HEAT-RESISTANT COBALT-BASED ALLOYS

Cobalt-based alloys are widely used for manufacturing of various components of gas turbine engines and gas turbines such as vanes and combustion chambers both in wrought state and as cast parts. They have been designed for improving the heat resistance due to solid solution and carbide-strengthening mechanisms. In order to obtain satisfactory oxidation resistance and hot corrosion resistance cobalt-based alloys are doped with sufficient amount of chromium (above 15 % wt.). Recently additive manufacturing has started to use cobalt-based alloys. The paper considers the features of the structure of high-temperature cobalt-based alloys and their application in various branches of industry.

Mechanical Performance and Microstructural Evolution of (NiCo)75Cr17Fe8Cx (x = 0~0.83) Medium Entropy Alloys at Room and Cryogenic Temperatures

We investigated the effects of the addition of Co and carbon on the deformation behavior of new medium-entropy alloys (MEAs) designed by increasing the entropy of the conventional NiCrFe-type Alloy 600. The strength/ductility combination of carbon-free (NiCo)75Cr17Fe8 MEA was found to be 729 MPa/81% at 298 K and it increased to a remarkable 1212 MPa/106% at 77 K. The excellent strength and ductility of (NiCo)75Cr17Fe8 at cryogenic temperature is attributed to the increased strain hardening rate caused by the interaction between dislocation slip and deformation twins. Strength/ductility combinations of carbon-doped (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at cryogenic temperature were observed to be 1321 MPa/96% and 1398 MPa/66%, respectively, both of which are superior to those of other high-entropy alloys (HEAs). Strength/ductility combinations of (NiCo)75Cr17Fe8C0.34 and (NiCo)75Cr17Fe8C0.83 at room temperature were found to be 831 MPa/72% and 942 MPa/55%, respectively and both are far superior to 676 MPa/41% of the commercial Alloy 600. Yield strengths of carbon-free and carbon-doped alloys comprised strengthening components from the friction stress, grain size strengthening, carbide strengthening and interstitial strengthening and excellent agreement between the predictions and the experiments was obtained. A design strategy to develop new MEAs by increasing the entropy of the conventional alloys was found to be effective in enhancing the mechanical performance.

Microstructure and micro-hardness of Ni-based surfacing coating on the Cr12MoV die steel

Ni-based surfacing coating was fabricated on the Cr12MoV die steel utilizing the ENiCrFe-2 alloy. The defect-free surfacing coating was obtained with a good metallurgical bonding between the coating and the substrate. The microstructure, element distribution, and micro-hardness were analyzed. Results showed that the microstructure was composed of planar crystals at the bottom of coating, and it consisted of columnar crystals at the middle region. There existed a diffusion layer with a width of about 100 μm at the ENiCrFe-2–Cr12MoV interface. Three regions were formed in the heat-affected zone with various microstructure morphologies. The average micro-hardness of coating was 275 HV0.5, which was more than that of Cr12MoV. Micro-hardness was dramatically increased (765 HV0.5) in the middle of the heat-affected zone. This was caused by the combined effect of grain refinement, solid solution strengthening, second-phase strengthening, and primary carbide strengthening. Moreover, the formation mechanism of middle heat-affected zone was discussed in this paper in detail.

Research on the Properties of Co-Tic and Ni-Tic Hip-Sintered Alloys

Three types of sintered alloys were fabricated based on cobalt, nickel and high-temperature alloy ZhS32-VI matrix with titanium carbide strengthening phase. TiC content was in a range of 30–50 vol. %. The melting temperatures of alloys are higher than 1320°C, and they may undergo undamaged through all technological procedures together with turbine blades, including soldering and outgassing. DSC analyses indicates no additional thermal effects until melting, which confirms their structural stability. The examinations of microstructure revealed three types of constituents – TiC particles, matrix solid solution and blow outs – structural defects having negative effects on all the studied properties. It was found that heat resistance of nickel based sintered alloys at the temperature of 1100°C is superior as compared with the alloys based on cobalt and alloy ZhS32-VI. It has been established that wear resistance in conditions of fretting wear at temperatures of 20, 850, 950 and 1050°C of sintered alloy with ZhS32-VI matrix is mostly superior as compared with the other alloys. The properties of produced alloys allow to use them for manufacturing of components of friction couples operating in conditions of high temperature fretting wear, including protective pads of turbine blades top shrouds contact faces.