Texture and anisotropy in Al-Li alloy 2195 plate and near-net-shape extrusions

MATERIALS SCIENCE & ENGINEERING

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Texture and anisotropy in Al-Li alloy 2195 plate and near-net-shape extrusions S.J. Hales *, R.A. Hafley NASA

Langley

Reseamh

Center,

MS

18SA, 2 IV. Reid Street,

Hampton,

VA 23681-0001,

USA

Abstract Low density aluminum-lithium (Al-Li) alloys, such as 2195, are candidate materials for reducing the structural weight of cryogenic tanks on launch vehicles. The objective of this investigation was to establish the relationship between mechanical anisotropy, microstructure and texture in a near-net-shape ‘T-stiffened’ extrusion and in two gauges of plate. In both product forms, in-plane properties were measured as a function of orientation to the extrusion/rolling direction; tensile properties in increments of 15” and fracture toughness properties at 0, 45 and 90” to the principal axis. Mechanical behavior was evaluated at two locations in the skin of the extrusion, namely midway between and directly beneath the stiffeners. The microstructures of the plate and extruded product were characterized using cross-polarized light imaging of anodically etched metallographic sections, The textural characteristics were quantified as a function of location within the respective cross-sections using orientation distribution function (ODF) analysis. Tensile and fracture properties were correlated with microstructural and textural characteristics in the 2195 near-net-shape extrusion and compared with plate product. Published by Elsevier Science S.A. Ke~~lorn’s:AI-Li alloy; Extrusion; Orientation; Texture; Anisotropy

1. Introduction Alloy 2195 (Al-4.0Cul.OLi-0.4Mg-0.4Ag0.12Zr) has a 5% lower density than 2219, the alloy used on the space shuttle external tank [l]. The skinstiffened tank is currently manufactured by integral machining of thick plate with a high scrap rate and the potential exists to reduce manufacturing costs by using near-net-shape extrusions or built-up structures (incorporating thin plate and attached stiffeners) [2]. Mechanical anisotropy is an important issue in pressure vessel materials due to the complex service loads involved. A large volume of information is available on structure/ property relationships in Al-L1 products, but not on extrusions of the 2195 alloy composition. The mechanical behavior is expected to be strongly dependent on tensile axis orientation as a result of the highly directional, unrecrystallized microstructures and strong textures common in Al-Li alloy materials. In this investigation a 2195 integrally ‘T-stiffened’ extruded panel, a 7.5 mm plate and a 45 mm plate were evaluated in parallel. + Corresponding author. 0921-5093/98/%- see front matter Q Published by Elsevier ScienceSA PZZSO921-5093C98)00834-X

The geometry of the extruded cross-section and the nomenclature adopted to describe both the principal axes and the different locations examined in the extrusion are summarized in Fig. l(a). The principal axes are defined with respect to the original ‘stovepipe’ extruded product, namely (A)xial, which is parallel to the extrusion direction and (C)ircumferential and (R)adial, which are perpendicular. In contrast to the plate, through-thickness locations in the extruded product are not symmetrical about the mid-plane. Therefore, ti and t, refer to the inner and outer surfaces of the stovepipe extrusion, with the intermediate locations being self-explanatory. The ‘T’ stiffeners are located on the outer surface of the extrusion. Commensurate with the use of the product in pressurized tank applications, two locations in the extruded cross-section were of interest; the skin mid-way between the stiffeners and the skin at the base of the stiffeners. These regions will hereafter be referred to as the ‘skin’ and ‘base’ locations in discussion. The base region is of interest because of the microstructural and textural inhomogeneities introduced during the extrusion process and their effect on mechanical properties.

