Effects of niobium particles on the wear behavior of powder metallurgical γ-TiAl alloy in different environments

The wear behavior of a powder metallurgical gamma-TiAl alloy, Ti-45Al-5Nb-0.4W + 2Nb (in at. %), containing Nb particles was investigated using pin-on-disk wear tests at room temperature. The dry sliding wear tests were performed against a zirconia disk in different environments: argon, a hydrogen in nitrogen mixture, air and oxygen. The wear rates of the TiAl pins were significantly greater in the presence of oxygen but little affected by water vapor or molecular hydrogen. The worn surface of the TiAl pins and the wear debris were examined using both a scanning electron microscope and an X-ray diffractometer. The tribolayer in the Nb rich area near the worn tip of the pin was investigated using transmission electron microscopy on specimens prepared by focused ion beam milling. It was found that both pure Nb particle residues present in the beta phase of the alloy and new compounds precipitated in the Nb-rich area around the Nb particles substantially enhanced the wear-resistance. The zirconia disk underwent a phase transformation induced by the high stresses and temperature during the wear tests. The abrasive particles produced during the wear testing, which largely consisted of the zirconia particles, were found to be embedded into a tribolayer on the worn tips of the TiAl pins. The results indicate that the main wear mechanisms of the (Ti-45Al-5Nb-0.4W) + 2Nb alloy were two-body and three-body abrasive wear, some delamination, and plastic deformation.