R. M. Imayev

Formation of submicrocrystalline structure in TiAl intermetallic compound

The TiAl intermetallic compound was used to illustrate an approach which enables the creation of a submicrocrystalline structure (d≃0.1 μm) in massive semifinished products made of hard-to-deform materials by means of their deformation at elevated temperatures. Tensile mechanical properties of the TiAl intermetallic compound with a mean grain size of 0.4 μm were tested. In this state, the lower temperature limit of superplasticity in TiAl was found to be 800°C. At this temperature and at an initial strain rate of 8.3×10−4s−1, the relative elongation to rupture attains 225%.

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.

Principles of Fabrication of Bulk Ultrafine-Grained and Nanostructured Materials by Multiple Isothermal Forging

On the basis of generalization of research results obtained at the Institute for Metals Superplasticity Problems, principles of fabrication of bulk ultrafine-grained and nanostructured materials by multiple isothermal forging are formulated. Multiple isothermal forging is shown to be a universal high-performance deformation technique for the grain refinement in metals and alloys maximally exploiting the potential of dynamic recrystallization.

Mechanical properties of thermomechanically treated Ti-rich γ+α2 titanium aluminide alloys

Abstract Tensile mechanical properties of Ti-rich γ+α 2 titanium aluminide alloys, Ti-45.2Al-3.5(Nb,Cr,B) and Ti-44.2Al-3(Nb,Cr,B), have been investigated in thermomechanically treated fine-grained and lamellar conditions. These alloys exhibited excellent superplastic properties in the fine-grained condition, high strength coupled with high ductility at elevated temperatures, reasonable ductility and fracture toughness at room temperature in the lamellar condition.

Processing and Deformation Behavior of Gamma TiAl Alloys with Fine-Grained Equiaxed Microstructures

This paper describes how a submicrocrystalline structure can be produced by isothermal deformation of cast and powder metallurgy γ-TiAl-based alloys at temperatures 1000°C and below using relatively inexpensive nickel-based superalloy die tooling. A detailed analysis of the effects of initial microstructure, chemical compositions, conditions of heat treatment and hot working on the formation of a homogeneous fine-grained microstructure in γ-TiAl-based alloys is presented.

Effect of grain size on superplasticity of an intermetallic Ti3Al compound

Abstract The effect of grain size on the high temperature mechanical behavior of an intermetallic Ti 3 Al compound was investigated. An unusually strong dependence of material ductility on the structural parameter was revealed. Once submicro-crystalline grain size ( d μm ) is reached or approached, the superplastic characteristics displayed by Ti 3 Al are considerably higher than those of the intermetallics previously investigated. The most probable reason for such unusual behaviour is that unlike the other intermetallics studied, this compound is not prone to the formation, on heating, of annealing twins which ‘suppress’ superplastic flow.