- Email: [email protected]
New aspect of development of high strength aluminum alloys for aerospace applications
Materials Science and Engineering A285 (2000) 62 – 68 www.elsevier.com/locate/msea
New aspects of development of high strength aluminum alloys for aerospace applications Manabu Nakai, Takehiko Eto * Al and Cu Research Section, Technical Department, Al and Cu Di6ision, Kobe Steel Ltd., 1 -5 -5 Takatsukadai, Nishi-ku, Kobe 651 -2271, Japan
Abstract New aspect of developing the materials in lights of interrelationship among alloy/process microstructure (mainly the second phase particles) — properties combined with micromechanics has been carried out in 2024 aluminum alloys. The fracture toughness is proportional to the square root of the spacing of constituents, Cu2FeAl7, in 2024 aluminum alloys. Broadening the spacing 75–140 mm, the fracture toughness increases by 20%. In a low DK region, the fatigue cracks propagate much slowly by broadening the spacing of dispersoids, Cu2Mn3Al20. Broadening the spacing, the crack propagation rate decreases by 50%. The larger size of dispersoids, Cu2Mn3Al20, makes the rate much slower than smaller ones, Cr2Mg3Al18 or ZrAl3. On the other hand, in a high DK region, the rate becomes slower mainly by broadening the constituents spacing. The newly developed 2 ×24 aluminum alloy sheets, in these concepts in mind, have excellent fracture toughness and fatigue crack propagation characteristics to provide lower weight, higher damage tolerance, and longer-term durability for aerospace applications. © 2000 Elsevier Science S.A. All rights reserved. Keywords: 2024 Aluminum alloy; Constituents; Dispersoids; Fracture toughness; Fatigue crack propagation rate; Damage tolerance
1. Introduction Advanced aluminum alloys for aerospace application have been required to allow high fracture toughness, higher fatigue performance, high formability, and superplasticity to meet the needs for lower structural weight, higher damage tolerance, and higher durability [1,2]. Conventional approach was merely based on relation between alloy/process-properties. In 2024 aluminum alloys, e.g. it is well known that decreasing the volume fraction of constituents such as Cu2FeAl7 increases fracture toughness of the materials [3]. Rich et al. [4] have proposed the following equation for the relationship among fracture toughness, KIC, and constituents volume fraction, Vf, and mean diameter of constituents, D.
KIC = 2sY · E
n p 6
1/3
1/2
D
* Corresponding author.
1/6 V− f
(1)
where sY is the yield strength and E is Young’s modulus. Constituents are distributed from fine as 0.1 mm or less as coarse as 10 mm or over. Vf and D are no more than the microstructural parameters including the average morphology of constituents. Thus, they cannot constitute the macrostructural parameters determining fracture toughness. With respect to fatigue crack propagation characteristics, only a few researches are available in which the mutual effect of constituents and dispersoids are separated. Of these, Staley et al. [5] reported that fatigue crack propagation rate decreases as fracture toughness increases. However, the microstructural parameter subjected to the investigation is volume fraction of constituent only. The authors have studied microstructural parameters determining toughness and fatigue crack propagation characteristics in 2024 aluminum alloys [6]. The object of the present paper was to clarify the interrelationship among alloy/process-microstructure (mainly the second phase particles) — properties combined with micromechanics has been concerning the fracture toughness and fatigue crack propagation rate
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 6 6 7 - 5
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
63
Table 1 Chemical composition of 2024 aluminum alloys (mass percent)
Table 3 Mechanical properties of 2024-T3 aluminum alloys
Alloys
Si
Fe
Cu
Mn
Mg
Al
Alloys
Ftu (MPa)
Fty (MPa)
e (%)
KQ (MPa m1/2)
High purity (H) Low purity (L)
0.03 0.08
0.05 0.20
4.0 4.5
0.55 0.55
1.5 1.5
bal. bal.
