Ilya Nikulin

Low-Cycle Fatigue Behavior and Microstructural Evolution of the Fe–30Mn–4Si–2Al Alloy

Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δε=2% was found in the Fe–30Mn–4Si–2Al high-Mn alloy, as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys with fatigue life of 2×103 cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe–30Mn–4Si–2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe–30Mn–4Si–2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced ε-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation ε-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe–30Mn–4Si–2Al alloy.

Mechanical Properties of an Al-5.4%Mg-0.5%Mn-0.1%Zr Alloy Subjected to ECAP and Rolling

Superplasticity and microstructural evolution of a commercial Al-5.4%Mg-0.5%Mn-0.1%Zr alloy subjected to severe plastic deformation through equal-channel angular pressing (ECAP) and subsequent rolling was studied in tension at strain rates ranging from 1.4×10-4 to 5.6×10-2 s-1 in the temperature interval 400-550°C. The alloy had an unrecrystallized microstructure with an average crystallite size less than 5 m. The alloy exhibited the yield strength of ~370 MPa, ultimate strength of ~450 MPa and elongation-to-failure of ~15% at ambient temperature. In spite of small crystallite size the alloy shows moderate superplastic properties. The highest elongation-to-failures of ~450% appeared at a temperature of ~500°C and an initial strain rate of ~1.4×10-3 s-1, where the strain rate sensitivity coefficient, m, is of about 0.57. The relationship between superplastic ductilities and microstructure is discussed.

Effect of Microstructure on the Low-Cycle Fatigue Properties of a Fe–15Mn–10Cr–8Ni–4Si Austenitic Alloy

In the present study, the effect of the initial austenitic structure on the low-cycle fatigue (LCF) properties and e-martensitic transformation (e-MT) was studied in the Fe–15Mn–10Cr–8Ni–4Si seismic damping alloy under an axial strain control mode with total strain amplitude, Δet/2, of 0.01. The microstructures with various grain size and texture conditions were produced by warm rolling followed by annealing at temperatures that ranged from 600 to 900 °C. It was found that the increase in the austenitic grain size observed in the studied temperature interval generally does not affect martensitic transformation and the LCF resistance of the studied alloy. However, strong texture and substructure remaining in the alloy after low temperature annealing at T ≤ 700 °C inhibit strain-induced phase transformation and reduce fatigue resistance of the studied alloy. As a result, the alloy annealed at T ≥ 800 °C shows higher fatigue resistance than that one annealed at T ≤ 700 °C.

Low-Cycle Fatigue Properties of the Fe-30Mn-(6-x)Si-xAl Trip/Twip Alloys

Low-cycle fatigue (LCF) properties of the Fe-30Mn-(6-x)Si-xAl alloys ascribed to TRIP/TWIP high-Mn austenitic alloys were examined in relation with tensile properties. It was found that fatigue resistance of these alloys correlates with tensile yield strength and strain hardening rate, while tensile ductility are not affects LCF properties of examined alloys. At total fatigue strain range of 2% the highest fatigue life of about 8000 cycles was found in the Fe-30Mn-4Si-2Al alloy with higher yield strength (YS) and lower hardening rate. Decreasing of YS and increasing of hardening rate lead to considerable decrease of fatigue life of examined alloys. In the presentation we will discuss the relation between LCF properties and deformation microstructure.

Effect of Static Aging on Mechanical Properties of an 18Cr-8Ni-W-Nb-V-N Stainless Steel

The interrelations between microstructure, precipitation and mechanical properties of the 18Cr-8Ni-W-Nb-V-N austenitic stainless steel were examined under long-term aging at 650°C. It was shown that aging leads to decreasing strength characteristics with increasing aging time despite the fact that hardness tends to increase. In none-aged condition the present steel exhibits superior impact toughness of about 255 J/cm-2. This values decreases gradually at the early stage of the aging. After 1000 hours exposure the impact toughness is 195 J/cm-2 and decreases sharply to 135 J/cm-2 at 3000 hours. However, an evidence for ductile fracture was found even after long-term aging. Degradation in impact toughness and mechanical properties with aging is discussed in relation to microstructure evolution, precipitations of the secondary phase and fracture mechanisms.

