Design of high temperature cobalt-based alloys processed by powder metallurgy route

Resumo

The range of application for superalloys has increased in many areas since their development for use in the aviation industry. An increasing market demand requires new finding associates with such alloys. The cobalt-based superalloys have been studied in the literature since the discovery of the stable ternary Co3(Al,W) compound with an ordered L12 precipitates structures by Sato et al. in 2006. It is well known that the Co-Al-W system has provided a good alternative for these demanding applications due to an ability to retain most of their strength even after long exposition times and temperatures. Xue et al. reported that the novel ternary system has been designed to form rafted γ'-cuboidal precipitates embedded in a continuous γ-matrix, with many similarities with the γ/γ' dual phase of nickel based superalloys. However, to compete better with nickel based alloys it is necessary to increase the temperature range of stability of γ’-Co3(Al,W). Yan et al. reported that ternary Co-Al-W alloys were previously discarded due to their lower high temperature strength, limited by the γ’ solvus temperature, about 100 °C - 300 °C lower than the present nickel based alloys. Bauer et al. shows how addition of 2 at. % -Ta as well as -Ti on the ternary alloy produced a substantial increase in the γ’ volume fraction. However, Suzuki et al. determined how cobalt-based with -Ta element addition has a strength comparable to a conventional nickel-based alloys at 900 °C. Suzuki and Pollock also studied that -Ta element is effective for stabilization of the γ’ phase, while an addition of Cr decreases the γ’ solvus temperature. Pollock et al. reported that cobalt-based alloys have significantly higher solidus and liquidus temperatures compared to the nickel-based alloys, typically 100 to 150 °C higher. Bauer et al. studied how different alloying elements, as W, Ta, Ti, Nb and V, increase the γ’ volume fraction and γ’ solvus temperature. The tensile creep behavior was studied by Xue et al. indicating that the creep properties of experimental alloy exceeded commercial 1st generation nickel-based single crystal superalloy. Two alloying elements, such as -Ti and –Ta, are of special interest as they are known to increase the range of stability of γ’ and the volume fraction of the phase. Ishida reported that the addition of alloying elements was found to be very similar to that of nickel-based alloys, where Ti, -Ta, -Nb and V are the γ’ stabilizing elements. Xue et al. remarked that γ’-phase stabilize better by adding -Ti and -Ta elements. It is also shown that -Ta addition in the cobalt system can improve the high temperature strength. The mechanical response of cobalt-based alloys has been usually measured at low strain rates, such as, compression and tensile tests. No evidences of high strain rates studies have been found in the cobalt superalloys literature. The majority of the material data properties were obtained under quasi-static loading conditions but in a wide range of situations, materials are subjected to impact or explosion conditions. A better knowledge of the fundamentals of dynamic behavior in these materials could be critical for enhancing alloy development. The main objective of this thesis is to develop a novel cobalt-based superalloy processed by powder metallurgy (PM) route and to study the dynamic behavior of this alloy at high strain rates. With this aim, a powder route based on the mix of elemental powders processed by Mechanical Alloying (MA) and consolidated by Field Assisted Sintering Techniques (FAST) was proposed. Previously, the ternary system was designed by thermodynamic simulation (CALPHAD), to predict the nominal composition Co-12Al-10W. Based on prior research works, another composition with -Ti and -Ta (2 at. %) was also developed. Subsequently, different alternatives of heat treatments have been suggested to investigate their effect in the γ/γ’ two phase microstructures and to optimize the morphological properties of the desired alloys. In addition, a conventional casting route was also determined to compare the morphological properties, as well as mechanical properties. Finally, a dynamic study with Hopkinson bar test was performed with the aim of characterizing the impact behavior of cobalt-based specimens under high strain rates (1000 s-1) and different temperatures, allowing to identify an anomalous positive flow stress in the range of temperatures of (700 – 800 ºC). A modified Johnson-Cook model was considered with the aim of adjusting the mathematical model with the experimental tests.