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In the plate products. the standard (L)ongitudinal, which is parallel to the rolling direction, (T)ransverse and (S)hort-transverse notation is applied. The 7.5 mm plate is the same gauge as the skin of the extrusion and is comparable to the built-up structure concept. The 45 mm plate is approximately the same thickness as the extruded cross-section and is comparable to manufacture by integral machining. In the extrusion and the 7.5 mm plate, structure/property correlations are important at the mid-plane (f/2) location, but in the 45 mm plate the t/8 location is of interest because it corresponds to the ‘skin’ location in machined product. The objectives of this study involve: (i) determination of the mechanical anisotropy exhibited by the extrusion compared with 7.5 mm and 45 mm plates; and (ii) correlation of microstructural and textural characteristics with tensile and fracture properties in the two product forms. 2. Experimental

procedures

Details concerning the manufacture of the near-netshape extrusion have been furnished elsewhere [3]. An extrusion ratio of 18: 1 and a hot working temperature range of 38.5 to 415°C were employed and after reducing to flat panels, a nominal 3% stretch was used prior to aging to a T8 temper condition. The 7.5 and 45 mm rolled plates evaluated were also in a similar T8 condition. In all but the 45 mm plate, in which the r/8 plane was of interest, evaluation of mechanical behavior was conducted on specimens extracted from the mid-plane (t/2), with the loading axis oriented with respect to the extrusion/rolling direction at 0” (L), 15, 30, 45, 60, 75 and 90” (T) for tensile and 0” (L-T)? 45” (45-45) and 90” (T-L) for toughness evaluation. Tensile testing was conducted in accordance with ASTM E8 using sub-size flat specimens with a 25.4 mm gage length x 6.4 mm wide x 3.8 mm thick [4]. Tests were performed at a constant cross head speed of 4.2 x 10 - ’ mm s - ’ in a servo-hydraulic test frame, Strain-to-Failure was measured using back-to-back extensometers. Fracture toughness tests were conducted in accordance with ASTM E813 and El 152 using compact tension (C(T)) specimens with W= 50.8 mm and B = 3.8 mm [5,6]. Crack lengths were determined using either compliance or DC potential drop methods. Initiation toughness (I& defined as the toughness at 0.1 mm stable crack extension, and tearing modulus (T,), which is proportional to the slope of the J-R curve during the first 0.5 mm of stable crack extension, were determined from J-R curves using prescribed methods

[7,81. Microstructural assessment was based on triplanar views and through-thickness montages of cross-polarized images assembled for the locations of interest. Quantitative texture measurements involved calculating

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orientation distribution functions (ODF’s) from partial { 11 l}, 1200) and 1’220) pole figures. Variations in textural composition as a function of product form and through-thickness location were addressed by plotting orientation density, f(g), along selected fibers in Euler space (Bunge notation) which isolate most of the principal texture components defined in Table 1 [9,10].

3. Results

The variation in macrostructure observed within the extruded cross-section at the locations of interest is illustrated in Fig. l(b) and (c). In the skin region a layered structure exists which lies in the plane of the section and shows little through-thickness variation. In the base region, a similar layered structure exists but the layers are distorted out-of-plane in the direction of the stiffener. In the web of the stiffener, the layers form a ‘herring bone’ pattern and the area from the midplane (t/2) of the base up into the stiffener reflects a transition in macrostructure from that in the skin to that characteristic of the web region. The micrographs in Fig. 2, taken at t/2, show that the extrusion has a partially recrystallized microstructure which is strongly directional throughout the cross-section. The grain structure consists of alternating layers of elongated, deformed grains containing substructure and layers containing finer recrystallized grains. The R-C aiid A-R views reveal that there is a difference in grain morphology between the skin and the base regions. Fig. 2(a) and (b) show that in the skin, most of the elongated grains are pancake-shaped in cross-section resultgrain ing in a ribbon-like morphology. Through-thickness variations in grain morphology are minimal and the microstructural characteristics documented for the t/2 location are representative of 90% of the cross-section in the skin. Fig. 2(c) and (d) show that in the base, ribbon-shaped grains comprise about 50% of the microstructure with the remainder consisting of fibrous (cigar-shaped) grains. The R-C view reveals how the grain structure is distorted out of the plane of the skin as the microstructure transitions to the ‘herringbone’ pattern observed in the web of the stiffener. The microstructural characteristics vary little from the mid-plane toward the inner surface of the extrusion opposite the stiffener and are similar to the majority of the skin. Fig. 3(a) and (b) show that the 7.5 mm plate (at t/2) has an almost lamellar microstructure. consisting of highly elongated grains varying in thickness from 5 to 50 pm. The microstructure is predominantly unrecrystallized and the majority of the deformed grains contain very little substructure. The L-S view shows that there are some thin laths of recrystallized grains interspersed among the deformed grains. Adjacent to the plate

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Fig. 1. Schematic of the ‘T-stiffened’ extrusion showing the nomenclature adopted to describe location and orientation within the cross-section (a) and optical macrographs showing variations in macrostructure between the skin (b) and base (c) regions.

