Refractory metals and alloys have high melting points and high strength at elevated temperatures. But one of the main challenges for their applications is the lack of room-temperature tensile ductility and fracture toughness. While the introduction of the MPEA concept has largely expanded the design flexibility of the composition and properties of refractory alloys, the tensile ductility is still limited in general. Conventional strategies, such as twinning and phase transformation, have been demonstrated in refractory MPEAs to improve the ductility, which, however, results in the reduction of the strength, reflecting the commonly known strength–ductility trade-off in alloys. MPEAs are prone to developing local CFs, such as the local chemical order in the range of interatomic distances that has been widely reported as a microstructural characteristic in MPEAs. Local chemical order can be promoted by ageing treatment and be taken advantage of to improve the mechanical properties of MPEAs, including their tensile ductility.
Han and colleagues propose a strategy of negative mixing enthalpy (abbreviated as negative enthalpy) in solid solution to produce chemical affinity heterogeneities in a higher degree and in multiple levels to navigate the strength–ductility trade-off dilemma. As a demonstration, they show that by adding Al into HfNbTiV, hierarchical CFs form in the alloy, consisting of lamellar-shaped regions with a width of a few nanometres and length up to 100 nm (referred to as long-range CFs), nanoclusters with a diameter of 1–3 nm (medium-range CFs) and noticeable chemical variation at the atomic scale (A-CFs). Different levels of CFs help to pin the dislocations, and enhance the dislocation interaction and multiplication, thus promoting the strain hardening and plastic deformation. It is seen that both the strength and ductility increase with the higher fraction of Al element introduced in the HfNbTiVAlx alloy from x = 0 to 10, along with the augmented content of CFs. Ultimately, the HfNbTiVAl10 alloy shows tensile ductility of around 20% and yield strength of 1,390 MPa, beyond the regime that current bcc refractory MPEAs can achieve.
This is a preview of subscription content, access via your institution