Recrystallization textures in metals and alloys

Transcription

1 Recrystallization textures in metals and alloys

2 Uniaxial deformation Aluminium wire F.C.C. Metals and alloys FCC wires retain deformation texture ([111]+[100]) upon recrystallisation Composition / Purity plays an important role: CP Aluminium (99.95%) with ([111] + [100]) deformation texture when recrystallised at C, it leads to (i) sometimes random texture, or (ii)major component [103] to [113]+minor component [111] when recrystallised at 600 C, pure aluminium leads to secondary recrystallisation texture - [103] to [113]

3 Aluminium after compression deformation texture is retained on recrystallisation, as a whole but the individual grains will alter their orientation. secondary recrystallisation leads to [111] texture in aluminium, [110] texture in silver Copper when recrystallised below 400 C [100], [112] when recrystallised above 400 C may be random when recrystallised above 950 C [112] / [111] mixed or random, depending on deformation texture

4 Annealing of F.C.C. sheet materials can produce the following textures: The cube texture Sheet texture A texture approximately same as the rolling texture One or more new types of texture Approximate randomness

5 Rolled aluminium partially retains its rolling texture on recrystallisation, in addition to developing a cube texture Retained component is due to recrystallisation insitu in some grains (without orientation change) Rolled silver, Cu-33Zn, Cu-5Sn, Ag-1Zn, Ag-30Au (with {110}<112> rolling texture) changes to a new texture {113}<211> or {225}<734> at low temperature upon long annealing at 433 to 533 C by secondary grains having the orientation of deformation texture becomes random texture with annealing above 800 C. Recrystallisation texture of Brass, Cu-Ge, Cu-Sn gradually change with composition

6 The cube texture Most remarkable recrystallisation texture Extremely sharp texture when fully developed Resembles a single crystal with subgrains may contain a minor amount of material with different orientation Arrangement of Crystallites in cube texture (111) Pole figure RD TD (001) ND (Sheet Normal Direction) [100] RD (Sheet Rolling Direction)

7 Conditions for Cube texture Percentage of cube oriented grains vary with (a) processing variables, and (b) composition For example, for a well developed cube texture well developed deformation texture is required 85-90% cold reduction is necessary percentage of cube orientation in copper increases with: (a) annealing temperature (b) annealing time (c) high reduction prior to final anneal (d) a small penultimate grain size (e) small thickness of cold rolled grains prior to final anneal

8 Theory of Cube textures in FCC metals Mechanism of formation of Cube nuclei A large driving pressure provided by higher stored energy* of adjacent S component. Orientation Size of subgrains ( m) Stored energy (mjcm -3 ) Brass S Copper Cube A large subgrain size cube subgrains tend to be larger than other orientations have a larger size spread which results in the presence of some exceptionally large cube subgrains initiation of Strain Induced Boundary Migration of the cube bands * Stored energy within a grain is due to the accumulation of dislocation and other defects during deformation

9 Theory of Cube textures in FCC metals (continued) A higher mobility of a 40 o <111> Cube/S boundary would promote the early stages of growth- having boundaries that are curved to favour continued growth at the expense of the others. A lower energy of a special boundary such as 40 o <111> would reduce the retarding pressure due to boundary curvature Pc = 2 b /R

10 How to minimise a cube texture? (a) Intermediate anneal and small reduction (b) Large penultimate grain size (c) Low final annealing temperature (d) Addition of alloying elements, particularly the ones which change the deformation texture from copper type to brass type e.g. In copper, following elements minimise or suppress cube texture 5 wt % Zn 1 wt % Sn 4 wt % Al 0.5 wt% Be /Cd at% P 0.3 at% Sb 1.5 at% Mg 4.2 at% Ni 0.18 at% Cd at% As Size effect strong solid solution hardening In Aluminium, conditions for optimising cube texture are different: Increasing rolling reduction suppresses cube texture; retains rolling texture Ratio of Fe to Si in Al is critical for cube texture Recovery is easier in Al. Recrystallisation is replaced by recovery (a) on annealing at high reductions, and (b) with high iron contents

11 B.C.C. Metals and alloys Uniaxial deformation BCC wires retain deformation texture [110] upon recrystallisation - grains of other orientations are formed on secondary recrystallisation Composition plays an important role: Tungten has [320], [321], [531], or [421] - as secondary recrystallisation texture Fe, Steels, Vanadium recrystallise with retention of [110] Iron after compression deformation texture [111] is retained on recrystallisation as a major component, C - the minor component [100] is lost on recrystallisation at 580 and 850

12 Sheet texture Rolled Iron and steel on recrystallisation at 540 and 840 C show three principal texture components: 1. {111}[211] 2. {001}[uvw], where [uvw] is 15 from [110] 3. {112}[uvw], where [uvw] is 15 from [110] relatively weaker Recrystallisation texture of V, Fe-Si, Zr-Nb has been found similar to Fe Mo retains its deformation texture with somewhat spread ~5 Ta has recrystallisation texture {111}[211] (~2400 C) W recrystallises (~1800 C) to {001}[uvw], where [uvw] is 15 from [110]

13 H.C.P. Metals and alloys Uniaxial deformation Be wires retain deformation texture [10 10] upon recrystallisation - Ti & Zr change to [11 20] Sheet textures In Zn, Mg, Ti (annealed below 500 C ), Be (annealed at 700 C ) deformation texture is retained on recrystallisation. - Zr recrystallised between C has orientation with orientations having [11 20] rotated to the position near rolling direction.

14 Questions 1. The texture component seen dominantly during FCC recrystallization is (a) {110} <001>; (b) 100} <001>; (c) {123} <634>; (d) {100} <013> 2. Which of the following is true. (a) High cube is due to the lowest stored energy (b) Cube formation is independent of amount of deformation and temperature (c) Fraction of cube increases with alloying additions (d) High cube can be attributed to its high boundary migration 3. Which of the following is true regarding nucleation. (a) Surface energy should be larger than strain energy (b) Strain energy should be larger than surface energy (c) Both surface and strain energies should be equal. (d) Surface and strain energies are independent. 4. Recrystallization, in general, is characterized by (a) High hardness and increased HAGB* (b) Low hardness and increased HAGB (c) High hardness and decreased HAGB (d) Low hardness and decreased HAGB 5. Can reducing the SFE of a material by alloying additions suppresses cube texture. Yes/No *HAGB High angle grain boundary