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Damage behaviour of Al matrix composite reinforced with Ti–6Al–4V meshes under the hypervelocity impact
Materials Science and Engineering A 535 (2012) 136–143
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Damage behaviour of Al matrix composite reinforced with Ti–6Al–4V meshes under the hypervelocity impact Q. Guo, D.L. Sun ∗ , X.L. Han, W.S. Yang, L.T. Jiang, G.H. Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 9 December 2011 Accepted 14 December 2011 Available online 21 December 2011 Keywords: Composites Impact behaviour Microstructures Shear bands Interfaces
a b s t r a c t 15 vol.% Ti–6Al–4V meshes reinforced 5A06Al matrix composites (TC4m /5A06Al) were fabricated by pressure infiltration method. The resistance of TC4m /5A06Al composites to hypervelocity impact was tested by a two-stage light gas gun. Moreover, damage behaviour and residual microstructure of the composite were investigated. The depth of crater in TC4m /5A06Al composite was decreased 9.4–12.81% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. Numerous dimples from plastic deformation were observed on the fracture surface of TC4 fibre near the pithead. Adiabatic shearing fracture was occurred in the middle part of crater. Porous structure and cracks were found at the bottom of crater. Interface separation of fibre and Al matrix along impact direction were observed in the region near the pithead. The original horizontal fibres deformed along the outline of the crater in the region around crater. Moreover, some fibres were sheared into two sections along 45◦ direction. Below the crater, fibre was adiabatically sheared and slid along 45◦ direction. Significant adiabatic shear band containing ultra-fined grains (grain size < 200 nm) was observed in the Al matrix at the bottom of the crater. The blocky (Fe, Mn)Al6 phase was sheared into two parts completely along the impact direction, followed with several cracks. Formation and fracture of twins in (Fe, Mn)Al6 were also observed. TiAl amorphous structure was formed at the interface far away from TC4 fibre. The inner layer close to TC4 fibre was composed of TiAl nanocrystalline and amorphous phase. Deformed band with elongated grains and transformed band with equiaxed recrystallized grains were observed at the interface between TC4 fibre and Al matrix at the bottom of crater. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The shelter of space vehicles is very important in order to protect them from the impact of micrometeoroids and space debris [1]. Moreover, it is very important to investigate the dynamic response of materials under the impact of hypervelocity projectile with progressive increase of space activities [2,3]. Therefore, the research on the damage behaviours of materials under the impact of hypervelocity projectiles have been carried out for several decades, and deformation and fracture of steel [4,5], copper alloys [6,7], titanium alloys [8], magnesium alloys [9,10] and aluminium alloys [11–14] have been reported. It was observed by Sunwoo et al. that dominant failure mechanism of AerMet-100 (Fe–Ni–Co) alloy appeared to be dynamic fracture along adiabatic shear bands and shear bands differed in size and morphology depending on the heat-treated conditions [4]. Three kinds of micro-damages in the region around the crater (namely, micro-cracks, microvoids and adiabatic shear bands) were
∗ Corresponding author. Tel.: +86 0451 86418635; fax: +86 0451 86413922. E-mail address: [email protected] (D.L. Sun). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.12.055
observed in 30CrMnSiA steel by Zhou et al. [5]. However, solidstate plastic flow associated with dynamic recrystallization was the prominent mechanism in the penetration process of oxygenfree high conductivity (OFHC) copper alloy [6]. Furthermore, Rivas et al. [7] found that a wide zone exhibiting microband clusters with unidirectional, elongated, cell-like microstructures was coincident with trace of {1 1 1} crystal plane in OFHC copper. The investigation on TC4 alloy indicated that deformed shear bands were formed at an early stage of the localization, which was transformed into the white-etching shear bands with increase of shear strain [8]. Furthermore, the deformed microstructure below the crater of AM60B Mg alloy was classified into the dynamic recrystallization zone, the high density deformation twin zone and the low density deformation twin zone as described by Zou et al. [9,10]. Moreover, micro-cracks, microvoids and adiabatic shear bands were produced by hypervelocity impacts in Al–6Mg alloy [11]. Additionally, the effect of rare earth element (Sc) into Al alloys was also investigated. Al3 Sc precipitates suppressed dislocation motion and promoted an increase in the work hardening stress [12]. Ye et al. [13] assumed that secondary Al3 Sc phase lost its coherency and recrystallized due to the huge amount of energy from high-speed impact. Gao et al. [14] concluded that adiabatic shear lines, arc-like shearing
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Fig. 1. Microstructure of TC4m /5A06Al composite. (a) SEM microstructure of composite; (b) interface between matrix and TC4 fibre; (c) EDX analysis of interface.
