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Initiation and slow stable growth of brittle cracks in TiAl
S c r i p t a METALLURGICA e t MATERIALIA
Vol. 26, pp. 8 1 3 - 8 1 7 , 1992 P r i n t e d in t h e U.S.A.
Pergamon P r e s s p l c All rights reserved
INITIAI'ION AND SLOW STABLE GROWTH OF BRITrLE CRACKS IN TiAI Ke-wei Gao, Yan-Bin Wang, Wu-yang Chu, Chi-mei Hsiao Dept. of Materials Phys. Univ. of Sci. and Tech. Beijing 100083, China Anthony W. Thompson Dept. of Metall. Eng. and Mater. Sci. Carnegie Mellon Univ., Pittsburgh, PA 15213 ( R e c e i v e d November 26, 1991) ( R e v i s e d J a n u a r y 2, 1992)
Introduction
Typically, brittle cracks propagate unstably at high speed to catastrophic failure (1). In that situation, the crack growth resistance, or R curve, under plane strain conditions should be saturated at two percent crack extension, after which the crack shhoud propagate rapidly (1). The toughness would be given by K~c2= ERc/(1-F), where Rc is the saturated resistance value, E is Young's Modulus, and ~ is Poi.sson's ratio. Chu and Thompson, however, found that a cleavage-like fracture in the titanium aluminide alloy Ti-24Al-llNb occurred by slow stable crack growth characterized by a well-defmed resistance or R curve (2). The universality of this behavior for titanium aluminides was checked for a TiA1 alloy in this work. Often, for metals, the nucleation of cleavage fracture occurs as a result of inhomogeneous plastic deformation in regions of microscopic dimensions (1). Cleavage cracks extend, in general, so rapidly that it is difficult to trace the local plastic deformation and its relationship with crack initiation. However, if the cleavage or cleavage-like fracture occurs by slow stable crack growth, the process of the local plastic deformation ahead of a crack and its relationship to crack initiation can be traced by means of a light microscope. Such a study was the other goal of this work.
Ex~rimental Procedur~
The TiA1 material for these experiments had the following composition (in atomic %): AI=49.9, Fe=0.16, Offi0.12, Nffi0.005, Hffi0.14, balance Ti. It was in the form of a cast and homogenized ingot. Specimens were heat treated after finish machining. The specimens included WOL, notched, and modified WOL specimens (3), the latter with width W and height 2H in the customary ratio H/W--0.486, and thickness of 10ram. The notch was made by electric spark cutting using a Mo wire, and the radius of the notch root was 0. lmm. All specimens were sealed in evacuated tubes and heat treated at 1250"C to 1425"C for 40 rain to obtain different microstructures (4), air cooled, then annealed at 900"C for 2 hours to stabilize the microstructures. After heat treatment some 813 0 0 3 6 - 9 7 4 8 / 9 2 $5.00 + .00 C o p y r i g h t (c) 1992 Pergamon P r e s s p l c
GROWTH OF CRACKS
814
IN TiA1
Vol.
26, No.
specimens were precharged with hydrogen at 900°C in a Sievertsapparatus (5). Three microstructures were produced in the Ti-50at% AI alloy through heat t~eatment at the temperatures already described (4,6): equiaxed grains of single-phase 7, a lameUar structure of 7+a2, or mixed structures with various volume fractions of 3' and lamellar constituent. The microstructure and hydrogen amount for each specimen are listed in Table 1, where L designates the lameUar constituent.
