American Journal of Materials Engineering and Technology, 2017, Vol. 5, No. 1, 14-23 Available online at http://pubs.sciepub.com/materials/5/1/3 ©Science and Education Publishing DOI:10.12691/materials-5-1-3 On the Role of Boride in the Structural Integrity of a Turbine Disc Superalloy’s Solid State Weld K.M. Oluwasegun 1,* , J.O Olawale 1 , M.D. Shittu 1 , O.O. Ige 1 , P.O. Atanda 1 , O.O. Ajide 2 , L.O. Osoba 3 1 Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria 2 Department of Mechanical Engineering, University of Ibadan, Nigeria 3 Department of Metallurgical and Materials Engineering, University of Lagos, Nigeria *Corresponding author: excetom@gmail.com Abstract This work reports the melting of boride precipitates along the grain boundary of a supposedly solid state welding of a polycrystalline superalloy, and discusses its attendant effect on the hot ductility behaviour of the alloy. Nickel-based superalloy used for this study was previously processed by hot extrusion of argon atomized powered followed by forging. The alloy was solution heat treated at 1120 °C, aged at 760 °C and subsequently air cooled to room temperature. Thereafter, it was welded by inertial friction welding (IFW) at a forging pressure of 250 MPa and finally stressed relieved at 760 °C for 8 hours. The microstructures of welded samples were studied by scanning and scanning transmission electron microscopes. Gleeble hot ductility test was carried out on tensile specimen machined from the welded sample. The microstructures of the welded alloy shows that boride precipitates liquated along the grain boundary within the heat affected zone (HAZ) as a result of rapid heating of IFW. The results of hot ductility test revealed that the melting of boride lowered the hot ductility of the alloy. It was concluded that the boride precipitates liquated along the grain boundary of the nickel-based superalloy during solid state welding and lowered its hot ductility. Keywords: superalloy, solid state welding, boride precipitates, grain boundary, hot ductility, welding Cite This Article: K.M. Oluwasegun, J.O Olawale, M.D. Shittu, O.O. Ige, P.O. Atanda, O.O. Ajide, and L.O. Osoba, “On the Role of Boride in the Structural Integrity of a Turbine Disc Superalloy’s Solid State Weld.” American Journal of Materials Engineering and Technology, vol. 5, no. 1 (2017): 14-23. doi: 10.12691/materials-5-1-3. 1. Introduction The need for more heat resistant materials in aircraft engine turbo superchargers prompted the development of superalloys in 1930s. It has been driven since the early 1940s by the increasing demands of advancing gas turbine engine technology [1]. In addition to aircraft applications, superalloys are now used in space vehicles, rocket engines, nuclear reactors, submarines, steam power plants, petrochemical equipment and other high-temperature applications. The largest use of superalloys, however, is the gas turbine industry [1]. The recent global demand in the reduction of emissions is also pertinent to aerospace industry. Achieving this goal of reducing emission by aerospace industry and consequently lowering its burden on the environment significantly requires a generation of jet engines that will burn fuel more effectively at higher temperature [1]. This stems the need for the development of new superalloys that offer heat resistance of which nickel base superalloys are candidate superalloys. Nickel- based superalloys among others have emerged as the choice for high-temperature application because of their FCC crystal structure, which confers good toughness and ductility, due to a considerable cohesive energy arising from the bonding provided by the outer d electrons [2]. This crystal structure is stable from room temperature to the melting point, so that there are no phase transformations leading to expansion and contraction, which might complicate its use for high temperature components. Their low rate of thermally activated processes (e.g. creep) and moderate cost have also contributed to their choice as candidate materials for high temperature applications. The high corrosion resistance observed in these alloys stems from the high level of chromium, as chromium forms an oxide layer which protects the material from further oxidation. Addition of boron to nickel-base superalloys has been proposed to influence the chemistry and structure of the grain boundary precipitates [3]. It is generally known that the solid solubility of boron in austenitic γ alloys is very low [4]. For example, it was reported that the solubility of boron in 18%Cr-15%Ni stainless steel was 97 ppm at 1125 °C. This solubility decreased rapidly with decreasing temperature, becoming less than 30 ppm at 900 °C [25]. In addition to this, boron atoms are larger than the common interstitial elements (e.g carbon) but smaller than substitutional elements like Co and Cr. This misfit in size of boron atoms for substitutional and interstitial sites in austenitic lattices suggests that it could be energetically favorable for boron atoms to segregate to loosely packed regions like grain boundaries and incoherent interphase boundaries [5,6]. Kurban et al [7] have been able to report recently from their ion mass spectroscopy study of boron segregation that boron tends to have a stronger affinity for