Quasi-static and dynamic deformation behavior of Ti–6Al–4V alloy containing fine α2-Ti3Al precipitates

Materials Science and Engineering A366 (2004) 25–37

Quasi-static and dynamic deformation behavior of Ti–6Al–4V alloy containing fine ␣2 -Ti3Al precipitates Dong-Geun Lee, Sunghak Lee∗ , Chong Soo Lee Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 22 October 2002; received in revised form 14 August 2003

Abstract Widmanstätten and equiaxed microstructures containing very fine ␣2 particles were obtained by aging a Ti–6Al–4V alloy, and their quasi-static and dynamic deformation behavior was investigated in comparison to that of unaged microstructures. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar, and torsionally deformed areas beneath fracture surfaces were observed to investigate various microstructural factors determining the deformation behavior and effects of ␣2 precipitation. The dynamic torsional test results indicated that maximum shear stress and fracture shear strain of the aged Widmanstätten and equiaxed microstructures were higher than those of the unaged microstructures. The number of voids initiated in the aged Widmanstätten and equiaxed microstructures was five times greater than those in unaged microstructures because of the ␣2 precipitation. This indicated that the aging treatment had a homogenizing effect, i.e., less likelihood of developing a region of concentrated strain that preceded the adiabatic shear band formation, thereby reducing the possibility of the adiabatic shear band formation. Fine ␣2 precipitation by aging was effective in the improvement of quasi-static and dynamic torsional properties and in the reduction of the adiabatic shear banding, which provided a new idea to improve ballistic performance of Ti alloy armor plates. © 2003 Elsevier B.V. All rights reserved. Keywords: Dynamic deformation behavior; Ti–6Al–4V alloy; ␣2 -Ti3 Al precipitate; Widmanstätten microstructure; Equiaxed microstructure; Dynamic torsional test; Adiabatic shear band

1. Introduction Ti–6Al–4V alloy has high specific strength and stiffness, outstanding corrosion resistance and high-temperature properties as well as having low density, thereby providing great potential to be used as structural armor materials [1–5]. Its microstructure is largely divided into Widmanstätten, equiaxed, and bimodal microstructures according to heat treatment conditions. The equiaxed microstructure has high tensile strength and elongation, and excellent resistance to fatigue crack initiation [6,7], while the Widmanstätten microstructure has high creep strength and fracture toughness, and excellent resistance to crack propagation. Many researches have focused on obtaining desired mechanical properties by controlling microstructural factors such as prior ␤ grain size, colony size, thickness of boundary ␣ phase, and volume fractions of ␣ and ␤ through heat treat∗ Corresponding author. Tel.: +82-54-279-2140; fax: +82-54-279-2399. E-mail address: [email protected] (S. Lee).

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.061

ments or thermo-mechanical treatments [8–12]. When the Ti–6Al–4V alloy is aged at 500–600 ◦ C, nanometer-sized, ordered ␣2 (Ti3 Al) phases can be homogeneously precipitated inside the ␣ phase, thereby leading to the additional improvement of mechanical properties [13,14]. Al in the Ti–6Al–4V alloy plays a role in increasing the ␣/␤ transformation temperature and in forming a region coexisting ␣ and ␣2 phases in the phase diagram because it works as an ␣ stabilizing element [15]. ␣2 has a structure of DO19 , and is homogeneously precipitated in a form of very fine particles having coherent relationship with ␣ during aging. The size and inter-particle spacing of ␣2 are mainly affected by aging temperature and concentration of Al. However, deformation and fracture behaviors of the Ti–6Al–4V alloy under dynamic loading are rarely studied because most of studies are related to phenomena occurring under static or quasi-static loading. Thus, it is required to obtain information on dynamic deformation and fracture behaviors of the Ti–6Al–4V alloy so that it can be effectively applied to strategic fields such as defense, aerospace, precision machinery, and automotive industries. Under