bands and DRX grains due to the concentrated stress and heat were observed in 2519A aluminium alloy plate impacted at 573 K. Nesterenko and Goldsmith’s investigation [15] showed that The branching and bifurcation of shear bands and interaction with one another are a distinct and a common feature of the hot isostatically pressed Ti–6Al–4V targets relative to the commercially available Ti–6Al–4V alloy (bar, forged, annealed). They consider this feature being mainly responsible for the enhanced ballistic performance of hot isostatically pressed Ti–6Al–4V targets materials. The shear bands are the preferred sites for nucleation, growth, and coalescence of voids and are, as such, precursors to failure. The evolution of shear-band pattern during the deformation process reveals a self-organization character [16]. Unfortunately, the damage bahaviour of aluminium matrix composites, an important candidate for aerospace application, has been rarely reported. Since TC4 (Ti–6Al–4V) fibres present low density, high modulus, strength and good ductility, it would be very promising for aluminium matrix composite reinforced with TC4 fibre (TC4f /Al) for shelter application. Therefore, in present work, 15 vol.% TC4 meshes reinforced 5A06Al matrix composites
(TC4m /5A06Al) were fabricated by pressure infiltration method. The resistance of TC4m /5A06Al composites to hypervelocity impact was tested by a two-stage light gas gun, whereas damage behaviour and residual microstructure of the composite were also investigated. 2. Experimental TC4 meshes (15 vol.%) were used as reinforcement to obtain isotropic properties in planes in the composites. The fibre diameter was 100 m while the spacing between fibres in mesh was 500 m. 5A06Al alloy was selected as matrix, whose composition was 5.8–6.8 Mg, 0.5–0.8 Mn, 0.4 Fe, 0.4 Si, 0.05 Cu, 0.02–0.10 Ti and the balance was Al. Later the TC4m /5A06Al composites were fabricated by pressure infiltration method [17]. Temperatures for melting Al alloy and mould were 750 and 500 ◦ C, respectively. During the infiltration process, a pressure of 5 MPa was applied and maintained for 10 min, and then the composites were solidified in air. Afterward, the TC4m /5A06Al composites for hypervelocity impact testing were annealed at 330 ◦ C for 30 min.
Fig. 2. Macro-morphologies of craters in (a) and (c) 5A06Al alloy, and (b) and (d) TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s.
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The hypervelocity impact testing was performed on a twostage light gas gun at the Hypervelocity Aerodynamics Institute (China Aerodynamics R&D Center) using 2 mm 2024Al projectile with a speed of 3.79 km/s and normal angle. The thickness of TC4m /5A06Al composite target was 15 mm. Residual microstructure of TC4m /5A06Al composites was observed by ZEISS-40MAT optical microscope, Olympus SZX7 stereomicroscope and S-4700 scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX). The measurement of geometric dimension of craters was presented as following: The top and corresponding cross-section photos of craters were taken by Olympus SZX7 stereomicroscope. Then, photos were opened by Micro-image Analysis & Process software and geometric measurement function in Measurement Menu was used to measure the geometric dimension of craters. Measurement accuracy is 0.001 mm.The further observation was carried out on Philips CM-12 transmission electron microscopy (TEM) equipment with an accelerated voltage of 120 kV and Tecnai G2 F30 high-resolution transmission electron microscopy (HRTEM) with an accelerated voltage of 300 kV. 3. Results and discussion 3.1. Microstructure of TC4m /5A06Al composite before impact Fig. 1 shows the microstructure of TC4m /5A06Al composite before impact. Since high pressure was applied during fabrication, the TC4 meshes were well infiltrated by the molten Al and no apparent porosity or significant casting defects were observed. Moreover, TC4 fibres distributed uniformly and some secondary phases ((Fe, Mn)Al6 ) were found in the Al matrix. However, a thin interface (5 m) was noticed between fibre and Al matrix (Fig. 1b). It was reported that a series of Ti–Al intermetallic compounds, such as Ti3 Al phase, TiAl phase and TiAl3 phase would be formed between Ti fibre and Al above 600 ◦ C [18,19]. However, TiAl3 phase was formed to be the main phase in the layer when Al was excessive [20], which is similar to present work. It was revealed by EDX that the atomic ratio of Al/Ti in the interface layer was 3:1, which corresponds to TiAl3 . 3.2. Microstructure of TC4m /5A06Al composite after impact 3.2.1. Microstructure of crater The top and corresponding cross-section views of craters in 5A06Al and TC4m /5A06Al composite targets after impact are shown in Fig. 2. The shape of the craters in both targets is hemispheric since the density of projectiles (2024Al) is close to that of targets (5A06Al or TC4m /5A06Al composite) [21]. Moreover, the traces
Table 1 Geometric dimension of craters in 5A06Al alloy and TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and velocity of 3.79 km/s. Experiments number
Target
Depth of craters (mm)
Diameter on the initial impact surface (mm)
Flanging diameter (mm)
1 2 3 4 5 6
5A06Al
3.34 3.37 3.30 2.99 2.94 2.97
5.77 5.70 5.79 6.37 6.44 6.38
7.31 7.28 7.42 7.90 7.99 7.93
TC4m /5A06Al
of the molten Al (from projectiles or targets) could be found on surface of the craters both in alloy (Fig. 2a) and composite (Fig. 2b). Furthermore, flange around pithead on the top surface, which was generated from deformation of the front face along the reverse direction of impact due to surface wave during hypervelocity impact [21], was more significant in 5A06Al alloy than in TC4m /5A06Al composite. Table 1 summarizes the geometric dimension of craters in 5A06Al alloy and TC4m /5A06Al composite. Depth of crater is one of the most important indexes to evaluate the resistance of materials to the hypervelocity impact [21]. Actually, three repeated high velocity experiments for the 5A06Al alloy and TC4m /5A06Al composite were done in order to obtain the convincing results, which were shown in Table 1. According to Table 1, the depth of crater in TC4m /5A06Al composite was decreased 9.4–12.8% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. Mechanism of better performance of composite lies in the following two aspects. Firstly, enhanced projectile erosion. Impact pressures of Al-5A06Al and Al-TC4m /5A06Al are 36.3 GPa and 38.1 GPa, respectively, according to Eq. (1) [21]. It demonstrates that Al projectile in the impact process of Al-TC4m /5A06Al subjects to higher pressure than that in the initial impact process of Al-5A06Al. Thus, higher pressure enhanced Al projectile erosion in the impact process of Al-TC4m /5A06Al. Secondly, introduction of high strength, modulus and ductility of TC4 fibres to 5A06 alloy can improve the strength of composite. Thus, TC4m /5A06Al composite can resist the penetration of Al projectile more effectively than 5A06 alloy in the following penetration process. P = (CUp + SUp2 )
(1)
where P is the impact pressure; is the density of target or projectile; C is the sound velocity in target or projectile; Up is the particle velocity in target or projectile; S is the empirical parameter of target or projectile.
