Thermal stability of laser track microstructures in an Al-Cr-Co-Mn-Zr I-phase alloy

Abstract

Laser glazing of solid metal surfaces is an efficient and cost-effective way to evaluate the potential of an alloy system for use in metal additive manufacturing (MAM), but there are concerns about the stability of the resulting microstructures. Here it is shown that in situ transmission electron microscopy heating experiments on specimens cut from laser glazing tracks using focused ion beam methods can provide a useful insight into the transformation phenomena that occur upon reheating. Examples of such experiments are presented for laser tracks in a powder-processed Al-Cr-Mn-Co-Zr alloy, which contains a complex distribution of icosahedral quasicrystal (I-phase) dispersoids in a polycrystalline FCC Al matrix. Initial ramped heating experiments were performed to identify suitable conditions for isothermal studies of transformation pathways. Tracks with the lowest laser energy input exhibited supersaturated solid solution microstructures. These were stable during ramped heating to around 375 °C. For isothermal exposures at 400 °C there was precipitation of Al4(Cr,Mn) both at grain boundaries and within the FCC Al matrix phase. In tracks with higher laser energy input, equiaxed I-phase dispersoids were formed in the matrix. These microstructures were stable during ramped heating to around 400 °C. In isothermal experiments at 425 °C, there was precipitation of Al4(Cr,Mn), followed by the onset of I-phase decomposition. The partially decomposed dispersoids exhibited an I-phase core surrounded by a shell of Al45(Cr,Mn)7. In isothermal experiments at 450 °C, the dispersoids decomposed completely to form Al4(Cr,Mn), indicating that Al45(Cr,Mn)7 is a metastable intermediate transformation product. These observations provide a useful guide to the transformations that might occur during MAM processing of this alloy system.