High purity (H) Low purity (L)
435
310
25
134
460
320
20
112
Table 2 The morphology of constituents and dispersoids in 2024 aluminum alloya Alloys
Constituents
High purity (H) Low purity (L) a
Dispersoids
Vf (%)
D (mm)
s (mm)
s (A, )
0.46 2.0
1.8 2.6
140 75
7000 5300
Vf, volume fraction; D, diameter; s, spacing.
in 2024 aluminum alloys, keeping new aspect of developing the materials in light of the above mentioned.
2. Experimental Table 1 shows the two chemical compositions of specimens, high purity (H) and low purity (L). These
alloy ingots were homogenized at 480°C for 24 h, hot and cold rolled to 5.1-mm thick sheets, solution treated in salt bath at 535°C for 35 min, quenched in cold water, and then stretched around 2% to the aged T3 conditions. Tensile tests were carried out at a strain rate of 1.6 ×10 − 3 s − 1 in long transverse (LT) direction. The fracture toughness testing was in accordance with ASTM-E651 and ASTM-B646 using 400 mm wide panel in the T–L direction. Fatigue crack propagation testing was in accordance with ASTM-E647 using center notched tensile specimens (CCT) in the T–L direction. Tensile yield strength for these two alloys were almost the same as shown in Table 2, and it is assumed that fracture toughness is primarily affected by microstructures. Fig. 1 shows the constituents of high purity (H) and low purity (L). The major constituent was Cu2FeAl7 the spacing was 75 and 140 mm, respec-
Fig. 1. Optical micrographs of constituents in high purity (H) and low purity (L) 2024 aluminum alloys.
Fig. 2. Transmission electron micrographs of dispersoids in high purity (H) and low purity (L) 2024 aluminum alloys.
64
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
Fig. 5. Schematic illustrations of fracture modes for 2024 aluminum alloys with high purity (H), wide spacing of constituents and low purity (L), narrow spacing of constituents.
Fig. 3. Fracture toughness (KQ) versus average spacing (s) of constituents for 2024 aluminum aloys with high purity (H) and low purity (L). The data of Thompson et al. [8] is also plotted.
tively. Fig. 2 shows these two alloys. The major dispersoid was Cu2Mn3Al20. The dispersoid spacing was about 7000 and 5300 A, , respectively.
values in Table 3. Value of 20 and 19 MPa m − 1/2 were obtained, respectively, indicating almost equal. Therefore, it was suggested that fracture toughness is not merely calculated using Eq. (1) with the microstructural parameters, Vf and D. The fracture toughness of high purity (H) and low purity (L) alloys were plotted against the square root of the constituent spacing as shown in Fig. 3 [6] after the microstructural and fractographical considerations as mentioned later. There is a linear relationship between the fracture toughness and the square root of the constituent spacing of the materials. Firrao et al. [7] have proposed the following equation for the relationship between fracture toughness KIC and the particles for steels. KIC = [sY · o*f · E · f(n)]1/2s 1/2
3. Result and discussion
3.1. Effect of constituents and dispersoids morphologies on fracture toughness Table 3 shows that fracture toughness KQ of high purity (H) was higher than that of low purity (L) by about 20%. In order to identify the microstructural parameters, fracture toughness KIC of high purity (H) and low purity (L) were calculated by Eq. (1) and the
(2)
where o *f is the maximum strain acting at the crack tip, n is the strain-hardening exponent, and s is the average non-metallic inclusion spacing in the matrix. Eq. (2) assumed that crack tip blunts, up to achieving a finite radius, which is of the same order of magnitude as the inclusions spacing, s, and indicates KIC increases in proportion to the square root of s. Fig. 3 shows that Eq. (2) can be applied to 2024 aluminum alloys, too. In order to clarify the reason why fracture toughness increases with an increase in constituents spacing, frac-
Fig. 4. Fractographs of specimens after fracture toughness test for 2024 aluminum alloys with high purity (H) and low purity (L).
