Materials Science Forum Vols. 667-669

Abstract: The composite filament structure was produced in Cu-5.7%Cr as-cast alloy ingots, subjected to equal channel angular pressing (ECAP) and cold rolling (CR) at room temperature. Microstructure, tensile properties and electrical conductivity before and after the severe plastic deformation (SPD) processing have been investigated here. The results point out that the rotation and spreading of Cr particles took place during ECAP and the additional rolling resulting in long thin in situ filaments. The average grain size of a Cu phase is equal to about 200 nm after eight ECAP passes. The formation of finer equiaxed grains of the Cu phase has been revealed after the additional CR. The tensile strength 403 MPa and 507 MPa have been achieved after one and eight ECAP passes respectively and increased up to 517 MPa and 607 MPa after the subsequent CR deformation. The enhancement of the tensile strength and the deterioration of the electrical conductivity have been explained by the microstructure evolution of Cu matrix and the dendritic Cr phase.

Abstract: Experiments were conducted on AZ80 magnesium alloy by using a procedure of back pressure equal-channel angular pressing (BP-ECAP) in order to achieve submicron grain size. Microstructure was effectively refined by BP-ECAP. The gain size was found around 100~500 nm after 4 passes using both route A and route Bc at pressing temperatures of 200°C and 150 °C. The grain size was much finer in comparison with the same alloy but received conventional procedure of ECAP without back pressure, which maintains around 2~3 μm after 8 passes at relatively high temperatures. Compression test results showed the yield strength increased with increasing applied pass. In addition, the samples processed using route A had a increasing of yield strength more obvious than that in samples processed using route Bc. The highest yield strength from the sample pressed by route A at 150 °C was more than twice of the yield strength in the solution-treated and forged condition.

Abstract: Deformation behavior and structural changes were studied in a 304-type austenitic stainless steel subjected to large strain multiple forging at an ambient temperature. The number of forging passes was 10, leading to the total cumulative strain of 4.0. The yield stress rapidly increased to about 1000 MPa after the first forging pass and then gradually approached a saturation level of about 2000 MPa in large strains. The grain/subgrain size decreased to about 50 nm at total strain of about 2. This grain/subgrain size reduced a little upon further processing; and comprised 35 nm after a total strain of 4.0. The fast kinetics for grain refinement was associated with deformation twinning and strain-induced martensitic transformation. The both of them resulted in fast grain subdivision at relatively small strains.

Abstract: Severe plastic deformation (SPD) was applied to pure titanium powder by mechanical milling (MM) process with stearic acid, added as a process control agent (PCA), by using a vibrational ball mill, and MMed powders possessing large strains were subsequently consolidated into bulk materials by spark plasma sintering (SPS) in order to enhance the hardness and strength of pure titanium. Changes in the hardness and constituent phases of the MMed powders have been examined by microhardness measurements and X-ray diffraction. The hardness and microstructure of the SPS materials have also been studied by hardness measurements and optical microscopy. The Vickers microhardness of the pure titanium powders with PCA 0.25 g increased sharply from 189 HV to 513 HV after 8 h of the mechanical milling (MM) process. Formation of TiH2 as the solid-state reaction product occurred in the MMed powder during 4 and 8 h of the MM process. The near full density was obtained for the SPS materials under the condition of an applied pressure at 49 MPa with a sintering temperature at 1073 K for 0.5 h. The Vickers hardness of the SPS material fabricated from 8 h MMed powder with PCA 0.50 g exhibited a maximum value of 1253 HV.

Abstract: The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.

Abstract: In the present research, the microstructural features of ultrafine grained Cu-30 Zn alloy via ARB at room temperature were investigated by X-ray diffraction peak profile analysis. The character of dislocations was determined by analyzing the dislocation contrast factors. The average contrast factors for the different reflections obtained by determination of the type of dislocations and Burgers vectors in crystals. Also, using the modified Williamson–Hall and Warren–Averbach procedure size parameters, the effective outer cut-off radius and density of dislocations were determined. Assuming that the grain size distribution is log-normal, the median and the variance of the size distribution of sub grains were obtained. It was found that the crystallite size is reduced substantially, while the dislocation density increases up to 2 cycles of ARB. After 2nd cycle, dislocation density decreases. This is attributed to the occurrence of dynamic restoration process which takes place during next ARB cycles.

Abstract: The aim of the present work is to analyze the evolution of texture after ECAP and post deformation heat treatment of an AA1050 alloy produced by roll casting. The initial plate exhibited a weak cube texture at the surface, decreasing in its intensity toward the sheet center, where the deformation texture consisted of Brass and Copper components. ECAP-deformation employed one to four passes at room temperature, following route A in a = 120o die. Texture evaluation was performed by x-ray analysis and by EBSD, using transverse cross section scans. Only the central part of the plate was studied. Results were analyzed by regular texture and orientation distribution function calculations. After 1, 2 and 4 passes the texture changed to a strong {111}// TD and after recrystallization at 350oC for 1 h this main orientation was maintained.

Abstract: In the present research, a combined forward extrusion-equal channel angular pressing (FE-ECAP) was developed and used for production of bulk ultrafine grained steel in the high temperature conditions. In this method, two different deformation steps including forward extrusion and equal channel angular pressing takes place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930, and 1100 °C). In addition, 3D finite element simulation was used to predict the hot/warm deformation parameters such as strain and temperature variations within the samples during deformation. The results show that the EF-ECAP process is effective in refining the grains from initial size of 32 m to final size of 0.9 m after executing of extrusion and ECAP on as received samples. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain-induced transformation, and continuous dynamic recrystallization.

Abstract: SUS 304 austenitic stainless steel (ASS) was deformed by high pressure torsion (HPT) to obtain 100% volume fraction of martensite (α') from a fully austenitic (γ) matrix. Deformation caused an increase in hardness (Hv) from 1.6 GPa in the as annealed state to 6.4 GPa　after HPT. Deformed samples were then annealed in the range 200 – 600oC and peak hardness of 7.8 GPa was observed after annealing at 400oC for 1 hour. Differential scanning calorimetry (DSC) and electrical resistivity tests showed that the deformed alloy undergoes a two stage phase transformation on heating from room temperature up to 700oC. The first stage of transformation was associated with hardening behavior while the second one which is reverse α' → γ transformation resulted in a reduction in hardness. Annealing at 400oC after deformation was found to increase the magnetization saturation (Msat) values.

Abstract: Equal channel angular pressing (ECAP) has been conducted on as-extruded Mg-Zn-Y alloy containing quasicrystal phase at a temperature of 523 K. The optical images indicate that after 8 ECAP passes through route BA, the grain size of the extruded alloy is decreased sharply; and the coarse eutectic icosahedral quasicrystal phases (I- phases) are broken and dispersed in the alloy; and the distributions of Zn and Y elements become more homogeneous. These can be attributed to the shear effect during the ECAP processing. TEM micrographs show the grain refinement, the evolution of broken and dispersed I- phases and dispersion precipitation of nano I-phases during 1- ,4- and 8- pass ECAPed Mg-Zn-Y alloy. And the mechanism of grain refinement is also discussed.