MICROSTRUCTURAL EVOLUTION, MECHANICAL PROPERTIES AND CORROSION BEHAVIOUR OF LOW COST Ti-Fe-O-N (alpha-beta) ALLOYS
Titanium and its alloys are highly recognized materials used for advanced applications such as aerospace, biomedical and chemical industries . This is attributed to the excellent physical and mechanical properties exhibited by these alloys. Some of these properties include low density, specific strength, corrosion resistance, non magnetic and high temperature resistance. The potentials these materials offer in terms of biocompactibility, increased efficiency in fuel consumption, low thermal coefficient and environmental friendliness has attracted it use to other general applications such as automobile, jewelry industries, marine industries just to mention a few [1-2].
The most common type of titanium alloy used in wide range of applications is known as the “titanium work horse-Ti-6Al-4V”. The alloys have good combination of mechanical properties owing to the possibility of modifying its complex dual phase (hcp α+ bcc β) microstructure through heat treatment . Other types of titanium alloys produced to meet specific requirements are the alpha alloys (commercially pure alloys), near alpha alloys and metastable beta alloys [2-4]. All these alloys are considered as materials capable of replacing conventional structural and industrial materials such as steel and cast iron. However, the manufacturing cost of these alloys has been a limiting factor [5-8]. Consequently, efforts are been made by researchers to develop different grades of low cost titanium alloys.
Two major approaches have been adopted to reduce the manufacturing cost of titanium alloys . The first approach entails replacing expensive alloying elements like Mo, Co, Ta, V, Al with cheaper elements (O2, N, Fe, C, Cr, Mn etc) while the second approach involves adopting cheaper processing routes such as the recently developed powder metallurgy techniques used to produce near net shapes and the use of impurities to refine grain structures instead of thermomechanical processing [9-10]. The alloy of interest in this research work was designed based on the first approach. The Ti-Fe-O-N alloy is a dual phase (hcp α+ bcc β) type of alloy in which all the expensive elements (alpha and beta phase stabilizers) of conventional titanium alloys are replaced by inexpensive ones. Other low cost titanium alloys that have been developed by replacing the alloying elements usually contain aluminium as the alpha phase stabilizer [4-5]. Aluminium has been reported to increase hot deformation resistance and decrease hot workability, this reduces yield and increases manufacturing cost due to lots of work needed to remove surface defects [4-5]. Therefore, it is worth mentioning that one unique attribute of the Ti-Fe-O-N alloy series is the replacement of aluminium with oxygen and nitrogen as the alpha phase stabilizers making the alloy much more inexpensive than the other...