The effect of cold work on the properties of high temperature fastener materials

The polycrystalline nickel-based superalloy ATI Allvac 718Plus\(^{TM}\) (718Plus) has been identified as a potential aero-engine fastener material for use in intermediate temperature applications, circa 650°C. Cold working operations are used widely in the manufacturing of aerospace fasteners for lower temperature applications. However, the benefits of work hardening can disappear at high temperatures. This thesis investigates the effects of cold work on the mechanical properties of alloy 718Plus from 300 to 750°C. This includes: tensile; creep; stress rupture; stress relaxation; notched low cycle fatigue life; and, fatigue crack growth resistance. The stresses and temperatures considered in mechanical tests were dictated by engine service conditions. All material studied in this investigation had been cold drawn initially to 30% area reduction (HT1). Direct aged (HT2) and, annealed + direct aged (HT3) heat treatment conditions are compared to investigate the effects of this prior cold working. A comprehensive microstructural characterisation was conducted prior to testing. Hardness, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), electron backscatter diffraction (EBSD) and scanning transmission electron microscopy (STEM) analysis techniques were used to examine the microstructure, deformation and (gamma-prime and eta) phases. The heat treatments affected the grain and phases differently. This was in addition to the amount of cold work retained. HT2 retained a worked microstructure; whereas HT3 was partially recrystallized. Grains in HT2 were relatively larger and also elongated in the drawing direction. HT3 produced a finer equiaxed grain structure. HT3 showed a significantly higher volume fraction of eta phase which precipitated both at grain boundaries and intragranularly. The amount of gamma-prime precipitation in HT2 was unchanged from the as-received cold worked microstructure and precipitation was limited to grain boundaries. HT2 showed a higher volume fraction of gamma-prime precipitates than HT3. HT3 showed greater ductility than HT2. It was observed that the HT2 treatment was superior to HT3 in terms of: tensile strength; creep resistance; notched fatigue crack initiation life; and, fatigue crack growth resistance, despite its tendency to fail intergranularly