Typical Textures part 2 Thermomechanical Processing TMP of

Typical Textures, part 2: Thermomechanical Processing (TMP) of bcc Metals 27 -750 Advanced Characterization and Microstructural Analysis A. D. Rollett 1

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. 2

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. 3

Why does deformation result in texture development? • Deformation means that a body changes its shape, which is quantified by the plastic strain, ep. • Plastic strain is accommodated in crystalline materials by dislocation motion, or by re-alignment of long chain molecules in polymers. 4

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 5

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. 6

Texture Development in Low C Steel During Cold Rolling & Annealing rolling and recrystallization texture - to - transformation Transformed hot band texture Cold Rolling Cold rolling texture RD fiber, <110> RD ND fiber, <111> ND ND fiber sharpens Annealing texture RD fiber weakens (R. K. Ray et al. , International Materials Reviews, 1994, Vol. 39, p. 129) 7

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, Mc. Graw-Hill, New York, 1990. ) 8

Axisymmetric deformation: Extrusion, Drawing 9

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). 10

Rolling ND RD Rolling ~ plane strain deformation means extension or compression in a pair of directions with zero strain in the third direction: a multiaxial strain. 11

Rolling Textures bcc {110} and {100} pole figures (equal area projection; rolling direction vertical) for (a) lowcarbon 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] 12

{100} Pole figure for certain components of rolled bcc metals Note how very different components tend to overlap in a pole figure. 13

Fiber Texture in bcc Metals 14

bcc fibers: the f 2 = 45° section 1 e, a, <110>||RD g, <111>||ND <110>||TD Goss Bunge Euler angles 15

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 16

Dependence on Rolling Reduction RD TD 17

Crystallite Orientation Distribution in Polar Space h-fiber a-fiber g-fiber RD TD 18

Alpha fiber plot, <110> // RD 19

Gamma Fiber, <111> ND 20

Ta, Fe rolling textures Note: in these plots, the Euler angles are Roe angles: axes transposed with Q horizontal, y (= 1 -90°)vertical. 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 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% a, <110>//RD b a g, <111>//ND max c d max 21

Fe, Fe-Si rolling, fiber plots Note the marked alloy dependence in the alpha fiber; smaller variations in the gamma fiber. 22

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) 23

Mechanical Properties Plastic Strain Ratio (r-value) Rolling Direction 0 90 45 Wi Li Large rm and small ∆r required for deep drawing 24

Correlation between rm and I{111}/I{100} texture ratio in Steels (J. F. Held, 'Mechanical Working and Steel Processing IV', 1965) 25

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 26 fiber tend to lower the r-value and make it anisotropic

Shear Texture • Shear strain means that displacements are tangential to the direction in which they increase. • Shear direction=1, Shear Plane 2 -axis 2= Torsion Axis = {hkl} e 12 1 = Shear Direction = <uvw> 27

Torsion Textures: twisting of a hollow cylinder specimen (a) (c) Torsion Axis (b) 28

{100} Pole figures Montheillet et al. , Acta metall. , 33, 705, 1985 fcc bcc 29

bcc torsion textures: Fe Ideal |{100} pole figures 30

bcc torsion textures: Ta (a) initial texture from swaged rod; (b) torsion texture Ideal |{100} pole figures 31

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). 32