G. A. Salishchev

Formation of a submicrocrystalline structure in TiAl and Ti3Al intermetallics by hot working

A method based on initiation of dynamic recrystallization (DRX) during hot working has been developed to produce a submicrocrystalline (SMC) structure (d < 1 µm) in massive work-pieces of hard-to-deform materials, like titanium aluminides, The method involves continuous grain refinement due to dynamic recrystallization at a decreasing temperature. A microstructure with a grain size of 0.1 to 0.4 µm and no porosity was produced in different TiAl and Ti3Al based alloys. Partial disordering was detected in a Ti3Al alloy with the SMC structure. The grain refinement hardened the intermetallic alloys at room temperature (RT). In a fully ordered Ti3Al alloy RT ductility increased when the grain size decreased, while the ductility of a partially disordered SMC Ti3Al and TiAl alloys was close to zero.

Low-temperature superplasticity of titanium aluminides

Data on superplastic behavior of intermetallic alloys with high ordering energy, such as stoichiometric TiAl and Ti3Al and a number of alloys based on TiAl, with submicron grain size are summarized. The small grain size resulted in a temperature range for superplasticity of 600–900°C that is 200–400°C lower than that for material with micron-sized grains. This paper reports on the effects of grain size, composition and superlattice type on the low-temperature and high temperature superplastic properties of titanium aluminides. Low temperature and high-temperature superplastic properties of the titanium aluminides are compared.

Low-temperature superplasticity of submicrocrystalline Ti-48Al-2Nb-2Cr alloy produced by multiple forging

Gamma titanium aluminides are attractive for elevated temperature applications because of their high specific strength, modulus retention, good oxidation and creep resistance. However they are inherently brittle at temperatures below 600 C due to their strong covalent interatomic bonding, which makes fabrication difficult and has restricted commercial applications. In this research work the authors have studied superplastic (SP) forming of a gamma alloy. Grain refinement is the most common method of decreasing the temperature at which superplasticity is observed, while the second approach is applicable only to gamma alloys containing less than 47 at.% Al. In the present work, a low-temperature superplasticity of a gamma TiAl-based alloy was achieved by producing a submicrocrystalline structure via multistep forging. Mechanical behavior and microstructural evolution of the submicrocrystalline gamma titanium aluminide were studied and possible mechanisms of the low-temperature superplasticity were discussed.

Superplasticity and hot rolling of two-phase intermetallic alloy based on TiAl

The recent investigations of superplasticity (SP) in intermetallic alloys indicate that these materials exhibit lower indices of SP (the relative elongation to rupture) at high enough homologous temperatures and low strain rates compared to conventional alloys. This behavior inhibits application of SP effects in intermetallics. The results of two-phase titanium alloys indicate that the combination of a high stable microstructure with a submicron grain size is necessary to realize the effect of SP at relatively high strain rates. The aim of the present work is to examine the SP behavior of a Ti-46at.%Al intermetallic alloy (TiAl + Ti{sub 3}Al) with micro- and submicron grain sizes and to apply obtained results in hot rolling.

On two stages of brittle-to-ductile transition in TiAl intermetallic

Abstract The present paper has shown that the brittle-to-ductile transition in TiAl intermetallic ought to be considered as a two-stage phenomenon: (1) first noticeable increase in ductility (at T 1 °C) is controlled by thermally activated relaxation processes in grain boundaries. In this case, the brittle fracture type is retained; (2) following an increase in ductility (at T 2 = T 1 + ΔT ) is caused by thermally activated relaxation processes within the grains (in addition to ones in grain boundaries) which lead to the transition from brittle fracture to ductile one.

Effect of Grain Size on Mechanical Properties of Commercially Pure Titanium

The grain-size dependence of certain mechanical properties of commercially pure titanium under deformation at room temperature is examined. A decrease in the grain size is found to provide a continuous improvement in strength, lower work hardening, and nonmonotonic dependence of the length of the uniform deformation stage. Furthermore, localized deformation in the neck and total plasticity before fracture exhibit a low sensitivity to the grain size. A yield tooth and plateau occur in the flow curve as the structure is reduced down to a certain grain size. The grain-size dependence of the mechanical behavior of the material and its relation to the dislocation redistribution are discussed.

Porosity of TiAl intermetallic compound with micro- and submicrocrystalline structure after superplastic deformation

Abstract Comparative investigations of porosity in micro- (d = 8 μm) and submicrocrystalline (d = 0.4 μm) TiAl after superplastic (SP) deformation have been performed. The experiments showed that the transition from microcrystalline (d ~ 1 μm) to submicrocrystalline (d

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 Mechanical Properties of Nanostructured Intermetallic Alloy Based on Ti2AlNb

Homogeneous nanocrystalline structure with the average grain size of about 300 nm was produced in Ti2AlNb-based intermetallic alloy by a thermomechanical processing which included multistep isothermal forging at temperatures below the β-transus and intermediate annealings. Nanostructured material possessed excellent mechanical properties. 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 σ50 of about 125 MPa were obtained at 900°C and strain rate of 4.2×10-3 s-1. The rolling temperatures of nanostructured alloy were defined from analysis of its mechanical behavior at a typical rolling strain rate of about 10-1 s-1 and intermetallic sheets with improved mechanical properties were produced.

Superplastic properties of γ+α2 titanium aluminide alloy Ti-43Al-(Nb, Mo, B) in cast + post-solidification heat treated condition

A novel approach to fabrication of globularized fine-grained structure in γ+α2 titanium aluminide alloys has been proposed. The approach included the use of a specially designed alloy Ti-43Al-X(Nb,Mo,B) and heat treatment. It was found that the ingot structure of the alloy might be partially globularized on a scale of bulk material using only globularization anneal excluding any hot working procedure. The microstructure and tensile mechanical properties of the alloy in the cast + heat treated condition were investigated. The tensile mechanical tests were performed in air in the temperature range of T=900-1130°C at an initial strain rate of ε′=1.7×10-4 s-1. High elongation (δ=160-230%) and low flow stresses (σ=36-100 MPa) typical of superplastic behavior were measured at T=1050-1130°C. It was demonstrated that the sheet material produced by spark cutting of the cast + heat treated alloy might be successfully hot formed.