Papers by Author: Rustam Kaibyshev

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: The liquid hot isostatic pressure in superplastic conditions was applied to eliminate macroporosity in a die cast Al-12%Si eutectic alloy. The removing of macroporosity provides the yield stress increase of 56%, the ultimate tensile strength increase of 46%, the total elongation increase of 21%, the fracture toughness increase of 340% and the fatigue strength increase of 58%. Remarkable improvement of mechanical properties is caused by suppression of crack initiation on internal coarse pores.

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 (UFG) structure with an average size of ∼0.8 μm was produced in an Al-Li-Mg-Sc alloy by equal-channel angular extrusion (ECAE) at 325oC with a total strain of ~16. Superplastic behavior was examined in the temperature range 150-250oC at strain rates ranging from 10^-5 to 10^-2 s^-1. A maximum elongation-to-failure of 440% was recorded at 175oC (~0.5 T_m, where T_m is the melting point) and a strain rate of 2.8×10^-5 s^-1 with the corresponded strain rate sensitivity coefficient of 0.32. Mechanisms of low-temperature superplasticity (LTSP) and high-strain-rate superplasticity (HTSP) are essentially the same. The difference between superplastic behaviors at low and high temperatures is attributed to applied stress.

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 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: An AA2139 alloy belonging to Al-Cu-Mg-Ag system was subjected to thermomechanical processing (TMP) included solution treatment at 525°C for 1 h, quenching in water, over-aging at 380°C for 3 h followed by equal-channel angular pressing (ECAP) at 250°C to a total strain of ~12 via route B_C. This TMP produced a partially recrystallized structure with fine crystallites having an average size of ~0.9 μm and a fraction of high-angle boundaries of ~56%. Tensile test was performed in the temperature interval 300-500°C at initial strain rates ranging from ~1.8×10^-4 to ~1.7×10^-1 s^-1. It was shown that the highest elongation-to-fracture of ~660% appears at a temperature of 400°C and an initial strain rate of 1.4×10^-3 corresponding with the coefficient of sensitivity coefficient, m, of 0.34.

Abstract: The recent studies on grain refinement in austenitic stainless steels during large strain deformations are critically reviewed. The paper is focused on the mechanism of structural changes that is responsible for the development of submicrocrystalline structures that can be interpreted as continuous dynamic recrystallization developing under conditions of warm working. The final grain size that is attainable by large strain warm working can be expressed by a power law function of temperature compensated strain rate with an exponent of about -0.15. The development of submicrocrystalline structures is assisted by the deformation microbanding and dynamic recovery, which are characterized by opposite temperature dependencies. The grain refinement kinetics, therefore, are characterized by a weak temperature dependence for a wide range of warm working conditions.

Abstract: A commercial AA2519 alloy with a chemical composition of Al-5.64Cu-0.33Mn-0.23Mg-0.15Zr (in wt. %) was subjected to two-step thermomechanical processing (TMP) providing the formation of fully recrystallized structure with an average grain size of ~7 mm in 3 mm thin sheets. Superplastic tensile tests were performed in the temperature interval 450-535°C and initial strain rates ranging from ~2.8 x 10^-4 to ~6.0 x 10^-1 s^-1. The highest elongation-to-failure of ~750% appears at a temperature of ~525°C and an initial strain rate of ~1.4 × 10^-4 s^-1 with the corresponding strain rate sensitivity coefficient of ~0.46.

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.