THERMEC 2006

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Authors: Buddhisagar Naik
Abstract: Different models have been put forward to explain superplasticity. Most of the models predict the independency of activation energy (Q) on stress. Superplasticity is observed in region II of creep curve (logε Vs σ/E). The most commonly considered mechanism for superplastic flow involves Grain Boundary Sliding (GBS), and it is necessary for an accommodation process to accompany GBS. The accommodation process might be grain boundary migration, recrystalisation, diffusional flow or some dislocation slip process. But the Arrhenius type of equation given by Becker gives the dependency of activation energy on stress. Here in this work we have considered this equation and relation between Q and σ is found out using genetic algorithm. The present model development studies the parameter optimization, where parameters appearing in the stress and energy relationship equation e.g. relationship between Q(σ) and σ for the Q(σ) equation given in present work as well as pre-exponential factors are optimized with the objective function being the error minimization of model predicted values and experimental data of strain are available from open literature.
2865
Authors: Young Hoon Chung, Jong Woo Park, Kyong Hwan Lee
Abstract: As the surface friction between feeding rolls and metal sheet generates the feeding power of ECAR, the generated feeding power is low, and the friction between the metal sheet and ECAR die should be minimized. However, for obtaining a large shear deformation by ECAR, the metal sheet should be tightly contacted with the wall of ECAR die. In this condition, the thickness of the metal sheet is continuously increased during ECAR. A new ECAR apparatus is developed for maximizing the shear deformation and obtaining sheet thickness uniformity, and succeeding continuous ECAR with such a limited feeding power. By controlling the outlet gap of the ECAR die with elastic unit, the thickness of the metal sheet is kept uniform. Detailed thickness control mechanism during the new ECAR process is analyzed. A sheet of Al 6063 alloy that is 1-pass deformed with the new ECAR apparatus shows below ±0.037 mm of thickness variation and 0.61 of shear strain.
2872
Authors: Paolo Matteazzi
Abstract: Assembling Nanostructures in 3D objects is actually the most relevant challenge in nanomanufacturing, opening the route to full industrial impact of nanomaterials. Titanium based systems are of great interest in several applications due to combination of strength, density, corrosion resistance and biocompatibility. Nanostructured Titanium alloys can be synthesized by high energy milling and assembled in 3D products by different routes.
2878
Authors: Zuo Gui Zhang, Eiji Akiyama, Yoshimi Watanabe, Yasuyuki Katada, Kaneaki Tsuzaki
Abstract: In this study, an Al-7 wt% Si-1.5 wt% Cu alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. The ECAP process was repetitively carried out up to 8 passes using a strain introduction method of route BC, at a temperature of 25 °C and a pressing rate of 0.33 mm s-1. Microstructures of the samples before and after ECAP were observed by a scanning electron microscopy (SEM). Electrochemical properties of the Al-Si-Cu alloy fabricated by ECAP have been investigated in a borate-boric acid buffer solution containing Cl¯ ions at pH 8.3 and 25 °C by potentiodynamic polarization test. Corrosion pits on the sample surface after anodic polarization were investigated by means of SEM. The anodic polarization showed that as-cast Al-Si-Cu alloy with plate-shaped Si particles has poor resistance against pitting corrosion comparing to quenched sample without ECAP. Pitting potentials of ECAPed Al-Si-Cu alloy samples were higher than that of the sample without ECAP. In the Al-Si-Cu alloy, the corrosion pits were found in the region of Si particles and the size of pits formed on the ECAPed samples became smaller than that without ECAP. It is considered that the improvement of the pitting resistance of ECAPed Al-Si-Cu alloy is due to homogenous distribution of spherical Si particles generated during ECAP process.
