In order to investigate the mechanical properties of the developed wrought γ'-hardened Ni-based superalloy, LESS alloy (Low Eta Sigma Superalloy), and to understand the deformation behavior, tensile tests were carried out from room temperature to 800oC in comparison with IN 740H alloy. Like other superalloys, the tensile properties of the LESS alloy were found to be dependent on temperature. As the temperature rose between room temperature and 700oC, the ultimate tensile strength (UTS) decreased slightly, and then decreased sharply. The yield strength (YS) of the LESS alloy was found to be about 40% higher than that of IN 740H alloy from room temperature to 700oC. On the basis of microstructure observations, the high yield strength of the LESS alloy is thought to result from the dense grain boundary, where the fine carbides (MC, M₂₃C₆) and the γ' phase form a continuous film structure, which more effectively suppressed dislocation movements during deformation. It was found that the deformation mode of the LESS alloy changed from γ'- shearing to mixed mode (γ'-shearing + climb) at around 700~800oC. Although the main fracture mode of the LESS alloy was ductile dimple fracture, elongation decreased with temperature due to the local brittle region caused by micro-twins.

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Toughening behavior of semicrystalline polymers was investigated using syndiotactic polystyrene (sPS)/polyamide 6(PA-6) blends compatibilized with maleic-anhydride functionalized poly (styrene-b-(ethyleneco-butylene)-b-styrene) SEBS-MA triblock copolymer. The effect of interparticle distance and crystal microstructure near the particle/matrix interface of the blends were studied. The morphology studies revealed that the size and interparticle distance of the dispersed PA-6 particles decreased with increasing SEBS-MA concentration. sPS/ PA-6 blends exhibited sharp brittle-ductile transitions at a critical interparticle distance of 0.25 lm. With the increase of the compatibilizer concentration beyond a certain level, it was observed that the further addition resulted in decreased impact strength. This could be attributed to the formation of a separate phase in the matrix by the additional SEBS added. The TEM studies showed that when the interparticle distance is below 0.25 lm, the matrix ligaments between the inclusions will be filled with welloriented crystalline material of reduced plastic resistance. From DSC and X-ray diffraction studies of model thin films, it was found that the fraction of small and imperfect crystallites near the particle/matrix interface increased with decreasing interparticle distance. This resulted in decreased yield stress of the whole matrix with increasing concentration of SEBS-MA accompanied by changes in the fracture mode from brittle to tough.

The alloys required for fossil power plants are altered from stainless steel that has been used below 600 o C to Ni-based alloys that can operate at 700 o C for Hyper Super Critical (HSC) steam turbine. The IN740 alloy (Special Metals Co. USA) is proposed for improved rupture strength and corrosion resistance at high temperature. However, previous studies with experiments and simulations on stable phases at about 700 o C indicated the formation of the eta phase with the wasting of a gamma prime phase, which is the most important reinforced phase in precipitation hardened Ni alloys, and this resulted in the formation of precipitation free zones to decrease the strength. On the basis of thermodynamic calculation, the new Ni-based superalloy named LESS 1 (Low Eta Sigma Superalloy) was designed in this study to improve the strengthening effect and structure stability by depressing the formation of topologically close packed phases, especially sigma and eta phases at high temperature. A thermal exposure test was carried out to determine the microstructure stability of LESS 1 in comparison with IN740 at 800 o C for 300 hrs. The experimental results show that a needle-shaped eta phase was formed in the grin boundary and it grew to intragrain, and a precipitation free zone was also observed in IN740, but these defects were entirely controlled in LESS 1.

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