Flexural strength of gamma base titanium aluminides at room and elevated temperatures

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A197 (1995) 69 77

A

Flexural strength of gamma base titanium aluminides at room and elevated temperatures R. Gnanamoorthy

a'*, Y . M u t o h a, Y . M i z u h a r a b

~Department of" Mechanical Engineering, Nagaoka University of Technology, Nagaoka, Japan bNippon Steel Corporation, Kawasaki, Japan Received 2 September 1994; in revised form 2 November 1994

Abstract

The influences of microstructure, alloying additions and processing method on the flexural strength and fracture behavior of gamma base titanium aluminides were investigated at room and elevated temperatures. The flexural strength strongly depends on the microstructure, alloying addition and processing method. The lamellar microstructure material exhibited high strength and ternary chromium or niobium addition improved the strength. Isothermal forging resulted in a fine grain microstructure and influenced the bend properties. The flexural strength increased up to a 'critical temperature' and thereafter it decreased. This 'critical temperature' seemed to depend on the chemical composition. The room-temperature fracture was mostly transgranular cleavage and intergranular fracture regions were dominant with increasing temperature. At high temperatures ductile tearing was observed, irrespective of the microstructure.

Keywords: Flexural strength; Titanium aluminides; Fracture; Microstructure

I. Introduction

Titanium aluminide base intermetallic compounds are light in weight and possess excellent high-temperature properties such as strength, stiffness, and modulus retention [1]. However, their low room-temperature ductility and toughness restrict any practical application. The high-temperature tensile behavior of gamma base titanium aluminide intermetallic compounds has been studied by many investigators and such work has shown that the strength and ductility depend on the composition and microstructural parameters [1 5]. The room-temperature yield strengths vary from 350 MPa to 650 MPa, and ultimate strengths vary from 400 MPa to 720 MPa, depending on alloy composition, processing method and microstructure [1]. Huang and Hall [4] have investigated the effects of the Ti/A1 ratio on the mechanical behavior of binary TiA1 base alloys. Their results show that the duplex structure is more de* Present address: Institute for Materials Research, Tohoku University, Katahira 2-1-1, 980 Sendai, Japan. Elsevier Science S.A.

SSDI 0921-5093(94)09737-2

formable than a single-phase gamma structure which, in turn, is more deformable than a fully transformed structure. The duplex structure also had highest strength among the materials investigated by them [4]. The maximum tensile plastic elongation observed in the binary gamma alloys at room temperature was about 2% [1]. Prasad Rao and Tangri [5] have shown that increasing the lamellar constituent in two-phase alloys increases the yield strength. The tensile ductility can be improved by alloying additions. Addition of chromium [6], vanadium [7], or manganese [8] was found to improve the ductility at room temperature. However, addition of these elements decreases the oxidation resistance [1] and so niobium, tungsten or tantalum has to be added to improve the oxidation resistance [1]. Effects of annealing on the microstructure and fracture properties of niobium alloyed materials have been reported recently [9]. However, there is still a lack of systematic investigation delineating the effects of chromium or niobium addition on the high-temperature strength and fracture behavior. In general, the fine grain size increases the strength and ductility. Secondary hot-work-

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70

ing processes, such as hot-forging, extrusion etc., may refine the microstructure and lead to improved mechanical properties. Pu et al. [10] have shown that the ductility of gamma alloys can be enhanced through refinement of duplex structure grain size. Previous investigations by the authors indicated the significant effect of chromium addition on the hightemperature fracture behavior of gamma base titanium aluminides [11]. The objective of this paper is to present the results of a study which investigates the effects of microstructure and alloying additions on the hightemperature strength using the heat-treated and isothermally forged gamma base titanium aluminides. Fracture behavior at room and elevated temperatures is also discussed.

crostructures of the test materials are shown in Fig. 1. Test specimens were made from uniformly recrystallized regions by multi-wire cutting and machining to the required dimensions. Flexural strength was estimated using three-point bend specimens of dimensions 3 mm x 4 mm x 36 mm according to the test standard for estimating flexural strength of high-performance ceramic materials [12]. The span length for three-point bending was 30mm. Experiments were conducted using an Instron-type universal testing machine at a crosshead speed of 0.5 mm min ~. High temperature tests were carried out in a vacuum of better than 2 mPa. The specimens were soaked at the test temperature for at least 20 min before testing. Load crosshead displacement curves were recorded using an X - Y plotter and flexural strength ar was estimated from:

2. Test materials and experimental procedure

3PS ar-2wt2

Cast and heat-treated binary gamma base titanium aluminides, Ti-45A1 (all compositions are stated in atomic percentages), and Ti-50A1 (hereafter Ti 45A1 H T and T i - 5 0 A I - H T , respectively) and ternary Ti 49A1-3Cr and T i - 4 4 A 1 - 3 N b (hereafter T i A 1 C r - H T and TiAINb HT, respectively) were used. Some results of the materials T i - 5 0 A 1 - H T and T i A 1 C r - H T have been reported [11]. Subsequently isothermally forged Ti-50A1 and TiA1Cr (hereafter Ti 50A1 TF and TiA1Cr ITF, respectively) were also used. Ingots were prepared by plasma arc melting of high-purity materials. The ingots were 80 mm in diameter and 300 mm in height. The cast materials were homogenized for 96 h at 1050 °C in a vacuum of better than 1.3 mPa and fabricated into cylindrical rods of 35 mm diameter and 42 mm height using electric discharge machining for isothermal forging. The chemical compositions of the test materials after homogenization treatment are shown in Table 1. The impurity levels of the materials were controlled to very low concentrations. The homogenized cylindrical block was forged to 70% of its original height at 1200 °C at low strain rates for obtaining fine grains. The forged materials were cooled in the vacuum chamber to room temperature. The final miTable l Chemical composition of test materials Material

Chemical composition Ti

A1

Cr/Nb

O

(at/A) Ti-45Al Ti 50AI TiA1Cr TiA1Nb

55.3 49.1 48.1 53.5

44.7 50.9 49.2 43.5

H

C

(wt. ppm)

2.75 3.06

120 170 250 170

1.6 5.0 7.0 8.9

< 30 70 120 <30

(1)

where P is the maximum load on the load displacement curve, S the span length, w the specimen width and t the specimen thickness. At lower temperatures, brittle fracture occurred and the flexural strength was estimated from the fracture load. At higher temperatures (above 800 °C), test materials exhibited high plasticity and the flexural strength was calculated from the maximum load in the load crosshead displacement plot. Fractographical investigations were carried out using optical and scanning electron microscopes.

3. Results

3.1. Microstructure The microstructures of the materials are shown in Fig. 1 and the microstructural parameters are summarized in Table 2. In spite of identical heat treatment conditions, the final microstructures after homogenization treatment considerably differed from one another depending on composition. Very coarse lamellar grains with alternate layers of ~2/Y laths were obtained after homogenization treatment of Ti-45A1 HT. The average grain size was about 400 ~tm. Some fine equiaxed gamma grains were also observed at the lamellar grain boundaries. The grain size of the T i A 1 N b - H T after homogenization treatment was relatively finer compared with the binary material. The average grain size was about 60 ~tm. Isothermal forging after homogenization treatment resulted in a fine grain microstructure for both binary and chromium alloyed material. The binary Ti 50A1 ITF after homogenization treatment contained fine equiaxed 7 grains with an average grain size of about 25 lam. The X-ray diffraction (XRD) studies indicated

R. Gnanamoorthy et al./ Materials Science and Engineering A 197 (1995) 69 77

71

Fig. 1. Optical micrographs of the test materials: (a) Ti-45AI-HT; (b) TiAINb-HT; (c) Ti 50AI ITF; (d) TiA1Cr ITF.

the presence of ?,-phase peaks only [13]. The final grain size of the TiAICr ITF was about 19 ~tm and the X R D results showed the presence of fl and 7 phases [13]. No :~2 phase was observed after isothermal forging in both the materials. The transmission electron micrograph of the TiA1Cr ITF showed that the fl phase was present in agglomeration at triple points [13]. The computational image analysis and X R D analysis showed that the volume fraction of the second phase was about 7% [131.

Table 2 Microstructural details and room-temperature three-point bend test results of test materials Material

Microstructure Grainsize (p.m)

Flexural strength (MPa)

Ti 46A1 HT TiAINb HT Ti 50A1 HT" Ti 50AI ITF TiA1Cr- HT~' TiAICr ITF

Lamellar Lamellar Equiaxed Equiaxed Duplex Equiaxed

836 864 394 420 740 713

~From Ref. [14].