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
ture surfaces were investigated. Fig. 4 shows scanning electron microscope (SEM) fractographs of specimens after fracture toughness testing of high purity (H) and low purity (L) alloys. All show equiaxed dimples initiating from cracked constituents spacing, therefore, the fracture-nucleation site spacing in high purity (H) is wider than in low purity (L). Fig. 5 shows schematic illustrations of fracture modes for high purity (H) and
65
low purity (L) alloys, respectively. In high purity (H), the dimple linkage is delayed with an increase in fracture-nucleation site spacing. As a result, dimples are likely to increase in size. The above description suggests that widened constituent spacing delays dimple growth and linkage and absorbs much more deformation energy before rupture, resulting in high fracture toughness in the materials. These have been reconfirmed by frac-
Fig. 6. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with wide constituents and dispersoids, H, and narrow constituents and dispersoids, L.
Fig. 7. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with various constituents and dispersoids.
66
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
Fig. 8. Fractographs of specimens after fatigue crack propagation test for 2024 aluminum alloys with wide (H) and narrow (L) spacing of dispersoids.
Fig. 9. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with Mn-(Cu2Mn3Al20), Cr-(Cr2Mg3Al15), and Zr-(ZrAl3) bearing dispersoids, respectively.
tographical observations. Even when the dispersoid spacing is varied from 6400 to 4500 A, with the same spacing of constituents, 140 mm [6] as shown in Fig. 3, fracture toughness does not change. This shows that dispersoid spacing has little or no effect on fracture toughness.
3.2. Effect of constituents and dispersoids morphologies on fatigue crack propagation characteristics Fig. 6 shows fatigue crack propagation (da/dn) versus stress intensity factor range (DK) curves for high purity (H) and low purity (L) alloys. In the DK range from 5 to 30 MPa m1/2 or higher, the propagation rate of high purity (H) with wide constituent and dispersoid spacing decreased considerably, about half of that of low purity (L). Fig. 7 shows da/dn – DK curves for 2024
aluminum alloys with variant constituents and dispersoid spacing [6]. At high DK level, 15 MPa m1/2 or higher, the propagation rate of wide constituents spacing alloy (140 mm) was about half of that of the narrow constituents spacing (58 mm). The fatigue crack propagation rate of two alloys with dispersoid spacing of 4500 and 6400 A, is nearly the same, so dipersoid spacing has no effect on the propagation rate. Whereas, at a low DK level equal to 15 MPa m1/2 or lower, for the wide spacing of dispersoid alloy, 6400 A, , the fatigue crack propagation rate decreased further. Therefore, the decrease of propagation rate of high purity (H) in the DK range from low to high level results from the widened spacing of both constituents and dispersoids. At a high DK level, the propagation rate decreased as constituent spacing was widened. Moreover, Staley [5] stated that the fatigue crack propagation characteristics
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
could be increased by increasing fracture toughness. The microstructural parameter at a high DK level, therefore, is constituent spacing as in the case of fracture toughness. At a low DK level, the propagation rate decreased as dispersoid spacing was broadened. Fig. 8 shows the striation in high purity (H) and low purity (L) alloys. The striation width in low purity (L) with dispersoid spacing as narrow as 5300 A, is nearly smooth. On the other hand, high purity (H) with
Fig. 10. Effect of stress ratio on fatigue crack propagation rate (da/dn) in the range of Paris’ region, DK= 7 MPa m1/2, for 2024 type aluminum alloys with Cu2Mn3Al20, Cr2Mg3Al18, ZrAl3 dispersoids, respectively.
