Evolution of Strain-Induced Microstructure and Texture in

Commercial Aluminum Sheet under Balanced Biaxial

Stretching

S.W. BANOVIC and T. FOECKE

The evolution of surface topography and crystallographic texture was investigated under balanced

biaxial stretching in sheets of the aluminum alloy 5052-H32. Two different lots of material, with an

initial nominal thickness of 1 mm, were tested in the as-received condition. Samples with increasing

levels of balanced biaxial strain were deformed using a modified Marciniak in-plane stretching test.

In general, the sheet materials were microstructurally and crystallographically anisotropic. Between

the two lots, the initial microstructure and mechanical properties were found to be equivalent; however,

the sheet texture was appreciably different. This latter variation was observed to have an effect on

the additional roughening of the surface subsequent to deformation. For a given lot of material, the

surface roughness was found to be proportional to the magnitude of the strain. However, while the

roughening rates for the two lots were comparable, the lot having a stronger initial {220} texture

component was found to roughen to a higher degree. Corresponding changes in the sheet texture

were observed to have two regimes as a function of the strain level. In the first regime, typically, for

strains () up to 0.05, the orientations were found to rotate quickly away from the initial cube

{001}100 orientation observed in the as-received sheet toward positions along the  fiber. Above

a strain level of 0.05, the {220} texture component continued to increase with deformation, but at a

decreasing rate up to failure of the sheet. The difference in the grain rotation rates observed did not

appear to have an effect on the surface roughening, as the relative change of the crystallographic

orientations with increasing plastic strain was similar for both heats of material. Instead, it is believed

that localized grain or grain-grouping interactions may play a more important role in the surface

roughening process.

I. INTRODUCTION alloys has generally been observed to increase in linear

proportion to the magnitude of the plastic deformation[1–7]

THE replacement of conventional steel sheet used for and to the average grain size,[4–9] but is independent of strain various automobile components with aluminum alloys rate[4,5] and stress type (mode of deformation).[4–7,9] The would lead to reduced vehicle weight. This would provide predominant mechanism cited for the roughening process is one effective approach to reaching the gas-mileage goals set associated with the different orientations of slip systems in forth by the Partnership for a New Generation of Vehicles. adjacent grains.[2,4–7,10–13] This can lead to mismatch strains However, widespread application of these lightweight mate- at the grain boundaries under multiaxial stretching. Upon rials by the automotive industry is presently limited, due to deformation, neighboring grains, which typically have dif- their poor formability in terms of limiting strains, wrinkling, ferent crystallographic orientations, will deform by slip on and overall final surface appearance. These forming defects particular crystallographic systems. The differences in the arise from both materials issues (i.e., a limited number of deformation of adjacent grains will lead to strain incompati- slip systems for fcc aluminum compared to bcc ferritic steel; bilities between them. As the effects of mutual constraint solution-strengthened aluminum alloys strain-rate soften, are partially relaxed at the free surfaces, the outermost grains while steels strain-rate harden) and a lack of experience in are allowed to move normal to the surface to accommodate forming aluminum. Automotive manufacturers are the strain at the grain boundaries, resulting in the formation employing finite-element simulations to contend with these of peaks and valleys. Thus, the higher the amount of defor- stamping problems by carrying out intensive die-design mation or the larger the initial grain size, the greater the development prior to die fabrication. Unfortunately, this surface roughness. The emergence of slip steps is thought approach has been relatively unsuccessful at predicting the to play a lesser role in roughening,[4,11] as their height magni- behavior of the material in terms of the complex stress states tude is on a much smaller scale when compared to that of occurring within the sheet and the frictional forces occurring grain rotation. along the die-workpiece interfaces. This is believed to be a Becker investigated the relationship between the initial result of the in-situ development of surface roughness.

At room temperature, the surface roughness of aluminum crystallographic texture of an aluminum sheet and surface

roughening as a result of deformation through detailed

numerical simulations.[1] He observed that a sample with

random grain orientations near the surface developed more S.W. BANOVIC and T. FOECKE, Materials Research Engineers, are with roughening than sheets with higher-texture components, par- the Metallurgy Division, National Institute of Standards and Technology,

Gaithersburg, MD 20899-8553. Contact e-mail: swbanovic@nist.gov ticularly those associated with the initially rolled texture.

