Ripening of L12 nanoparticles and their effects on mechanical properties of Ni28Co28Fe21Cr15Al4Ti4 high-entropy alloys

Development and characterisation of new high entropy alloy systems of dual and/or multiple phases

“…The volume fraction used in Figure 11.15 was 25% for 15 nm diameter and gets its maximum value at 36% for 60 nm, with linear increment. These values were picked to match the 160h specimen diameter and area fraction in the current experiment and the values reported in the literature [35,144,303,337]. As the histograms in Figure 11.7 suggest, the precipitates in both areas possess about the same diameters, which translates into both areas get mainly strengthened with the shearing mechanism with regards to 𝜎 𝑝 (1 st Mark).…”

“…It was reported that L12 nano-precipitates formed in the Al, Ti-containing alloy were correlated to the extremely high strength/ductility ratio value of 42600 MPa%, which is much higher than this of many TRIP steels (exhibiting values in the range of 15000-30000 MPa%) [141,142]. Another work based on the same high-entropy alloy system (the Ni28Co28Fe21Cr15Al4Ti4 HEA) was reported by Peng et al [144]. The samples were manufactured by vacuum arc melting and then homogenized at 1150 °C for 2 hours, water quenched and cold worked until 70% reduction of their initial thickness (followed by recrystallization at 1150 °C for 5 minutes and water quenching).…”

“…Precipitation can lead to significant strengthening of an alloy, through different morphologies and size as a function of time. Fundamentally, the aforementioned phenomenon can happen via either of the following well established mechanisms; (i) by dislocation cutting through precipitates, (ii) or/and dislocation bypass looping mechanism (Orowan), which interestingly are also reported in many HEAs [35,144,195,337]. The occurrence of each mechanism is mainly governed via the diameter of the precipitates.…”

“…Hence, particle shearing will dominate when the particle sizes are below dc (usually between 5 and 50 nm), giving its place to Orowan when exceeding a critical value. The shearing stress for the precipitates (σsh) and Orowan stress (σOr) can be expressed as: where 𝑀 is the Taylor factor of 3.06 for an FCC alloy, 𝛾 𝐴𝑃𝐵 is the antiphase boundary (APB) energy of the L12 Ni3Al with a value of 184 mJ/m 2 , b is the Burgers vector with a value of approximately 0.262 nm, l is the average distance between particles given by the following empirical equation proposed by Leskovsek et al [338] : 𝑙=𝑑×(1−𝑓)×√23•𝑓 (11.6), r0 is the dislocation core radius (r0≈b), and 𝐺 is the shear modulus of 78.5 GPa, as it was used by Peng et al [144].…”

“…The nature of the above matrices permits the extended solubility for many alloying additions and displays favorable characteristics for the precipitation of a variety of secondary phases. The most common strengthening secondary phases are the gamma prime (γ') intermetallic and ideally intergranular nano-precipitates [35,138,139,144,[292][293][294][295][296][297][298][299][300][301][302][303][304][305]. Elements like Al and Ti (along with Ni present in much greater concentrations) are usually selected as γ' stabilizers for the desirable L12 (formula: X3Y, here: Ni3(Al,Ti) ordered FCC phases) and B2 (formula: XY, here: Ni(Al,Ti) ordered BCC phases) phases.…”