Microstructural and fracture characterization of Nb-Cr-Ti mechanically alloyed materials

Three materials containing Nb, Cr, and Ti were fabricated by consolidating powders made by mechanical alloying. The Nb/Ti ratio was maintained at about 1.3 and Cr was increased to form the intermetallic Cr₂Nb. X-ray diffraction, metallography, and transmission electron microscopy were used to thoroughly characterize the microstructure and substructure of the materials. Fatigue and fracture toughness properties were also evaluated at ambient temperature. The alloyed powders contained only small amounts of intermetallic, but during the consolidation heat treatment, two of the materials precipitated large volume fractions of Cr₂Nb. In the third material, Cr₂Nb was precipitated by heat treatment, although this was not expected from the composition based on the Nb-Cr-Ti phase diagram. Maximum fracture toughness of the composutes was ≈ 11 MPa √m. The low fracture toughness was attributed to the high plastic constraint of matrix deformation by the Cr₂Nb and compositional change in the matrix.