Bernard Viguier

Modelling the flow stress anomaly in γ-TiAl I. Experimental observations of dislocation mechanisms

Abstract Mechanical tests on a polycrystalline γ Ti47A151,Mn2 alloy between ‐ 150 and 1000°C and subsequent microstructure analysis clearly distinguish between three temperature domains which correspond to different deformation mechanisms. At low temperatures, the motion of superdislocations dragging faulted dipoles is rate controlling. At intermediate temperatures where a stress anomaly is observed, screw simple dislocations are observed with cusps, the density of which increases with increasing temperature. A description of the simple dislocation motion based on these observations is developed. The glide of a screw simple dislocation in two planes through a Peierls mechanism is believed to be the intrinsic source for the formation of pinning points. It is proposed that these pinning points can be erased by lateral motion of superkinks: an unzipping process. A model for this pinning-unzipping mechanism will be fully developed in part 11. At high temperatures, the climb of simple dislocations appears to c...

Modelling the flow stress anomaly in γ-TiAl II. The local pinning-unzipping model: Statistical analysis and consequences

Based on experiments carried out on γ-TiAl polycrystalline samples and presented in part I of this work, a statistical model is proposed in order to account for stress anomalies observed in this material in the intermediate-temperature range. This statistical analysis is based on a particular mechanism in which dislocations are successively pinned on localized intrinsic obstacles and unzipped by lateral motion of cusps. Owing to the increase in the pinning-point density with increasing temperature, the probability of operation of the unzipping mechanism is shown to depart from unity above a critical temperature. The resulting strong exhaustion mechanism of mobile dislocations naturally yields a stress anomaly. The model also accounts for some other characteristic features of TiAl in this temperature range, such as the high work-hardening rate and the very small strain rate sensitivity.

Characterizing faulted dipoles in deformed gamma TiAl

Abstract 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...

TEM Study of High-Temperature Precipitation of Delta Phase in Inconel 718 Alloy

Inconel 718 is widely used because of its ability to retain strength at up to 650∘C for long periods of time through coherent metastable 𝛾 Ni3Nb precipitation associated with a smaller volume fraction of 𝛾 Ni3Al precipitates. At very long ageing times at service temperature, 𝛾 decomposes to the stable Ni3Nb 𝛿 phase. This latter phase is also present above the 𝛾 solvus and is used for grain control during forging of alloy 718. While most works available on 𝛿 precipitation have been performed at temperatures below the 𝛾 solvus, it appeared of interest to also investigate the case where 𝛿 phase precipitates directly from the fcc matrix free of 𝛾 precipitates. This was studied by X-ray diffraction and transmission electron microscopy (TEM). TEM observations confirmed the presence of rotation-ordered domains in 𝛿 plates, and some unexpected contrast could be explained by double diffraction due to overlapping phases.

The mechanical properties of single phase γ Ti47Al51Mn2 polycrystals

Abstract The mechanical properties of polycrystalline samples of the single phase γ-Ti 47 Al 51 Mn 2 alloy have been studied during compression tests in a wide range of temperatures (120–1270 K). The flow stress and the work hardening rate are measured during imposed strain rate tests, while the strain rate sensitivity of the stress is examined using both strain rate jumps and stress relaxation experiments. From the temperature, strain and stress dependence of these parameters, it is shown that the investigated temperature domain can be divided into three regimes corresponding to different deformation mechanisms. The results are compared to the data available in the literature and are found to be in good agreement with the dislocation structures and dislocation motion mechanisms that we have previously reported.

Thermal fatigue of single-crystalline superalloy CMSX-4 ® :a comparison of epitaxial laser-deposited material with the base single crystal

The thermal fatigue resistance of cast single-crystalline CMSX-4 samples featuring an edge composed of a 1-mm-deep layer of laser-deposited epitaxial single-crystalline CMSX-4 is compared with that of samples made entirely of the virgin base material. Tests are conducted by cycling between 200 and 1100°C, and recording the progression of the largest crack growing in each sample. It is found that crack growth rates in samples with laser claddings are around twice those in the base material; this is chiefly attributed to their different thermal history. Resistance to crack initiation was lower in two out of three laser-clad samples than in the base material; this is attributed to interdendritic flaws in laser-clad layers of these two samples. The performance of the third coated sample was comparable with that of the base material.

A re-examination of the liquidus surface of the Al-Fe-Ti system

Abstract The liquidus surface of the Al–Fe–Ti system has been reexamined. 34 different alloys have been studied by differential thermal analysis, and the temperatures of invariant reactions have been determined. By examining the as-cast samples by optical microscopy, X-ray diffraction and energy-dispersive spectrometry in a scanning electron microscope, the crystal structures and chemical compositions of the primary phases and two-phase aggregates have been established. In some cases, additional investigations were carried out in the transmission electron microscope. From the combined results of these investigations the solidification paths of the alloys were established leading to a revised representation of the liquidus surface of the Al–Fe–Ti system. A reaction scheme has been set up which shows phase equilibria that are consistent with previous solid-state isothermal sections.

High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

Creep tests were performed on MC2 single crystal superalloy at 950°C/200 MPa and 1150°C/80 MPa under isothermal and thermal cycling conditions with a tensile axis along the [0 0 1] direction. It was found that the thermal cycles strongly affect the creep behavior at 1150°C but not at 950°C. This was related to the repetitive precipitation and dissolution of small γ′ rafts at the higher temperature, as revealed by quantitative characterization of the γ/γ′ microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[1 0 0]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appears that the density of a[1 0 0] dislocations, and not their climb velocity or diffusion rate, is the key parameter for the control of creep rate. The thermal cycles, which imply the creation and subsequent dissolution of rafts, provided new dislocations, which explains the acceleration of creep observed under such conditions.

Ordinary dislocations in γ-TiAl: cusp unzipping, jog dragging and stress anomaly

Abstract Two models recently proposed for strength anomalies in TiAl were based on different behaviours of jogs: in the first, jogs are supposed to move conservatively along the screw direction, whereas in the other, screw dislocation motion is supposed to be controlled by jog or dipole dragging. In the present paper, we compute the flow stresses expected in both cases. The former model is shown to operate up to the stress peak temperature, and to be replaced by the latter as temperature is raised above the peak.

Dislocation densities and strain hardening rate in some intermetallic compounds

The strain hardening rate (SHR) of some intermetallic compounds is examined in regards of dislocation density. It is shown that a Taylor-type relation between stress increases and dislocation density is obeyed and that the SHR rate can be explained in terms of dislocation storage due to a very short mean free path of dislocations.