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Effects of heating rate during solid-solution treatment on microstructure and fatigue properties of AA2524 T3 Al–Cu–Mg sheet
Materials and Design 104 (2016) 116–125
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Effects of heating rate during solid-solution treatment on microstructure and fatigue properties of AA2524 T3 Al–Cu–Mg sheet Fanghua Shen a,c, Bin Wang a,⁎, Danqing Yi a,b,c,⁎⁎, Huiqun Liu a, Cong Tang a, Wenbin Shou a a b c
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China Light Alloy Research Institute, Central South University, Changsha, Hunan 410083, China National Collaborative Innovation Center of Advanced Nonferrous Structural Materials and Manufacturing, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 5 March 2016 Received in revised form 21 April 2016 Accepted 2 May 2016 Available online 03 May 2016 Keywords: Aluminum alloy Heating rates Recrystallization mechanisms Fatigue crack growth Crack closure
a b s t r a c t T3 AA2524 (Al–Cu–Mg) aluminum alloy sheets were fabricated using two different heating rates (air furnace and salt bath) during the solid-solution treatment. The corresponding microstructure, recrystallization mechanisms, and fatigue behavior were investigated by means of transmission electron microscopy, scanning electron microscopy, electron back scattering diffraction, and optical microscopy. The results showed that the heating rate had a significant effect on the microstructure, owing to the different recrystallization mechanisms induced, namely, strain-induced boundary migration and particle-stimulated nucleation. Elongated fibrous grains (approximately 75 μm) with Cube, Goss, and S orientations were obtained for low heating rate, while fine equiaxed grains structure (approximately 13 μm) with a P-type orientation were obtained for high heating rate. The rate of fatigue crack growth in the case of former was lower than that in the latter during the stable stage. The effects of crack closure, induced by the large grain size and a high Schmid factor, would be responsible for such difference in fatigue property. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Al–Cu–Mg aluminum alloy are used widely for aerospace structural components owing to their high damage tolerance, and outstanding strength and workability [1–4]. A new generation of 2524 T3 sheets have been used as the skin sheets in Boeing and Airbus aircraft since the 1990s [5]. However, the safety of aircraft is always threatened by fatigue failure. Further, with the growth in air transportation, increasingly stringent requirements have been to be met, including those related to cost economy, and weight-reduction. Therefore, efforts to improve the fatigue performance of Al–Cu–Mg alloys have been a topic of continuous interest [6–8]. Heat treatment is an important and practical way of controlling and adjusting the microstructure and properties of the heat-treatable metallic materials, which generally contains the holding temperature, and time two factors. Recently, the effects of the heating rate during annealing on the microstructure and properties have attracted more and more attentions [9–13]. For instance, a high heating rate results in rapid recrystallization and a fine-grained microstructure, which corresponds to an increasing yield strength and ultimate tensile strength [14–17]. Wang et al. have also found the similar conclusion in Al-Mg-Si-Cu T4 alloy with different heating rate during solution treatment [18]. ⁎ Corresponding author. ⁎⁎ Correspondence to: D. Yi, School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China. E-mail address: [email protected] (B. Wang).
http://dx.doi.org/10.1016/j.matdes.2016.05.002 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Solution is one of the most important strengthening heat treatments in precipitation-hardening aluminum alloys. The temperature and time during solution exert significant influence on the properties of alloy. For instance, Sadeler have found that the fatigue strength of 2014 T6 alloy can be enhanced by the increase of solution temperature [19]. Similar conclusion has also obtained in the fatigue life of Al0Cu-Mg T3 alloy [20]. Although the majority of studies have tended to focus on the effects of such on the microstructure and mechanical property of the alloys, yet, few studies have investigated the effects of the heating rate on the microstructure and fatigue properties of alloy sheets. The effects of heating rate during solution treatment on the microstructure and fatigue property of alloy have not been clear. The aim of the present work is to investigate the effects of heating rate during solid solution on the microstructure of 2524 T3 skin sheets, including grain size and texture components, and the behaviors of fatigue crack growth (FCG) in the sheets. The underlying mechanisms responsible for the observed effects are also discussed. 2. Experimental 2.1. Materials The cold-rolled AA2524 (Al-Cu-Mg) sheets used in this study had a thickness of 1.75 mm and a nominal composition of 4.2% Cu, 1.4% Mg, 0.56% Mn, 0.08% Fe, 0.06% Si, and Al balance (all in wt%). The coldrolled sheets were provided by Southwest Aluminum Co. Ltd. They
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directions. The tensile tests were performed on an Instron1342 testing machine at a cross-head speed of 2 mm/min. The FCG rate tests were performed on an MTS Landmark testing machine using so-called middle-tension (M(T)) specimens, which had dimensions of 300 × 100 × 2.5 mm3 (length × width × thickness, as shown in Fig. 2). These were taken from the T3 sheets along the longitudinaltransversal direction as per the ASTM E647 standard. A pre-crack with a central hole (R, the length of radius is 1 mm) and groove (6 × 0.2 mm2) was formed by electrical-discharge machining. Next, the surfaces of the specimens were polished. The FCG rate tests were conducted at room temperature in the laboratory. A sinusoidal load (stress control, maximum stress is 14.4 kN, minimum stress is 1.44 kN) with a frequency of 10 Hz was used at a stress ratio of 0.1. The length of the fatigue crack was measured with an optical microscopy (OM) system (±1 μm) attached to the tester. The Eq. 1 was used to determine the stress intensity factor [6], ΔK. Fig. 1. Microstructure of the cold-rolled 2524 sheets investigated in this study.
were not subjected to intermediate annealing, and had been shelved for over a year. The original structural characteristics of the cold-rolled sheets were such that they consisted of multiple layers of deformation bands, as shown in Fig. 1. Next, the cold-rolled sheets were subjected to a solid-solution treatment at 500 °C for 0.5 h at two different heating rates (using in air furnace and salt bath).This was followed by water quenching, pre-deformation(5–10%), and natural aging for 96 h. The heat- transfer medium used in salt bath was composed of 50% KNO3 and 50% NaNO2 (in wt%).
