Solid State Phenomena Vol. 114

Abstract: During the last decade severe plastic deformation (SPD) has become a widely known method of materials processing used for fabrication of ultrafine-grained materials with attractive properties. Nowadays SPD processing is rapidly developing and is on the verge of a transition from lab-scale research to commercial production. This paper focuses on several new trends in the development of SPD techniques for effective grain refinement, including those for commercial alloys and presents new SPD processing routes to produce bulk nanocrystalline materials.

Abstract: The Cyclic Extrusion Compression (CEC) is one of the methods of severe plastic deformation (SPD), used for producing nanomaterials. The CEC method allows materials to be deformed to arbitrarily large strains without changing the initial shape of sample. Large hydrostatic compressive stresses are exerted during deformation avoiding sample cracking. Using the CEC method Cu and aluminum alloys nanomaterials were produced. It has been found that, after exerting true strain of about ϕ = 14, only some part of the sample changes into a nanomaterial, while the remainder volume still shows the ultrafine microstructure. The nanometric microstructure is created generally inside the areas of intersecting microbands. Large misorientation has been found between the microbands and the surrounding material, facilitating the formation of nanograin boundaries. The hardness of samples increases with the increase of deformation, however only to a boundary level of about 100 MPa. The stabilization of hardening, above a deformation of about ϕ = 4, suggests the activation of softening processes. Independently to the stabilization of properties, the refinement of nanograins is continued, indicating the development of anomalies in the hardening – grain size relationship.

Abstract: The aim of this study was to investigate the influence of large plastic strain on the microstructure and mechanical properties of aluminium processed by severe plastic deformation (SPD) by the Equal Channel Angular Pressing (ECAP) method. Polycrystalline high purity aluminium (99,99%) was pressed at room temperature to produce samples subjected to 4, 8 and 12 ECAP passes. The microstructure of aluminium was examined using a light polarized microscope. Microhardness measurements and tensile tests were undertaken to determine the mechanical properties of the material processed by ECAP. The results obtained show the relationship between the microstructure and the mechanical properties of the material.

Abstract: XRD, TEM, microhardness and thermal analysis were carried out on a series of Ni samples produced by high-pressure torsion (HPT). The evolution of microstructures and their inhomogeneity were investigated. The local microstrain showed dynamical oscillations as a function of the HPT rotations, demonstrating dynamical evolution of lattice defects during the procedure. Both XRD and TEM showed that a small difference in grain sizes remains even after 5 revolutions of HPT with smaller grain sizes at the peripheral region of the sample. The higher microhardness at the peripheral region is the result of the smaller grain sizes and the higher density of lattice defects, compared with the central region. Thermal treatment at a heating rate of 20K/min from room temperature to 473K did not result in decreased microhardness, but increased by about 10% for samples treated with not more than 3 rotations of HPT. The increase in microhardness was attributed to further grain refinement, the formation of a larger fraction of high-angle grain boundaries and grain boundaries being closer to equilibrium after recovery.

Abstract: An ultra-fine grained microstructure was obtained in high purity nickel by a combination of (a) equal-channel angular pressing (ECAP) and (b) hydrostatic extrusion (HE) with a cumulative true strain of ~11.2. The resulting microstructure was examined by light and TEM microscopy. Mechanical properties have been measured by tensile and hardness tests. It was found that HE of ECAP-ed samples leads to a significant grain size refinement (from 330 to 160nm) and to an increase in microstructural homogeneity. SPD nickel, made by a combination of the ECAP and hydrostatic extrusion methods, has high strength and ductility (i.e.: YS=1120MPa and εf = 11%). The microstructure transformation was accompanied by a strength increase of 78% compared to ECAP alone. The results obtained fit well with the Hall-Petch relationship. A combination of ECAP and HE has achieved much better properties than either single process and show it to be a promising procedure for manufacturing bulk UFG nickel.

Abstract: The paper presents the results of investigations into the mechanical properties and tribological characteristics of 316 LVM processed by hydrostatic extrusion (HE). The mechanical properties were characterized by microhardness measurements and compression tests. The wear properties were investigated using a pin-on-disc tribometer under dry and lubricated conditions. The friction coefficient was measured as a function of the time of the wear test. The results indicate that the hydrostatic extrusion process significantly improves the mechanical properties and the wear resistance of 316 LVM stainless steel. The results are discussed in terms of the microstructural changes induced during processing by hydrostatic extrusion.

Abstract: The material examined was commercially pure titanium with intermetallic Ti-Al layers produced by magnetron sputtering followed by glow discharge assisted treatment. This material was subjected to hydrostatic extrusion at room temperature. This resulted in substantial grain size refinement in the titanium accompanied by significant property improvement. The intermetallic Ti- Al layers reduced the pressure required during hydroextrusion and also increased the microhardness and frictional wear resistance of the material.

Abstract: During the last decade it has been shown that severe plastic deformation (SPD) is a very effective for obtaining ultra-fine grained (UFG) and nanostructured materials. The basic SPD methods are High Pressure Torsion (HPT) and Equal Channel Angular Extrusion (ECAE). Recently several new methods have been developed: 3D deformation, Accumulative Roll Bonding, Constrained Groove Pressing, Repetitive Corrugation and Straightening, Twist Extrusion (TE), etc. In this paper the twist extrusion method is analyzed in terms of SPD processing and the essential features from the “scientific” and “technological” viewpoint are compared with other SPD techniques. Results for commercial, 99.9 wt.% purity, copper processed by TE are reported to show the effectiveness of the method. UFG structure with an average grain size of ~0.3 μm was produced in Cu billets by TE processing. The mechanical properties in copper billets are near their saturation after two TE passes through a 60º die. Subsequent processing improves homogeneity and eliminates anisotropy. The homogeneity of strength for Cu after TE is lower than after ECAE by route BC, but higher than after ECAE by route C. The homogeneity in ductility characteristics was of almost of inverse character. The comparison of mechanical properties inhomogeneity in Cu after TE and ECAE suggests that alternate processing by ECAE and TE should give the most uniform properties.

Abstract: The mechanical properties and deformation behaviour of ultra-fine grained (UFG) Ni subjected to severe plastic deformation (SPD) were investigated. The UFG Ni characterized possessed an homogeneous structure with a grain size of 120 nm and high angle nonequilibrium grain boundaries. This nickel possessed an ultimate tensile stress of 1270 MPa. Investigation of the deformation relief on the polished surface of the UFG Ni sample by HRSEM revealed grain boundaries (GBs) steps already at small degrees of tension that testifying to the involvement of grain boundaries in the deformation process at room temperature. After significant tensile strain the deformation relief revealed a network of crossed shear bands oriented at an angle in the range 35°- 45° to the axis of tensile deformation. Shear bands propagated along GBs parallel to the plane of maximal shear stress. The formation of shear bands occurred due to a strong shift and rotation of grain groups. This led to a deformation mechanism involving collective relative displacement of grain groups, with extensive grain boundary sliding at room temperature. Over a length of a few micrometers the material can be regarded as uniform and therefore the local strain distribution becomes more uniform than in coarse-grained materials. It is plausible that this mode of deformation may contribute to the enhanced ductility. The deformation behaviour of Ni having different grain sizes and various grain boundary states are also considered. The Opportunity to achieve a combination high strength and good ductility by control of the microstructure of in metals and alloys opens perspectives industrial applications, particularly, for mirco-systems and for items of complex geometry to be produced by superplastic forming.