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Characteristic microstructure and microstructure evolution in Al–Cu–Mn alloy under projectile impact
Materials Science and Engineering A 531 (2012) 12–17
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Characteristic microstructure and microstructure evolution in Al–Cu–Mn alloy under projectile impact Qing Hua Zhang, Bo Long Li, Xi Chen, Jia Ming Yin, Zuo Ren Nie ∗ , Tie Yong Zuo College of Materials Science and Engineering, Beijing University of Technology, 100# Pingleyuan, Chaoyang District, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 26 July 2011 Accepted 27 September 2011 Available online 6 October 2011 Keywords: Aluminum alloy Projectile impact Microstructure Dynamic recrystallization
a b s t r a c t The microstructure and microstructural evolution were investigated near crater wall in Al–Cu–Mn alloy using optical and transmission electron microscopy (TEM) after projectile impaction. The results show that three characteristic zones around the crater can be classified based on the different microstructure, i.e. deformation bands, dynamic recovery zone and adiabatic shear bands (ASBs). The TEM observation indicates that the dislocation glide plays a crucial role in the formation of that microstructure during projectile impact. The adiabatic shear bands were formed near the crater wall and extend into matrix. It can be found that fine grains were formed within the adiabatic shear bands by dynamic recrystallization occurring during projectile impact. The micro-cracks have been developed along the adiabatic shear bands. However, It is demonstrated that the formation of deformation bands are favorable for improving anti-impact property of Al–Cu–Mn alloy, but adiabatic shear bands are easily to initiate micro-cracks, leads to the failure of target material. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Al–Cu–Mn alloy has high strength and toughness than most other aluminum alloys after solution treated and rolled 7% reduction before artificial aging, which named T87 treatment [1]. So, it has become a kind of prospective armor material to be used in chariot, aero-craft and other commercial machines for its excellent anti-stress corrosion property, better welding performance and good processing property [2]. Many scholars and experts have profoundly investigated the microstructure of adiabatic shear bands, and classified to two kinds of categories: one is the character of high strain concentration, which have fiercely broken grains, mostly generated in nonferrous metals and alloys [3,4]; the another is the phase transformation or recrystallization, usually showed in steel and titanium alloy [4,5]. On the other hand, the adiabatic shear band with recrystallization microstructure has also been found in the 7039 aluminum alloy target after dynamic deformation [6], and even some amorphous structure are formed in the centre of the adiabatic shear bands in a high strength steel target [7]. In addition, for Al–Cu–Mn alloy, there is a research report on microstructure evolution at high strain rates impact by using split Hopkinson pressure bar (SHPB), focused on the precipitation behavior [8]. However, the SHPB experiment is different from projectile impact experiment,
∗ Corresponding author. Tel.: +86 10 6739 6439; fax: +86 10 6739 1536. E-mail addresses: [email protected], [email protected], [email protected] (Z.R. Nie). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.109
because the specimen in the SHPS is much smaller than target plate in projectile impact. Up to now, the characteristic microstructure and microstructural evolution in the Al–Cu–Mn (2519) alloy target after high velocity projectile impact are still not fully clarified, and the research works on this issue in the material have not been found yet. So, the present study focused on the characteristic microstructures formed by high velocity impact in the 2519-T87 aluminum alloy target, with optical microscope (OM) and transmission electron microscope (TEM), in order to shed light on the failure mechanism of this material during high velocity projectile impact. 2. Experimental procedure The composition of the target material used in this study was 5.8% Cu, 0.27% Mn, 0.20% Zr, 0.1% Fe, 0.057% V, 0.052% Mg, and 0.044% Si and balance Al, in mass percent. The projectile impact experiment has carried out by using Model No. 54 style armorpiercing incendiary (API) projectile, which vertically shooting the 2519-T87 Aluminum alloy plate with an initial velocity of 584 m/s, and then obtained a crater for inspection. The crater specimens for microstructure observation were taken from half sections along the impact axis, then grinded and polished samples were etched with Keller reagent (1 ml HF + 1.5 ml HCl + 2.5 ml HNO3 + 95 ml H2 O) for optical metallographic observation. In order to perform a systematic TEM investigation in the shear localization regions, a special technique has carried out in the present work. At first, the metallographic specimen was sectioned
Q.H. Zhang et al. / Materials Science and Engineering A 531 (2012) 12–17
13
Fig. 1. The OM image of the starting material in 2519-T87 aluminum alloy target.
