Physics /2. Solid state physics

 

 

Ò.À. Lychagina1, D.I. Nikolayev1, H.-G. Brokmeier2

1Joint Institute for Nuclear Research, Dubna, Russia

2GKSS Forschungszentrum, Geesthacht, Germany

Texture investigation of magnesium alloy MA2-1 after equal channel angular pressing

 

Magnesium alloys find wide application in various branches of mechanical engineering,  first of all  in aerospace, automobile and electronic industries. Ductility of semi-finished products from these alloys is not rather high [1] especially at deformation temperatures below 250°C in spite of the high strength. It is connected to features of the structure and texture formation during plastic deformation of these alloys. Situation can considerably be improved if to find ways of magnesium products deformation allowing generating the equiaxed fine-grained structure and diffuse or tilted basal texture in these materials. One of the promising ways to solve the problem is application of the severe plastic deformations with help of equal channel angular pressing (ECAP) [2-3]. The ECAP process is characterized by development of deformation textures. Moreover, plastic response in the samples after ECAP is highly anisotropic and some directions actually exhibit significantly lower ductility than the materials undergone by conventional treatment (extrusion) [4]. So, plastic deformation of magnesium alloys strongly affected by crystallographic texture [5].

Crystallographic texture is defined as the preferred orientations of crystallites of a polycrystalline sample with respect to a sample orientation system. In this work the texture evolution of  ECAPed samples from the alloy MA2-1 (Mg-4.5% Al-1% Zn) has been investigated by means of neutron diffraction. The texture measurements have been carried out at TEX-2 instrument situated at reactor FRG-1(GKSS, Germany). The deformation experiments were done following the three important routes of  ECAP, namely A, Bc and C up to six passes in a 90° die. Texture evolution with double initial fibers was analyzed and related to the processing routes.

 

Experimental

 

     We investigated the texture formed after ECAP for Mg-4.5%Al-1%Zn (MA2-1) alloy. The texture of the eight samples has been investigated by measuring the (002), (100), (101), (110) pole figures using the TEX-2 neutron diffractometer at the FRG-1 neutron source.  Neutron diffraction is one of the powerful techniques of texture measurements. The main advantage of thermal neutrons is their high penetration power which allows to measure global textures over the whole cross-section of the ECAP samples [6]. Due to high penetration depth of neutrons, the spherical sample method could be used, so that it is possible to obtain complete pole figures, and for most materials data correction could be neglected. The information about texture of material is extracted from the measured pole figures. The pole figures undergo the experimental error. So it is very important to study sources of the pole figure measurement errors to look after their minimization. The pole figure measurement errors have been studied on the example of MA2-1 alloy in the following works [7-8]. 

The bar extruded with backpressure was input for ECAP. ECAP combining with backpressure enables considerably improve strength as well as ductility of  material. The ECAP scheme is presented in Fig. 1.

 

Fig.1.  The ECAP scheme.

The ECAP has been carried out by the following routes.

The pressing in the route A is repeated  without sample rotation, i.e. a sample orientation is not changed after each pass. The pressing for one pass has been done with strain  at 260ºC. The pressing for two passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second pass. The pressing for four passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second and third passes, 220ºC in the fourth pass.

The pressing in the route C is repeated  with rotation a sample about  its pressing direction by 180º after each pass. The pressing for two passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second pass.  The pressing for four passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second and third passes, 220ºC in the fourth pass.

The pressing in the route Bc is repeated with sample rotation by 90º about its axis and simultaneous rotation by 180º about pressing direction. The pressing for four passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second and third passes, 220ºC in the fourth pass. Third and foutrh passes have been done after sample rotation by 90º about pressing direction. The pressing for six passes resulted in the strain  at 260ºC in the first pass and 240ºC in the second and third passes, 220ºC in the fourth and fifth passes, 200ºC in the sixth pass. Third, foutrh, fifth and sixth passes have been done after sample rotation by 90º about pressing direction.