67
dispersoid spacing as wide as 6400 A, showed a large number of small cracks. At the tip end of these cracks, the second phase particles were observed and showed the presence of Al, Mn, and Cu elements. Furthermore, the particle size is nearly the same as that of dispersoids shown in Fig. 2. So, the alloys with the second phase particles at the tip end of these bends are Cu2Mn3Al20 dispersoids. In the alloys with widened dispersoid spacing, the fatigue crack propagation rate was lower. Moreover, striation bending at these dispersoids was observed in the fracture surface. Therefore, the lower fatigue propagation rate at a low DK level can be attributed to the widened dispersoid spacing. The microstructural parameter determining the fatigue crack propagation rate at a low DK level is the dispersoid spacing as mentioned above. Finally two additional experiments were done to understand the effect of dispersoids further. The first, 2024 aluminum alloys containing Cr-dispersoids, Cr2Mg3Al18, and Zr-dispersoids replacing Cu2Mn3Al20 dispersoids. The sizes were 350 and 800 A, , respectively. In order words, these dispersoids bearing materials have narrower dispersoid spacing than Mn-dispersoid materials. Fig. 9 shows da/dn –DK curves for 2024 aluminum alloys containing these three different dispersoids, respectively. The fatigue crack propagation rate of Mn-dispersoid materials was lower about half or one-third of that of Cr-, Zr-dispersoid materials, respectively. These results coincide with that of Fig. 7. The second, the fatigue crack propagation test under different stress ratios (R= smin/smax) were carried out. Fig. 10 shows the crack propagation rates in the range Paris’ region, DK = 7 MPa m1/2, in the three different dispersoid bearing materials under R= + 0.10 and +0.33, respectively. The effect of dispersoid size on crack propagation rate decreased with an increase of stress ratio. Suresh [9] reported that the bridging effect of particles tend to delay the fatigue crack propagation rate. These effect suggests that the delay will become smaller when the crack opens under the higher stress ratios such as R= +0.33 as shown in Fig. 11. Indication of the figure meets the experiment as shown in Fig. 10. It is concluded that the delay of fatigue crack propagation rate in a low DK level is due to the bridging effect of larger dispersoids.
4. Conclusions
Fig. 11. Schematic illustrations of the effect of dispersoid size on fatigue crack closure (bridging effect). This effect becomes smaller with an increase of stress ratio.
In the light of interrelationship among alloy/process microstructure (mainly the second phase particles) properties combined with micromechanics, 2024 series aluminum alloy sheets have been investigated. The fracture toughness is proportional to the square root
68
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
of the spacing of constituents, Cu2FeAl7. By broadening the spacing from 75 to 140 mm, the fracture toughness increases by 20%. In a low DK region, the fatigue crack propagates much slowly by broadening the spacing of dispersoids, Cu2Mn3Al20. By broadening the spacing, the crack propagation rate decreases by 50%. The larger size of dispersoids, Cu2Mn3Al20, makes the rate much slower than smaller ones, Cr2Mg3Al18 or ZrAl3. On the other hand, in a high DK region, the rate becomes slower, mainly by broadening the constituents spacing. The newly developed 2×24 aluminum alloy sheets, keeping these concepts in mind have excellent fracture toughness and fatigue crack propagation characteristics to provide lower weight, higher damage tolerance, and longer-term durability for aerospace applications.
References [1] M. Hyatt, R. Caton, D. Lovell, Proc. Of AIAA/AHS/Aircraft Design, Systems and Operations Conference paper No. AIAA 89-2127 (1989), AIAA. [2] E.A. Starke, J.T. Staley, Prog. Aerospace Sci. 32 (1995) 131. [3] M. Speidel, Sixth International Light Metals Congress, Leoven, Vienna, 1975, pp. 67 – 68. [4] J.R. Rice, M.A. Johonson, Inelastic Behaviour of Solid, 1970, p. 641. [5] T. Staley, W.G. Truckner, R.J. Bucci, A.B. Thakker, Aluminum 53 (1977) 667. [6] M. Nakai, T. Eto, J. Jpn. Inst. Light Met. 45 (1995) 677. [7] D. Firrao, R. Roberti, in: J.M. Wells, J.B. Landers (Eds.), Proceedings of the 113th AIME Conference, AIME, 1985, pp. 165–177. [8] D.F. Thompson, S.A. Levy, Symposium on Aluminum Alloy for Aircraft Industry, 1985, p. 165. [9] S. Suresh, R.O. Ritchie, Int. Met. Rev. 29 (1984) 455.
.