400 60 200 25 60 19

3.2. B e n d i n g t e s t s

Fig. 2 shows the load-displacement curves of specimens subjected to three-point bending tests are room temperature. Ternary niobium addition refined the lamellar grain size and contributed to the bending strength. The flexural strength of TiAICr ITF with equiaxed fine grain microstructure exhibited high strength compared with the TiA1 ITF, indicating the strength improvement due to the addition of chromium. The load-displacement curves indicate large plastic deformation in the chromium alloyed materials. The room-temperature bending strength of lamellar microstructure materials was very high compared with duplex and equiaxed microstructure materials. The strength and plastic deformation of the Ti 5 0 A I - I T F were superior compared with the Ti 50A1 HT. The temperature dependence of flexural strength of the cast and heat-treated materials is shown in Fig. 3. Results o f T i - 5 0 A 1 HT and T i A 1 C r - H T from Ref. [11] are also shown for comparison. The bending strength was high for the Ti 4 5 A 1 - H T at 600 °C compared with that at 400 °C but it was more or less constant below 600 °C for all the other materials (Fig. 3). The tempera-

72

R. Gnanamoorthy et al. / Materials Scienee and Engineering A 197 (1995) 69 77 100

I

,

,

,

I

J

'

'

'

I

Three-point

'

l

,

Bend

,

1500 -~O-%--~ - . O ,~

Test

Temperature

Room

I I Ti-50AI-ITF TiAICr-ITF T i - 5 0 A I - H T [11] TiAICr-HT [11]

I

I

I

8O

(Lamellar)X ..X/" TiAICr-HT

riAINb-HT

¢-

(Duplex)

..:f?Ti-45AI-HT (Lamellar)

5oo1::

/ . .

60

~:.. ..... x

~

._..__...--~-~---~o

'I'" ~

TiAICr-ITF (Equiaxed)

_.o

U_

O •- J

40

I/~._.__.

x Ti-50AI-ITF

o/

J

0

200

I

I

I

I

400

600

800

1000

Temperature

(Equiaxed)

i-50AI-HT (Equiaxed)

1200

(°C)

Fig. 4. Flexural strength versus test temperature for isothermally forged materials compared with cast and heat-treated materials.

20

0

i

i

s

i

I

0

i

~

i

i

J

0.5

J

,

K

1.5

displacement

Crosshead

i

1

but the flexural strength of ternary chromium alloyed material was not influenced significantly by the isothermal forging.

(mm)

3.3. Fractography

Fig. 2. Load displacement curves of three-point bend specimens tested at room temperature. ture at which the maximum strength was obtained seems to be increased by niobium addition. The temperature dependence of flexural strength of the isothermally forged materials are compared with that of the corresponding cast and heat-treated materials in Fig. 4. Similarly to the cast and heat-treated materials, the flexural strength was higher at high temperatures compared with that at room temperature. Isothermal forging improved the flexural strength at room and elevated temperatures of the binary Ti-50A1,

1500 (I~

I I - - - - ~ - - - T i - 4 5 A I - HT

I

I

I

/%

~..

- . . . . . n .....

TiAINb-HT

"-"

---O-- ~

T i - 5 0 A I - H T [11] /" ~...', TiAICr-HT [11],, "<~ D"" . . . . . . . I~.,

,"-'~, ts

,-,

= 1 o o o -

D

.

........

-,

:.,. 7.~......a........

\",,

500

L,.

x

_.o

LL

o

I 0

200

I I 400 600 Temperature

I

I

800

1000

1200

(°C)

Fig. 3. Flexural strength versus test temperature for cast and heattreated materials.

Fractographs of the fractured specimens are shown in Figs. 5-8. The room-temperature fracture features of the materials reflected their original microstructure. The lamellar microstructure, Ti 45A1-HT, exhibited 'pillar-shaped' fracture featuers (Fig. 5(a)). At 600 °C and 800 °C, mixed transgranular and intergranular fracture features were observed (Fig. 5(b) and (c)). Specimens tested at much higher temperatures showed dominant ductile fracture features (Fig. 5(d)). The fractographs of the TiA1Nb are shown in Fig. 6. Increased intergranular fracture features were observed at room temperature compared with the binary lamellar material (Fig. 6(a)). With increasing temperature, intergranular fracture became dominant (Fig. 6(b) and (c)). At 1000°C, 'bubble-like' fracture features were observed which were not found in other materials investigated (Fig. 6(d)). Ductile fracture features were observed only at a much higher temperature (at 1100 °C), similarly to the fracture features of the T i 45A1-HT at 1000 °C. Fine recrystallized gamma grains were observed in the specimen fractured at 1000 °C in TEM studies [14]. The fracture surfaces of equiaxed microstructure T i 5 0 A I - I T F are shown in Fig. 7. At room temperature, the fracture surfaces were characterized by a dominant transgranular cleavage with a typical 'river-pattern' type of fracture (Fig. 7(a)). The fracture features at 600 °C contained increased intergranular fracture features, as shown in Fig. 7(b). At 800 °C, the fracture surfaces were characterized by a dominant intergranular fracture (Fig. 7(c)). At 1000 °C, the isothermally