Fig. 3. SEM images of different regions on the surface of crater in TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s. (a) Surface morphology of region 1 in Fig. 2b; (b) surface morphology of region 2 in Fig. 2b; and (c) surface morphology of region 3 in Fig. 2b.
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Fig. 4. Microstructure of different regions around the crater in TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s. (a) Optical microstructure; (b) micro-damage near the pithead; (c) micro-damage around the middle of crater; and (d) micro-damage below the crater.
Fig. 3 illustrates the SEM observation of different regions (region 1, 2 and 3 in Fig. 2b) on the surface of crater in TC4m /5A06Al composite. Severe separation of TC4 fibre and matrix occurred in the flange region (region 1, Fig. 3a). Furthermore, numerous dimples were observed on the fracture surface of TC4 fibre (Fig. 3a), which was generated from plastic deformation during impact. However, a fibre was cut off and its two parts slid with a certain distance in region 2 (Fig. 3b). Moreover, fractography of fibre was not characterized by dimples but melting phenomenon, which indicates the occurrence of adiabatic shearing fracture. Additionally, trace of molten jet fluid along the impacting direction was observed (Fig. 3b). Solidification feature of molten droplets and spreading liquid can be clearly seen in Fig. 3c. From this point of review, melting was occurred at the bottom of crater during hypervelocity impact. Porous structure and cracks were found at the bottom of crater. Projectile, Al matrix and even fibre would be molten under high temperature and pressure during hypervelocity impact. Porous structure and cracks would be formed due to the shrinkage during subsequent rapid solidification. 3.2.2. Microstructure around crater Microstructure of different regions around the crater in TC4m /5A06Al composite is shown in Fig. 4. Three deformation characteristics could be classified in the region near the pithead (Fig. 4b), around (Fig. 4c) and below the crater (Fig. 4d), respectively. In the region near the pithead (Fig. 4b), interface separation of fibre and Al matrix along impact direction were found. Furthermore, holes or cracks at the interface tended to connect with each other. As a
result, some parts near the pithead have separated from the target along the reverse direction of impact direction (Fig. 4b). It should be attributed to the combination of lateral compress and surface wave [21]. In the region around crater (Fig. 4c), severe shear deformation was occurred in fibre. The horizontal fibres deformed along outline of the crater. Moreover, some fibres were sheared into two sections while the angle between shear plane and fibre axis was about 45◦ (Fig. 4c). Below the crater (Fig. 4d), a fibre was adiabatically sheared and slid along 45◦ direction. Moreover, adiabatic shear band whose thickness was less than 1 mm, was observed below the crater (Fig. 4d). The deformation of Al matrix at the bottom of the crater is illustrated in Fig. 5. Significant adiabatic shear band was observed in the Al matrix (Fig. 5a). Further observation revealed the ultra-fined grains (grain size < 200 nm) in the shear band (Fig. 5c), and electron diffraction pattern within the shear band was discontinued diffraction rings. The high temperature, large plastic deformation and high stain energy stored in the shear band led to the formation of these ultra-fine dynamic recrystallized grains [9]. There are two possible shear localization. Firstly, as indicated in oval-shaped region in Fig. 5a, a classical adibatic shear banding due to instability of plastic flow of Al matrix is formed. Secondly, as indicated in oval-shaped region in Fig. 5b, abiabatic shear localization of TC4 fibres in the direction about 45◦ to impact line may trigger forced shear localization in matrix. Such behaviour was also observed in impacted Ti–6Al–4V alloys with alumina tubes and rods inclusions. Gu and Nesterenko believed that [22], for composites, the significant difference from the homogeneous materials is that the fracture of the
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Fig. 5. Deformation microstructure in Al matrix at the bottom of the crater. (a) Optical microstructure, (b) SEM and (c) TEM microstructure of adiabatic shear band; and (d) SEAD within the adiabatic shear band.
Fig. 6. Deformation of the second phase in Al matrix at the bottom of the crater. (a) SEM photo; (b) TEM photo; and (c) SAED of second phase.