in some thin slices parallel to the section plane containing impact axis. One side of the slice was polished and etched, so that the shear band location was revealed. This location was marked by a light scratch, and then the slice of sample was thinned by grinding from the opposite side to a thickness of 120 m. A 3 mm disk in diameter was subsequently punched out from the sheet, making sure that the marked location was located close to the centre of the disk. The disk was then carefully ground to 60 m in thickness from the side which has been marked. Finally, the disc was electro-polished by StruersTenupol-5, with the electrolyte (30% HNO3 + 70% CH3 COOH), at the temperature about −30 ◦ C, voltage of 16 V and current flow of 80–100 mA. The characteristic microstructures of these samples have been observed by JEOL-JEM2010 TEM, at accelerating voltage of 200 kV. 3. Results and discussion 3.1. Deformation bands zones The starting target plate was solution treated followed by artificial aging process, provide homogeneous grain structure with an average size of 100 m, as seen in Fig. 1. The material adjacent to the crater suffers the most severe plastic deformation as seen in Fig. 2, which shows the OM and TEM images of deformation bands. In this zone, the grains suffer severe plastic deformation during impact, and evolved into deformation bands with parallel strips along the impact direction (Fig. 2a). The deformation bands are lamella structure at TEM observations, as seen in Fig. 2b. With increasing the distance from the crater, the thickness of the lamella decreased gradually (Fig. 2b), showing that the shock wave and stress decreases when the bullet was traveling in the target, leading to decrease of compressive deformation in the material (Fig. 2a). The boundary regions between the deformation bands and the matrix are consisted of dislocation cells owing to the low deformation, marked by P1 in Fig. 2b. These lamella structures are uniform, so it is benefit to dissipate the impact energy and can improve the anti-impact property of material. Furthermore, Fig. 2b shows the lamella structure containing high dislocation density, which was similar to typical low strain rate deformation structure, such as cold rolling deformation process. This provides evidence that the original structures change to a thin lath microstructure through the dislocation gliding during impact process as well. The deformation begins with the establishment of cell blocks, delineated by “geometrically necessary” boundaries (GNBs). At high strain these structures became the lamellar structures parallel to the main stress direction [9].
Fig. 2. Microstructure in the deformation bands near the crater wall in 2519-T87 aluminum alloy target impacted at a velocity of 584 m/s. (a) OM image and (b) TEM image.
3.2. Dynamic recovery zones Fig. 3 shows TEM images of microstructure in dynamic recovery zone near crater wall in target plate. This zone consists of dynamic recovery microstructure (Fig. 3a). A certain dislocation density in the region can be observed, which indicates that the dislocation gliding plays a crucial role for the grain refinement during deformation process. The zone adjacent to the crater wall consists of sub-grains and the average grain size is about 0.2 m (Fig. 3b). The formation of subgrains should be attributed to the dynamic recovery mechanism. The temperature rising caused by shock wave and plastic deformation plus high stain energy stored in the material leads to the formation of the subgrains.
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Fig. 3. (a) TEM image shows deformation microstructure in dynamic recovery zone near the crater wall in 2519-T87 aluminum alloy target. (b) The subgrains in the zone adjacent to the crater wall.
In the dynamic recovery process, the dislocations were fully activated for the high temperature. Fig. 4 shows an example of long thin laths observed in the deformation bands, in which the dislocations tend to accumulate at some locations to form transverse dislocation walls marked by the arrows in Fig. 4. The formation of these interface led to the breakdown of the laths into elongated segments and finally to form nearly equiaxed subgrains through lattice rotations near the interface. This similar with formation mechanism of equiaxed nanosized (sub) grains at large strains found by Yang et al. [10]. With increasing the distance from the crater wall, the stress and temperature within the matrix caused by shock wave decreased and plastic deformation decreased gradually. Therefore, dynamic recovery is not sufficient in the region (marked T1 in Fig. 3a) away from the crater wall and adjacent to the deformation bands. In the process, the dislocation density decreases for the dislocation gliding leads to the dislocation reorganization including dislocations reaction and annihilation. Finally, the microstructure changes into substructure of the dislocation cells which have the irregular borders.
Fig. 4. TEM image shows thin laths structure breaking down into subgrains.
3.3. Adiabatic shear bands zones Fig. 5 shows the ASB and the micro-crack below the crater wall in 2519-T87 aluminum alloy target. It has been observed that the ASB extends from the crater wall fringe and its width is about 10 m. The front region of the ASB marked with arrow A in Fig. 5, has developed into a long micro-crack, extended to interior of the matrix, and the width of the micro crack become narrower until the crack get ceased. The end region of the crack still presents the characteristic image of the ASB. The formation of ASBs and propagation of cracks are the process of consumption of impact energy, so the width of the ASBs become narrower with increasing the distance from the crater and finally the ASBs terminate extension, with the impact energy gradually consumed. Fig. 6 shows the TEM images of fine grains in ASBs. The width of the fine grain zone is about ten micros and almost same as the width of the ASB, as seen in Fig. 6a. The detail fine grain structure can be clearly visualized in the Fig. 6b with illustrated grain boundaries, and the corresponding selected area diffraction shows a poly-crystal rings, indicating large misorientation associated with these recrystallized grains. It can be observed that the grain size distribution in this zone is non-uniform, ranging from several tens nanometer to several micrometer, which was unambiguously demonstrated by TEM observations in present experiments, as seen in Fig. 6. Compared with original grains, the size of dynamic recrystallized grains decreased at least two orders of magnitude. The formation of fine grains should be attributed to the dynamic recrystallization mechanism. It seems that, in the 2519-T87 target, adiabatic shear deformation occurred in some local areas of crater wall, and induced ASB with dynamic recrystallization grains. The experimental results of the microstructure observation in the ASB in 2519-T87 target have somewhat similar to that observed in some other impacted targets. According to Ref. [4], the initial cracks easily generated at some microstructure inhomogeneous regions in the target material, such as the grain boundaries at the contact surface with bullet. Because of the temperature rising in the contact surface, the grain boundaries in the contact surface are of weak parts during impact, being easily generated the cracks, and
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Fig. 5. OM image shows adiabatic shear bands and the micro-crack near crater wall in 2519-T87 aluminum alloy target.
