Typical Textures, part 2: Thermomechanical Processing (TMP) of bcc Metals

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Nov 29, 2013 (3 years and 8 months ago)

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1

Typical Textures, part 2:

Thermomechanical Processing (TMP)
of bcc Metals

27
-
750

Advanced Characterization and
Microstructural Analysis

A.D. Rollett

Carnegie
Mellon
MRSEC
2

Objectives


Introduce you to experimentally observed
textures in a wide range of (bcc) materials.


Develop a taxonomy of textures based on
deformation type.


Prepare you for relating observed textures to
theoretical (numerical) models of texture
development, especially the
Taylor

model.


See chapter 5 in Kocks, Tomé & Wenk.

3

Taxonomy


Deformation history more significant than
alloy.


Crystal structure determines texture through
slip (and twinning) characteristics.


Alloy (and temperature) can affect textures,
e.g. through planarity of slip; e.g. through
frequency of shear banding.


Annealing (recrystallization) sometimes
produces a drastic change in texture,
although this is less important than in fcc
alloys.

4

Why does deformation result in
texture development?


Deformation means that a body
changes its shape, which is quantified
by the plastic strain,
e
p
.


Plastic strain is accommodated in
crystalline materials by dislocation
motion, or by re
-
alignment of long chain
molecules in polymers.

5

Dislocation glide


grain
reorientation


Dislocation motion at low (homologous
temperatures) occurs by glide of loops on
crystallographic planes in crystallographic
directions:
restricted glide
.


Restricted glide throughout the volume is
equivalent to uniform shear.


In general, shear requires lattice rotation in
order to maintain grain alignment:
compatibility

6

Re
-
orientation



Preferred orientation


Reorientations experienced by grains depend on the
type of strain (compression versus rolling, e.g.) and
the type of slip (e.g. {110}<111> in bcc).


The Taylor model is a useful first order (crystal
plasticity) model that correctly predicts the main
features of deformation textures, although more
sophisticated models are required for quantitative
matching.


In general, some orientations are unstable (
f(g)

decreases) and some are stable (
f(g)

increases) with
respect to the deformation imposed, hence
texture
development
.

7

Texture Development in Low C Steel
During Cold Rolling & Annealing

(R.K. Ray et al., International Materials Reviews, 1994, Vol.39, p. 129)



rolling and recrystallization texture




-

to
-



瑲慮sf潲m慴a潮

Tr慮sf潲med


h潴b慮d瑥硴xre

†††
C潬d⁲潬lin朠瑥硴xre

C潬d⁒潬ling

NfiberⰠ,11ㄾ




RfiberⰠ,1㄰1





nne慬in朠瑥硴xre

Nfiber⁳h慲pens

Rfiber⁷e慫ens

8

Deformation systems (typical)

In deformed materials, texture or preferred orientation exists due to the
anisotropy of slip. While slip in bcc metals generally occurs in the <111>
type direction, it may be restricted to {110} planes or it may involve
other planes

(
T. H. Courtney, Mechanical Behavior of Materials, McGraw
-
Hill, New York, 1990
.)

9

Axisymmetric deformation:
Extrusion, Drawing


















5
.
0
0
0
0
5
.
0
0
0
0
e
10

bcc uniaxial textures

92% rolled Ta

Tensile test in

original RD to

strain of 0.6:

<110> fiber

(a) Normal and rolling direction
inverse pole figures (equal area
projection) of 92% rolled Ta and
(b) Prior normal and rolling
direction inverse pole figures for
(a) tested in tension to a strain
of 0.6 (tensile direction
coincident to prior rolling
direction).

11

Rolling

RD

ND

Rolling ~ plane strain deformation means
extension or compression in a pair of
directions with zero strain in the third
direction: a
multiaxial strain
.
















0
0
0
0
0
0
0
e
12

Rolling
Textures
bcc

{110} and {100} pole figures
(equal area projection; rolling
direction vertical) for (a) low
-
carbon steel cold rolled to a
reduction in thickness of 80%
(approximate equivalent
strain of 2); (b) tantalum,
unidirectionally rolled at room
temperature to a reduction in
thickness of 91%.

[Kocks]

13

{100} Pole
figure for
certain
components of
rolled bcc
metals

Note how very different
components tend to overlap in
a pole figure.

14

Fiber Texture in bcc Metals

15

bcc fibers: the
f
2

= 45
°

section


,

<111>||ND

,

<110>||RD


e,

<110>||TD

Goss

F

f
1

Bunge Euler angles

16

Theoretical
bcc rolling
texture

Calculated using LApp,
starting from a random
texture with a strain of 50%
(about 35% reduction). The
gamma fiber is approximately
in the center of each section.

