Effect of deformation mode on hot ductility of a γ′ precipitation strengthened nickel-base superalloy O.T. Ola, O.A. Ojo n , M.C. Chaturvedi Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 5V6 article info Article history: Received 11 May 2013 Received in revised form 15 June 2013 Accepted 18 June 2013 Available online 31 July 2013 Keywords: Nickel-base Hot ductility Hot workability Liquation Forming Joining abstract Disparity in hot ductility of a γ′ precipitation strengthened nickel-base superalloy, IN 738 LC, subjected to non-equilibrium heating and compressive and tensile stresses, was investigated. The alloy, which shows considerable hot ductility at temperatures ranging from 1160 1C to 1250 1C under compressive loading, exhibits zero ductility under tensile loading within this temperature range. The difference is attributed to the fact that while compressive loading permits plastic deformation in spite of non-equilibrium liquid phase dissolution of γ′ precipitates in the alloy, the occurrence of the liquation reaction results in inhibition of plastic deformation under tensile loading. Accordingly, while grain refinement through strain-induced dynamic recrystallization occurred under compressive loading, within the same tem- perature range, the formation of new grains was prevented under tensile loading. This behavior is crucial during high temperature processing of γ′ precipitation strengthened nickel-base superalloys. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Manufacturing of components for applications in the high temperature operating environment of aero and land-based gas turbine engines requires the use of materials with excellent high temperature mechanical properties and reliable hot corrosion resistance. Nickel-base superalloys are usually used for hot- section gas turbine applications. The high temperature perfor- mance of nickel-base superalloys can be attributed to a combina- tion of solid solution strengthening by a careful selection of alloy chemistry, precipitation strengthening of the matrix by γ′ and/or γ″ phases, and improvements in grain boundary characteristics by the formation of various carbide phases. Cast nickel-base super- alloys that are precipitation strengthened by γ′ precipitates, such as IN 738 LC, are particularly suited for higher operating tempera- tures due to better microstructural stability. Joining and forming processes are important in the manufactur- ing of gas turbine components. Recent developments in the joining of high temperature materials have led to the use of friction joining processes, including friction stir, linear friction, etc., which have produced excellent joints in materials that are very difficult to join by other conventional methods [1–4]. Also, forming processes, such as forging, extrusion and rolling, remain critically important in the manufacturing of gas turbine components due to the possibility of achieving high production volumes and various shapes [5,6]. Fric- tion joining and forming involve two major events. Firstly, the materials are rapidly heated (non-equilibrium heating) to the processing temperature, which usually results in drastic changes in microstructure. Secondly, the materials undergo significant plastic deformation under externally imposed stresses. Studies have shown that plastic deformation and hot workability of γ′ precipita- tion strengthened nickel-base superalloys depend on the dissolu- tion behavior of the precipitates at processing temperatures [7,8]. Precipitation strengthened nickel-base superalloys, such as IN 738 LC, are usually hot and difficult to work due to their high flow stresses. This necessitates an increase in hot working temperatures, where the precipitates essentially dissolve in order to enhance workability. One of the phenomena controlling the flow stress and the rate of crack propagation in hot-worked materials is dynamic recrystalliza- tion, which occurs in several alloys, including precipitation strength- ened nickel-base superalloys [7]. During hot working deformation, several metals tend to exhibit a microstructure consisting of disloca- tion sub-boundaries similar to the structure obtained after softening of cold-worked materials by annealing but without recrystallization [7]. This is referred to as dynamic recovery and has been observed in different materials [7,9]. High strains and strain rates increase the density of dislocations at the sub-boundaries and cause the dis- locations to become more tangled [10]. Due to greater misorienta- tion between the sub-grains, created by the network of dislocations, new grains eventually nucleate, lowering the flow stress and resulting in a steady-state condition where deformation, recovery Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.06.088 n Corresponding author. Tel.: +1 2044747972. E-mail address: olanrewaju.ojo@umanitoba.ca (O.A. Ojo). Materials Science & Engineering A 585 (2013) 319–325