It has long been recognized by Gas Turbine manufacturers that increasing the turbine entry temperature offers attractive advantages in turbine performance and fuel economy. A significant amount of improvement in the turbine entry temperature has been made possible by advances in material capability. One of the most demanding parts in gas turbine is the first stage turbine blade, which is subjected to the most severe combinations of stress and temperature in a hostile environment. Oxide dispersion strengthened (ODS) alloys attract great attention as advanced high temperature materials, because they can retain useful strength up to a relatively high melting points. The high temperature strength of ODS alloys is due to the presence of fine, uniformly dispersed, stable oxide particles in the coarse elongated grains. Production of a coarse elongated grain structure is very important in ODS alloys, since the creep rupture strength is usually increasing with the increasing grain aspect ratio (GAR). The formation of long grains is possible during secondary recrystallization by a process of directional zone annealing. The combination of oxide dispersion strengthening and gamma prime precipitate strengthening is made by mechanical alloying process, which was developed by INCO Alloys International in the late 1960. Mechanical alloying is a dry, high-energy ball milling operation that produces composite metal powders with controlled, extremely fine, micro-structures. The powders are produced in high-energy ball mills. Mechanically alloyed powders are consolidated by placing them in sealed cans for extrusion or hot pressing, followed by conventional hot and cold working processes. A final anneal at very high temperatures is required to develop the stable, coarse grain structure suitable for the most demanding stress-rupture applications.
The only commercially available ODS plus gamma prime strengthened alloy is MA 6000, with the nominal composition of Ni / 15Cr / 4W / 2Mo / 4.5Al / 2.5Ti / 2Ta / 0.15Zr / 0.05C / 0.01B / $1.1Y_2O_3$. MA6000 has good oxidation resistance and excellent sulfidation resistance similar to alloy 713C and IN-792, respectively. Development of the MA 6000 was performed by INCO Alloys International under the NASA Contract CR-159493 from 1977 to 1979. The commercial production of MA 6000 is conducted by INCO Alloys International, England. The commercially produced MA 6000 has been selected and used in 1991 as the first stage solid blades for the Hurricane industrial gas turbine engine, a product of European Gas Turbine Ltd. (EGT), Lincoln, England. Alloy MA 6000 was selected as the first stage turbine blade alloy because its creep resistance at above 2000℉ is superior to those of other Ni-base alloys including single crystal superalloys. Although MA 6000 exhibits superior strength at above 2000℉, the intermediate temperature strength of MA 6000 is inferior to those of single crystal Ni-base superalloys. The volume fraction of gamma prime precipitate in MA 6000 was about 55% by containing the total amount of Al plus Ti as 7 w/o %. It is possible to increase further the amount of gamma prime precipitate by increasing the amounts of Al plus Ti while lowering the content of Cr. As an initial effort to increase the intermediate temperature strength, an experimental alloy of Alloy 92 has been recently developed at the INCO Alloys International, Huntington, West Virginia [4]. The nominal composition of Alloy 92 is Ni / 8Cr / 6.5Al / 6W / 3Ta / 1.5Mo / 3Re / 5Co / 1Ti / 0.15Zr / 0.01B / 0.05C / $1.1Y_2O_3$. The volume fraction of gamma prime precipitate is about 65%.
The purposes of this work were to characterize the recrystallization behaviour and evaluate the mechanical properties of the Alloy 92 by varying heat treatment conditions.
From this work, we obtained the results as follows. The as-extruded material consisted of extremely fine grains(0.3∼0.6μm), which were primary recrystallized during the hot extrusion process. The highly elongated coarse grains were developed through the secondary recrystallization by zone annealing treatment of extruded alloy in order to enhance the high temperature creep resistance. Elongated coarse grains shows strong [022] texture. Grain aspect ratio was increased with increasing hot zone temperature up to 1315℃ and with decreasing furnace travel speed down to 2cm/hr. The effect of solution treatment on the $\gamma$' precipitate's size and the stress-rupture property of ODS Ni-base superalloy was investigated at the intermediate of 760℃. When $\gamma$' size was about 0.41μm by 1/2hr/1280℃/AC(HTC2) condition, the $\gamma$' precipitates were sheared by 1/3<211> type dislocations with forming superlattice stacking faults. When the $\gamma$' size was decreased to 0.3μm by 1/2hr/1232℃/AC(HTC1) condition, the deformation was occurred by extensive slip through the matrix. As the precipitate's size increased to 0.53μm by 2hr/1280℃/AC(HTC3) condition, the Orowan bowing of dislocation around the gamma prime precipitates was promoted. The highest stress-rupture property of Alloy 92 was abtained at cuboidal $\gamma$' size of about 0.41μm by solution treatment 1280℃ for 0.5hr (HTC2). The shear deformation mode at about 0.41μm gamma prime size showed the highest creep strength than extensive matrix slip at 0.3μm (HTC1) and Orowan bowing at 0.53μm (HTC3). These showed that the sizes of $\gamma$' precipitates were closely related to the heat treatment for solutioning and this parameter could significantly affect the stress-rupture property of ODS Ni-base superalloy at the intermediate temperature. The grain structure after zone annealing was strongly depended on grain size of as-extruded mechanically alloyed oxide dispersion strengthened alloys. The optimum grain sizes for production a coarse elongated structure ranged from 0.45μm to 0.55μm in this alloy system. When the primary grain size was less than 0.45μm, preannealing heat prior to zone annealing brought into the optimum grain size, thereby producing a coarse elongated structure. In this work, preannealing temperature was chosen as 1135℃ and time was varied. Increased GAR above some critical value about 20 by applying preannealing heat treatment resulted in the increasing of rupture life more than a hundred-fold at 950℃.
