Texture Evolution in Severe Plastic Deformation Severe plastic deformation processes involve large...

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Transcript of Texture Evolution in Severe Plastic Deformation Severe plastic deformation processes involve large...

  • Texture Evolution in Severe Plastic Deformation Processes

    Satyam Suwas and Soumita Mondal

    Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

    Severe plastic deformation processes involve large grain rotations due to the action of different modes of plastic deformation and other microstructural changes which lead to characteristic texture formation. The present review deals with the evolution of texture during the most important severe plastic deformation processes, namely Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Friction Stir Processing (FSP), Accumulative Roll Bonding (ARB) and Multi-Axial Forging (MAF). First three of the processes are shear based, while the latter two are plane-strain based. The textures formed during ECAP are visually different from simple shear textures due to (i) the inclination of the shear plane, (ii) additional contribution of non-shear based deformation. The relative intensities of texture components are function of deformation micro-mechanisms, amount of straining and configuration of the strain path. The texture evolved during HPT is very similar to simple shear texture, with additional consequences of microstructural changes that occur due to very large deformations. The textures formed in FSP process also resemble shear textures. On the other hand, texture evolution during ARB and MAF can be described using plane strain deformation. The present review deals with texture evolution during severe plastic deformation as a function of nature of processes and type of materials. [doi:10.2320/matertrans.MF201933]

    (Received February 25, 2019; Accepted March 18, 2019; Published June 21, 2019)

    Keywords: texture, severe plastic deformation

    1. Introduction

    The relevance for SPD processing is the production of bulk ultrafine grained materials without changing the dimensions. Severe plastic deformation is generally carried out for grain size reduction, which in turn leads to the enhancement in strength and hardness of a material. SPD techniques introduce a large amount of strain in the material, and a variety of strain path changes. As a consequence, substantial modification/evolution of texture also occurs, that plays an equally important role in the mechanical response of the deformed material, together with grain size. Texture too plays a major role in the grain refinement process during SPD.

    Texture is defined as the orientation distribution of crystallites in a polycrystalline aggregate. Texture changes are produced by the rotation of grains. During deformation, small regions within a grain start rotating by the activity of different slip systems, which lead to the formation of substructures within the grain. The orientation within these substructures will have a slight deviation from that of its parent grain. Gradually these substructures rotate to preferred orientations which are characteristic to the applied deforma- tion. The same mechanism is applicable when deformation is mediated through twinning, except for the fact that the twinned region will exhibit a high misorientation angle with respect to its parent grain. The characteristic orientations are also dependent on the rate of rotation of the crystallites.

    2. Texture Evolution during Severe Plastic Deformation

    Many severe plastic deformation techniques have been proposed. The most important among them are equal channel angular pressing (ECAP), high pressure torsion (HPT), friction stir processing (FSP), accumulative roll bonding (ARB) and multi-axial forging (MAF). Out of these processes, first three are shear based processes, and the latter two involve plane strain deformation. Accordingly, the texture evolved during severe plastic deformation can also

    be categorised in two types: (i) shear based, and (ii) plane strain based.

    2.1 Ideal orientations after shear based SPD processes As mentioned earlier, textures evolved during SPD are

    basically deformation textures. The parameters that control the deformation texture are the imposed strain path, crystallographic factors governing the mechanism of deformation (slip, twinning etc.) and the initial microstructure and texture. In the following subsections, the evolution of texture has been described on the basis of strain path selected for each type of materials. ODF sections showing the location of ideal orientations during shear based and plane-strain based deformation processing are shown in Fig. 1, and the corresponding components are listed in Table 1. 2.1.1 FCC materials

    The ideal orientations formed under simple shear deformation in an FCC crystal has been widely studied and reported.1,2) The texture in ECAP materials is described with respect to the shear plane (SP) and the shear direction (SD). The texture typically consists of fibers, designated as the A fiber with {111} ¬ SP and the B fiber with ©110ª ¬ SD. The A fiber contains the A, �A, A�1, A