New aspects of development of high strength aluminum alloys for aerospace applications Manabu Nakai, Takehiko Eto * Al and Cu Research Section, Technical Department, Al and Cu Di6ision, Kobe Steel Ltd., 1 -5 -5 Takatsukadai, Nishi-ku, Kobe 651 -2271, Japan
Abstract New aspect of developing the materials in lights of interrelationship among alloy/process microstructure (mainly the second phase particles) — properties combined with micromechanics has been carried out in 2024 aluminum alloys. The fracture toughness is proportional to the square root of the spacing of constituents, Cu2FeAl7, in 2024 aluminum alloys. Broadening the spacing 75–140 mm, the fracture toughness increases by 20%. In a low DK region, the fatigue cracks propagate much slowly by broadening the spacing of dispersoids, Cu2Mn3Al20. Broadening the spacing, the crack propagation rate decreases by 50%. The larger size of dispersoids, Cu2Mn3Al20, makes the rate much slower than smaller ones, Cr2Mg3Al18 or ZrAl3. On the other hand, in a high DK region, the rate becomes slower mainly by broadening the constituents spacing. The newly developed 2 ×24 aluminum alloy sheets, in these concepts in mind, have excellent fracture toughness and fatigue crack propagation characteristics to provide lower weight, higher damage tolerance, and longer-term durability for aerospace applications. © 2000 Elsevier Science S.A. All rights reserved. Keywords: 2024 Aluminum alloy; Constituents; Dispersoids; Fracture toughness; Fatigue crack propagation rate; Damage tolerance
1. Introduction Advanced aluminum alloys for aerospace application have been required to allow high fracture toughness, higher fatigue performance, high formability, and superplasticity to meet the needs for lower structural weight, higher damage tolerance, and higher durability [1,2]. Conventional approach was merely based on relation between alloy/process-properties. In 2024 aluminum alloys, e.g. it is well known that decreasing the volume fraction of constituents such as Cu2FeAl7 increases fracture toughness of the materials [3]. Rich et al. [4] have proposed the following equation for the relationship among fracture toughness, KIC, and constituents volume fraction, Vf, and mean diameter of constituents, D.
KIC = 2sY · E
n p 6
1/3
1/2
D
* Corresponding author.
1/6 V− f
(1)
where sY is the yield strength and E is Young’s modulus. Constituents are distributed from fine as 0.1 mm or less as coarse as 10 mm or over. Vf and D are no more than the microstructural parameters including the average morphology of constituents. Thus, they cannot constitute the macrostructural parameters determining fracture toughness. With respect to fatigue crack propagation characteristics, only a few researches are available in which the mutual effect of constituents and dispersoids are separated. Of these, Staley et al. [5] reported that fatigue crack propagation rate decreases as fracture toughness increases. However, the microstructural parameter subjected to the investigation is volume fraction of constituent only. The authors have studied microstructural parameters determining toughness and fatigue crack propagation characteristics in 2024 aluminum alloys [6]. The object of the present paper was to clarify the interrelationship among alloy/process-microstructure (mainly the second phase particles) — properties combined with micromechanics has been concerning the fracture toughness and fatigue crack propagation rate
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 6 6 7 - 5
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
63
Table 1 Chemical composition of 2024 aluminum alloys (mass percent)
Table 3 Mechanical properties of 2024-T3 aluminum alloys
Alloys
Si
Fe
Cu
Mn
Mg
Al
Alloys
Ftu (MPa)
Fty (MPa)
e (%)
KQ (MPa m1/2)
High purity (H) Low purity (L)
0.03 0.08
0.05 0.20
4.0 4.5
0.55 0.55
1.5 1.5
bal. bal.
High purity (H) Low purity (L)
435
310
25
134
460
320
20
112
Table 2 The morphology of constituents and dispersoids in 2024 aluminum alloya Alloys
Constituents
High purity (H) Low purity (L) a
Dispersoids
Vf (%)
D (mm)
s (mm)
s (A, )
0.46 2.0
1.8 2.6
140 75
7000 5300
Vf, volume fraction; D, diameter; s, spacing.
in 2024 aluminum alloys, keeping new aspect of developing the materials in light of the above mentioned.
