Search

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]

Designing engines with higher power densities to comply with ever increasing emissions and fuel economy regulations requires the development of cost-effective alloys with superior properties than existing high-performance alloys. A new class of Nb-bearing austenitic heat-resistant cast steels showed promise to withstand exhaust gas temperatures of as high as 1000 °C. This paper describes the development of this new class of alloys using combined CALPHAD (CALculation of PHAse Diagrams) and experimental approaches to investigate the effect of C and N additions on the microstructure and tensile properties. Composition ranges were first established based on CALPHAD predictions to achieve the desired phases, and then four alloys of varying N/C ratios were cast and experimentally characterized to determine quantitatively the microstructure and mechanical properties. Microstructural characterization revealed that the NbC/Nb(C,N) phase transformed from “Chinese-script”, a mixed flake-blocky morphology to faceted-blocky morphology as the N/C ratio increased. The area fractions of the different phases were quantified and agreed favorably with the calculated results. Alloys with the “Chinese-script” NbC/Nb(C,N) showed favorable tensile properties at room temperature and 1000 °C. Excessive δ-ferrite diminished the tensile resistance at 1000 °C, while limited quantity of (Cr, Fe) 23 C 6 did not show adverse effects. [ABSTRACT FROM AUTHOR]