Iaroslava Shakhova

Development of Ultrafine Grained Austenitic Stainless Steels by Large Strain Deformation and Annealing

The development of ultrafine grained structures in 316L and 304-type austenitic stainless steels subjected to large strain cold working and subsequent annealing and their effect on mechanical properties were studied. The cold rolling was accompanied by a mechanical twinning and a partial martensitic transformation and resulted in the development of elongated austenite/ferrite grains with the transverse size of about 50 nm at a strain of 4. The grain refinement by large strain cold working resulted in an increase of tensile strength above 2000 MPa in the both steels. Annealing at temperatures above 500°C resulted in ferrite-austenite reversion. However, the transverse grain/subgrain size remained on the level of about 100-150 nm after annealing at temperatures up to 700°C.

Effect of SPD Processing Technique on Grain Refinement and Properties of an Austenitic Stainless Steel

The formation of nanocrystalline structures and mechanical properties were studied in a nitrogen-bearing 304-type stainless steel subjected to severe plastic deformation (SPD). The steel samples were processed at ambient temperature using three different methods, i.e., caliber rolling, multidirectional forging and high pressure torsion. All these techniques resulted in pronounced grain refinement. The microstructures consisting of austenite/ferrite crystallites with transverse dimensions of 50 and 30 nm evolved in the rolled and forged samples, respectively. The austenite fractions comprised approximately 0.4. In contrast, the microstructure consisted mainly of austenite with an average grain size of about 25 nm evolved after high pressure torsion. All samples of the stainless steel subjected to severe plastic deformation demonstrated significant strengthening. The ultimate tensile strengths of 2065 MPa and 1950 MPa, were obtained after rolling and high pressure torsion, respectively. The ultimate tensile strength of samples subjected to multidirectional forging was 1540 MPa.

Regularities of Microstructure Evolution and Strengthening Mechanisms of Austenitic Stainless Steels Subjected to Large Strain Cold Working

The deformation microstructures and their effects on mechanical properties of austenitic stainless steels processed by cold rolling at ambient temperature to various total strains were studied. The cold working was accompanied by the development of strain-induced martensitic transformation because of meta-stable austenite at room temperature. The strain-induced martensitic transformation and deformation twinning promoted the grain refinement during cold rolling, leading to nanocrystalline structures consisting of a mixture of austenite and martensite grains with their transverse grain sizes of 50-150 nm containing high dislocation densities. The rolled samples experienced substantial strengthening resulted from high density of strain induced grain/phase boundaries and dislocations. The yield strength of austenitic stainless steels could be increased to 2000 MPa after rolling to total strains of about 4. The martensite and austenite provided almost the same contribution to overall yield strength. The dislocation strengthening was much higher than the grain boundary strengthening at small to moderate strains of about 2, whereas the latter gradually increased approaching the level of dislocation strengthening with increasing the strain.

Submicrocrystalline Structures and Tensile Behaviour of Stainless Steels Subjected to Large Strain Deformation and Subsequent Annealing

Tensile behaviour of two steels with submicrocrystalline structures, i.e. a 304-type austenitic steel and an Fe-27%Cr-9%Ni austenitic-ferritic steel, was studied. The starting materials were subjected to large strain rolling and swaging to a total strain of ∼4 at ambient temperature. The severe deformation resulted in a partial martensitic transformation and the development of highly elongated austenite/ferrite (sub) grains aligned along the deformation axis. In the cold worked state, the transverse grain/subgrain size was about 100 nm in the 304-type steel and about 150 nm in the Fe-27%Cr-9%Ni steel. The grain refinement by severe plastic deformation resulted in increase of ultimate tensile strength to 2000 MPa and 1800 MPa in 304-type and Fe-27%Cr-9%Ni steels, respectively. The phase transformation and recrystallization took place concurrently upon annealing, leading to the development of submicrocrystalline structure consisting of austenite and ferrite grains. No significant softening took place under annealing at temperatures below 600°C. The tensile strength was 1920 MPa in 304-type steel and 1710 MPa in Fe-27%Cr-9%Ni steel after annealing at 500°C for 2 hours.

