Effect of boron on microstructures and mechanical properties of Ni-46Ti-4Al alloy

The effects of B (0~1%) addition on the microstructure Ni-46Ti-4Al alloy were studied through electron probe microanalysis (EPMA), transmission electron microscopy (TEM) measurement and X-ray diffraction (XRD), and the mechanical properties of the alloys were investigated by compressive tests at room and elevated temperatures. The results indicated that Ni-46Ti-4Al alloy was composed of NiTi matrix and (Ti, Al)₂Ni phase. TiB₂ compounds were precipitated by adding a small quantity of B (0.02%) and existed in the grain boundaries together with (Ti, Al)₂Ni phase. The volume fraction of TiB₂ increased with the increasing of B content, while the fraction of (Ti, Al)₂Ni phase decreased. The hardness values of both TiB₂ and (Ti, Al)₂Ni phases were higher than that of NiTi matrix and they were strengthening phases in Ni-46Ti-4Al-B alloy. The yield strengths of Ni-46Ti-4Al alloy at room and elevated temperatures could be remarkably increased by proper addition of B. The yield strength of Ni-46Ti-4Al-B alloy with 1% B addition at room temperature was 2553 MPa, which was 128% higher than that of Ni-46Ti-4Al alloy. At 600 °C, the yield strength of Ni-46Ti-4Al-B alloys with 0.02% B and 1% B addition were 382 and 498 MPa, which were 35% and 70% higher than that of Ni-46Ti-4Al alloy, respectively. The yield strength of Ni-46Ti-4Al-B alloy was directly proportional to the total volume fraction of strengthening phases (TiB₂ and (Ti, Al)₂Ni phases). The addition of B in Ni-46Ti-4Al alloys caused the solid solution hardening effect, and, on the other hand, accelerated the formation of TiB₂ strengthening phase, which was the main reason for the enhancement of the alloys at room and elevated temperatures.