Manuscript submitted November 29, 2001. An important point found through the simulations was that

METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 34A, MARCH 2003—657

NOT PROTECTED BY U.S. COPYRIGHT
Table I. Alloy Compositions, as Mass Fractions Multiplied by 100, for the Two Lots of AA5052-H32 and the Acceptable

Limits; Balance is Aluminum

Material Mg Fe Cr Si Cu Mn Ni Ti Zn

Lot 1 2.50 0.40 0.22 0.23 0.10 0.09 0.01 0.05 0.06

Lot 2 2.30 0.28 0.21 0.13 0.06 0.02 0.01 0.01 0.01

Limits 2.2 to 2.8 0.40 max 0.15 to 0.35 0.25 max 0.10 max 0.10 max — — 0.10 max

a more uniform strain distribution occurred along the surface

of the more highly textured materials as a result of strain

variations between localized regions and the immediate surroundings being minimal. This resulted in less grain rotation,

and, hence, the roughening of the surface decreased. This

dependence of surface roughening on the neighboring grain

misorientations suggests the possibility of reducing

roughness by the development of certain textures within the

sheet.[1,13,14] However, as Becker stated, the model used to

obtain these results was not fully developed, as it is limited

by its two-dimensional character. In addition, experiments

indicating the effect of initial crystallographic texture on the

surface roughness of an aluminum sheet, which could be

used to assess the accuracy of the model, were not available

in the literature. Thus, this article will focus on the experimental determination of the relationship between the initial,

Fig. 1—Schematic of the modified Marciniak in-plane stretching test. as-received microstructure and crystallographic texture of

the sheet and the evolved surface roughness as a function

of balanced biaxial strain. the 6 m diamond finish, the sheet thickness averaged 0.98

mm. A circle with a 28 mm diameter was stenciled in the

exact center of the sheet for strain determination. II. EXPERIMENTAL PROCEDURE Drawing quality–special killed steel, with dimensions

The commercially available aluminum alloy 5052-H32 of 30  30  0.1 cm, was used as a washer (driver blank)

(in sheet form, of 1 mm nominal thickness) was received to prevent failure in the sidewall of the aluminum test sheet.

from two different suppliers. Table I gives the chemical A 10-cm-diameter center cut out of the blank was used

composition for the two lots. Mechanical-property values to maximize the area of uniformly, biaxially strained test

were obtained through uniaxial tension testing for compari- material. The washer was placed between the Marciniak

son of the as-received materials. These tests were conducted tooling and the aluminum sheet, with the unpolished side

in accordance with ASTM E8-99, with the stress axis paral- of the sample in contact with the washer (Figure 1). Lubrilel to the rolling direction. Microstructures were revealed cant was placed on the sides in contact with the tooling.

by mechanical polishing using 0.25 m diamond paste, This configuration for testing resulted in a 15-cm-diameter

followed by a conventional anodizing technique using a area (the central portion of the specimen, which remained

solution of 2 mL of HBF and 98 mL of H2O immersed in flat and free from any frictional forces) that was uniformly

an ice bath. The samples were anodized for 45 to 60 seconds deformed in an equibiaxial mode with an unmarred sheet

with a preset voltage of 30 V d.c. and were subsequently face for surface characterization. The load of the holdviewed using polarized light. down ring was maintained at approximately 340 kN for

Equibiaxial deformation of the sheets was conducted us- the duration of testing, so that movement of material inward

ing a modified Marciniak in-plane stretching test (Figure 1). over the draw bead was restricted. A constant central ram

This test was first proposed by Marciniak and Kuczynski, speed of 0.5 mm/s was used for testing, and the ram load [15]

and a detailed description of the experimental equipment was observed to remain below 320 kN. Varying amounts

used in this research can be found in Reference 16. This of stretching were obtained by setting the limit control

type of intrinsic testing was chosen over the more com- of the central ram (Marciniak tooling) to predetermined

monly used limiting-dome-height (LDH) test, so as to elim- deflections. The true in-plane strain was determined by

inate (1) the out-of-plane deflections associated with the measuring the change in diameter of the stenciled circle

bending/bulging of the sheet and (2) the unquantifiable along three directions: parallel, perpendicular, and 45 deg

frictional forces that exist between the sheet and punch. to the rolling direction of the sheet. The true throughSample preparation of the as-received aluminum sheets thickness strain was calculated after measuring the final

consisted of cutting to a 30  30 cm sample size. In order sheet thickness, using calipers with a resolution of 20 m.

to remove both the native oxide layer and remnants of Pre- and postcharacterization of the aluminum sheet conthe deformation marks that developed during the rolling sisted of measuring both external and internal characterisprocedure, a central portion of one face was polished to a tics of the material. Changes in the surface topography of

6 m diamond finish. Cleaning was subsequently done the polished face were observed through roughness measusing acetone and cotton. After preparing the surface to urements via a mechanical stylus profilometer fitted with

658—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
a 5-m-diameter probe tip. A minimum of 15 traces per

sample was conducted, with a trace length of 5.6 mm and

a 0.8 mm cutoff length. Measurements were made along

three directions of the sheet: parallel, perpendicular and 45

deg to the rolling direction. The parameters of the arithmetical mean of the areas of all profile values of the roughness

profile (Ra) and the vertical distance between the highest

peak and the deepest valley of a single roughness profile

(Ry) were chosen to characterize the surface; however, it

is recognized that these parameters do not give a complete

description of the roughness of a surface. Scanning electron

microscopy of the surface was also conducted to determine

the change in surface topography.

