Fracture toughness and yield strength of boron-doped, high (Ti+Mn) L12 titanium trialuminides

Three batches of L12-base titanium trialuminide alloys, one with low Ti content: 9 Mn–25 Ti, and the other two with high Ti content: 14 Mn–29 Ti and 18 Mn–32 Ti (at.%), referred to as 9 Mn, 14 Mn and 18 Mn, respectively, some doped with 0.004 to 0.66 at.% boron, were induction melted under high purity argon, homogenized and subsequently HIP-ed at 1250°C/2 h/180 MPa. Both Vickers microhardness and compressive 0.2% offset yield strength at room temperature increase primarily with increasing Ti concentration in the L12 matrix and secondarily with boron doping, attaining 550 MPa. The yield strength increment by boron doping for high Ti 14 Mn is much higher (∼1.74 MPa/0.01 at.% B) than that for low Ti 9 Mn (∼0.4 MPa/0.01 at.% B). At room temperature, a combination of high Ti concentration and boron doping increases chevron-notched beam (CNB) fracture toughness to ∼8 MPa m1/2 (a 100% increase with respect to low Ti 9 Mn alloy) but fracture mode remains transgranular cleavage regardless of the composition. At 800–1000°C the toughness increase of high Ti 14 Mn is mainly determined by a high Ti concentration. For boron-free and boron-doped high Ti 14 Mn the transgranular cleavage/intergranular failure transition temperature approaches ∼600°C and at 1000°C the fraction of intergranular failure mode is suppressed to barely ∼45%.