MICROSTRUCTURES AND MECHANICAL PROPERTIES OF ULTRAFINE GRAINED AlMgSi ALLOY PROCESSED BY ECAP AND IT S THERMAL STABILITY.

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1 MICROSTRUCTURES AND MECHANICAL PROPERTIES OF ULTRAFINE GRAINED AlMgSi ALLOY PROCESSED BY ECAP AND IT S THERMAL STABILITY. Kovářík Tomáš a Zrník Jozef b a ZČU, Univerzitní 22, Plzeň, ČR, kt3@seznam.cz b COMTES FHT s.r.o., Průmyslová 995, Dobřany, ČR, comtes@comtesfht.cz Abstract It has been shown that ultrafine grained structure can be introduced into metals via severe plastic deformation (SPD). Equal Channel Angular Pressing (ECAP) is a very popular SPD technique recently used to refine metal and alloy structure in great extent. After processing by ECAP at ambient temperature, the microstructures are quite complex and represent a mixture of segments with misorientation angles ranging from low to high angles (<15 ). Comparatively little is known about the thermal stability of these severely deformed structures. In the present work the microstructure and mechanical properties of Equal Channel Angular Pressing (ECAP) processed AlMgSi1 alloy (route Bc), as well as their evolution during post annealing were investigated. The three different initial structural states of alloy, as a result of alloy preliminary treatment, were prepared for ECAP pressing. The work hardening evolution in dependence on the number of die passes was evaluated by hardness (HV10) measurements and structure analysis. Two different temperatures of 270 C and 350 C and different time holds have been chosen in order to evaluate the annealing effect on structure stability and mechanical properties changes in dependence on the strain introduced. The effect of annealing was monitored by structural changes and hardness measurements of annealed specimens. Upon annealing, the hardness of the UFG samples decreased gradually with hold time for both applied annealing temperatures. Any additional hardening peak after annealing was not detected in the alloy. The results demonstrate, regardless of the number of passes applied, that ECAP enhanced for both annealing temperatures the driving force for the recovery and recrystallization of alloy. As regards the effect initial structure prior to ECAP there was not observed substantial difference in recrystallization behavior of AlMgSi1 alloy. 1. INTRODUCTION Although the mechanical and physical properties of all crystalline materials are determined by several factors, the average grain size of the material generally plays a very important and often a dominant role. The strength of all polycrystalline materials is related to the grain size (d). According to the Hall Petch equation which states that the yield stress (σ y ) is given by σ y = σ 0 + k y d -1/2 (1) σ 0 = friction stress, k y = constant of yielding. It follows from equation that the strength increases with a reduction in the grain size and this leads to an ever-rising interest in fabricating materials with extremely small grain sizes [1, 2]. The grain sizes of commercial alloys are generally tailored for specific applications by making use of pre-determined thermomechanical treatments. The alloys are 1

2 subjected to specified regimes of temperature. However, these procedures cannot be used to produce materials with submicrometer grain sizes because there is always a lower limit of the order of a few micrometers, which represents the minimum grain size easily achieved using these procedures. Therefore attention has been directed to the development of new and different techniques that may be used to fabricate ultrafine-grained materials with grain sizes in the submicrometer and the nanometer range. Ultrafine-grained (UFG) materials processed by severe plastic deformation (SPD) are defined as polycrystals having very small grains with average grain sizes less than ~ 1 µm (including nanocrystalline materials having a grain size of typically <100 nm). For bulk UFG materials, there are the additional requirements of fairly homogeneous and reasonably equiaxed microstructures and with a majority of grain boundaries having high angles of misorientation. The presence of a high fraction of high-angle grain boundaries is important in order to achieve advanced and unique properties. For example an increasing of strength, low temperature and high strain rate superplasticity [3]. Several methods available for producing ultrafine-grained structure in bulk materials are based mostly (as mentioned above) on severely large plastic deformation. One of them is equal channel angular pressing (ECAP), the most promising and interesting method. ECAP process is especially attractive because it can economically produce bulk UFG materials that are 100% dense, contamination free and large enough for real structural applications. It is a relatively simple procedure that is easily effective on a wide range of alloys. The processing by ECAP uses equipment that is readily available in most laboratories. ECAP may be developed and applied to materials with different crystal structures and to many materials ranging from precipitation-hardened alloys to intermetallics and metal matrix composites [4]. 2. EXPERIMENTAL PROCEDURE The material used in this study is an AlMgSi1 alloy. The chemical composition (according to the norm DIN ) is noted in table 1. Chemical composition of the experimental alloy was confirmed by the GD-OES analysis and is presented in table 2. Element Si Fe Cu Mn Mg Zn Ti Al In mass [%] 0.70 až až rest Table 1. The chemical composition of material according to the norm DIN Element Si Fe Cu Mn Mg Zn Ti Al In mass [%] rest Table 2. The natural chemical composition of experimental material Bars with dimension of mm for ECAP were cut from the center part of continual cast rods with a diameter of 20 mm. The microstructure is presented by Fig. 1, 2. The heat treatments were applied to rods in two modes and the first mode (S1) was only in initial conditions with mechanical fabrication of forward pressing. In the second mode (S2) the rod was solution-annealed at 540 C for 1.5 h followed by water quenching. In the third mode (S3) it was quenched to room temperature (with the same parameters) and followed by temper hardening at 160 C for 12 h. ECAP was carried out using a pressing die which has 2