The texture of the sheets was evaluated using neutron

diffraction, X-ray diffraction, and electron-backscattered

diffraction (EBSD) techniques. The neutron-diffraction

studies were conducted at the NIST Center for Neutron (a) Research. For these experiments, sample sizes were 5 mm

on a side, stacked five high, which resulted in the beam

irradiating approximately 125 mm3 of deformed material.

The samples were mounted in the center of a four-circle

diffractometer with the rolling direction parallel to the beam

(wavelength of 1.884 A ̊ ). The pole-figure data of the three

reflections ({111}, {200}, and {220}) were measured over

an entire orientation hemisphere ranging from 0 deg to 360

deg in  angle and 0 deg to 90 deg in  angle, each in 5

deg steps using standard procedures.[17]

The X-ray diffraction technique used to obtain the texture

along the rolling direction of the sheet is described in detail

in References 18 and 19. A conventional powder X-ray

diffractometer was used for both scan collections (-2

scans and  scans) with incident and receiving slits of 0.68

and 0.53 deg, respectively. The tube electric current was

40 mA at a voltage of 45 kV. These conditions resulted in

a sampled area of 225 mm2 across the surface at a maximum

depth of approximately 50 m, thus allowing for a large (b)

area of the surface to be analyzed. Rocking curves, or 	

scans, for the {111}, {200}, and {220} reflections were

obtained by correcting the  scans for defocusing and

absorption using the WINDOWS*-based software package

*WINDOWS is a trademark of the Microsoft Corporation, Redmond,

WA.

called TexturePlus, which is available on the World Wide

Web.[20]

For the EBSD technique, a JEOL* 6400 scanning electron

*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

microscope with a tungsten filament was used, with settings

of 20 kV, a magnification of 750 times, and a working

distance of 28 mm. The samples were tilted 68 deg with

respect to the beam. Scans were collected using the Channel

5 software from HKL Technologies, with subsequent generation of pole figures and orientation distribution functions (c) from the data.

Fig. 2—Microstructures of material from lot 1: (a) short transverse direction, (b) long transverse direction, and (c) second-phase constituents in the

short transverse direction.

III. RESULTS

A. Microstructural Characteristics

(Figure 2) and mechanical properties (Table II). Figure 2(a)

As-received materials from the two different lots of shows the relatively equiaxed grains in the short-transverse

AA5052-H32 were found to have similar microstructures direction (plane of the sheet) that were elongated when

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—659
Table II. Comparison of Mechanical Properties for the

Two Lots of AA5052-H32

Total Uniform Total

0.2 Pct YS UTS Elongation Elongation

(MPa) (MPa) (Pct) (Pct)

Lot 1 182 8 239 5 8.1 0.6 9.5 0.5

Lot 2 185 7 242 4 8.0 0.6 10.0 1.0

Typical 195 230 — 12

Table III. Average Planar Grain Size (in the Short

Transverse Direction) and Transverse Aspect Ratio for

Samples of Various Strain Levels

Average Planar Transverse

Grain Size Aspect

(m) Ratio

0.0 (as-received) 31 9 1.9 0.5

0.104 35 13 4.0 0.5

0.198 41 14 7.2 1.7 Fig. 3—Plot of through-thickness strain vsin-plane strain for AA5052-H32.

examined in the long-transverse direction (Figure 2(b)). can be attributed to the relatively large amount of material

(approximately 180 cm2 Table III gives the average planar grain size, as determined ) which was sampled via the current

by the linear-intercept method, and the aspect ratio for the method. This allows more defects in the material to be

long-transverse direction. There was no statistical difference sampled, leading to a higher probability of failure at lower

in grain size between the two lots of material. However, strains. Also, it has been shown that out-of-plane straining,

based upon Figures 2(a) and (b), the anisotropy of the sheet which occurs during the LDH test, is different from the

on a microstructural basis was evident. in-plane straining due to the effects of curvature, normal

pressure, and friction from the tooling.[33] Second-phase constituents were found to be dispersed These factors may

throughout the matrix as well as in the grain boundaries retard the in-situ formation of defects in the sheet and can

(Figure 2(c)). There was no significant difference noted lead to higher limiting strains during out-of-plane deformabetween the two lots. As both lots of material fell within the tion when compared to values obtained via in-plane techniques. acceptable composition range for the commercially available [34] When failure of the sheet did occur, the fracture

sheet, further analysis of the particles was not conducted. was primarily oriented parallel to the rolling direction and

However, the size, shape, and coloring suggested that they initiated within the center of the sample (away from the area

were most likely (FeCr)3SiAl12, Mg2Si, and other intermetal- where the ram was in contact with the specimen). Failure

lic phases typically found in the 5xxx-series alloys. of the sheet along this path was anticipated, as more defects [31]

Postdeformation analysis revealed that the grain shape (thickness variations) and microstructural variations (elonwas found to have further elongated, when the microstructure gated grains and second-phase constituents) are expected to

was examined in the long-transverse direction. Table III be aligned parallel to the rolling direction. Also, since failure

shows that while the average planar grain size did not dra- occurred away from the edges of the sample, the intrinsic

matically change, the transverse aspect ratio was found to strength of the material was tested, and extraneous variables

increase significantly. from the testing procedure were not found to influence it.

