Gennady A. Salishchev

Formation of submicrocrystalline structure in TiAl intermetallic compound

The TiAl intermetallic compound was used to illustrate an approach which enables the creation of a submicrocrystalline structure (d≃0.1 μm) in massive semifinished products made of hard-to-deform materials by means of their deformation at elevated temperatures. Tensile mechanical properties of the TiAl intermetallic compound with a mean grain size of 0.4 μm were tested. In this state, the lower temperature limit of superplasticity in TiAl was found to be 800°C. At this temperature and at an initial strain rate of 8.3×10−4s−1, the relative elongation to rupture attains 225%.

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.

Effect of Al on structure and mechanical properties of AlxNbTiVZr (x = 0, 0.5, 1, 1.5) high entropy alloys

Abstract The crystal structure, microstructure, microhardness and compression mechanical properties of AlxNbTiVZr (x = 0, 0.5, 1, 1.5) high entropy alloy were examined. In the as solidified conditions, the alloys consisted from bcc matrix and C14 Laves phase. After homogenisation, the NbTiVZr alloy was bcc solid solution, whereas in Al containing alloys, C14 Laves phase and Zr2Al particles were found in the bcc matrix. Volume fraction of second phase increased with Al concentration. Increase in Al content results in gradual decrease in density of the alloys from 6.49 g cm− 3 of the NbTiVZr to 5.55 g cm− 3 of the Al1.5NbTiVZr alloy. The microhardness of the alloys was higher in the alloys with higher Al content and was generally proportional to the volume fraction of second phase particles. The compression yield strength of the alloys was of 960–1320 MPa, and NbTiVZr alloy was stronger than Al containing alloys. The ductility of the alloys gradually decreased with increase in Al content. The factors determining phase formation in the AlxNbTiVZr alloys and effect of phase composition and chemical composition of individual phases on the mechanical properties are discussed.

Superplasticity of AlCoCrCuFeNi High Entropy Alloy

An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fine equiaxed structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3 s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect ductility: elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.

Laves-phase formation criterion for high-entropy alloys

In this study, we have analysed Laves-phase formation in high-entropy alloys (HEAs). For that purpose, the AlCrxNbTiV and AlxCrNbTiVZr (x = 0, 0.5, 1, 1.5) alloys were produced and examined. It was found that the AlNbTiV and AlCr0.5NbTiV alloys had single-phase body-centred cubic structure, while the other alloys contained Laves phase. Analysis has demonstrated that Laves-phase formation in the produced and in the other HEAs, which are predominantly composed of Al and the elements of 4–6 groups and tend to form body-centred cubic solid solutions, can be predicted by the atomic size mismatch, δr, and the Allen electronegativity difference, ΔχAllen, parameters. It was shown that Laves-phase formation is observed when δr > 5.0% and ΔχAllen > 7.0%.

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.

Characterization of Submicron-Grained Ti-6Al-4V Sheets with Enhanced Superplastic Properties

The microstructure, texture, room temperature and superplastic properties of the Ti-64 sheets with submicrocrystalline (SMC) structure were investigated. For evaluate the superplastic performance of submicron-grained sheets the tensile tests were carried out in the temperature range of 600 800°C. Submicron-grained sheets possessed a strong basal texture and isotropic mechanical properties in the rolling plane, higher yield strength and ultimate tensile strength than that of commercial sheet. At exceptionally low temperature range of 650 750oC the submicron-grained sheets demonstrated enhanced superplastic properties, which are defined as an initial flow stress of 15-30 MPa and about 1000% elongation at the strain rates of 3×10 -4 7×10 -4 /s and temperature of 750°C. At the same time, the flow stress increased considerably due to grain growth during deformation at low strain rates and deformation temperatures above 700oC.

Formation of Grain Boundary Misorientation Spectrum in Alpha-Beta Titanium Alloys with Lamellar Structure under Warm and Hot Working

Microstructure evolution in alpha-beta Ti-64 alloy samples with lamellar structure deformed to a height reduction of 70% at temperatures between 450 and 800°C has been investigated. The deformation led to a distinctly globularized structure of α- and β-phase in the whole temperature interval. The dependence of globular grain size on deformation temperature is of a linear character up to the temperature of warm deformation at which formation of an SMC structure takes place. Continuous recrystallization was observed in the α-and β-phases. Different types of defects responsible for splitting of α-lamellae such as low and high angle boundaries, shear bands and twins were found. An investigation of boundary misorientation spectra in the α-and β-phases deformed to different strains at 550 and 800°C was carried out. Typical boundary misorientation spectra for single phase metals with the same lattice were obtained. The boundary misorientation spectrum depends weakly on strain and deformation temperature. The results of this study show the importance of transformation of semi-coherent interphase boundaries to non-coherent ones for globularization of lamellar microstructures.

Microstructure evolution of commercial-purity titanium during cryorolling

Electron backscatter diffraction and transmission electron microscopy have been used to analyze the microstructural evolution of titanium during rolling at a cryogenic temperature (T = −196°C). It has been found that intensive twinning at the cryogenic deformation temperature accelerates the kinetics of microstructure refinement. The quantitative analysis of microstructure evolution in titanium upon cryorolling has shown that structure evolution is mainly related to mechanical twinning in the initial stages. A substructure is developed and deformation-induced high-angle boundaries are formed in the range of mean and high degrees of deformation. It was established that rolling to the total degree of deformation ɛ = 93 % (e = 2.6) at T = −196°C leads to the formation of a titanium microstructure with a grain/subgrains size of approximately 80 nm. The contribution of mechanical twinning and dislocation gliding in structural transformations in titanium with increasing degree of deformation during cryorolling is discussed.

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.