Phase selections and mechanical properties of ternary Cr-Nb-Ti alloys under rapid solidification

Phase selection under far-from-equilibrium condition has been investigated in three Cr₂Nb-20/30/40Ti (at. %) alloys processed by laser surface melting (LSM) with scanning speeds of 50-200 mm/s. Phase identification was performed by micro-area XRD technique combined with SEM plus EDS methods. The results show that, at all scanning speeds of 50-200 mm/s, the microstructure of Cr₂Nb-20Ti alloy consisted of single-phase Cr₂Nb, whereas it evolved to monolithic β-Ti in the Cr₂Nb-40Ti alloy. Meanwhile, the microstructure of Cr₂Nb-30Ti alloy solidified with eutectic β-Ti/Cr₂Nb at 50 mm/s, which transited to monolithic β-Ti as scanning speeds increased to 100-200 mm/s. Microstructures evolution was interpreted based on Trivedi-Magnin-Kurz (TMK) and Kurz-Giovanola-Trivedi (KGT) models incorporating the effect of non-equilibrium solidification. After the process of LSM, the micro-hardness of Cr₂Nb-20Ti alloy was improved to a level of 9.0 GPa, and the fracture toughness was reduced to about 3.1 MPa m^1/2; while for the Cr₂Nb-30/40Ti alloys, the micro-hardness decreased within a range of 5.5-6.6 GPa, but with a great increase of facture toughness of 21-23 MPa m^1/2.