3 two channels having the equal square cross-section with a dimension of 8 8 mm and intersect at an angle of 90. The numbers of passes for each mode were set to 1 4. The last heat treatment (after ECAP) has been applied to set of all specimens ( 3 modes, 1 4 passes,). Annealing has been used in range of temperatures at 270 C and 350 C, for 0.5 h, 2 h, 4 h, 6 h with a view to find out the thermal stability of particular specimens. Fig. 2. Image of the cast rod in initial conditions (center); Polarized-light effect Fig. 1. Image of the cast rod in initial conditions (side area); 3. EXPERIMENTAL RESULT AND DISCUSSION 3.1 Microstructure analysis, First mode, 4 passes. Influence of deformation and its character of homogeneity in capacity of forming material is displayed on kinetics and extent of re-crystallization process. Analyses of microstructure by optical microscope demonstrate it. In this mode is the noticeable effect of ECAP process well-noticeable. The pattern of the billet produced by severe deformation of 4 passes creates a homogeneity structure with markedly extend grains. It is evident in all cross sections of the billet and show in the Fig. 3 (which is snapped from the center of the billet). 3.2 Hardness, 3 modes, 1 4 passes Annealing has been used in temperature at 270 C and 350 C for 0.5 h, 2 h, 4 h, 6 h. with a view to find out the thermal stability of the billets under examination. Fig. 5, 6 presenting hardness depending on number of passes and heat treatment. The highest grow of strengthening (hardness) was achieved in all three modes after one pass. Fig. 3. After 4 passes; Polarized-light effect Fig. 4. Mode S1 4P 270 C / 0.5 h; Polarized-light effect 3

4 The hardness increace was lesser after application other deformation (2 4 passes) and the smallest increace was observed after fourth pass. Relations of hardness and annealing time in temperature 270 C confirm significant decrease hardness values after shortest annealing time (0.5 h) for all modes. Figure 4. represents mode S1 after 4 passes and annealing at 270 C for 0.5 h. The steadiest behavior was observed in mode S1 (initial conditions: cast and ECAP). The mildest decrease of hardness values with increases number of passes was monitored in this mode. The strongest (unexpected) softening was observed in mode S3. It was expected that thermal stability should be higher with a view to heat treatment before deformation. The increasing of annealing time (over 2 hours) didn t have any effect on degradation and properties of the structure. Fig. 5. Hardness depending on the number of passes 4

5 Fig. 6. Hardness depending on the annealing time 4. CONCLUSIONS The analyses of microstructure by optical microscope, in the meantime, demonstrated that nucleation of recrystallization process and homogeneity of recrystallized structure directly relating to level of deformation and homogeneity transformed structure. REFERENCES [1] HALL, E. O. Proc Roy Soc B, 64, 1951 [2] PETCH, N. J. J Iron Steel Inst, 174, 1953 [3] VALIEV, R. Z. Nature Mater, 3, 2004 [4] VALIEV, R. Z., LANGDON, T. G. Science and Direct, 51,