R. Gnanamoo~thy et al./ Materials Science and Engineering A 197 (1995) 69-77

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Fig. 5. Fractographs of three-point bend Ti 45A1 HT specimens tested at (a) 20 °C, (b) 600 °C, (c) 800 °C and (d) 1000°C.

forged materials exhibited high plasticity and did not fracture. The room-temperature fracture of the TiA1Cr-ITF was also characterized by a transgranular cleavage type of fracture with typical 'river-patterns' as shown in Fig. 8(a). Increased intergranular fracture regions compared with the binary material were observed at 600 °C (Figs. 7(b) and 8(b)). Specimens tested above 600 °C exhibited high plasticity and did not fracture.

4. Discussion 4.1. Bending properties at room temperature

From the experimental results it is clear that the room-temperature bending behavior is strongly influenced by the microstructure. Fully lamellar microstructure materials exhibited high bending strength compared with the duplex microstructure material, which in turn possessed high strength compared with the equiaxed microstructure materials. An increase in the amount of lamellar constituent in the cast and heat-treated materials increases the strength. Recent

work by Prasad Rao and Tangri [5] has also shown that increasing the lamellar constituent in two-phase alloys increases the yield strength. The increase in the amount of hard ~2 phase contributes to the strength of the material. Fractographical investigations indicated the dominant transgranular cleavage fracture features with typical 'river-patterns' in the case of equiaxed microstructure materials (Figs. 7(a) and 8(a)). However, the fracture surfaces of a lamellar microstructure material consisted of 'pillar-shaped' fracture features (Fig. 5(a)). The presence of hard lamellar laths in the lamellar colony of the duplex and lamellar microstructure materials contributes to the increased strength. The T i - 5 0 A I - I T F exhibited high strength compared with the T i - 5 0 A 1 - H T [11], but the TiA1Cr-ITF possessed low strength compared with the TiA1Cr-HT. The marginally low flexural strength of the TiA1Cr ITF can be attributed to the absence of lamellar grains. The beneficial effect of grain size refining contributing to the strength might have been subdued by the absence of hard c~2 phase. The marginally high flexural strength of T i - 5 0 A I - I T F may be due to the fine grain size. Grain refinement due to isothermal forging might have contributed positively to strength but the effect seems

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R. Gnanamoorthy et al. / Materials" Science and Engineering A 197 (1995) 69- 77

Fig. 6. Fractographs of three-point bend TiA1Nb HT specimens tested at (a) 20 °C, (b) 600 °C, (c) 800 °C and (d) 1000°C.

to be small. A similar effect of grain size has been reported by Kawabata et al. [15]. Though isothermal forging after casting the heat treatment refined the grain size, the phase contents of the materials after isothermal forging were different in the materials investigated. It is therefore difficult to figure out the influence of grain size refining due to isothermal forging. Further investigations are needed to clarify the effects of grain size for materials with similar phase contents. The ternary gamma base titanium aluminides investigated exhibited high strength compared with binary materials with similar microstructure. Solid-solution strengthening due to the additions of niobium and chromium [1] might have resulted in high strength in ternary alloys compared with binary materials with similar microstructure. The bend deformation also depends strongly on the ternary addition. Among the materials investigated, the chromium alloyed material showed large plastic deformation. The effect of ternary addition on the strength and ductility has been interpreted in terms of the difference in phase content, deformation mechanisms [16-18], variations of unit-cell volume [1], site occupancy [19], enhanced microtwinning [20] and changes in the electronic structure [21]. The high strength of T i A 1 N b - H T compared with TiAICr H T can be attributed to the

increased :~2-phase content and fully lamellar microstructure of the TiA1Nb-HT. In spite of the fine gain size, the lamellar microstructure T i A 1 N b - H T exhibited less deformation compared with the coarse grained binary lamellar microstructure Ti-45A1 HT, indicating the decrease in plasticity due to niobium addition. Addition of niobium also decreases the tensile ductility of gamma base titanium aluminides [1]. A decrease in the unit-cell volume will strengthen the chemical bonding and may improve the ductility. Addition of chromium, manganese or vanadium, each of which improves the ductility, decreases the unit-cell volume to a greater extent compared with niobium, which marginally decreases the unit-cell volume [1]. Fractographical observations indicated the increased intergranular or grain boundary fracture in the niobium alloyed lamellar microstructure material (Fig. 6(a)). This can be attributed to the weak grain boundary strength of the niobium alloyed material compared with the binary or chromium alloyed material. Soboyejo et al. [9] have found that the niobium alloyed material contained grain boundary precipitates which were not observed in the binary material. The presence of grain boundary precipitates may decrease the grain boundary cohesive strength and result in increased intergranular fracture.

