O.R. Valiakhmetov’s research while affiliated with Institute for Metals Superplasticity Problems of Russian Academy of Sciences and other places

An alloy design approach for titanium alloys is presented. New alloys are isolated, manufactured and tested with an emphasis on the superplastic response. The superplastic effect is found to be optimal between 650 to 750∘C at strain rates between 8.3×10⁻² and 8.3×10⁻³/s – this is a substantial improvement in terms of temperature and deformation rates over traditional titanium alloys such as Ti–6Al–4V. Elongations approaching ∼2000% are demonstrated. Electron backscatter diffraction studies confirm a randomisation of texture and absence of significant intragranular dislocation density, confirming superplasticity and thus grain-boundary sliding as the overarching deformation mechanism. At strain rates faster than 0.01/s, the alloys exhibit large elongations (∼200–500%) but softening is evident and lower ductility results. Our results reveal that the physical factors controlling the alloy composition/property/manufacturing interrelationship are understood and quantified. Physically-based constitutive equations are presented and used to demonstrate the practical advantages of the designed alloys.

Optimization of the chemical composition of Ti2AlNb-based alloy resulted in development of high-strength Ti-20.3Al-22.1Nb-1.2Zr-1.3V-0.9Mo-0.3Si (at.%) intermetallic with rather low density ρ≈5.1 g/cm3. Mechanical properties of the alloy were considerably improved due to formation of the homogeneous ultrafine-grained structure. The ultrafine-grained material exhibited both high strength and ductility at room temperature (δUTS=1400 MPa; σ=25%) and superplastic behavior in the temperature range of 850-1000°C (σmax=930% at T=900°C). The metal-intermetallic composites consisting of alternating layers of the orthorhombic intermetallic alloy and the commercial high-temperature titanium alloy were produced. The 3-layered composite had high strength and reasonable ductility both at room and elevated temperatures: δUTS=1235 MPa and σ=4% at T=20°C; δUTS=875 MPa and σ=21% at T=600°C.

The paper describes the results of comprehensive studies of the microstructure, mechanical and technological properties (formability and weldability in the solid state) of the titanium sheet alloy VT6 (Ti-6Al-4V) with improved superplastic properties produced by PSC «VSMPO-AVISMA». In the first part the initial microstructure of the alloy and its mechanical properties over a wide temperature range from 650 to 900°C at strain rates of 4×10–4, 4×10–3 and 4×10–2 s–1 have been investigated. It is found that the initial microstructure of the sheet is uniform and ultrafine grained with an average size of 1.2 μm. The initial grain size of the microstructure is varied from 0.1 to 4 microns. Most of the grains have a size from 0.5 to 1.5 microns. Mechanical tensile tests revealed that the ultrafine grained titanium alloy VT6 possesses higher superplastic characteristics as compared with those of the standard VT6 sheet. The microstructure of deformed samples and data on changes in the grain size depending on the temperature and strain rate are investigated and represented. The comparison of the superplastic characteristics of the samples cut along and across to the rolling direction indicates the absence of anisotropy. The studied alloy exhibits the best superplastic properties at temperatures ranging from 700 to 850°C and strain rates of 4×10–4 – 4×10–3 s⁻¹, which corresponds to the manifestation of the low-temperature superplasticity. Superplastic elongations are from 650 to 1075 %. That allows us to recommend the produced sheet alloy for development of low temperature processing methods based on superplastic forming and diffusion welding to manufacture the parts for aerospace industry [in Russian].

Summarizing the results of the studies performed at IMSP RAS, the principles of processing of a uniform ultrafine-grained (UFG) structure in large-section billets by means of the technique of multiple isothermal forging are formulated. These principles imply a uniform introduction of plastic strain energy into large-section billets, minimization of contact friction between the die-set and the billets, and deformation processing in the temperature and strain-rate conditions that are most favourable for the development of dynamic recrystallization.

Laminated composite materials consisting of an orthorhombic Ti2AlNb based alloy and an (a+b) titanium alloy have been fabricated at a laboratory scale using a two-step process involving diffusion bonding and hot-pack rolling. The feasibility of fabrication of two- and three-layered materials with high quality bonding between layers has been demonstrated. Preliminary assessment of the tensile mechanical properties of the obtained composite materials at room and elevated temperatures showed that, on the whole, they were superior to those of the titanium alloy and insignificantly inferior to the orthorhombic alloy.

The durability of chemically deposited Ni based coatings was studied under heat treatment and hot deformation of Zr alloy ingots (Э125) over the temperature range of 700–1000°C. It was shown that a single-layer Ni coating retains its satisfactory protective-lubricating properties up to 800°C. Double-layer Ni-glass lubricant coatings efficiently protect Zr alloy against oxidation throughout the temperature range; the low friction factor of the coating allowed hot working to be carried out under low load values.

Commercially pure (99.9%) copper was severe plastically deformed by equal-channel angular pressing (ECAP) following route-Bc in three different processing regimes in order to obtain ultrafine-grained (UFG) microstructures leading to improved mechanical properties. In regime-1, the billets were processed at room temperature up to eight passes. The billets were processed at 200°C up to eight passes in regime-2. Regime-3 is a hybrid regime by which the billets were deformed at 200°C up to four passes first, and these billets were then processed at room temperature for one, two and four passes. In all regimes, the ECAP processing results in a refinement of the conventionally grained (CG) initial microstructure of copper down to sub-micron level leading to a great improvement in the strength as compared to CG copper. Among the regimes applied, regime-3 was found to be the best regime for improvement in strength along with adequate ductility. The samples showed more than eight times increases in yield strength after processing in regime-3 for 4 + 4 passes, from 47 MPa for CG copper to about 408 MPa for the UFG sample. Such improvement in strength was accompanied by a 16.9% total elongation and 6% uniform elongation. The processing in regime-2 resulted in the best elongation to failure of about 22% after eight passes, but it gave the lowest strength values among others.

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.