SUMMARY

In the present work, grain refinement taking place in coarse grained aluminum alloys Al-3%Cu and AA 2219 was studied during equal channel angular pressing in the temperature interval from 523 to 748 K. The effect of strain and increasing the deformation temperature on microstructural changes and deformation-induced grain formation in the both of alloys was examined in detail.

The main results obtained in each chapter are summarized here.

Severe plastic deformation (SPD) has been used with intent to produce ultrafine grain structure with size ranging from submicro- to nanoscale resulting in improvements of some physical and mechanical properties of metallic materials. The relationship between ultrafine grained structure and the nature of its unique proprieties is a central theme in scientific research works and became more important recently. Accordingly, the question of fine grained structure evolution is of great interest for researchers working in the field of solid state physics and material science. Structural changes which occur during and after deformation need to be clarified. Thus, the main subject of present work is how the fine grained microstructure develops with strain and how the characteristics of the boundaries increase with strain. The aim of present study was to examine the microstructural developments with strain taking place in Al-Cu based alloys during equal channel angular pressing (ECAP) in wide temperature interval. The main tasks were summarized in Chapter 1 as following:

1. To investigate the microstructure evolution in a model dilute Al-3%Cu alloy during ECAP as a basic study.

2. To study the effect of increasing deformation temperature on grain refinement taking place in Al-3%Cu alloy and to reveal main features of deformed microstructures.

3. To investigate the microstructural changes taking place in a commercial base Al alloy 2219 during ECAP .

4. To examine the effect of rising temperature on fine grain development during ECAP in AA 2219 alloy and to clarify main factors affecting grain refinement under hot ECAP conditions.

In Chapter 2, the materials used in the present study and the deformation conditions were described. A dilute aluminum-copper alloy, denoted here as Al-3%Cu, in mass percent, was produced by direct chill casting and subjected to a homogenization at 793 K for 4 hours followed by slowly cooling in a furnace. A commercial based aluminum alloy 2219, denoted here as AA 2219 has a following chemical composition Al − 6.4%Cu − 0.3%Mn − 0.18%Cr − 0.19%Zr- 0.06%Fe − 0.01% Si, Ti. The alloy was manufactured by semicontinuous chill casting followed by homogenization at 803 K for 6 hours and subsequently aging at 683 K for 8 hours. The deformation by ECAP was conducted on the rod-type samples via route A, i.e. without any

rotation of sample in the die at a temperatures ranging from 523 to 748 K up to total strain of 12 for both the alloys.

In Chapter 3, the microstructural evolution of coarse grained Al-3%Cu alloy was studied in ECAP up to high strain of 12 mainly at a temperature of 523 K (~0.6Tm). It has been shown that new fine grains are developed by grain subdivision due to formation of microshear bands (MSBs) having moderate angle misorientation. Strain heterogeneity can be introduced during ECAP due to operation of grain boundary sliding with different rate in the grain interiors, resulting in local lattice rotation and frequent formation of deformation bands at low strains. The particles of θ-phase are nonuniformly distributed in grain interiors and may cause the heterogeneity in deformed structure. The number and average misorientation angle of the boundaries increase with repeated ECAP, finally leading to development of a new fine-grained structure with high angle boundaries (HABs) at high strains. It can be concluded, therefore, that ECAP of Al-3%Cu alloy with total strain of about 12 at 523 K provides a partial formation of non-uniform new fine-grained structure with an average grain size ∼6 μm, with the volume fraction of about 50%. The main process of fine grain formation is directly connected with development and evolution of deformation bands during deformation by ECAP. With increasing temperature of ECAP from 523 to 748 K (~0.75Tm), the formation of strain–induced fine grained structure takes place much more heterogeneously in the whole material, that results in dramatically reduction of the volume fraction of deformation-induced grains accompanying with increasing the average size. Increase in the deformation temperature makes the formation of DBs less frequently in early deformation. In addition, the dissolution of θ-phase particles (Al2Cu) taking place with rising temperature of deformation results in decrease the thermal stability of the deformation-induced substructure. These events leads to formation of numerous statically recrystallized coarse grains evolved at the highest temperature of 748 K. It can be concluded, in addition, that deformation by ECAP of dilute Al-3%Cu alloy at lower temperatures (≤ 573 K) is not effective for obtaining grain refinement in this alloy at higher temperatures.

In Chapter 4, the microstructural changes taking place in a coarse grained 2219 aluminum alloy were studied during ECAP to strains of 12 at 523 K. It is shown that the microstructural changes with strain can be subdivided into three stages. ECAP at low strains, in stage 1, results in development of conventional dislocation substructures with misorientation angles less than 5^o which are homogeneously developed in initial grain interiors accompanying with a few embryos of deformation bands with boundaries of low to moderate misorientation angles, i.e. 5^o ≤ Θ ≤ 15^o. These bands are formed roughly parallel to pressing direction (PD) in several local places in grain interiors due to large strain gradient introduced by ECAP deformation. Elongated subgrains are developed in DBs interiors while conventional equiaxed subgrains with LABs of Θ ≤ 5^o are formed in the exteriors of DBs. With repeated ECAP from 2 to 4, in stage 2, such embryos of DBs can transform into large-scale ones due to increasing the boundary misorientations. The density of boundaries of DBs as well as their average misorentation angle rapidly grows with further straining, leading to fragmentation of original coarse grains and development of fine grains. New deformation-induced grains surrounded by HABs are formed firstly in the regions closed to DBs interiors and subsequently the exteriors at medium to high strains. In stage 3, ECAP of 2219 Al alloy, leads to a fully and relatively homogeneous development of fine grained structure with an average grain size of about 1.3 μm in a whole volume.