extended to the interior of the materials. Most of cracks have been originated from the contact surface and extend along the ASB or blocked at the fringe of the shear band, as seen in Fig. 5. However, the ASB occur in 2519-T87 target are black color without
clear width, which was different from steel target with white color and clear outline. This attributed to the size of recrystallized grains within ASB in 2519-T87 alloy target are smaller than that formed in steel target. It is hard to be etched for steel target by metallographic
Fig. 6. (a) TEM image showing dynamic recrystallization grains in ASBs near the crater wall in 2519-T87 aluminum alloy target. (b) A detail inspection of area marked Z in (a).
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chemical reagent, so the ASB of steel target with white color can distinguish with the surround matrix in steel [4]. The critical condition of the adiabatic shear is closely relevant to the stress, strain, strain rate and the temperature in the process of impact deformation. Chen and Kocks [11] observed that the activation energy for the dynamic recrystallization process is highly stress dependent. Xu et al. [12] reported that there are critical values of strain for the occurrence of the deformed shear band and white shear band, and they form at different stages of deformation. The deformed shear band appears in the first stage of localization, and the white shear band is a result of further shear strain. Li et al. [13] show that the plastic strain rate and the density of mobile dislocation play the key role in the formation of new recrystallized grains and growth during the plastic deformation at high strain rate. If the temperature raise in the shear band is high enough by adiabatic heating, the recrystallized grain growth is also controlled by atomic thermal migration. In this process, the strain rate effect is still noticeable. Zener and Hollomon proposed the model for the mechanical collapsing with the thermoplasticity theory [14]. In the model the strain rate has been neglected, because the strain rate has caused both strain hardening effect and adiabatic softening effect simultaneously, so the complex effect of strain rate could involve both the hardening and softening effects in target material. The formula relating shear stress and the strain is:
˛
= (0 + h) exp −
2c
(20 + h)
(1)
And the formula of elevated temperature and the strain is: =
1 ˛
˛
1 − exp −
2c
(20 + h)
(2)
The critical shear strain value of the adiabatic shear is:
c =
(hc/˛) − 0 h
(3)
where, 0 is the shear proportion limit in static load ( 0 = 0.2 /2); h is the coefficient of the strain-strengthening; ˛ is the coefficient of thermal softening; is the coefficient of thermal transition; is the density; c is the heat capacity. The relevant parameters of 2519 aluminum alloy are [15]: h = 4 GPa, ˛ = 1/(1000 ◦ C), = 0.9, 0 = 175 MPa, = 2.84 × 103 kg m−3 , c = 1005 J (kg ◦ C)−1 , From the formulae with all relevant parameters of 2519-T87 alloy, we can obtain the critical condition of occurring the adiabatic shear phenomenon: the critical shear strain is c = 0.85, the corresponding shear stress is = 2145 MPa, and the critical temperature is = 400 ◦ C. That is, the critical temperature exceed the recrystallization temperature of the 2519-T87 alloy (350 ◦ C) [16]. Therefore, it is possible that the dynamic recrystallization occur in the adiabatic shear bands, which is correspondent to the phenomenon by TEM experimental observation. In the ASBs, the energy concentrates in the local region and the temperature rapidly increases. So, the dynamic recrystallization occurred in the ASBs under several combined condition, such as the high strain rate, large deformation, local higher temperature and applied shear stress. The dynamic recrystallization mechanism in the ASBs caused by projectile impact is different from the conventional thermoplastic deformation recrystallization. The main difference is that the recrystallization process is completed promptly in short time. McQueen and Bergerson [17] suggested that a different mechanism may take place at higher strain rates. They state that under high strain rates, recrystallization nuclei may be forming at highly misoriented cell walls rather than exclusively at pre-existing high angle boundaries. Gil Sevillano et al. [18] suggested that the diffusion-controlled dislocation climbing process will be
Fig. 7. Schematic illustration of the characteristic microstructure distribution near the crater wall in 2519-T87 aluminum alloy target.
