High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that favour the primary creep regime. The study has revealed the detailed structure of extended a2〈112〉 dislocations as they shear the γ′ precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a〈1 1 2〉 dislocations was also found to distort the γ/γ′ interface, changing its appearance from linear to a "saw tooth" pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a2〈110〉 dissociated dislocations were also observed in the γ channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the γ′ and intrinsic stacking faults in the γ, and to make estimates of their energies

Prediction of Mechanical Behaviour in Ni-Base Superalloys Using the Phase Field Model of Dislocations

The "Phase-Field Model of Dislocations" (PFMD) was used to simulate shearing of gamma-prime precipitate arrays in single crystal turbine blade superalloys. The focus of the work has been on the cutting of the L12 ordered precipitates by a{111} dislocation ribbons during Primary Creep. The Phase Field Model presented incorporates specially developed Generalised Stacking Fault Energy (γ-surface) data obtained from atomistic simulations. The topography of this surface determines the shearing mechanisms observed in the model. The merit of the new γ-surface, is that it accounts for the formation of extrinsic stacking faults, making the model more relevant to creep deformation of superalloys at elevated temperatures

Deformation twinning during high temperature compression tests of the Ni-base superalloy ATI 718Plus®

This study discusses the deformation mechanisms active during high temperature compression tests of the Ni-base superalloy ATI 718Plus R. Deformation twins were observed in deformed grains across a range of temperatures and strain rates by use of transmission electron microscopes (TEM). Even at strain rates as low as 0.01 s-1 and temperatures up to 1025C the microstructure contained of grains which deformed by deformation twinning. It was concluded that the low stacking fault energy of the alloy, which was measured to be 15mJm-2 caused the formation of the deformation twins. In addition, several examples of the early stages of twin formation were captured. The twinning partials were in most cases emitted from grain boundaries. In a second instance cross-slip events from a Frank-Read source lead to the formation of partials which formed stacking faults

Stress orientation dependence for the propagation of stacking faults and superlattice stacking faults in nickel-based superalloys

Superlattice stacking fault propagation dominates the creep deformation behaviour of nickel-based superalloys at intermediate temperatures. These planar defects may appear under many different configurations depending on the dislocation arrangements and their interactions with the precipitates. Whilst these have been spotted and described before, no systematic way to explain their configurations has been provided. The current study quantifies the types of faults in multiple grains within a tensile crept polycrystalline alloy via a combination of scanning transmission electron microscopy and electron backscatter diffraction. A new defect consisting of a superlattice intrinsic stacking fault in the precipitates and an extrinsic stacking fault in the matrix is observed and a mechanism for its formation is proposed. In combination with data from the literature on single crystals, the results are incorporated into a robust framework to discern the orientation dependencies of these faults. A comprehensive analytical model based on a series of one-dimensional force balances on different dislocation configurations is developed first for the case of athermal stacking fault propagation for the cases of cuboidal and spherical precipitates. The model is then extended to include six configurations of superlattice faults and microtwinning. This results in novel mechanistic maps that account for stress, orientation and microstructure, with excellent qualitative agreement with experiments.Consejo Nacional de Ciencia y Tecnologia Cambridge Commonwealth, European and International Trust Rolls-Royce pl

Role of the Secondary Phase η During High-Temperature Compression of ATI 718Plus®

Abstract High-temperature compression tests were performed on a Ni-base superalloy with a multi-phase microstructure. Particular attention was given on the influence of the η phase on recrystallization of ATI 718Plus®. The compression tests were performed at two temperatures over a variety of strains and strain rates. Meta-dynamic recrystallization was studied by exposing the samples to a set dwell time at the test temperature after deformation. Electron backscatter diffraction (EBSD) was used to investigate the microstructures after the tests. Secondary electron imaging (SEI) and scanning transmission electron microscopy (STEM) were utilized in order to investigate the deformation behavior of η and obtaining a detailed understanding of the recrystallization mechanism. The secondary η phase was found to increase the recrystallized fraction compared to η free tests. However, clusters of thin lamellar η inhibited recrystallization. The flow curve softening was distinctly stronger in the microstructure containing precipitates. It could be shown by SE images that this was due to the breakage and realignment of η. In addition, η was also found to accommodate the stresses by a remarkable deformation without breaking up. This was considered to be due to the composite nature of the precipitate as well as the ongoing recrystallization in the surrounding matrix.</jats:p

Detailed Analysis of the Solution Heat Treatment of a Third-Generation Single-Crystal Nickel-Based Superalloy CMSX-10K^®

