Growth and microstructural characterisation of CoTi and CoTi(Zr) crystals

Methods have been developed to produce CoTi and CoTi(Zr) large-grained polycrystals and single crystals by reducing the formation of titanium oxide particles during melting and crystal growth. Oxide particles were found to form when powdered starting materials were used, even though the metals were handled and melted under an inert gas atmosphere. Alloys produced from bulk starting materials contained fewer oxide particles than alloys from powders, but adding a small amount of Al to getter the oxygen was not sufficient to prevent TiO2 formation. However, the use of a slightly reducing atmosphere during initial melting was effective in reducing the formation of oxide. Crystal growth carried out in Ar did not reduce the total amount of oxide but only resulted in redistribution of TiO2 particles to grain boundaries, where they obstructed grain growth. Crystal growth in a vacuum was found to be essential in producing oxide-free crystals. A seed selection technique was developed and used to grow CoTi single crystals. The microstructures of these materials were examined by optical microscopy, scanning electron microscopy and transmission electron microscopy, the B2 crystal structure was confirmed and the morphologies, grain sizes and oxide distributions were determined.