Joining Si₃N₄ to FA-129 iron aluminide

Joints between dissimilar materials are characterized particularly by compositional gradients and microstructural changes, which yield large variations in chemical, physical and mechanical properties across the joint. The joining of dissimilar materials is therefore more complex than the joining of similar materials. In this project, the joining procedure, from the interaction between the different components in a joint to the determination of the mechanical properties was applied to the Si3N4/FA-129 system. This iron aluminide intermetallic alloy (FA-129), was developed by Oak Ridge National Laboratories (ORNL) to have high temperature properties with good room temperature ductility. This intermetallic is replacing high strength ferritic stainless steel (SS) in moderate strength applications due to cost and property reasons. Joints between SS and Si3N4 are already used industrially and this project was to evaluate the potential to replace these Si3N 4/SS joints by those of Si3N4/FA-129.Broadly stated, the results obtained during this project are as follows: (I) The E2 energy for Si3N4 ceramic was calculated to be 3.01 keV. (II) The wetting of iron aluminide alloy by copper has been achieved and the spreading and reaction kinetics are influenced by the presence of Cr as alloying element. (III) The penetration and decohesion of the FA-129 microstructure is significantly reduced by the utilization of a Cu alloy containing a high titanium concentration. (IV) An active brazing alloy containing a high active element content can be fabricated by an electroless deposition technique. (V) The melting behavior of the powder was characterized and complete melting occurs in a multi-step process at different temperatures, which are a function of the heating rate. (VI) The strength of joint produced by brazing Si3N4 to itself using the composite powder reached 400 MPa. (VII) Direct brazing of Si 3N4 to FA-129 was shown to be unsuccessful and therefore a soft Cu interlayer was inserted to absorb residual stresses. The maximum joint strength reached was 160 MPa. (VIII) Partial Transient Liquid Phase Bonding was successfully applied to the Si3N4/FA-129 system using a nickel interlayer. The conventional silicide and nitride layers were not observed as the silicide layer dissolved into the nickel core at high temperature. The strength of the assembly was measured and a strength of 80 MPa was obtained, independent of the joining parameters