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Fig. 7. Deformation at the interface (TiAl phase) of TC4 fibre and Al matrix at the bottom of crater. (a) Interface morphology; (b) EDX analysis of interface; (c) SAED of the interface as indicated by solid line circle; and (d) SAED of the interface as indicated by dotted line circle.
alumina rods and tubes creates a larger volume involved in interaction with the projectile. The forced shear bands was crated by fracture of alumina tubes in the direction about 45◦ to impact line or initialed fracture of Al2 O3 tubes accompanied spontaneous shear bands resulting from instability of matrix materials. In this paper, abiabatic shear failure of TC4 fibres along 45◦ may cause shearing and cracks in Al matrix, as shown in Fig. 5b and c. The deformation of the second phase in Al matrix at the bottom of the crater is shown in Fig. 6. The blocky (Fe, Mn)Al6 phase has been sheared into two parts completely along the impact direction, followed with several cracks (Fig. 6a). Furthermore, formation of twins in (Fe, Mn)Al6 was observed by TEM (Fig. 6b). A crack propagated to cut the twin was also found by TEM analysis (Fig. 6b). Interface of TC4 fibre and Al matrix at the bottom of crater is illustrated in Fig. 7. There were two layers within the interface (Fig. 7a). EDX analysis (Fig. 7b) revealed the chemical composition of the interface layer are titanium and aluminium with atomic ratio of 43:40, respectively, which is close to that of TiAl phase. The halo pattern (Fig. 7c) of the outer layer (solid line circle region in Fig. 7a) demonstrates the amorphous structure formed at interface after impact as stable compound is prone to form amorphous structure [23]. However, halo and rings diffraction patterns (Fig. 7d) were observed in the inner thin layer (dotted line circle region in Fig. 7a). After calibration, diffraction rings of {1 1 0} and {2 2 0} crystalline plane of TiAl phase have been confirmed within the patterns, implying that the inner layer was composed of TiAl nanocrystalline and amorphous phase. Both heating and cooling time of interface are on the order of fraction of milliseconds and nucleation–growth processes are inhibited during hypervelocity impact [24,25], which
lead to the formation of amorphous structure. However, a thin layer close to TC4 fibre would have the opportunity to nucleate since fibre surface would greatly decrease the energy for nucleation [26]. Deformation of TiAl3 phase at the interface between TC4 fibre and Al matrix at the bottom of crater is shown in Fig. 8. Deformed (Fig. 8a) and transformed (Fig. 8b) band were observed at interfacial layer. It is obvious that the TiAl3 grains are elongated severely in the deformed band (Fig. 8a). Adiabatic shear band is the typical microstructure characteristic on the condition of local shear deformation and can be classified as deformed band based on the characteristic of severely elongated and fine grains [11] while transformed band based on the characteristic of recrystalline grains [27,28], phase transformation [25,29] and amorphization [25]. Microstructure in transformed band is illustrated in Fig. 8b. Maximum shear stress at the bottom of crater determines strain rate and temperature raise of adiabatic shear band [11], which finally makes the microstructure of adiabatic shear band vary from deformed to transformed band. However, different regions undertake different shear stress, which leads to different structure at the interface. It was proposed by Perez-Prado and Hines [27] that numerous vacancies would be generated during formation process of adiabatic shear band. These vacancies would affect the diffusion, especially the phase transformation and recrystallized process in the transformed band. Therefore, formation of equiaxed grains from dynamic recrystallization could be accomplished in very short time with association of high-density vacancies at high temperature. Furthermore, paralleled edge dislocations, which result in the deflection of (1 1¯ 2) plane with 10◦ , were observed in a subgrain boundary in nano-scale TiAl3 phase by HRTEM (Fig. 8d).
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Fig. 8. Deformation at the interface (TiAl3 phase) of TC4 fibre and Al matrix at the bottom of crater. (a) Deformed band of TiAl3 phase at the interface; (b) nano-sized TiAl3 phase at the interface; (c) SAED of nano-sized TiAl3 phase; and (d) subgrain boundary in nano-sized TiAl3 phase.
4. Conclusions 1. The depth of crater in TC4m /5A06Al composite was decreased 9.4–12.8% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. 2. Numerous dimples from plastic deformation were observed on the fracture surface of TC4 fibre near the pithead. Adiabatic shearing fracture was occurred in the middle part of crater. Porous structure and cracks were found at the bottom of crater. 3. Interface separation of fibre and Al matrix along impact direction was observed in the region near the pithead. The original horizontal fibres deformed along outline of the crater in the region around crater. Moreover, some fibres were sheared into two sections along 45◦ direction. Below the crater, fibre was adiabatically sheared and slid along 45◦ direction. 4. Significant adiabatic shear band containing ultra-fined grains (grain size < 200 nm) was observed in the Al matrix at the bottom of the crater. The blocky (Fe, Mn)Al6 phase has been sheared into
two parts completely along the impact direction, followed with several cracks. Formation and fracture of twins in (Fe, Mn)Al6 were observed. 5. TiAl amorphous structure was formed at the interface far away from TC4 fibre. The inner layer close to TC4 fibre was composed of TiAl nanocrystalline and amorphous phase. 6. Deformed band with elongated grains and transformed band with equiaxed recrystallized grains were observed at the interface between TC4 fibre and Al matrix at the bottom of crater.
