Evaluation of Al3Mg2 Precipitates and Mn-Rich Phase in Aluminum-Magnesium Alloy Based on Scanning Transmission Electron Microscopy Imaging

Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) were used to observe intergranular and intragranular ??-phase (Al₃Mg₂) formation and growth in as-received sample and long-term (~1?year) thermally treated samples of 5083-H131 alloy. Rod-shaped and equiaxed particles rich in Mn, Fe, and Cr were present in the as-received and heat treated samples. The ??-phase precipitated along grain boundaries as well as around and between preexisting Mn-Fe-Cr rich particles. The measured thickness of ??-phase along grain boundaries was lower than Zener?CHillert diffusion model predicted value and the potential reasons were theoretically analyzed. Dislocation networks, grain boundaries, and different preexisting particles were observed to contribute to Mg diffusion and ??-phase precipitation.     

Resistivity and Transmission Electron Microscopy Investigations of Ordering Transformation in Stoichiometric Ni2(Cr0.5Mo0.5) Alloy

The evolution of long-range order (LRO) from short-range order (SRO) and vice versa was studied by combining resistivity measurement and transmission electron microscopy (TEM) investigations on solution-treated samples of composition Ni₂(Cr_0.5Mo_0.5). The progress of microstructural evolution during the isochronal run for the resistivity measurement was followed. The measured variation in the intensity of superlattice spots during SRO to LRO transition was correlated with the resistivity profile obtained during the isochronal run. The reappearance of SRO was observed after the disappearance of LRO. The isothermal transformation temperature for the appearance of the long-range ordered state, the beginning of disappearance of the long-range ordered state, and the reappearance of the short-range ordered state in the solution-treated sample were determined from the resistivity curves obtained for different heating rates. The resistivity profile was qualitatively explained on the basis of ordering and clustering of solute atoms associated with the formation of different ordered phases obtained from microscopic evidence.