Oscar A. Kaibyshev

Mechanical behaviour of fine grained TiAl intermetallic compound—I. Superplasticity

Abstract TiAl was used to show that superplasticity is possible in intermetallic compounds with a high ordering energy. Mechanical properties and structural changes are found to be strongly affected by the original grain size and grian boundary structure. It was established that the regularities of superplastic flow in TiAl are those typical of the structural superplasticity in metals.

Fundamental aspects of superplastic deformation

Abstract The results of studies of superplastic deformation mechanism on microscopic, mesoscopic and macroscopic scales are presented. On a microscopic scale the main mechanism of superplastic deformation is ‘stimulated’ grain boundary sliding, while on a mesoscopic scale it is the cooperative grain boundary sliding. On the basis of these experimental data a physical model describing the phenomenon of superplasticity as result of realization of a special deformation mechanism has been developed.

Superplastic roll forming of Ti alloys

Abstract The high cost of aerospace alloys and their components makes them prime candidates for net-shape manufacturing techniques. Conventional processes for manufacturing disk components include hammer, hot die, and isothermal forging. This paper will examine the potential of a revolutionary approach for the manufacture of aircraft engine disks, superplastic roll forming. The process of superplastic roll forming, developed at the Institute for Metals Superplasticity Problems, Ufa, employs pairs of small opposed rollers to shape a cylindrical workpiece into a complex axisymmetric shape by simultaneously adjusting the roll gap and by moving the rolls radially outward on the workpiece while it is rotated about its axis of symmetry. Both the workpiece and the rolls are maintained at temperatures close to the beta transus. This paper will describe metallurgical evaluations of superplastically roll formed disks of alloy VT25. The evaluations of the disks included microstructure, crystallographic texture, heat treatment response, tensile strength, stress rupture resistance, and ultrasonic characteristics. The disk microstructures were found to be uniform and without any strongly textured colonies. Mechanical properties of the roll formed VT 25 were compared with those of Ti-6242S, IMI834, and conventionally forged VT25. The RF VT25 disk was found to possess low ultrasonic noise and high inspectability, which provided an increase in signal to noise for synthetic flaws.

Self-organization of cooperative grain boundary sliding in aluminium tricrystals

Grain boundary sliding (GBS) being an important mechanism of high temperature deformation of polycrystals acquires a leading role in microcrystalline materials providing a superplasticity effect in them. Self-organization of change (increase) in angles of triple junctions providing CGBS realization was experimentally studied in a number of works. In particular, it was established that long range chains of grain boundaries are formed in structure of different materials. That is due to superplastic deformation. These chains are arranged almost in a straight line. Many well-known models on accommodation of GBS during superplastic deformation suggest the straightening of triple junctions. However, driving force and mechanisms of such transformation remain unclear in many cases. The aim of the present work is to design a transmission of GBS via a triple junction, to show physical origin of the triple junction transformation and to suggest a mechanism of the process. Well attested tricrystals are the best two dimensional model objects for such investigations.

Advanced superplastic forming and diffusion bonding of titanium alloy

Abstract This paper reports on investigations of the solid state formability and weldability of Ti–6Al–4V titanium alloy sheet with submicrocrystalline structure (grain size 0·5 and 0·8 μm) under superplastic conditions at temperatures of 700 and 800°C. It has been established that the submicrocrystalline condition of the sheet material reduces the temperature of superplastic forming (SPF) by 200°C and of diffusion bonding (DB) by 100°C, compared to the conventional superplastic forming Ti–6Al–4V material with microcrystalline structure (grain size 3 μm). Unlike commercially available sheet, the submicron grained sheet is characterised by isotropic formability. Submicrocrystalline processed Ti–6Al–4V sheet can be successfully used in SPF/DB technology. The temperature range of superplastic forming can be between 700 and 800°C, and diffusion bonding at 800°C.