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Table 1 Principal texture components in Al alloys Name

{ hkl} (uvw)

{011)(211) { 123}(634) {123](412) {146}(211) {112}(111) {011}(111) {123)(111) {OOl)(llO) {111)(110) {112}(110)

Euler angles CPI

4

$3

35 60 15 55 90 55 7s 45 0 0

45 35 60 80 35 45 40 0 55 35

0 6.5 20 35 45 0 65 0 45 45

Deformation

Bs s s; S; cu J% Exz Shear, Shear, Shear,

Recrystallization

Goss {011}<100) Cube {001)<100) RC RD1 10133(1OO) RC RD2 {0231(100) RC ND, {001}<310) WVD2 {001)(320)

0 0 0 0 20 35

45 0 20 35 0 0

0 0 0 0 0 0

L R %X

70 55 55 75

45 20 75 25

0 25 45

{013)(231) {011)(122) {124}(211) {113}(211)

surfaces the lath thickness decreases and the frequency of recrystallized laths increases, but the microstructure shown is representative of the central 60% of the plate cross-section. Fig. 3(c) and (d) show that the grain

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structure in the 45 mm plate (at t/8) is coarser and less lamellar in appearance than in the thinner plate. Again, the grains are highly elongated in the rolling direction with a pancake-shaped cross-section and vary in thickness from 15 to 100 pm. The L-S and T-S views show that the grain structure is more irregular and many of the grains contain deformation bands. The microstructure shown in the figures illustrates the grain morphology observed throughout the plate cross-section. In contrast to the 7.5 mm plate, the presence of any recrystallized grains is not readily apparent because of the distorted nature of the microstructure. The textural characteristics of the regions of interest in the extruded cross-section are shown in Fig. 4. Variations in the intensity of the constituent texture components are not symmetrical about the mid-plane (r/2). The skin region, Fig. 4(a) and (b), has a similar texture composition to rolled product comprising an incomplete ,/?-fiber with some recrystallization components. The dominant deformation-related component is Brass (Bs), with evidence of some Si. Intensity is very weak at the S, S’, and Cu orientations. The major recrystallization-related components consist of Cube, RCND2 and Goss, with minor contributions from the P and Bs,, components. All of these texture components vary in intensity from the outer surface (r,) through t/4, t/2 and 3t/4 to the inner surface (rJ of the cross-section. The intensity of the Bs component is strong from t, to 3t/4 and weak at ti. The S.h component has moderate

Fig. 2. Cross-polarized optical micrographs (Barker’s anodic etch) showing rhe grain morphology in the skin of the ‘T-stiffened’ extrusion mid-way between the stiffeners (a) and (b), and at the base of the stiffeners (c) and (d). in the R-C and A-R planes, respectively.

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Fig. 3. Cross-polarized optical micrographs (Barker’s anodic etch) showing the grain morphology in 7.5 mm plate (a) and (b), and 45 mm plate (c)-and cd), in the T-S and L-S planes, respectively.

