The statistical-thermodynamic theory of ordering and electronic theory of ordering in the pseudo-potential approximation was used to study the influence of ternary addition of some transition metals on the atomic ordering behavior of Co0.5(Ti1–xMx)0.5 alloys with M = Fe, Pt, Re, V, Cr, Mn, Ni, Cu, Zn, Zr, Ag, Hf or Au up to a concentration of 1 at.%. The partial ordering energies, order-disorder phase transformation temperatures and partial short range order parameters have been calculated for these alloys. The analysis shows that the impurity elements in Co0.5(Ti1–xMx)0.5 alloys can be divided into two main groups on the basis of lattice site occupancy i.e. M = V, Cr, Mn, Cu, Zn, Zr, Ag, Hf and Au mainly substitute for Co sublattice sites whereas M = Fe, Ni, Pt or Re mainly substitute for Ti sublattice sites. Further, the order-disorder transformation temperatures were found to either increase or remain nearly unchanged by the addition of ternary impurities in the CoTi alloy depending on the absolute value of the partial ordering energies. Alloys of Ti with V, Cr, Mn, Cu, Zn, Zr, Ag, Hf or Au in place of Co and alloys of Co with Fe, Ni, Pt or Re in place of Ti can be predicted for future. The results of the present analysis are in good agreement with the available experimental data on these alloys.

One of the most important factors directly affecting microstructure and mechanical properties in directional solidification process is secondary dendrite arm spacing (SDAS). It is very important to measure the SDAS and examine the factors that may affect them. To investigate the effect of growth rate on the SDAS, the alloy specimens were directional solidified upward with different growth rates ( V = 8.3-83.0 μm/s) at a constant temperature gradient ( G = 4 K/mm) in a Bridgman-type growth apparatus. After the specimens are directionally solidified, they were exposed to metallographic processes in order to observe the dendritic solidification structure on the longitudinal section of the specimens. Coarsen secondary dendrite arm spacings (λ _2C) were measured excluding the first arms near the tip of the dendrite. Local solidification times ( t_f) were calculated by ratio of spacings to growth rates. It was determined that the t_f values decreased with increasing V values. The relationships between t_f and λ _2C were defined by means of the binary regression analysis. Exponent values of tf were obtained as 0.37, 0.43, 0.46 and 0.47 according to increasing V values, respectively. These exponent values are close to the exponent value (0.33) predicted by the Rappaz-Boettinger theoretical model and good agreement with the exponent values (0.33-0.50) obtained by other experimental studies.