For some samples, the test was halted prior to the fracture

reaching the edge of the sample. In these circumstances, B. Marciniak Testing wide and deep grooves (on the order of 1 and 0.3 mm,

Figure 3 shows the through-thickness strain of the sam- respectively) were observed in the areas directly ahead of

ples, calculated from the thickness measurements, as a func- the fractured regions.

tion of the in-plane strain. A linear relationship was observed

between the two measurements, consistent with the con- C. Surface Roughness straints of volume conservation. The individual in-plane

strain level for each sample was found to be extremely During the stretching process, the sample went from havuniform across the area of interest. Also, the initial anisot- ing a mirror-like finish to having a hazy appearance as the

ropy of the sheet was not observed to affect the formability amount of deformation increased. Qualitatively, there was

of the sheet, below the limiting strain, as there was no a transition around a strain level of 0.05, in which one could

statistically significant difference in the in-plane strain meas- no longer make out clear features in the reflection off the

ured along the three directions. The variability between the sheet. As mentioned previously, failure of the sheet was

two lots of material was also negligible. preceded by the formation of small undulating grooves

A limiting strain of 0.21 0.01 was found for this mate- across the surface that were consistently oriented parallel to

rial, while published values obtained using the LDH test the rolling direction. These features did not occur until the

report a strain of approximately 0.30. final stages of straining (levels above 0.20), after which they [32] This difference

660—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 4.—Secondary electron micrographs revealing the evolution of surface topography on the free surface as a function of the in-plane strain for material

from lot 1. The strain levels shown are (a) 0.0 (polished sheet), (b) 0.051, (c) 0.104, and (d ) 0.198. Arrows in (d) point out the near vertical lines discussed

in the text.

developed into diffuse grooves across the surface. Failure slip lines and the rolling direction, as their orientation was

of the sheet typically resided within one of these grooves. randomly distributed on the surface, similar to the results

Microscopically, the evolution of surface topography on of Guangnan et al.[2] In terms of their appearance, the slip

the free, polished surface was tracked as a function of the bands had a “wavy” characteristic (according to the definistrain level using scanning electron microscopy. Figure 4 tion of Williams et al.,

[35] i.e., bands which are broad, diffuse,

shows this evolution for samples from lot 1. The polished branched, or curved), which would be indicative of a crosssheet (Figure 4(a)) shows the second-phase constituents as slip mode of deformation. While transmission electron

well as some light, straight scratches on the surface from microscopy observations were conducted to confirm or

the polishing procedure. As the material was progressively refute the occurrence of cross slip, the results were

deformed in a balanced biaxial mode, slip bands began to inconclusive.

emerge out of the surface and were readily apparent, at this Images with increased magnifications of samples with

resolution, after a strain level of 0.026. Initially, the slip higher strain levels showed further evidence of the change

bands were not homogeneously distributed across the struc- in the surface. Figure 5(a) shows either a grain or group of

ture, in terms of their number, dispersion, or severity grains that have moved normal to the surface, producing

(height), as localized regions appeared devoid of the defor- peaks and valleys. With continued deformation, the differmation marks (Figure 4(b)). With larger strains, the slips ences between the peaks and valleys intensified as the latter

steps were found to be more uniform across the surface and became deeper and wider. Both decohesion and cracking of

their number and height increased (Figures 4(c) and (d)). the second-phase constituents was noted (Figure 5(b)) for

There was no apparent orientation relationship between the strains as low as 0.018. Also of particular interest was the

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—661
obtained along the three directions, averages were calculated

and comparisons made between the two lots of material

(Figures 6(c) and (d)). The plots showed that for equivalent

strains, the roughness measurements of material from lot 1

were consistently lower than those obtained from lot 2. The

individual roughening rates calculated for the materials are

located in Table IV and do not show a significant difference.

In addition, it appears that the roughness values may be

saturating near the limiting strain, above strain levels of 0.20.

For samples that failed and had diffuse grooves ahead of

the crack tip, roughness measurements were made in the

valleys, parallel to its length. These values were found to

be higher than those obtained for material outside of the

groove (diamond symbols in Figures 6(c) and (d)).

D. Crystallographic Texture

The pole figures obtained from neutron-diffraction techniques are shown in Figure 7 for strain levels of 0, 0.104,

and 0.198 from material of lot 1. Similar intensities were

observed for lot 2. The as-received material (Figure 7(a))

displayed a strong {200} component. With increasing strain

in the material, the development of a prominent {220} component and the decrease of the {111} and {200} components

were observed. In addition, planes of the form {200} were

found to be more uniformly dispersed along the rim of the

projections after deformation (Figure 7(c)), indicating a more

homogeneous distribution of 200 components that were

parallel to the plane of the sheet than in that seen in the asreceived condition (Figure 7(a)). The asymmetry of the plots

indicates the anisotropy of the sheet on a crystallographic

basis.

