward in establishing g-TiAl blading technology and developing a better understanding of the performance
1. The susceptibility and sensitivity of the TNB g-TiAl alloy to defects, impact damage and pre-loading has been assessed for plain and sub-element aerofoil-shaped specimen geometries. 2. Defect sensitivity testing and modelling have established a series of tensile and fatigue failure limit diagrams (Kitigawa plots) for Ti–4522XD and TNB that show the alloy strengths as a function of defect size. A fatigue strength reduction was observed as a consequence of defects greater than w10 mm. 3. Thermal exposure at 700 8C appears to reduce the HCF stress limit at that temperature. 4. It was found that HCF is not greatly affected by preexposure to creep strains of up to 3%. Pre-exposure to low-cycle fatigue was also found to have little effect on the HCF properties of TNB. 5. Sub-component tests on undamaged and ballistic impacted aerofoil test-pieces have found that the AF fatigue strength of TNB to be better and more defecttolerant than both Ti–4522XD and the existing blade alloy IN718. The cast Ti–4522XD was shown to be unsuitable because of excessive scatter in properties, probably due to variable microstructural features.
References [1] Kitagawa H, Takahashi S. Applicability of fracture mechanics to very small cracks or cracks in the early stage. In: Proceedings of the second international conference on mechanical behaviour of metals. Boston, MA: American Society of Metals; 1976. p. 627–31. [2] Draper SL, Lerch BA, Locci IE, Shazly M, Prakash V. Effect of exposure on the mechanical properties of gamma MET PX. Proceedings of the second international conference on mechanical behaviour of metals. [3] Pather R, Mitten WA, Holdway P, Ubhi HS, Wisbey A, Brooks JW. The effects of high temperature exposure on the tensile properties of r TiAl alloys. Intermetallics 2003;11:1015–27.