intensity from r,, to 3t/4 and is also weaker at ti. The Cube and Goss components show maximum intensity at t/2 decreasing toward both surfaces. The RCKD2, P and Bs,, components show a similar trend to the deformation-related components, being weakest at t,. The texture in the base region essentially comprises a double fiber texture consisting of strong (111) and (100) fibers. Fig. 4(c) shows that most of the deformation-related components are contained within the (111) fiber with maximum intensity at the Ex, orientation and moderate intensity at the Exz orientation. The intensity of the Ex, and Ex, components is highest at the t/4, t/2 and 3t/4 and decreases toward the t, and ti locations. The Cu component has moderate intensity at t, and gradually decreases in intensity toward ti. There is also a shear component present which is weak from 6, to t/2 and moderate in intensity from t/2 to ti. Fig. 4(d) shows that the recrystallization-related components are substantially contained within the (100) fiber, comprising Cube and RC,,, types. The Cube component increases in intensity from t, to 3t/4 and then rapidly decreases towards ti. The GOSS and RC,, components are strongest at t, and decrease in intensity toward ti. The RC,,? component fluctuates in intensity through the cross-section, exhibiting a maximum at the t/2 location. Fig. 5 reveals that the textural characteristics of the 7.5 and 45 mm plates are symmetrical about the midplane (t/2). For 7.5 mm plate, Fig. 5(a), deformation-related elements of the texture comprise a well-developed

P-fiber in the core (t/4 and t/2) which weakens toward the surfaces. The ,/?-fiber consists of a strong Bs component with moderate intensity at the S, S’,, and Si orientations and a weak Cu component. The recrystallization-related components, Fig. 5(b), are weaker with RC ND2> R and Cube being the most significant and some intensity at the P orientation. The intensity of the RC NDZcomponent is highest at the surfaces and rapidly decreases to a reduced level in the core (t/4 and t/2). The through-thickness intensity profile at the R and cube orientations is ‘M’ shaped with highest intensity at t/4. The P component is the weakest, with gradually increasing intensity from the surfaces toward t/2. Fig. 5(c) shows that the deformation-related /?-fiber is not as well developed in the 45 mm plate. The Bs component is of comparable strength, but the S and S variant components are weaker. In contrast to the thinner plate, the through-thickness intensity profile of the P-fiber is ‘W’ shaped, with minima around the t/S-t/4 locations. Fig. 5(d) shows that the R and Cube are again the most significant recrystallization-related components with intensities similar to the thinner plate. Intensities at the R and cube orientations follow roughly an ‘M’ shaped profile with maxima at 3t/8, minima at t/8 and intermediate intensity at t/2. The RCND, component is much weaker with highest intensity at t/2 and the P component is absent. The trends in tensile behavior as a function of orientation are summarized in Fig. 6. In the extruded skin,

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10

0 (4

1

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,

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I

ti

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0 Depth Location Fig. 4. Textural through-thickness

characteristics of the extrusion in the skin region (a) and variation in the intensities of the major deformation-related

strength is at a maximum when the tensile axis is perpendicular to the extrusion direction (90”). Strength is comparable in the 0” orientation. but is 15-20% lower at all other orientations of the tensile axis. Strainto-failure is highest in the 45” orientation and gradually decreases by - 30% toward the 0 and 90” orientations. In the extruded base, strength is at a maximum when the tensile axis is parallel to the extrusion direction (0’). Strength levels are 13-23% lower at all other orientations, including when the tensile axis is perpendicular to the extrusion direction (90”). Strain-to-Failure fluctuates with tensile axis orientation, being higher at the 30 and 60” orientations and lower at the 0, 45 and 90” orientations. In the 7.5 mm plate at t/2, strength is at a maximum when the tensile axis is parallel to the rolling

(b), and in the base region and recrystallization-related

(c) and (d), categorized components, respectively.

in terms

of

direction (0”). Strength levels in the transverse direction (90”) are slightly lower, but the ultimate and yield strength drop by 15-l@? in the 45 and 60” orientations. Strain-to-failure is at a maximum in the 45” orientation and decreases toward the 0 and 90” orientations with minima at 15 and 75”, respectively. There is a 30% drop in strain-to-failure from the 45” orientation to the 15” orientation of the tensile axis. In the 45 mm plate at t/S, the tensile properties are more isotropic compared to the 7.5 mm plate at t/2. The strength levels are intermediate between the maximum and minimum observed in the thinner plate and vary little with orientation. Strain-to-failure is generally lower than in the 7.5 mm plate and varies little for tensile axis orientations between 0 and 60”. However, the strain-to-fail-