2.2. Tensile and fatigue crack growth rate tests Dog-bone-shaped specimens for the tensile testes were taken from the as-T3-treated sheet samples along the rolling and transverse
ΔK ¼
ΔP B
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi πα πα sec 2W 2
ð1Þ
where P is the load; B and w are the thickness and width of the sample being tested, and α = 2a/w (a is the half-crack length). 2.3. Microstructural analysis The microstructures of the specimens were observed under polarized light using an OM system after they had been polished and anodized in 2.5% HBF4 solution, at 18 V for 2 min. The secondary phase and precipitates were observed using a FEI Helios Nanolab 600i focused ion beam/scanning electron microscopy (FIB/SEM) system in the back scanning (BS) mode. The morphology of the fracture surfaces were observed by means of a SEM system (FEI QUANTA-200). Thin foil samples for transmission electron microscopy (TEM) analysis were prepared by twinjet polishing with an electrolyte solution consisting of 25% HNO3
Fig. 2. Schematic diagram and geometry of the M(T) specimens (dimensions in mm).
Fig. 3. Three-dimensional microstructures of the 2524 T3 sheet specimens: (a) treated in an air furnace) and (b) treated in a salt bath.
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and 75% methanol (volume fraction) at temperatures lower than −25 °C. The TEM observations were performed on a Tecnai G2 20 system at 200 kV. Standard metallographic techniques were used to prepare the specimens for the electron backscatter diffraction scans (EBSD). The specimens were ground and polished and subsequently electro-polished using a solution of 10% perchloric acid and 90% ethanol at a temperature of −30 °C. The electro-polishing was done at 18–22 V for 30–60 s using direct current power. The EBSD measurements were made on a FEI Helios Nanolab 600i FIB/SEM system with an accelerating voltage of 20 kV. The step size was 1–2 μm. The success rate for identifying the Kikuchi patterns was 90–95%. The structures and Schmid factors of the grains were analyzed using the software TSL OIM Analysis 5. 3. Results and discussion 3.1. Microstructure Fig. 4. Holding time during simulation as a function of the immediate temperature for various heating rates.
Fig. 3a and b show the microstructures of the two types of 2524 T3 sheets, namely, those were subjected to the solid-solution treatment in an air furnace and a salt bath, respectively. The sample that was treated in an air furnace exhibited a structure with coarse elongated
Fig. 5. Secondary phase and precipitates (a, b, c, and d) in cold rolled sheet and as-annealed cold rolled sheet (e: in air furnace; f: in salt bath, 500 °C/6 min).
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Table 1 Statistical result of phase in 2524 sheet sample (in Fig. 5). Sample
Fraction /%
Average size /μm
≤0.2 μm /%
0.2~0.5 μm /%
0.5~1 μm /%
1~2 μm /%
Cold rolled sheet, Fig. 5a Cold rolled sheet, Fig. 5d As-annealed, in air furnace, 500 °C/6 min, Fig. 5e As-annealed, in salt bath, 500 °C/6 min, Fig. 5f
3.1 7.2 5.1 2.1
1.9 0.14 0.1 0.09
19 5.6 1
53 2 1 0.5
17
78 93 95
grains structure (approximately 75 μm), while fine equiaxed grain structure (approximately 13 μm) were observed in the case of the sheets treated in the salt bath. A comparison of Fig. 3a and b reveals that the heating rate strongly affected the size and shape of the grains. A similar effect of heating rate on grain size and shape has been reported in AA3015, Al-Mg-Si-Mg [18], Al-Mn [21], and Al-Mn-Mg [22] alloys, which may be attributed to the so-called concurrent precipitation phenomenon on recrystallization behavior. Humphreys hold the view that there are two competing mechanisms in recrystallization of alloys, namely strain-induced boundary migration (SIBM) and particlestimulated nucleation (PSN) [15]. Therefore, the difference of microstructure in current paper might also be related to the above two recrystallization mechanisms, especially in the initial stage of recrystallization.
3.2. Heating rate and recrystallization mechanism Owing to the cold-rolled sheet samples were put in the furnace at the same temperature (500 °C), and the equal holding time were adopted (30 min), the situations of heat transfer on the sample in the initial stage of solution treated should be the main focus. Therefore, numerical heat-transfer simulations in the initial stage of solution treatments were performed. We assumed that radiative heat transfer was dominant in the case of the air furnace and convective heat transfer was dominant in the case of the salt bath. The convection coefficient in the case of the salt bath was considered to be approximately 425 W/(m2/K) [23].
Fig. 4 shows the simulated immediate temperature of the surface of a cold-rolled 2524 sheets as a function of the holding time in the initial stage of solid-solution treatment. It was observed that the time to reach the solution temperature (500 °C) using air furnace (approximately 160 s) was longer than that taken in the salt bath (approximately 6 s) in the initial stage of treatment. In addition, the time taken to reach the recrystallization temperature (approximately 0.6 Tm) in the salt bath (approximately 3 s) was much shorter than that in the air furnace (approximately 20 s). This meant there was more time for recovery in the case of the air furnace during the early stage of the solid-solution treatment. In general, the strain-induced boundary migration (SIBM) and particle-stimulated nucleation (PSN) are the two primary coexisting mechanisms responsible for recrystallization in alloys, which would result in continuous and discontinuous recrystallization respectively. The course of PSN is influenced by the coarse second phase and precipitated phase [15]. In order to elucidate the course of recrystallization in the cold-rolled sheets at various heating rates during the solid-solution treatments, SEM/BS observations were performed on the cold-rolled and as-annealed sheet samples. As shown in Figs. 5a, b, and c, the coarse secondary phases with an average size of 1.9 μm (see Table 1), including residual θ (Al2Cu) phase, S (Al2CuMg) phase, and impurity phase [20, 24], were distributed along the original grain boundaries, which generally would promote the recrystallization. Owing to the fact that an intermittent annealing process was not used, the fine precipitates of the cold-rolled 2524 sheets were mainly composed of the broken needlelike precipitated, formed by deformations, including θ (Al2Cu) phase, S
Fig. 6. TEM images of the (a) cold rolled 2524 sheet samples and the sheets (b) annealed in the air furnace (500 °C/6 min), (c) in the salt bath (500 °C/6 min), and (d) in the air furnace, 500 °C/30 min.
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Fig. 7. TEM images of the 2524 T3 alloy sheets: (a, and c) solid-solution treated in the air furnace and (b and d) treated in the salt bath.
