Abstract:The effect of the amount of Sn on the formation of fcc phase in Ti-13Ta-xSn (x=3, 6, 9 and 12, at.%) alloys was studied. The alloys were synthesized by mechanical alloying using a planetary mill, jar and balls of stabilized yttrium. Using Rietveld refinement, it was found that the obtained fcc phase has crystallite size smaller than 10 nm and microstrain larger than 10^-3. Both conditions are required to form an fcc phase in Ti-based alloys. For all samples, the microstructure of the fcc phase consists of equiaxial crystallites with sizes smaller than 10 nm. The largest presence of fcc phase in the studied Ti alloy was found with 6 at.% Sn, because this alloy exhibits the largest microstrain (1.5×10^-2) and crystallite size of 6.5 nm. Experimental data reveal that a solid solution and an amorphous phase were formed during milling. The necessary conditions to promote the formation of solid solution and amorphous phases were determined using thermodynamic calculations. When the amount of Sn increases, the energy required to form an amorphous phase varies from approximately 10 to approximately -5 kJ/mol for 3 and 12 at.% Sn, respectively. The thermodynamic calculations are in agreement with XRD patterns analysis and HRTEM results.

Key words: Ti-based alloy; phase transformation; fcc phase; Rietveld analysis; mechanical alloying