Structure and properties of stainless steels subjected to severe plastic deformation

The physical, mechanical, and corrosion properties of stainless steels with microcrystalline and submicrocrystalline structures obtained with the use of different techniques of thermal deformation treatment are considered. A comparative study of formation of a dispersed structure in austenitic and ferritic steels is performed. The effect of the type of structure on mechanical properties is discussed. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 27–32, February, 2006.     

Structure and deformaton behaviour of Armco iron subjected to severe plastic deformation

Structural evolutions in an Armco iron subjected to severe plastic deformation by torsion under high pressure are anlysed with conventional and high resolution electron microscopes. The substructure observed at low strains appears to shrink with increasing deformation and transforms at very high strains into grain boundaries. The resulting grain size decreases down to a constant submicrometric value. Meanwhile, the material strength, as revealed by micro hardness measurements, levels out. Dislocation densities and internal stress levels are used to discuss the structural transformations. Hydrostatic pressure and deformation temperature are believed to modify the steady-state stress level and structural size by impeding the recovery processes involving diffusion.     

Microstructure and mechanical properties of medium-carbon steel subjected to severe plastic deformation

Metal Science and Heat Treatment - The microstructure and properties of medium-carbon steel (0.45% C) are studied after torsional severe plastic deformation (SPD) at a high quasi-hydrostatic...     

Internal friction and evolution of ultrafine-grained structure during annealing of Grade-4 titanium subjected to severe plastic deformation

Electron microscopy, X-ray diffraction, tensile tests, and measurements of internal friction (IF) have been used to study the transformation of the structure and properties of commercial Grade-4 titanium subjected to deformation using equal-channel angular pressing combined with the Conform process (ECAP-Conform), drawing, and subsequent annealing. It has been found that the ECAP-CONFORM with drawing leads to a decrease in the grain size of titanium to about 150 nm. During annealing at 400°C for 1 h, the growth of separate grains was observed; with an increase in the annealing temperature to 450°C or higher, the growth of grains in the whole volume of a specimen occurred. It has been shown that titanium in the ultrafine-grained (UFG) state is characterized by an internal-friction peak at temperatures of 450–500°C (under the selected measurement conditions), which results from the processes of recovery and recrystallization. During repeated heating, no such effect was observed. At higher temperatures, a thermally activated relaxation internal-friction peak was found, which appears to be a grain-boundary internal-friction peak.     

Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation

Bulk components made of a Ni-base C-2000 alloy with a face-centered cubic crystal structure and a very low stacking fault energy have been severely plastically deformed at the surface region to attain a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk. The evolution of microstructural characteristics has been studied as a function of the processing time employing a variety of analytical techniques, including extensive conventional and high-resolution transmission electron microscopy analyses. The thickness of the nanocrystalline surface layer is found to increase with the processing time. Deformation twinning is ubiquitous and occurs at the earliest stage of deformation and the deepest region of the material where plastic deformation has taken place in the surface severe plastic deformation process. A grain-refinement mechanism led by deformation twins and complemented by dislocation activity has been put forth to explain the nanocrystallization of the coarse-grained material employed in this investigation.     

Surface integrity of nickel-based alloys subjected to severe plastic deformation by abusive drilling

Surface integrity of nickel-based superalloys after machining operations has become a topic of major interest in the aerospace sector. In the present work, the characteristics of nickel-based alloys (Alloy 718, Waspaloy, Alloy 720Li, and RR1000) subjected to abusive drilling conditions have been investigated using experimental methods such as FEG-SEM, EBSD, XRD, TEM and nano-indentation. The results indicated the presence of nano-sized grains typical of severe plastic deformation in the machined surface while the presence of plastic slip bands was observed in the sub-surface layers. Correlations between the thermo-mechanical properties of the nickel-based alloys and the severe plastic deformation features of the machined surfaces are presented.     

