S.V. Zherebtsov

Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read–Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T = 800°C.

Mechanical Properties of Ti–6Al–4V Titanium Alloy with Submicrocrystalline Structure Produced by Multiaxial Forging

A comparative investigation of mechanical properties of Ti–6Al–4V titanium alloy with coarse-grained (400 m), microcrystalline (10 µm) and submicrocrystalline (0.4 µm) structures in the temperature range 20–500°C has been carried out. The submicrocrystalline structure was obtained by multiaxial isothermal forging. The alloys with the coarse-grained and microcrystalline structures were used in a heat-strengthened condition. The microstructure refinement increases both the strength and fatigue limit of the alloy at room temperature by about 20%. The strength of the submicrocrystalline alloy is higher than that of the microcrystalline alloy in the range 20 - 400°C. Long-term strength of the submicrocrystalline specimens below 300°C is also considerably higher than that of the other conditions. However, the creep strength of the submicrocrystalline alloy is slightly lower than that of the heat-strengthened microcrystalline alloy already at 250°C. The impact toughness in submicrocrystalline state is lower especially in the samples with introduced cracks. Additional surface modification of submicrocrystalline alloy by ion implantation gives a considerable increase in the fatigue limit. Advantages of practical application of submicrocrystalline titanium alloys produced by multiaxial isothermal forging have been evaluated.

Low Temperature Superplasticity of Ti-6Al-4V Processed by Warm Multidirectional Forging (Preprint)

Multidirectional forging has been developed to produce an ultrafine-grain (UFG) microstructure in the two-phase titanium alloy Ti-6Al-4V. A microstructure with a grain size of 135 nm was attained, enabling low-temperature superplasticity (LTSP) at 550°C. A total elongation of 1000% and strain-rate-sensitivity coefficient m=0.47 were obtained at the optimal strain rate of 2×10-4 s-1. Important features of the microstructure and superplastic behavior of the alloy are summarized in the present work. It is shown that microstructure evolution during low-temperature deformation plays a key role in superplastic flow behavior.

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.

Grain Refinement Kinetics in a Low Alloyed Cu-Cr-Zr Alloy Subjected to Large Strain Deformation

This paper investigates the microstructural evolution and grain refinement kinetics of a solution-treated Cu–0.1Cr–0.06Zr alloy during equal channel angular pressing (ECAP) at a temperature of 673 K via route BC. The microstructural change during plastic deformation was accompanied by the formation of the microband and an increase in the misorientations of strain-induced subboundaries. We argue that continuous dynamic recrystallization refined the initially coarse grains, and discuss the dynamic recrystallization kinetics in terms of grain/subgrain boundary triple junction evolution. A modified Johnson–Mehl–Avrami–Kolmogorov relationship with a strain exponent of about 1.49 is used to express the strain dependence of the triple junctions of high-angle boundaries. Severe plastic deformation by ECAP led to substantial strengthening of the Cu–0.1Cr–0.06Zr alloy. The yield strength increased from 60 MPa in the initial state to 445 MPa after a total strain level of 12.

Mechanical Behaviour and Microstructure Evolution of Severely Deformed Two-Phase Titanium Alloys

Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V (VT6) and near-beta Ti-5Al-5Mo-5V-1Cr-1Fe (VT22) titanium alloys during uniaxial compression at 600°C to a high strain of 70% was studied. The plastic-flow response for both alloys is characterized by successive stages of strain hardening, flow softening, and steady-state flow. During compression the lamellae spheroidized to produce a partially globular microstructure. Globularization in VT6 is associated with the loss of the initial Burgers-type coherency between the alpha and beta phases and the subsequent individual deformation of each phase. The misorientations of boundaries increase to the high-angle range by means of the accumulation of lattice dislocations. In VT22 alloy the alpha phase evolves similar to that in VT6 alloy, while in the beta phase mainly low-angle boundaries are observed even after 70 pct. reduction.

Effect of Multiaxial Forging on Structure Evolution and Mechanical Properties of Oxygen Free Copper

Evolution of micro- and macrostructure and mechanical properties of oxygen-free copper after MAF at room temperature was studied. MAF included sequential upsetting and drawing with total cycles number equal to 20 and maximum strain ≈50. MAF causes the formation of homogenous UFG structure with a grain/subgrain size of 0.3 m and fraction of high angle boundaries 50%, but macrostructure is heterogeneous. Rough shear macrobands areas of different orientation are observed. MAF results in significant strengthening from 280 MPa to 445 MPa, but samples remain very ductile even after large strains. Mechanisms of UFG structure formations during MAF are discussed.

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.

Production, Properties and Application of Ultrafine-Grained Titanium Alloys

Some features ofstructure, way of production and mechanical properties of titanium andtwo-phase titanium alloys with ultrafine-grained (UFG) microstructure werediscussed. Various methods of UFG structure formation was considered andoptimal conditions for attaining of a homogeneous microstructure with thesmallest grain size were determined. The kinetics of microstructure evolutionin titanium and two-phase titanium alloys during large (severe) plasticdeformation in a wide temperature interval and the mechanisms of ultrafinegrain formation were analyzed. The influence of grain size on static mechanicalproperties was determined. Significantly reduced temperatures of superplasticforming of the UFG conditions were shown. Some examples of practical applicationsof the UFG two-phase titanium alloys were discussed.

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.