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dynamic loading such as ballistic impact, machining, and high-speed metal forming, resistance to deformation and fracture is generally lower than under quasi-static loading [16–19], and plastic deformation is often highly localized in a narrow region, which is called an adiabatic shear band. This is believed to initiate crack due to the development of a thermo-mechanical plastic instability. Since localization generally induces failure of structural components through a loss in load-carrying capacity within the shear band [20–22], the adiabatic shear banding is of great interest to the ballistic performance of armors. Therefore, studies on dynamic deformation behavior are essential for evaluation of ballistic performance, alloy designing, microstructural modification, and process control in high-speed manufacturing in order to improve dynamic properties, but only limited information is available on the dynamic properties of the Ti–6Al–4V alloy. In this study, Widmanstätten and equiaxed microstructures containing fine ␣2 particles were obtained by heat treating a Ti–6Al–4V alloy, and their quasi-static and dynamic deformation behavior was investigated. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar, and deformed microstructures and fracture surfaces were observed to investigate microstructural factors affecting both deformation behavior and adiabatic shear banding behavior. Since these experiments allow direct comparison and analysis of the deformation behavior occurring under quasi-static and dynamic loading, it is expected to contribute to fundamental understanding of the deformation mechanisms.

2. Experimental The material used in this study was a Ti–6Al–4V alloy plate (thickness; 50 mm) obtained from Supra Alloys Inc., Camarillo, CA, and its chemical composition was 6.19Al – 4.05V– 0.19Fe– 0.12O– 0.02C–0.01N– 0.004H– Ti (wt.%). This alloy plate was subjected to different heat

treatments as shown in Fig. 1(a)–(c) to obtain Widmanstätten and equiaxed microstructures and to precipitate fine ␣2 phases inside ␣ phases [23]. The Widmanstätten microstructure was obtained by holding at 1050 ◦ C, above the ␤ transformation temperature, for 1 h followed by furnace cooling (Fig. 1(a)), while the equiaxed microstructure by holding at 950 ◦ C, the ␣ + ␤ region, for 1 h followed by furnace cooling (Fig. 1(b)). These two microstructures were aged at 545 ◦ C for 200 h to precipitate very fine ␣2 phases (Fig. 1(c)). Specimens were etched using a Kroll solution (H2 O 100 ml, HF 3 ml, HNO3 5 ml) for examination by optical microscopy. The size of each phase was measured using an image analyzer. Tensile bars were machined with a gage length of 30 mm and a gage diameter of 6 mm, and tensile tests were conducted at a strain rate of 10−3 s−1 . Fracture surfaces were observed using a scanning electron microscope (SEM) after the tests. Hardness of the microstructures aged for 200–300 h was also measured using a Vickers hardness tester under a 2 kg load. Thin-walled tubular specimens used for quasi-static and dynamic torsional tests have a gage length of 2.5 mm and a gage thickness of 280 ␮m as shown in Fig. 2(a). The torsional Kolsky bar consists of a pair of 2 m long 2024-T6 aluminum bars with a diameter of 25.4 mm (Fig. 2(b)) [24]. In the dynamic torsional test, a certain amount of torque is stored between a clamp and a dynamic loading pulley, and then the clamp is fractured, at which time an elastic shear wave is momentarily transmitted into the specimen, deforming it. In this process, incident wave, reflected wave, and transmitted wave are detected respectively at strain gages, and recorded at an oscilloscope. Among the recorded wave signals, average shear strain expressed as a function of time, γ(t), is measured from the reflected wave, while shear stress, τ(t), from the transmitted wave. A dynamic shear stress–shear strain curve is obtained from these γ(t) and τ(t) by eliminating the time term, and is smoothened by low-pass filtering since bumps and wiggles occur in the

Fig. 1. Schematic representation of heat treatments for (a) Widmanstätten, (b) equiaxed microstructures, and for (c) precipitation of fine ␣2 phases.

