Papers by Author: Alexei Vinogradov

Abstract: High-alloy cast CrMnNi steels exhibit depending on the chemical composition either transformation induced plasticity (TRIP-effect) or twinning induced plasticity (TWIP effect). Whereas the TRIP effect is caused by a martensitic phase transformation from the f.c.c austenitic phase into the b.c.c α-martensite phase via the formation of deformation bands with high stacking fault density the so-called ε-martensite, the TWIP effect is the result of mechanical twinning during plastic deformation. The occurrence of both effects as well as the underlying microstructural processes are strongly affected by the austenite stability, the stacking fault energy and/or the test temperature. However, the onset stress and the kinetics of these deformation processes are still unknown. The in-situ measurement of acoustic emission signals during the plastic deformation of materials is a powerful tool for the investigation of such dynamic microstructural processes and their kinetics. In the present study acoustic emission measurements were performed during tensile deformation at room temperature on a high-alloy cast TWIP steel. The AE investigations were completed by SEM investigations on the deformed specimens. The statistical and cluster analysis of acoustic emission signals reveals different patterns of acoustic emission signals, which are correlated with underlying microstructural processes.

Abstract: The pure copper single crystals with specific crystallographic orientated were subjected to ECAP for one pass at room temperature. Two types of shear bands were observed. Type 1 shear bands were constructed with clusters of distorting micro shear bands and matrix. Micro shear band and matrix were delineated by large-angle grain boundaries, and these two orientations are in a twinning relationship. Parallel sets of deformation twins were observed in the matrix. Type 2 shear bands had no crystallographic feature, and shear band and matrix were considered as low-angle grain boundaries. Deformation twin was not observed both in matrix and the shear bands.

Abstract: In this study, susceptibility to SCC of nanostructured Cu-10wt%Zn alloys, produced by equal-channel angular pressing (ECAP) was investigated under the constant stress test in ammonia vapour, which has been well-known typical environment for IGSCC of Cu-Zn alloy. Billets having diameter of 20 mm and length of 100 mm were subjected to ECAP for eight passes at room temperature to obtain structure with grain size of about 100 nm. After ECAP, some of the billets were flush-annealed in 473 K for 60 seconds to decrease excessive unequilibrium dislocations at grain boundaries. Coarse grained specimens without ECAP and one-pass specimens were also tested for comparison. The specimens for SCC were tensioned by a constant load in ammonia vapour inside a glass chamber for 24 hours at room temperature. After the SCC tests, maximum length of cracks was evaluated by SEM. Specimen having UFG structure by 8-passes exhibited cracks in lower applied stress ratio, (=σa/σys) compared with 0- and 1-pass samples, where σa is applied stress and σys is yield stress, respectively. Most importantly, the specimen with annealed at 473K for 60s after ECAP cracked in higher applied stress. It became less sensitive to SCC after flush annealing although mechanical properties were not changed considerably. In our previous studies, we reported that the SCC of UFG copper produced by ECAP, and the sensitivity to SCC becomes lower by flush annealing. Results are discussed in terms of grain boundary state with or without extrinsic grain boundary dislocations

Abstract: The role of the deformation pre-history in high-cycle fatigue properties of copper produced by severe plastic deformation is discussed. The focus is placed on comparison of the structures and mechanical behaviours of two types specimens produced either by rolling or by ECAP, i.e. by the pure shear or simple shear mode, respectively. It is shown that the deformation either by simple or pure shear mode to the same equivalent strain results in alike mechanical properties, both monotonic and cyclic. The significant influence of the initial stages of strain hardening on fatigue is highlighted.

Abstract: This paper describes the influence of initial crystallographic orientation on the formation of dense shear bands in pure copper single crystals subjected to equal-channel angular pressing (ECAP) for one pass at room temperature. Local orientation change during simple shear by ECAP traced by electron backscatter diffraction (EBSD) indicated that the shear bands were formed when twinning plane and direction become parallel to the macroscopic shear plane and shear direction of simple shear strain, respectively. Orientation splitting associated with shear bands have a twinning relation. The shear bands were delineated by large-angle grain boundaries, having close relation to twinning relation with matrix, suggesting the role of deformation twinning as their nucleation sites. The activation of deformation twinning is suggested and can be rationalized by favorable crystallographic orientation and critical dislocation density as indicated elsewhere by the present authors.

Abstract: Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that of recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which attack dislocations and grain boundaries preferentially. UFG copper have exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on its surface is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.

Abstract: The relationship between acoustic emission and internal friction is investigated during cyclic deformation of copper single- and poly-crystals at intermediate amplitudes. Good agreement between these two phenomena has been demonstrated in all materials tested as long as the stress relaxation occurs uniformly within the sample whereas the difference between them becomes pronounced when strain/fracture localisation takes place. The similarity between acoustic emission and internal friction can be extended to materials deforming without appreciable plastic deformation, such as metal matrix composites where the main source of stress relaxation and strain accumulation is the particle breakage. The significance of the relation between internal friction and acoustic emission for understanding of fundamentals of AE is discussed.

Abstract: The present work is aimed at linking the microstuctutral features obtained after severe plastic deformation via ECAP to the tensile behavior and thermal stability of pure (99.98%) copper processed by routes A and Bc to different number of passes. The main conclusion one can draw unambiguously from the currently available results is that the strain path exerts relatively little effect on the resultant tensile properties when the number of pressing is sufficiently large, although there have been some marked differences in crystallographic textures and distribution of grain-boundaries. The effect of the number of pressings on the tensile ductility is considerable.

Abstract: Strengthening is a complex process involving such basic mechanisms as dislocation accumulation (work hardening), Hall-Petch hardening due to grain refinement, solid solution hardening and precipitation hardening in various combinations. The contribution of different mechanisms into resultant strength can vary significantly depending on chemical composition and processing. The purpose of the present work is to explore the significance of SPD for hardening and to clarify the role of different strengthening mechanisms. The model Au-based system was employed using pure Au, single phase solid solution Au-25Ag and precipitation hardenable Au-12.5Ag-12.5Cu (in mass %) alloy subjected to ECAP. The additive character of different strengthening mechanisms is clearly demonstrated. The extremely high strength exceeding 1 GPa is achieved in the Au-Ag-Cu after ECAP followed by aging, which has never been attained in conventional processing schemes.

Abstract: Anisotropy of mechanical properties, fatigue and fracture resistance of precipitation hardened CuCrZr alloy ultrafine (UFG) grained by equal-channel angular pressing (ECAP) is in focus of the present communication. Fracture toughness was estimated in terms of J-integral and the fatigue crack growth rate was quantified. It was found that although the estimated JIC-value appeared lower than that reported in the literature for a reference alloy, the ductility, fracture and crack growth resistance remained satisfactory after ECAP while the tensile strength and fatigue limit improved considerably. The stable crack growth rate did not differ very much for ECAP and reference conventional CuCrZr and no remarkable anisotropy in the stable crack growth was noticed.