Papers by Keyword: Deformation Structure

Abstract: The microstructure and mechanical properties of friction stir welded Al-5.4Mg-0.2Sc-0.1Zr alloy were studied. Defect-free welds were produced in hot extruded, hot rolled and cold rolled initial conditions. Friction stir welding led to the formation of ultrafine-grained structure in stir zone that contributes to overall strengthening. Coherent Al₃(Sc,Zr) dispersoids retain partially during welding process that provides a joint efficiency close to 100% in the hot extruded and hot rolled materials. In the cold-rolled state the joint efficiency was found to be only 64%. The relatively low weld strength of the cold rolled material was attributed to the elimination of strain hardening due to the formation of recrystallized structure. It was shown that full strength weld can be achieved in semi-finished products of Al-Mg-Sc alloys in cold-worked and stabilized states being equal to H323 and H341 tempering by friction stir welding.

Abstract: The microstructural evolution and mechanical properties of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to cold rolling with a total strain up to ~1.6 was studied using high resolution EBSD analysis and TEM. It was shown that cold rolling induces elongation of initial grains and the formation of deformation bands along rolling direction in addition to dramatic increase in density of lattice dislocations. Two types of deformation bands evolve. Deformation bands initially bounded by low-angle boundaries (LABs) with misorientation higher than 2^o and spacing ranging from 0.8 to 4 μm gradually transform to lamellas delimited by high-angle boundaries (HABs). Thin deformation bands delimited by LABs with misorientation of 2^o or less evolve within these coarse bands. First type of deformation bands is subdivided due to mutual intersection with second order deformation bands or shear bands to elongated crystallite evolving to micron scale grains with strain. The thin deformation bands may be also subdivided to nanoscale crystallites. It was shown that the formation of well-defined deformation bands yield very high anisotropy in strength and ductility, while a strong increase in lattice dislocation density with strain diminishes this anisotropy.

Abstract: Heterogeneous deformation during rolling is a crucial issue for elucidating recrystallization behavior. The progress thus far in our understanding of heterogeneity has been reviewed focusing on grain boundary and shear band. A statistical study on heterogeneous deformation structure using EBSD revealed that heterogeneity along the grain boundary can be classified into three types: 1) relatively flat boundary, 2) irregularly serrated boundary, and 3) boundary associated with fine grains. The fine grains in type 3 seem to be dynamically recovered as a cold-rolled state. Shear band formation is considered to be caused by plastic instability that is accelerated, for instance, by dynamic strain aging. A shear band is revealed to have a feature of recovered fine cells with Goss orientation already embedded in the shear band. The application of the phase-field method is exploited to predict recrystallization behavior and texture evolution during annealing based on the subgrain growth model. In simulation, a bulging mechanism seems to be dominant. Thus, a more rigorous description of the heterogeneous deformation structure is needed in the future

Abstract: A pure Ni sheet was heavily deformed up to an equivalent strain of 6.4 at room temperature and then annealed to obtain highly Cube textured material, which is a polycrystal subdivided by many low-angle grain boundaries. The highly Cube-oriented sheets were cold-rolled to various reductions up to 90%. It was found that large fraction of Cube oriented grains remained stable in cold-rolling although the orientation is theoretically unstable. The stability of Cube orientation was considered to be associated with the constraint by grain boundaries in the materials.

Abstract: Deformation structures produced by high pressure torsion (HPT) and accumulative roll-bonding (ARB) were characterized by transmission electron microscopy and electron backscatter diffraction, and the mechanical properties of the ARB samples were determined by uniaxial tensile testing. The structural evolution during HPT in high purity nickel has been examined and an extended lamellar boundary structure was observed at high strains. For ARB samples deformed to high strains, an almost similar structural morphology has been observed in both interstitial free steel and in commercial purity aluminum, whereas a relatively equiaxed structural morphology was observed in high purity aluminum samples. In all samples, both deformed by HPT and ARB, the deformation structures were composed of a large fraction of high-angle boundaries, together with low-angle boundaries and isolated dislocations between the boundaries. Common characteristics have been identified in the mechanical behavior of the ARB samples, namely a very high strength, a small uniform elongation and a relatively large post-uniform elongation after necking. For HPT and ARB the structural morphology and structural parameters are compared, and for the ARB samples structure-property relationships are also discussed.