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Fig. 2. (a) Shape and dimensions of the thin-walled tubular specimen used for the torsional test (unit, mm). (b) Schematic drawing of the torsional Kolsky bar.

experimental curves. Shear strain rate during the test was about 1750 s−1 , and the test was done at room temperature. In the quasi-static torsional test, the incident bar is fixed by a clamp, and then the specimen is deformed slowly at a strain rate of about 10−4 s−1 by transmitting power to the bar by a motor and two speed-reducing motors. Output voltage from the static bridge attached on the transmitter bar and those from two linear variable differential transformers are respectively recorded at an XYt-recorder and an oscilloscope. Each recorded data is transformed into shear stress and shear strain as a function of time, and the time term is eliminated to obtain a quasi-static shear stress–shear strain curve. Detailed descriptions of the dynamic and quasi-static torsional testing are provided in references [20,24–26]. Fracture surfaces were observed in an SEM after the quasi-static and dynamic torsional tests.

3. Results 3.1. Microstructure Fig. 3(a) and (b) are optical micrographs of the Widmanstätten and equiaxed microstructures. In the Widmanstätten microstructure, ␣ phases are formed along prior ␤ grain boundaries, and colonies of lath-type ␤ and ␣ lamel-

lar structure are present inside prior ␤ grains (Fig. 3(a)). Prior ␤ grain size, colony size, and thickness of ␣ platelets were measured to be 300–800 ␮m, 100–350 ␮m, 8–10 ␮m, respectively. In the equiaxed microstructure, about 10 vol.% of ␤ is present at triple points of ␣ grains, and volume fraction and grain size of ␣ are about 91% and 19 ␮m, respectively (Fig. 3(b)). These two microstructures are similar to aged ones. Table 1 shows the quantitative analysis data of the microstructural factors of unaged and aged microstructures. Since these factors are within the error range, the aging treatment hardly affects the optical microstructures. There are some differences in the high-magnification SEM micrographs of the aged Widmanstätten and equiaxed microstructures as shown in Fig. 4(a) and (b). The insets Table 1 Quantitative analysis results of the four microstructures of the Ti–6Al–4V alloy Microstructure

Parameter

Widmanstätten Colony Size (␮m) GB ␣ thickness (␮m) ␣ Platelet thickness (␮m) Equiaxed

Unaged 100–350 8–10 5–6

Primary ␣ grain size (␮m) 19 Primary ␣ Volume fraction (%) 90 ␤ Volume fraction (%) 10

Aged 120–400 7–10 5–7 20 91 9

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Fig. 3. Optical micrographs of the (a) Widmanstätten and (b) equiaxed microstructures; Kroll etched.

are optical micrographs (like Fig. 3(a) and (b)), and the regions marked by four-pointed stars indicate ␣ phases, which are magnified in the SEM micrographs. Fine ␣2 phases of 50–200 nm in size are homogeneously distributed in the ␣ phase of the Widmanstätten and equiaxed microstructures. This matches well with the results of Margolin et al. [13] and Welsch et al. [14].

Fig. 4. SEM micrographs of the aged (a) Widmanstätten and (b) equiaxed microstructures, showing very fine ␣2 phases homogeneously distributed in the ␣ phase. Insets are optical micrographs (like Fig. 3(a) and (b)), and the regions marked by four-pointed stars indicate ␣ phases, which are magnified in the SEM micrographs. Kroll etched.

3.2. Hardness and tensile properties Fig. 5 shows the variation of hardness as a function of aging time at 545 ◦ C. The hardness of the Widmanstätten microstructure is higher than that of the equiaxed microstructure, and the hardness increases with increasing the aging time because of the increased precipitation of ␣2 . The tensile test results of the Widmanstätten and equiaxed microstructures before and after aging for 200 h are listed in Table 2. Before aging, the equiaxed microstructure shows excellent yield strength, tensile strength, and elongation of 872 MPa, 959 MPa, and 15%, respectively, due to the presence of equiaxed, fine ␣ grains.

Fig. 5. Variation of hardness as a function of aging time.

