The role of chemistry in controlling the bonding and fracture properties of grain boundaries in L12 intermetallic compounds

Abstract

Among the main limitations of ordered intermetallic compounds such as Ni aluminides and silicides are their low room temperature ductility and propensity for intergranular fracture, behavior which has been attributed to both poor intergranular cohesion and environmental embrittlement. Microalloying is one of the approaches used to improve the mechanical properties of these compounds, with the most successful example being the addition of B to Ni-rich (76 at.% Ni) Ni 3 Al which results in a dramatic improvement in ductility and a change in fracture mode from intergranular to transgranular. However, the addition of B to Ni 3 Si and Zr to Ni-rich (77.3 at.% Ni) Ni 3 Al does not prevent intergranular fracture. The goal of the present study is to understand why B and Ni-enrichment are effective in improving boundary properties in Ni 3 Al, while Zr and B are ineffective in preventing intergranular fracture in Ni 3 Al and Ni 3 Si, respectively. Insight into the influence of local chemistry on the fracture properties of boundaries in intermetallic compounds may contribute to a better general understanding of the influence of solute segregation on the properties of grain boundaries in polycrystalline solids. The chemistry and electronic structure at grain boundaries in Ni 3 Al with and without B were examined using energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). B was shown to restore bulk-like bonding to grain boundaries in Ni 3 Al. The changes in bonding induced by B segregation were shown to result in lower grain boundary energy and improved grain boundary cohesive strength in Ni-rich Ni 3 Al. Similar studies were conducted on Ni 3 Si with and without B and Zr-doped Ni 3 Al, where the bonding at the boundaries was shown not to be significantly affected by the addition of dopants consistent with their ineffectiveness in preventing intergranular fracture. The relationship between the observed changes in bonding and the grain boundary energy, cohesive strength and fracture properties is discussed. To examine the effect of Ni-enrichment on grain boundary properties in Ni 3 Al, the chemistry and energy of antiphase boundaries (APB) at small angle boundaries were examined. Ni-enrichment at APBs was shown to lower their energy by replacing high energy Al-Al interactions with low energy Ni-Ni and Al-Al interactions. The implications of these observations for grain boundary properties are discussed.