(Al2CuMg) phase, and the so-called T-phase (Al20Cu2Mn3) [4,25]. Furthermore, these fine precipitates, which had an average size of 0.14 μm were distributed evenly within the original grains; this always retards recrystallization, owing to the so-called Zener-pinning force, as shown in Fig. 5c and d. In fact, the phases and grains were the two main interactional structural features of the 2524 alloy sheets. The relation between of the two features during the solid-solution is discussed below using both the high-resolution FIB/SEM and TEM, owing to some soluble precipitates induced by deformations and natural precipitation of equilibrium phase during storage period. First, in order to observe the evolution of the above-mentioned coarse residual secondary phase and precipitates in the alloy sheets, annealing experiments were performed at 500 °C for 6 min on the cold rolled 2524 sheets at two different heating rates. As shown in Fig. 5 and Table 1, the statistical volume fractions and average sizes of above-mentioned phases decreased with the annealing process, meaning that the total volume of coarse secondary phases and soluble precipitates (S and θ phases) decreased with the hold time in the annealing process. The corresponding bright-field TEM images are shown in Fig. 6. It can be observed that a mass of dislocations were concentrated within the sub-grains and near the rod-like T-phase [26] in the matrix of the cold rolled sheets. In addition, the grain boundaries of the cold rolled sheet annealed in the air furnace were pinned by the in-soluble T-
phase bars, as can be seen in Fig. 6b. However, the dislocations in the matrix of the sheet annealed in the salt bath, as shown in Fig. 6c, were annihilated rapidly, owing to the high heating rate. Fig. 6d shows an image of the cold rolled sheet annealed in the air furnace at 500 °C for 30 min, indicating that the migration of boundary is the main mechanism responsible for in the subsequent grain growth during the solidsolution treatment. Fig. 7 shows bright field images of the 2524 T3 sheets subjected to the solid-solution treatment at different heating rates. The T-phase bars distributed within the grains and on the grains boundaries. It shows the grains may growth by grain boundary migration. In addition, the T-phase bar can exert a pinning force on the grain boundaries, which would inhibit the excessive growth of grains. Furthermore, owing to its unshearable nature and dispersed distribution, the dislocations resulting from the pre-deformation process piled up near the T-phase bars. That is the reason why T phase can prevent the continuous fractures of 2524 T3 sheets [27]. Fig. 8 shows the maps of the orientation distribution function (ODF) for the 2524 T3 sheet samples, while Table. 2 list the corresponding intensities of the texture components. As can be seen, the Goss and Cube texture were the dominant in the case of the sample treated at a low heating rate (in the air furnace).Further, a certain degree of the deformation texture S orientation ({123} b 634N) was observed, which corresponds to the SIBM mechanism of recrystallization. However, the P-type ({110} (110)) texture was dominant in the case of sample treated in the
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Fig. 8. ODF maps of the 2524 T3 sheet treated in the air furnace (a) and salt bath (b);
Table 2 Intensities of the texture components of the 2524 T3 sheets treated at different heating rates. Component/Intensity
Cube
Goss
S
P
In the air furnace In the salt bath
2.85 2.53
4.31 1.16
3.52 1.24
1.49 4.53
salt bath. The P-type texture is, in general, related to the PSN mechanism of recrystallization, which is used widely for preparation of fine-grained materials in the industry [15].This meant that the PSN mechanism was the dominant recrystallization mechanism in the 2524 T3 sheets treated at a high heating rate in this study. Constant φ2 equals to 0°, 45°, and 60° respectively. According to above analysis, the evolution of the grains during solidsolution with different heating rates can be described by the schematic diagram, as shown in Fig. 9. In the early stage of the solid-solution treatment, the heating of sample in the air furnace results in an obvious
course of recovery. During this course, the coarse soluble secondary phases on the original grain boundaries and the precipitates inside dissolve in a step-by-step manner, owing to the increasing dissolvability. The dissolvability may be related to the saturation of matrix and the diffusion of elements through the vacancy or dislocations. These coarse residual phases, however, still exert significant pinning force on the subgrains. Subsequently, the nucleation of grains occurs, mainly because of the merging of neighboring sub-grains over time, as indicated in Fig. 9b. Finally, the grains grow up further with a decrease of the above-mentioned pinning force, as well as because of the migrations of the grain boundaries (see in Fig. 9c). However, in the case of the treatment that uses the salt bath, most of the precipitates dissolve rapidly, owing to the high heating rate and dissolvability. On the one hand, the coarse secondary phase particles presents at the original grain boundaries decrease in number and nucleate preferentially. On the other hand, owing to the rapid consumption of the deformation-stored energy and a decrease in the pinning force, the sub-grains within the original grain boundaries grow quickly to the nucleation related size required for recrystallization (see Fig. 9d). Finally, the grains grow
Fig. 9. Schematic diagram of the course of recrystallization during the solid-solution treatment of 2524 sheet samples in an air furnace (a, b, and c) and salt bath (a, d, and e).
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Fig. 10. Tensile properties of the AA2524 T3 sheet specimens annealed at different heating rates.
evenly, as shown in Fig. 9e. Therefore, in contrast to the case with a low heating rate (treatment in air furnace), a greater number of sites form during the early stage of recrystallization at a high heating rate. According to the equation given by Johnson-Mehl (Eq. 2) [28], the relation between the final average grain size dave, and the grain nucle_ as well as the grain growth rate, G, can be derived as ation rate, N, shown below in Eq. 3 [16,29] π _ 3 t4 Xr ¼ 1− exp − NG 3
ð2Þ
14 G dave ∝ _ N
ð3Þ
where Xr is the fraction of recrystallization and t is time. When the grain growth rate, G, is constant, the average size, dave, de_ As stated above, the sheet annealed creases with the nucleation rate N. at a high heating rate possessed a greater number of nucleation sites, _ As a result, a suggesting that it exhibited a higher nucleation rate, N. 2524 T3 sheet with smaller grains was obtained. 3.3. Tensile properties
3.5. Fatigue fracture analysis The fracture morphologies of the specimens were observed during the initial stage, the stable stage, and the final stage of the fatigue fracture extension process. In the initial stage, as shown in Fig. 12a, d, and g, cleavage facets were dominant. Fig. 12b, e, and h show the fracture morphology in the stable stage. It was observed that continuous striations were predominant in the fracture. The morphology of the striations in this stage was related to the grain size and the underlying mechanism of recrystallization. Serrated striations were observed in the as-annealed cold rolled 2524 sheet, as shown in Fig. 12b. Chain-like pits and short striations were present in the fine grains formed by PSN in the fracture of the sample solid -solution treated in the salt bath (see Fig. 12e). However, long and fine-grained striations (see Fig. 12h) were seen in the sheet treated at a low heating rate. In contrast, the spacing of the fatigue striations in the 2524 T3 sheet sample treated in the air furnace was smaller than that of the sample treated in the salt bath. The length of such striations is related to the grain size, owing to the effects of deflection and retardation by grain boundaries [24,31]. These often mean the former exhibited better fatigue resistance. Thus it can be seen that the FCG rate in the stable stage was affected significantly by the grain size. This was probably because of the hindrance and deflection caused by sub-grain boundaries and grain boundaries. Fig. 12c, f, and i show the dimples formed in the final stage of the fatigue fracture process. The size of dimples corresponded to the size of the grains in alloy matrix.