D.-G. Lee et al. / Materials Science and Engineering A366 (2004) 25–37 Table 2 Room-temperature tensile results of the four microstructures of the Ti–6Al–4V alloy Microstructure

Yield strength (MPa)

Unaged Widmanstätten Unaged equiaxed Aged Widmanstätten Aged equiaxed

829 872 877 908

± ± ± ±

6.2 5.0 6.0 4.4

Ultimate tensile strength (MPa) 897 959 908 924

± ± ± ±

7.0 8.3 5.7 4.0

Elongation (%) 12.7 15.1 12.8 15.3

± ± ± ±

0.5 0.3 0.4 0.3

Elongation of the Widmanstätten microstructure is relatively low because deformation at colony boundaries and boundary ␣ phases occurs with ease [27,28]. Yield and tensile strengths of the aged microstructures are higher than those of the unaged microstructures because of the Orowan-type strengthening, while elongation is similar, and thus over-all tensile properties are better in the aged microstructures. Fig. 6(a)–(d) are SEM fractographs of the fractured tensile specimens, and show a typical ductile fracture mode composed of dimples. Dimple size were measured using an image analyzer, and the results are shown in Table 3. The dimple size of the Widmanstätten microstructure is

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Table 3 Dimple sizes measured from fractured tensile, quasi-static torsional, and dynamic torsional specimens for the four microstructures of the Ti–6Al–4V alloy Microstructure

Dimple size (␮m) Tensile loading

Unaged Widmanstätten Unaged equiaxed Aged Widmanstätten Aged equiaxed

5.2 19.5 3.6 10.8

± ± ± ±

Quasi-static torsional loading 1.0 2.5 1.5 1.4

5.6 10.9 3.0 7.5

± ± ± ±

0.5 1.0 1.7 2.4

Dynamic torsional loading 6.3 12.9 3.5 9.8

± ± ± ±

1.2 2.0 1.0 1.6

about 5 ␮m, about the same as the thickness of ␣ platelets (Fig 6(a)), whereas that of the equiaxed microstructure is about 20 ␮m, roughly consistent with the spacing of ␤ phases present at the triple points of ␣ grains (Fig. 6(b)). Thus, dimple size increases in the order of Widmanstätten and equiaxed microstructures because the initiation sites and number of voids vary with the microstructures. Dimple sizes of the aged Widmanstätten and equiaxed microstructures are slightly smaller than those of the unaged microstructures (Fig. 6(c) and (d)).

Fig. 6. SEM fractographs of the fractured tensile specimens of the unaged (a) Widmanstätten, (b) equiaxed microstructures, and the aged (c) Widmanstätten and (d) equiaxed microstructures.

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Fig. 7. Shear stress–shear strain curves obtained from the quasi-static torsional test.

3.3. Quasi-static torsional properties Fig. 7 presents shear stress–shear strain curves obtained from the quasi-static torsional test. From these curves, maximum shear stress, shear strain at maximum shear stress

point, and fracture shear strain were measured, and are summarized in Table 4. All the microstructures show a low level of strain hardening after yielding, and stress continues increasing with straining, reaching eventual fracture. Maximum shear stress and fracture shear strain of the equiaxed microstructure are higher than those of the Widmanstätten, and shows a similar tendency for tensile strength and elongation. Also, the aged microstructures have more excellent torsional properties than the unaged microstructures. Here, maximum √ shear strength roughly satisfies the relationship of σ = 3τ, when compared with tensile strength. SEM fractographs of quasi-statically fractured torsional specimens of the unaged and aged microstructures are shown in Fig. 8(a)–(d). As in the fractured tensile specimens, they all show a typical ductile fracture mode. Fig. 9(a)–(d) are SEM micrographs of the deformed area (the side area of the gage center) beneath the fracture surface of the quasi-statically fractured torsional specimens. In the Widmanstätten microstructure, many voids are initiated at ␣/␤ interfaces or boundary ␣ phases, and their number decreases gradually as it gets deeper from the fracture surface

Fig. 8. SEM fractographs of the quasi-statically fractured torsional specimens for the unaged (a) Widmanstätten, (b) equiaxed microstructures, and the aged (c) Widmanstätten and (d) equiaxed microstructures.

