Quantification of the minor precipitates in UDIMET™ alloy720(LI) using electrolytic extraction and X-ray diffraction1

A study on the quantification of the minor precipitates such as σ-phase, carbide and boride in UDIMET™ alloy720 (U720) and alloy720LI (U720LI) was carried out. A quantitative X-ray diffraction technique combined with electrolytic extraction has been used to quantify the micro-constituents. It was shown that U720LI is much less prone to σ formation than U720. At 800oC, the weight fraction of σ in U720LI remained at a level as low as ≈0.1%, even after 3000 h of exposure. In U720, on the contrary, it increased remarkably with exposure up to 1000 h and then saturated at ≈5%. A close examination of the σ formation mechanism using transmission electron microscopy (TEM), shows that σ forms and grows in consumption of γ′-phases. M23C6-type carbide and M3B2-type boride precipitates did not show any significant variations of amount in both alloys, U720LI and U720, during long time exposure at 800oC.