The faulted dipoles that are associated with the low temperature deformation of gamma TiAl have been characterized with conventional, high resolution and in situ heating transmission electron microscopy. The results of this analysis indicate that the formation of these faulted dipoles involves the localized pinning of a superdislocation, the bypassing of that pinning point and the drawing out of a dipole whose energy is subsequently reduced by the passage of partial dislocations and the formation of an extrinsic stacking fault. The extrinsic nature of the stacking fault was determined through comparisons with computer generated image simulations and verified by atomic resolution lattice images. These faults are bounded by either single or double Shockley partial dislocations, but the latter is a rare occurrence and the former was identified to be most relevant to the mechanical behaviour of this alloy. The low temperature stability of this defect has been attributed to the ordered structure of Ti...

Characterizing faulted dipoles in deformed gamma TiAl

The deformation behavior of polysynthetically twinned (PST) crystals of TiAl alloyed with V and Zr has been studied at room temperature both in compression and in tension as a function of the angle (o) between the lamellar boundaries and loading axis. The yield stress as well as tensile elongation strongly depends on the angle o. When compared to binary PST crystals, both V- and Zr-alloyed PST crystals, in particular, Zr-alloyed ones show high yield stress for all orientations. Environmental loss in ductility of TiAl PST crystals is reduced by the addition of V without affecting their intrinsic ductility. Deformation structures of the two ternary TiAl PST crystals have been investigated by transmission electron microscopy. In addition to deformation twins and ordinary dislocations, numerous superlattice dislocations and faulted dipoles associated with them are observed for both V- and Zr-alloyed TiAl PST crystals. Which of these deformation structure constituents prevails in each of the TiAl domains depends on the crystal orientation of the domain. Based on the results obtained, the effects of V and Zr additions on the strength and ductility of two-phase TiAl alloys are discussed.

Plastic deformation of V- and Zr-alloyed PST TiAl in tension and compression at room temperature

The role of microstructural stability on compression creep of fully lamellar γ-TiAl alloys

Abstract Transmission electron microscopic study on a α2 + γ two-phases Ti-40 at.% Al alloy demonstrates that (011)γ/(101)γ twin does occur in TiAl. Differently-orientated γ regions are observed to coexist within the same γ lamella with the (011)γ/(101)γ twin plane as their boundary. The variants pair corresponding to these differently-orientated γ regions should come from variants which have the same stacking sequence. The mechanism for the formation of (011)γ/(101)γ twin plane is suggested to grow from the ‘APB-induced fault’ formed in the γ lamella during the precipitation transformation of α2→α2 + γ. Total strain induced by the structure change (h.c.p.→f.c.c.) and by the tetragonality of γ phase during the precipitation transformation can be minimized by a composite of {111}γ and {011}γ twins coexisting in γ lamellae.

(011)γ/(101)γOrder Twin of γLamella in a Ti-40at.% Al Alloy

On the massive transformation in TiAl-based alloys

The output stability of the displacement sensors is essential to enhance the reliability of active magnetic bearings in the high-temperature applications. This paper proposes a novel temperature drift compensation method for exponential hysteresis characteristics of high-temperature eddy current displacement sensors. The transient thermal analysis for the sensing assemblies demonstrate that the temperature drift errors are mainly induced by the temperature variation hysteresis effect: one is the dynamic lag between the probe coil and the air gap, and the other is the dynamic lag between the target material and the air gap. An auxiliary device consisting of compensation probe and compensation plate is designed to solve the hysteresis effect. The thickness of the compensation plate is determined by simulating the transient thermal analysis of the compensation plate and the measured target region. The model analyses also find that the exponential temperature drift at different determined displacement states is similar under the same temperature gradient. With this error characteristic, the differential circuit is used to eliminate the exponential hysteresis temperature drift. Experimental investigations show the effectiveness of the proposed solution.

Temperature Drift Compensation for Exponential Hysteresis Characteristics of High-Temperature Eddy Current Displacement Sensors

Observations and formation mechanism of type faulted dipoles in TiAl Deformed at room temperature

Diffusion ledge mechanism of massive γ transformation in quenched TiAl alloys

The invention discloses a three-degree-of-freedom magnetic bearing and eddy current sensor integrated structure. The integrated structure comprises a radial magnetic bearing stator module, a sensor module, split permanent magnets and an axial magnetic bearing stator module, wherein the radial magnetic bearing stator module comprises a radial magnetic bearing stator iron core, a coil skeleton and a coil; and the axial magnetic bearing stator module comprises an axial magnetic bearing stator iron core and a coil. The radial magnetic bearing module, the axial magnetic bearing module and the permanent magnet form a three-degree-of-freedom magnetic bearing; and the sensor module comprises a sensor probe and a circuit board. The sensor module is mounted in middle of the three-degree-of-freedom magnetic bearing; and the probe is between the split permanent magnets. The structure integrates the three-degree-of-freedom magnetic bearing with a radial displacement sensor, so that the axial length of the magnetic bearing can be greatly reduced, and the working frequency of a motor of the magnetic bearing is improved. Meanwhile, the integrated structure can reduce the assembly difficulty in the assembly process, and improves the assembly precision.

Three-degree-of-freedom magnetic bearing and eddy current sensor integrated structure

The TiAl based alloy is one of the most promising candidate materials for high temperature applications in the next decade. However, its low-temperature brittleness hinders its wide usage. In the last twenty years the deformation mechanisms of TiAl alloys have been intensively investigated. It is well known that the extremely common feature of deformation microstructures at room temperature is the existence of numerous stacking faults surrounded by 1/6 partials. Further investigations by Hug et al showed that these planar faults were extrinsic faulted dislocation dipoles which were produced by the rearrangement of the core structure of a pinned 1/2 superdislocation. In their previous paper, the authors reported an experimental observation of a new type of faulted dipoles. The surrounding partials of these faulted dipoles have a Burgers vector of 1/3 rather than 1/6 . This is striking. One could argue that it seems impossible to identify unambiguously the Burgers vector of partial dislocations bounding a stacking fault by extinction rules only because the contrast of partial dislocations is often affected significantly by that of the bounded stacking faults. In order to check the previously reported result, image simulation has been performed and is presented in this note.

Y. G. Zhang

Papers

Temperature Drift Compensation for Exponential Hysteresis Characteristics of High-Temperature Eddy Current Displacement Sensors

Observations and formation mechanism of type faulted dipoles in TiAl Deformed at room temperature

Diffusion ledge mechanism of massive γ transformation in quenched TiAl alloys

Three-degree-of-freedom magnetic bearing and eddy current sensor integrated structure

Further verification of 1/3 type faulted dipoles in TiAl