too slow to create a large misorientation boundary, so, the subgrain with high misorientation observed here must be dominated by mechanical rotation mechanism. Flaquer and Gil Sevillano [19] have proposed a mechanically assisted subgrain rotation/coalescence model to explain subgrain growth at low temperatures. Based on above theories and combined with experimental results at present work, it can be supposed that this process of dynamic recrystallization is mainly divided into two stages under hypervelocity impact. In the early stage of dynamic recrystallization, the original grains are broken into the deformed sub-grain structure under high stress, as shown in Fig. 2b. And then the sub-grains mechanically rotate results in the formation of grain boundary. The dynamic recrystallization behavior found in our investigation is of interest, for it is opposite to the results of previous research report [20], where the authors claimed that no evidence for dynamic recrystallization have been found in 6061-T6 aluminum impacted by spherical soda-lime glass projectiles, even though both of the 2519-T87 aluminum and the 6061-T6 aluminum are precipitation strengthening alloys. Further more, in contrast to the previous report [20], there are clear evidence for the ASBs leading to the cracks in the 2519-T87 aluminum, as shown in Fig. 5. It is obvious that the cracks are likely to form in the ASBs. The cracks generate in the ASB in the hardening process, and not in the thermal softening process which was described in the literature [21]. Contrast to the ASBs, which easily developed to micro-cracks and lead to the failure of material, the deformation bands can consume the impact energy in a homogeneous way, therefore formation of the deformation bands in target materials are favorable to anti-impact property. According to the experiment results the microstructure near the crater wall can be characterized as follows. Fig. 7 shows the schematic description of the characteristic microstructure distribution near the crater wall in 2519-T87 aluminum alloy target. Three characteristic zones including deformation bands, dynamic recovery zone and adiabatic shear bands (ASBs) can be classified based on the different microstructure characteristics. And also the micro-cracks are seen within the ASBs. The distribution boundary between the deformation zones is not clear because of the continuous variation of the strain and strain rate in the material.
3.4. Microhardness distribution Fig. 8 shows the microhardness distribution near crater wall. The microhardness values beneath the crater wall are lower than at the surrounding matrix. The microhardness value increases with the distance from the crater edge to the matrix. This is attributed to the thermal softening behaviors caused by heating effect near crater edge during high speed impact, which may exceeding the plastic deformation hardening effect.
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Fig. 8. The microhardness distribution around the crater wall in 2519-T87 aluminum alloy target impacted at a velocity of 584 m/s. (a) The paths of microhardness test. (b) The microhardness values corresponding the four paths in (a), respectively.
4. Conclusions
References
1. Under hypervelocity impact, the characteristical microstructure have clarified near crater wall in 2519-T87 aluminum alloy target, including deformation band, dynamic recovery zone and adiabatic shear bands (ASBs). 2. Lamella structure has formed in the deformation band and subgrains have generated in the dynamic recovery zone. The ASBs consisted of fine grains of dynamic recrystallization ranging from several tens nanometer to several micrometer. 3. The dislocation gliding plays a critical role in the microstructure evolution. The dynamic recrystallization process in the ASBs is completed promptly in short time and the mechanical rotation/coarsening mechanism plays an essential role in the process. 4. The micro-cracks which formed in the ASBs under the concentration of stress in the hardening process, not in the thermal softening process. The formation of the deformation bands are benefit to improve the anti-impact property of material, but ASBs are easily to develop micro-cracks, lead to the failure of material.
[1] Q.Q. Lin, H. Zhang, D.S. Peng, G.Y. Lin, Hot Work. Technol. 3 (2002) 3. [2] S.M. Devicent, J.H. Devletian, S.A. Gedon, Weld. J. 67 (1988) 33. [3] M.A. Meyers, Y.B. Xu, Q. Xue, M.T. Pérez-Prado, T.R. McNelley, Acta Mater. 51 (2003) 1307. [4] H. Dong, G.F. Li, B.Z. Wang, N.P. Chen, J. Iron Steel 6 (1995) 7. [5] L.E. Murr, E.A. Trillo, A.A. Bujanda, N.E. Martinez, Acta Mater. 50 (2002) 121. [6] C. Kennedy, L.E. Murr, Mater. Sci. Eng. A 325 (2002) 131. [7] C.G. Fan, D. Han, Y.L. Liu, Q.L. Yong, Y.Q. Weng, Ordnance Mater. Sci. Eng. 29 (2006) 29. [8] Z.G. Gao, X.M. Zhang, M.A. Chen, Scripta Mater. 59 (2008) 983. [9] C.Y. Barlow, N. Hansen, Y.L. Liu, Acta Mater. 50 (2002) 171–182. [10] D.K. Yang, P. Cizek, P.D. Hodgson, C.E. Wen, Acta Mater. (2010). [11] S.R. Chen, U.F. Kocks, Scripta Metall. Mater. 27 (1992) 1587–1592. [12] Y.B. Xu, W.L. Zhong, Y.J. Chen b, L.T. Shen, Q. Liu, Y.L. Bai, M.A. Meyers, Mater. Sci. Eng. A 299 (2001) 287–295. [13] Q. Li, Y.B. Xu, Z.H. Lai, L.T. Shen, Y.L. Bai, Mater. Sci. Eng. A 276 (2000) 250–256. [14] C. Zener, J.H. Hollomon, Contributed original research, J. Appl. Phys. 15 (1) (1944) 22–32. [15] Z.T. Wang, R.Z. Tian, Manual of Aluminum alloy and Working, third ed., Hunan, China, 2005. [16] A.L. Ning, B.S. Peng, S.S. Jiang, Hot Work. Technol. 35 (2006) 17. [17] H.J. McQueen, S. Bergerson, Mater. Sci. Eng. A 6 (1972) 25–29. [18] J. Gil Sevillano, P. van Houtte, E. Aernoudt, Prog. Mater. Sci. 25 (1980) 69–412. [19] J. Flaquer, J. Gil Sevillano, Mater. Sci. 19 (1984) 423–427. [20] L.E. Murr, A. Ayala, C.S. Niou, Mater. Sci. Eng. A 216 (1996) 69. [21] L.E. Murr, A.C. Ramirez, S.M. Gaytan, M.I. Lopez, E.Y. Martinez, D.H. Hernandez, E. Martinez, Mater. Sci. Eng. A 516 (2009) 205–216.