The Effect of Temperature on Microstructure Evolution in a 7055 Aluminum Alloy Subjected to ECAP

Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular pressing (ECAP) was examined in the temperature interval 250375°C. It was shown that the formation of recrystallized grains occurs through continuous dynamic recrystallization (CDRX). At 250°C, a low rate of dynamic recovery and high volume fraction of second phase particles provide the rapid formation of stable three-dimensional arrays of low-angle boundaries and their gradual transformation into high-angle boundaries. Increasing temperature leads an increase in the average crystallite size produced by ECAP from 0.7 μm at 250°C to 1.3 μm at 375°C. The effect of temperature on CDRX kinetic is discussed.

Low-Temperature Superplasticity in an Al–Mg–Mn Alloy Subjected to ECAP and Subsequent Isothermal Rolling

An ultra-fine grained structure with an average size of ~ 1 μm was produced in a commercial Al–5.4%Mg–0.5%Mn–0.1%Zr–0.12%Si–0.014%Fe alloy by hot equal-channel angular pressing (ECAP) followed by isothermal rolling (IR). It was found that in the strain rate interval from 5.6×10-4 to 2.8×10-2 s-1 the alloy exhibits a low-temperature superplasticity with elongation-to-failure exceeding 400% and the strain rate sensitivity coefficient of ~0.3. The highest elongation-to-failure of ~ 620% appeared at a temperature of ~ 275°C and an initial strain rate of ~ 5.6×10-3 s-1. The relationship between superplastic properties and microstructural evolution of the examined alloy is discussed.

Superplasticity of a Sc and Zr Modified Al-6%Cu Alloy Subjected to ECAE

Superplasticity in an Al-6%Cu-0.45%Mg-0.4%Mn-0.16%Sc-0.12%Zr alloy subjected to intense plastic straining through equal-channel angular extrusion (ECAE) was studied in tension at strain rates ranging from 5.6×10-4 to 5.6×10-3 s-1 in the temperature interval 350-450°C. The alloy had a non-uniform microstructure with an average crystallite size of 1.2 m. The volume fraction of high-angle grain boundaries was about 57%. In spite of small crystallite size the alloy shows moderate superplastic properties. The highest elongation-to-failures of 320% appeared at a temperature of ~425°C and an initial strain rate of ~1.410-3 s-1, where the strain rate sensitivity coefficient, m, was about 0.33. The relationship between superplastic ductilities and microstructure stability is analyzed.

Development of Ultra-Fine Grained Structure in an Al-5.4%Mg-0.5%Mn Alloy Processed by ECAP

Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of ~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 m. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.

Synergetic Effect of ECAP and Friction Stir Welding on Microstructure and Mechanical Properties of Aluminium Sheets

Friction stir welding (FSW) was used to join the submicrocrystalline (SMC) grained Al-Cu-Mg-Ag sheets produced by equal channel angular pressing (ECAP) followed by hot rolling (HR). The effect of SPD and FSW on the microstructure and mechanical properties in the zone of base metal, as well as in the stirred zone (SZ) were examined. In addition, effect of standard heat treatment on microstructure and mechanical properties in these zones was considered. A refined microstructure with an average grain size of ~ 0.6 m and a portion of high-angle grain boundaries (HAGBs) of ~0.67 was produced in sheets by ECAP followed by HR at 250°C. The microcrystalline grained structure with average grain size of ~2.3 mm was found in joint weld. The moderate mechanical properties were revealed in SMC sheets and joint welds. Heat treatment considerably increases strength of the base metal as well as the joint welds. The higher strength of the alloy after T6 temper is attributed to the dense precipitations of  dispersoids having plate-like shape which are uniformly distributed within aluminum matrix. It was observed that FSW can produce full strength weld both in the tempered and in the un-tempered conditions.