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Damage behaviour of Al matrix composite reinforced with Ti–6Al–4V meshes under the hypervelocity impact Q. Guo, D.L. Sun ∗ , X.L. Han, W.S. Yang, L.T. Jiang, G.H. Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 9 December 2011 Accepted 14 December 2011 Available online 21 December 2011 Keywords: Composites Impact behaviour Microstructures Shear bands Interfaces
a b s t r a c t 15 vol.% Ti–6Al–4V meshes reinforced 5A06Al matrix composites (TC4m /5A06Al) were fabricated by pressure infiltration method. The resistance of TC4m /5A06Al composites to hypervelocity impact was tested by a two-stage light gas gun. Moreover, damage behaviour and residual microstructure of the composite were investigated. The depth of crater in TC4m /5A06Al composite was decreased 9.4–12.81% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. Numerous dimples from plastic deformation were observed on the fracture surface of TC4 fibre near the pithead. Adiabatic shearing fracture was occurred in the middle part of crater. Porous structure and cracks were found at the bottom of crater. Interface separation of fibre and Al matrix along impact direction were observed in the region near the pithead. The original horizontal fibres deformed along the outline of the crater in the region around crater. Moreover, some fibres were sheared into two sections along 45◦ direction. Below the crater, fibre was adiabatically sheared and slid along 45◦ direction. Significant adiabatic shear band containing ultra-fined grains (grain size < 200 nm) was observed in the Al matrix at the bottom of the crater. The blocky (Fe, Mn)Al6 phase was sheared into two parts completely along the impact direction, followed with several cracks. Formation and fracture of twins in (Fe, Mn)Al6 were also observed. TiAl amorphous structure was formed at the interface far away from TC4 fibre. The inner layer close to TC4 fibre was composed of TiAl nanocrystalline and amorphous phase. Deformed band with elongated grains and transformed band with equiaxed recrystallized grains were observed at the interface between TC4 fibre and Al matrix at the bottom of crater. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The shelter of space vehicles is very important in order to protect them from the impact of micrometeoroids and space debris [1]. Moreover, it is very important to investigate the dynamic response of materials under the impact of hypervelocity projectile with progressive increase of space activities [2,3]. Therefore, the research on the damage behaviours of materials under the impact of hypervelocity projectiles have been carried out for several decades, and deformation and fracture of steel [4,5], copper alloys [6,7], titanium alloys [8], magnesium alloys [9,10] and aluminium alloys [11–14] have been reported. It was observed by Sunwoo et al. that dominant failure mechanism of AerMet-100 (Fe–Ni–Co) alloy appeared to be dynamic fracture along adiabatic shear bands and shear bands differed in size and morphology depending on the heat-treated conditions [4]. Three kinds of micro-damages in the region around the crater (namely, micro-cracks, microvoids and adiabatic shear bands) were
∗ Corresponding author. Tel.: +86 0451 86418635; fax: +86 0451 86413922. E-mail address: [email protected] (D.L. Sun). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.12.055
observed in 30CrMnSiA steel by Zhou et al. [5]. However, solidstate plastic flow associated with dynamic recrystallization was the prominent mechanism in the penetration process of oxygenfree high conductivity (OFHC) copper alloy [6]. Furthermore, Rivas et al. [7] found that a wide zone exhibiting microband clusters with unidirectional, elongated, cell-like microstructures was coincident with trace of {1 1 1} crystal plane in OFHC copper. The investigation on TC4 alloy indicated that deformed shear bands were formed at an early stage of the localization, which was transformed into the white-etching shear bands with increase of shear strain [8]. Furthermore, the deformed microstructure below the crater of AM60B Mg alloy was classified into the dynamic recrystallization zone, the high density deformation twin zone and the low density deformation twin zone as described by Zou et al. [9,10]. Moreover, micro-cracks, microvoids and adiabatic shear bands were produced by hypervelocity impacts in Al–6Mg alloy [11]. Additionally, the effect of rare earth element (Sc) into Al alloys was also investigated. Al3 Sc precipitates suppressed dislocation motion and promoted an increase in the work hardening stress [12]. Ye et al. [13] assumed that secondary Al3 Sc phase lost its coherency and recrystallized due to the huge amount of energy from high-speed impact. Gao et al. [14] concluded that adiabatic shear lines, arc-like shearing
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Fig. 1. Microstructure of TC4m /5A06Al composite. (a) SEM microstructure of composite; (b) interface between matrix and TC4 fibre; (c) EDX analysis of interface.
