The severe plastic deformation method known as constrained groove pressing was used to produce ultrafine-grained microstructure in recrystallized aluminium (99.99%) at room temperature. The impact of ...repeated groove pressing, upon microstructure refinement was investigated by transmission electron microscopy of thin foils. Changes in mechanical properties measured by tensile and by hardness tests were related to microstructure development. The formation of banded subgrain microstructure with dislocation cells, and appearance of polygonal subgrains was a common feature observed in deformed plate subjected to the first pass. The substantial impact of strain upon strength increase was observed after the first pressings. The yield stress and ultimate tensile strength reached a maximum after four passes. A loss of ductility was observed in all processed plates. Hardness values measured in different areas of the deformed plates indicated heterogeneous strain distribution even after large degrees of straining.
The main emphasis of this study has been placed on thermomechanical processing simulations performed using press forging. In order to develop a comprehensive understanding of the effect of large ...deformation of bulk specimens, introduced in the austenite recrystallized, nonrecrystallized and intercritical temperature regions, on the microstructure evolution and mechanical properties, the C–Si–Mn transformation induced plasticity (TRIP)-aided steel was used for experimental. The choice of applied strain and thermal parameters had strong impact on austenite transformation kinetics and obtained final multiphase structure, which clearly modified the mechanical properties of TRIP steel. To rationalize the retained austenite volume fraction employing different thermomechanical schedules, the effect of austenite conditioning on ferrite transformation was considered at the first step and the subsequent isothermal bainite transformation treatment was optimized in the second step. The microstructures of treated samples were characterized by optical metallography and scanning electron microscopy. The tensile tests were conducted to characterize the development of TRIP effect with respect to volume fraction of retained austenite in multiphase structure. The volume fraction of retained austenite was measured using X-ray and neutron diffraction methods. The specific design of thermomechanical schedule with varying of strain and temperature values affected multiphase structure development in TRIP bulk steel and yielded in good combination of high tensile strength over 900
MPa and elongation over 20% nearly at all experimental schedules. The complex relationship among the volume fraction of the retained austenite, the morphology and distribution of phases present in the microstructure, and mechanical properties of TRIP steel were revealed. These findings suggested that the intercritical treatment conditions prior austenite transformation appeared to have decisive effect on steel refinement and forming of convenient multiphase structure, which should provide both, the high strength and sufficient ductility of bulky products.
SPD methods are used to convert coarse grain metals and alloys into ultrafine grained (UFG) materials. Obtained UFG materials then possess improved mechanical and physical properties which destine ...them for a wide commercial use. This paper, in one direction, looks into historical development of SPD processes and their effect at obtaining fine crystalline structure, and on the other side also partially focuses on development of UFG structure and its stability in commercial pure aluminium as a function of strain and post-deformation annealing applied.
Advanced High Strength (AHS) steels, among them especially Dual Phase (DP) steels, Transformation Induced Plasticity (TRIP) steels, Complex Phase (CP) steels, Partially Martensite (PM) steels, ...feature promising results in the field. Their extraordinary mechanical properties can be tailored and adjusted by alloying and processing. The introduction of steels with a microstructure consisting at least of two different components has led to the enlargement of the strength level without a deterioration of ductility. Furthermore, the development of ultra fine-grained AHS steels and their service performance are reviewed and new techniques are introduced. Various projects have been devoted to develop new materials for flat and long steel products for structural applications. The main stream line is High Strength, in order to match the weight lightening requirements that concern the whole class of load bearing structures and/or steel components and one of the most investigated topics is grain refinement.