The misorientation angle distribution for the strain-induced boundaries shows a single peak at relatively low misorientations and changes to a bimodal distribution with two peaks at low and high misorientations at moderate strains. Pressing to a total strain of 12 provides full development of new fine grains which are almost surrounded by HABs. It was shown that formation of fine grained structure is directly connected with formation and development of boundaries of DBs. In the other words, repetitive ECAP leads to a transformation of the boundaries of DBs with moderate in misorientation into HABs and evolution of deformation-inducecd grains. It is concluded that grain refinement occurs by a deformation-induced continuous reactions which is similar to cDRX. ECAP at lower temperature of 523 K (~0.6Tm) results in considerable grain refinement and a new fine grained microstructure develops more homogeneously at a strain of about 12 in a 2219 Al alloy compared to the Al-3%Cu, deformed at

a similar conditions. The average grain size obtained by ECAP was reduced to around 1.3 μm and the volume fraction of new fine grains is dramatically increased to ~90% in AA 2219. It can be concluded, that significant grain refinement takes place in the 2219 Al alloy due to presence of second phase particles and dispersoids, which assiste a rapid increase in the number and the misorientation angle of deformation-induced boundaries with repeated ECAP.

In Chapter 5, the process of microstructural changes and temperature effect on it were investigated at temperatures from 523 K to 748 K (0.6~0.75Tm). The samples of AA 2219 were pressed additionally at 573, 673 and 748 K by using ECAP via route A up to strains of 12. The evolution of strain-induced microstructures at each temperature can be classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by fragmentation of original grains at moderate strains; and (3) full or partial development of fine grained structure at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected by higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. Thus, in the early two stages, i.e. in low-to-moderate strains, deformation-induced dislocation subboundaries are developed accompanying with embryos of DBs followed by its transformation into large scale of DBs. These processes are scarcely affected by deformation temperature and may be considered as mechanically induced events and so athermal ones.

Mixed microstructure developed at high strains in stage 3 contains strain−induced fine grains with HABs and unrefined areas containing subgrains with LABs. It was found that misorientation characteristics of deformation-induced grains are hardly affected by temperature of deformation; however, with increasing temperature, fine grained regions are inhomogeneously distributed in a whole volume and reduced in the fraction. This can be attributed to a relaxation of strain compatibility between grains due to activation of dynamic recovery with higher rate.

However, high fraction of dispersoids phases Al6Mn and mostly Al3(Zr, Cr) formed in the present alloy is effective in dislocation pinning and so retard or prevent relaxation of strain gradient even

at 748 K and serve the formation of DBs which can develop with strain and results in grain fragmentation followed by new fine grain formation with HABs. It is concluded, therefore, that strain−induced grain formation results from dynamic formation of HABs in stages 1 and 2, followed by frequent operation of dynamic recovery in the HABs developed in stage 3.

Based on the experimental results summarized and described above, a model for grain refinement process taking place during ECAP in wide temperature interval (0.6-0.75 Tm) for Al alloys can be proposed and summarized as follows:

1. Process of microstructural changes with strain can be subdivided into the following stages: Stage1, an incubation period for formation of DBs; Stage 2, original grain fragmentation by large-scale DBs followed by formation of deformation induced grains;

Stage3, full development of fine grained structure in a whole material.

2. In stage 1, embryos of deformation bands form inhomogeneously in several local places of original coarse grain interiors and caused by high strain gradient introduced by deformation due to operation of grain boundary sliding. In stage 2, rapid transformation of embryos into large scale DBs occurs. The number and misorientation angle of DBs grows with strain , followed by a full formation of new grains with high angle boundaries in stage 3.

3. Increasing the temperature of deformation activates the process of dynamic recovery and affects the full and homogeneous formation of new fine grains in a whole volume in stage 3, while the formation of DBs and its evolution is hardly affected by temperature of deformation in stages 1 and 2.

ACKNOWLEDGEMENTS

The author would like to express her hearty thanks to the Japanese Governement for providing the Monbusho scholarship and to Professors Rustam Kaybishev and Taku Sakai for giving an opportunity to perform the present work in the Sakai-Miura laboratory at the University of Electro-Communications (The UEC, Tokyo).

Much gratitude is due to her supervisor, Professor Taku Sakai for his strong consultation, stimulation and support during preparation of the thesis.

Author would like to express her hearty thanks to the professors of the UEC H. Miura, Y.

Ochi, M. Murata, T. Matsumura and Dr. Oleg Sitdikov for their advice, discussion and help.

Many thanks to family, especially to husband Pavel Fedoseev for moral support, advices and help during the writing of the thesis.