Texture evolution of five wrought magnesium alloys during route A equal channel angular extrusion: Experiments and simulations S.R. Agnew a, * , P. Mehrotra a , T.M. Lillo b , G.M. Stoica c , P.K. Liaw c a Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 24590, USA b Environmental and Energy Sciences Division, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415, USA c Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA Received 20 January 2005; received in revised form 9 February 2005; accepted 10 February 2005 Abstract Equal channel angular extrusion (ECAE) has been demonstrated to induce unusual deformation textures and resulting properties in magnesium alloys, such as the remarkably enhanced room temperature ductility first reported by Mukai et al. [Mukai T, Yamanoi M, Watanabe H, Higashi K. Scr Mater 2001;45:89]. This paper documents a wide range of textures which evolve during ECAE of magnesium alloys. The fact that different alloys exhibit different texture evolutions is an indication of distinctions in the balance of deformation mechanisms which operate within the different alloys. Polycrystal plasticity modeling is used to develop explanations for these texture distinctions in terms of the relative activities of non-basal secondary slip modes, involving Æaæ and Æc + aæ type dis- locations. AZ alloys appear to exhibit balanced secondary slip of non-basal Æaæ and Æc + aæ dislocations, while ZK60 and WE43 appear to favor non-basal Æc + aæ slip. A binary Mg–Li alloy exhibits a radically distinct texture evolution, which is associated with large-scale strain accommodation by non-basal Æaæ slip. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium; Texture; ECAE; ECAP; Crystal plasticity; Ductility 1. Introduction Texture can result in strong anisotropy and asymme- try, particularly in the case of the plastic behavior of non-cubic materials, such as hexagonal close packed (hcp) magnesium and its alloys. One example is the ten- sion/compression strength asymmetry exhibited by mag- nesium extrusions, where the yield strength in compression along the prior extrusion axis may be as lit- tle as half that of the tensile yield strength along the same direction (e.g., Refs. [1,2]). If the flow stresses and hardening behaviors of the individual deformation mechanisms are known, poly- crystal-plasticity models provide a means of predicting the evolution of texture given boundary conditions appropriate for the prescribed deformation [3]. Con- versely, if the texture evolution and strain path are well characterized, an investigation of the underlying defor- mation mechanisms is possible [4]. Both avenues are examined in the context of equal channel angular extru- sion of magnesium alloys in the present paper. 1.1. Equal channel angular extrusion A novel metal forming process, which has come to be known as equal channel angular extrusion (ECAE) (e.g., Ref. [5]) or pressing (ECAP) (e.g., Ref. [6]), can impart enormous levels of strain without significantly changing the overall dimensions of the sample. In this regard, it is similar to torsion, which has also been used extensively in the study of strain hardening [7,8] and hot working 1359-6454/$30.00 Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2005.02.019 * Corresponding author. Tel.: +1 434 924 0605; fax: +1 434 982 5660. E-mail address: [email protected](S.R. Agnew). Acta Materialia 53 (2005) 3135–3146 www.actamat-journals.com
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Acta Materialia 53 (2005) 3135–3146
www.actamat-journals.com
Texture evolution of five wrought magnesium alloys during routeA equal channel angular extrusion: Experiments and simulations
S.R. Agnew a,*, P. Mehrotra a, T.M. Lillo b, G.M. Stoica c, P.K. Liaw c
a Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 24590, USAb Environmental and Energy Sciences Division, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415, USA
c Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA
Received 20 January 2005; received in revised form 9 February 2005; accepted 10 February 2005
Abstract
Equal channel angular extrusion (ECAE) has been demonstrated to induce unusual deformation textures and resulting properties
in magnesium alloys, such as the remarkably enhanced room temperature ductility first reported by Mukai et al. [Mukai T, Yamanoi
M, Watanabe H, Higashi K. Scr Mater 2001;45:89]. This paper documents a wide range of textures which evolve during ECAE of
magnesium alloys. The fact that different alloys exhibit different texture evolutions is an indication of distinctions in the balance of
deformation mechanisms which operate within the different alloys. Polycrystal plasticity modeling is used to develop explanations
for these texture distinctions in terms of the relative activities of non-basal secondary slip modes, involving Æaæ and Æc + aæ type dis-locations. AZ alloys appear to exhibit balanced secondary slip of non-basal Æaæ and Æc + aæ dislocations, while ZK60 and WE43
appear to favor non-basal Æc + aæ slip. A binary Mg–Li alloy exhibits a radically distinct texture evolution, which is associated with
large-scale strain accommodation by non-basal Æaæ slip.� 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
a 445 kN universal testing machine. Billets were ma-
chined from the as-received materials with dimensions
of 22 · 22 · 100–150 mm for ECAE processing. It was
necessary to process all of these alloys at elevated-tem-
peratures ranging from 175 to 325 �C (see Table 3) in or-
der to avoid fracturing the billets. Depending upon the
alloy, a temperature between 200 and 300 �C was first
attempted in view of hot-workability data from the liter-ature [1]. The temperature adopted was the lowest at
which successful extrusions were possible, in order to
promote grain refinement (see Table 3). The whole
ECAE die is heated to the desired temperature using
built-in cartridge heaters. The billets were lubricated
with MoS2 paste, inserted into the die, held until they
reached the processing temperature (approximately
25 min), and, then, pressed at a rate of 25 mm/min (or12 mm/min in the case of alloy WE43). Samples sub-
jected to multi-pass processing were machined slightly
to allow them to fit back in the entrance channel.