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Fig. 5. Textural characteristics of 7.5 mm plate (a) and (b), and 45 mm plate (c) and (d), categorized in terms of through-thickness variation in the intensities of the major deformation-related and recrystallization-related components, respecti\refy.

ure is lower in the 75 and 90” orientations and 50% less than in the thinner plate. The trends in fracture toughness behavior as a function of orientation are summarized in Fig. 7. In terms of initiation toughness, Fig. 7(a), the extruded skin has the highest level in the 45-45 orientation and the extruded base has the lowest in the T-L orientation. The extruded skin, extruded base and 7.5 mm plate show the same trends with respect to orientation. Initiation toughness is highest in the 45-45 orientation and decreases toward the L-T and T-L orientations, being lowest in the T-L orientation. In contrast, initiation toughness in the 45 mm plate varies little with orientation, but tends to be at a reduced level overall. In terms of tearing modulus, Fig. 7(b), the extruded skin and 7.5 mm plate are at

similar levels and exhibit the same trend with respect to orientation. In both cases, tearing modulus is highest in the 45-45 orientation and decreases toward the L-T and T-L orientations, being lowest at the L-T orientation. The level of tearing modulus in the extruded base and the 45 mm plate are generally lower than in the extruded skin and 7.5 mm plate and there is a different trend in orientation dependence. Tearing modulus is highest in the L-T orientation and decreases from L-T to 45-45 and again from 45-45 to the T-L orientation. 4. Discussion

In comparing the anisotropic behavior of the different product forms and correlating microstructure and

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Extruded Skin @ t12 Extruded Base @ t/2 45 mm Plate @ t/8 7.5 mm Plate @ ti2

0 l

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c 500 t,..l..,..,..,..,.. 0 15 (4

4501 30 45 60 Orientation, degrees

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“““8’.‘I”1 0 15

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90

‘, 15

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30 Orientation,

45

60

,I”’ 75

”

” ‘, ” 30 45 60 Orientation, degrees

3 ” 75

” 90

90

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Fig, 6. Variations in tensile properties with orientation in the skin and base regions of the extrusion (t/2), 7.5 mm plate (t/2) and 45 mm plate (t/8); (a) ultimate tensile strength (b) yield strength and (c) strain-to-failure.

texture with mechanical properties, the characteristics at the mid-plane (r/2) are of interest in the extruded skin, extruded base and the 7.5 mm plate. However, in the 45 mm plate, the t/8 location is of interest because it corresponds to the mid-plane of the ‘skin region’ in integrally machined product. A comparison of the tensile behavior reveals that the general trends comprise high strength parallel to the rolling/extrusion direction and low strength in the 45-60” orientations. (The exception is the 45 mm plate at the t/8 location which exhibits fairly isotropic tensile properties.) In the extruded skin, low strength extends to the 15, 30 and 75” orientations of the tensile axis. In the extruded base the trend extends to the 15 and 30” orientations only, with a temporary increase in strength at the 75” orientation.

The trends also reveal that the 45 mm plate at t/S had the highest overall level of strength and was the most isotropic. In most orientations, both the skin and the base regions of the extrusion had low strength, with the base being the most anisotropic. The 7.5 mm plate h$d intermediate strength levels with similar anisotropy, but the lowest yield strength of all at the 55” orientation. In terms of strain-to-failure, the 7.5 mm plate was the most ductile and the most anisotropic and the extruded base was the least ductile, but more isotropic. The microstructure of the 2195 extrusion was partially recrystallized consisting of a mixture of highly elongated, deformed grains interspersed with layers of finer recrystallized grains throughout the cross-section. The differences in grain structure and texture between

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the skin and the base reflect the geometry-dependent variations in material flow during the extrusion process. Section thickness governs the local extrusion aspect ratio which controls the mode of deformation and the resulting microstructural and textural characteristics [l l-151. At t/2, the skin had a ribbon-shaped grain morphology and a texture comprised of strong intensity at the Bs and Cube-type orientations and moderate intensity at the Sk orientation. The skin is a thin section extrusion which undergoes deformation closer to plane strain resulting in a rolling-type texture, i.e. an incomplete /?-fiber with maximum intensity at the Bs orientation [14,15]. In contrast, the base at t/2 had a mixture of ribbon- and cigar-shaped grains and a texture consisting of a well-developed (111) fiber (dominated by

(a)

Orientation ‘-/C

01

(b)

1

I

L-T

45-45

I

T-L

Orientation

Fig. 7. Variations in initiation toughness (a), and tearing modulus (b), with orientation in the skin and base regions of the 2195 extrusion (r/2), 7.5 mm plate (t/2) and 45 mm plate (t/8).

