Production and Mechanical Properties of Nanocrystalline Intermetallics Based on TiAl_3-X

摘要

Production of intermetallic materials in the system TiAl_3-X (X = Cr, Mn, Fe) has been achieved by means of mechanical milling and sintering techniques. Spark plasma sintering is used since it reduces time at high temperature and inhibits grain growth. The produced materials have grain sizes in the nano and microscale depending on the material and processing variables. The TiAl_3-X alloys are formed mostly by the cubic Ll_2 phase. The average grain size ranges between 30 and 50 nm in the as sintered condition. Aging at elevated temperature has been used to promote grain growth. Compression tests have been performed to evaluate mechanical properties as a function of temperature and grain size. In all cases yield stresses higher that 700 MPa are obtained together with a ductility that depends upon temperature and grain size. No ductility is found for the smallest grains sizes tested (30 nm) at room temperature. Above 673 K, these materials show ductility and additionally they present a quasi superplastic behavior at temperatures higher that 973 K. On the other hand ductility can also be developed in the TiAl_3-X alloys by inducing grain growth via annealing. Alloys with grains sizes around 500 nm show high ductility and a large density of microcraks after deformation suggesting that the yield strenght becomes lower than the stress to propagate the cracks. In such materials, a considerably high strength is retained up to 873 K.