Abstract: The microstructural development of cold-rolled lath martensite structure in the low carbon steels and ultra-low carbon steels are studied and compared. In low carbon steel of as-quenched specimens, very thin austenite films exist at boundaries of adjacent laths, but do not exist in ultra-low carbon steel. After cold rolling for the low carbon steel, the lamellar dislocation cells, irregularly bent laths and kinked laths regions are frequently observed and, in some instances, the disappearance of initial lath boundaries is observed. The existence of retained austenite films suggests that the lath boundaries rarely disappear during cold-rolling in the low carbon steel.

Abstract: Microstructural observations are presented for different metals deformed from low to high strain by both traditional and new metal working processes. It is shown that deformation induced dislocation structures can be interpreted and analyzed within a common framework of grain subdivision on a finer and finer scale down to the nanometer dimension, which can be reached at ultrahigh strains. It is demonstrated that classical materials science and engineering principles apply from the largest to the smallest structural scale but also that new and unexpected structures and properties characterize metals with structures on the scale from about 10 nm to 1 μm.

Abstract: The feasibility of a novel continuous severe plastic deformation (SPD) technique, continuous frictional angular extrusion (CFAE), for producing ultra-fine grained strip material, has been studied. The CFAE technique takes advantage of facets of rolling and equal channel angular extrusion (ECAE) and is designed to produce bulk ultra-fine grained (UFG) metals with high productivity and low cost. A process setup was established through the modification of a standard rolling mill. CFAE processing of commercially pure aluminium AA1050 sheets was successfully carried out at room temperature, using a 120o die angle. A uniform UFG structure with an average grain size of ~0.6μm was achieved after 10 CFAE passes, at an equivalent strain of ~ 6.6. Evolution of the deformation structure and texture during processing was examined as a function of strain and characterized using high resolution EBSD.

Abstract: Isothermal low cycle fatigue (LCF) behaviours of a third generation titanium aluminide based γ-TiAl alloy with duplex microstructure were investigated under the various test conditions, including temperature (550°C-750°C), total strain amplitude (0.3%-0.6%) and environment (air and vacuum), in order to clarify the fatigue life, deformation characters and fracture process of the alloy during LCF. The plastic strain accumulation has a great contribution to LCF damage. With increasing total strain range, LCF life decreases distinctly. Under the small total strain amplitude (≤0.4%), the increase of test temperature enforces microstructure resistance to LCF fracture. However, the increase of test temperature together with large total strain amplitude (>0.5%) accelerates the microstructural degradation, which behaves the dissolution of α2 lamellae and recrystallization of γ phase, resulting in great LCF damage. Moreover, environment brittlement during high temperature exposure to air influences the initiation process of fatigue cracks. The fracture mechanisms at various test conditions were analyzed.

Abstract: Severe plastic straining is an established method for producing submicron grain (SMG) structures in alloys. However, the development of such a fine grained structure in single-phase alloys is usually futile if they are to be exposed or processed at elevated temperatures. This is a direct consequence of the natural tendency for rapid and substantial grain coarsening which completely removes the benefits obtained by grain refinement. This problem may be avoided by the introduction of nanosized, highly stable particles in the metal matrix. In this work, a SMG structure was generated in an Al-0.3 wt.% Sc alloy by Equal Channel Angular Pressing (ECAP). The alloy was prepared initially to produce a fine grained microstructure exhibiting a large fraction of high angle grain boundaries and a dispersion of nanosized Al3Sc particles. The evolution of microstructure during annealing at temperatures up to 550 °C was examined in detail and grain size distributions generated from the data. It was shown that grain coarsening is rapid at temperatures above 450 °C and the initial log-normal grain size distribution exhibiting low variance and skewness was altered considerably. The statistical information generated from the grain size distributions confirms that discontinuous grain coarsening occurs in this alloy only at temperatures greater than 500 °C.