Fig. 10 shows the room-temperature tensile strength, yield strength, and elongation of the 2524 T3 aluminum alloy sample. The tensile tests reveal that the sample annealed at a high heating rate exhibit a higher tensile strength and yield strength, but a slightly lower elongation than did the sample annealed at a slow heating rate. The strength decreases with a decrease in heating rate, which was in accordance with the prediction of Hall-Petch relationship [30]. 3.4. Fatigue properties The curves of FCG rate (da/dN) as a function of the stress intensity factor (ΔK) for 2524 T3 sheets and as-annealed cold rolled sheet are shown in Fig. 11. In the initial stage, the as-annealed cold rolled sheet exhibited the lowest FCG rate. During the stable stage, the FCG rate of the T3 sheet solution treated in the air furnace was lower than that of sheet treated in the salt bath. When viewed in combination with the grain sizes shown in Fig. 3a and b, this result suggested that the formation of small or sub-grains decreased the FCG rate in the early stage of the fatigue crack extension process. Larger grains, however, increased the fatigue resistance in the stable stage. In order to elucidate the fatigue mechanism further, the fatigue fracture and the area near the fatigue crack path were observed and analyzed using SEM, OM, and EBSD.
Fig. 11. Fatigue crack growth (FCG) rate (da/dN) as a function of the stress intensity factor (ΔK).
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Fig. 12. Fatigue fractures in (a, b, and c) as-annealed cold rolling 2524 sheet and (d, e, and f) T3 sheet sample (treated in the salt bath), and (g, h, and i)T3 sheet samples (treated in the air furnace) in the initiate, stable, and final stage respectively.
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Fig. 13. OM and EBSD images of the area near the fatigue crack growth path for 2524 T3 sheets solid-solution treated at different heating rates: in a salt bath (a, b, c, and h) and (d, e, f, g, and i) in an air furnace; a, g: IPF map; b, f: derived-(111)b1−10N Schmid factor maps; c, d: OM images of the area near the fatigue crack growth path; e: image of the background; h, i: statistical charts of Schmid factors.
3.6. Fatigue crack path analysis The tortuosity of the crack path can often reflect the fatigue resistance in a sense, owing to the so-called crack closure [32,33]. Fig. 13 shows the microscopic features along the fatigue crack path in the case of two 2524 T3 sheet samples after 50,000 loading cycles using the low-magnification OM and EBSD. As shown in Figs. 13 c and d, the tortuosity of the fracture surface (in the initial stage) in the case of the 2524 T3 sample heated in the air furnace was markedly higher than that of the fracture surface of sample treated in the salt bath. In addition, Fig. 13a and b show the inverse pole-figure (IPF) maps and the corresponding Schmid factors of the fatigue crack tip in sable stage for the sample heated rapidly (in salt bath). Similarly, Fig. 13e–g shows the images of the background, Schimd factor, as well as the inverse pole-figure respectively for the area near the tip for the sample heated at a low rate (in the air furnace). It can be observed that the fatigue crack tends to extend along the grain
boundary (among the fine grains) in the path of inter-granular fracture, which showed some characteristics of brittle fracture, in keeping with previously reported results [4]. However, the fatigue crack propagated across the grains with high Schmid factors by means of a typical transgranular fracture in the case of the sample heated in an air furnace. Furthermore, significant deflections occurred, as denoted by the marks α1, α2, and α3 (see Fig. 13e), when the crack extended within grains and across the grain boundaries. Most of all, the effects of crack closure were visible in the initial stage (as highlighted by ellipses in Fig. 13d) and the stable stage of the fatigue crack extension process (as shown in Fig. 13f). The phenomenon of crack closure delaying fatigue crack propagation has been observed and reported in the previous studies as well [34,35]. Thus, the crack closure observed in this study may be related to the high Schmid factors and large sizes of the grains, as per the statistical chart of the Schmid factors (see in Fig. 13h and i), which would be responsible for the effects on the fatigue properties.
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4. Conclusions The effects of the heating rate during solid-solution treatment on the microstructure and fatigue behavior of T3 AA2524 Al–Cu–Mg T3 sheets were investigated. The main conclusions are as follows: 1) The fatigue properties of the 2524 T3 (Al–Cu–Mg) sheets were affected by the heating rate during the solid-solution treatment. The T3 sheet solution treated at a slow heating rate (in an air furnace) exhibited better resistance to fatigue crack propagation. 2) Different heating rates during the solid-solution treatment can result in two different recrystallization mechanisms, namely, PSN and SIBM, which affect the structural properties of the T3 sheets, including their grain size and texture components. The sheet treated at a low rate consisted of long fibrous grains with Goss, Cube, and S orientations. The sheet heated at a high rate consisted of equiaxed grains with the P-type orientation. 3) The effects of crack closure, induced by a large grain size and high Schmid factor, are probably responsible for the observed fatigue properties. Acknowledgments The authors are grateful for the support provided by the National Key Basic Research Program of China (973 Program, No. 2012CB619506) and the 2011 Program of the Ministry of Education in China. References [1] S.P. Ringer, T. Sakurai, I.J. Polmear, Acta Mater. 45 (1997) 3731–3744. [2] Y. Nagai, M. Murayama, Z. Tang, Acta Mater. 49 (2001) 913–920. [3] T. Dursun, C. Soutis, Mater. Des. 56 (2014) 862–871.