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the Ex, component) and (100) fiber (dominated by the Cube component). The (111) + (100) double fiber texture is typical of Al-Li thick section extrusions which undergo axisymmetric deformation [ 14,151. In the highly localized area beneath the stiffener, the base region is effectively a thick section extrusion due to the presence of the stiffener. Consistent with other findings, recrystallization during the extrusion processing has resulted in the formation of the Cube and RC-type components. The higher intensity of the recrystallization-related components in the base compared to the skin suggests that the degree of recrystallization is higher in the base than in the skin. Al-Li extruded products with a strong (111) + (100) fiber texture tend to exhibit highly anisotropic mechanical behavior [ 1 1- 151. Tensile properties are characterized by high longitudinal yield strength, low transverse ductility and a small spread between the ultimate and yield strength. In contrast, extrusions with a rolling-type texture tend to be more isotropic, possess lower longitudinal strength and have an acceptable difference between ultimate and yield [12]. In the 2 195 extrusion it was observed that longitudinal yield strength is higher in the base than in the skin, but transverse yield strength is much higher in the skin than in the base, the yield strength at 45” being approximately the same. In addition, the average spread between ultimate and yield strength in the skin and the base is comparable (45-50 MPa) considering all orientations. The spread varies with orientation in both cases, but in a different manner. In the skin, the gap is smallest at 0” and largest at the 60” orientation, whereas in the base, the gap is smallest at 75” and largest at the 15” orientation. The anisotropy in tensile behavior is consistent with the earlier findings, but it is difficult to correlate the observations regarding the ultimate-yield differential with previous results. It has been shown that as the section thickness decreases, the degree of recrystallization increases and tensile properties decrease due to a decreasing contribution from substructure strengthening [16]. However in this study, the amount of recrystallization appears higher in the base than in the skin. This suggests that texture may play the dominant role with regards to this aspect of mechanical behavior. It has also been suggested that substructure causes strengthening but usually at the expense of ductility [17]. Other reports indicate that increasing aspect ratio has little or no effect on longitudinal ductility [14,15]. The results presented show that this is only true for the L orientation, since strain-tofailure in the base is 50% lower than in the skin at the 45” orientation. In the plate products, the microstructure was predominantly unrecrystallized. The 7.5 mm plate (at r/2) had an almost lamellar grain structure, whereas the 45 mm plate (at t/8) had a coarser microstructure with

