Abstract
This study investigates the feasibility of thermophysical parametric calculations and CALPHAD-based predictions in the alloy design of novel septenary Ti-Al-based high entropy alloys (HEAs), aimed at improving the mechanical properties, high-temperature stability, and low-density for high-temperature applications in the aerospace industry. The research employed spark plasma sintering (SPS), an advanced powder metallurgy technique, for the fabrication of the designed alloys. The CALPHAD-based computational tool was leveraged to reduce production time and material waste, facilitating innovative multicomponent alloy development characterized by complex structures and diverse compositions. Key thermophysical parameters, including valence electron concentration (VEC), enthalpy of mixing (π₯π»πππ₯), configurational entropy (π₯ππππ₯), atomic size difference (πΏ), interaction parameters (πΊ), and electronegativity difference (π₯π), were computed to predict crystal structures and solid-solution phase formations. The equiatomic alloy (Ti14.286Al14.286Cr14.286Nb14.286Ni14.286Cu14.286Co14.286) was identified as a single-phase (BCC) HEA, while other non-equiatomic HEAs- Ti20Al20Cr5Nb5Ni19Cu12Co19 (6.88), Ti20Al20Cr5Nb5Ni18Cu14Co18 (6.91) and Ti20Al20Cr5Nb5Ni17Cu16Co17 (6.94) exhibited dual-phase characteristics (BCC+FCC) influenced by π₯π»πππ₯ values slightly exceeding the required optimal range (-22 β€ π₯π»πππ₯ β€ 7). intermetallic phases were strategically incorporated to enhance the mechanical strength of the alloys. ThermoCalc software 2021b with TCHEA5 HEAs encrypted database utilized as a reliable CALPHAD-based tool to perform thermodynamic simulation of the HEAs, multi-phase crystal structures, predominantly BCC with other phases C15-laves, FCC_L12, Sigma and Heusler were predicted. The softwareβs Property Model Calculator indicated that total hardness increased with temperature, while intrinsic hardness decreased, highlighting the materials' behaviour during processing versus service conditions respectively. Notably, the increase in copper (Cu) content among the non-equiatomic HEAs prompted a phase transition from BCC to FCC, correlating to variations in predicted hardness. To streamline experimental validations, Design Expert software was employed for optimization, applying response surface methodology (RSM) to determine the influence of milling time (MT) and sintering temperature (ST) on relative density (RD), percentage porosity (PP), and microhardness (MH). The optimized conditions yielded RD between 99.72% and 99.9%, minimal PP (0.02% to 0.28%), and MH values ranging from 580.1 HV to 859.6 HV. The low porosity levels are advantageous for enhancing mechanical properties and durability, making these materials suitable for demanding engineering
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applications. Experimental assessments conducted through XRD characterization, SEM-EDS examination, microhardness testing and nanoindentation study confirmed the crystallite sizes, microstrain, phase characteristics and mechanical properties as predicted by ThermoCalc. The XRD result shows that Ti14.286Al14.286Cr14.286Nb14.286Ni14.286Cu14.286Co14.286 has the largest crystallite size of 32.20E-3 and the least microstrain of 0.1689, whereas Ti20Al20Cr5Nb5Ni19Cu12Co19 exhibited the smallest crystallite size (10.38E-3) and largest microstrain (0.3045). With all the component elements spotted in the EDS spectra, the phases identified were also quantified with the aid of the SEM-EDS showing the BCC phase as the predominant phase among other noticeable phases (FCC_L12, Sigma, Heusler, and C15_Laves). The XRD and SEM examination confirmed the presence of all phases predicted by the ThermoCalc software. At a least penetration depth of 427.822 nm, the nanohardness and elastic modulus results of Ti20Al20Cr5Nb5Ni19Cu12Co19 HEA exhibited the highest value (15.185 GPa and 246.92 GPa respectively) compared to 13.905 GPa and 188.22 GPa obtained for the equiatomic grade Ti14.286Al14.286Cr14.286Nb14.286Ni14.286Cu14.286Co14.286 with a penetration depth of 462.57 nm. The microhardness measurements also correspond with the trend of the hardness values observed with the ThermoCalc predictions. Comparatively, an optimum value of microhardness was also observed in Ti20Al20Cr5Nb5Ni19Cu12Co19 HEA (859.08 HV), and this can be attributed to the effective distribution of the intermetallics within the BCC matrix, thereby enhancing dislocation density