T. G. Khismatullin

Microstructure and mechanical properties of the intermetallic alloy Ti-45Al-6(Nb, Mo)-0.2B

The microstructure and tensile properties of the intermetallic γ-TiAl + α2-Ti3Al alloy Ti-45Al-6(Nb, Mo)-0.2B are studied after various heat and thermomechanical treatments. Its cast state is found to be characterized by a homogeneous, relatively fine-grained structure in the cases of both a model 30-g ingot and a bulk laboratory ∅120 × 180-mm ingot. The cast material subjected to heat treatment with furnace cooling and aging at T = 1100°C exhibits strength properties at T = 20–900°C that are relatively high for cast γ + α2 alloys. Due to the rational choice of the alloy, its thermomechanical treatment is facilitated, a fine-grained structure can easily be formed upon hot deformation, and a superplastic state with elongations in the test-temperature range T = 900–1000°C that are very large for γ + α2 alloys is reached. However, the high contents of niobium and molybdenum in the Ti-45Al-6(Nb, Mo)-0.2B alloy hinder the formation of an equilibrium lamellar structure upon heat treatment, and an increase in the aging temperature to T = 1100°C leads to the development of the α2 → ß(B2) phase transformation, which makes it impossible to reach a high level of the mechanical properties in the temperature range of the potential application of γ + α2 alloys. Our study has revealed that the compositions of the γ + α2 alloys need further optimization of the way of refining the niobium and molybdenum contents.

New approaches to designing alloys based on γ-TiAl + α2-Ti3Al phases

Based on the investigations of the microstructure of ingots depending on the content of aluminum and alloying additives and cooling rate, a new concept of alloying of γ-TiAl + α2-Ti3Al alloys has been developed, which is directed on the production of a chemically uniform cast material with a fine-grained structure. The results obtained open new opportunities in the designing of γ+α2 alloys with an improved processing plasticity.

Microstructure and processing ability of β-solidifying TNM-based γ-TiAl alloys

Microstructure and hot workability have been considered for a number of -TiAl alloys including -solidifying TNM alloys. All TNM alloys under study showed improved hot workability in cast condition. As was shown for the Ti-45Al-5Nb-1Mo-0.2B alloy, a critical issue of TNM alloys is room temperature ductility in the conditions with lamellar structure.

Mechanical properties of the cast intermetallic alloy Ti-43Al-7(Nb,Mo)-0.2B (at %) after heat treatment

AbstractA new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α2-Ti3Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate

Grain refinement in small-size ingots of intermetallic alloys Ti-46Al-8Nb and Ti-46Al-8Ta with the use of a massive transformation

\dot \varepsilon

Superplastic behavior of Ti-43Al-7(Nb, Mo)-0.2B alloy in the cast + heat-treated condition

= 1.7 × 10−4 K−1. Under these temperature-strain-rate conditions, relative elongations such as δ = 160–230% and low flow stresses such as σ = 36–100 MPa characteristic of superplastic flow have been obtained. It has been shown for the first time for the intermetallic γ-TiAl + ga2-Ti3Al alloy that a sheet semifinished product cut out from an ingot subjected only to heat treatment can have plasticity acceptable for press forming.

Grain Refinement in Cast Ti‐46Al‐8Nb AND Ti‐46Al‐8Ta Alloys via Massive Transformation

This work is devoted to the estimation of the technological plasticity of binary and alloyed γ titanium aluminides by conducting compression tests at T = 1000°C. The technological plasticity was shown to grow with decreasing size of grains and grain colonies and with increasing amount of β-stabilizing elements in the alloys. The best technological properties are characteristic of the alloys that solidify completely through the β phase, containing β-stabilizing additions of niobium and molybdenum and microadditions of boron. These alloys are characterized by a small size of crystallites in the cast state; the use of special heat treatments makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase in them. For the most technological alloy, tensile tests in the cast state have been carried out. In the temperature range of T = 900–1100°C, superplastic elongations have been achieved.

Refining of the microstructure of cast intermetallic alloy Ti-43% Al-X (Nb, Mo, B) with the help of heat treatment

An estimation of the efficiency of the refinement of colonies/grains of the as-cast structure of ingots of size Ø13 × 150 mm in the Ti-46Al-8Nb and Ti-46Al-8Ta (at %) alloys with the aid of a heat treatment including massive transformation has been performed. It is shown that the initial coarse-grained as-cast structure of these alloys can be refined using massive transformation without the use of a labor-consuming procedure of hot working. The method proposed is efficient for the alloys containing alloying elements that retard diffusion in the alloys, which makes it possible to obtain massive γm phase at moderate cooling rates. It is shown by X-ray diffraction that the massive γm phase is characterized by a reduced parameter of tetragonality, which can be restored via subsequent high-temperature aging.