X-ray 
-2
 scans of the as-received material showed a

variation in the initial texture of the two lots (Figure 8). An

average difference of approximately 30 pct in the maximum

intensity of the individual peaks was found, with material

from lot 2 initially having a stronger {220} component.

The subsequent rocking curves obtained for both lots were Fig. 5—Secondary electron micrographs of (a) a grain or a group of grains found to replicate the trends of the neutron-diffraction data that have shifted normal to the surface producing peaks and valleys and

(b) decohesion and cracking of the second-phase constituents next to the (Figure 9). These 	 scans were taken from lot 1 material

initially straight polishing scratch (lot 1 material with a strain level of 0.198). and are aligned parallel to the rolling direction. Therefore,

the line A-B in Figure 9(b) corresponds directly to the line

A-B on the {200} pole figure, shown as an example in

appearance of the initial scratch lines produced from the 6 Figure 7(c). Due to the configuration of the X-ray equipment,

m diamond polish. In all cases observed, these scratches only partial scans (e.g., from 0 deg to 31 deg for the 220

were fairly straight prior to deformation (Figure 4(a)). After peak) could be obtained and not the full scans along the line

deformation of the sample, these lines also took on a wavy A-B (0 deg to 90 deg), as acquired for the neutron results.

appearance. Note the scratch running near vertical in Figure However, values taken from the symmetric position (	 

4(c) and in the higher-magnification image in Figure 5(b). 0 deg) again showed that an increase in strain led to a

This change in appearance would be indicative of a change decrease in grains oriented with their 111 and 200 compoin the surface topography. nents normal to the sheet plane, while the overall number

To corroborate the change in surface characteristics, of grains with their {220} components parallel to the plane

roughness parameters were quantified as a function of the of the sheet increased.

in-plane strain level. Figures 6(a) and (b) display the Ra and When comparing the rocking curves between the two lots

Ry values for the deformed material from lot 2. With the of material, it was observed that equivalent amounts of strain

exception of the lowest strain levels, there was no significant did result in similar texture profiles, but the intensities of the

difference between the measurements along the three direc- scans were different. Figure 10 shows the average intensity

tions. The lower strain levels were found to have variations, values for the symmetric position (	  0 deg) for both sets

as the remnants from the initial rolling marks were not of materials as a function of the strain level. The offset

completely removed during the polishing procedure. This in intensities was associated with the difference in initial

resulted in higher values for the probe traces perpendicular texturing of the materials, as seen in Figure 8. While direct

to the rolling direction. However, this deviation was not comparison of these average intensities at 	  0 deg could

seen to affect the data after a strain of 0.018. From the data not be made, quantitative changes in the orientation of the

662—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
(a) (b)

(c) (d )

Fig. 6—Roughness parameters as a function of the in-plane strain level. (a) and (b) Results obtained for lot 2 along three directions. (c) and (d ) Averaged

values for the three directions plotted and compared between the two lots of material. The values obtained in the diffuse grooves ahead of the crack tip

are also shown as diamonds.

Table IV. Roughening Rates, in m/p , for the Two Lots curve, with a break in the slope between strain levels of

of AA5052-H32 as Calculated from Figure 5; Correlation 0.04 to 0.05. The rate of texture development (R) for these Coefficients for the Fit of the Data Are Also Indicated two regimes can be found by determining the slopes of the

two portions of the curve (Table V), with the initial slope Ra Ry

about 5.5 times larger than the latter. Roughening Correlation Roughening Correlation Orientation distribution functions were generated from Rate Coefficient Rate Coefficient the EBSD data collected. Examples from lot 2 material are

Lot 1 5.9 0.99 41.0 0.99 found in Figure 12 for strain levels of 0.0 and 0.219. The Lot 2 6.1 0.99 42.6 0.98 as-received material shows the cube {100}001 rollingtexture component typically developed during processing of

fcc materials.[28] As the material is deformed, the grains are

sheets, when compared to their initial intensities, could be observed to rotate toward positions along the alpha fiber,

evaluated. The absolute values of the changes in the maxi- defined as common orientations with the 110 component

mum intensities at 	  0 deg are shown as a function of normal to the plane of the sheet. There appears to be a strong

the strain level in Figure 11 for the three reflections. In concentration near the brass {110}112 component. Similar

general, the data for both lots appear to lie along the same results were obtained for lot 1 material.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—663
(a)

(b)

(c)

Fig. 7—Pole figures obtained via neutron diffraction for strain levels of (a) 0.0, (b) 0.104, and (c) 0.198 from material of lot 1. Isolines are 0.5 to 6.0 in

steps of 0.5 with their minimum and maximum values indicated. The rolling and transverse directions are shown. The line A-B in the {200} reflection for

the sample with 0.198 indicates the slice of data acquired by the X-ray diffraction technique.

