Accelerating advancements in technological systems have demonstrated a need for alloys with drastically improved thermomechanical and chemical properties, called superalloys. Ceramic molds are typically used in near-net shape investment casting processes of superalloy components due to their chemical inertness and high-temperature capabilities. Ceramic molds, however, often suffer from shortcomings in vital properties including flexural strength, thermal shock resistance, permeability, dimensional stability, corrosion resistance, and leachability, which have restricted their ability to adequately process modern alloy castings. This study analyses these limitations and illustrates how to address them, particularly regarding ceramic mold and slurry design, processing of shells and cores, material selection, and testing and characterization. By utilizing advanced processing methods including additive manufacturing and gel-casting, more dimensionally accurate and preferentially built molds can be formed. Additionally, by varying the mold composition to achieve more chemically inert structures, reactions with the mold can be mitigated to reduce chemically induced defects.

An overview of ceramic molds for investment casting of nickel superalloys

Evaluation of microstructural degradation in a failed gas turbine blade due to overheating

Al2O3/Y3Al5O12(YAG) directionally solidified eutectic (DSE) crystal was prepared by optical floating zone technique. Al2O3/YAG DSE consists of continuous entangled Al2O3 and the YAG forming a three-dimensional networks structure. The volume fraction of porosity is ultra-low (0.013%) and the average equivalent diameters of most pores (>84%) are smaller than 4 μm. The Al2O3/YAG DSE shows excellent high-temperature elastic stiffness. The Young’s modulus at 1500 °C maintains more than 85% of the value at room temperature. Bending strength exhibits excellent retention up to high temperature as well. High-temperature ball indentation testing shows plastic deformation involving dislocations and twinning, which predominantly occur in Al2O3 phase, while the YAG phase is stable. Evaluation on Hv/E index predicts Al2O3/YAG DSE with moderate capability to accommodate damages. Our results highlight Al2O3/YAG DSE as excellent high-temperature structural materials.

Mechanical properties of directionally solidified Al2O3/Y3Al5O12 eutectic ceramic prepared by optical floating zone technique

This paper investigates the root cause of a failure in gas-turbine blades, made of Nimonic-105 nickel-based superalloy. The failure was reported in two blades in the second stage of a turbine-compressor of a gas turbine in the hot section. Two failed blades were broken from the root and from the airfoil. The failure took place after 20 k h of service exposure in the temperature range 700–850 °C, with the rotating speed being in the range 15,000–16,000 rpm. The microstructures of the failed blades were studied using optical/electron microscopes. Energy dispersive X-ray spectroscopy (EDS) was employed for phase identification. Results showed that failure first initiated from the root. The dominant failure mechanism in the root was concluded to be corrosion-fatigue. The failure scenario was suggested based on the results obtained.

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N. Tabrizi

Papers

Failure analysis of gas turbine first stage blade made of nickel-based superalloy