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Table 4 Quasi-static and dynamic torsional properties of the four microstructures of the Ti–6Al–4V alloy Loading condition Microstructure

Maximum shear stress (MPa) Shear strain at maximum shear stress point (%) Fracture shear strain (%)

Quasi-static

Unaged Widmanstätten Unaged equiaxed Aged Widmanstätten Aged equiaxed

470 523 493 596

± ± ± ±

11 7.4 6.8 8.5

13.2 14.8 15.1 26.4

± ± ± ±

0.7 1.0 0.7 0.9

14.1 16.0 17.0 28.1

± ± ± ±

1.2 1.7 0.9 1.6

Dynamic

Unaged Widmanstätten Unaged equiaxed Aged Widmanstätten Aged equiaxed

608 663 654 751

± ± ± ±

5.5 5.2 13 11

10.6 11.4 12.8 23.8

± ± ± ±

0.6 0.8 0.5 0.6

17.3 18.4 24.8 31.7

± ± ± ±

1.5 1.0 1.8 2.1

(Fig. 9(a)). Some voids are observed even at considerable distance from the surface. Voids are initiated mainly at ␣/␤ interfaces distributed at triple points of ␣ grains for the equiaxed microstructure (Fig. 9(b)). The number of voids of the aged Widmanstätten and equiaxed microstructures increases over the unaged microstructures because of the additional precipitation of ␣2 (Fig. 9(c) and (d)).

3.4. Dynamic torsional properties Fig. 10 shows shear stress–shear strain curves obtained from the dynamic torsional test. Maximum shear stress, shear strain at maximum shear stress point, and fracture shear strain under both dynamic and quasi-static loading conditions are compared as shown in Table 4. Maximum

Fig. 9. SEM micrographs of the deformed area (the side area of the gage center) of the quasi-statically fractured torsional specimens for the unaged (a) Widmanstätten, (b) equiaxed microstructures, and the aged (c) Widmanstätten and (d) equiaxed microstructures; Kroll etched.

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Fig. 10. Shear stress–shear strain curves obtained from the dynamic torsional test.

shear stress is higher and maximum shear strain is lower under dynamic loading, but the same tendency over-all is shown. Stress increases but strain decreases under dynamic loading over the case of quasi-static loading because of the

strain rate hardening effect. Maximum shear stress and maximum shear strain of the equiaxed microstructure are higher than those of the Widmanstätten microstructure, as in the trends of tensile strength and elongation of the quasi-static tensile test. Maximum shear stress and maximum shear strain of the aged microstructures increase over the unaged microstructures, and over-all dynamic shear properties of the equiaxed microstructure are more excellent than those of the Widmanstätten microstructure. SEM fractographs of dynamically fractured torsional specimens are shown in Fig. 11(a)–(d). All the microstructures show a ductile fracture mode as in the quasi-statically fractured specimens, but dimples are elongated more in the shear direction than under quasi-static loading. The aged microstructures tend to have larger dimples than the unaged microstructures. Fig. 12(a)–(d) are SEM micrographs of the deformed area (the side area of the gage center) just beneath the fracture surface of the dynamic torsional specimen. Voids are initiated at ␣/␤ interfaces or boundary ␣ phases, and the number of voids initiated near the fracture surface is largely reduced, compared with the quasi-static torsional test. In the unaged

Fig. 11. SEM fractographs of the dynamically fractured torsional specimens for the unaged (a) Widmanstätten, (b) equiaxed microstructures, and the aged (c) Widmanstätten and (d) equiaxed microstructures.