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Effects of heating rate during solid-solution treatment on microstructure and fatigue properties of AA2524 T3 Al–Cu–Mg sheet Fanghua Shen a,c, Bin Wang a,⁎, Danqing Yi a,b,c,⁎⁎, Huiqun Liu a, Cong Tang a, Wenbin Shou a a b c
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China Light Alloy Research Institute, Central South University, Changsha, Hunan 410083, China National Collaborative Innovation Center of Advanced Nonferrous Structural Materials and Manufacturing, Central South University, Changsha 410083, China
a r t i c l e
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Article history: Received 5 March 2016 Received in revised form 21 April 2016 Accepted 2 May 2016 Available online 03 May 2016 Keywords: Aluminum alloy Heating rates Recrystallization mechanisms Fatigue crack growth Crack closure
a b s t r a c t T3 AA2524 (Al–Cu–Mg) aluminum alloy sheets were fabricated using two different heating rates (air furnace and salt bath) during the solid-solution treatment. The corresponding microstructure, recrystallization mechanisms, and fatigue behavior were investigated by means of transmission electron microscopy, scanning electron microscopy, electron back scattering diffraction, and optical microscopy. The results showed that the heating rate had a significant effect on the microstructure, owing to the different recrystallization mechanisms induced, namely, strain-induced boundary migration and particle-stimulated nucleation. Elongated fibrous grains (approximately 75 μm) with Cube, Goss, and S orientations were obtained for low heating rate, while fine equiaxed grains structure (approximately 13 μm) with a P-type orientation were obtained for high heating rate. The rate of fatigue crack growth in the case of former was lower than that in the latter during the stable stage. The effects of crack closure, induced by the large grain size and a high Schmid factor, would be responsible for such difference in fatigue property. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Al–Cu–Mg aluminum alloy are used widely for aerospace structural components owing to their high damage tolerance, and outstanding strength and workability [1–4]. A new generation of 2524 T3 sheets have been used as the skin sheets in Boeing and Airbus aircraft since the 1990s [5]. However, the safety of aircraft is always threatened by fatigue failure. Further, with the growth in air transportation, increasingly stringent requirements have been to be met, including those related to cost economy, and weight-reduction. Therefore, efforts to improve the fatigue performance of Al–Cu–Mg alloys have been a topic of continuous interest [6–8]. Heat treatment is an important and practical way of controlling and adjusting the microstructure and properties of the heat-treatable metallic materials, which generally contains the holding temperature, and time two factors. Recently, the effects of the heating rate during annealing on the microstructure and properties have attracted more and more attentions [9–13]. For instance, a high heating rate results in rapid recrystallization and a fine-grained microstructure, which corresponds to an increasing yield strength and ultimate tensile strength [14–17]. Wang et al. have also found the similar conclusion in Al-Mg-Si-Cu T4 alloy with different heating rate during solution treatment [18]. ⁎ Corresponding author. ⁎⁎ Correspondence to: D. Yi, School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China. E-mail address: [email protected] (B. Wang).
http://dx.doi.org/10.1016/j.matdes.2016.05.002 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Solution is one of the most important strengthening heat treatments in precipitation-hardening aluminum alloys. The temperature and time during solution exert significant influence on the properties of alloy. For instance, Sadeler have found that the fatigue strength of 2014 T6 alloy can be enhanced by the increase of solution temperature [19]. Similar conclusion has also obtained in the fatigue life of Al0Cu-Mg T3 alloy [20]. Although the majority of studies have tended to focus on the effects of such on the microstructure and mechanical property of the alloys, yet, few studies have investigated the effects of the heating rate on the microstructure and fatigue properties of alloy sheets. The effects of heating rate during solution treatment on the microstructure and fatigue property of alloy have not been clear. The aim of the present work is to investigate the effects of heating rate during solid solution on the microstructure of 2524 T3 skin sheets, including grain size and texture components, and the behaviors of fatigue crack growth (FCG) in the sheets. The underlying mechanisms responsible for the observed effects are also discussed. 2. Experimental 2.1. Materials The cold-rolled AA2524 (Al-Cu-Mg) sheets used in this study had a thickness of 1.75 mm and a nominal composition of 4.2% Cu, 1.4% Mg, 0.56% Mn, 0.08% Fe, 0.06% Si, and Al balance (all in wt%). The coldrolled sheets were provided by Southwest Aluminum Co. Ltd. They
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directions. The tensile tests were performed on an Instron1342 testing machine at a cross-head speed of 2 mm/min. The FCG rate tests were performed on an MTS Landmark testing machine using so-called middle-tension (M(T)) specimens, which had dimensions of 300 × 100 × 2.5 mm3 (length × width × thickness, as shown in Fig. 2). These were taken from the T3 sheets along the longitudinaltransversal direction as per the ASTM E647 standard. A pre-crack with a central hole (R, the length of radius is 1 mm) and groove (6 × 0.2 mm2) was formed by electrical-discharge machining. Next, the surfaces of the specimens were polished. The FCG rate tests were conducted at room temperature in the laboratory. A sinusoidal load (stress control, maximum stress is 14.4 kN, minimum stress is 1.44 kN) with a frequency of 10 Hz was used at a stress ratio of 0.1. The length of the fatigue crack was measured with an optical microscopy (OM) system (±1 μm) attached to the tester. The Eq. 1 was used to determine the stress intensity factor [6], ΔK. Fig. 1. Microstructure of the cold-rolled 2524 sheets investigated in this study.
were not subjected to intermediate annealing, and had been shelved for over a year. The original structural characteristics of the cold-rolled sheets were such that they consisted of multiple layers of deformation bands, as shown in Fig. 1. Next, the cold-rolled sheets were subjected to a solid-solution treatment at 500 °C for 0.5 h at two different heating rates (using in air furnace and salt bath).This was followed by water quenching, pre-deformation(5–10%), and natural aging for 96 h. The heat- transfer medium used in salt bath was composed of 50% KNO3 and 50% NaNO2 (in wt%).