The article focuses on the severe plastic deformation (SPD) of low carbon steel AISI 1010 performed at increased temperature. The grain refinement of ferrite structure is monitored and described with ...respect to different initial steel structure modified by thermal and thermomechanical (TM) treatment (TM) prior severe plastic deformation. The refinement of coarse initial ferrite structure with grain size in range of 30 – 50 gm resulted from solutioning was conducted then in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process and quite uniform ferrite structure with grain size of the order of gm was obtained. The further grain refinement steel structure was then accomplished during warm Equal Channel Angular Pressing (ECAP φ 120°) at 300°C, introducing different strain in range of ϵef 2.6 -4. The change of microstructure in dependence of the effective strain was evaluated by SEM and TEM study of thin foils. The high straining of steel resulted in extensive deformation of ferrite grains and formation of mixture of submicron grains structure in banded deformed structure with dense dislocation network and subgrains. The dynamic polygonization process, due to increased ECAP temperature, modified the submicrocrystalline structure formation. There was only indistinctive difference observed in structure refinement when considering different initial structure of steel. The tensile behaviour was characterized by strength increase followed by softening. None work hardening phenomenon appeared at tensile deformation of deformed bars.
Tensile deformation behavior of two transformation-induced plasticity (TRIP)–assisted multiphase steels with slightly different microstructures due to different thermomechanical treatment conditions ...applied was investigated by
in-situ
neutron diffraction. The steel with lower austenite volume fraction (
f
γ
= 0.04) and higher volume fraction of needlelike bainite in the
α
-matrix exhibits higher yield stress (sample B, 600 MPa) but considerably lower elongation in comparison to the steel with higher austenite volume fraction (
f
γ
= 0.08), granular bainite, and polygonal ferrite matrix (sample A, 500 MPa). The neutron diffraction results have shown that the applied tensile load is redistributed at the yielding point in such a way that the retained austenite bears a significantly larger load than the
α
matrix during the TRIP-assisted steel deformation.
When exposed for long time at elevated temperatures of 430 and 650
°C the nickel base superalloy EI 698 VD can experience a significant decrease in creep resistance. The cause of the creep ...degradation of nickel base superalloy is generally attributed to the microstructural instability at prolonged high temperature exposure. In this article, the creep-life data, generated on long thermally exposed nickel base superalloy EI698 VD were related to the local microstructural changes observed using SEM and TEM analysing techniques. While structure analysis provided supporting evidence concerning the changes associated with grain boundary carbide precipitation, no persuasive evidence of a morphological and/or dimensional gamma prime change was showed. For clarifying of the role of gamma prime precipitates on alloy on creep degradation, the SANS (small angle neutron scattering) experiment was crucial in the characterization of the bulk-averaged gamma prime morphology and its size distribution with respect to the period of thermal exposure.
The aim of this paper is to consider the features of structure evolution during severe plastic deformation (SPD) of steels and its influence on mechanical properties. The investigation have been ...carried out mainly on low carbon steels as well as on austenitic stainless steels after SPD by torsion under high pressure (HPT) and equal channel angular (ECA) pressing. Structure formation dependencies on temperature deformation conditions, strain degree, chemical composition, initial state and pressure are considered. The role of phase transformations for additional grain refinement, namely, martensitic transformation, precipitation of carbide particles during SPD and heating is underlined.
This research provides an attempt to achieve uniform grain structure by recrystallization process in NiMoCr alloy after different hot- and cold-working application. The changes in microstructure ...resulting from thermomechanical processing were investigated by optical microscopy. The results showed that homogeneity of the recrystallized structure is strongly related to the amount of sample reduction imposed during hot working and to the period of annealing time. Furthermore, the results illustrate that the recrystallized structure at the surface was finer, than in the interior of the sample. This is due the specimen surface, which is more strain exposed for applied deformation and driving force for primary recrystalization in sample surface area is therefore higher than in central part of samples. Then, using conventional two-step hot rolling, the expected uniform recrystallized microstructure could not be obtained across the width of slab. An unfavourable contribution at recrystalization process was observed from initial grain size heterogeneity of as-cast ingot structure. However, after additional introduction of cold rolling, which followed hot working the more uniform recrystallized grain structure was achieved after recrystallization annealing.
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