Studies on microstructure, mechanical and high-temperature oxidation behaviour of tungsten containing high entropy alloys

Abstract

Novel AlCrFeMnNiWx (x= 0, 0.05, 0.1 & 0.5 mol) high-entropy alloys (HEAs) have been synthesized by both powder metallurgy and vacuum arc melting routes. The addition of tungsten on the phase evolution in the spark plasma sintered and arc melted sample was investigated using X-Ray diffractometry (XRD). HEAs' morphology and composition were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The detailed phase and microstructural characterization of arc melted HEAs reveal the presence of BCC Fe-Cr Mn-rich (𝞫1) primary phase and BCC Ni-Al-rich (𝞫2) secondary dendritic phase. In the powder metallurgy route, during mechanical alloying, solid solution was formed with the formation of major BCC and minor FCC phases fraction in AlCrFeMnNiWx (x=0, 0.05, 0.1 & 0.5 mol) HEAs while additional ordered B2 and σ phases were found after the SPS. The phase stability with respect to temperature has been studied using a differential scanning calorimeter (DSC) and thermal gravimetric analyzer (TGA). Thermodynamic parameters for AlCrFeMnNiWx alloys are calculated and studied to explain the formation of a solid solution. The formation and growth of the AlFe phase in AlCrFeMnNiWx HEAs have been described schematically, mainly due to the high enthalpy difference between the binary elements. Furthermore, microhardness has been evaluated in alloy systems. In addition, to establish the relation between the alloying composition and hardness values, an artificial neural network (ANN) model has been approached to predict the alloys' hardness with different elemental compositions. The designed spark plasma sintered HEAs exhibit excellent hardness (8.31 – 13.57 GPa) and high elastic modulus (165.52 GPa - 202.3 GPa), strongly dependent upon the tungsten content while experimented by the nanoindentation. Further, The HEAs are utilized for heat treatment to better understand the alloy behavior due to different processing routes. A set of heat treatments has been performed at 800℃, 1000℃, and 1200℃. HEAs' microstructure and crystal structure have been compared by using a Scanning Electron microscope (SEM) and X-ray diffractometer. This study reveals a drastic change in the morphology in arc melted samples. Admittedly, the spark plasma sintered sample has coarsening appropriately in the presence of tungsten in the alloy system. Afterward, microhardness testing has been performed to understand the alloys' softness behavior. The alloys prepared by different routes have become softer and more homogeneous after heat treatment. A separate study was also performed to evaluate the possibility of hydrogen storage in the equiatomic high entropy alloy system. A novel high entropy alloy (AlCrFeMnNiW) is synthesized via high-energy planetary ball milling with an average crystallite size of 10.37 nm. The hydrogen storage behavior of this alloy is investigated through the gravimetric method. The hydrogen storage capacity is observed to be 0.615 wt% at atmospheric pressure and temperature. The microstructural characterization of an alloy is carried out utilizing X-Ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray (EDX) analysis to determine its lattice parameters, crystallite size, chemical composition, etc. The unit cell volume of the as prepared alloy is estimated as 0.03131 nm3, whereas the average crystallite size is 10.37 nm. It is observed that the unit cell volume is increased by 0.67%, whereas the crystallite size decreased by ~10.8% upon hydrogenation. The dehydrogenation of the sample is performed using thermogravimetry analysis (TGA) with different scanning rates. Activation energy during hydrogen desorption is found to be – 8.161 kJ/mol. The enthalpy and entropy of the mixing are estimated to be -2.645 kJ/mol and 1.793R J/mol K. For the oxidation study, an extensive experiment has been performed, and every oxidation experiment was repeated to check the reproducibility of the oxidation results. Initially, spark plasma sintered (SPS) AlCrFeMnNiWx (x = 0, 0.05, 0.1, 0.5 mol) HEA oxidized isothermally at 200℃, 500℃, 700℃, 800℃, and 850℃ using thermal gravimetric analyzer (TGA) for 50 hr. XRD, SEM, and Raman spectroscopy were utilized to investigate the oxidized samples. The HEAs exhibited multifarious behavior while adding tungsten and shows the formation of various oxides. Admittedly, alloying constituent significantly affects the oxidation behavior due to complex oxide layers and outstanding oxidation resistance at high temperatures confirms that the proposed HEAs would be applicable for high-temperature applications. Furthermore, an artificial Neural Network (ANN)-based model has been developed for the prediction of the hardness of arc melted and powder metallurgy route samples. A dataset of 16 samples has been utilized for the arc melted HEAs in which sensitivity analysis has also been reported due to less available dataset. While A particular class of HEAs by using 36 HEAs available data from the literature in case of powder metallurgy route HEAs. The model simulates the data by utilizing training, validation, and testing methods in a useful way. A backpropagation ANN has been used to predict the hardness value with an accuracy of 95.9% and 93.54% for powder metallurgy and arc melted HEAs, respectively. The developed model's predicted capability also provides the freedom to choose the HEA composition with the required hardness of HEA without any experimental trials. Eventually, ANN has also been explored to understand the oxidation behavior of the proposed HEAs.