FORMATION OF NOVEL MICROSTRUCTURES BY SPRAY DEPOSITION PROCESS

摘要

Due to the relative high cooling rate, the characteristic microstructure of as-spray deposits typically consist of a fine scale microstructure and might also exhibit some extended solid solubility and metastable phases. Our group has studied alloys of several different systems with the aim of investigating the potential of achieving the formation of novel microstructures by the high cooling rate involved in this near-net-shape process. The glass former Al-Y-Ni-Co and Al-Y-Ni-Co-Zr alloys, including also a Al-Y-Ni-Co-Zr + SiC composite have been processed. Nevertheless the formation of amorphous phase, which was observed in the overspray powders (droplets which do not impact on the deposit top surface or/and bounce off impacting droplets or powder from the deposit surface) of all alloys, only for the Al-Y-Ni-Co alloy amorphous phase was observed in the deposit. High volume fraction of solid droplets type deposition and a very high glass forming ability might be necessary to avoid the crystallization of amorphous phase during the deposition process. The spray deposited Ni-Al-Cr-B-C nickel based casting alloys showed a strong microstructural refinement, with carbides about one order of magnitude smaller than those obtained in the conventionally cast materials, dispersed in a predominantly gamma-prime matrix. The spray deposited 2.9%C-22%Cr white cast iron showed a microstructure formed by fine M_7C_3 carbides (< 10 μm in length) in a martensitic matrix. This is a novel microstructure considering that the conventional microstructure for this alloy consists of interconnected M_7C_3 carbides (about 300 μm in length) embedded in a matrix of austenite and martensite dendrites. The spray deposited Fe-6.5wt%Si alloy presented in the as cast condition a random crystallographic texture and a coarse B_2 domain antiphase structure typical of heat treated, recrystallized ribbons obtained by melt spinning. These results showed that the spray deposition is a very interesting near-net-shape process for achieving novel microstructures in a variety of alloy systems.