Acknowledgements This work was financially supported by the National 973 Project (2005CB623706) and 863 key Project (2009AA033801).
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Characteristic microstructure and microstructure evolution in Al–Cu–Mn alloy under projectile impact Qing Hua Zhang, Bo Long Li, Xi Chen, Jia Ming Yin, Zuo Ren Nie ∗ , Tie Yong Zuo College of Materials Science and Engineering, Beijing University of Technology, 100# Pingleyuan, Chaoyang District, Beijing 100124, China
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 26 July 2011 Accepted 27 September 2011 Available online 6 October 2011 Keywords: Aluminum alloy Projectile impact Microstructure Dynamic recrystallization
a b s t r a c t The microstructure and microstructural evolution were investigated near crater wall in Al–Cu–Mn alloy using optical and transmission electron microscopy (TEM) after projectile impaction. The results show that three characteristic zones around the crater can be classified based on the different microstructure, i.e. deformation bands, dynamic recovery zone and adiabatic shear bands (ASBs). The TEM observation indicates that the dislocation glide plays a crucial role in the formation of that microstructure during projectile impact. The adiabatic shear bands were formed near the crater wall and extend into matrix. It can be found that fine grains were formed within the adiabatic shear bands by dynamic recrystallization occurring during projectile impact. The micro-cracks have been developed along the adiabatic shear bands. However, It is demonstrated that the formation of deformation bands are favorable for improving anti-impact property of Al–Cu–Mn alloy, but adiabatic shear bands are easily to initiate micro-cracks, leads to the failure of target material. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Al–Cu–Mn alloy has high strength and toughness than most other aluminum alloys after solution treated and rolled 7% reduction before artificial aging, which named T87 treatment [1]. So, it has become a kind of prospective armor material to be used in chariot, aero-craft and other commercial machines for its excellent anti-stress corrosion property, better welding performance and good processing property [2]. Many scholars and experts have profoundly investigated the microstructure of adiabatic shear bands, and classified to two kinds of categories: one is the character of high strain concentration, which have fiercely broken grains, mostly generated in nonferrous metals and alloys [3,4]; the another is the phase transformation or recrystallization, usually showed in steel and titanium alloy [4,5]. On the other hand, the adiabatic shear band with recrystallization microstructure has also been found in the 7039 aluminum alloy target after dynamic deformation [6], and even some amorphous structure are formed in the centre of the adiabatic shear bands in a high strength steel target [7]. In addition, for Al–Cu–Mn alloy, there is a research report on microstructure evolution at high strain rates impact by using split Hopkinson pressure bar (SHPB), focused on the precipitation behavior [8]. However, the SHPB experiment is different from projectile impact experiment,
∗ Corresponding author. Tel.: +86 10 6739 6439; fax: +86 10 6739 1536. E-mail addresses: [email protected], [email protected], [email protected] (Z.R. Nie). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.109
because the specimen in the SHPS is much smaller than target plate in projectile impact. Up to now, the characteristic microstructure and microstructural evolution in the Al–Cu–Mn (2519) alloy target after high velocity projectile impact are still not fully clarified, and the research works on this issue in the material have not been found yet. So, the present study focused on the characteristic microstructures formed by high velocity impact in the 2519-T87 aluminum alloy target, with optical microscope (OM) and transmission electron microscope (TEM), in order to shed light on the failure mechanism of this material during high velocity projectile impact. 2. Experimental procedure The composition of the target material used in this study was 5.8% Cu, 0.27% Mn, 0.20% Zr, 0.1% Fe, 0.057% V, 0.052% Mg, and 0.044% Si and balance Al, in mass percent. The projectile impact experiment has carried out by using Model No. 54 style armorpiercing incendiary (API) projectile, which vertically shooting the 2519-T87 Aluminum alloy plate with an initial velocity of 584 m/s, and then obtained a crater for inspection. The crater specimens for microstructure observation were taken from half sections along the impact axis, then grinded and polished samples were etched with Keller reagent (1 ml HF + 1.5 ml HCl + 2.5 ml HNO3 + 95 ml H2 O) for optical metallographic observation. In order to perform a systematic TEM investigation in the shear localization regions, a special technique has carried out in the present work. At first, the metallographic specimen was sectioned
Q.H. Zhang et al. / Materials Science and Engineering A 531 (2012) 12–17
13
Fig. 1. The OM image of the starting material in 2519-T87 aluminum alloy target.
