Oleg R. Valiakhmetov

Formation of submicrocrystalline structure in the titanium alloy VT8 and its influence on mechanical properties

It is shown that the formation of microstructure with grain size up to 0.06 μm may occur during the course of plastic deformation of the Ti-6Al-3.2Mo (α+β)-alloy with the initial coarse-grained lamellar structure. The formation of submicrocrystalline structure results from the development of dynamic recrystallization concurrent with the process of spheroidization. The temperature of superplastic deformation significantly decreases while strength characteristics at room temperature sharply increase in the alloy with such a microstructure.

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.

Improved Mechanical Properties of Ti2AlNb-Based Intermetallic Alloys and Composites

Mechanical properties of a Ti2AlNb-based intermetallic alloy both at room and elevated temperatures were considerably improved due to formation of a homogeneous microstructure with the average grain size of about 300 nm. At room temperature, elongations up to 25% were obtained and the ultimate strength reached 1400 MPa. The alloy exhibited superplastic behavior in the temperature range of 850-1000°C. The maximum elongation of 930% and steady state flow stress 50 of about 125 MPa were obtained at 900°C and strain rate of 4.210-3 s-1. The nanostructured material was used for production of intermetallic sheets and multilayer composite plates consisting of alternating layers of orthorhombic intermetallic and commercial high temperature titanium alloy. Ti2AlNb-based sheets and composites exhibited improved mechanical properties.

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.

Production of Ti-6Al-4V Sheets for Low Temperature Superplastic Forming

The availability to produce Ti-6Al-4V sheet material with submicron-grained microstructure for superplastic forming (SPF) has been studied. The laboratory scale sheets with an average grain size of 0.3 μm and the commercial size sheets with an average grain size of 0.65 μm were produced by pack rolling manufacturing technique from the forgings with pre-formed submicrocrystalline (SMC) structure. The sheets possessing isotropic mechanical properties in the sheet plane had higher yield strength, ultimate tensile strength. Over the exceptionally low temperature range of 700-750°C the SMC sheets demonstrated enhanced superplastic properties, namely an initial flow stress of 20-25 MPa and elongation more than 600% at the strain rate of 3×10-4/s. The sheet material with SMC structure was characterized by well formability compared to a conventional sheet under low temperature superplastic conditions.

Formation of Submicrocrystalline Structure in Large Size Billets and Sheets out of Titanium Alloys

The structure evolution and mechanical behavior of titanium and Ti-64 alloy during successive compression of sample along three orthogonal directions or “abc” deformation were studied. It is shown formation of submicrocrystalline structure in the both materials under warm “abc” deformation. In titanium the structure evolution occurs via formation of deformation induced high angle boundaries and interaction between them that leads to formation of submicron-grained structure and strengthening. In two-phase lamellar alloy plates of phases divide into fragments, which afterwards are spheroidizied due to formation of high angle grain boundaries and transformation of semicoherent interphase boundaries to noncoherent ones. In this case superplastic flow and softening is observed. Large-scale billets and sheets with homogeneous submicrocrystalline structure were produced by “abc” deformation. Advantages of their mechanical properties were showed.

Microstructure and properties of a titanium alloy-orthorhombic titanium aluminide layered composite

The microstructure and tensile properties of a layered composite material fabricated by solid-state bonding of its components using pressure welding are studied at room and elevated temperatures. The components are made of a high-temperature VT25U titanium alloy and an intermetallic alloy (O alloy) based on orthorhombic titanium aluminide of the composition Ti-23Al-22.7Nb-1.1V-0.6Zr-0.2Si-0.3C (at %). The study of the microstructure and chemical composition of the composite by scanning electron microscopy and energy dispersive X-ray analysis demonstrates that this method of producing a layered composite provides high-quality poreless bonding of materials of different types. The solid-state bonding zone has an intermediate chemical composition. Mechanical tests demonstrate that the room-temperature strength of the composite is comparable with that of the O alloy and is higher than that of the titanium alloy; as the fraction of the titanium alloy in the composite decreases, this strength increases. The relative elongation of the layered composite is found to be higher than that of the O alloy and lower than that of the titanium alloy. In the temperature range 500–700°C, the strength of the composite material is 25% higher than that of the titanium alloy, and its plasticity is lower than that of the titanium alloy. Our method is shown to be promising for producing layered composite materials that have high mechanical properties over a wide temperature range.