The ECAP procedure was carried out using a specially designed die with an angle 90º between two channels of square cross section (20×20mm) without any rounding of the corners. In each route the sample  have to be turned by 90º around its transverse axis in order to set the deformed sample back into the die. The samples size was 10×10×10mm. The all samples have  been  measured so that the extrusion direction E_D (the direction of ECAP) is in the center of the pole figures. After ECAP the samples have been annealed at 345ºC during one hour in the air. The ECAP resulted in the ultrafine-grained structure in the investigeted alloy with the mean grain size 2,0-2,4 µm.

 

 Results and discussion

  The experimental PFs measured for routes A, C and Bc are given in Fig. 2-4. The first rows present the PFs for the input extruded sample on each figure. In the second row it can be seen the PFs for sample after one ECAP pass. So the PFs in the second row (after one ECAP pass) are the same for the Fig. 2-3 (routes A and C). The second row in Fig.4 presents PFs after two passes in the route C.

Fig. 2. The experimental pole figures for the input sample and the samples in the route A of ECAP. All samples are from alloy MA2-1. The extrusion direction (ED) is in the center for all PFs. The position of the sample coordinate system (extrusion direction, transverse direction, normal direction) is the same for all samples.

This corresponds to the first step of  ECAP procedure for the route Bc in our case. Also the PFs in the third row (Fig.4) describe the next step of the ECAP procedure, i.e. PFs after four passes in the route Bc . Here two passes are carried out in route C and two passes in route Bc, i.e. with rotation after second and third passes about longitudinal axis of the sample by 90º clockwise.  

 

Fig. 3. The experimental pole figures for the input sample and the samples in the route C of ECAP. All samples are from alloy MA2-1. The extrusion direction (ED) is in the center for all PFs.

 

The initial texture (for input sample) is characterized by two strong axial components (basal and prismatic). The intensity of the basal component is about 10 mrd, and the intensity of the prismatic component is about 18 mrd. The pole density is given in the units of random distribution. It should be underlined that for common magnesium (AZ31, AZ61, MA2-1) alloys samples after extrusion only one prismatic texture component is typical. The reason for the second component is backpressure combining with extrusion. After the first pass we observe the moving of the basal component (the intensity maximum) on the angle about 45° respectively to extrusion direction. After the second pass in the route A this component moved on the angle about 90° respectively to extrusion direction. After the fourth pass in the route A the intensity of this component increased from the 6.06mrd (after the second pass) up to 8.97 mrd.

 

Fig. 4. The experimental pole figures for the input sample and the samples in the route Bc of ECAP. All samples are from alloy MA2-1. The extrusion direction (ED) is in the center for the PFs.

 

The moving of the texture components during the route C is rather close to one in the route A. The difference is velocity of intensity redistribution. So during the route C the maximum of basal texture component on the angle about 45° respectively to extrusion direction is more stable than in the route A. It presents on the PFs for the samples after second and fourth passes as well in the route C. The maximum of this texture component  increased from 6.35mrd after the second pass up to 8.42mrd after the fourth pass in the route C. As for the route Bc  we also observe the same component moving after the fourth pass. The maximum of this component  increased from 8.61mrd after the fourth pass up to 11.9 mrd after the sixth pass. 

The ECAP drastically changes the initial axial texture characterized by sharp basal and prismatic components by splitting them into several more scattered orientations. New texture consist of the basal component inclined to the extrusion direction on 45-55˚ with high degree of scattering and prismatic component. The degree of the orientation scattering depends on the ECAP regime and route. The ECAP route Bc  after four passes results in the strongest scattering of the texture maximum.  Such texture type favors for low temperature ductility of magnesium alloys.

Samples were produced by V.N. Serebryany  within  the  frame of  the Russian State contract ¹ 02.513.11.3340.

 

Summary

The plastic anisotropy can be rationalized in terms of the strong crystallographic texture induced by the ECAP.  The ECAP as a kind of severe plastic deformation results in the essential texture changing in magnesium alloy MA2-1. The quantitative study of texture influence on the plastic properties of these alloys after different ECAP routes and annealing can be done by using orientation distribution function. This analysis will allow finding the optimal deformation regimes provided increasing of low temperature ductility.

 

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