RARE EARTH METALS IN TITANIUM ALLOYS – A SYSTEMATIC STUDY

Titanium alloys combine outstanding mechanical properties with corrosion resistance and biocompatibility. Thermo-mechanical processing of titanium alloys is difficult to perform above the  -transus-temperature as severe grain growth occurs. In addition, the machinability of titanium alloys is generally poor and component manufacturing is costly as automation of turning and drilling processes is often impossible due to the formation of long chips. In the current study, the grain size stability at elevated temperature and the chip formation process of different commercially available titanium alloys as well as of several experimental alloys have been studied. To improve their machinability and grain size stability during heat treatments above the  -transus-temperature alloy development has been carried out. Small amounts of different rare earth metals (cerium, lanthanum and erbium) have been added to the standard alloys to distribute micrometre-size particles in the titanium matrix. Rare earth metals are almost insoluble in titanium at room temperature. Consequently, the microstructure of the modified alloys consists of a titanium matrix and metallic precipitates mostly located on the grain boundaries. Iron supports a more homogeneous particle distribution. Whenever tin is present in the alloys intermetallic compounds like La 5 Sn 3 form. Several benefiting effects have been observed in the rare earth metal containing titanium alloys: Metallic cerium and lanthanum particles enhance machining operations as short breaking chips develop during metal cutting. Hence, automation of drilling and turning is enabled. In addition, grain size stability is ensured up to temperatures of 1100°C by all three rare earth metals. The mechanical properties of selected modified alloys have been tested and compared to the standard alloys. The static strength of the particle containing alloys is similar, the fatigue limit is slightly diminished, some alloys show reduced ductility.