DIFFUSION IN Mo-SILICIDES ALLOYED WITH Re AND Al


Prof. M.A. Dayananda, with Graduate Student Ed Ciecko,
School of Materials Engineering
Supported by ONR Grant: N00014-97-1-0757; 1997-2000.

Phase formation and interdiffusion have been examined for the systems Al-Mo, Al-Mo-Si, and Al-Mo-Re-Si through the construction of solid-liquid and solid-vapor diffusion couples annealed in the temperature range of 700 - 1000oC. Solid-liquid diffusion couples examined include Mo/Al(l), MoSi2/Al(l), and (Mo,Re)Si2/Al(l) annealed in the temperature range 700 - 800oC. Solid-vapor diffusion couples included Mo/Al(v), columnar-MoSi2Al(v), and Mo5Si3B5/Al(v), where Mo3Al8 was employed as an Al vapor source at 1000oC. Phases identified as interdiffusion layers or solidification structures include the binary phases Mo3Al, Mo3Al8, MoAl4, Mo4Al17, MoAl5, MoAl6, and MoAl12, the ternary phases t-Mo(Si,Al) 2, h-Mo(Si,Al) 2, Mo(Al,Si)4, and Mo(Al,Si)5, and the quaternary phases t-(Mo,Re)(Si,Al)2,h-(Mo,Re)(Si,Al)2, (Mo,Re)(Al,Si)4,(Mo,Re)(Al,Si) 5Mo3(Al,Si) 8Bx, and h-Mo(Si,Al) 2Bx. Integrated and average effective interdiffusion coefficients are reported for the binary Mo3/Al and Mo3/Al8, the ternary t-Mo(Si,Al) 2/ and h-Mo(Si,Al)2, the quaternary Mo3/(Al,Si) 8/Bx/, and a two-phase region containing the quaternary Mo3(Al,Si)8Bx and h-Mo(Si,Al) 2/Bx. The main conclusions are: (1) the growth of Mo3Al is slow relative to that of Mo3Al8; (2) the growth of the Mo3(Al,Si)8Bx phase relative to that of the binary Mo3Al8 phase is retarded by the presence of Si; (3) interdiffusion of Al, Mo, and Si is faster in a two-phase region comprised of Mo3(Al,Si)8Bx and h-Mo(Si,Al)2Bx phases than in Mo3(Al,Si)8Bx alone; and (4) interdiffusion of Al and Si is more rapid in t-Mo(Si,Al)2 than in h-Mo(Si,Al)2. In addition, relative extrema were observed to develop in the Al and Si concentration profiles within the columnar t-Mo(Si,Al)2 phase for columnar MoSi2/Al(v) couples. Such extrema in concentrations are considered to arise from enhanced diffusion along grain boundaries and triple junctions oriented parallel to the diffusion direction and possible accumulation of the diffusant at grain boundaries and triple junctions oriented perpendicular to the main axis of the columnar grains.

Publications

  1. E.J. Ciecko and M.A. Dayananda: "Phase formation and interdiffusion in Al-Mo, Al-Mo-Si systems with Mo and MoSi2 in contact with liquid Al or Al vapor" -to be submitted for publication.
  2. E.J. Ciecko and M.A. Dayananda: "Phase formation between (Mo,Re)Si2 and liquid Al at 800o C" -- under preparation.
  3. E.J. Ciecko and M.A. Dayananda: " Interdiffusion betwee Mo5Si3Bx exposed to Al vapor at 1000oC " -- under preparation.
  4. E.J. Ciecko and M.A. Dayananda: "Interdiffusion of Al and its build-up in columnar-grained MoSi2 at 1000oC " -- under preparation.