Alloying and Hardness of Eutectics with Nbss and Nb5Si3 in Nb-silicide Based Alloys

In Nb-silicide based alloys, eutectics can form that contain the Nbss and Nb5Si3 phases. The Nb5Si3 can be rich or poor in Ti, the Nb can be substituted with other transition and refractory metals, and the Si can be substituted with simple metal and metalloid elements. For the production of directionally solidified in situ composites of multi-element Nb-silicide based alloys, data about eutectics with Nbss and Nb5Si3 is essential. In this paper, the alloying behaviour of eutectics observed in Nb-silicide based alloys was studied using the parameters Delta H-mix, Delta S-mix, VEC (valence electron concentration), ffi (related to atomic size), Delta chi (related to electronegativity), and W (= Tm Delta S-mix/vertical bar Delta H-mix vertical bar). The values of these parameters were in the ranges -41.9 < Delta H-mix < -25.5 kJ/mol, 4.7 < Delta S-mix < 15 J/molK, 4.33 < VEC < 4.89, 6.23 < delta < 9.44, 0.38 < Omega < 1.35, and 0.118 < Delta chi < 0.248, with a gap in Delta chi values between 0.164 and 0.181. Correlations between Delta S-mix, Omega, Delta S-mix, and VEC were found for all of the eutectics. The correlation between D Hmix and ffi for the eutectics was the same as that of the Nbss, with more negative Delta H(mix)for the former. The delta versus Delta chi map separated the Ti-rich eutectics from the Ti-poor eutectics, with a gap in Delta chi values between 0.164 and 0.181, which is within the Delta chi gap of the Nb-ss. Eutectics were separated according to alloying additions in the Delta chi versus VEC, Delta chi versus < Si >, delta versus < Si >, and VEC versus < Si > maps, where < Si > = Al + Ge + Si + Sn. Convergence of data in maps occurred at delta approximate to 9.25, VEC approximate to 4.35, Delta chi in the range approximate to 0.155 to 0.162, and < Si > in the range approximate to 21.6 at.% to approximate to 24.3 at.%. The convergence of data also indicated that the minimum concentration of Ti and maximum concentrations of Al and Si in the eutectic were about 8.7 at.% Ti, 6.3 at.% Al, and 21.6 at.% Si, respectively, and that the minimum concentration of Si in the eutectic was in the range 8 < Si < 10 at.%.