Results and Discussion
I. Crack growth resistance curve in TiAI The fracture strain in TiAI at room temperature was 1.5 to 1.7% and the uniform plastic strain only 0.05 to 0.12% (4). The fracture surfaces of single phase 7 consisted predominantly of cleavage-like and blocky fracture areas, with occasional intergranular fracture (4,6,7). The WOL specimens were loaded by a slow strain rate testing machine with a displacement rate of 2x10" 4mm/s, i.e., under a rising-K, condition. A "pop-in', corresponding to nucleation of a macrocrack from the notch tip (1), was always observed. The pop-in was followed by gradual crack extension with rising load. The crack growth resistance or R curve was measured using the modified WOL notch specimen. The K~ necessary for initiation of a crack from the notch tip was a minimum value, which can be identified as I~, and is listed in Table 1. In order to extend the crack further, IQ must be increased. Thus, the resistance to crack growth Rffi(1-~K~/E increased with crack propagation, as shown in Fig. 1. Surprisingly these R curves did not approach saturation, even at 40 to 50 percent crack extension, which is different from the case for Ti3AI+Nb as Ti-24AI-11Nb (2,8). Table 1. Kq for WOL Specimens with Different Microstructures and Hydrogen
Tensile WOL
Modified WOL
sample,No
1-2
1-5
1-6
1-9
1-i0
1-4
1-3
structure
7
7
7
50%L
50%L
7
7
200
i000
H, wppm KQ,MPam m
16.1
14.9
13.0
16.5
14.8
11.7
10.2
sample,No
2-4
2-5
2-9
2-8
2-I0
2-2
2-3
7
7
50%L
7
7
580
58O
1190
10.6
10.4
15.8
structure
100%L 100%L
H, wppm r,Q,Mpam ''~
13.2
16.3
14.3
18.0
19.2
2-1 lO0%L
2. Initiation of brittle cracks A polished pro-cracked WOL specimen was slowly loaded by the use of a bolt and loading pin (8). The plastic zone ahead of the original crack increased in size with increasing K~, see Fig. 2(a) to Fig. 2('o). In Fig. 2(b), slip bands B were observed beside the slip bands A. The new crack initiated along the slip band B, when the plastic deformation developed to a certain extent, Fig. 2(c). Further increases in Kz caused a zigzag extension of the crack
5
Vol.
26, No.
5
GROWTH
OF CRACKS
IN TiAI
815
along the slip band A, Fig. 2(d). The slip bands C appeared and the zigzag crack propagated stably with increasing Kx, Fig. 2(e). In general, new cracks were observed to initiatediscontinuously within the plasticzone along the slipbands, when the plasticdeformation developed to a criticalextent (8), as shown in Fig. 3(a). Sometimes, a macroscopic crack along a slipband consisted of many small cracks inclined at 35 deg to the slipband, as shown in Fig.3(b). The cleavage fracture planes in TiAI single crystals have been identified(9)as {III}, {I00}, and {110}. Initiationand propagation of the cracks along slip bands means that the cleavage-like fracture occurs on {I I I} planes. The small cracks inclined at 35 deg to slipbands apparently indicates that fracture occurs also on {II0} planes. For single-phase7, the minimum K~ corresponding to initiationof a crack from the notch tip was 14.7MPam 1:2. Therefore, the minimum resistance R=(I-~K:/E
=ll401/m 2, where E=l.73x10SMPa, v=0.29 (I0). This is
much higher than the true surface energy 7°, which for metals is typicallyof the order of i or 2 J/m 2. Thus, using the interpretation(I) thatR is the sum of 27, and the plasticenergy consumed in forming the plasticzone, 7P, one sees that 7v is far larger, and local plastic deformation plays a dominant role in initiationand propagation of cleavage-like cracks in TiAI. The same is true in Ti-24AI-IINb (8). R increased with crack extension, likelydue to work hardening and increasing 7P values. However, why the R curve did not approach saturationcan not be explained. In general, the fracturetoughness for notched specimens with a notch radius ofo is I~cf.O)= Av/p (I i), which was typicallylarger than K~c for a pre-cracked specimen. For TiAI, a notched specimen with a notch radius of 0. I m m can be used to obtain a good estimate of K m or R curve, because the K o for initiationof a cleavage-like crack from the notch tip was nearly the same as that from a pre-crack tip.
Acknowledgement
This project is supported by NNSF of China.