bands and DRX grains due to the concentrated stress and heat were observed in 2519A aluminium alloy plate impacted at 573 K. Nesterenko and Goldsmith’s investigation [15] showed that The branching and bifurcation of shear bands and interaction with one another are a distinct and a common feature of the hot isostatically pressed Ti–6Al–4V targets relative to the commercially available Ti–6Al–4V alloy (bar, forged, annealed). They consider this feature being mainly responsible for the enhanced ballistic performance of hot isostatically pressed Ti–6Al–4V targets materials. The shear bands are the preferred sites for nucleation, growth, and coalescence of voids and are, as such, precursors to failure. The evolution of shear-band pattern during the deformation process reveals a self-organization character [16]. Unfortunately, the damage bahaviour of aluminium matrix composites, an important candidate for aerospace application, has been rarely reported. Since TC4 (Ti–6Al–4V) fibres present low density, high modulus, strength and good ductility, it would be very promising for aluminium matrix composite reinforced with TC4 fibre (TC4f /Al) for shelter application. Therefore, in present work, 15 vol.% TC4 meshes reinforced 5A06Al matrix composites
(TC4m /5A06Al) were fabricated by pressure infiltration method. The resistance of TC4m /5A06Al composites to hypervelocity impact was tested by a two-stage light gas gun, whereas damage behaviour and residual microstructure of the composite were also investigated. 2. Experimental TC4 meshes (15 vol.%) were used as reinforcement to obtain isotropic properties in planes in the composites. The fibre diameter was 100 m while the spacing between fibres in mesh was 500 m. 5A06Al alloy was selected as matrix, whose composition was 5.8–6.8 Mg, 0.5–0.8 Mn, 0.4 Fe, 0.4 Si, 0.05 Cu, 0.02–0.10 Ti and the balance was Al. Later the TC4m /5A06Al composites were fabricated by pressure infiltration method [17]. Temperatures for melting Al alloy and mould were 750 and 500 ◦ C, respectively. During the infiltration process, a pressure of 5 MPa was applied and maintained for 10 min, and then the composites were solidified in air. Afterward, the TC4m /5A06Al composites for hypervelocity impact testing were annealed at 330 ◦ C for 30 min.
Fig. 2. Macro-morphologies of craters in (a) and (c) 5A06Al alloy, and (b) and (d) TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s.
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The hypervelocity impact testing was performed on a twostage light gas gun at the Hypervelocity Aerodynamics Institute (China Aerodynamics R&D Center) using 2 mm 2024Al projectile with a speed of 3.79 km/s and normal angle. The thickness of TC4m /5A06Al composite target was 15 mm. Residual microstructure of TC4m /5A06Al composites was observed by ZEISS-40MAT optical microscope, Olympus SZX7 stereomicroscope and S-4700 scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX). The measurement of geometric dimension of craters was presented as following: The top and corresponding cross-section photos of craters were taken by Olympus SZX7 stereomicroscope. Then, photos were opened by Micro-image Analysis & Process software and geometric measurement function in Measurement Menu was used to measure the geometric dimension of craters. Measurement accuracy is 0.001 mm.The further observation was carried out on Philips CM-12 transmission electron microscopy (TEM) equipment with an accelerated voltage of 120 kV and Tecnai G2 F30 high-resolution transmission electron microscopy (HRTEM) with an accelerated voltage of 300 kV. 3. Results and discussion 3.1. Microstructure of TC4m /5A06Al composite before impact Fig. 1 shows the microstructure of TC4m /5A06Al composite before impact. Since high pressure was applied during fabrication, the TC4 meshes were well infiltrated by the molten Al and no apparent porosity or significant casting defects were observed. Moreover, TC4 fibres distributed uniformly and some secondary phases ((Fe, Mn)Al6 ) were found in the Al matrix. However, a thin interface (5 m) was noticed between fibre and Al matrix (Fig. 1b). It was reported that a series of Ti–Al intermetallic compounds, such as Ti3 Al phase, TiAl phase and TiAl3 phase would be formed between Ti fibre and Al above 600 ◦ C [18,19]. However, TiAl3 phase was formed to be the main phase in the layer when Al was excessive [20], which is similar to present work. It was revealed by EDX that the atomic ratio of Al/Ti in the interface layer was 3:1, which corresponds to TiAl3 . 3.2. Microstructure of TC4m /5A06Al composite after impact 3.2.1. Microstructure of crater The top and corresponding cross-section views of craters in 5A06Al and TC4m /5A06Al composite targets after impact are shown in Fig. 2. The shape of the craters in both targets is hemispheric since the density of projectiles (2024Al) is close to that of targets (5A06Al or TC4m /5A06Al composite) [21]. Moreover, the traces
Table 1 Geometric dimension of craters in 5A06Al alloy and TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and velocity of 3.79 km/s. Experiments number
Target
Depth of craters (mm)
Diameter on the initial impact surface (mm)
Flanging diameter (mm)
1 2 3 4 5 6
5A06Al
3.34 3.37 3.30 2.99 2.94 2.97
5.77 5.70 5.79 6.37 6.44 6.38
7.31 7.28 7.42 7.90 7.99 7.93
TC4m /5A06Al
of the molten Al (from projectiles or targets) could be found on surface of the craters both in alloy (Fig. 2a) and composite (Fig. 2b). Furthermore, flange around pithead on the top surface, which was generated from deformation of the front face along the reverse direction of impact due to surface wave during hypervelocity impact [21], was more significant in 5A06Al alloy than in TC4m /5A06Al composite. Table 1 summarizes the geometric dimension of craters in 5A06Al alloy and TC4m /5A06Al composite. Depth of crater is one of the most important indexes to evaluate the resistance of materials to the hypervelocity impact [21]. Actually, three repeated high velocity experiments for the 5A06Al alloy and TC4m /5A06Al composite were done in order to obtain the convincing results, which were shown in Table 1. According to Table 1, the depth of crater in TC4m /5A06Al composite was decreased 9.4–12.8% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. Mechanism of better performance of composite lies in the following two aspects. Firstly, enhanced projectile erosion. Impact pressures of Al-5A06Al and Al-TC4m /5A06Al are 36.3 GPa and 38.1 GPa, respectively, according to Eq. (1) [21]. It demonstrates that Al projectile in the impact process of Al-TC4m /5A06Al subjects to higher pressure than that in the initial impact process of Al-5A06Al. Thus, higher pressure enhanced Al projectile erosion in the impact process of Al-TC4m /5A06Al. Secondly, introduction of high strength, modulus and ductility of TC4 fibres to 5A06 alloy can improve the strength of composite. Thus, TC4m /5A06Al composite can resist the penetration of Al projectile more effectively than 5A06 alloy in the following penetration process. P = (CUp + SUp2 )
(1)
where P is the impact pressure; is the density of target or projectile; C is the sound velocity in target or projectile; Up is the particle velocity in target or projectile; S is the empirical parameter of target or projectile.