Some of the ECAE runs were performed with a back-
pressure enforced by a second ram and hydraulic cylin-
der on the exit channel. The back-pressure superimposes
a hydrostatic compressive stress on top of the primarily
shear stress at the intersection of the channels in order toprevent shear failures. For example, an initial attempt to
process alloy AZ31 at 200 �C or below resulted in cata-
strophic shear failures (Fig. 2). However, processing the
alloy at 300 �C yielded only marginal grain refinement
Fig. 2. Shear failures exhibited by the alloy AZ31 ECAE processed at (a) 20
Use of a backpressure during 200 �C processing prevented such failures. (Th
[51]. Enforcing a back-pressure of 58–77 MPa (as com-
pared with the �300–350 MPa entrance pressure) pre-
vented the shear failures mentioned above and enabled
processing at a temperature of 200 �C, which resulted
in substantial grain refinement [32]. Attempts to process
at lower temperatures (175 �C) were unsuccessful evenwith a back-pressure (Fig. 2(b)).
2.4. Modeling texture evolution
It will be shown that ECAE processing results in dif-
ferent texture evolutions for the different classes of
alloys. The viscoplastic self-consistent (VPSC) polycrys-
tal modeling scheme originally proposed by Hill [54] andimplemented later by Hutchinson [55] has been shown to
be very effective for modeling the plastic response and
texture evolution of non-cubic metals [56]. The details
of the particular VPSC algorithm employed in the pres-
ent work are explained in the paper by Lebesohn and
Tome [56]. In brief, the texture of the polycrystalline
aggregate is represented by assigning a finite number
(�1000) of discrete crystallographic orientations a vol-ume fraction. The resulting discrete texture approxi-
mates the more continuous orientation distribution
measured using X-ray diffraction. Each orientation is as-
signed an anisotropic viscoplastic constitutive response
characteristic of the single-crystal, e.g. 57, where all
the slip and twinning mechanisms have been assigned
a high stress exponent, n = 20. This exponent is not in-
tended to explicitly reflect the overall rate-sensitivity ofthe material. Rather, it serves to round the vertices of
the single crystal �yield surface�, providing a mathemati-
cally convenient and unambiguous connection between
the stressing direction and the resulting strain rate.
The coupling of the grain-level response with the
aggregate is accomplished using a self-consistent
homogenization scheme based upon Eshelby�s inclusionformalism [58]. The interaction strength is assignedthrough the parameter neff = 10, which is an intermedi-
ate value between the compliant tangent modulus
0 �C without any back-pressure, and (b) 175 �C with a back-pressure.
basal planes aligned with the extrusion axis and a ten-
dency for the h10�10i directions to align with the extru-
sion axis. Alloys ZK60 and Mg–Li have the strongest
initial textures, and WE43 is the most weakly textured.
The texture of the large AZ80 extrusion is distinct, in
that there is also a secondary Æ0001æ fiber aligned with
extrusion axis. The effect of initial texture on the texture
evolution during ECAE was examined using two otherinitial conditions of alloy AZ31: a plate with an axis-
symmetric Æ0001æ fiber aligned with the plate normal
with an intensity of four multiples of a random distribu-
tion (MRD) and a weakly textured direct-chill cast
billet.
3.2. The effect of alloying
The initial textures (labeled 0) and the texture ob-
served after 1, 2 and 4 passes by route A are shown in
Fig. 4. All the texture results are presented in terms of
recalculated (0002) and ð10�10Þ pole figures from the
flow plane (i.e., the plane which contains the entrance
and exit channels) with the exit direction (the x-axis in
Fig. 1) to the right. Due to the 2-fold rotational symme-
try of the process (about the flow plane normal, thez-axis in Fig. 1), and the symmetry of the experimental
pole figures, the orientation distributions and full pole
figures were calculated assuming monoclinic symmetry.
The pole figures are, thus, completely represented by
their upper half in order to save space.
