Alloy Design and Optimization of Mechanical Properties of High-Entropy Alloys

Licentiatavhandling, 2016

High-entropy alloys (HEAs) are described as alloys containing multi-principal elements in equal or close to equal atomic percentage. HEAs are considered as potential structural materials for high-temperature applications; where alloy design and optimization of mechanical properties is extremely critical. In this regard, achieving both high strength and high tensile ductility is still a great challenge. Compared to conventional alloys, HEAs have high configurational entropy, which tends to stabilize the solid solution formation, mainly face-centered-cubic (fcc) and/or body-centered-cubic (bcc) solid solutions. Generally, fcc-type HEAs are ductile but soft, while bcc-type HEAs are hard but brittle.     One part of this work is to understand the solid solubility in HEAs. The need for single-phase solid solution and controlling the formation of TCP/GCP phases, is addressed through the molecular orbital approach. The output of this approach is the Md parameter, the d-orbital energy level, which can well describe the solubility limit in fcc HEAs comprising of only 3d transition metals. However, Md alone cannot describe the solid solubilities in fcc HEAs, which also contain 4d elements. Alloying of 4d elements with 3d elements will cause a large increase of bond order, Bo, which is the measure of the strength of covalent bonds. The use of two-parameter Md - Bo plot can improve the prediction of solid solubility limit when 4d elements are alloyed, but needs further work. The Md approach for bcc HEAs containing 4d elements is also encouraging, but requires more evidence to support this alloy design approach. The second part of this work is to ductilize HEAs containing group IV (Ti, Zr, Hf), V (V, Nb, Ta) and VI (Cr, Mo, W) refractory elements where inadequate ductility puts a limit on their mechanical performance for structural applications. A strategy is proposed here to design refractory HEAs with yield strength reaching 900 MPa, and importantly with sufficient ductility at room temperature. Ductility is introduced by maintaining the number of total valence electrons low, which can be controlled by adjusting the alloy compositions. These findings will shed light on the design of refractory HEAs with optimal mechanical properties.