Superalloys 2021: Monday Part III - Blade Alloy Behavior
Program Organizers: Sammy Tin, University of Arizona; Christopher O'Brien, ATI Specialty Materials; Justin Clews, Pratt & Whitney; Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346; Qiang Feng, University of Science and Technology Beijing; Mark Hardy, Rolls-Royce Plc; John Marcin, Collins Aerospace; Akane Suzuki, GE Aerospace Research

Monday 4:10 PM
September 13, 2021
Room: Live Session Room
Location: Virtual Event

Session Chair: Justin Clews, Pratt & Whitney; Andrew Elliott, Consarc Corporation

4:10 PM
Rationalisation of the Micromechanisms Behind the High-temperature Strength Limit in Single Crystal Nickel-based Superalloys: Daniel Barba Cancho¹; Ashton Egan²; Yilun Gong¹; Michael Mills¹; Roger Reed¹; ¹University of Oxford; ²The Ohio State University
    The peculiar atomic structure of γ′γ′ precipitates [Ni33(Al/Ti)-L122] in Ni-based superalloys produces high-energy faults when dislocations glide them, giving their significant strength at high temperatures. The mechanisms behind the strength failure of these alloys above 700–800 ° C are still controversial. Recent advances in atomic resolution microscopy have allowed to study these mechanisms with unprecedented detail. In our study, we have characterised in a careful systematic study a SX-[001] superalloy from RT to 1000 ° C. Multiscale microscopy (TEM and SEM) has been combined with physical metallurgy and atomistic modelling to fully understand the correlation between the strength drop and the observed changes in the γ′γ′ shearing mechanism. Our results show that, far from previous beliefs, the initial failing of alloy strength is not a consequence of the activation of dislocation climbing. Instead, there is a transition between three different mechanisms: (T<750 ° T<750 ° C) continuous planar stacking faults below, (T = 750 ° C) APB shearing at the strength peak anomaly and (T>800 ° T>800 ° C) extensive twin deformation after the yield drop. Local chemical changes around the γ′γ′ shearing dislocations boost these changes, thus producing the sudden drop of strength.

4:35 PM
Local Mechanical Properties at the Dendrite Scale of Ni-base Superalloys Studied by Advanced High Temperature Indentation Creep and Micropillar Compression Tests: Lukas Haussmann¹; Steffen Neumeier¹; Markus Kolb¹; Johannes Ast²; Gaurav Mohanty²; Johann Michler²; Mathias Göken¹; ¹FAU Erlangen; ²EMPA Thun
    Chemical inhomogenities due to dendritic solidification of Ni base superalloys result in different local microstructures with varying mechanical properties. New indentation creep test methods allow probing of the local creep properties at the dendrite scale at high temperatures. The as-cast single crystalline Ni-base superalloy ERBO1A (a derivative alloy of CMSX-4) was investigated and electron-probe microanalysis (EPMA) measurements revealed strong segregation of e.g. Re and W in the dendritic region and e.g. Ta in the interdendritic region. Indentation creep experiments at 750 °C and micropillar compression tests at 785 °C were conducted in both regions and a higher creep strength was found in the dendritic region compared to the interdendritic region. Theoretical models for solid solution hardening as well as ′ precipitation hardening confirm these results, since they predict a higher strength in the dendritic region than in the interdendritic region. Compared with the fully heat treated state, a smaller difference in the local mechanical properties or even a reverse strength ratio of the dendritic and interdendritic region can be expected.

5:00 PM
Creep, Fatigue, and Oxidation Interactions during High and Very High Cycle Fatigue at Elevated Temperature of Nickel-base Single Crystal Superalloys: Alice Cervellon¹; Jianzhang Yi²; Fabien Corpace³; Zéline Hervier⁴; Joe Rigney⁵; P. Wright⁵; Chris Torbet¹; Jonathan Cormier⁶; J. Jones²; Tresa Pollock¹; ¹University of California Santa Barbara; ²The University of Michigan; ³Safran Aircraft Engines; ⁴Safran Helicopter Engines; ⁵GE Aviation; ⁶Institut Pprime
    High-temperature fatigue of Ni-based single crystal superalloys is studied at 1000 °C in a wide range of loading conditions (-1 ≤ R ≤ 0.8) and number of cycles (103 – 109). Under fully reversed conditions, a competition between crack initiation from the surface – assisted by oxidation – and from internal metallurgical defects – mostly large casting pores – is observed. Increasing the testing frequency shifts the competition to a higher number of cycles. Conversely, decreasing the casting pore size or coating the specimen promotes surface initiations. When a positive mean stress is added (R ≥ 0), a creep deformation/damage mechanism mainly controls fatigue life, despite fracture surfaces presenting a variety of initiation types. Fatigue life can be predicted by a simple creep law if the contribution of the alternating stress is considered. A linear damage summation method that considers pure fatigue and pure creep damage is used to predict the fatigue lives and Haigh diagrams for different alloys are presented.

5:25 PM Question and Answer Period