Role of alloying elements in Mo5Si3 intermetallics

Abstract: The role of alloying elements in the intermetallic compound Mo5Si3 was studied in the alloying systems Mo5-xMxSi3 (M = Cr, Ti, Nb, Ni, or Co, x = 0.5-2). The effects of the alloying elements were examined by characterising the microstructures and identifying the resulting phases. In the case of Cr and Nb alloying, a solubility of 25% (at.) can be obtained. However, equilibrium conditions are very hard to reach, and segregation is pronounced. The Ti solubility in Mo5Si3 is slightly lower than 25%, and Ti does not seem to segregate such as Cr and Nb. In contrast, the solubility of Ni and Co in the Mo5Si3 phase is limited, less than 3% respectively. As a result, Mo(Ni/Co)Si Laves phases form as major phases in the Mo3(Ni/Co)2Si3 alloys. Homogenisation heat treatment shows that these ternary Laves phases have relatively low melting points of less than 1150°C and 1350°C for MoNiSi and MoCoSi, respectively.

Site occupancy of Cr and Ti in the Mo5Si3 unit cell was determined using Rietveld refinement of neutron powder diffraction data. In addition, the effect of alloying on the CTE was investigated employing high temperature diffraction. Single crystal X-ray diffraction data indicates that Cr occupies solely the 4b site of the D8m unit cell of Mo5Si3 in the as-cast condition, while annealing results in Cr occupying both 16k and 4a sites. In contrast, Ti seems to preferably occupy the 16k site in the annealed condition. Cr and Ti additions can reduce the CTE-mismatch of Mo5Si3 to 1.45 and 1.32, respectively.

Vickers hardness measurements and compression tests at 1300°C were performed to evaluate the influence of alloying on the mechanical properties of Mo5Si3. Cr additions reduce the hardness of Mo5Si3 drastically compared to the other alloying elements, as do small amounts of Ni dissolved in the Mo5Si3 phase. Indentation fracture toughness increases with alloying content for Cr, Ti, and Nb additions, and Nb and Ni show considerable toughness improvements even for low additions. Finally, Cr additions lower the high temperature strength of Mo5Si3 by a factor of two, whereas both Ti and Nb additions seem to increase the brittle to ductile transition temperature of Mo5Si3 to over 1300°C.