Referenc¢~
(1) LF.Knott, Fundamentals of Fracture Mechanics, Ch. 5 and 7, Butterworths, London, 1973, pp. 126-203. (2) W.Y.Chu, and A.W.Thompson, Metall Trans A, 22A(1991), 71-81. (3) S.R.Novak and S.T.Rolfe, LMater., 4(1969), 701-728. (4) W.Y.Chu and A.W.Thompson, Scripta MetaU. Mater., 25(1991), 641-644. (5) W.Y.Chu and A.W.Thompson, "Hydrogen solubility in Ti-24AI-I1Nb', Acta Metall. Mater., in press. (6) W.Y.Chu and A.W.Thompson, Scripta Metal1. Mater., 25(1991), 2133-38.
816
GROWTH OF CRACKS IN TiAI
Vol. Z6, No. 5
(7) A.W.Thompson and W.Y.Chu, in Microstructure/Pmper~ Relationships in Titanium Alloys and Titanium Alumi~Lides, Y. W. Kim and R. R. Boyer, eds., TMS, Warrendale, PA, 1991,pp. 165-177. (8) W. Y. Chu and A. W. Thompson, "Hydrogen Effects on Brittle Fracture of the Titanium Aluminides Alloy Ti-24AI-IINb', Metall. Trans.A, submitted.
(9) T.Kawabata and O.Izumi, Scripta MetaU., 21(1987), 435-440. (10) R.E.Schafrik, Metall. Trans.A, 8A(1977), 1003-1007. (11) W.Y.Chu, C.M.Hsiao, S.Q.IA, Scripta Metall, 13(1979), I059-I062.
L~o~
20 q
mm
1
2
3
I
I
I
4
18 q,I
.16
14
2-3 12
I0 0
I
I
I
I
I0
20
30
40
/x a / a
,.50
,
Fig. 1. Resistance or R curves for slow stable crack extension
Vol.
26, No.
5
GROWTH OF CRACKS
IN TiAI
81
Fig.2. Plastic zone enlargement and crack initiation on the polished surface of the pre-cracked modified WOL specimen with increasing K~
J
fl J
b |
.I
SO vm
(a)
(b)
Fig.3. Discontinuous nucleation of crack within the plastic zone along the slip bands (a) or inclined at 35 deg. to the slip bands (b)
Vol. 26, pp. 8 1 3 - 8 1 7 , 1992 P r i n t e d in t h e U.S.A.
Pergamon P r e s s p l c All rights reserved
INITIAI'ION AND SLOW STABLE GROWTH OF BRITrLE CRACKS IN TiAI Ke-wei Gao, Yan-Bin Wang, Wu-yang Chu, Chi-mei Hsiao Dept. of Materials Phys. Univ. of Sci. and Tech. Beijing 100083, China Anthony W. Thompson Dept. of Metall. Eng. and Mater. Sci. Carnegie Mellon Univ., Pittsburgh, PA 15213 ( R e c e i v e d November 26, 1991) ( R e v i s e d J a n u a r y 2, 1992)
Introduction
Typically, brittle cracks propagate unstably at high speed to catastrophic failure (1). In that situation, the crack growth resistance, or R curve, under plane strain conditions should be saturated at two percent crack extension, after which the crack shhoud propagate rapidly (1). The toughness would be given by K~c2= ERc/(1-F), where Rc is the saturated resistance value, E is Young's Modulus, and ~ is Poi.sson's ratio. Chu and Thompson, however, found that a cleavage-like fracture in the titanium aluminide alloy Ti-24Al-llNb occurred by slow stable crack growth characterized by a well-defmed resistance or R curve (2). The universality of this behavior for titanium aluminides was checked for a TiA1 alloy in this work. Often, for metals, the nucleation of cleavage fracture occurs as a result of inhomogeneous plastic deformation in regions of microscopic dimensions (1). Cleavage cracks extend, in general, so rapidly that it is difficult to trace the local plastic deformation and its relationship with crack initiation. However, if the cleavage or cleavage-like fracture occurs by slow stable crack growth, the process of the local plastic deformation ahead of a crack and its relationship to crack initiation can be traced by means of a light microscope. Such a study was the other goal of this work.
