Papers by Keyword: Grain Fragmentation

Abstract: The evolution process of ultrafine grains during hot severe plastic deformation (SPD) was studied in several aluminum alloys. The structural changes can be characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be categorized into the three stages irrespective of deformation mode and temperature: i.e. i) an incubation period for new grain evolution in low strain; ii) a grain fragmentation by frequent development of MSBs and subsequently new grains in medium strain, and iii) a full development of fine grains in large strain. Temperature effect on the new grain formation in aluminum alloys is also analysed in detail and the mechanism operating is discussed.

Abstract: Features of the deformation process by cold radial forging of tube billets from Zr-1%Nb alloy were reconstructed on the basis of X-ray texture data. The cold radial forging intensifies grain fragmentation in the bulk of billet and increases significantly the latent hardening of potentially active slip systems, so that operation only of the single slip system becomes possible. As a result, in radially-forged billets unusual deformation and recrystallization textures arise, differing from usual textures of a-Zr by the mutual inversion of crystallographic axes, aligned along the axis of tube.

Abstract: To understand and model grain refinement in severe plastic deformation, some analysis of Nb single crystals has been carried out in previous work. To bridge the gap with normal polycrystalline materials, supplementary experiments on large polycrystals, deformed at moderate strains appear to be necessary to explain the grain subdivision step by step. In the present work, successive uniaxial compression tests have been carried out on a large grained Niobium polycrystal up to height reductions of 30% with small strain increments. Electron backscatter diffraction (EBSD) analysis was done after each compression step to characterize the evolution of orientation and microstructures. It is observed that a “rotation front” forms inside the grain and moves with increasing strain from one side to the other side of the grain. In one grain, this process results in a grain boundary affected zone in the vicinity of the grain boundary. Both static orientation evolution inside the grain and historical evolution of the average orientation have been studied, which indicates that the grain orientation rotates around one of the (110) poles at low strain.

Abstract: For systematic study of the distribution of residual deformation effects in textured metal materials the method of Generalized Pole Figures was developed, combining texture measurement with X-ray line profile measurements in such a manner that the profile of the same X-ray line (hkl) is registered by each successive position of the sample. The obtained totality of profiles after that or another treatment is used for construction of Generalized Pole Figures (GPF), i.e. distributions of measured diffraction parameters or calculated substructure parameters in the stereographic projection of the sample depending on the orientation of reflecting crystallographic planes. As applied to metal materials with developed rolling textures it was found that any volume ~1 mm3 is characterized by an extremely wide spectrum of substructure conditions. Three laws of substructure anisotropy were revealed for the first time. The 1st law: Residual deformation effects are minimal along directions, corresponding to maximal density of crystallographic axes, i.e. texture maxima, and increase up to highest values by passing to texture minima. The 2nd law concerns variation of lattice parameters in metal products due to elastic microstrain: For each grain with crystalline lattice, extended along axis by (+ε), there is its pair with the symmetric orientation, where along axis crystalline lattice is compressed by (-ε), so that accompanying elastic microstresses are equilibrated. The 3rd law: By passing from residual tension of the crystalline lattice to its compression, grain fragmentation changes depending on indexes of the reference axis, i.e. tensile and compressive elastic deformations of the crystalline lattice differ in their uniformity.

Abstract: The evolution mechanisms of ultrafine grains processed by severe plastic deformation are studied in ferritic steel, copper and aluminum alloys. The structural changes are characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be subdivided in the following three stages irrespective of deformation temperature: i.e. an incubation period for new grain evolution in low strain; grain fragmentation by frequent development of MSBs in medium strain, and a full development of new grains in large strain. A mechanism of new grain formation during SPD, i.e. the MSB-based model, is proposed and discussed comparing with the subgrain-based model.

Abstract: Equal channel angular extrusion (ECAE), involving intense plastic straining under high applied pressure is generally recognized and extensively studied top down approach for producing bulk ultra-fine grained (UFG) metallic materials, and even going down in size to the nanometer range. In this research efforts are made to identify conditional under which grains with size less than 100 nm form after ECAE. Evolution of microstructure of Al-Li based alloy processed by ECAE is analyzed using transmission electron microscopy (TEM). Observations on the effect of precipitates/second phase particles in the sample on the deformation characteristics and their role on the increased degree of grain fragmentation process is highlighted. Samples of Al-Li based alloy are solutionized, quenched and aged at different temperatures to obtain well formed precipitate laths/plates before subjecting to ECAE. During the deformation process these precipitates disintegrate into fragments and get dispersed into the Al matrix. The fragments of a few nanometers size bring about drastic changes in the flow as well as the recovery characteristics of processed samples. Evidence for dynamic recrystallisation taking place during the ECAE processing is presented. It was observed that optimal thermal treatment leads to more effective grain refinement and consequently an ultra-fine grained microstructure could be achieved even after single pass in Al-Li based alloy containing precipitates and second phase particles.

Abstract: The texture of Ti and Zr rods, subjected to equal-channel angular pressing (ECAP) by routes C and BC, is considered as a source of information about the actual loading scheme, operating mechanisms of plastic deformation, the structure condition of material. Processes of grain reorientation under rolling and ECAP are compared and distinguishing features of the latter are revealed. Effects of grain fragmentation and dynamic recrystallization on the texture are discussed.

Abstract: The development of deformation substructure and texture has been studied up to large plastic strains in some simple Al base alloys by multiple forging. The experiments involve successive forging strains on near-cube samples along 3 orthogonal axes up to cumulative strains of 3 or more (and temperatures from 20 to 400°C). The alloys include the commercial AA 3103 (Al- 1%Mn) and a laboratory Al-3%Mn-Sc-Zr alloy for the high temperature tests. Some complementary experiments have been carried out on oriented single crystals of Al-0.3%Mn. During 3D cross forging of fcc metals a clear texture composed of three symmetrical components is formed; they are the 3 possible variants of the <110> <110> <100> crystal axes along the 3 forging axes. This macroscopic texture is demonstrated by X-ray pole figure analysis, EBSD mapping and confirmed by crystal plasticity (CP) simulations. At room temperature the alloys (particularly Al-Mn) exhibit significant grain refinement by grain fragmentation leading to "grain sizes" of less than 103m. However, at temperatures ≥ 300°C in the stable Al-3%Mn-Sc-Zr alloy the lattice rotations towards just 3 texture components leads to a high frequency of grain "fusions"; each grain becomes surrounded by 3-5 neighbours of the same orientation so that long interpenetrating chains of the texture components are formed; they are also confirmed by FEG-SEM EBSD and spatially resolved texture simulations. The behaviour of stable (Goss) and unstable (cube) single crystal orientations during the same deformation processing is also investigated and shown to agree with the CP simulations.

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.

Abstract: Grain refinement taking place in a magnesium alloy AZ31 was studied in a single- and multi-directional compression at a temperature of 573K. The structural changes observed by SEM/EBSD analysis can be characterized by the evolution of many mutually crossing kink bands at low strains, continuous increase in their number and misorientation angle in moderate strain and finally full formation of a fine-grained structure in high strain. The characteristics of new grain evolution process are sensitively affected by initial grain size (D0) and strain path. New grains are developed faster with decrease in D0. Multi-directional compression accelerates the evolution of fine grains and the improvement of plastic workability. The mechanism of new grain formation is discussed in detail.