2. Experimental Table 1 shows the two chemical compositions of specimens, high purity (H) and low purity (L). These
alloy ingots were homogenized at 480°C for 24 h, hot and cold rolled to 5.1-mm thick sheets, solution treated in salt bath at 535°C for 35 min, quenched in cold water, and then stretched around 2% to the aged T3 conditions. Tensile tests were carried out at a strain rate of 1.6 ×10 − 3 s − 1 in long transverse (LT) direction. The fracture toughness testing was in accordance with ASTM-E651 and ASTM-B646 using 400 mm wide panel in the T–L direction. Fatigue crack propagation testing was in accordance with ASTM-E647 using center notched tensile specimens (CCT) in the T–L direction. Tensile yield strength for these two alloys were almost the same as shown in Table 2, and it is assumed that fracture toughness is primarily affected by microstructures. Fig. 1 shows the constituents of high purity (H) and low purity (L). The major constituent was Cu2FeAl7 the spacing was 75 and 140 mm, respec-
Fig. 1. Optical micrographs of constituents in high purity (H) and low purity (L) 2024 aluminum alloys.
Fig. 2. Transmission electron micrographs of dispersoids in high purity (H) and low purity (L) 2024 aluminum alloys.
64
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
Fig. 5. Schematic illustrations of fracture modes for 2024 aluminum alloys with high purity (H), wide spacing of constituents and low purity (L), narrow spacing of constituents.
Fig. 3. Fracture toughness (KQ) versus average spacing (s) of constituents for 2024 aluminum aloys with high purity (H) and low purity (L). The data of Thompson et al. [8] is also plotted.
tively. Fig. 2 shows these two alloys. The major dispersoid was Cu2Mn3Al20. The dispersoid spacing was about 7000 and 5300 A, , respectively.
values in Table 3. Value of 20 and 19 MPa m − 1/2 were obtained, respectively, indicating almost equal. Therefore, it was suggested that fracture toughness is not merely calculated using Eq. (1) with the microstructural parameters, Vf and D. The fracture toughness of high purity (H) and low purity (L) alloys were plotted against the square root of the constituent spacing as shown in Fig. 3 [6] after the microstructural and fractographical considerations as mentioned later. There is a linear relationship between the fracture toughness and the square root of the constituent spacing of the materials. Firrao et al. [7] have proposed the following equation for the relationship between fracture toughness KIC and the particles for steels. KIC = [sY · o*f · E · f(n)]1/2s 1/2
3. Result and discussion
3.1. Effect of constituents and dispersoids morphologies on fracture toughness Table 3 shows that fracture toughness KQ of high purity (H) was higher than that of low purity (L) by about 20%. In order to identify the microstructural parameters, fracture toughness KIC of high purity (H) and low purity (L) were calculated by Eq. (1) and the
(2)
where o *f is the maximum strain acting at the crack tip, n is the strain-hardening exponent, and s is the average non-metallic inclusion spacing in the matrix. Eq. (2) assumed that crack tip blunts, up to achieving a finite radius, which is of the same order of magnitude as the inclusions spacing, s, and indicates KIC increases in proportion to the square root of s. Fig. 3 shows that Eq. (2) can be applied to 2024 aluminum alloys, too. In order to clarify the reason why fracture toughness increases with an increase in constituents spacing, frac-
Fig. 4. Fractographs of specimens after fracture toughness test for 2024 aluminum alloys with high purity (H) and low purity (L).