2.2. Tensile and fatigue crack growth rate tests Dog-bone-shaped specimens for the tensile testes were taken from the as-T3-treated sheet samples along the rolling and transverse
ΔK ¼
ΔP B
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi πα πα sec 2W 2
ð1Þ
where P is the load; B and w are the thickness and width of the sample being tested, and α = 2a/w (a is the half-crack length). 2.3. Microstructural analysis The microstructures of the specimens were observed under polarized light using an OM system after they had been polished and anodized in 2.5% HBF4 solution, at 18 V for 2 min. The secondary phase and precipitates were observed using a FEI Helios Nanolab 600i focused ion beam/scanning electron microscopy (FIB/SEM) system in the back scanning (BS) mode. The morphology of the fracture surfaces were observed by means of a SEM system (FEI QUANTA-200). Thin foil samples for transmission electron microscopy (TEM) analysis were prepared by twinjet polishing with an electrolyte solution consisting of 25% HNO3
Fig. 2. Schematic diagram and geometry of the M(T) specimens (dimensions in mm).
Fig. 3. Three-dimensional microstructures of the 2524 T3 sheet specimens: (a) treated in an air furnace) and (b) treated in a salt bath.
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and 75% methanol (volume fraction) at temperatures lower than −25 °C. The TEM observations were performed on a Tecnai G2 20 system at 200 kV. Standard metallographic techniques were used to prepare the specimens for the electron backscatter diffraction scans (EBSD). The specimens were ground and polished and subsequently electro-polished using a solution of 10% perchloric acid and 90% ethanol at a temperature of −30 °C. The electro-polishing was done at 18–22 V for 30–60 s using direct current power. The EBSD measurements were made on a FEI Helios Nanolab 600i FIB/SEM system with an accelerating voltage of 20 kV. The step size was 1–2 μm. The success rate for identifying the Kikuchi patterns was 90–95%. The structures and Schmid factors of the grains were analyzed using the software TSL OIM Analysis 5. 3. Results and discussion 3.1. Microstructure Fig. 4. Holding time during simulation as a function of the immediate temperature for various heating rates.
Fig. 3a and b show the microstructures of the two types of 2524 T3 sheets, namely, those were subjected to the solid-solution treatment in an air furnace and a salt bath, respectively. The sample that was treated in an air furnace exhibited a structure with coarse elongated
Fig. 5. Secondary phase and precipitates (a, b, c, and d) in cold rolled sheet and as-annealed cold rolled sheet (e: in air furnace; f: in salt bath, 500 °C/6 min).
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Table 1 Statistical result of phase in 2524 sheet sample (in Fig. 5). Sample
Fraction /%
Average size /μm
≤0.2 μm /%
0.2~0.5 μm /%
0.5~1 μm /%
1~2 μm /%
Cold rolled sheet, Fig. 5a Cold rolled sheet, Fig. 5d As-annealed, in air furnace, 500 °C/6 min, Fig. 5e As-annealed, in salt bath, 500 °C/6 min, Fig. 5f
3.1 7.2 5.1 2.1
1.9 0.14 0.1 0.09
19 5.6 1
53 2 1 0.5
17
78 93 95
grains structure (approximately 75 μm), while fine equiaxed grain structure (approximately 13 μm) were observed in the case of the sheets treated in the salt bath. A comparison of Fig. 3a and b reveals that the heating rate strongly affected the size and shape of the grains. A similar effect of heating rate on grain size and shape has been reported in AA3015, Al-Mg-Si-Mg [18], Al-Mn [21], and Al-Mn-Mg [22] alloys, which may be attributed to the so-called concurrent precipitation phenomenon on recrystallization behavior. Humphreys hold the view that there are two competing mechanisms in recrystallization of alloys, namely strain-induced boundary migration (SIBM) and particlestimulated nucleation (PSN) [15]. Therefore, the difference of microstructure in current paper might also be related to the above two recrystallization mechanisms, especially in the initial stage of recrystallization.
3.2. Heating rate and recrystallization mechanism Owing to the cold-rolled sheet samples were put in the furnace at the same temperature (500 °C), and the equal holding time were adopted (30 min), the situations of heat transfer on the sample in the initial stage of solution treated should be the main focus. Therefore, numerical heat-transfer simulations in the initial stage of solution treatments were performed. We assumed that radiative heat transfer was dominant in the case of the air furnace and convective heat transfer was dominant in the case of the salt bath. The convection coefficient in the case of the salt bath was considered to be approximately 425 W/(m2/K) [23].
Fig. 4 shows the simulated immediate temperature of the surface of a cold-rolled 2524 sheets as a function of the holding time in the initial stage of solid-solution treatment. It was observed that the time to reach the solution temperature (500 °C) using air furnace (approximately 160 s) was longer than that taken in the salt bath (approximately 6 s) in the initial stage of treatment. In addition, the time taken to reach the recrystallization temperature (approximately 0.6 Tm) in the salt bath (approximately 3 s) was much shorter than that in the air furnace (approximately 20 s). This meant there was more time for recovery in the case of the air furnace during the early stage of the solid-solution treatment. In general, the strain-induced boundary migration (SIBM) and particle-stimulated nucleation (PSN) are the two primary coexisting mechanisms responsible for recrystallization in alloys, which would result in continuous and discontinuous recrystallization respectively. The course of PSN is influenced by the coarse second phase and precipitated phase [15]. In order to elucidate the course of recrystallization in the cold-rolled sheets at various heating rates during the solid-solution treatments, SEM/BS observations were performed on the cold-rolled and as-annealed sheet samples. As shown in Figs. 5a, b, and c, the coarse secondary phases with an average size of 1.9 μm (see Table 1), including residual θ (Al2Cu) phase, S (Al2CuMg) phase, and impurity phase [20, 24], were distributed along the original grain boundaries, which generally would promote the recrystallization. Owing to the fact that an intermittent annealing process was not used, the fine precipitates of the cold-rolled 2524 sheets were mainly composed of the broken needlelike precipitated, formed by deformations, including θ (Al2Cu) phase, S
Fig. 6. TEM images of the (a) cold rolled 2524 sheet samples and the sheets (b) annealed in the air furnace (500 °C/6 min), (c) in the salt bath (500 °C/6 min), and (d) in the air furnace, 500 °C/30 min.
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Fig. 7. TEM images of the 2524 T3 alloy sheets: (a, and c) solid-solution treated in the air furnace and (b and d) treated in the salt bath.