IV. DISCUSSION of these factors rely heavily upon the composition of the

alloy and the processing parameters (e.g., rolling tempera- In this research, commercially available aluminum sheet tures, amount of deformation, and postheat treatment) of the was tested in the as-received condition to determine the

sheet. However, it has been suggested that the deformation relationship between strain levels and evolving surface texture may change only slightly from the as-received texture roughness and sheet texture. The sheets were manufactured during the stretching process, as the amount of deformation by a combination of mechanical working and thermal treat- is limited.[21] ments and, thus, are anisotropic in both the microstructural If this is the case, then it is important to study

and crystallographic sense. Microstructural anisotropy is the impact of the anisotropy from the initial texture, inherited

typically caused by variations in the grain size or shape, from the rolling process, on the evolved surface roughness

while crystallographic anisotropy arises from some preferred and developed strain texture of the sheet subsequent to defororientation or texturing of the grains in the material. Both mation. The activation of slip systems and the rotation of

664—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
(a)

(a)

(b)

(b)

(c)

Fig. 8—(a) through (c) Portions of -2 scans, acquired with X-ray diffrac- (c)

tion techniques, of the as-received material showing a variation in the initial

texture of the two lots. Fig. 9—Rocking curves obtained using X-ray diffraction for the three

reflections (a) {111}, (b) {200}, and (c) {220}. Various strain levels are

shown for samples from lot 1.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—665
(a)

(a)

(b) (b)

(c)

(c) Fig. 11—Quantitative changes in the orientation of the sheets when compared to the initial condition. The absolute values of the changes in maxi- Fig. 10—Average intensities for 	  0 deg position as a function of strain mum intensities at the symmetry point (	  0 deg) are shown as a function for the individual reflections (a) {111}, (b) {200}, and (c) {220} from of the strain level for both lots of material: (a) {111}, (b) {200}, and both lots of material. (c) {220}.

666—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
Table V. The Rate of Texture Development, in Percent Change in cps/p, for the Two Lots of AA5052-H32 as Calculated

from Figure 10 for Each Reflection; Correlation Coefficients for the Fit of the Data Are also Indicated

Strain Levels  0.05 Strain Levels  0.05

Analyzed Rate of Texture Correlation Rate of Texture Correlation

Peak Development Coefficient Development Coefficient

111 992 0.98 161 0.96

200 923 0.90 189 0.95

220 934 0.96 151 0.96

(a) (b)

Fig. 12—Orientation distribution function for material from lot 2: (a) as-received and (b) 0.219 strain level.

the crystals from plastic deformation will lead directly to itself on a larger scale that has low-, medium-, and highthe redistribution of crystallographic texture and changes in frequency components on a roughness profile. These various

surface roughening. Many studies have been found which frequencies can be found in two different phases of roughening, identified by Wilson and Acselrad.[36] indicate the effect of strain on surface roughening They are termed [1–7] and

texture[13,14,22–30] individually, but no one study has quantita- the microscopic and macroscopic phases of roughening. The

tively tracked all three. In addition, the materials typically microscopic phase encompasses the process of strain localused in these experiments were fully recrystal- ization, in which inhomogeneities will localize on the scale

lized, of the microstructure. This occurs at the outset of straining, [1,10,13,14,22,27] which annihilates the initial microstructure and texture of the sheet developed during rolling. From in which the grains most susceptible to deformation will

a technological point of view, it is essential to study these develop slip bands within each individual grain (Figure 4).

materials in the as-received condition, as this is the state in These bands contribute to the high-frequency portion of the

which they will be used in fabricating components. roughness profile. While the number and severity of the

slip bands increased with deformation, the in-grain surface

roughness was observed to be on a much smaller scale than A. Development of Surface Roughness the grain-to-grain surface roughness. This latter portion of

On a fundamental level, the cause of surface roughening the roughness profile constitutes the medium-frequency

is due to nucleation and movement of dislocations through range and typically comprises most of the roughness in

the measured parameters.[4] the structure in order to accommodate straining of the mate- Since strain is still somewhat

rial. However, the result of deformation typically manifested localized at this stage, the grain-to-grain surface roughness

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—667
is also in the microstructural phase. As deformation of the continue to roughen and the leveling off of the roughness

material continues to increase, mismatch strains develop at parameters outside of this sheet defect.

the grain boundaries due to the different orientations of slip

systems in adjacent grains.[2,4–7,10–13] In order to accommodate these strain incompatibilities, grains will begin to rotate B. Evolution of Crystallographic Texture with respect to each other. The resulting out-of-plane component of the relative rotation contributes to surface roughening Plastic deformation of the sheet will lead to mismatch

at the free surface (Figure 5(a)), as the effects of mutual strains at the grain boundaries due to the different orientations of slip systems in adjacent grains.[2,4–7,10–13] constraint are partially relaxed in this area. With continued In order

deformation, the rotation is aggravated and the valleys to accommodate these strain incompatibilities, grains will

become deeper and wider. Neighboring grains may also slide rotate with respect to each other to orient themselves in

more stable positions for deformation to occur.[14] relative to each other, forming steps at the grain boundaries Literature

and increasing the surface topography as well. has shown that underbalanced biaxial stretching, the main

