Microstructural inhomogeneities and sea water corrosion in laser-deposited Ti-6Al-4V alloy matrix/carbide particulate composite surfaces

Metal matrix/carbide particulate composite surface layers were produced on Ti-6Al-4V alloy samples by injecting metal carbide particles into laser-melted surfaces followed by rapid solidification. Hard, wear-resistant surfaces were produced on a strong alloy which normal ly has poor wear resistance. The corrosion behaviour of the composite surface was evaluated after a month's exposure to flowing sea water. A variety of solidification products was found in the laser-deposited surface layers, but corrosion was observed only in the carbide particulate phase in the WC-injected sample. No corrosion was observed in the TiC-injected sample nor in the Ti-6Al-4V base alloy. Corrosion in the WC-injected sample was related to the formation of a narrow interphase zone surrounding the particulate phase and a thin reaction zone on the surface of the particulate phase during solidification. The titanium-rich interphase zone formed a galvanic couple with the WC particulate. Crevice-type corrosion initiated at the interface between the two phases and proceeded into the particulate phase assisted by the reaction zone. Electrochemical test results revealed a high corrosion rate for the WC-injected sample and almost none for both the TiC-injected sample and the Ti-6A1-4V base alloy, confirming the microstructural observations. (C) 1998 Kluwer Academic Publishers.