After a single pass, alloy AZ31 essentially exhibits a
Æ0001æ fiber texture (Fig. 4(a)) with the dominant c-axis
fiber is oriented approximately 20� aft of vertical (they-axis in Fig. 1). Subsequent passes serve to strengthen
this texture, although it is noted that there are enormous
rigid body rotations (Eqs. (2) and (3)) and plastic rota-
tions inherent to the process. Therefore, this result is
not evidence of texture stabilization, as might occur dur-
ing monotonic straining, such as multi-pass rolling.
Rather, it demonstrates reorientation to a very similar
texture after each subsequent pass. After subsequent
passes, there is also a notable strengthening of the
h10�10i parallel to (or i) z. Finally, there is a trend for
the dominant Æ0001æ texture component to split into
two components, one closer to the z-axis. Alloy AZ80
processed under the same conditions adopted an essen-
tially identical texture, which is not shown in the interest
of space.
The texture evolution of alloy ZK60 (Fig. 4(b)) showsthree major distinctions from that observed for the AZ
alloys after a single pass. (i) The major Æ0001æ fiber is
oriented closer to the y-axis; (ii) there is a secondary fi-
ber Æ0001æ iz; and (iii) h11�20ik z, placing maxima in the
ð10�10Þ pole figure 30� away from the z-axis. During
subsequent passes, the secondary texture component
Æ0001æ iz weakens, and the dominant fiber �5� aft of
the y-axis. The texture after 8 passes by route A is verysimilar to that shown after 4 passes, albeit with higher
texture strength, with peak intensities in the basal pole
figure of �8 multiples of a random distribution
(MRD), in comparison with �6 MRD for 4 passes. Al-
loy WE43 is designed for high temperature strength and,
thus, required the highest processing temperature
(325 �C) of all the alloys examined. Although it exhibits
slightly weaker textures throughout (maximum peakintensities of �4 in the basal pole figure), the texture
evolution of alloy WE43 (Fig. 4(c)) is most similar to
that of the other fine-grained Zr-containing alloy in this
study, ZK60.
The experimental Mg–Li alloy exhibits the most dis-
tinct texture evolution (Fig. 4(d)), which is not surpris-
ing in view of the unique properties and deformation
mechanisms already reported for this alloy (e.g., Refs.[46–49]. The initial extrusion texture is similar to the
other alloys due to the high symmetry of the extrusion
process itself. The lower symmetry of the ECAE process
reveals a strong distinction between Mg–4wt%Li and
the other alloys. The dominant texture component in
all the other alloys (Æ0001æ iy) becomes secondary, while
the Æ0001æ iz dominates the texture by the 4th pass
through the die. This appears consistent with the prior
Fig. 4. (0002) and ð10�10Þ pole figures (upper half) show initial extrusion textures (labeled 0) and ECAE textures after 1, 2, and 4 passes by route A.
The distinctions suggest that different deformation mechanisms operate in the different alloys. The extrusion axis is to the right, and the intensity scale
is the same as in Fig. 3 in this figure and all that follow.
observation of enhanced non-basal slip in this alloy, in
particular, the observation that prismatic slip dominated
the single-crystal deformation of Mg at elevated
temperatures [47].
3.3. The effect of initial texture
The hot-rolled AZ31 plate has a very distinct initial
texture compared to the extrusions discussed above,
however, the resulting ECAE textures (Fig. 5) are simi-
lar. The primary distinction is the complete absence of
any Æ0001æ iz, which most likely results from the factthat the initial texture places essentially all the grains
with their c-axes far from this orientation. A second dis-
tinction is that the dominant Æ0001æ fiber component is
closer to the z-axis after ECAE. Taking a most simplistic
view, the ECAE textures shown in Fig. 5 are similar to
the initial plate texture. However, the initial plate tex-
ture exhibits an orthotropic symmetry characteristic of
the rolling process, while the ECAE textures exhibit alower (monoclinic) symmetry characteristic of ECAE
[25]. Proceeding to 8 passes by route A only succeeds
in strengthening the texture. Beginning with a very
weakly textured, but coarse-grained, as-cast AZ31 mate-
rial resulted in an ECAE texture evolution almost iden-
tical to that shown for the extruded AZ31.
3.4. Modeling the texture evolution during ECAE, route A
In all cases, it has been assumed that the basal Æaæ slipmechanism is the easy slip mechanism and, therefore, it
is shown to accommodate most of the strain and leads to
the overall similarity between the textures of the various
alloys (see Figs. 3–5). In most of the simulations, the
three slip modes explored were basal Æaæ, prismatic Æaæ,and pyramidal Æc + aæ slip. In a few cases, the pyramidalÆaæ slip and f10�12g tension twinning modes were
included.