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highly elongated grains containing deformation bands. The texture of unrecrystallized Al-Li rolled products is best characterized by the p-fiber, which contains the deformation-related components Bs, S, S;, Sh and Cu [IO]. The p-fiber becomes increasingly well-developed with increasing rolling strain (decreasing gauge) and maximum intensity tends to shift from the Bs to the S orientation [lS]. As the degree of recrystallization is increased via processing, intensity along the P-fiber decreases and the recrystallization-related Cube, RC and R components increase in intensity [19]. Consistent with this, the 7.5 mm plate had a well-developed p-fiber texture containing strong Bs, S and S variant components and the intensity of the recrystallization-related components was higher. A large through-thickness texture gradient was observed with the major components being strongest at t/2 and weakest at the surfaces. At t/8 in the 45 mm plate, the texture was essentially random, with ,/J-fiber and Cube-type components at the same (weak) level of intensity. A large through-thickness texture gradient was again observed with the major components being strongest at t/2, but still weaker than those observed in the 7.5 mm plate at t/2. In Al-Li rolled product, variations in longitudinal yield strength have been correlated with the strength of the p-fiber and in particular the intensity of the Bs component [20]. Other considerations of the orientation dependence of yield strength with respect to the rolling direction have revealed that deformation-related components along the P-fiber result in a yield strength minimum in the 30-60” range, while recrystallization-related components such as Cube and Goss, result ~J.Ia maximum in the same orientation range [21]. The counteracting effect of these factors explains the difference in mechanitrends observed between the cal anisotropy strongly-textured 7.5 mm (at r/2) and the weakly-textured 45 mm plate (at t/8). It is interesting to note that the orientation dependence of yield strength in 7.5 mm 2195 plate follows the same trend as observed in 2090 sheet [22] and the orientation dependence of yield strength and ductility in 45 mm 2195 plate follows the trends observed in 8090 plate [23,24]. The textural and microstructural characteristics of the three alloys are similar, but the dispersion of second phases differs [25]. This cotirms that texture governs anisotropy in mechanical behavior, with grain morphology and strengthening precipitates playing secondary roles in rolled product. It is important to note that the extruded skin and both plate products show a concommitant increase in ductility with decrease in yield strength in the off-axis orientations. In contrast, the extruded base shows a drop in both yield strength and ductility in off-axis orientations. The trend in yield strength can be accounted for through the texture, Bs in the extruded skin and both plate materials and (111) fiber in the ex-

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truded base. This anomalous tensile behavior in the extruded base is probably related to grain morphology. Even though the textural characteristics are similar to that of an axisymmetric extrusion? the microstructural characteristics are complex. The layers of ribbon- and cigar-shaped grains are distorted out of the plane of the skin into the stiffener region. In extruded product with a similar cross-sectional geometry, it has been observed that tensile fracture path tends to follow the contours of the grain structure in such regions [26]. Low off-axis ductility in highly directional microstructures has also been correlated with fracture occurring by a mixture of transgranular failure along shear bands (on { 11 l} planes) and intergranular failure along grain boundaries oriented parallel to the plane of maximum shear stress [27]. The more fibrous nature of the grain structure in the base would support the notion of an increased area fraction of unfavorably oriented grain boundaries leading to reduced ductility. In agreement with previous work [28], current observations suggest that there is no simple correlation between tensile behavior and fracture toughness in Al-Li alloy materials. All of the materials exhibited anisotropic behavior with respect to initiation toughness and/or tearing modulus. In the extruded skin, the best overall levels of initiation toughness and tearing modulus correlate well with both the high level and orientation dependence of ductility observed. The extruded base exhibited lower and more isotropic behavior in initiation toughness, which is consistent with the reduced level of ductility. In contrast, the tearing modulus behavior is very anisotropic, but the orientation dependence does track with the trends in ductility observed. The minimum in ductility correlates with the lowest initiation toughness and tearing modulus in the T-L orientation in the base. The higher level of tearing modulus in the L-T orientation with little change in ductility suggests that different microstructural features are controlling each behavior. In Al-Li extruded plate it has been documented that fracture toughness decreases with section thickness and increasing degree of recrystallization [16]. Other observations show that unrecrystallized microstructures result in high longitudinal toughness, but low transverse properties and recrystallized microstructures result in lower longitudinal toughness, but less anisotropy [29]. The present results appear contrary to these findings because, in general, initiation toughness and tearing modulus are higher in the skin than in the base independent of orientation. The complex cross-sectional geometry and the nature of the grain structure in the extruded base is most likely responsible for this discrepancy. The 7.5 mm plate exhibits isotropic behavior with respect to initiation toughness, but the same anisotropic behavior as the extruded skin with respect to tearing modulus. The overall level and orientation dependence

