Texture development during processing

Transcription

1 Texture development during processing Course objectives: 1. To introduce the typical deformation textures in metals and alloys with FCC, BCC and HCP crystal structures 2. To explain the micro-mechanism of texture formation during simple deformation processes 3. To introduce the typical annealing textures that form in FCC, BCC and HCP materials 4. To explain the mechanism of annealing texture formation 5. To understand texture formation in non-metals

2 Deformation texure of FCC Metals and alloys

3 Texture development during uniaxial deformation In uni-axial deformation like tension, compression, extrusion etc., textures are of fibre type. Texture is represented using inverse pole figure. Here only the crystallographic direction parallel to the loading axis is presented. Since there is no well defined RD and TD in these deformation processes unlike rolling, direction parallel to RD is neglected while representing textures. The pole figure below shows a typical fibre texture obtained after tension test. FCC metals tend to show a double fibre texture comprising dominantly <111> and <100> fibres. 111 pole figure of Ni after tensile test

4 Texture development during rolling Rolling textures in FCC materials can be classified into two categories: Copper-type texture (also pure metal texture) : This texture consists of copper-type texture constitutes higher fraction of S {123} <634> and equal fraction of Bs {110}<112> and Cu {112}<111> texture components. High and medium stacking fault energy materials like Al, Ni, Cu etc. develop this type of texture during deformation. Brass-type texture (also alloy type texture): This texture constitutes higher fraction of Bs {110}<112> and Goss {110} <001> and negligible fraction of S {123} <634> and Cu {112}<111> texture components. Low stacking fault energy materials like Ag, austenitic stainless steel, Cu- 30Zn (α-brass), Ni-60Co etc.

5 Copper type texture A typical 111 pole figure for FCC showing a characteristic Cu-type texture, after high strain rolling deformation is shown below. RD {112} <111> {110} <112> {123} <634> {100} <001> ND TD ND This pole figure was obtained after rolling of Ni-20Co alloy to 95% thickness reduction. The above figure schematically shows the locations of ideal rolling texture components in a 111 pole figure. It is to be mentioned here that the name copper component (Cu), brass component (Bs) etc. were given due to the fact that these orientations were first identified in these materials. However, in the texture of any FCC material, significant fractions of grains with all the orientations depicted in the above figure are found. It is only the question of dominant fraction of grains with some set of orientations.

6 Brass type texture A typical 111 pole figure for FCC showing a characteristic Bs-type texture is shown below. RD {112} <111> {110} <112> {123} <634> {100} <001> ND TD ND This pole figure was obtained after rolling of Ni-60Co alloy to 95% thickness reduction. The above figure schematically shows the locations of ideal rolling texture components in a 111 pole figure. A comparison of the pole figure on the previous slide with the one given above clearly depicts the difference in the two types of texture. In the pole figure of material displaying Bs type texture, the Cu component is almost non-existent. However, in the materials displaying copper type texture, in addition to Cu as the main component, even Bs and S components are present. The mechanism behind the formation of these two types of texture is described in next slides.

7 Mechanism of Rolling texture formation We have seen that stacking fault energy plays a major role in deciding the deformation mechanism in FCC materials. In addition, it is also established that texture evolution is a function of imposed strain also in addition to alloying. The results of various experimental observations indicate that texture evolution is a strong function of initial texture, and the stable end orientation can develop only after large deformation, sufficient enough to annul the effect of initial texture.

8 As mentioned earlier, every orientation in an Euler space has an unique Euler angles, φ 1, φ, φ 2. The following table lists the Euler angles for the ideal FCC deformation texture components. Deformation texture components Euler angles ϕ 1, φ, ϕ 2 (Bunge notation) Cu {112} <111> 90, 35, 45 Bs {110} <112> 35, 45, 0 S {123} <634> 59, 29, 63 Goss {110} <001> 0, 45, 0 Cube {100} <001> 0, 0, 0 Goss Goss Brass Different deformation texture components in Euler space are connected. The lines connecting them are called Texture Fibres. These fibres help in tracking the evolution of ideal components as a function of deformation or any processing conditions Brass Brass S 1 Goss 90 Cu

9 Following are the texture fibres defined for FCC systems: α - fibre: Extends from Goss {011}<001> to Brass orientation {011}<112>. β - fibre: 90 -fibre 2 Extends from Copper {112}<111> to S {123}<634> to Brass orientation τ - fibre: Extends from {001}<110> to Copper {112}<111> to Goss orientation Goss -fibre Goss 0 90 Brass Brass Brass S 1 -fibre Goss Caution: Due to the symmetry of Euler space, same orientation and fibres can appear multiple times. 90 Cu -fibre

10 Bs Cu Courtesy: R. Madhavan S

11 How the texture transition is manifested in ODF? The figures show the complete 2 sections of ODF for Ni-20Co (top) and Ni-60Co (bottom). Just recall the locations of Bs, S, Cu, Goss components in Euler space listed earlier for better understanding. For ease, let us compare only 2 = 0 o, 45 o, 65 o sections. We could see that Goss and Bs components (red circles) strengthened as Co additions increases, i.e with lower SFE. We also could observe the complete vanishing of Cu (green circles) and S (purple circles) components from 2 = 45 and 65 o sections respectively. Ni-20 Co Ni-60 Co

12 Questions 1. Texture transition during rolling in FCC occurs because (a) Stacking fault energy (b)temperature of deformation (c) Amount of deformation (d)strain per pass. 2. Which of the following are true. (a) Low SFE materials show higher fractions of Bs than S & Cu. (b)α-fibre weakens during rolling of low SFE materials. (c) Higher Bs content is attributed to strain heterogeneities like twinning and banding. (d)cube is one of the major rolling texture component in FCC. 3. Which of the following component(s) disappear during the rolling of a typical low SFE FCC material (a) Bs (b) Goss (c) Cu (d) S 4. In Cu-Zn alloys, addition of Zn increases the SFE and hence lead to Bs-type texture. True/False 5. Fibre textures in FCC are commonly seen in uni-axial deformation. True/False 6.What is the effect of stacking fault energy on the texture evolution in FCC materials. 7.How does the impurities in the samples affect the texture of FCC materials? 8.List the texture fibres in FCC materials resulting from different types of deformation. 9.What factors contributes to the deviation in texture components from the ideal position in FCC materials? 10.List set of ideal texture components during rolling, wire drawing, extrusion and compressions texture in FCC materials.