M. Klimova

Microstructure and Mechanical Properties Evolution of the Al, C-Containing CoCrFeNiMn-Type High-Entropy Alloy during Cold Rolling

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20–23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (<40%), while grain (twin) boundary strengthening prevailed at higher strains.

Twinning induced nanostructure formation during cryo-deformation

In the present work the influence of cryo-rolling to a true strain e=2.66 on twinning and formation of ultrafine-grained/nanostructure in commercial-purity titanium and Fe-0.3C- 23Mn-1.5Al TWIP steel was quantified using scanning and transmission electron microscopy. Different influence of twinning on the kinetics of microstructure refinement and nanostructure formation in titanium and steel was revealed. In titanium twin boundaries during deformation transform into arbitrary high-angle grain boundaries thereby promoting the microstructure refinement to a grain/subgrain size of 80 nm. In steel twinning has less pronounced influence on the microstructure refinement. However, very fine grains/subgrains with the size of 30-50 nm was observed in the microstructure after rolling at 77K to a true thickness strain of 2.66.

Twinning-Induced Formation of Nanostructure in Commercial-Purity Titanium

In the present work the influence of various parameters on formation of nanoor ultrafine-grained structure in commercial-purity titanium during large deformation was quantified using TEM and EBSD. The beneficial effect of twinning on the kinetics of microstructure refinement in titanium was revealed. It was shown that deformation twinning (and therefore nanostructure formation) can be intensified via decrease in temperature, increase in the initial grain size and decrease in the impurities content. The minimum grain size at which twinning can still operate in commercial-purity titanium was determined to be ~1μm. It was shown that rolling to a thickness strain of 93% at-196°C resulted in the formation of a microstructure with a grain/subgrain size ~80 nm.

Microstructure Refinement in the CoCrFeNiMn High Entropy Alloy under Plastic Straining

The effect of plastic deformation under various conditions of the equiatomic CoCrFeNiMn alloy with single face-centered cubic phase structure was studied. The alloy was rolled at room and cryogenic temperatures, and uniaxially compressed at room temperature and temperatures of 600-1100°C with different height reductions. In addition, multiaxial forging at 900-1000°C was performed. Scanning and transmission electron microscopy, including EBSD analysis, was widely employed to characterize microstructure of the deformed alloy. At room and cryogenic temperatures, mechanical twinning and shear banding plays play dominant role in microstructure evolution. Extensive refinement of the microstructure occurs as the result of rolling with reduction of 80%. During deformation at 600-1100°C, discontinuous dynamic recrystallization takes place. The recrystallized grains size and their volume fraction increases with increase of deformation temperature. Multiaxial forging at 900-1000°C was used to produce fully recrystallized structure with average grain size of 6.7 μm. The alloy in the initial condition had low yield strength of 160 Mpa but remarkable tensile ductility of 68%. Rolling substantial increases yield strength to 1120-1290 MPa, but results in loss of ductility. After multiaxial forging the alloy has balanced combination of properties – yield strength of 280 MPa and elongation of 56%.