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Fig. 12. SEM micrographs of the deformed area (the side area of the gage center) of the dynamically fractured torsional specimens for the unaged (a) Widmanstätten, (b) equiaxed microstructures, and the aged (c) Widmanstätten and (d) equiaxed microstructures; Kroll etched.

microstructures, a few adiabatic shear bands are formed weakly as marked by arrows in Fig. 12(a) and (b). The number of voids of the aged Widmanstätten and equiaxed microstructures increases over the unaged microstructures (Fig. 12(c) and (d)). Fig. 13(a)–(d) are SEM micrographs of the interior region (the central area of the gage section perpendicular to the fracture region) of the deformed area of the dynamically fractured torsional specimens. The samples were prepared by sectioning, mounting, polishing, and etching the deformed

area. Beneath the fracture surfaces of the four microstructures, localized shear zones exist in the torsional stress direction. The arrowed zone width corresponds to half of the total width of the adiabatic shear band formed at the gage center, and the interior microstructure of the zone is not clearly visible because it was severely deformed. The zone widths of the unaged Widmanstätten and equiaxed microstructures are 8 and 10 ␮m, respectively (Fig. 13(a) and (b)), while those of aged ones are 12 and 18 ␮m, respectively (Fig. 13(c) and (d)).

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Fig. 13. SEM micrographs of the interior region of the deformed area of the dynamically fractured torsional specimens for the unaged (a) Widmanstätten, (b) equiaxed, and the aged (c) Widmanstätten and (d) equiaxed microstructures. These samples were prepared by sectioning, mounting, polishing, and etching the deformed area; Kroll etched.

4. Discussion One of the characteristics that appear when dynamic loading is applied to the Ti–6Al–4V alloy is the formation of adiabatic shear bands [29–31]. In the Widmanstätten and equiaxed microstructures, ␣ and ␤ platelets or ␣ grains are severely elongated along the shear direction to obscure

grain boundaries, forming localized shear zones as shown in Fig. 13(a)–(d). Cho et al. [22] photographed the formation processes of adiabatic shear bands from dynamic torsional specimens of plain carbon steels and ultra-high strength steels using a high-speed camera. They observed that an adiabatic shear band was formed along the center of the gage section at a later stage of dynamic deformation, and that a

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Fig. 14. Number of voids per unit area as a function of distance from the fracture surface for the unaged and aged microstructures.

crack propagated along it. Consequently, the localized shear zone in the Widmanstätten and equiaxed microstructures indicates a half of an adiabatic shear band width formed at the gage section center of the dynamically fractured torsional specimen. In order to quantitatively interpret the possibility of the adiabatic shear band formation, the number of voids per unit area in the deformed region beneath the fracture surface was measured, and the results are shown in Fig. 14. In the quasi-static torsional specimens, the number of voids of the unaged Widmanstätten microstructure is larger than that of the unaged equiaxed microstructure. This indicates that void initiation is related to the number or the area of interfaces initiating voids. Voids are mostly initiated at ␣/␤ interfaces or boundary ␣ phases for the Widmanstätten microstructure [32,33] and at interfaces between ␣ and ␤ distributed at triple points of ␣ grains for the equiaxed microstructure. Void initiation sites are few because of the meager presence of ␤ in the equiaxed microstructure. Since deformation under dynamic loading occurs by a short duration, the time factor required for deformation and the energy needed for plastic deformation should also be considered in addition to the number of void initiation sites and the interfacial areas as considered in the quasi-static loading case. Under dynamic loading, which does not allow enough time for voids to initiate at void initiation sites and to grow, the number of voids in the two microstructures is smaller than that under quasi-static loading. In addition, the amount of shear strain at regions away from the adiabatic shear band is lower than that under quasi-static loading, and thus there are fewer voids under dynamic loading. In the aged Widmanstätten and equiaxed microstructures, the number of voids initiated is five times greater than those in unaged microstructures (Fig. 14). Fig. 12(a)–(d) confirm that the void initiation is more frequently observed in the deformed area of the aged microstructures than in that of the

Fig. 15. SEM micrographs of the deformed area (the central area of the gage section) of the dynamically fractured torsional specimens for the aged (a) Widmanstätten and (b) equiaxed microstructures, showing many voids located inside ␣ phases; Kroll etched.