Kinetics of Submicrocrystalline Structure Formation in a Cu-Cr-Zr Alloy during Large Plastic Deformation

The grain refinement and kinetics of submicrocrystalline structure formation in a Cu-0.3%Cr - 0.5%Zr alloy during large plastic deformation were investigated. The fraction of high-angle boundaries and the fraction of ultrafine grains were used to estimate the kinetics of grain refinement and submicrocrystalline structure evolution during large plastic deformation. The multidirectional forging (MDF), equal channel angular pressing (ECAP), and high pressure torsion (HPT) were used as methods of large plastic deformation. Comparative analysis showed that the grain refinement process occurred faster during HPT and MDF in comparison with ECAP. The fraction of ultrafine grains achieved almost 1 after 3 HPT turns and after MDF to the total strain of 4; while the one reached only 0.29 after 4 ECAP passes. The modified Johnson-Mehl-Avrami-Kolmogorov equation could be applied to determine the kinetics of grain refinement in copper alloy during large plastic deformation as a function of true strain.

Effects of Initial Microstructure and Deformation Method on Grain Refinement in a Cu-Cr-Zr Alloy

The development of submicrocrystalline structure in a Cu-0.3wt.%Cr-0.5wt.%Zr during multidirectional forging (MDF) and equal channel angular pressing (ECAP) was investigated in comparison. A large number of strain-induced subboundaries with low-angle misorientations appeared at early deformation. The subsequent straining led to an increase in the misorientations of these subboundaries, resulting in the formation of submicrocrystalline structure at sufficiently large strains. The process of microstructural evolution can be considered as continuous dynamic recrystallization. MDF provided faster kinetics of new ultrafine grain formation as compared to ECAP. The fraction of ultrafine grains with a size below 2 μm comprised 0.59 or 0.23 after MDF or ECAP to a total strain of 4, respectively. The grain refinement kinetics could be accelerated by the presence of second phase precipitates. The fraction of ultrafine grains after MDF to a strain of 4 achieved 0.36 or 0.59 in the solution treated or aged samples, respectively.

Superplastic Behavior of a Cu-Cr-Zr Alloy Subjected to ECAP

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.

Ultrafine Grain Evolution in a Cu-Cr-Zr Alloy during Warm Multidirectional Forging

The microstructure evolution and the deformation behavior of a Cu-0.3%Cr-0.5%Zr alloy subjected to multidirectional forging at a temperature of 673 K under a strain rate of about 10-3 s-1 were studied. Following a rapid increase in the flow stress during straining to about 1, the strain hardening gradually decreases, leading to a steady-state flow behavior at total strain above 2. The multidirectional forging led to the development of ultrafine grained microstructures with mean grain sizes of 0.9 μm and 0.64 μm in the solution treated and aged samples, respectively. The presence of second phase precipitates promoted the grain refinement. After processing to a total strain of 4, the fractions of ultrafine grains (D < 2 μm) comprised 0.36 and 0.59 in the solution treated and aged samples, respectively.

Effect of multidirectional forging and equal channel angular pressing on ultrafine grain formation in a Cu- Cr-Zr alloy

The microstructure evolution was investigated in a Cu-0.3%Cr-0.5%Zr alloy subjected to large plastic deformation at temperature of 400 °C. Two methods of large plastic deformation, i.e., equal channel angular pressing (ECAP) and multidirectional forging (MDF) were used. The large plastic deformations resulted in the development of new ultrafine grains. The formation of new ultrafine grains occurred as a result of continuous reaction, i.e., progressive increase in the misorientations of deformation subboundaries. The faster kinetics of microstructure evolution was observed during MDF as compared to ECAP. The MDF to a total strain of 4 resulted in the formation of uniform ultrafine grained structure, while ECAP to the same strain led to the heterogeneous microstructure consisting of new ultrafine grains and coarse remnants of original grains. Corresponding area fractions of ultrafine grains comprised 0.23 and 0.59 in the samples subjected to ECAP and MDF, respectively.

Microstructure evolution in a Cu-Cr-Zr alloy during warm intense plastic straining

The effect of equal channel angular pressing at a temperature of 200 °C to a total strain of 12 on microstructure evolution and mechanical properties of a Cu-0.87wt.%Cr- 0.06wt.%Zr was investigated. New ultrafine grains resulted from gradual increase in the misorientations of strain-induced low-angle boundaries with increasing number of passes. Therefore, the development of ultrafine grains is considered as a kind of dynamic recrystallization. The equal channel angular pressing to a total strain of 12 resulted in the formation of almost equiaxed ultrafine grained structure with an average grain size of 0.5 dm and 0.7 dm in the solution treated and aged samples, respectively. At the same time, the fraction of ultrafine grains comprises 0.77 in the solution treated samples and 0.72 in the aged samples. Significant grain refinement led to the remarkable increase of the ultimate tensile strength up to 550 MPa.