Dai and Chang[4] studied roughness profiles from strained component of the evolved texture from Al-based alloys, was

sheets and determined that the grain-to-grain roughness of represented as 110 parallel to the normal direction of the

sheet.[26,28–30,38] a composite profile is the dominant factor that contributes to By assuming this configuration, the grains

surface topography. They found that although the roughness are aligned for optimum slip conditions along their slip

systems {111}110. Results from the diffraction experi- computed for the high-frequency component (from slip

bands) is roughly one-third of that computed for the compos- ments confirm that the initial cube texture component was

found to deteriorate during biaxial stretching, and the weight ite profile, in fact, it contributes far less to the actual surface- of components in the alpha fiber increased relatively quickly. roughness parameters than it appears (approximately 1 pct).

This is primarily due to the fact that the roughness parameter The strengthening of the {220} component parallel to the

plane of the sheet was shown by the large initial slope in for a composite profile is not a simple addition of the differ- Figure 11. Once oriented near the fiber, the grain positions ent frequency components. This indicates that the in-grain stabilize, and rotation as a result of deformation occurs at surface roughness caused by plastic deformation (i.e., slip a slower rate (Table V). Similar textural evolutions were bands) is not the major factor in the measured surface- observed by Kohara[38] as well. Investigating a high-purity roughness parameters, but rather, the grain-to-grain interac- aluminum sheet, he found that a large distribution of grains tions and their subsequent out-of-plane rotation are the pri- were oriented within the alpha fiber after deformation and, mary components of the roughening. This reinforces the additionally, contained a very strong component of brass trend that the roughness parameters scale with the grain size {110}112. of the material.[4–9]

It was previously reported that the relative grain rotation As these microstructural-scale strain inhomogeneities of a strained aluminum sheet increased linearly with plastic (peaks and valleys) grow and coalesce with increased defor- strain.[4] The results of Figure 11 clearly do not support this mation, they eventually form coherent regions of strain con- finding. Instead, orientations rotated very quickly toward centration that span the sheet thickness. These features are the alpha fiber below a strain range of 0.04 to 0.05, after similar to the roping or ridging inhomogeneities observed which the rate of texture development was observed to during other modes of deformation.[10,12] This now comprises decrease. Dai and Chang hypothesized that at low strains the macroscopic phase of roughening. The wavelength of (up to 0.04), plastic deformation is small and some grains these defects (on the order of 1 mm) is much larger than tend to be bonded together to rotate as a group with respect the wavelength of the grain rotation and, concomitantly, has to the other grains or grain groups. Thus, a larger number the lowest frequency of the surface roughening. The present of grains would rotate together in unison. As the strain experimental results indicate a steady, near-linear growth in increases above this level, fewer and fewer grains remain surface roughness with plastic deformation and that a gradual bonded together, resulting in an apparently slower rate of change in the surface roughness occurred near the limiting rotation. Another possibility is that there are fewer grains strain (around 0.20). It was at these levels of strain that the not located in stable orientations at the later stages of defor- diffuse grooves were visually observed on the surface paral- mation; thus, the apparent decrease in rotation rates at the lel to the rolling direction prior to failure of the sheet. From higher strain levels. The rates of texture development also the roughness plots (Figure 6), the parameters appeared to appear to be relatively fast (Table V) and high (nearly 75 level off at these later stages of deformation. However, meas- pct at failure), considering that the maximum amount of urements from inside of the groove (parallel to the channel) deformation introduced in the plane of the sheet was 0.22. show that the roughening continues to increase (diamond However, when considering that the largest strains take place symbols in Figures 6(c) and (d)), similar to the findings of in the thickness of the sheet and are upward of 0.40, the Kobayashi et al.[37] Increased roughness values due to the relatively high rate of texture development can be justified. low-frequency component are not observed, due to the cutoff

length used to define the roughness parameters. Thus, it

appears that the sheet is no longer experiencing uniform C. Correlation of Surface Roughness and deformation at this point, and that plane-strain conditions Crystallographic Texture have been reached (within the groove). Continued deformation of the sheet leads to a gradual decrease in the strain of Becker’s analysis of various crystallographic textures

the material outside of the groove and an increase in the showed the evolution of localized strain bands through the

stretching of the material located within the groove. This material thickness and their effect on surface topography

development.[1] process is manifested by the deepening of the grooves that Three ideal texture components, strong cube,

668—VOLUME 34A, MARCH 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 13—Correlation between relative change in grain orientation and surface roughening for both lots of material.

strong Goss {011}001, and an unstable {111}[011] orien- to the rolling plane of the sheet are less favorable for evolved

tation, were analyzed along with a recrystallized grain struc- surface roughness as well.

