Structural defects and thermal stability of Ti(Al) solid solution obtained by mechanical alloying

The structure of a metastable Ti(Al) hexagonal close packed (h.c.p.) solid solution obtained by mechanical alloying of the Al50Ti50 powder, was investigated using X-ray diffractometry (XRD). An analysis of the profiles shows physical broadening of (100), (002) and (101) diffraction lines. The probability of stacking faults on the basal (0001) and prismatic (10 (1) over bar 0) planes was determined from the differences between the physical broadening of the (100) and (101) lines which are influenced by the stacking faults and the (002) line which is not affected by the stacking faults. The total concentration of stacking faults is comparable to the values obtained earlier for AI-Ti solid solutions deformed by filing. Thermo-desorption and mass-spectrometry showed that, during mechanical alloying, about 1 at.% of hydrogen is dissolved in the alloy. A significant amount of this hydrogen contributes to the formation of stacking faults on the basal and prismatic planes. The temperature range within which the metastable h.c.p. structure is transformed inter the equilibrium AlTi (L1(0)) structure was determined by DSC calorimetry. The transition of the Ti(Al) solid solution into the AlTi intermetallic proceeds through an intermediate stage of the metastable f.c.c. phase with a lattice parameter a = 0.4012 nm. (C) 1998 Elsevier Science S.A.