Ex~rimental Procedur~
The TiA1 material for these experiments had the following composition (in atomic %): AI=49.9, Fe=0.16, Offi0.12, Nffi0.005, Hffi0.14, balance Ti. It was in the form of a cast and homogenized ingot. Specimens were heat treated after finish machining. The specimens included WOL, notched, and modified WOL specimens (3), the latter with width W and height 2H in the customary ratio H/W--0.486, and thickness of 10ram. The notch was made by electric spark cutting using a Mo wire, and the radius of the notch root was 0. lmm. All specimens were sealed in evacuated tubes and heat treated at 1250"C to 1425"C for 40 rain to obtain different microstructures (4), air cooled, then annealed at 900"C for 2 hours to stabilize the microstructures. After heat treatment some 813 0 0 3 6 - 9 7 4 8 / 9 2 $5.00 + .00 C o p y r i g h t (c) 1992 Pergamon P r e s s p l c
GROWTH OF CRACKS
814
IN TiA1
Vol.
26, No.
specimens were precharged with hydrogen at 900°C in a Sievertsapparatus (5). Three microstructures were produced in the Ti-50at% AI alloy through heat t~eatment at the temperatures already described (4,6): equiaxed grains of single-phase 7, a lameUar structure of 7+a2, or mixed structures with various volume fractions of 3' and lamellar constituent. The microstructure and hydrogen amount for each specimen are listed in Table 1, where L designates the lameUar constituent.
Results and Discussion
I. Crack growth resistance curve in TiAI The fracture strain in TiAI at room temperature was 1.5 to 1.7% and the uniform plastic strain only 0.05 to 0.12% (4). The fracture surfaces of single phase 7 consisted predominantly of cleavage-like and blocky fracture areas, with occasional intergranular fracture (4,6,7). The WOL specimens were loaded by a slow strain rate testing machine with a displacement rate of 2x10" 4mm/s, i.e., under a rising-K, condition. A "pop-in', corresponding to nucleation of a macrocrack from the notch tip (1), was always observed. The pop-in was followed by gradual crack extension with rising load. The crack growth resistance or R curve was measured using the modified WOL notch specimen. The K~ necessary for initiation of a crack from the notch tip was a minimum value, which can be identified as I~, and is listed in Table 1. In order to extend the crack further, IQ must be increased. Thus, the resistance to crack growth Rffi(1-~K~/E increased with crack propagation, as shown in Fig. 1. Surprisingly these R curves did not approach saturation, even at 40 to 50 percent crack extension, which is different from the case for Ti3AI+Nb as Ti-24AI-11Nb (2,8). Table 1. Kq for WOL Specimens with Different Microstructures and Hydrogen
Tensile WOL
Modified WOL
sample,No
1-2
1-5
1-6
1-9
1-i0
1-4
1-3
structure
7
7
7
50%L
50%L
7
7
200
i000
H, wppm KQ,MPam m
16.1
14.9
13.0
16.5
14.8
11.7
10.2
sample,No
2-4
2-5
2-9
2-8
2-I0
2-2
2-3
7
7
50%L
7
7
580
58O
1190
10.6
10.4
15.8
structure
100%L 100%L
H, wppm r,Q,Mpam ''~
13.2
16.3
14.3
18.0
19.2
2-1 lO0%L
2. Initiation of brittle cracks A polished pro-cracked WOL specimen was slowly loaded by the use of a bolt and loading pin (8). The plastic zone ahead of the original crack increased in size with increasing K~, see Fig. 2(a) to Fig. 2('o). In Fig. 2(b), slip bands B were observed beside the slip bands A. The new crack initiated along the slip band B, when the plastic deformation developed to a certain extent, Fig. 2(c). Further increases in Kz caused a zigzag extension of the crack
5
Vol.
26, No.
