Materials Science Forum Vols. 838-839

Abstract: Recently, friction stir processing (FSP), as an effective tool, has been employed to modify microstructures and mechanical properties of metal sheet. A novel technique of double surface friction stir processing (FSP) was proposed in this study. A 7050-T7451 aluminum alloy sheet of 6.5mm in thickness was processed by FSP with 30% overlap in each side. The position deviation of the stir pin from the upper surface to the bottom surface is 35%. Significant grain refinement with an average grain size of 2.5μm from the as-received 50μm, can be obtained after double-sided FSP. In comparison with the conventional FSP, no HAZ (heat affected zone) or TMAZ (thermal mechanical affected zone) was formed between the adjacent stir zones of the aluminum alloy sheet. At a strain rate of 10^-3 s^-1, a maximum elongation of 29.8% and 327% in transverse direction of FSP passes have been achieved at room temperature and 400°C, respectively.

Abstract: The influence of thermo-mechanical treatment consisting of cold rolling followed by recrystallization annealing on the grain size and mechanical properties of a high-Mn TWIP steel was studied. An Fe-23Mn-0.3C-1.5Al TWIP steel (wt. %) was subjected to extensive cold rolling with a reduction of 80% (true strain of ∼1.6) and then annealed in the temperature interval ranging from 400 to 900 °C during 20 minutes. Recovery processes took place below 500 °C, partial recrystallization was evident at ~550°C and fully recrystallized structure evolved after annealing at 600 °C and higher. The static recovery resulted in a slight decrease in the yield strength from 1400 MPa to 1250 MPa and the ultimate tensile strength from 1540 MPa to 1400 MPa whereas the total elongation of 4% did not changed. The recrystallization development led to a drastic drop of strength and an increase in ductility. The yield strength of 225 MPa, the ultimate tensile strength of 700 MPa and the total elongation of 79% was obtained after annealing at 900 °C. Correspondingly, the grain size increased from 0.2 μm to 6.2 μm with increase in anneal temperature from 550 to 900°C.

Abstract: The formation of submicrocrystalline structure during severe plastic deformation and its effect on mechanical properties of an S304H austenitic stainless steel with chemical composition of Fe – 0.1C – 0.12N – 0.1Si – 0.95Mn – 18.4Cr – 7.85Ni – 3.2Cu – 0.5Nb – 0.01P – 0.006S (all in mass%) were studied. The severe plastic deformation was carried out by high pressure torsion (HPT) at two different temperatures, i.e., room temperature or 400°C. HPT at room temperature or 400°C led to the formation of a fully austenitic submicrocrystalline structure. The grain size and strength of the steels with ultrafine-grained structures produced by cold or warm HPT were almost the same. The ultimate tensile strengths were 1950 MPa and 1828 MPa after HPT at room temperature and 400°C, respectively.

Abstract: A Cu-0.87%Cr-0.06%Zr alloy was subjected to equal channel angular pressing (ECAP) at a temperature of 400 °C up to a total strain of ~ 12. This processing produced ultra-fine grained (UFG) structure with an average grain size of 0.6 μm and an average dislocation density of ~4×10¹⁴ m^-2. Tensile tests were carried out in the temperature interval 450 – 650 °C at strain rates ranging from 2.8´10^-4 to 0.55 s^-1. The alloy exhibits superplastic behavior in the temperature interval 550 – 600 °C at strain rate over 5.5´10^-3 s^-1. The highest elongation-to-failure of ~300% was obtained at a temperature of 575 °C and a strain rate of 2.8´10^-3 s^-1 with the corresponding strain rate sensitivity of 0.32. It was shown the superplastic flow at the optimum conditions leads to limited grain growth in the gauge section. The grain size increases from 0.6 μm to 0.87 μm after testing, while dislocation density decreases insignificantly to ~10¹⁴ m^-2.

Abstract: The microstructure evolution and mechanical properties of 316L and 304L austenitic stainless steels subjected to large strain cold bar rolling and subsequent annealing were studied. The cold working was accompanied by mechanical twinning and strain-induced martensitic transformation. The latter was readily developed in 304L stainless steel. The uniform microstructures consisting of elongated austenite and martensite nanocrystallites evolved at large total strains, resulting in tensile strength above 2000 MPa in the both steels. The subsequent annealing at temperatures above 700°C was accompanied by the martensite-austenite reversion followed by recrystallization, leading to ultrafine grained austenite.

Abstract: The superplastic behavior of a commercial aluminum alloy denoted as 1570 Al with a chemical composition of Al-6%Mg-0.5%Mn-0.2%Sc-0.07%Zr (in wt. %) and ultrafine-grained (UFG) structure produced by equal channel angular pressing at 300°C to a true strain ~12 was studied after final cold or warm rolling. The tensile specimens were machined along rolling direction and pulled up to failure in the temperature range of 250 to 500°C at strain rates ranging from 10^-4 s^-1 to 10^-1 s^-1. The specimens produced by warm or cold rolling exhibit different superplastic behavior. The material subjected to warm rolling exhibits excellent superplastic properties; the highest elongation-to-failure of ~1970% was recorded at a temperature of ~450°C and an initial strain rate of 1.4×10^-1 s^-1. On the other hand, the material subjected to cold rolling demonstrates moderate superplastic properties; the highest elongation-to-failure of ~755% appears at a temperature of ~300°C and an initial strain rate of 1.4×10^-2 s^-1.

Abstract: The ultrafine grained structure of an AA5024 with an average size of ∼0.7 μm was produced by equal-channel angular pressing (ECAP) at 300°C with a total strain of ~12. Superplastic behavior of this alloy was examined in the temperature interval 175 - 300°C at strain rates ranging from 10^-4 to 10^-1 s^-1. The maximum elongation-to-failure of ~1200% with the corresponding strain rate sensitivity coefficient, m, of ∼0.49 was attained at a temperature of 275°C and a strain rate of 5.6×10^–3s^–1. At 175°C (~0.53T_m, where T_m is the melting point), the elongation-to-failure of ~370% with the m value of ~0.3 was found at ε̇=1.4×10^–4 s^–1.

Abstract: The superplastic behavior of an 5024 alloy, subjected to equal-channel angular pressing (ECAP) followed by extensive cold rolling (CR), was studied in the temperature range 250-500°C and at strain rates ranging from 10^–4 to 10^–1 s^–1. The maximum elongation-to failure of ~1440% with a corresponding strain rate sensitive coefficient m of ~0.42 was attained at 450°C and a strain rate of ~1.4×10^–1 s^–1. The relationship between superplastic properties and microstructure of the alloy was discussed.

Abstract: The regularities of static recrystallization in an Fe-0.3C-17Mn-1.5Al TWIP steel subjected to cold rolling and annealing were studied. The cold rolling led to noticeable increase in the dislocation density, extensive mechanical twinning and shear banding. The subsequent annealing resulted in the development of recovered or recrystallized microstructure depending on the rolling reduction and the annealing temperature. An increase in the rolling reduction promoted the recrystallization development, which led to ultrafine-grained microstructure with a grain size below 10 μm. The developed ultrafine-grained steel samples are characterized by beneficial mechanical properties.

Abstract: The effect of hot deformation behavior on austenite grain size refinement of low carbon multi-microalloyed steel was investigated. The morphology of austenite grains was revealed by thermal etching and observed using optical microscope. The results showed that single pass compression can only marginally refine austenite grain size by dynamic recrystallization, even under severe plastic deformation. However, when the specimens were held for a while after hot deformation, the fine austenite grain size can be obtained due to static recrystallization behavior.