Microstructure and Mechanical Properties of TixNbMoTaW Refractory High-Entropy Alloy for Bolt Coating Applications

Abstract

High-strength bolts are prone to crack initiation from the threaded hole during fastening due to large loads, which can compromise their performance and reliability. To enhance the durability of these bolts, coatings are often employed to strengthen their surfaces. NbMoTaW refractory high-entropy alloy coatings are widely used in hard coating applications due to their exceptional mechanical properties. However, the brittleness of this alloy at room temperature limits its performance in high-stress environments. To enhance the ductility of NbMoTaW alloys, this study systematically investigates the effect of varying titanium (Ti) content on the alloy’s properties. First-principles calculations were employed to analyze the elastic properties of TixNbMoTaW alloys, including elastic constants, the elastic modulus, the bulk modulus (B)-to-shear modulus (G) ratio (Pugh’s ratio), Poisson’s ratio (ν), and Cauchy pressure (C12–C44). The results indicate that the addition of Ti significantly improves the alloy’s plasticity. Specifically, when the Ti content is x = 2, the B/G ratio increases to 3.23, and Poisson’s ratio increases to 0.39, indicating enhanced deformability. At x = 0.75, the elastic modulus (E) increases to 273.78 GPa, compared to 244.99 GPa for the original alloy. The experimental results further validate the computational findings. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses indicate that all alloys exhibit a single body-centered cubic (BCC) phase. Room-temperature compression tests show that as the Ti content increases, the yield strength, fracture strength, and plasticity of the alloys significantly improve. Specifically, for a Ti content of x = 0.75, the yield strength reaches 1551 MPa, the fracture strength is 1856 MPa, and the plastic strain increases to 14.6%. For Ti1.5NbMoTaW, the yield strength is 1506 MPa, the fracture strength is 1893 MPa, and the plastic strain is 17.3%. Overall, TixNbMoTaW refractory high-entropy alloys demonstrate significant improvements in both plasticity and strength, showing great potential for coating applications in high-stress environments.