in some thin slices parallel to the section plane containing impact axis. One side of the slice was polished and etched, so that the shear band location was revealed. This location was marked by a light scratch, and then the slice of sample was thinned by grinding from the opposite side to a thickness of 120 m. A 3 mm disk in diameter was subsequently punched out from the sheet, making sure that the marked location was located close to the centre of the disk. The disk was then carefully ground to 60 m in thickness from the side which has been marked. Finally, the disc was electro-polished by StruersTenupol-5, with the electrolyte (30% HNO3 + 70% CH3 COOH), at the temperature about −30 ◦ C, voltage of 16 V and current flow of 80–100 mA. The characteristic microstructures of these samples have been observed by JEOL-JEM2010 TEM, at accelerating voltage of 200 kV. 3. Results and discussion 3.1. Deformation bands zones The starting target plate was solution treated followed by artificial aging process, provide homogeneous grain structure with an average size of 100 m, as seen in Fig. 1. The material adjacent to the crater suffers the most severe plastic deformation as seen in Fig. 2, which shows the OM and TEM images of deformation bands. In this zone, the grains suffer severe plastic deformation during impact, and evolved into deformation bands with parallel strips along the impact direction (Fig. 2a). The deformation bands are lamella structure at TEM observations, as seen in Fig. 2b. With increasing the distance from the crater, the thickness of the lamella decreased gradually (Fig. 2b), showing that the shock wave and stress decreases when the bullet was traveling in the target, leading to decrease of compressive deformation in the material (Fig. 2a). The boundary regions between the deformation bands and the matrix are consisted of dislocation cells owing to the low deformation, marked by P1 in Fig. 2b. These lamella structures are uniform, so it is benefit to dissipate the impact energy and can improve the anti-impact property of material. Furthermore, Fig. 2b shows the lamella structure containing high dislocation density, which was similar to typical low strain rate deformation structure, such as cold rolling deformation process. This provides evidence that the original structures change to a thin lath microstructure through the dislocation gliding during impact process as well. The deformation begins with the establishment of cell blocks, delineated by “geometrically necessary” boundaries (GNBs). At high strain these structures became the lamellar structures parallel to the main stress direction [9].
Fig. 2. Microstructure in the deformation bands near the crater wall in 2519-T87 aluminum alloy target impacted at a velocity of 584 m/s. (a) OM image and (b) TEM image.
3.2. Dynamic recovery zones Fig. 3 shows TEM images of microstructure in dynamic recovery zone near crater wall in target plate. This zone consists of dynamic recovery microstructure (Fig. 3a). A certain dislocation density in the region can be observed, which indicates that the dislocation gliding plays a crucial role for the grain refinement during deformation process. The zone adjacent to the crater wall consists of sub-grains and the average grain size is about 0.2 m (Fig. 3b). The formation of subgrains should be attributed to the dynamic recovery mechanism. The temperature rising caused by shock wave and plastic deformation plus high stain energy stored in the material leads to the formation of the subgrains.
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Fig. 3. (a) TEM image shows deformation microstructure in dynamic recovery zone near the crater wall in 2519-T87 aluminum alloy target. (b) The subgrains in the zone adjacent to the crater wall.
In the dynamic recovery process, the dislocations were fully activated for the high temperature. Fig. 4 shows an example of long thin laths observed in the deformation bands, in which the dislocations tend to accumulate at some locations to form transverse dislocation walls marked by the arrows in Fig. 4. The formation of these interface led to the breakdown of the laths into elongated segments and finally to form nearly equiaxed subgrains through lattice rotations near the interface. This similar with formation mechanism of equiaxed nanosized (sub) grains at large strains found by Yang et al. [10]. With increasing the distance from the crater wall, the stress and temperature within the matrix caused by shock wave decreased and plastic deformation decreased gradually. Therefore, dynamic recovery is not sufficient in the region (marked T1 in Fig. 3a) away from the crater wall and adjacent to the deformation bands. In the process, the dislocation density decreases for the dislocation gliding leads to the dislocation reorganization including dislocations reaction and annihilation. Finally, the microstructure changes into substructure of the dislocation cells which have the irregular borders.
