Stress Corrosion Testing of CMSX-4, CM247LC DS and IN6203DS Ni-Base Superalloys
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Stress Corrosion Testing of CMSX‑4, CM247LC DS and IN6203DS Ni‑Base Superalloys Neil Chapman1,2 · Simon Gray2 · Joy Sumner2 · John Nicholls2 Received: 20 October 2020 / Revised: 20 October 2020 / Accepted: 5 November 2020 © The Author(s) 2020
Abstract The combination of stress and hot corrosion may result in Ni-base superalloys experiencing stress corrosion cracking, of which, the mechanisms are little understood. The aim of this research was to enhance the understanding by performing a series of stress corrosion exposures, at temperatures of 550, 500 and 450 °C, on: CMSX-4, CM247LC DS and IN6203DS superalloys. After completing the exposures, the superalloys were ranked with respect to the severity of the cracking experienced (CMSX-4 showing the worst severity, followed by CM247LC DS and then IN6203DS which showed no evidence of cracking at all) and the ranking appeared to be correlated to the gamma prime volume fraction. This suggests the gamma prime volume fraction is associated with the crack mechanism with lower values increasing the resistance to stress corrosion cracking. From the findings of this research, a new crack initiation/propagation mechanism is proposed which is based on a summation of stresses that includes those associated with the gamma prime. Keywords Lattice mismatch · Interstitial atoms · Embrittlement
Introduction Rotor blades are critical components of industrial gas turbines (IGT) which are subjected to high temperatures and stresses. This necessitates the use of materials for these components that have suitable high temperature mechanical properties, such as precipitation-hardened Ni-base superalloys. These materials have been developed over the years to optimise the mechanical properties [1], and this has enabled IGTs to operate at higher temperatures and efficiencies thus reducing the C O2 emissions [2]. * Neil Chapman [email protected] 1
Siemens Industrial Turbomachinery Ltd, Ruston House, PO Box 1, Waterside South, Lincoln LN5 7FD, UK
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Cranfield University, College Road, Cranfield, Wharley End, Bedfordshire MK43 0AL, UK
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Oxidation of Metals
The precipitates, which have an ordered L 12 fcc crystal structure [2], are N i3Al based particles (known as gamma prime) and may be strengthened by substituting the Al with other elements such as Ti, Ta and Nb [1]. The gamma prime precipitates are homogeneously dispersed and coherent within the disordered [3] fcc matrix (referred to as gamma) of the superalloy [2]. Due to a difference in the lattice constants between the gamma prime and gamma, a lattice mismatch ( 𝛿 ) exists at the interface between the two. Providing the lattice constants of the gamma prime ( a𝛾 ′ ) and gamma ( a𝛾 ) are known, the mismatch may be calculated using Eq. 1 [3–6]: ( ) 2 a𝛾 � − a𝛾 𝛿= (1) a 𝛾 � + a𝛾 The lattice mismatch tends to be negative, with an approximate magnitude of 1 0−3, at room temperature [6]. Owing to a difference in the thermal expansion properties between the two phases, the magnitude of the lattice mism
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