Materials Science Forum Vols. 667-669

Abstract: Dispersed nanoparticles are introduced from stabilized suspensions during the accumulative roll bonding process in aluminium AA1050A by air gun spraying up to a final volume fraction of 0.1 % after eight cycles. Additional strengthening caused by particle insertion is observed and strongly depends on the suspension medium and stabilizing agent as both influence interfacial bonding of the particles to the matrix. The particle insertion furthermore results in reduced peel strength of the sheets irrespective of particle material and size caused by a reduction of effective metal to metal bonding area during rolling through the presence of the particles.

Abstract: This work presents the possibility of strength enhancement in the -alloy Ti-6.8Mo-4.5Fe-1.5Al via equal channel angular pressing at room temperature without precipitation of a secondary -phase. Influence of the initial structure of the alloy and ageing temperature on the density of precipitations and dimensions of the secondary -phase is revealed. It has been stated that combination of the solid solution treatment with the subsequent ECAP processing and final ageing is an effective way to achieve superior strength and ductility.

Abstract: The effects of asymmetric and symmetric rolling at room temperature on mechanical properties and microstructure of the commercial purity Ti were investigated by means of mechanical test, optical microscopy, X-ray diffraction and transition electron microscopy. The results show that through asymmetric and symmetric rolling processes the ultimate tensile strength is substantially increased from 450 MPa to 960 MPa. Microstructure observation illustrates this variation in mechanical property is caused by the grain refinement and work hardening.

Abstract: Al-2 wt. % Si alloys with and without 0.25 wt. % scandium additions were processed by high-pressure torsion up to five turns at room temperature under a pressure of 6.0 GPa. Microstructural examination of the as-cast Al-2Si-0.25Sc alloy revealed the presence of Al3Sc precipitates which refined the Al grain structure, whereas no major changes were observed in the morphology of the Si particles. Processing by HPT of both experimental alloys revealed submicrometer grains with uniformly distributed Si particles. The mechanical properties were obtained using hardness measurements and the ball-indentation technique. The results show the hardness increased in the first turn of HPT and further increased with increasing numbers of turns. In addition, the hardness values were lower at the centers and continuously increased towards the edges of the disks. The difference in hardness values between the centre and the edge decreased with increasing turns, thereby suggesting an increasing homogeneity with increasing processing. The scandium addition and HPT processing of the Al-2Si alloy strongly influences the grain refinement and mechanical properties. The grain size reduction in the Al-2Si alloy was similar to Al whereas the presence of Sc in Al-2Si during HPT processing was responsible for large precipitation networks and a submicrometer grain formation.

Abstract: A grain size is known to be one of the factors which define mechanical properties of metallic materials. At the same time the mechanisms which regulate the deformation behavior of bulk ultrafine-grained (UFG) metals produced by the severe plastic deformation method are still a subject for intensive study and fixed ambiguously. The report presents the developed model and the results of its application for kinetic modeling of the deformation behavior of coarse-grained (CG) and UFG Ti. Modeling has been carried out considering the possible contribution of dislocation slip and ageing. Conclusions about the role of the investigated mechanisms in the appearance of the peculiarities of the deformation behavior of CG and UFG Ti have been made.

Abstract: Copper based materials are still the most attractive low resistivity materials for microelectronics and electrotechnics applications, though, all variants developed to combine strength and conductivity, such as solid solutions and composites, suffer from decay in electric conductivity while strength is increased . In a addition, linear decay was also conjectured for pure copper when grain size is refined below the UFG and nanostructured domains (except when grain boundaries are pure twins). Copper alloys with low content of silver and chromium were prepared by high pressure torsion (HPT) with various annealing conditions. Vickers hardness and electric resistivity in the temperature range of 4K-340K, were measured as well as microstructural characterizations were performed using quantitative X-ray diffraction. Depending on the annealing conditions the alloys exhibit from 25% to 75% of IACS electric conductivity at room temperature and hardness in the range of 200 Hv. Origins of both high strength and high electric conductivity were investigated from microstructures analysis, using transmission electron microscopy and mechanical testing.

Abstract: Ultra-fine-grained (UFG)/Nanocrystalline (NC) materials usually show reduced strain hardening and limited ductility due to formation of adiabatic shear band (ASB) under dynamic loading. In the present study, evolution of ASB in UFG Fe under dynamic shear loading was investigated. The UFG Fe was processed by equal-channel angular pressing (ECAP) via route Bc. After 6 passes, the grain size of UFG Fe reaches ~ 600 nm, as confirmed by means of Electron Back Scatter Diffraction (EBSD). Examination of micro-hardness and grain size of UFG Fe as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization at 500 0C. The high-strain-rate response of UFG Fe was characterized by hat-shaped specimen set-ups in Hopkinson bar experiments. The characteristics of ASB as a function of shear displacement, such as thickness of shear band and micro-hardness inside the shear band, were examined by SEM and Vickers micro-indentation respectively.

Abstract: Mg-3wt.%Y alloys were processed by cyclic extrusion and compression (CEC) up to 7 passes at different temperatures from 375 to 450 °C, respectively. The microstructure was effectively refined and the mean grain size was decreased from 800 μm to 3–15 μm. Tensile and compressive tests were performed at room temperature at an initial strain rate of 1.11 × 10-3 s-1. The experimental results show that after 7-pass CEC processing the tensile yield strength and elongation-to-failure of Mg-3Y alloy increased simultaneously. Furthermore, the strength differential effect (SDE) of tension-compression of the alloy was weakened, especially the SDE value was only 3.3% when processed at 400 °C.

Abstract: Atomic force microscopy provides a useful tool for examining the flow processes occurring during the tensile testing of ultrafine-grained metals processed by equal-channel angular pressing (ECAP). This paper describes the results obtained from experiments conducted on a Zn-22% Al eutectoid alloy and high-purity (99.99%) aluminium. The results show these two materials behave in different ways in terms of the development of mesoscopic shear planes.

Abstract: A series of experiments were conducted to evaluate the feasibility that commercially pure titanium (CP-Ti) was pressed for multiple passes by equal channel angular pressing (ECAP) at room temperature. Samples of CP-Ti were processed at room temperature using the dies with channel angles of 90° and 120°, respectively. First, each billet was processed 4 passes by ECAP using a die with an angle of 120° and a ram speed of 0.5mm s-1. And in order to eliminate residual stress, immediate annealing at 473 K for an hour was conducted between two adjacent passes. Second, CP-Ti was successfully processed by ECAP for up to 8 passes using the same die and a ram speed of 2 mm s-1 by controlling the flow of metal. Finally, CP-Ti was successfully achieved using a conventional die with an angle of 90° between the channels at room temperature. Each billet was processed for two passes with a ram speed of 26 mm s-1. These experiments show that CP-Ti may be processed by ECAP at room temperature and special attention was paid on improvements in the yield stress, ultimate strength and micro-hardness of ECAPed-Ti that are slightly higher than the improvements attained after pressing at elevated temperatures.