Synthesis and structural characterization of Co-Al based superalloys

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2019
Dinler, Ali Fırat
Co-based superalloys are commonly used in static components such as stationary vanes because of their superior hot corrosion, oxidation, sulfidation resistance and phase stability at elevated temperatures. They possess relatively lower strength values due to limits in strengthening with solid solution and/or carbide precipitation that makes them unusable for blade and disk applications requiring higher strength values. However, Co-based superalloys with L12 ordered γ’-Co3(Al,W) microstructure has recently drawn attention as a suitable alloy structure which could be a substitute for the now-days extensively-used Ni-based superalloys. In the first part of the investigation on the synthesis and structural characterization of Co-Al based superalloys; synthesis route, heat treatment regime and structural characterization of binary Co-Al alloys were presented. Co-rich side of Co-Al binary system was investigated by preparing 6 different alloy compositions (1.23, 8.40, 16.72, 19.50, 21.50 and 25 at. % Al), which were produced by arc melting technique. It has been found that relatively high Al content (≥ 16.72 at. % Al) seemed to stabilize fcc-Co in as-cast state. For low Al containing alloys (≥ 8.40 at. % Al) CoAl precipitation was not detected upon cooling from liquid state. It was shown that for these alloys fcc to hcp transformation can occur via martensitic manner not requiring fast cooling rates. It is interesting to note that, microstructure and composition of as-cast and annealed (1300°C for 24 hours followed by furnace cooling) states of Co-Al binary alloys disagree and are not consistent with current equilibrium phase diagrams published in literature. The superior properties of Co-based superalloys are originated from coherently distributed L12 type ordered Co3(Al,W)-γ’ precipitates within disordered γ matrix of Co-Al-W alloys. The phase stability of these ordered γ’ precipitates in the extremely harsh service environment strongly depends on the ordering characteristics as well as coherency between γ and γ’ phases. The aim of the second part of this study is to promote precipitation hardened Co-based superalloy microstructure for high-temperature applications. Therefore, the effects of tertiary alloying “W” element addition and heat treatments (including different aging times) on the solidification microstructures, structural stability and the mechanical properties of the Co-Al-W alloy systems with different Al & W content were presented. Through solutionizing and aging procedure until 256 h, γ/γ’ microstructure was achieved. During the aging treatment, γ/γ’ dual microstructure disturbed beneath the oxide layer and phases, namely Co3W and Co7W6, that can degrade microstructural properties were formed. Structural investigations were performed by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. Micro-hardness (Vickers) measurements were performed on as cast and heat-treated samples by using a Shimadzu Micro Hardness Tester.