Seeing transformations as they happen
State-of-the-art heating stages for transmission electron microscopy (TEM), based on MEMS technology, give close control of sample temperature and stability of sample position. Yurii Ivanov and colleagues in Lindsay Greer’s group in the Department, together with Kostas Georgarakis of Cranfield University, have exploited this in-situ heating to characterize the complex sequence of transformations as a gold-based metallic glass (MG) is heated towards equilibrium. The MG composition, Au76.3Ag4.7Pd1.9Cu13.5Si3.6 (wt.%) has a gold content surpassing that required for the 18 carat hallmark. MGs are attractive for jewellery because they can be thermoplastically formed and their hardness (much higher than for conventional alloys) confers scratch resistance.
The studied MG (developed by Schroers et al. at Yale) suffers from tarnishing. In adapting the composition to solve this problem, knowledge of the crystallization is vital. Crystallization may also bring benefits in allowing for control of alloy colour and in improved mechanical properties exploiting the remarkable stability of nanoscale structures seen in this TEM study.
Figure caption: the sequence of transformations is mapped over a wide range of heating rate using both conventional and ultra-fast scanning calorimetry. The map (on the right) allows the calorimetric signal to be correlated with the in-situ TEM at the appropriate local temperature and heating rate. ‘As it happens’ imaging (stills and videos) and quantification of heats of transformation allow secure identification of the processes involved. Successive images (on the left) show (i) the uniform, fully glassy phase at room temperature (RT), (ii) complete nanocrystallization to (Au,Cu)7Si, (iii) precipitation of (Pd,Ag)Si inside the primary phase, (iv) decomposition of the matrix into three intermetallic phases and coarsening of the silicide precipitates.
YuP Ivanov, CM Meylan, NT Panagiotopoulos, K Georgarakis, AL Greer, "In-situ TEM study of the crystallization sequence in a gold-based metallic glass", Acta Materialia196 (2020) 52‒60