5
GROWTH
OF CRACKS
IN TiAI
815
along the slip band A, Fig. 2(d). The slip bands C appeared and the zigzag crack propagated stably with increasing Kx, Fig. 2(e). In general, new cracks were observed to initiatediscontinuously within the plasticzone along the slipbands, when the plasticdeformation developed to a criticalextent (8), as shown in Fig. 3(a). Sometimes, a macroscopic crack along a slipband consisted of many small cracks inclined at 35 deg to the slipband, as shown in Fig.3(b). The cleavage fracture planes in TiAI single crystals have been identified(9)as {III}, {I00}, and {110}. Initiationand propagation of the cracks along slip bands means that the cleavage-like fracture occurs on {I I I} planes. The small cracks inclined at 35 deg to slipbands apparently indicates that fracture occurs also on {II0} planes. For single-phase7, the minimum K~ corresponding to initiationof a crack from the notch tip was 14.7MPam 1:2. Therefore, the minimum resistance R=(I-~K:/E
=ll401/m 2, where E=l.73x10SMPa, v=0.29 (I0). This is
much higher than the true surface energy 7°, which for metals is typicallyof the order of i or 2 J/m 2. Thus, using the interpretation(I) thatR is the sum of 27, and the plasticenergy consumed in forming the plasticzone, 7P, one sees that 7v is far larger, and local plastic deformation plays a dominant role in initiationand propagation of cleavage-like cracks in TiAI. The same is true in Ti-24AI-IINb (8). R increased with crack extension, likelydue to work hardening and increasing 7P values. However, why the R curve did not approach saturationcan not be explained. In general, the fracturetoughness for notched specimens with a notch radius ofo is I~cf.O)= Av/p (I i), which was typicallylarger than K~c for a pre-cracked specimen. For TiAI, a notched specimen with a notch radius of 0. I m m can be used to obtain a good estimate of K m or R curve, because the K o for initiationof a cleavage-like crack from the notch tip was nearly the same as that from a pre-crack tip.
Acknowledgement
This project is supported by NNSF of China.
Referenc¢~
(1) LF.Knott, Fundamentals of Fracture Mechanics, Ch. 5 and 7, Butterworths, London, 1973, pp. 126-203. (2) W.Y.Chu, and A.W.Thompson, Metall Trans A, 22A(1991), 71-81. (3) S.R.Novak and S.T.Rolfe, LMater., 4(1969), 701-728. (4) W.Y.Chu and A.W.Thompson, Scripta MetaU. Mater., 25(1991), 641-644. (5) W.Y.Chu and A.W.Thompson, "Hydrogen solubility in Ti-24AI-I1Nb', Acta Metall. Mater., in press. (6) W.Y.Chu and A.W.Thompson, Scripta Metal1. Mater., 25(1991), 2133-38.
816
GROWTH OF CRACKS IN TiAI
Vol. Z6, No. 5
(7) A.W.Thompson and W.Y.Chu, in Microstructure/Pmper~ Relationships in Titanium Alloys and Titanium Alumi~Lides, Y. W. Kim and R. R. Boyer, eds., TMS, Warrendale, PA, 1991,pp. 165-177. (8) W. Y. Chu and A. W. Thompson, "Hydrogen Effects on Brittle Fracture of the Titanium Aluminides Alloy Ti-24AI-IINb', Metall. Trans.A, submitted.
(9) T.Kawabata and O.Izumi, Scripta MetaU., 21(1987), 435-440. (10) R.E.Schafrik, Metall. Trans.A, 8A(1977), 1003-1007. (11) W.Y.Chu, C.M.Hsiao, S.Q.IA, Scripta Metall, 13(1979), I059-I062.
L~o~
20 q
mm
1
2
3
I
I
I
4
18 q,I
.16
14
2-3 12
I0 0
I
I
I
I
I0
20
30
40
/x a / a
,.50
,
Fig. 1. Resistance or R curves for slow stable crack extension
Vol.
26, No.
5
GROWTH OF CRACKS
IN TiAI
81
Fig.2. Plastic zone enlargement and crack initiation on the polished surface of the pre-cracked modified WOL specimen with increasing K~
J
fl J
b |
.I
SO vm
(a)
(b)
Fig.3. Discontinuous nucleation of crack within the plastic zone along the slip bands (a) or inclined at 35 deg. to the slip bands (b)