(Al2CuMg) phase, and the so-called T-phase (Al20Cu2Mn3) [4,25]. Furthermore, these fine precipitates, which had an average size of 0.14 μm were distributed evenly within the original grains; this always retards recrystallization, owing to the so-called Zener-pinning force, as shown in Fig. 5c and d. In fact, the phases and grains were the two main interactional structural features of the 2524 alloy sheets. The relation between of the two features during the solid-solution is discussed below using both the high-resolution FIB/SEM and TEM, owing to some soluble precipitates induced by deformations and natural precipitation of equilibrium phase during storage period. First, in order to observe the evolution of the above-mentioned coarse residual secondary phase and precipitates in the alloy sheets, annealing experiments were performed at 500 °C for 6 min on the cold rolled 2524 sheets at two different heating rates. As shown in Fig. 5 and Table 1, the statistical volume fractions and average sizes of above-mentioned phases decreased with the annealing process, meaning that the total volume of coarse secondary phases and soluble precipitates (S and θ phases) decreased with the hold time in the annealing process. The corresponding bright-field TEM images are shown in Fig. 6. It can be observed that a mass of dislocations were concentrated within the sub-grains and near the rod-like T-phase [26] in the matrix of the cold rolled sheets. In addition, the grain boundaries of the cold rolled sheet annealed in the air furnace were pinned by the in-soluble T-
phase bars, as can be seen in Fig. 6b. However, the dislocations in the matrix of the sheet annealed in the salt bath, as shown in Fig. 6c, were annihilated rapidly, owing to the high heating rate. Fig. 6d shows an image of the cold rolled sheet annealed in the air furnace at 500 °C for 30 min, indicating that the migration of boundary is the main mechanism responsible for in the subsequent grain growth during the solidsolution treatment. Fig. 7 shows bright field images of the 2524 T3 sheets subjected to the solid-solution treatment at different heating rates. The T-phase bars distributed within the grains and on the grains boundaries. It shows the grains may growth by grain boundary migration. In addition, the T-phase bar can exert a pinning force on the grain boundaries, which would inhibit the excessive growth of grains. Furthermore, owing to its unshearable nature and dispersed distribution, the dislocations resulting from the pre-deformation process piled up near the T-phase bars. That is the reason why T phase can prevent the continuous fractures of 2524 T3 sheets [27]. Fig. 8 shows the maps of the orientation distribution function (ODF) for the 2524 T3 sheet samples, while Table. 2 list the corresponding intensities of the texture components. As can be seen, the Goss and Cube texture were the dominant in the case of the sample treated at a low heating rate (in the air furnace).Further, a certain degree of the deformation texture S orientation ({123} b 634N) was observed, which corresponds to the SIBM mechanism of recrystallization. However, the P-type ({110} (110)) texture was dominant in the case of sample treated in the
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Fig. 8. ODF maps of the 2524 T3 sheet treated in the air furnace (a) and salt bath (b);
Table 2 Intensities of the texture components of the 2524 T3 sheets treated at different heating rates. Component/Intensity
Cube
Goss
S
P
In the air furnace In the salt bath
2.85 2.53
4.31 1.16
3.52 1.24
1.49 4.53
salt bath. The P-type texture is, in general, related to the PSN mechanism of recrystallization, which is used widely for preparation of fine-grained materials in the industry [15].This meant that the PSN mechanism was the dominant recrystallization mechanism in the 2524 T3 sheets treated at a high heating rate in this study. Constant φ2 equals to 0°, 45°, and 60° respectively. According to above analysis, the evolution of the grains during solidsolution with different heating rates can be described by the schematic diagram, as shown in Fig. 9. In the early stage of the solid-solution treatment, the heating of sample in the air furnace results in an obvious
course of recovery. During this course, the coarse soluble secondary phases on the original grain boundaries and the precipitates inside dissolve in a step-by-step manner, owing to the increasing dissolvability. The dissolvability may be related to the saturation of matrix and the diffusion of elements through the vacancy or dislocations. These coarse residual phases, however, still exert significant pinning force on the subgrains. Subsequently, the nucleation of grains occurs, mainly because of the merging of neighboring sub-grains over time, as indicated in Fig. 9b. Finally, the grains grow up further with a decrease of the above-mentioned pinning force, as well as because of the migrations of the grain boundaries (see in Fig. 9c). However, in the case of the treatment that uses the salt bath, most of the precipitates dissolve rapidly, owing to the high heating rate and dissolvability. On the one hand, the coarse secondary phase particles presents at the original grain boundaries decrease in number and nucleate preferentially. On the other hand, owing to the rapid consumption of the deformation-stored energy and a decrease in the pinning force, the sub-grains within the original grain boundaries grow quickly to the nucleation related size required for recrystallization (see Fig. 9d). Finally, the grains grow
Fig. 9. Schematic diagram of the course of recrystallization during the solid-solution treatment of 2524 sheet samples in an air furnace (a, b, and c) and salt bath (a, d, and e).
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Fig. 10. Tensile properties of the AA2524 T3 sheet specimens annealed at different heating rates.
evenly, as shown in Fig. 9e. Therefore, in contrast to the case with a low heating rate (treatment in air furnace), a greater number of sites form during the early stage of recrystallization at a high heating rate. According to the equation given by Johnson-Mehl (Eq. 2) [28], the relation between the final average grain size dave, and the grain nucle_ as well as the grain growth rate, G, can be derived as ation rate, N, shown below in Eq. 3 [16,29] π _ 3 t4 Xr ¼ 1− exp − NG 3
ð2Þ
14 G dave ∝ _ N
ð3Þ
where Xr is the fraction of recrystallization and t is time. When the grain growth rate, G, is constant, the average size, dave, de_ As stated above, the sheet annealed creases with the nucleation rate N. at a high heating rate possessed a greater number of nucleation sites, _ As a result, a suggesting that it exhibited a higher nucleation rate, N. 2524 T3 sheet with smaller grains was obtained. 3.3. Tensile properties
3.5. Fatigue fracture analysis The fracture morphologies of the specimens were observed during the initial stage, the stable stage, and the final stage of the fatigue fracture extension process. In the initial stage, as shown in Fig. 12a, d, and g, cleavage facets were dominant. Fig. 12b, e, and h show the fracture morphology in the stable stage. It was observed that continuous striations were predominant in the fracture. The morphology of the striations in this stage was related to the grain size and the underlying mechanism of recrystallization. Serrated striations were observed in the as-annealed cold rolled 2524 sheet, as shown in Fig. 12b. Chain-like pits and short striations were present in the fine grains formed by PSN in the fracture of the sample solid -solution treated in the salt bath (see Fig. 12e). However, long and fine-grained striations (see Fig. 12h) were seen in the sheet treated at a low heating rate. In contrast, the spacing of the fatigue striations in the 2524 T3 sheet sample treated in the air furnace was smaller than that of the sample treated in the salt bath. The length of such striations is related to the grain size, owing to the effects of deflection and retardation by grain boundaries [24,31]. These often mean the former exhibited better fatigue resistance. Thus it can be seen that the FCG rate in the stable stage was affected significantly by the grain size. This was probably because of the hindrance and deflection caused by sub-grain boundaries and grain boundaries. Fig. 12c, f, and i show the dimples formed in the final stage of the fatigue fracture process. The size of dimples corresponded to the size of the grains in alloy matrix.
