Effects of Transient Thermal Shock on the Microstructure and Mechanical Properties of CoCrFeNiMn High-Entropy Alloy Coatings

CoCrFeNiMn high-entropy alloy (HEA) has great potential for engineering application due to its good ductility and high fracture toughness at low temperature. It can be deposited on components as coatings to take advantage of its excellent properties and reduce the cost. In this study, CoCrFeNiMn HEA coatings were deposited on 316L stainless steel substrates by atmospheric plasma spraying (APS) technique, and a series of transient thermal shock tests were performed. It was found that the coatings contained two main phases: a face-centered cubic (FCC) solid solution phase and a flocculent oxides phase. The elemental contents of Co, Cr, Fe, and Ni were close to equal atomic percentage in the coating, while Mn was reduced significantly. The oxygen was mainly distributed in the dark flocculent oxides phase. After transient thermal shock tests, these two phases remained stable, but some tiny cracks appeared on the surface. Meanwhile, the microhardness of the coating after transient thermal shock tests also showed stable, ∼ 420 HV. Weibull statistics were used to analyze the reliability of the microhardness, and the Weibull modulus m was distributed from 9 to 15. The CoCrFeNiMn HEA coating exhibited high phase stability and excellent properties under transient thermal shock, making it have service advantages in extreme environments, especially in the fields of the development of future nuclear and aerospace structural materials.

Fracture Toughness Characteristics of High-Manganese Austenitic Steel Plate for Application in a Liquefied Natural Gas Carrier

High-manganese austenitic steel was developed to improve the fracture toughness and safety of steel under cryogenic temperatures, and its austenite structure was formed by increasing the Mn content. The developed high-manganese austenitic steel was alloyed with austenite-stabilizing elements (e.g., C, Mn, and Ni) to increase cryogenic toughness. It was demonstrated that 30 mm thickness high-manganese austenitic steel, as well as joints welded with this steel, had a sufficiently higher fracture toughness than the required toughness values evaluated under the postulated stress conditions. High-manganese austenitic steel can be applied to large offshore and onshore LNG storage and fuel tanks located in areas experiencing cryogenic conditions. Generally, fracture toughness decreases at lower temperatures; therefore, cryogenic steel requires high fracture toughness to prevent unstable fractures. Brittle fracture initiation and arrest tests were performed using 30 mm thickness high-manganese austenitic steel and SAW joints. The ductile fracture resistance of the weld joints (weld metal, fusion line, fusion line + 2 mm) was investigated using the R-curve because a crack in the weld joint tends to deviate into the weld metal in the case of undermatched joints. The developed high-manganese austenitic steel showed little possibility of brittle fracture and a remarkably unstable ductile fracture toughness.

The Fabrication and Mechanical Properties of Laminated ZrB2-Mo5SiB2 Ceramics with an Mo-Mo5SiB2 Interlayer

The excellent physical and chemical properties of ultra-high temperature ceramics make them suitable for many high-temperature structural components, while their poor toughness and high sintering temperature become key limitations to their application. Laminated toughening has long been considered an effective toughening method to improve the mechanical properties of ceramics. In this study, laminated ZrB2-Mo5SiB2 ceramics with an Mo-Mo5SiB2 interlayer were fabricated by tape casting and hot press sintering at 1900 °C for 2 h. Different layer thickness ratios between the matrix layer and the interlayer were designed to illustrate the toughening mechanism. Both the fracture toughness and flexural strength of the laminated ceramics showed a trend of first increasing and then decreasing with the increase of the layer thickness ratio. High fracture toughness (9.89 ± 0.26 MPa·m1/2) and flexural strength (431.6 ± 15.1 MPa) were obtained when the layer thickness ratio was 13. The improvement in fracture toughness of the laminated ceramics could be attributed to the generation of the residual stress, the deflection and the bifurcation of the cracks. Residual stress that developed in the laminated ceramics was also evaluated.

Properties of Spark Plasma Sintered Compacts and Magnetron Sputtered Coatings Made from Cr, Mo, Re and Zr Alloyed Tungsten Diboride

To enhance the properties of tungsten diboride, we have synthesized and characterized solid solutions of this material with chromium, molybdenum, rhenium and zirconium. The obtained materials were subsequently deposited as coatings. Various concentrations of these transition metal elements, ranging from 0.0 to 24.0 at.%, on a metals basis, were made. Spark plasma sintering was used to synthesize these refractory compounds from the pure elements. Elemental and phase purity of both samples (sintered compacts and coatings) were examined using energy dispersive X-ray spectroscopy and X-ray diffraction. Microindentation was utilized to measure the Vickers hardness. X-ray diffraction results indicate that the solubility limit is below 8 at.% for Mo, Re and Zr and below 16 at.% for Cr. Above this limit both diborides (W,TM)B2 are created. Addition of transition metals caused decrease of density and increase of hardness and electrical conductivity of sintered compacts. Deposited coatings W1−xTMxBy (TM = Cr, Mo, Re, Zr; x = 0.2; y = 1.7–2) are homogenous, smooth and hard. The maximal hardness was measured for W-Cr-B films and under the load of 10 g was 50.4 ± 4.7 GPa. Deposited films possess relatively high fracture toughness and for WB2 coatings alloyed with zirconium it is K1c = 2.11 MPa m1/2.

Influence of Ti Addition on the Strengthening and Toughening Effect in CoCrFeNiTix Multi Principal Element Alloys

Multi principal element alloys have attracted interests as a promising way to balance the bottleneck of the “inverse relationship” between high hardness and high fracture toughness. In the present study, the authors demonstrate the effects of Ti addition on the microstructures and mechanical properties of the CoCrFeNiTix alloys (x values in molar ratio, x = 0.7, 1.0, and 1.2), which exhibits a multi-phase structure containing face-centered cubic phase and various secondary phases, such as sigma, Laves, and (Cr,Fe)-rich phase. Throughout the combined experimental examination and modeling, we show that superb hardness (~9.3 GPa) and excellent compressive strength (~2.4 GPa) in our alloy system are attributed to solid-solution strengthening of the matrix and the formation of hard secondary phases. In addition, high indentation fracture toughness is also derived from the toughening mechanism interplay within the multiple-phase microstructure. At the fundamental level, the results suggest that multi-principal element alloys containing dual or multi-phase structures may provide a solution for developing structural alloys with enhanced strength-toughness synergy.

Fracability Evaluation Method and Influencing Factors of the Tight Sandstone Reservoir

Fracability evaluation is the basis of reservoir fracturing and fracturing zone optimization. The tight sandstone reservoir is characterized by low porosity and low permeability, which requires hydraulic fracturing to improve industrial productivity. In this study, a systematic model was proposed for the fracability evaluation of tight sandstone reservoirs. The rock mechanics tests and sonic tests demonstrated that tight sandstone reservoir is characterized by high brittleness, high fracture toughness, and weak development of natural fractures. Numerical simulation was used to analyze the change of reservoir parameters during hydraulic fracturing and the influence of in situ stress on fracture propagation. The results showed that when the horizontal stress anisotropy coefficient is small, natural fractures may lead hydraulic fractures to change direction, and complex fracture networks are easily formed in the reservoir. The horizontal stress anisotropy coefficient ranges from 0.23 to 0.52, and it is easy to produce fracture networks in the reservoir. A new fracability evaluation model was established based on the analytic hierarchy process (AHP). The fracability of tight sandstone reservoir is characterized by the fracability index (FI) and is divided into three levels. Based on the model, this study carried out fracability evaluation and fracturing zone optimization in the study area, and the microseismic monitoring results verified the accuracy of the model.