Effects of boron doping on the grain growth behaviour of γ/γ nickel-aluminium alloy

Abstract

Effects of 0.5 at. 9c boron doping on Nigs Al y/f superalloys were investigated for the first !ime. yielding a number of observations not previously observed in the literature. First, the grant growth kinetics of both the doped and undoped alloys were found to disobey the simple Nielsen law d = Of. but instead follow an equation of the type d = C ln(r) + C0. The constants C and Co were found to be respectively 10.2 and 23.2 for the boron-free alloy, and 6.2 and 14.2 for ire boron-doped alloy, i.e., the grain growth rate was retarded significantly upon boron doping. Such a retarding effect is thought to be due to the formation of a boron-nickel cosegregated zone observed at the grain boundaries of the doped alloy: the width of the zone, in juni's, is two to three orders of magnitude larger than the boron induced disordered layer found in nickel rich Ni3Al. compounds doped with boron. Other associated effects of the cosegregated zone include a -harp increase in toughness, much better slip transmission across grains and reduced workhardening rates. Another intriguing point is that the Y precipitates were found to segregate to the grain boundaries in the boron-free alloy after cold rolling, but no such segregation of y precipitates, has been observed in the boron-doped alloy. The different deformation microstructures and the retarded grain growth rates upon boron doping will be discussed.