Fig. 10 shows the room-temperature tensile strength, yield strength, and elongation of the 2524 T3 aluminum alloy sample. The tensile tests reveal that the sample annealed at a high heating rate exhibit a higher tensile strength and yield strength, but a slightly lower elongation than did the sample annealed at a slow heating rate. The strength decreases with a decrease in heating rate, which was in accordance with the prediction of Hall-Petch relationship [30]. 3.4. Fatigue properties The curves of FCG rate (da/dN) as a function of the stress intensity factor (ΔK) for 2524 T3 sheets and as-annealed cold rolled sheet are shown in Fig. 11. In the initial stage, the as-annealed cold rolled sheet exhibited the lowest FCG rate. During the stable stage, the FCG rate of the T3 sheet solution treated in the air furnace was lower than that of sheet treated in the salt bath. When viewed in combination with the grain sizes shown in Fig. 3a and b, this result suggested that the formation of small or sub-grains decreased the FCG rate in the early stage of the fatigue crack extension process. Larger grains, however, increased the fatigue resistance in the stable stage. In order to elucidate the fatigue mechanism further, the fatigue fracture and the area near the fatigue crack path were observed and analyzed using SEM, OM, and EBSD.
Fig. 11. Fatigue crack growth (FCG) rate (da/dN) as a function of the stress intensity factor (ΔK).
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Fig. 12. Fatigue fractures in (a, b, and c) as-annealed cold rolling 2524 sheet and (d, e, and f) T3 sheet sample (treated in the salt bath), and (g, h, and i)T3 sheet samples (treated in the air furnace) in the initiate, stable, and final stage respectively.
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Fig. 13. OM and EBSD images of the area near the fatigue crack growth path for 2524 T3 sheets solid-solution treated at different heating rates: in a salt bath (a, b, c, and h) and (d, e, f, g, and i) in an air furnace; a, g: IPF map; b, f: derived-(111)b1−10N Schmid factor maps; c, d: OM images of the area near the fatigue crack growth path; e: image of the background; h, i: statistical charts of Schmid factors.
3.6. Fatigue crack path analysis The tortuosity of the crack path can often reflect the fatigue resistance in a sense, owing to the so-called crack closure [32,33]. Fig. 13 shows the microscopic features along the fatigue crack path in the case of two 2524 T3 sheet samples after 50,000 loading cycles using the low-magnification OM and EBSD. As shown in Figs. 13 c and d, the tortuosity of the fracture surface (in the initial stage) in the case of the 2524 T3 sample heated in the air furnace was markedly higher than that of the fracture surface of sample treated in the salt bath. In addition, Fig. 13a and b show the inverse pole-figure (IPF) maps and the corresponding Schmid factors of the fatigue crack tip in sable stage for the sample heated rapidly (in salt bath). Similarly, Fig. 13e–g shows the images of the background, Schimd factor, as well as the inverse pole-figure respectively for the area near the tip for the sample heated at a low rate (in the air furnace). It can be observed that the fatigue crack tends to extend along the grain
boundary (among the fine grains) in the path of inter-granular fracture, which showed some characteristics of brittle fracture, in keeping with previously reported results [4]. However, the fatigue crack propagated across the grains with high Schmid factors by means of a typical transgranular fracture in the case of the sample heated in an air furnace. Furthermore, significant deflections occurred, as denoted by the marks α1, α2, and α3 (see Fig. 13e), when the crack extended within grains and across the grain boundaries. Most of all, the effects of crack closure were visible in the initial stage (as highlighted by ellipses in Fig. 13d) and the stable stage of the fatigue crack extension process (as shown in Fig. 13f). The phenomenon of crack closure delaying fatigue crack propagation has been observed and reported in the previous studies as well [34,35]. Thus, the crack closure observed in this study may be related to the high Schmid factors and large sizes of the grains, as per the statistical chart of the Schmid factors (see in Fig. 13h and i), which would be responsible for the effects on the fatigue properties.
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4. Conclusions The effects of the heating rate during solid-solution treatment on the microstructure and fatigue behavior of T3 AA2524 Al–Cu–Mg T3 sheets were investigated. The main conclusions are as follows: 1) The fatigue properties of the 2524 T3 (Al–Cu–Mg) sheets were affected by the heating rate during the solid-solution treatment. The T3 sheet solution treated at a slow heating rate (in an air furnace) exhibited better resistance to fatigue crack propagation. 2) Different heating rates during the solid-solution treatment can result in two different recrystallization mechanisms, namely, PSN and SIBM, which affect the structural properties of the T3 sheets, including their grain size and texture components. The sheet treated at a low rate consisted of long fibrous grains with Goss, Cube, and S orientations. The sheet heated at a high rate consisted of equiaxed grains with the P-type orientation. 3) The effects of crack closure, induced by a large grain size and high Schmid factor, are probably responsible for the observed fatigue properties. Acknowledgments The authors are grateful for the support provided by the National Key Basic Research Program of China (973 Program, No. 2012CB619506) and the 2011 Program of the Ministry of Education in China. References [1] S.P. Ringer, T. Sakurai, I.J. Polmear, Acta Mater. 45 (1997) 3731–3744. [2] Y. Nagai, M. Murayama, Z. Tang, Acta Mater. 49 (2001) 913–920. [3] T. Dursun, C. Soutis, Mater. Des. 56 (2014) 862–871.
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