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
ture surfaces were investigated. Fig. 4 shows scanning electron microscope (SEM) fractographs of specimens after fracture toughness testing of high purity (H) and low purity (L) alloys. All show equiaxed dimples initiating from cracked constituents spacing, therefore, the fracture-nucleation site spacing in high purity (H) is wider than in low purity (L). Fig. 5 shows schematic illustrations of fracture modes for high purity (H) and
65
low purity (L) alloys, respectively. In high purity (H), the dimple linkage is delayed with an increase in fracture-nucleation site spacing. As a result, dimples are likely to increase in size. The above description suggests that widened constituent spacing delays dimple growth and linkage and absorbs much more deformation energy before rupture, resulting in high fracture toughness in the materials. These have been reconfirmed by frac-
Fig. 6. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with wide constituents and dispersoids, H, and narrow constituents and dispersoids, L.
Fig. 7. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with various constituents and dispersoids.
66
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
Fig. 8. Fractographs of specimens after fatigue crack propagation test for 2024 aluminum alloys with wide (H) and narrow (L) spacing of dispersoids.
Fig. 9. Fatigue crack propagation rate (da/dn) versus stress intensity factor range (DK) for 2024 aluminum alloys with Mn-(Cu2Mn3Al20), Cr-(Cr2Mg3Al15), and Zr-(ZrAl3) bearing dispersoids, respectively.
tographical observations. Even when the dispersoid spacing is varied from 6400 to 4500 A, with the same spacing of constituents, 140 mm [6] as shown in Fig. 3, fracture toughness does not change. This shows that dispersoid spacing has little or no effect on fracture toughness.
3.2. Effect of constituents and dispersoids morphologies on fatigue crack propagation characteristics Fig. 6 shows fatigue crack propagation (da/dn) versus stress intensity factor range (DK) curves for high purity (H) and low purity (L) alloys. In the DK range from 5 to 30 MPa m1/2 or higher, the propagation rate of high purity (H) with wide constituent and dispersoid spacing decreased considerably, about half of that of low purity (L). Fig. 7 shows da/dn – DK curves for 2024
aluminum alloys with variant constituents and dispersoid spacing [6]. At high DK level, 15 MPa m1/2 or higher, the propagation rate of wide constituents spacing alloy (140 mm) was about half of that of the narrow constituents spacing (58 mm). The fatigue crack propagation rate of two alloys with dispersoid spacing of 4500 and 6400 A, is nearly the same, so dipersoid spacing has no effect on the propagation rate. Whereas, at a low DK level equal to 15 MPa m1/2 or lower, for the wide spacing of dispersoid alloy, 6400 A, , the fatigue crack propagation rate decreased further. Therefore, the decrease of propagation rate of high purity (H) in the DK range from low to high level results from the widened spacing of both constituents and dispersoids. At a high DK level, the propagation rate decreased as constituent spacing was widened. Moreover, Staley [5] stated that the fatigue crack propagation characteristics
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
could be increased by increasing fracture toughness. The microstructural parameter at a high DK level, therefore, is constituent spacing as in the case of fracture toughness. At a low DK level, the propagation rate decreased as dispersoid spacing was broadened. Fig. 8 shows the striation in high purity (H) and low purity (L) alloys. The striation width in low purity (L) with dispersoid spacing as narrow as 5300 A, is nearly smooth. On the other hand, high purity (H) with
Fig. 10. Effect of stress ratio on fatigue crack propagation rate (da/dn) in the range of Paris’ region, DK= 7 MPa m1/2, for 2024 type aluminum alloys with Cu2Mn3Al20, Cr2Mg3Al18, ZrAl3 dispersoids, respectively.