unaged microstructures. In the higher-magnification SEM micrographs of the deformed area (Fig. 15(a) and (b)), many voids exist inside ␣ phases as well as at boundary ␣ phases, ␣/␤ interfaces, and triple points of ␣ grains. It is also noted that dimple sizes in tensile and torsional specimens of the aged microstructures are smaller than those of the unaged microstructures (Figs. 6, 8, and 11). This implies that ␣2 particles can work as void initiation sites, although they are very fine. The fact that the number of voids formed in the aged microstructures drastically increases because of the ␣2 precipitation is probably associated with the higher strain occurring prior to fracture. This implies that the aging treatment has a homogenizing effect, i.e., less likelihood of developing a region of concentrated strain that precedes the adiabatic shear band formation, thereby reducing the possibility of the adiabatic shear band formation. Fig. 13(a)–(d) indicate that the widths of the localized shear zone of the unaged Widmanstätten and equiaxed microstructures are 8 and 10 ␮m, respectively, which increase to 12 and 18 ␮m after aging. Here, the narrower width implies the larger concentration of shear deformation in the localized shear zone. In

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addition, adiabatic shear bands are not found near the fracture surface in the aged microstructures (Fig. 12(c) and (d)), although a few adiabatic shear bands are formed weakly in the unaged microstructures (Fig. 12(a) and (b)). Thus, it is confirmed that the possibility of the adiabatic shear band formation is lower in the aged microstructures than in the unaged microstructures. The precipitation of very fine ␣2 phases through aging of the Widmanstätten and equiaxed microstructures increases dynamic torsional properties as well as quasi-static tensile properties, and reduces the possibility of the adiabatic shear band formation, thereby providing a new idea to improve ballistic performance and to find optimal process conditions in high-speed metal forming. However, since the data of this study were obtained from investigations into the some basic microstructures, the effects of critical microstructural factors such as the size and volume fraction of each phase on the quasi-static and dynamic deformation behavior were not studied here. Thus, further studies on microstructures and heat treatment conditions, under which adiabatic shear banding is suppressed while excellent mechanical properties are promoted, are required in the future.

5. Conclusions Quasi-static and dynamic torsional deformation behavior of Widmanstätten and equiaxed microstructures containing fine ␣2 particles was investigated, and the results were analyzed in relation to void initiation and adiabatic shear banding to reach the following conclusions. 1. Quasi-static torsional properties of the Widmanstätten and equiaxed microstructures showed a similar tendency to tensile properties, and ductile fracture occurred in all of them. Voids were initiated at ␣/␤ interfaces or boundary ␣ phases in the Widmanstätten microstructure and at ␣/␤ interfaces distributed at triple points of ␣ grains in the equiaxed microstructure. The number of voids was larger in the Widmanstätten microstructure than in the equiaxed microstructure. 2. Under dynamic torsional loading, maximum shear stress of the unaged microstructures was higher and maximum shear strain was lower than under quasi-static loading, although the over-all tendency was similar. Ductile fracture was observed in all of them, and the number of voids under dynamic loading was smaller than that under quasi-static loading. 3. Under dynamic loading, the number of voids initiated in the aged Widmanstätten and equiaxed microstructures was five times greater than those in unaged microstructures because of the ␣2 precipitation. This indicated that the aging treatment had a homogenizing effect, i.e., less likelihood of developing a region of concentrated strain that preceded the adiabatic shear band formation, thereby

reducing the possibility of the adiabatic shear band formation. 4. The precipitation of very fine ␣2 phases through aging of the Widmanstätten and equiaxed microstructures improved dynamic torsional properties as well as quasi-static properties, and reduced the possibility of adiabatic shear banding, thereby providing a new idea to improve ballistic performance of Ti alloy armor plates.

Acknowledgements This work was supported by the 2003 National Research Laboratory Program of the Ministry of Science and Technology of Korea. The authors thank Dr. Sunmoo Hur of Agency for Defense Development and Mr. Yoohan Lee of POSTECH for their help on the heat treatment of the Ti–6Al–4V alloy.