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of ductility are the same for the 7.5 mm plate and the skin, suggesting little correlation between ductility and initiation toughness. This is reinforced by the isotropic behavior in the 45 mm plate with respect to initiation toughness and ductility. However, the level of ductility is comparable to the 7.5 mm plate in the L-T and T-L orientations, whereas the initiation toughness is 60% lower. In the 45 mm plate, the tearing modulus behavior is anisotropic and is very similar to the extruded base. There is a reasonable correlation between the orientation dependence of tearing modulus and ductility in all but the 45 mm plate. The tearing modulus is 50% higher in the L-T orientation compared to the T-L orientation while the difference in ductility is small. In rolled Al-Li products, fracture behavior is influenced by texture, grain morphology and grain boundary strength [23]. Fracture tends to occur by a mixture of transgranular, intergranular and inter-subgranular failure, the proportions of which depend on texture intensity, degree of recrystallization and temper condition [28,30]. Texture seems to play the dominant role, so the essentially random texture observed in the 45 mm plate at the r/8 location may account for this behavior. It is interesting that the orientation dependence of tearing modulus in 45 mm 2195 plate follows the same trend as fracture toughness in 8090 plate [24]. It has been suggested that microstructural characteristics which are considered undesirable for toughness may be beneficial for tearing modulus [31]. This could explain the isotropic nature of initiation toughness, but not the anisotropic nature of tearing modulus in both the 45 mm plate and the extruded base. As a consequence of this work, there are several factors which raise concerns about the use of near-netshape extruded product in pressure vessel applications in an unrecrystallized or partially recrystallized condition. First, the narrow ultimate-yield strength differential at certain orientations in both the skin and the base regions is of concern due to the risk of catastrophic failure under service conditions. Second, the level of yield strength in the extruded skin and base were lo-15% lower in most orientations than the 45 mm plate at t/8. Third, although the best combination of initiation toughness and tearing modulus was found in the extruded skin (at 45-45), the poorest was observed in the extruded base (at T-L). Acceptable levels for these two properties have yet to be clearly defined for the purposes of design criteria. Additional processing to produce a fully recrystallized microstructure should reduce texture and microstructural directionality resulting in more isotropic properties in these 2195 near-net-shape extrusions. Considerable work has been performed on both AI-Li rolled products [32-351 and extruded products [12,29]

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to reduce mechanical anisotropy, but usually at the expense of longitudinal properties. Most of these efforts have involved reducing the texture gradients and modifying the grain morphology via secondary thermal or thermomechanical processing.

Conclusions The extruded skin at t/2 has a ribbon-shaped grain morphology with a strong Bs and Cube texture. Yield strength is high at the L and T orientations, but 15-20% lower in the off-axis directions with a minimum at the 75” orientation. The overall levels of ductility, initiation toughness and tearing modulus are high with maxima at the 45” (45-45) orientation and minima at the 0” (L-T) orientation. The microstructure of the extruded base at t/2 comprises a mixture of ribbon- and cigar-shaped grains with a double (11 l)/(lOO) fiber texture. Yield strength is high at L, intermediate at T and 75” and low elsewhere. Ductility is generally low and fluctuates with orientation being lowest at 45” and T. Initiation toughness is fairly isotropic but low. Tearing modulus is very anisotropic being high at the 0” (L-T) orientation and very low at 90” (T-L) orientation. The 45 mm plate at t/S has an elongated, pancake cross-section grain structure and a weak texture. Yield strength and ductility are isotropic at intermediate levels. Initiation toughness is isotropic at a low level. Tearing modulus is very anisotropic and displays the same trend with orientation as the extruded base; very high at 0” (L-T) and low at the 90” (T-L) orientation. The 7.5 mm plate at t/2 has a highly elongated, almost laminar grain structure with a strong P-fiber texture. Yield strength is high at L and T, but decreases by 15-20% to a minimum at 55”. Ductility is high showing a similar trend to the extruded skin with orientation. Initiation toughness is high and fairly isotropic. Tearing modulus is high exhibiting the same orientation dependence as the extruded skin; high at 45” (45-45) and low at the 0” (L-T) orientation. Compared to plate properties, the small ultimateyield strength differential in some orientations, the overall lower yield strength, and the low levels of initiation toughness and tearing modulus beneath the stiffeners, raise concerns about the use of nearnet-shape extrusions in pressure vessel applications. It is likely that secondary processing to reduce texture and modify grain morphology will be required to produce better and more isotropic properties.

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Acknowledgements The authors would like to thank Joel Alexa and Jim Baughman of Analytical Services and Materials, Hampton, VA, for assistance with the experimental work.

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