Fig. 4. TEM image shows thin laths structure breaking down into subgrains.
3.3. Adiabatic shear bands zones Fig. 5 shows the ASB and the micro-crack below the crater wall in 2519-T87 aluminum alloy target. It has been observed that the ASB extends from the crater wall fringe and its width is about 10 m. The front region of the ASB marked with arrow A in Fig. 5, has developed into a long micro-crack, extended to interior of the matrix, and the width of the micro crack become narrower until the crack get ceased. The end region of the crack still presents the characteristic image of the ASB. The formation of ASBs and propagation of cracks are the process of consumption of impact energy, so the width of the ASBs become narrower with increasing the distance from the crater and finally the ASBs terminate extension, with the impact energy gradually consumed. Fig. 6 shows the TEM images of fine grains in ASBs. The width of the fine grain zone is about ten micros and almost same as the width of the ASB, as seen in Fig. 6a. The detail fine grain structure can be clearly visualized in the Fig. 6b with illustrated grain boundaries, and the corresponding selected area diffraction shows a poly-crystal rings, indicating large misorientation associated with these recrystallized grains. It can be observed that the grain size distribution in this zone is non-uniform, ranging from several tens nanometer to several micrometer, which was unambiguously demonstrated by TEM observations in present experiments, as seen in Fig. 6. Compared with original grains, the size of dynamic recrystallized grains decreased at least two orders of magnitude. The formation of fine grains should be attributed to the dynamic recrystallization mechanism. It seems that, in the 2519-T87 target, adiabatic shear deformation occurred in some local areas of crater wall, and induced ASB with dynamic recrystallization grains. The experimental results of the microstructure observation in the ASB in 2519-T87 target have somewhat similar to that observed in some other impacted targets. According to Ref. [4], the initial cracks easily generated at some microstructure inhomogeneous regions in the target material, such as the grain boundaries at the contact surface with bullet. Because of the temperature rising in the contact surface, the grain boundaries in the contact surface are of weak parts during impact, being easily generated the cracks, and
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Fig. 5. OM image shows adiabatic shear bands and the micro-crack near crater wall in 2519-T87 aluminum alloy target.
extended to the interior of the materials. Most of cracks have been originated from the contact surface and extend along the ASB or blocked at the fringe of the shear band, as seen in Fig. 5. However, the ASB occur in 2519-T87 target are black color without
clear width, which was different from steel target with white color and clear outline. This attributed to the size of recrystallized grains within ASB in 2519-T87 alloy target are smaller than that formed in steel target. It is hard to be etched for steel target by metallographic
Fig. 6. (a) TEM image showing dynamic recrystallization grains in ASBs near the crater wall in 2519-T87 aluminum alloy target. (b) A detail inspection of area marked Z in (a).
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chemical reagent, so the ASB of steel target with white color can distinguish with the surround matrix in steel [4]. The critical condition of the adiabatic shear is closely relevant to the stress, strain, strain rate and the temperature in the process of impact deformation. Chen and Kocks [11] observed that the activation energy for the dynamic recrystallization process is highly stress dependent. Xu et al. [12] reported that there are critical values of strain for the occurrence of the deformed shear band and white shear band, and they form at different stages of deformation. The deformed shear band appears in the first stage of localization, and the white shear band is a result of further shear strain. Li et al. [13] show that the plastic strain rate and the density of mobile dislocation play the key role in the formation of new recrystallized grains and growth during the plastic deformation at high strain rate. If the temperature raise in the shear band is high enough by adiabatic heating, the recrystallized grain growth is also controlled by atomic thermal migration. In this process, the strain rate effect is still noticeable. Zener and Hollomon proposed the model for the mechanical collapsing with the thermoplasticity theory [14]. In the model the strain rate has been neglected, because the strain rate has caused both strain hardening effect and adiabatic softening effect simultaneously, so the complex effect of strain rate could involve both the hardening and softening effects in target material. The formula relating shear stress and the strain is:
˛
= (0 + h) exp −
2c
(20 + h)
(1)
And the formula of elevated temperature and the strain is: =
1 ˛
˛
1 − exp −
2c
(20 + h)
(2)
The critical shear strain value of the adiabatic shear is:
c =
(hc/˛) − 0 h
(3)
where, 0 is the shear proportion limit in static load ( 0 = 0.2 /2); h is the coefficient of the strain-strengthening; ˛ is the coefficient of thermal softening; is the coefficient of thermal transition; is the density; c is the heat capacity. The relevant parameters of 2519 aluminum alloy are [15]: h = 4 GPa, ˛ = 1/(1000 ◦ C), = 0.9, 0 = 175 MPa, = 2.84 × 103 kg m−3 , c = 1005 J (kg ◦ C)−1 , From the formulae with all relevant parameters of 2519-T87 alloy, we can obtain the critical condition of occurring the adiabatic shear phenomenon: the critical shear strain is c = 0.85, the corresponding shear stress is = 2145 MPa, and the critical temperature is = 400 ◦ C. That is, the critical temperature exceed the recrystallization temperature of the 2519-T87 alloy (350 ◦ C) [16]. Therefore, it is possible that the dynamic recrystallization occur in the adiabatic shear bands, which is correspondent to the phenomenon by TEM experimental observation. In the ASBs, the energy concentrates in the local region and the temperature rapidly increases. So, the dynamic recrystallization occurred in the ASBs under several combined condition, such as the high strain rate, large deformation, local higher temperature and applied shear stress. The dynamic recrystallization mechanism in the ASBs caused by projectile impact is different from the conventional thermoplastic deformation recrystallization. The main difference is that the recrystallization process is completed promptly in short time. McQueen and Bergerson [17] suggested that a different mechanism may take place at higher strain rates. They state that under high strain rates, recrystallization nuclei may be forming at highly misoriented cell walls rather than exclusively at pre-existing high angle boundaries. Gil Sevillano et al. [18] suggested that the diffusion-controlled dislocation climbing process will be
Fig. 7. Schematic illustration of the characteristic microstructure distribution near the crater wall in 2519-T87 aluminum alloy target.
