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.) s olid 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) diffracti on lines. The probability of stacking faults on the basal (0001) and p rismatic (10 (1) over bar 0) planes was determined from the difference s between the physical broadening of the (100) and (101) lines which a re influenced by the stacking faults and the (002) line which is not a ffected by the stacking faults. The total concentration of stacking fa ults is comparable to the values obtained earlier for AI-Ti solid solu tions deformed by filing. Thermo-desorption and mass-spectrometry show ed that, during mechanical alloying, about 1 at.% of hydrogen is disso lved in the alloy. A significant amount of this hydrogen contributes t o 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 determine d 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) 19 98 Elsevier Science S.A.