P. Veyssière

On precipitate shearing by superlattice stacking faults in superalloys

Abstract Superlattice intrinsic stacking faults (SISFs) constitute one of the major features of the deformation microstructure of superalloys. It is pointed out that they may result from the dissociation, in the precipitates, of a single perfect matrix dislocation by a process essentially similar to the formation of SISFs in single-phase Ll2 ordered alloys. By using transmission electron microscopy under weak-beam conditions, it is demonstrated that SISFs originate mostly at the precipitate corners and that they may then interact attractively with secondary matrix dislocations, in order to form stable junctions. Energy calculations of the growth conditions of a SISF are performed as a function of precipitate size, SISF energy and applied stress, in the case of several alternative crystallographic configurations. It is shown that, although the dislocation dissociation is unfavourable on a per-unit-length basis, the nucleation and growth of SISFs is easy in superalloys since specific geometrical conditions ...

The fine structure of dislocations in Ni3V

Abstract The dissociation structure of dislocations resulting from deformation of Ni3V ordered alloy is studied, by electron microscopy, under weak-beam conditions. The energies of various surface defects are measured. Their dependence upon temperature are determined. These results are discussed in relation with the mechanical properties of alloys with the D022 structure.

c slip in Ti3Al

Abstract Strained Ti3Al polycrystals exhibit local evidence of slip in the c direction on a [20] prism plane. Dislocation mobility appears to be affected in the edge and several specific mixed orientations by their interaction with the lattice. Segmentation of the dislocation lines along the edge direction and, to a lesser extent, along the and mixed directions attests to pronounced first- and second-order Peierls valleys respectively. Contrary to what is usually observed in such processes, it is within about 30[ddot] from the screw orientation that dislocations appear to be the least affected by Peierls effects. Dislocation dissociation is not detectable under weak-beam conditions. It is pointed out that the favoured line orientations could originate from core spreading out of the prism slip plane possibly in the basal or second-order pyramidal planes for the edge orientation and in the first- and second-order pyramidal planes for the kinks.

On the effect of deviation from stoichiometry on the deformation microstructure of binary Ni3Al

Abstract Off-stoichiometric binary Ni3Al samples, deformed at varied temperatures, have been analysed in order to determine the sensitivity of the microstructures to a change in alloy composition. The results are compared with those obtained previously in a nearly stoichiometric alloy. It is found that the antiphase boundary energies are highly dependent upon the composition, especially after high-temperature deformation. In addition, orientational effects on dislocation lines are found to be dramatically sensitive to composition.

Calculation of antiphase boundaries on {110} planes in a b2 ordered compound by the cluster variation method

Abstract Antiphase boundaries (APBs) on {110} planes in a stoichiometric AB compound with the B2 ordered structure have been investigated using the cluster variation method. The basic irregular tetrahedron of the b.c.c. lattice is chosen as the maximum cluster for the entropy approximation, and the ordering energy of nearest-neighbour pairs is employed to evaluate the internal energy. The excess free energy due to the APB shows a monotonic decrease with increasing temperature and vanishes at T c as it should, considering that the B2↔disorder transition is second order. The excess internal energy exhibits a maximum at around 0.7T c before vanishing at T c. Up to about 0.3T c, the APB structure keeps the sharp profile of the pure shear structure at 0 K. Then it locally disorders and widens, the profile becoming flat and infinitely wide at T c. At each temperature, there exists a number of equilibrium APB configurations with almost the same energies corresponding to the same APB but located at different posi...

Weak-beam study of the dislocation microstructure of β-CuZn deformed in the temperature domain of the plastic anomaly

Abstract In the general framework of studies of the anomalous increase in plastic strength with temperature which is common to a number of ordered alloys, the effect of test temperature on the deformation microstructure has been investigated in β-CuZn (B2 structure). Single crystals have been deformed in compression at different temperatures between room temperature and 200[ddot]C along axes near [001] and [111]. The deformation microstructure has been observed under weak-beam conditions. The superdislocations with Burgers vector a dissociated into two superpartials bounding an antiphase boundary (APB) have been found to dominate the microstructure over the whole temperature range although, in the near [111] orientation of the applied stress, deformation involves glide in the a direction at the peak temperature. The relative density of climb-dissociated dislocations with mixed character has been observed to increase with temperature. In addition, the energies of the APBs on {110} and {112} plane...

Chapter 53 Microscopy and plasticity of the L12 γphase

Publisher Summary This chapter discusses the microscopy and plasticity of the L1 2 γ´ Phase. Single-phase ordered intermetallic alloys exhibit a variety of macroscopic mechanical properties that are dominated by the individual behavior of dislocations and, more precisely, by transformations that occur within the core of superdislocations. Positive temperature dependence of the flow stress has been reported in some metals and ceramics, but it is certainly in intermetallic systems that this behavior is the most frequently encountered. This chapter describes macroscopic mechanical properties, including deformation under constant strain rate, creep, and the analysis of transient tests. From one alloy composition to another, the measured orientation dependence may vary in a significantly inconsistent manner as shown by uncertainties on the position of maximum asymmetry. The actual orientation dependence appears, in fact, to be more complicated than a simple theoretical analysis could predict, unless of course this theory is appropriately parameterized.

Comparison between simulated weak-beam images : application to the extinction criterion in elastically anisotropic crystals

Abstract This paper investigates the reasons why the weak-beam contrast of dislocations is largely insensitive to elastic anisotropy. Particular attention is paid to the applicability of the g·b, = 0 invisibility criterion for Burgers vector determination to crystals with large elastic anisotropy factors. For this purpose, a method has been designed to allow for a direct comparison between weak-beam images simulated under different g·b, imaging conditions.

Some unusal properties of the fine structure of dislocations in Ni{sub 3}Al deformed at high temperature

In Ni{sub 3}Al polycrystals deformed at 850{degrees}C and quenched rapidly, surface defects exhibit two kinds of behaviour. On the one hand, dislocation coupling by antiphase boundary (APB) and complex stacking fault (CSF) is of normal magnitude for this alloy composition. On the other hand, separations are dramatically decreased in both cases. A method to visualize antiphase boundaries using a weakly excited superlattice reflection is introduced.