too slow to create a large misorientation boundary, so, the subgrain with high misorientation observed here must be dominated by mechanical rotation mechanism. Flaquer and Gil Sevillano [19] have proposed a mechanically assisted subgrain rotation/coalescence model to explain subgrain growth at low temperatures. Based on above theories and combined with experimental results at present work, it can be supposed that this process of dynamic recrystallization is mainly divided into two stages under hypervelocity impact. In the early stage of dynamic recrystallization, the original grains are broken into the deformed sub-grain structure under high stress, as shown in Fig. 2b. And then the sub-grains mechanically rotate results in the formation of grain boundary. The dynamic recrystallization behavior found in our investigation is of interest, for it is opposite to the results of previous research report [20], where the authors claimed that no evidence for dynamic recrystallization have been found in 6061-T6 aluminum impacted by spherical soda-lime glass projectiles, even though both of the 2519-T87 aluminum and the 6061-T6 aluminum are precipitation strengthening alloys. Further more, in contrast to the previous report [20], there are clear evidence for the ASBs leading to the cracks in the 2519-T87 aluminum, as shown in Fig. 5. It is obvious that the cracks are likely to form in the ASBs. The cracks generate in the ASB in the hardening process, and not in the thermal softening process which was described in the literature [21]. Contrast to the ASBs, which easily developed to micro-cracks and lead to the failure of material, the deformation bands can consume the impact energy in a homogeneous way, therefore formation of the deformation bands in target materials are favorable to anti-impact property. According to the experiment results the microstructure near the crater wall can be characterized as follows. Fig. 7 shows the schematic description of the characteristic microstructure distribution near the crater wall in 2519-T87 aluminum alloy target. Three characteristic zones including deformation bands, dynamic recovery zone and adiabatic shear bands (ASBs) can be classified based on the different microstructure characteristics. And also the micro-cracks are seen within the ASBs. The distribution boundary between the deformation zones is not clear because of the continuous variation of the strain and strain rate in the material.
3.4. Microhardness distribution Fig. 8 shows the microhardness distribution near crater wall. The microhardness values beneath the crater wall are lower than at the surrounding matrix. The microhardness value increases with the distance from the crater edge to the matrix. This is attributed to the thermal softening behaviors caused by heating effect near crater edge during high speed impact, which may exceeding the plastic deformation hardening effect.
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Fig. 8. The microhardness distribution around the crater wall in 2519-T87 aluminum alloy target impacted at a velocity of 584 m/s. (a) The paths of microhardness test. (b) The microhardness values corresponding the four paths in (a), respectively.
4. Conclusions
References
1. Under hypervelocity impact, the characteristical microstructure have clarified near crater wall in 2519-T87 aluminum alloy target, including deformation band, dynamic recovery zone and adiabatic shear bands (ASBs). 2. Lamella structure has formed in the deformation band and subgrains have generated in the dynamic recovery zone. The ASBs consisted of fine grains of dynamic recrystallization ranging from several tens nanometer to several micrometer. 3. The dislocation gliding plays a critical role in the microstructure evolution. The dynamic recrystallization process in the ASBs is completed promptly in short time and the mechanical rotation/coarsening mechanism plays an essential role in the process. 4. The micro-cracks which formed in the ASBs under the concentration of stress in the hardening process, not in the thermal softening process. The formation of the deformation bands are benefit to improve the anti-impact property of material, but ASBs are easily to develop micro-cracks, lead to the failure of material.
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Acknowledgements This work was financially supported by the National 973 Project (2005CB623706) and 863 key Project (2009AA033801).