Structure and microhardness of chromium-zirconium bronze subjected to severe plastic deformation by dynamic channel-angular pressing and rolling

Bulk samples of chromium-zirconium bronze have been subjected to severe plastic deformation by two methods, namely, high-strain-rate dynamic channel-angular pressing (DCAP) and quasistatic deformation by rolling. After deformation and additional aging using metallography and electron microscopy, the structure has been investigated and the microhardness of the samples has been measured. It has been shown that the high-strain-rate deformation by DCAP is of a periodic character. It has been established that, in the investigated bronze subjected to DCAP, in four passes, the structure of dynamic polygonization is predominantly formed, which is accompanied by processes of aging. Upon the rolling, cells of deformation origin and a structure with randomly distributed dislocations and numerous extinction contours are formed.     

Structure and mechanical properties of an aluminum alloy 1570 subjected to severe plastic deformation by high-pressure torsion

The effect of an ultrafine-grained (UFG) structure formed in an aluminum alloy 1570 using severe plastic deformation by high-pressure torsion (HPT) at room temperature and at temperatures of 100 and 200°C on the mechanical properties (strength and plasticity) has been investigated. The specific features of the UFG states obtained have been studied by transmission electron microscopy and X-ray diffraction analysis. The main regularities of changes in the structure characteristics of the alloy (the average grain size, size of coherent domains, magnitudes of microdeformations of the crystal lattice, dislocation density, and the lattice parameter) have been established depending on the temperature of the HPT treatment. The mechanical properties of the alloy after HPT have been estimated from the results of microhardness measurements and mechanical tests for tension. It has been established that after HPT performed at room temperature, the UFG alloy demonstrates an ultimately high level of strength (the microhardness, offset yield strength, and ultimate strength reach 2300, 905, and 950 MPa, respectively) and a marked plasticity (the relative elongation at fracture was 4.7%). The HPT treatment performed at higher temperatures insignificantly reduces the strength characteristics of the UFG material but leads to a substantial drop in its plasticity. This unusual mechanical behavior of the UFG alloy is discussed based on an analysis of the results of structural investigations.     

Effect of rolling-assisted deformation on the formation of an ultrafine-grained structure in a two-phase titanium alloy subjected to severe plastic deformation

The effect of rolling in the temperature range 450–650°C on the fragmentation of the primary phase in a hot-rolled VT6 alloy rod preliminarily subjected to severe plastic deformation by equal-channel angular pressing at 700°C (scheme B _c, the angle between the channels is 135°, 12 passes) is studied. Rolling at 450°C without preliminary ECAP is shown not to cause α-phase fragmentation and to favor intense cold working of the alloy due to multiple slip. ECAP provides partial fragmentation of the initial structure of the α phase and changes the morphology of the retained β phase: it transforms from a continuous matrix phase into separated precipitates located between α particles. This transformation activates the fragmentation of the α phase during rolling at 550°C owing to the development of twinning and polygonization processes apart from multiple slip. Both a decrease (to 450°C) and an increase (to 625–650°C) in the rolling temperature as compared to 550°C lead to the formation of a less homogeneous and fragmented structure because of weakly developed recovery and intense cold working in the former case and because of the beginning of recrystallization and the suppression of twinning in the latter case. A relation between the structure that forms upon SPD followed by rolling and the set of its properties is found. A general scheme is proposed for the structural transformations that occur during ECAP followed by rolling at various temperatures.     

Microstructural evolution in NiTi alloy subjected to surface mechanical attrition treatment and mechanism

Both nanocrystalline and amorphous phases are observed from the near surface of nickel titanium shape memory alloy (NiTi SMA) with the B2 austenite phase after surface mechanical attrition treatment (SMAT). The microstructure and phase changes are systematically studied by cross-sectional and plane-view transmission electron microscopy. The strain induces grain refinement and it is accompanied by increased strain in the surface layer triggering the onset of highly dense dislocations and dislocation tangles (DTs), formation of the martensite plate via stress-induced martensite (SIM) transformation (B2 to B19′), and dislocation lines (DLs) as well as dense dislocation walls (DDWs) inside the martensite plate leading to the subdivision of the martensite plate. In addition, reverse martensite transformation (B19′ to B2) and amorphization take place concurrently in the surface region, and successive subdivision and amorphization finally result in the formation of well separated nanocrystalline and amorphous phases in the near surface. The average grain size of the nanocrystallites is about 20 nm. Owing to the almost complete reverse martensite transformation as well as thermal stability, the strain-induced nanocrystalline structure has the B2 austenite phase in the surface layer and no transformation occurs.