2892
Authors: S.M. Lim, Christophe Desrayaud, S. Girard, Frank Montheillet
Abstract: Thermomechanical processing involving severe plastic deformation (SPD) is a popular approach to ultrafine grain formation in bulk samples. In the present study, two grades of highpurity α-iron were deformed within the ferritic domain in cold and warm torsion to large strains (>> 1). Examination of the deformed samples using orientation imaging microscopy revealed a highly fragmented, lamellar structure aligned almost parallel to the direction of shear. Between 37 and 54 % of boundaries detected are high angle ones (HAB). Some of these HAB are associated with the original grain boundaries. However, a good number are believed to originate from dislocation accumulation processes, during which the misorientation angle across certain strain-induced low angle boundaries (LAB) rises with increasing strain. The resultant microstructure is composed of ultrafine crystallites on the order of 1 – 2 1m. In fact, localised regions of equiaxed grains on the micron scale were observed within samples deformed between room temperature and 300 °C. Nonetheless, other areas remain relatively unfragmented despite persistent straining until failure. At higher temperatures, the microstructure is more homogeneous, but the average grain size is coarsened. Optimal grain refinement thus appears to be a compromise between several competing factors: large strains at relatively low temperatures for high dislocation density, higher temperatures to enable sufficient dynamic recovery, and low grain boundary mobility that is aided by low temperatures and/or pinning by solute atoms or second phase particles. Furthermore, the development of a torsion texture composed of a single ideal orientation at large strains is unfavourable towards the generation of HAB.
2898
Authors: Vàclav Sklenička, Jiří Dvořák, Marie Kvapilová, Milan Svoboda, Petr Král, Ivan Saxl, Z. Horita
Abstract: This paper examines the effect of equal-channel angular pressing (ECAP) on creep behaviour of pure aluminium, binary Al-0.2wt.%Sc alloy and ternary Al-3wt.%Mg-0.2wt.%Sc alloy. The ECAP was conducted at room temperature with a die that had a 90° angle between the channels and 8 repetitive ECAP passes followed route BC. Constant stress compression creep tests were performed at 473 K and stresses ranging between 16 to 80 MPa on ECAP materials and, for comparison purposes, on the initial coarse-grained materials. The results showed that the creep resistance of the ECAP processed Al-Sc and Al-Mg-Sc alloys was markedly deteriorated with respect to unpressed coarse-grained materials.
2904
Authors: Wayne Chen, David Ferguson, Gui Yan Li, Lie Ge Ao, Chun Ming Wang, Jian Zhang, Hui Xia Ma
Abstract: A series of severe strain compression tests of a pipe line steel were conducted using the MAXStrain® system. The tests were run at different thermal mechanical conditions including controlled cooling rates. 1-2 micron ultrafine grain microstructure was achieved. The characteristics of the ultrafine grain structure and precipitates were examined using optical microscope, SEM, and TEM, and the relationship between the processing conditions and microstructure and precipitates were studied.
2910
Authors: Yoshimi Watanabe, Zuo Gui Zhang, Hisashi Sato, Tomonari Inamura, Hideki Hosoda
Abstract: In this study, an Al-5.7wt% Ni eutectic alloy was subjected to severe plastic deformation (SPD) by an equal-channel angular pressing (ECAP) technique. ECAP technique was carried out by using two strain introduction methods, route BC and route A, at a temperature. It is found that the Al-Ni eutectic alloy specimens after ECAP technique by route BC and route A methods have very different microstructures which affected strongly the tensile properties of the specimens. Namely, after ECAP technique by route BC method, the fine Al3Ni dispersoids of about 300 nm homogeneously dispersed in α-aluminum matrix and the samples appeared no clear anisotropic in tensile properties, while the eutectic textures containing α-Al and Al3Ni fibrous dispersoids have highly anisotropy distribution and are proved to have significant anisotropy tensile properties after ECAP technique by route A method. Results of Young’s modulus and simultaneous internal friction measured by dynamic mechanical analysis (DMA) were also presented. Based on the experimental results, the fracture mechanism during tensile process of the Al-Ni eutectic alloy by different strain induction methods was discussed.
2916
Authors: Alexandre Goloborodko, Taku Sakai, Oleg Sitdikov, Hiromi Miura
Abstract: Microstructural changes taking place in an as-cast coarse-grained 7475 Al alloy was studied by using multidirectional forging (MDF) at a temperature of 250oC and at a strain rate of 3 × 10-4 s-1. The samples were deformed by MDF with a strain of 0.7 per pass up to cumulative strain (Σε) of 8.4. In the earlier stages of deformation, microstructural changes are mainly characterized by development of dislocation subboundaries with low-to-moderate misorientation angles. The misorientation angle initially increases with straining and reaches a plateau of around 3.7o in the strain range from 0.7 to 2.1, where new grain formation scarcely takes place in the original grain interiors. With further straining, grain fragmentation starts to occur accompanying with deformation bands developed at various directions, followed by rapid evolution of a new fine grain structure at large strain. The average grain size is around 1 μm at large strains and the average misorientation angle approaches a value of about 25o at Σε = 8.4.
2922

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