Papers by Keyword: Severe Plastic Deformation (SPD)

Abstract: Precipitation effects in ultra fine grained (UFG) lightweight Mg-based alloys were studied in the present work by means of positron lifetime spectroscopy, transmission electron microscopy, and microhardness. The UFG samples with grain size around 100 nm were fabricated by high pressure torsion (HPT). The UFG structure contains a significant volume fraction of grain boundaries and exhibits a high number of lattice defects (mainly dislocations) introduced by severe plastic deformation during the HPT processing. A high dislocation density and volume fraction of grain boundaries enhance the long range diffusion of solute elements. Moreover, dislocations and grain boundaries act as nucleation centers for precipitates. As a consequence, the precipitation effects are facilitated in the UFG alloys compared to the conventional coarse-grained samples. This phenomenon was examined in this work by comparison of the precipitation sequence in Mg alloys with UFG structure and solution treated coarse-grained alloys.

Abstract: Polycrystalline Cu has been deformed at room temperature by oscillatory compression method to true reduction εh = 0.6 and 1. Microstructure by using transmission electron microscopy (TEM) and texture evolution after deformations was investigated. Oscillatory compressed microareas contains two distinctive regions: fine grains inside banded microstructure with large misorientation and surrounding matrix with submicrometer subgrains with a fraction of both low and high angle boundaries. Moreover nucleation of new grains under recrystallization takes place at the local-regions. The study of the crystal orientation distribution during applied deformation showed that the pole figure registered for the sample after compression shows ring of pole density, which concentrates around projection of <011>. Oscillatory compression causes formation of two axial texture components: <001> and <011>.

Abstract: The correlation between the microstructure and the mechanical behavior of ultrafinegrained face centered cubic (f.c.c.) metals processed by equal-channel angular pressing (ECAP) was studied. It was found that the maximum value of the yield strength obtained at high strains is determined by the shear modulus and the saturation value of the dislocation density according to the Taylor equation. It was also revealed that the value of the parameter α in this equation decreases with decreasing stacking fault energy, indicating the effect of different geometrical arrangements of dislocations in the grain boundaries. In addition, it was shown that for ECAP processed Cu, the ductility decreases with increasing strain but at extremely high strains the ductility is partially restored due to a recovery of the grain boundary structure.

Abstract: Explosion welding has produced a large number of dissimilar joints. But the explosion welding of materials of low impact toughness and brittle nature is considered to be difficult. Magnesium, with its HCP structure, has a low capacity for plastic deformation at room temperature and moreover it has a high chemical reactivity. Therefore, successful explosion welding of magnesium alloys, demands careful attention to be paid to prevent failure or formation of brittle reacted zones at the weld interface. Explosive welding of the wrought Mg-Al-Zn alloy and commercially pure aluminum is performed. The welds are analyzed through metallographic characterization and shear tests. High quality welds of the two materials possessing shear strength higher than the softer layer has been obtained.

Abstract: Mechanical Milling (MM) is a Severe Plastic Deformation - Powder Metallurgy (SPD-PM) process which enables to produce a nano grain structure. A BCC layer with a nano grain structure appeared in the vicinity of the MM powder surface. Conventional cold work at room temperature never induces a strain-induced-martensitic-transformation in the SUS310S stainless steel. Therefore, a BCC layer formation from austenitic matrix is a specific phase transformation, and is attributed to the rise of the grain boundary energy by the nano grain formation. The hardness of this surface layer has approximately 540Hv, while that of the inner area has about 290Hv. As the MM powder anneals at 333K for 300s, the hardness of surface and inner area decreases to approximately 470Hv and 280Hv, respectively. Result of such a large hardness decrease in the surface of MM powder after annealing at near the room temperature indicates an existence of a huge number of defects, such as vacancy and interstitial atom, by the SPD-PM Process.

Abstract: It is now well-established that processing through the application of severe plastic deformation (SPD) leads to a significant reduction in the grain size of a wide range of metallic materials. This paper examines the fabrication of ultrafine-grained materials using high-pressure torsion (HPT) where this process is attractive because it leads to exceptional grain refinement with grain sizes that often lie in the nanometer or submicrometer ranges. Two aspects of HPT are examined. First, processing by HPT is usually confined to samples in the form of very thin disks but recent experiments demonstrate the potential for extending HPT also to bulk samples. Second, since the strains imposed in HPT vary with the distance from the center of the disk, it is important to examine the development of inhomogeneities in disk samples processed by HPT.

Abstract: Equal-channel angular pressing (ECAP) is a valuable technique for refining grain sizes to the submicrometer or the nanometer range. This study explores the reason for the difference in the grain refining behavior between pure Al and pure Cu. First, very high purity levels were adopted in order to minimize any effects of impurities: 99.999% for Al and 99.99999% for Cu. Second, high purity (99.999%) Au was also used in order to examine the effect of stacking fault energy. All three pure metals were subjected to ECAP and microstructural observations and hardness measurements were undertaken with respect to the number of ECAP passes. It is concluded that the stacking fault energy plays an important role and accounts for the difference in the grain refining behavior in the ECAP process.

Abstract: Recrystallization behavior of SPD processed high purity iron was studied. The 99.95% iron sheet was deformed by the accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. Subsequently, the ARB-processed specimens were annealed for 1.8ks at various temperatures from 300°C to 500°C. The microstructures of these specimens were characterized by TEM and SEM/EBSP. The microstructure of the as-ARB-processed specimens showed the lamellar boundary structure elongated along RD, which was the typical microstructure of the ARB-processed materials. The mean interval of the lamellar boundaries was about 100 nm. After annealing at 400°C, the ARB specimen showed a partially recrystallized microstructure composed of equiaxed grains with grain size larger than 10 5m and the recovered lamellar boundary structure. After annealing above 500°C, the microstructures were filled with equiaxed recrystallized grains. These results suggest that conventional discontinuous recrystallization characterized by nucleation and growth occurs during annealing at annealing temperature above 400 °C. In previous work reported about the annealing behavior of the low carbon IF steel ARB processed, the continuous recrystallization occurred during annealing at annealing temperature above 600 °C. The recrystallization temperature of the pure iron was much lower than the IF steel and the recrystallization process were significantly different. This difference was suggested to be caused by inhomogeneous microstructure in the pure iron ARB-processed.

Abstract: The structure and properties of oxygen-free copper (99,98%) were studied after different types of severe plastic deformation (SPD): equal-channel angular pressing (ECAP), multiaxial deformation (MD), and accumulative roll bonding (ARB) as a function of the strain at room temperature (to a true strain of 30-40). The SPD facilitates the formation of submicrocrystalline structure with a grain size of 200-250 nm and predominantly high angle boundaries (83-94%). ECA pressing leads to the formation of the most uniform submicrocrystalline structure.The strength characteristics increase with increasing strain and reach the steady stage at ε ≈ 5. At the steady stage, UTS = 460-480 MPa at ARB, and MD, while UTS at ECAP is somewhat lower, 430-440 MPa. The smallest "steady" values EL = 4 - 5% were obtained in the case of ARB, and the maximum EL = 18% was obtained at MD.

Abstract: Some of new severe plastic deformation processes as equal channel angular extrusion, and strip shearing are presented as very efficient techniques for grain refinement. Channel geometry, contact friction, strain rate and multi-pass processing versus microstructure and mechanical properties are shown.