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M. Heydari Nasab, R. Naghizadeh, H. Samadi, A. Nemati,
Volume 12, Issue 1 (march 2015 2015)

Ceramic-matrix composites containing TiC-TiN have been used in a variety of application because of their superior properties such as high hardness, good wear resistance and high chemical stability. In this research, effect of coke and coke/calcium beds in synthesis of Al 2O3-Ti(C, N) composites using alumino-carbothermic reduction of TiO 2 has been investigated. Al, TiO 2 and active carbon with additives of extra carbon and NaCl and without additives, in separate procedures, have been mixed. Afterwards, mixtures were pressed and synthesized in 1200oC for 4hrs, in coke and coke/calcium beds, separately. Al 2O3-Ti(C,N) composite was synthesized in ternary system of Al-TiO 2 -C with excess carbon and NaCl additives in calcium/coke bed in 1200 . X-ray diffraction patterns (XRD) results showed that existence of calcium in bed resulted in intensification of reduction atmosphere in samples and formation of Ti(C,N) phase enriched from carbon was accelerated. Crystallite sizes of synthesis Ti(C,N) at 1200 °C in reducing conditions were 22-28 nm.
A. Kermanpur, H. Ebrahimiyan, A. Heydari, D. Heydari, M. Bahmani,
Volume 14, Issue 4 (December 2017)

Formation of stray grain defects particularly around re-entrant features of the turbine blade airfoils is one of the major problems in directional and single crystal solidification processes. In this work, directional solidification tests of the GTD-111 Ni-based superalloy were conducted at different withdrawal velocities of 3, 6 and 9 mm.min-1 using various stepped cylindrical and cubic designs. The process was also simulated using ProCAST finite element solver to characterize the crystal orientations. The phase transformation temperatures of the superalloy were estimated by the differential scanning calorimetry test. A process map was developed to predict the formation of stray grains in the platform regions of the stepped cylindrical and cubic specimens using the experimentally-validated simulation model. The process map shows critical values of the platform size, withdrawal velocity and initial sample size for the stray grain formation. The withdrawal velocity, platform size and initial sample size all had an inverse effect on the formation of stray grains.

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