The model of grain-boundary sliding stimulated by intragranular slip

Abstract The theoretical model of grain-boundary sliding (GBS) stimulated by intragranular dislocation slip has been suggested on the basic of experiments on Zn and Cd bicrystals. Lattice dislocations of two grains dissociate in boundary during the spreading time t s and form two families of mobile gain-boundary dislocations (GBDs) with the opposite sign. The motion of GBDs containing a non-conservative component is the main mechanism of GBS. The GBS rate is determined by the GBD density and by the rate of their annihilation or sink to the surface. The increase in GBD density forms additional elastic fields in the vicinity of boundary, which inhibits the entering of lattice dislocations into the boundary and weaken the source efficiency. The GBD density balance determined by the ratio of influx of lattice dislocations and annihilation of the GBDs is established. Thus the boundary demonstrates the behaviour of system with negative feedback. The model allows one to describe the GBS-time dependence in Cd and...

Mechanical behaviour of fine grained TiAl intermetallic compound—II. Ductile-brittle transition

Abstract A systematic study of the influence of structure on the mechanical properties of a brittle TiAl intermetallic compound was undertaken in the temperature interval from 20 to 900°C. The area and structure of grain boundaries were shown to play an important role in the mechanical behaviour of the compound under study: the growing area of random boundaries provides for the enhancement of the intermetallic compound plasticity. The kinetics of the interaction of lattice dislocations with the grain boundaries were found to be related to the mechanical properties of the intermetallic compound.

The effect of superplasticity on the solid state weldability of the titanium alloy Ti4.5Al3Mo-1V

Abstract The systematic study of solid state joint (SSJ) formation under conditions of hot and superplastic (SP) deformation by the mode of uniaxial tensile elongation and during vacuum annealing has been conducted. The two-phase VT14 alloy (Ti4.5Al3Mo-1V) with the initial microcrystalline and coarse-grained lamellar structure was taken for investigations. It has been established that deformation processes are mainly responsible for the formation of a sound joint. Annealing at the temperature of SP deformation does not increase the shear strength of joints in the samples with the initial strength of 0.4 of that of the base alloy. The considerable effect of SP deformation rate on the kinetics of SSJ formation has been revealed and it is noted that the formation of SSJ is mostly accelerated during deformation in the SP (optimum) Region II. The given experimental results show that the acceleration of sound joint formation in the SP state is attributed to the influence of grain boundary sliding (GBS), the main mechanism of SP deformation.

Nature of Superplastic Deformation

As known, there was made a number of hypotheses for the nature of superplastic (SP) deformation and many physical models of this phenomenon were suggested [1-2]. The majority of authors tried to explain the σ=σ(e) dependence on the basis of a mechanism of SP deformation. And though their concepts on diffusion creep or dislocation slip in combination with grain boundary sliding were absolutely different, the final constitutional equations were rather similar differing only by coefficients [1-2]. It is not wonder since the experimental dependence σ=σ(e) was studied rather thoroughly and its parameters are known. However, the theory of SP deformation should explain not only the phenomenology but also all structural features of material during SP flow. It is also very important to predict and explain new effects, such as low temperature and high strain rate superplasticity. The aim of the present work is to investigate the main mechanism and characteristics of diffusion processes during SP deformation, and to develop a physical model of this phenomenon. The review of experimental data on superplastic alloys and model materials [3-10] obtained recently in the Institute for Metals Superplasticity Problems (IMSP) of the Russian Academy of Sciences is presented in this paper.

Superplasticity and solid state bonding of the TiAl intermetallic compound with micro- and submicrocrystalline structure

The use of superplastic phenomenon makes it possible to reduce considerably the temperature of solid state bonding of the TiAl intermetallic compound as compared with conventional diffusion bonding. At decreasing the superplastic deformation temperature due to the reduction of the grain size an adequate decrease of the temperature threshold of solid state weldability of the TiAl intermetallic compound was observed. In the TiAl intermetallic compound with submicrocrystalline grain size the formation of a sound solid state joint occurs in the process of deformation at the temperature t = 850 C and strain {var_epsilon} = 10%. Since the temperature-strain-rate conditions of solid state bonding and those of superplasticity coincide, it is assumed that the mechanism controlling the bonding process is grain boundary sliding, the main deformation mechanism of superplasticity.