Microstructure and Mechanical Properties of an Advanced Nickel-Based Superalloy in the as-HIP Form

Abstract

Abstract. This study evaluates the suitability of as-hot isostatically pressed (HIP) RR1000 for noncritical applications in aero-engine components. RR1000, an advanced powder nickel-based superalloy, was developed for disc rotor components in aero-engines. For these critical applications, the consolidated alloy powder particles are extruded to break down carbide and oxide networks, known as prior particle boundaries (PPBs), and to refine the structure into a fine grain size for isothermal forging. In this study, hot isostatically pressed compacts, made from two different powder particle size fractions have been assessed following heat treatments below and above the gamma prime solvus temperature. A microstructural evaluation shows a greater degree of PPB decoration occurs in the finer powder particle size fraction. Following a super-solvus heat treatment these PPBs pin grain boundaries of the fine powder particle size compacts, whilst the reduction of PPB decoration in coarse powder particle compacts allows significant grain growth. Tensile test results of as-HIP RR1000 show, good yield strengths, ultimate tensile strengths and ductility, which are comparable with extruded and isothermal forged RR1000 disc material. Dwell crack propagation tests show that finer powder particle size compacts, which have received a sub-solvus heat treatment, give the highest crack growth rates; whilst the remaining material conditions show markedly improved crack growth resistance. In conclusion, as-HIP RR1000 demonstrates clear potential for use in non-critical applications, employing either powder particle size fraction used in this study subject to the appropriate solution heat treatment.

Development of fatigue crack growth testing under thermo-mechanical fatigue conditions

“…Comparison of this RR1000 tested under bithemal/TMF conditions with other isothermal crack growth work on RR1000 showed that under isothermal conditions a transgranular fracture was seen for test conditions up to temperatures over 700 o C [16], similar to those seen here. Interestingly, despite the relatively high MATERIALS AT HIGH TEMPERATURES www.materialshightemp.co.uk maximum temperature of the current TMF/bithermal cycle (700 o C) and low test frequency, there was no evidence for intergranular fracture, as reported for Udimet 720Li at test temperatures of 700 o C [14]. This again supports the view that the crack growth seen here under TMF conditions is largely a result of the tensile loads at 400 o C during the thermal cycle, with little contribution from the rest of the cycle.…”

“…Although these initial isothermal tests were relatively trivial, the results suggested that this facility could produce acceptable conventional fatigue crack growth data using the same PD measurement system as planned for the TMF testing and there was no impact of using the radiant lamp furnace to provide the required temperature. It is worth noting at this point that the test piece design and general test approach has been used extensively for isothermal fatigue crack growth testing for many years [9,13,14]. Thus the crack length and PD measurement have been fully calibrated and demonstrated for isothermal testing, using this same test piece design.…”

Review and Analysis of Powder Prior Boundary (PPB) Formation in Powder Metallurgy Processes for Nickel-based Super Alloys

“…PPB networks tend to form on these larger less-deformed powder particles. However May et al [33] found that a greater degree of PPB decoration occurs in the finer powder particle size fraction, and following a super-solvus heat treatment these PPBs pin grain boundaries of the fine powder particle, while the reduction of PPB decoration in coarse powder particle compacts allows significant grain growth.…”

“…Since lower strain rates are used during deformation, isothermal forging at slow strain rates and high temperatures (slightly below the γ' solvus) [43] currently offers greater control on structure and enables quire complexity, nearnet shaped components to be formed [44]. May et al [33] stated that for critical applications such as disc rotor components in aero-engines, the consolidated alloy powder particles should be extruded to break down carbide and oxide networks, i. e. PPBs, and to refine the grain size for isothermal forging. Mourer et al [45] produced a billet of René 104 with a diameter of 235 mm, from which to manufacture a military aircraft engine high pressure turbine disk with dual microstructures.…”

On the diffusion mechanisms of fine-scaleγ′ in an advanced Ni-based superalloy

“…Excellent fatigue performance, damage tolerance, and good creep resistance at operation temperatures close to 750 • C [1][2][3] originate principally from the presence of high volume fraction (close to 50%) of gamma-prime (γ ) precipitates coherently embedded within the gamma (γ ) matrix [4,5]. The optimisation of the precipitate size distributions (PSDs) for these properties involves careful control of cooling conditions.…”