    � 2 texture components while

    the B fiber contains the A, �A, B, �B and C components. Most of the texture components lie along these fibers. Since the relative intensities of these components are dependent on the symmetry of the test being carried out, in the case of simple shear deformation, which exhibits a two-fold symmetry around the axis perpendicular to both shear plane normal (SPN) and shear direction (SD), the A/ �A, and B/ �B components have the same intensities. A�1 and A

    � 2

    components exhibit different intensities as they are not symmetric to the two fold symmetry operation of the SPN and SD. C components are self-symmetric. 2.1.2 BCC materials

    The ideal orientations formed during simple shear deformation of BCC materials were first calculated by Montheillet and Jonas.1) During ECAP of BCC materials,

    Materials Transactions, Vol. 60, No. 8 (2019) pp. 1457 to 1471 Special Issue on Severe Plastic Deformation for Nanomaterials with Advanced Functionality ©2019 The Japan Institute of Metals and Materials REVIEW

    https://doi.org/10.2320/matertrans.MF201933

  • the most commonly observed fibers are {110} ¬ SP containing the ideal components F, J, �J , E and �E, and ©111ª ¬ SD containing the ideal components D1, D2, E and

    �E.2) Additionally two more ideal orientations were also reported which appeared with the activation of slip on the {123} family of planes.3)

    2.1.3 HCP materials The ideal orientations formed in HCP materials with near

    ideal c/a ratio are fibers B with {0001} ¬ SP (basal fiber) and P ©11.0ª ¬ SD (prismatic fiber), Y, C1 and C2 fibers. However, the stability of the orientation is purely dependent on the c/a ratio of the metal which dictates the relative activation of the slip/twin families. Texture after simple shear deformation is represented in terms of shear direction SD, normal direction (ND) and shear plane normal (SPN) in pole figures (PFs) and inverse pole figures (IPFs).

    A point to be mentioned here is that initially the two leading groups, led by Tóth and Tomé have represented ECAP texture components with additional suffix ‘E’ and ‘ª’ added to the shear texture components, respectively. In later years, it was mutually decided to discard these additional “suffix” while representing the ECAP textures in order to bring uniformity of representation.

    3. Evolution of Texture during Shear Based SPD Processes

    3.1 Equal channel angular pressing (ECAP) Equal channel angular pressing (ECAP) is one of the

    extensively studied SPD processing techniques. In ECAP, specimens with square or a circular cross section are passed through two dies of equal cross-sectional area inclined at an angle, known as inter-channel angle typically ranging from 90°­150°. During consecutive ECAP passes, the sample is re-inserted multiple times with or without a rotation along the longitudinal axis of the billet in-between the passes. The rotation given between consecutive passes along the billet longitudinal axis are 0° in Route A, alternating between «90° in Route BA, 90° in the same direction in Route BC and 180° in Route C. The deformation and velocity gradient for ECAP processing by the routes mentioned above can be found in an extensive review by Beyerlein and Tóth.4)

    Owing to the complexity of deformation path in ECAP, texture varies significantly depending upon crystal structure, stacking fault energies and initial texture. However, in all the processes, textures are unique to the crystal structure. Although ECAP textures have been primarily described as shear textures, the shear plane is identified as the intersection plane of the inlet and exit channels.5) This causes the ideal texture components to rotate about the transverse direction (TD) by an angle of 45° for an ECAP die with inter-channel angle 90°. Further shift occurs along the TD (i.e. ¤1) with an increase in the die angle. The die angle has a great influence in the texture formed after each pass. Smaller the die angle, higher is the strain imposed per pass. As a result, rotation of grains to ideal orientations will be faster and the texture should appear stronger.

    Besides the die angle, other parameters that affect the texture are the crosshead speed of the punch, friction between the channel and sample, and the application of a back pressure. It has been concluded from flow line analysis that the material flow from top to bottom of the channel is not

    Fig. 1 Schematics of SPD processing techniques along with ODF sections the showing position of ideal orientations.33,59,83,117)

    S. Suwas and S. Mondal1458

  • identical. The material closer to the top surface of the channel undergoes deformation more severely than the one at the bottom.6) Ther