Fig. 3. SEM images of different regions on the surface of crater in TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s. (a) Surface morphology of region 1 in Fig. 2b; (b) surface morphology of region 2 in Fig. 2b; and (c) surface morphology of region 3 in Fig. 2b.
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Fig. 4. Microstructure of different regions around the crater in TC4m /5A06Al composite impacted by Al projectile with the diameter of 2 mm and the velocity of 3.79 km/s. (a) Optical microstructure; (b) micro-damage near the pithead; (c) micro-damage around the middle of crater; and (d) micro-damage below the crater.
Fig. 3 illustrates the SEM observation of different regions (region 1, 2 and 3 in Fig. 2b) on the surface of crater in TC4m /5A06Al composite. Severe separation of TC4 fibre and matrix occurred in the flange region (region 1, Fig. 3a). Furthermore, numerous dimples were observed on the fracture surface of TC4 fibre (Fig. 3a), which was generated from plastic deformation during impact. However, a fibre was cut off and its two parts slid with a certain distance in region 2 (Fig. 3b). Moreover, fractography of fibre was not characterized by dimples but melting phenomenon, which indicates the occurrence of adiabatic shearing fracture. Additionally, trace of molten jet fluid along the impacting direction was observed (Fig. 3b). Solidification feature of molten droplets and spreading liquid can be clearly seen in Fig. 3c. From this point of review, melting was occurred at the bottom of crater during hypervelocity impact. Porous structure and cracks were found at the bottom of crater. Projectile, Al matrix and even fibre would be molten under high temperature and pressure during hypervelocity impact. Porous structure and cracks would be formed due to the shrinkage during subsequent rapid solidification. 3.2.2. Microstructure around crater Microstructure of different regions around the crater in TC4m /5A06Al composite is shown in Fig. 4. Three deformation characteristics could be classified in the region near the pithead (Fig. 4b), around (Fig. 4c) and below the crater (Fig. 4d), respectively. In the region near the pithead (Fig. 4b), interface separation of fibre and Al matrix along impact direction were found. Furthermore, holes or cracks at the interface tended to connect with each other. As a
result, some parts near the pithead have separated from the target along the reverse direction of impact direction (Fig. 4b). It should be attributed to the combination of lateral compress and surface wave [21]. In the region around crater (Fig. 4c), severe shear deformation was occurred in fibre. The horizontal fibres deformed along outline of the crater. Moreover, some fibres were sheared into two sections while the angle between shear plane and fibre axis was about 45◦ (Fig. 4c). Below the crater (Fig. 4d), a fibre was adiabatically sheared and slid along 45◦ direction. Moreover, adiabatic shear band whose thickness was less than 1 mm, was observed below the crater (Fig. 4d). The deformation of Al matrix at the bottom of the crater is illustrated in Fig. 5. Significant adiabatic shear band was observed in the Al matrix (Fig. 5a). Further observation revealed the ultra-fined grains (grain size < 200 nm) in the shear band (Fig. 5c), and electron diffraction pattern within the shear band was discontinued diffraction rings. The high temperature, large plastic deformation and high stain energy stored in the shear band led to the formation of these ultra-fine dynamic recrystallized grains [9]. There are two possible shear localization. Firstly, as indicated in oval-shaped region in Fig. 5a, a classical adibatic shear banding due to instability of plastic flow of Al matrix is formed. Secondly, as indicated in oval-shaped region in Fig. 5b, abiabatic shear localization of TC4 fibres in the direction about 45◦ to impact line may trigger forced shear localization in matrix. Such behaviour was also observed in impacted Ti–6Al–4V alloys with alumina tubes and rods inclusions. Gu and Nesterenko believed that [22], for composites, the significant difference from the homogeneous materials is that the fracture of the
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Fig. 5. Deformation microstructure in Al matrix at the bottom of the crater. (a) Optical microstructure, (b) SEM and (c) TEM microstructure of adiabatic shear band; and (d) SEAD within the adiabatic shear band.
Fig. 6. Deformation of the second phase in Al matrix at the bottom of the crater. (a) SEM photo; (b) TEM photo; and (c) SAED of second phase.