R. Gnanamoorthy et al. / Materials Science and Engineering A 197 (1995) 69-77

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Fig. 7. Fractographs of three-point bend Ti 50AI-ITF specimens tested at (a) 20 °C, (b) 600 °C and (c) 800 °C.

4.2. Bending properties at intermediate temperatures (up to 800 °C) The bending strength of binary lamellar microstructure material, Ti-45AI HT, increased at a higher rate above 400 °C compared with that of the other materials tested and the bending strength of the lamellar microstructure material, Ti-45A1-HT, at 800 °C was much higher compared with the room-temperature strength. However, in the equiaxed microstructure material, Ti-50A1-HT, the strength at 800 °C was only marginally higher compared with room-temperature strength [11]. For both the binary cast and heat-treated materials, the maximum bending strength was observed around 700 °C, and does not seem to be affected by the microstructure. The flexural strength of isothermally forged materials increased at a higher rate at lower temperatures. The bending strength of isothermally forged materials are higher at 600 °C compared with the corresponding cast and heat-treated materials, as shown in Fig. 4. The fractographical investigations indicated a dominant intergranular fracture in isothermally forged materials. Ternary addition to gamma base titanium aluminides influenced the high-temperature behavior significantly. Bending strength was more or less the same up to 600 °C for both the chromium and niobium alloyed material, but increased rapidly above this temperature

similarly to the binary materials. The maximum bending strength was observed at 700 °C for the chromium alloyed material [11], but for the niobium alloyed material it was observed at a much higher temperature.

4.3. Bending properties at high temperatures (above 800 °C) The bending strength decreases with increasing temperature above 700 °C. This may be due to dynamic recrystallization occurring at higher temperatures. Fine recrystallized gamma grains were observed in the lamellar microstructure materials tested at 900 °C [14]. Lattice softening and dislocation annihilation may also cause the decrease in the bending strength. The fractographical investigations carried out indicated ductile tearing as a dominant mode of failure above 800 °C. All the materials tested showed 'dimple-like' fracture features. Therefore, it seems that the effect of the original microstructure becomes less at higher temperatures compared with that at room and low temperatures. The bending strength of chromium alloyed material decreased above 800 °C [l 1] but it decreased only after 900 °C for niobium alloyed material. Sobeyojo et al. [9] have determined the increased dislocation density in niobium alloyed material compared with the binary material. The increased slip activity was observed only above 800 °C for the niobium alloyed material.

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R. Gnanamoorthy et al. / Materials Seienee and Engineering A 197 (1995) 69 77

Fig. 8. Fractographs of three-point bend TiA1Cr-ITF specimens tested at (a) 20 °C and (b) 600 °C.

The fractographic observations carried out on c h r o m i u m alloyed materials above 800 °C indicated ductile tearing as the d o m i n a n t m o d e o f fracture [11]. In the case o f niobium alloyed material, however, interesting 'bubble-like' fracture features were observed in the specimens tested at 1000°C. The 'bubbles' contained curved striations which indicate that the bubbles might have been formed due to the extensive curvature o f lamellae. Soboyejo et al. [9] have studied the deform a t i o n mechanism in the niobium alloyed material and have shown the curvature o f lamellae at 982 °C.

minides. The temperature at which the strength was m a x i m u m was influenced by niobium addition.

Acknowledgements Support for one o f the authors (R.G.) from the Ministry o f Education, Science and Culture, Japan, during the course o f the research is gratefully acknowledged.

5. Conclusions

References

The flexural strength and fracture behavior o f binary and ternary g a m m a base titanium aluminides have been studied and the results are summarized as follows. (1) The beginning strength o f g a m m a base titanium aluminides strongly depends on the microstructure and alloying additions. The lamellar microstructure material exhibited the highest strength c o m p a r e d with the duplex and equiaxed microstructure materials. T e r n a r y c h r o m i u m or niobium addition improved the strength. (2) The r o o m - t e m p e r a t u r e fracture was mostly transgranular cleavage and the fracture feature depended on the original microstructure o f the material. With increasing temperature intergranular fracture regions were observed, and at high temperatures ductile tearing was dominant, irrespective o f the microstructure. (3) Isothermal forging resulted in fine grain microstructure and influenced the bend properties o f the material at r o o m temperature. At higher temperatures, isothermally forged materials exhibited high strength. (4) T e r n a r y addition significantly affected the hightemperature behavior o f g a m m a base titanium alu-

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