67
dispersoid spacing as wide as 6400 A, showed a large number of small cracks. At the tip end of these cracks, the second phase particles were observed and showed the presence of Al, Mn, and Cu elements. Furthermore, the particle size is nearly the same as that of dispersoids shown in Fig. 2. So, the alloys with the second phase particles at the tip end of these bends are Cu2Mn3Al20 dispersoids. In the alloys with widened dispersoid spacing, the fatigue crack propagation rate was lower. Moreover, striation bending at these dispersoids was observed in the fracture surface. Therefore, the lower fatigue propagation rate at a low DK level can be attributed to the widened dispersoid spacing. The microstructural parameter determining the fatigue crack propagation rate at a low DK level is the dispersoid spacing as mentioned above. Finally two additional experiments were done to understand the effect of dispersoids further. The first, 2024 aluminum alloys containing Cr-dispersoids, Cr2Mg3Al18, and Zr-dispersoids replacing Cu2Mn3Al20 dispersoids. The sizes were 350 and 800 A, , respectively. In order words, these dispersoids bearing materials have narrower dispersoid spacing than Mn-dispersoid materials. Fig. 9 shows da/dn –DK curves for 2024 aluminum alloys containing these three different dispersoids, respectively. The fatigue crack propagation rate of Mn-dispersoid materials was lower about half or one-third of that of Cr-, Zr-dispersoid materials, respectively. These results coincide with that of Fig. 7. The second, the fatigue crack propagation test under different stress ratios (R= smin/smax) were carried out. Fig. 10 shows the crack propagation rates in the range Paris’ region, DK = 7 MPa m1/2, in the three different dispersoid bearing materials under R= + 0.10 and +0.33, respectively. The effect of dispersoid size on crack propagation rate decreased with an increase of stress ratio. Suresh [9] reported that the bridging effect of particles tend to delay the fatigue crack propagation rate. These effect suggests that the delay will become smaller when the crack opens under the higher stress ratios such as R= +0.33 as shown in Fig. 11. Indication of the figure meets the experiment as shown in Fig. 10. It is concluded that the delay of fatigue crack propagation rate in a low DK level is due to the bridging effect of larger dispersoids.
4. Conclusions
Fig. 11. Schematic illustrations of the effect of dispersoid size on fatigue crack closure (bridging effect). This effect becomes smaller with an increase of stress ratio.
In the light of interrelationship among alloy/process microstructure (mainly the second phase particles) properties combined with micromechanics, 2024 series aluminum alloy sheets have been investigated. The fracture toughness is proportional to the square root
68
M. Nakai, T. Eto / Materials Science and Engineering A285 (2000) 62–68
of the spacing of constituents, Cu2FeAl7. By broadening the spacing from 75 to 140 mm, the fracture toughness increases by 20%. In a low DK region, the fatigue crack propagates much slowly by broadening the spacing of dispersoids, Cu2Mn3Al20. By broadening the spacing, the crack propagation rate decreases by 50%. The larger size of dispersoids, Cu2Mn3Al20, makes the rate much slower than smaller ones, Cr2Mg3Al18 or ZrAl3. On the other hand, in a high DK region, the rate becomes slower, mainly by broadening the constituents spacing. The newly developed 2×24 aluminum alloy sheets, keeping these concepts in mind have excellent fracture toughness and fatigue crack propagation characteristics to provide lower weight, higher damage tolerance, and longer-term durability for aerospace applications.
References [1] M. Hyatt, R. Caton, D. Lovell, Proc. Of AIAA/AHS/Aircraft Design, Systems and Operations Conference paper No. AIAA 89-2127 (1989), AIAA. [2] E.A. Starke, J.T. Staley, Prog. Aerospace Sci. 32 (1995) 131. [3] M. Speidel, Sixth International Light Metals Congress, Leoven, Vienna, 1975, pp. 67 – 68. [4] J.R. Rice, M.A. Johonson, Inelastic Behaviour of Solid, 1970, p. 641. [5] T. Staley, W.G. Truckner, R.J. Bucci, A.B. Thakker, Aluminum 53 (1977) 667. [6] M. Nakai, T. Eto, J. Jpn. Inst. Light Met. 45 (1995) 677. [7] D. Firrao, R. Roberti, in: J.M. Wells, J.B. Landers (Eds.), Proceedings of the 113th AIME Conference, AIME, 1985, pp. 165–177. [8] D.F. Thompson, S.A. Levy, Symposium on Aluminum Alloy for Aircraft Industry, 1985, p. 165. [9] S. Suresh, R.O. Ritchie, Int. Met. Rev. 29 (1984) 455.
.