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Fig. 7. Deformation at the interface (TiAl phase) of TC4 fibre and Al matrix at the bottom of crater. (a) Interface morphology; (b) EDX analysis of interface; (c) SAED of the interface as indicated by solid line circle; and (d) SAED of the interface as indicated by dotted line circle.
alumina rods and tubes creates a larger volume involved in interaction with the projectile. The forced shear bands was crated by fracture of alumina tubes in the direction about 45◦ to impact line or initialed fracture of Al2 O3 tubes accompanied spontaneous shear bands resulting from instability of matrix materials. In this paper, abiabatic shear failure of TC4 fibres along 45◦ may cause shearing and cracks in Al matrix, as shown in Fig. 5b and c. The deformation of the second phase in Al matrix at the bottom of the crater is shown in Fig. 6. The blocky (Fe, Mn)Al6 phase has been sheared into two parts completely along the impact direction, followed with several cracks (Fig. 6a). Furthermore, formation of twins in (Fe, Mn)Al6 was observed by TEM (Fig. 6b). A crack propagated to cut the twin was also found by TEM analysis (Fig. 6b). Interface of TC4 fibre and Al matrix at the bottom of crater is illustrated in Fig. 7. There were two layers within the interface (Fig. 7a). EDX analysis (Fig. 7b) revealed the chemical composition of the interface layer are titanium and aluminium with atomic ratio of 43:40, respectively, which is close to that of TiAl phase. The halo pattern (Fig. 7c) of the outer layer (solid line circle region in Fig. 7a) demonstrates the amorphous structure formed at interface after impact as stable compound is prone to form amorphous structure [23]. However, halo and rings diffraction patterns (Fig. 7d) were observed in the inner thin layer (dotted line circle region in Fig. 7a). After calibration, diffraction rings of {1 1 0} and {2 2 0} crystalline plane of TiAl phase have been confirmed within the patterns, implying that the inner layer was composed of TiAl nanocrystalline and amorphous phase. Both heating and cooling time of interface are on the order of fraction of milliseconds and nucleation–growth processes are inhibited during hypervelocity impact [24,25], which
lead to the formation of amorphous structure. However, a thin layer close to TC4 fibre would have the opportunity to nucleate since fibre surface would greatly decrease the energy for nucleation [26]. Deformation of TiAl3 phase at the interface between TC4 fibre and Al matrix at the bottom of crater is shown in Fig. 8. Deformed (Fig. 8a) and transformed (Fig. 8b) band were observed at interfacial layer. It is obvious that the TiAl3 grains are elongated severely in the deformed band (Fig. 8a). Adiabatic shear band is the typical microstructure characteristic on the condition of local shear deformation and can be classified as deformed band based on the characteristic of severely elongated and fine grains [11] while transformed band based on the characteristic of recrystalline grains [27,28], phase transformation [25,29] and amorphization [25]. Microstructure in transformed band is illustrated in Fig. 8b. Maximum shear stress at the bottom of crater determines strain rate and temperature raise of adiabatic shear band [11], which finally makes the microstructure of adiabatic shear band vary from deformed to transformed band. However, different regions undertake different shear stress, which leads to different structure at the interface. It was proposed by Perez-Prado and Hines [27] that numerous vacancies would be generated during formation process of adiabatic shear band. These vacancies would affect the diffusion, especially the phase transformation and recrystallized process in the transformed band. Therefore, formation of equiaxed grains from dynamic recrystallization could be accomplished in very short time with association of high-density vacancies at high temperature. Furthermore, paralleled edge dislocations, which result in the deflection of (1 1¯ 2) plane with 10◦ , were observed in a subgrain boundary in nano-scale TiAl3 phase by HRTEM (Fig. 8d).
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Fig. 8. Deformation at the interface (TiAl3 phase) of TC4 fibre and Al matrix at the bottom of crater. (a) Deformed band of TiAl3 phase at the interface; (b) nano-sized TiAl3 phase at the interface; (c) SAED of nano-sized TiAl3 phase; and (d) subgrain boundary in nano-sized TiAl3 phase.
4. Conclusions 1. The depth of crater in TC4m /5A06Al composite was decreased 9.4–12.8% as compared to 5A06Al, indicating that the addition of TC4 fibre is beneficial to the hypervelocity impact resistance. 2. Numerous dimples from plastic deformation were observed on the fracture surface of TC4 fibre near the pithead. Adiabatic shearing fracture was occurred in the middle part of crater. Porous structure and cracks were found at the bottom of crater. 3. Interface separation of fibre and Al matrix along impact direction was observed in the region near the pithead. The original horizontal fibres deformed along outline of the crater in the region around crater. Moreover, some fibres were sheared into two sections along 45◦ direction. Below the crater, fibre was adiabatically sheared and slid along 45◦ direction. 4. Significant adiabatic shear band containing ultra-fined grains (grain size < 200 nm) was observed in the Al matrix at the bottom of the crater. The blocky (Fe, Mn)Al6 phase has been sheared into
two parts completely along the impact direction, followed with several cracks. Formation and fracture of twins in (Fe, Mn)Al6 were observed. 5. TiAl amorphous structure was formed at the interface far away from TC4 fibre. The inner layer close to TC4 fibre was composed of TiAl nanocrystalline and amorphous phase. 6. Deformed band with elongated grains and transformed band with equiaxed recrystallized grains were observed at the interface between TC4 fibre and Al matrix at the bottom of crater.
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