Superalloys 2021: Tuesday Part II - Disk 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
Tuesday 11:10 AM
September 14, 2021
Room: Live Session Room
Location: Virtual Event
Session Chair: Catherine Rae, University of Cambridge; Nathalie Bozzolo, MINES-ParisTech
11:10 AM
The Effect of Shot Peening on the Ductility and Tensile Strength of Nickel-based Superalloy Alloy 720Li: Thomas Jackson1; James Rolph1; Ross Buckingham1; Mark Hardy1; 1Rolls Royce Plc
Alloy 720Li with a polished surface and with a 1000% coverage shot peened surface has been tensile tested at 200-650°C and two loading rates. Shot peened specimens had lower ductility than unpeened specimens across the temperature range, but the effect was greatest at 450 and 500°C. This reduction in ductility resulted in reduced tensile strength (TS) or fracture stress at 300-500°C. The shot peening effect was present at both loading rates. Shot peened specimens showed surface micro cracking all over the gauge length of tested specimens and at the point of fracture which explains their reduced ductility. The micro-cracks started at the surface and propagated along strain bands in the material. In contrast, unpeened specimens showed no surface micro-cracks. Interrupted testing on peened surfaces at 500°C 300 MPa/sec showed that the micro-cracks formed first at 2.3% plastic strain. The application of further strain caused new cracks to form and existing cracks to grow deeper until the specimen fractured at ~9% elongation. Further specimens were tested with different peening coverage levels of 125%, 200%, 400%, 600% and 1000%. Peening coverage of 125% caused a large reduction in ductility and small reduction in TS relative to the unpeened state. Higher coverage caused further reductions in ductility and TS. The reduced ductility of shot peened specimens is attributed to the low ductility of the shot peened layer which is heavily strained and work hardened.
11:35 AM
Experimental and Simulation Study of the Effect of Precipitation Distribution and Grain Size on the AD730TM Ni-based Polycrystalline Superalloy Tensile Behavior: Marco Panella1; Loïc Signor1; Jonathan Cormier1; Marc Bernacki1; Patrick Villechaise1; 1Institut Pprime, ISAE-ENSMA, University of Poitiers
The mechanical properties of nickel-based superalloys depend strongly on their microstructure, namely the grain size and the state of precipitation. Main design criteria in aeronautical turbine disks are the resistance to disk burst and low cycle fatigue in the bore, but also to creep in the rim part due to higher temperatures. The chosen microstructures result from a compromise between these contradictory requirements. Indeed, creep durability is improved using a coarse grain microstructure while the increase of static and fatigue strength requires a fine grain microstructure. Moreover, the volume fraction and the size distribution of ã' precipitates are the predominant parameters controlling mechanical properties at lower temperatures. A spatial optimization of the microstructure is reachable using specific technologies, e.g. dual microstructure heat treatment. The development of microstructure sensitive models is thus a major concern for the optimal design of these components including gradient of grain size and / or precipitate size. Full-field finite element simulations may be employed to predict the macroscopic behavior of polycrystalline aggregates using crystal plasticity constitutive equations whose parameters depend explicitly on the microstructural attributes. In this framework, the present study is devoted to the evaluation of the yield stress of polycrystalline AD730TM nickel-based superalloy, chosen as a model material. This work includes microstructural characterization and mechanical tests carried out on single crystals and polycrystalline specimens with well-controlled microstructures. Predictions of the macroscopic yield stress are provided by preliminary simulations carried out in the elastic regime combined with a specific post-processing.
12:00 PM
Modeling Creep of Ni-base Superalloys for Applications in Advanced Ultra-supercritical Power Generation: Chen Shen1; Monica Soare2; Vito Cedro3; 1GE Research; 2GE Global Research; 3National Energy Technology Laboratory
Steam turbine and boiler materials for Advanced Ultra-Supercritical (A-USC) coal-fired power plants are required to withstand steam conditions up to 760˚C and 35MPa for a typical service life of over 100,000 hours. Long-term creep evaluation of these materials is challenging and relies on extrapolation of data from accelerated tests under steady-state conditions and increased temperatures and/or stresses. This study is focused on improving accuracy of creep property predictions by using a microstructurally informed constitutive creep model. The model was recently developed within the framework of continuum damage mechanics (CDM) for ã' precipitation strengthened alloys Haynes 282, Inconel 740H, and Nimonic 105 at stresses between 100 and 400MPa, and temperatures between 700 and 850ºC. The model incorporated physical mechanisms including (1) growth of micro-voids and cracks to macroscopic cavities, (2) accumulation of dislocations toward excessive inelastic deformations, (3) microstructural changes (e.g., ã' coarsening). The model was validated by comparing creep strain evolution in time and rupture times to data from uniaxial creep tests. As damage mechanisms are captured by specific damage variables, the model aims at making predictions beyond the secondary creep regime, well into the tertiary regime by tracking evolution of the damage variables, eliminating the typical need of linear extrapolation.
12:25 PM Question and Answer Period