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The present article deals with effect of Cr addition (10 at.%) on the partitioning behavior and the consequent effect on mechanical properties for tungsten-free γ-γ′ cobalt-based superalloys with base alloy compositions of Co-30Ni-10Al-5Mo-2Ta (2Ta) and Co-30Ni-10Al-5Mo-2Ta-2Ti (2Ta2Ti). Cr addition leads to a change in the morphology of the strengthening cuboidal-shaped γ′ precipitates to a spherical shape. The site preference of Cr atoms in two alloy systems (with and without Ti) has been experimentally investigated using atom probe tomography with the supportive prediction from first principles DFT-based computations. Cr partitions more to the γ matrix relative to γ′. However, Cr also has a strong effect on the Ta and Mo partitioning coefficient across γ/γ′ interfaces. The value of partition coefficient for Mo ( K ) becomes <1 with Cr addition to the alloys. Results from ab initio calculations show that the Cr atoms prefer to replace Mo atoms in the sublattice sites of the L1 unit cell. The solvus temperature of about 1038 and 1078 °C was measured for 10Cr2Ta and 10Cr2Ta2Ti alloy, respectively, and these Cr-containing alloys have very low densities in the range of ~8.4-8.5 gm/cm. The 0.2% compressive proof strength of 10Cr2Ta2Ti alloy yields a value of 720 MPa at 870 °C, substantially better than most Co-Al-W-based alloys and many of the nickel-based superalloys (e.g., MAR-M-247). [ABSTRACT FROM AUTHOR]

Aluminium alloys can be strengthened significantly by nano-scale precipitates that restrict dislocation movement. In this study, the evolution of inhomogenously distributed trialuminide precipitates in two multi-component alloys was characterised by synchrotron small-angle X-ray scattering (SAXS). The appropriate selection of reference sample and data treatment required to successfully characterise a low volume fraction of precipitates in multi-component alloys via SAXS was investigated. The resulting SAXS study allowed the analysis of statistically significant numbers of precipitates (billions) as compared to electron microscopy (hundreds). Two cast aluminium alloys with different volume fractions of Al 3 Zr x V 1- x precipitates were studied. Data analysis was conducted using direct evaluation methods on SAXS spectra and the results compared with those from transmission electron microscopy (TEM). Precipitates were found to attain a spherical structure with homogeneous chemical composition. Precipitate evolution was quantified, including size, size distribution, volume fraction and number density. The results provide evidence that these multi-component alloys have a short nucleation stage, with coarsening dominating precipitate size. The coarsening rate constant was calculated and compared to similar precipitate behaviour. [ABSTRACT FROM AUTHOR]