Maria A. Murzinova

Application of reversible hydrogen alloying for formation of submicrocrystalline structure in (α + β) titanium alloys

Abstract A study of the effect of hydrogen on submicrocrystalline structure formation in Ti-6Al-3.5Mo-1.7Zr (α + β)-alloy is completed. It is shown that application of reversible hydrogen alloying enables one to obtain a microstructure with a grain size up to 0.04 μm during plastic deformation. Not only does hydrogen alloying decrease deformation temperature and thus promote submicrocrystalline structure, but it also promotes the formation of more fine globular structures at the same strain-rate and temperature conditions.

Superplasticity of hydrogen-containing VT6 titanium alloy with a submicrocrystalline structure

Concentration and temperature regimes of hydrogen treatment have been found, which in combination with severe plastic deformation allow one to obtain a microstructure with a size of constituents of about 0.1 μm in the VT6 titanium alloy. It has been shown that the alloy with such a structure has superplastic properties in a temperature range from 500 to 650°C. Characteristics of superplasticity of a hydrogenated submicrocrystalline VT6 alloy and a basic alloy, in which the submicrocrystalline structure was obtained without hydrogen alloying, have been compared. Particular features of a hydrogenated submicrocrystalline VT6 alloy in a superplastic state are considered.

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.

Influence of Reversible Hydrogen Alloying on Nanostructure Formation in Titanium Alloys Subjected to Severe Plastic Deformation

The method for production of a structure with a grain size of 30-40 nm in two-phase titanium alloys is proposed. It is shown, that the nanostructure can be formed in billets of 150×70×15 mm, and sheets of 250×150×1 mm. The method consists of several steps including hydrogen alloying of the alloy, heat treatment, warm deformation and finally dehydrogenating vacuum annealing. α-, α+β and β-titanium alloys have been investigated. Hydrogen content varied in the range 0.1– 30 at. %. Microstructure was examined using optical, scanning, transmission electron microscopy and X-ray analysis after every step of the treatment. The investigations have shown that a specific character of phase transformations in hydrogenated titanium alloys plays a leading role in formation of nanostructure. The effect of dissolved hydrogen on dynamic recrystallization in α- and β- phases is of a secondary importance. Additional refinement in structure is observed in the deformed alloys after vacuum annealing, if its temperature is less than the temperature of their deformation. The work was focused on the optimization of hydrogen content and deformation conditions with the aim to create the nanostructure in titanium alloys and to enhance their mechanical properties.

The Effect of Hydrogen on Dynamic Recrystallization in α-Titanium Alloys

The influence of hydrogen content on the mechanical properties and size of dynamically recrystallized grains in commercially pure (CP) titanium and Ti-5Al-2.5Sn alloy was investigated. The alloys with hydrogen contents from 0.1 to 5.2 at.% were deformed in the a-field at temperatures of 650°, 750°С with initial strain rates of 5×10-4 s-1. A decrease of the deformation temperature leads to a reduction in grain size and to a stress increase for all compositions. This is in good agreement with the well known relation between the recrystallized grain size (d) and the steady flow stress ss=kd-n. At a given test temperature the steady state flow stress is four times lower and the grain size is about ten times greater in CP titanium in comparison with the Ti-5Al- 2.5Sn alloy. Hydrogen alloying of the Ti-5Al-2.5Sn alloy does not lead to a noticeable change in ss and d. However, an increase in hydrogen content from 0.1 to 5.2 at.% in CP titanium leads not only to a decrease in grain size by a factor of 2 but also to a decrease in flow stress (about 28%). This result is not in agreement with the above relation. This unusual behaviour may be due to two reasons: the influence of hydrogen on grain growth and the hydrogen effect on dynamic strain ageing. Both these effects are stronger in CP titanium.

Mechanical Properties of Ultrafine Grained Two-Phase Titanium Alloy Produced by “abc” Deformation

The mechanical properties of two-phase Ti-6Al-4V titanium alloy with ultrafine grained microstructure were studied in the present work. Bulk ultrafine grained specimens of the alloy were produced by means of warm “abc” deformation. The final structure consisted of α/β particles with a size of 500 nm. Extensive studies of the mechanical properties of this material in comparison with conventionally heat-strengthened condition were conducted. A room-temperature strength and fatigue resistance of the ultrafine grained material was found to be 25-40% higher than that of heat-strengthened alloy. However such ductility related properties as tensile elongation and impact toughness noticeably decreased with decreasing grain size. Efficacy of ductility improvement and the strength/ductility balance optimization were analyzed.

Dependence of the specific energy of the β/α interface in the VT6 titanium alloy on the heating temperature in the interval 600–975°C

The specific energy of interphase boundaries is an important characteristic of multiphase alloys, because it determines in many respects their microstructural stability and properties during processing and exploitation. We analyze variation of the specific energy of the β/α interface in the VT6 titanium alloy at temperatures from 600 to 975°C. Analysis is based on the model of a ledge interphase boundary and the method for computation of its energy developed by van der Merwe and Shiflet [33, 34]. Calculations use the available results of measurements of the lattice parameters of phases in the indicated temperature interval and their chemical composition. In addition, we take into account the experimental data and the results of simulation of the effect of temperature and phase composition on the elastic moduli of the α and β phases in titanium alloys. It is shown that when the temperature decreases from 975 to 600°C, the specific energy of the β/α interface increases from 0.15 to 0.24 J/m2. The main contribution to the interfacial energy (about 85%) comes from edge dislocations accommodating the misfit in direction [0001]α || [110]β. The energy associated with the accommodation of the misfit in directions

Spheroidization of the lamellar microstructure in Ti–6Al–4V alloy during warm deformation and annealing

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