ture. From the numerical simulations, he observed that strain From Figure 13, it can be seen that the higher degree of

variations between localized regions and the immediate sur- roughening from lot 2 was not based upon the relative change

roundings were much less for the strongly cube-textured in grain orientation, as data from both lots tend to fall on

sheet than for the other combinations. This resulted in a the same curve. Since the difference in texturing of the

more uniform strain distribution along the surface and, sub- material did not affect the rotation rate of the grains (Figure

sequently, less roughening. Both the Goss and unstable con- 11), the higher degree of roughening for lot 2 must be

figuration developed greater surface roughening, due in part related to the localized interaction of the grains present in

to a significant amount of grain rotation early in deformation the material. For each strain increment, distortion of a grain

as a result of persistent bands of enhanced strain. From the is affected both by the crystallographic orientation of the

results obtained in this study, the material that had the grain and the local stress resulting from the deformation of

stronger {220} component was found to roughen to a greater neighboring grains.[7] Therefore, a grain of a given orientaextent, even though the roughening rates of the two lots tion could have a different imposed stress state with the same

were equivalent (Figures 6(c) and (d)). This sheet was con- macroscopic deformation state if the surrounding texture

was significantly different. Becker[1] sidered to have a stronger alpha fiber, which contains the showed this effect by

Goss component. With this in mind, it is thought that Beck- rearranging the original recrystallized texture grains into

er’s results of greater surface roughening occurring with a three random combinations and analyzing the results. By

strong Goss texturing may extend beyond just this single changing the crystal orientations, the deformation incompaticomponent to include other orientations within the alpha bility was enhanced and surface roughening increased over

fiber. However, closer study of this fact is needed to indicate the original array. Therefore, reorganizing of grains with

if these other orientations with their 110 component normal similar macroscopic textures can have a significant effect

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 34A, MARCH 2003—669
upon the surface roughening. This grain-to-grain interaction between the two sets of materials. However, the lot having

may be the cause of the higher degree of roughening found a stronger initial {220} texture component was found to

for lot 2. roughen to a higher degree.

A final comment is made upon the difference between the 3. The initial microstructural and crystallographic anisotmacroscopic and microscopic phases of roughening, taking ropy of the sheet was found to have no effect on the

into consideration the anisotropy of the sheet. While the stress microstructural roughening process. Macroscopically,

state was equibiaxial, the sheet was anisotropic in both a diffuse grooves formed just prior to failure were found

microstructural and crystallographic sense. This anisotropy to orient themselves parallel to the rolling direction.

of the sheet was observed only to affect the macroscopic phase 4. Relative changes in the sheet texture with plastic deforof roughening, as aspects of the microscopic roughening were mation were similar between the lots, with two rates of

homogeneous (Figures 6(a) and (b)). This was seen in the texture development occurring. For strains up to 0.05,

preferred orientation of the diffuse grooves that developed the orientations were found to rotate quickly away from

parallel to the rolling direction. This phenomenon may be the cube {001}100 orientation observed in the asattributed to the mechanical fibering of the texture compo- received sheet toward positions along the  fiber (011

nents along the rolling direction. This occurrence is not crystallographic direction parallel to the sheet normal).

uncommon in aluminum alloys that have been thermome- Subsequently, the {220} texture component continued to

chanically processed. Previous work[13,39] has shown that seg- increase with deformation, but at a decreasing rate up to

regation of textural components into elongated colonies failure of the sheet.

consisting of grains with similar orientations was particularly 5. The difference in the rate of texture development did not

damaging. This was due to the fact that colonies of grains appear to have an effect on the surface roughening, as

with preferred orientations will deform differently than a the relative change of the crystallographic orientations

group with random orientations and will eventually cause the with increasing plastic strain was similar for both heats

strain to localize in a nonuniform manner. If the texture colony of material. It is believed that localized grain interactions

is softer than the matrix, it will tend to deform to a greater may play a more important role in the surface roughextent, providing a microstructural-scale plastic inhomogene- ening process.

ity that can grow and coalesce with others to form coherent

regions of strain that span the thickness of the sheet. As

solution-strengthened aluminum alloys are strain-rate-soften- ACKNOWLEDGMENTS

ing materials, this will result in necking and eventual failure. M.D. Vaudin, Ceramics Division, NIST, is recognized for Marciniak and Kuczinski[15] based their approach describing his help with the X-ray diffraction technique. The neutron tensile instabilities and eventual necking failure of “real” diffraction experiments were conducted by T.H. Gnaeupel- materials on strain inhomogeneities developed from local Herold and V. Luzin, NIST Center for Neutron Research. regions of weakness within the material such as this. Further, Identification of equipment or software does not imply rec- numerical simulations[12] have shown that grains having simi- ommendation or endorsement by NIST, the Department of lar orientations can act collectively to form elevated or Commerce, or the United States Government, nor does it depressed regions during stretching. Therefore, it again brings imply that the identified equipment or software is the best to light the fact that statistical averaging conducted to obtain available.

the crystallographic texture within the sheet provides useful

trends for understanding the roughening rates of the material,

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