3.4.1. Modeling the effect of the non-basal slip mode
activity
The activities of the non-basal slip modes are var-
ied by assuming different critical resolved shear stress
(CRSS) values, relative to the CRSS for basal slip(sbasal = 1). A range of CRSS values from 2 to 8 is
explored for both prismatic and Æc + aæ slip modes.
Fig. 6 shows the simulated textures after a single
ECAE pass, where the initial texture was assumed
Fig. 5. (0002) and ð10�10Þ pole figures of AZ31, from initially (a) hot-rolled plate (labeled 0) or (b) cast billet and ECAE textures after 1, 2, and 4
passes by route A.
Fig. 6. (0002) and ð10�10Þ pole figures of simulated single-pass ECAE textures beginning with an initially random texture. Labels indicate relative
CRSS values of the basal, prismatic, and pyramidal Æc + aæ slip modes used in the simulations.
to be random. The contour plots presented have been
smoothed over 10� in keeping with the fact that the
initial discrete textures were determined for a
10� · 10� · 10� grid in Euler space. In all cases, basal
slip dominates the strain accommodation and causes a
dominant basal fiber texture to evolve. The clearest
case of this is labeled 1-8-8, indicating that there isa relatively high CRSS value for both of the non-ba-
sal slip modes sprismatic = sÆc + aæ = 8. The result is a
simple Æ0001æ fiber component tilted �20� aft of
the y-axis. Non-basal slip mechanisms accommodate
no more than �20% of the strain individually, with
the prismatic Æaæ slip mode being about twice as ac-
tive as the Æc + aæ.Increasing the activity of the Æc + aæ slip system up to
�20% by lowering its CRSS value to sÆc + aæ = 4 leads to
a splitting of the dominant Æ0001æ fiber into two compo-
nents, with one closer to the y-axis. There is a corre-
sponding weakening of the intensity in the ð10�10Þpole figure, with some tendency for h10�10ik z-axis. On
the other hand, if the prismatic slip mode becomes more
active (accommodating up to �40% of the strain for the
cases where sprismatic 6 4), orientations close toÆ0001æ iz are stabilized and the band in the ð10�10Þ polefigure splits into peaks corresponding to h11�20ik z.
Simulations incorporating the pyramidal Æaæ slip
mode yield very similar results to those involving
prismatic slip, since a combination of prismatic and
basal slip gives rise to the same shear strains and
crystallographic rotations as pyramidal Æaæ slip.
Therefore, to keep the simulation parameters to a
minimum, the non-basal slip of Æaæ dislocations is
considered collectively and only prismatic slip is mod-
eled in practice [71]. Additionally, while f10�12g ten-sion twinning is known to be a very important
deformation mechanism at low temperatures, it is less
active at elevated temperatures [2]. A few simulations
including tension twinning were performed, however,
the texture components which resulted are not ob-
served experimentally.
3.4.2. Modeling the effect of the initial texture
Simulations beginning with an initial texture, which
models the AZ31 extrusion or plate texture, exhibit
more non-basal slip mode activity than an initially ran-
dom texture. Even with the relative CRSS values set
When the CRSS for the prismatic slip mode is quite
low, sprismatic = 2, the Æ0001æ iz component becomes
the dominant feature in the texture labeled 1-2-8. Corre-spondingly, there are strong texture components within
the flow plane shown in the ð10�10Þ pole figure. The caseinvolving extensive Æc + aæ slip (�35% of the strain
accommodation) labeled 1-8-4 is similar to that simu-
lated from a random texture.
Since only alloy AZ31 was processed from plate mate-
rial, only CRSS conditions appropriate for simulating
the AZ31 texture evolution were explored. Fig. 8 showsthe impact of increasing the contribution of the Æc + aæslip mode. Again, increasing the activity of Æc + aæ causesthe dominant Æ0001æ fiber to split into two components
and with the stronger of the two rotated close to the
y-axis.
3.4.3. Modeling multi-pass ECAE texture evolution
The metallographic investigation of some ECAE pro-cessed magnesium alloy sample [51] reveals that the
grain shapes after ECAE are equiaxed. This observation
combined with the assumption of no slip system harden-
Fig. 9. Pole figures showing textures simulated after a second route A pass
simulations performed with the same CRSS ratios.
ing reduces the problem of multi-pass simulation to an
issue of initial texture. As for the case of the plate tex-
ture above, the focus here will be on simulating the tex-
ture evolution of alloy AZ31, thus the simulations wereall performed with the CRSS combination of 1-8-6 (i.e.,
sprismatic = 8 and sÆc+ aæ = 6).
The four cases in Fig. 9 show the texture evolution
during a second ECAE pass by route A, beginning with
simulated single pass textures with (a) random (Fig. 6),