Influence of manufacturing conditions and heat treatment on the microstructure of an Mg97Y2Zn1 alloyThursday (07.11.2019) 09:55 - 10:15 Part of:
In order to meet the calls for CO2 reduction, energy saving and resource efficiency, an important topic of the current research is the development of lightweight metals. Currently, magnesium alloys with additions of yttrium and zinc are of great interest. The presence of so-called Long Period Stacking Ordered structures (LPSO) gives Mg-Y-Zn alloys exceptional mechanical properties. For prediction of the forming capacity of a cast and a twin-roll cast (TRC) Mg97Y2Zn1 alloy with binary α-Mg/LPSO microstructure, comparative studies concerning morphology and present phases were carried out. The combination of scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and X-ray diffraction confirmed that the morphology and distribution of the LPSO precipitates strongly depend on the manufacturing conditions. In the conventionally cast material, different LPSO polytypes were present, whereas in the very quick solidified TRC state only the 18R structure was found. Moreover, the grains in the TRC alloy were preferentially oriented. Furthermore, different heat treatments were performed to explore, to what extent the LPSO structures can be dissolved and which morphology and distribution of the LPSO structures can be set up. At the cast state, the heat treatment at 400 °C for 120 h caused only minor changes in the microstructure, while heat treatments at 450 °C-500 °C for 120 h led to the transformation of the 18R LPSO structures to the 14H polytypes and to the agglomeration of fine dispersed LPSO structures in the matrix. Similarly, at the TRC state, heat treatments above 450 °C for 120 h also led to a phase transformation. The results show clearly that the LPSO structures, in either case, can neither be dissolved by heat treatments in the investigated temperature range nor precipitated in a finely disperse form and with a homogeneous distribution.