References [1] M.A. Greenfield, H. Margolin, Metall. Trans. A 3A (1972) 2649. [2] D. Eylon, J.A. Hall, C.M. Pierce, D.L. Ruckel, Metall. Trans. A 7A (1976) 1817. [3] A. Gysler, G. Lütjering, Metall. Trans. A 13A (1982) 1435. [4] H. Margolin, J.C. Williams, J.C. Chesnutt, G. Lutjering, in: Proceedings of the 4th International Conference on Ti, Kyoto, Japan, vol. 1, 1980, p. 169. [5] W. Lee, C. Lin, Mater. Sci. Eng. A 241 (1998) 48. [6] J.P. Hirth, F.H. Froes, Metall. Trans. A 8A (1977) 1165. [7] D. Eylon, S. Fujishiro, P.J. Postans, F.H. Froes (Eds.), Titanium Technology, 1985, p. 87. [8] C.C. Chen, J.E. Coyne, Metall. Trans. A 7A (1976) 1931. [9] P.S. Follansbee, G.T. Gray III, Metall. Trans. A 20A (1989) 863. [10] S.V. Kailas, Y.V.R.K. Prasad, S.K. Biswas, Metall. Mater. Trans. A 25A (1994) 2173. [11] M. Mier, A.K. Mukherjee, Scr. Metall. Mater. 24 (1990) 331. [12] S. Yadav, K.T. Ramesh, Mater. Sci. Eng. A 203 (1995) 140. [13] H. Margolin, J.C. Williams, J.C. Chesnutt, G. Lütjering, in: Proceedings of the 4th International Conference on Ti, Kyoto, Japan, vol. 1, 1980, p. 277. [14] G. Welsch, G. Lüetjering, K. Gazioglu, W. Bunk, Metall. Trans. 8A (1977) 169. [15] H. Margolin, J.C. Williams, J.C. Chesnutt, G. Lütjering, in: Proceedings of the 4th International Conference on Ti, Kyoto, Japan, vol. 1, 1980, p. 169. [16] D.-K. Kim, S. Lee, H.-S. Song, Metals Mater. Int. 5 (1999) 211. [17] H.J. Ryu, S.H. Hong, D.-K. Kim, S. Lee, Metals Mater. Int. 4 (1998) 367. [18] A. Marchand, J. Duffy, J. Mech. Phys. Solids 35 (1988) 252. [19] K.A. Hartley, J. Duffy, R.H. Hawley, J. Mech. Phys. Solids 35 (1987) 283. [20] K.J. Kim, C.G. Lee, S. Lee, Scr. Metall. 38 (1998) 27. [21] C.G. Lee, S. Lee, Metall. Mater. Trans. A 29A (1998) 227. [22] K. Cho, S. Lee, S.R. Nutt, J. Duffy, Acta Metall. 41 (1993) 923. [23] R. Boyer, G. Welsch, E.W. Collings (Eds.), Materials Properties Handbook, Titanium Alloys, ASM, Metals Park, OH, 1994, p. 491. [24] Metals Hand Book, ninth ed., ASM, Metals Park, OH, vol. 8, 1990, p. 218. [25] D.-K. Kim, S. Lee, H.-S. Song, Met. Mater. 5 (1999) 211.

D.-G. Lee et al. / Materials Science and Engineering A366 (2004) 25–37 [26] S. Kim, S. Lee, Metall. Mater. Trans. A 31A (2000) 1753. [27] D.-G. Lee, S. Kim, S. Lee, C. Lee, Metall. Mater. Trans. A 32A (2001) 315. [28] F.S. Lin, E.A. Starke, S.B. Chakrabortty, A. Gysler, Metall. Trans. A 15A (1984) 1229. [29] S.P. Timothy, I.M. Hutchings, Acta Metall. 33 (1985) 667.

37

[30] D.-G. Lee, S. Lee, C.S. Lee, S. Hur, Metall. Mater. Trans. A, in press. [31] Y. Me-bar, D. Shechtman, Mater. Sci. Eng. 58 (1983) 181. [32] S.L. Semiatin, V. Seetharaman, A.K. Ghosh, E.B. Shell, M.P. Simon, P.N. Fagin, Mater. Sci. Eng. A 256 (1998) 92. [33] T.R. Bieler, S.L. Semiatin, Int. J. Plasticity 18 (2002) 1165.