Plasticity and Brittleness of the Ordered β_o Phase in a TNM-TiAl Alloy

The aim of this work is to study the deformation mechanisms in the ordered β_o phase containing some ω_o precipitates of a TNM-TiAl alloy (Ti_51.05Al_43.9Nb₄Mo_0.95B_0.1 in at.%) and to look for the origin of the low ductility at room temperature of this alloy.<br/>The alloy produced by Spark Plasma Sintering exhibits a near lamellar microstructure made of lamellar γ/α₂ colonies surrounded by γ and β_o grains. Tensile tests were performed at room temperature to determine the mechanical properties and the ductility of the alloy. The rupture surface of the deformed material were investigated by scanning electron microscopy.<br/>Two types of TEM (Transmission Electron Microscopy) experiments were used to explore the behaviour of these dislocations and to study the deformation mechanisms. The first one lies in investigating the deformation microstructure in thin foils extracted from bulk samples previously deformed in tension at room temperature, while the second type consists in performing <i>in situ</i> tensile tests inside a TEM that allows the direct observation of the dislocation dynamics under stress and in real time.<br/>At room temperature, the material exhibits a limited ductility with cleavage surfaces along the lamellae interfaces of the colonies. The β_o grains contain nano-precipitates of the ω_o phase and deform plastically by both &lt;111&gt; superdislocations and &lt;001&gt; dislocations. The &lt;111&gt; superdislocations are dissociated into two superpartial dislocations separated by an antiphase boundary. They glide in {011} planes and have the tendency to be localized into pile-ups, an effect demonstrated as the result of the ω_o precipitation that harden the β_o grains and favour deformation concentration. The &lt;001&gt; dislocations are found to be split into two identical superpartial dislocations separated by a stacking fault. They are elongated along their screw orientation and also anchored at these small nano-precipitates of ω_o phase. <i>In situ</i> straining experiments performed at room temperature show that they move by jumps between these elongated configurations.<br/>These results are discussed to explain the low ductility of this alloy. The stress concentration due to the pile-up formation on grain boundaries is assumed to induce fracture along lamellae interfaces in neighbouring lamellar colonies.