The 45
°

section of
the COD shows a
strong alpha fiber
and only partial
development of the
gamma fiber at this
low strain

17

Dependence on Rolling Reduction

TD

RD

18

Crystallite Orientation

Distribution in Polar Space

RD

TD

h
-
fiber


-
fiber


-
fiber

19

Alpha fiber plot, <110>
// RD

0
2
4
6
8
10
12
14
0
20
40
60
80
Alpha fiber
20% CR
50 % CR
60% CR
80 % CR
70% CR BA
Intensity
Phi (°)
{001}<110>
{112}<110>
{111}<110>
{110}<110>
20

Gamma Fiber, <111>


ND

0
1
2
3
4
5
6
60
65
70
75
80
85
90
Gamma Fiber
20% CR
50% CR
60% CR
80% CR
70% CR BA
Intensity
f
1
(°)
{111}<110>
{111}<112>
21

Ta, Fe
rolling
textures

Note: in these plots, the

Euler angles

are Roe angles:

axes transposed

with
Q

F


horizontal,

y

(=
f
1
-
90
°
)vertical.

45
°

sections, contours at 1,2 … 7

(a)
low
-
C steel before cold rolling

(b)
Low
-
C steel reduced 90%

(c)
Tantalum rolled to 91%

45
°

sections, contours at 1,2,3,4 …

(a)
0% Si steel before cold rolling

(b)
2% Si steel before cold rolling

(c)
0% Si steel cold rolled 75 %;

{112}<110> strongest

(d)
2% Si steel cold rolled 75 %;

{111}<110> strongest

a

b

c

d

max

max


,

<111>//ND

,

<110>//RD

22

Fe, Fe
-
Si
rolling,

fiber plots

Note the marked

alloy dependence

in the alpha fiber;

smaller variations

in the gamma fiber.

23

Parameters for Optimizing <111>
Fiber Texture


Fine hot band grain size


Low concentrations of C, N, Mn


Additions of Ti, Nb


Long holding times in annealing


High annealing temperatures


Large cold reduction ratio


Control of coiling & rolling temperatures


Anneal before Cold Rolling (austenite)

24

Mechanical Properties


Plastic Strain Ratio (r
-
value)

)
2
(
2
1
)
(
)
2
(
4
1
)
(
)
/
ln(
)
/
ln(
)
/
ln(
)
/
ln(
90
45
0
90
45
0
r
r
r
anisotropy
planar
r
r
r
r
value
r
r
W
L
Wf
L
Wf
W
Tf
Ti
Wf
W
r
m
i
i
f
i
i











Large r
m

and small

r required for deep drawing

L
i

W
i

Rolling Direction

45

90

0

25

Correlation between r
m

and
I
{111}/
I
{100}

texture ratio in Steels

(J.F. Held, 'Mechanical Working and Steel Processing IV', 1965)

26

Variation of r
-
value with Texture

These plots make it clear that the two main gamma fiber components
complement each other to give high r
-
values; other components in the alpha
fiber tend to lower the r
-
value and make it anisotropic

27

Shear Texture


Shear strain means that displacements are tangential
to the direction in which they increase.


Shear direction=1, Shear Plane


2
-
axis

e
12

1 = Shear Direction = <uvw>

2 =

Torsion

Axis

= {hkl}


d
e

0

0
0
0
0
0
0
0










28

Torsion Textures: twisting of a
hollow cylinder specimen

(a)

(b)

(c)

Torsion Axis

29

{100} Pole figures

Montheillet et al.,

Acta metall.,
33
, 705, 1985

fcc

bcc

30

bcc torsion textures:
Fe

Ideal |{100}

pole figures

31

bcc torsion
textures: Ta

(a) initial texture

from swaged rod
;

(b) torsion texture

Ideal |{100}

pole figures

32

Summary: part 2


Typical textures illustrated for shear textures and for
bcc

metals.


Pole figures are recognizable for standard
deformation histories but orientation distributions
provide much more detailed information.


For bcc rolling textures, the 45
°

section often
provides most of the information needed.


As an example of the connection to mechanical
properties, the plastic strain ratio (r
-
value) is highly
sensitive to texture. For deep drawing, the gamma
fiber should be optimized. For use in motors as
electrical sheet steel, the <100>//ND fiber should be
optimized (not yet a solved problem).