Ravi Sankar Kottada

Creep-rupture behaviour of Nimonic263 alloy coated with 87.5 wt.%Na2SO4+5 wt.%NaCl +7.5 wt.%NaVO3(3SM) and 74 wt.%Na2SO4+21.7 wt.%NaCl+4.3 wt.%NaVO3(3SM-A) was investigated in the stress range of 180–350 MPa at 800 °C. The 3SMs are responsible for the reduction in creep-rupture life. Hot corrosion products found in the crack regions of the crept specimens deformed under the influence of 3SMs. Sulphidation followed by oxidation was dominant at 250 MPa, whereas oxidation was dominant at 215 and 180 MPa. Comprehensive microstructural characterization and analysis suggest that three-salt mixtures(3SMs) accelerate oxygen and sulphur penetration along the grain boundaries during creep, and enhance the stress assisted grain boundary oxidation resulting in a reduction in creep-rupture life.

The B2-Aluminide matrix (FeAl and NiAl) with in-situ Al2O3 reinforcement were synthesized using reactive milling. The oxides (Fe2O3 and NiO) were reduced by Al during high energy milling to form Al2O3. The 20 h ball milled powders were consolidated using spark plasma sintering (SPS). To understand the densification mechanisms during SPS, sintering was performed at various temperatures (750–850 °C) and applied pressures (25–100 MPa). The creep parameters are evaluated from the densification data obtained during SPS using the model proposed by Bernard and Granger. In addition, independent constant-stress compression creep studies were conducted on the dense SPS pellets. The creep studies were performed on the composites at 800 °C at different stresses (100–500 MPa). The densification studies and compression creep studies are correlated based on the creep parameters obtained from both these studies and corroborated by TEM studies of the crept samples. This correlation is found to be valid even for the in-situ composites. Thus, the analysis of densification data can be helpful in predicting the creep behavior and useful for designing the new creep resistant alloys or composites.

The densification kinetics during spark plasma sintering (SPS) of FeAl and NiAl powders were analyzed using a model proposed by Bernard-Granger and Guizard [10]. Creep parameters obtained through densification data are in good agreement with those obtained from conventional creep experiments. Validity of the model was illustrated for aluminides in the form of deformation mechanism maps. This validation assures plausible confidence to predict creep behavior using densification data obtained during pressure assisted sintering of metallic alloys.

This study reports the effect of 650 °C exposure on microstructural evolution and creep behavior of post-weld heat-treated dissimilar welds between the alloys T92 and Super304H. The dissimilar welds were exposed for durations of 24 h, 100 h, 250 h, 500 h, and 1000 h to investigate the thermal stability of the microstructure. Creep tests were carried out at 650 °C and 120 MPa on the post-weld heat-treated and on 1000 h exposed dissimilar metal welds. The isothermal exposure degraded the microstructure of T92-HAZ, resulting in type IV failure at an accelerated rate under creep conditions. Further, accelerated recovery of martensitic laths and extensive precipitation of Laves phase on M23C6 carbides sitting at the grain boundaries of the fine-grained heat-affected zone (FGHAZ) during creep deformation were observed. The isothermal exposure on Super304H-HAZ showed no significant microstructural changes except precipitation of thick M23C6 carbides at the grain boundaries. These prominent microstructural changes in the HAZ of T92 lead to the accelerated void formation in the FGHAZ and are responsible for consequent premature failure.