Positron annihilation study of boron-doped Ni 3Al

Abstract

Positron-lifetime and Doppler-broadening measurements were performed at room temperature on well-annealed polycrystalline Ni 75− x Al 25+ x alloys ( x = −1, 0, and + 1) containing 0, 100, and 500 wt. ppm boron. The large grain size ( ≈ 300–500μm) in these alloys rendered the grain-boundary contribution negligible in the positron annihilation process. Systematic dependences of the positron lifetimes and the Doppler-broadening lineshape parameter were observed as a function of both the aluminum and boron contents of the alloys. Positron trapping by defects was found in both the x = 0 and x = + 1 alloys, but was most pronounced in the nominally-stoichiometric alloys containing boron. No positron trapping was evident in the x = −1 alloys with or without boron. A model is suggested for the relationship between these results and the problem of ductilizing poly-crystalline Ni 3Al by boron doping.