The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti–3Ni alloy

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si lower the solidus of Ti–Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti–3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti5Si3 form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti5Si3 phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.

► Silicon is a potent sintering aid for Ti–Ni alloys revealed by predictions and confirmed by experiments. ► The addition of Si should be limited to ≤1% to avoid coarse Ti5Si3 phase and ensure good ductility. ► Liquid forms during heating at ∼988 °C due to reactions between Si and Ni and Ni and Ti. ► Silicon can be a unique addition to PM Ti alloys for significantly improved mechanical properties.