The effect of plastic deformation on fracture morphology of the sigma phase in the NbTiAl system

It has been observed that in bulk specimens indentation-induced plastic deformation at room temperature causes intergranular fracture in the σ-phse of the NbTiAl system. The TEM study of the indented regions has revealed that only intergranular microcracks, which develop when two slip bands impinge on both sides of a boundary or when a slip band arrives at a grain boundary triple point, are resulted by the dislocation—grain boundary interaction. Consequently, upon subsequent loading of the indented speciments the crack propagates intergranularly in these regions. In the undeformed σ-phase, the accidental cracks that develop during the thin foil preparation have been found to follower a predominantly transgranular path, similar to the bulk specimen, with no dislocation activity at their tips. In contrast, in the plastically deformed regions the accidental cracks follow the path along slip bands. This phenomenon is believed to be a thin-foil effect.     

Microstructure evolution of Ti48AlχNb γ intermetallics

The microstructures of Ti48Al alloys containing either 2 at.% or 8 at.% Nb have been studied in as-cast, as-HIPped and in heat-treated samples. The as-cast Ti48Al2Nb and Ti48Al8Nb are heterogeneous and lower Nb content has been detected in the interdentritic γ by EDX analysis. Depending on the heat-treatment temperature and cooling rate, the microstructure obtained vary from martensitic structure in the water quenched samples to duplex structure consisting of γ grains and lamellar α₂/γ and to fully lamellar structure in the furnace cooled samples. Nb was found to have a high solubility in both α and γ phases and to expand the γ phase region so that a fully γ structure is obtained in the Ti48Al8Nb sample by annealing at 1200°C. A dense array of planar defects (antiphase boundaries, stacking faults and microtwins) was obtained in the Ti48Al8Nb alloy water quenched from 1400°C.     

The coarsening of lamellae in Ti48Al2Mn2Nb

The thermal stability of as-cast Ti48Al2Mn2Nb alloys has been investigated as a function of time at temperatures of 1200, 1350 and 1420°C. The resultant structures have been characterised using optical microscopy, electron probe microanalysis and transmission electron microscopy. The as-cast structure consists mainly of lamellae of α₂ and γ. Annealing at 1350 and at 1420°C resulted in the continuous and discontinuous coarsening of the primary lamellae. Discontinuous coarsening, which often originates at the sample surface, results in branched irregular secondary lamellae. The average interlamellar spacing of the secondary lamellae is up to a thousand times larger than the primary interlamellar spacing. The extremum principles of maximum velocity and maximum rate of entropy production, used to predict the secondary interlamellar spacing, are inconsistent with the experimental observations. The theoretical predictions of the velocity of the discontinuous coarsening by Livingston and Cahn are used to estimate the value of grain boundary diffusivity at 1350°C. The temperature dependence of discontinuous coarsening is weak, in contrast to the strong effect of lamellar orientation on the rate of discontinuous coarsening.     

The influence of MN additions on the deformation behaviour of an AlMgSi alloy

The deformation behaviour at low plastic strains of three AlMgSi alloys in the fully aged condition has been studied. The MN-free alloy (BD3) plastically deforms to form narrow slip bands whose spacing decreases slightly with increasing strain. This alloy fails intergranularly at low strain because the high stress concentrations at the head of the slip bands nucleate voids at grain boundary precipitates which then propagate by coalescence within the PFZ.The two Mn-bearing alloys (BD3 and BDS) contain volume fractions of 0.004 and 0.011 respectively of 0.1 μm dia. incoherent particles of α-Al₁₂Mn₃Si. as well as the age-hardening phase. At small strains these alloys develop slip bands of slightly smaller spacing than those in BD3. out with increasing strain the slip band spacing decreases until at 9% strain are no longer resolvable in BD8.The Mn-phase thus causes lateral spreading of the slip bands, giving rise to significantly smaller stress concentrations at the head of the slip bands, and whose magnitude is further reduced by the smaller grain size of these alloys. A semi-quantitative assessment of these two contributions to the local grain boundary stress has been made. It is concluded that intergranular failure in BD6 and BD8 is suppressed at low strains because the associated stress concentrations are insufficient to produce decohesion at the grain boundary precipitates.     

On the formation of 2H stacking sequence in 18R martensite plates in a precipitate containing CuAlNiTiMn alloy

Basal planes stacking sequence variation in M18R martensite of CuAlNiTiMn alloy have been studied by means of electron diffraction and lattice imaging. Samples subjected to two different homogenisation temperatures (1173 and 973K) and two different cooling rates—water quenched and air cooled were analysed. The observed changes in stacking sequence have been attributed mainly to stress accomodation around X_L and X_S precipitates. Very small X_SS precipitates of the same phase have been observed and their high influence on the stacking sequence of martensite plates have been explained by means of a new “dislocation” mechanism related to their coherency loss into the matrix.     

Phase transformations in the (Ti,Nb)3 Al section of the TiAlNb system—II. Experimental tem study of microstructures

In Part I of this paper possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted for TiAlNb alloys cooled from the high temperature b.c.c./B2 phase field into close-packed orthorhombic or hexagonal phase fields. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti25Al12.5Nb (at.%), the path includes the formation of intermediate hexagonal phases, A3 and D0₁₉, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti25Al25Nb (at. %), path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing 700°C. The thermodynamics underlying the path selection and the two-phase formation are also discussed.