Abstract A detailed analysis of the response of as-cast third-generation single-crystal nickel-based superalloy CMSX-10K® to solution heat treatment (SHT) has been carried out, alongside an SHT optimization exercise. The analysis was conducted through microstructural characterization, differential scanning calorimetry, and compositional homogeneity measurements, quantifying (i) the dissolution and microstructural evolution of the inter-dendritic constituents, (ii) the shift in thermo-physical characteristics of the material, and (iii) the change in compositional homogeneity across the microstructure, in order to gain further understanding of these phenomena during the progression of the SHT. During the early stages of SHT, the coarse cellular γ′/narrow γ channel inter-dendritic constituents which were the last areas to solidify during casting, progressively dissolve; homogenization between these inter-dendritic areas and adjacent dendritic areas leads to a rapid increase in the incipient melting temperature T IM. The fine γ/γ′ morphology which were the first inter-dendritic constituents to solidify after primary γ dendrite solidification were found to progressively coarsen; however, subsequent dissolution of these coarsened γ/γ′ inter-dendritic areas did not result in significant increases in the T IM until the near-complete dissolution of these inter-dendritic areas. After the final SHT step, residual compositional micro-segregation could still be detected across the microstructure despite the near-complete dissolution of these remnant inter-dendritic areas; even so the T IM of the material approached the solidus temperature of the alloy.The authors would like to acknowledge funding through the EPSRC/Rolls-Royce Strategic Partnership (EP/H500375/1 and EP/M005607/1). The authors also wish to express appreciation to Dr. Chris Hayward at the School of Geosciences, University of Edinburgh for carrying out the EPMA composition measurements and to Mr. Kevin Roberts of Dept. of Materials Science and Metallurgy for assistance in carrying out the solution heat treatment runs. Requests for access to the underlying research data should be directed to the corresponding author and will be considered against commercial interests and data protection.This is the final version of the article. It was first available from Springer via http://dx.doi.org/10.1007/s11661-015-3252-

A model for dislocation creep in polycrystalline Ni-base superalloys at intermediate temperatures

A model for creep at intermediate temperatures in polycrystalline Ni-based superalloys is presented. The model is based on describing stacking fault nucleation, propagation and subsequent shear within the matrix and precipitates. The critical energy for stacking fault nucleation is obtained by minimising the energy to form a stacking fault from dislocation partials, which is promoted by local stress concentrations. The extent of stacking fault shear at a precipitate is estimated using a force balance at the interface to determine the critical shear distance The model results are validated against creep experimental data in several polycrystalline superalloys showing good agreement. Individual contributions to creep from key microstructural features, including grain size and distribution, are studied to identify which ones are more significant. Similarly, it is shown that one of the main factors controlling the creep rate is the stacking fault energy in the as it dictates the stacking fault nucleation and shear rates. Parameter analysis on alloying additions typically used in commercial superalloys demonstrates which elements have the strongest effect on creep, highlighting how the present model can be used as tool for alloy and microstructure design against dislocation creep

A multiscale study on the morphology and evolution of slip bands in a nickel-based superalloy during low cycle fatigue

Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700˚C for 1, 40 and 500 cycles.The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.Consejo Nacional de Ciencia y Tecnología Cambridge Commonwealth European and International Trust Roberto Rocca Education Program Royal Academy of Engineering Rolls-Royce plc Engineering and Physical Sciences Research Counci

On the oxidation behavior of titanium within coated nickel-based superalloys

Rutile precipitation within alumina scales grown on coated nickel-based superalloy CMSX-4 has been found to occur preferentially at grain boundaries within the scale. Misorientation analysis using Rodrigues–Frank space has revealed clustering of the misorientation between neighboring grains of corundum and rutile about the established 〈0 0 0 1〉_c{1 1 2¯ 0}_c//〈0 1 0〉_r{1 0 1}_r orientation relationship observed in Ti-containing sapphire crystals. The fraction of interfaces found to exist in this configuration is sufficient to explain the nucleation of rutile from a single corundum grain abutting the rutile grain. The diffusive behavior of Ti has been observed to vary considerably within three commercially used coatings, a plain aluminide coating, a plat-aluminide coating and a diffused platinum coating. Titanium diffusion is enhanced by the presence of Pt. However this did not lead to the precipitation of more rutile, which although observed in all three coatings, was present in sufficient quantity to be detected using XRD only within the plain aluminide coated samples.The work was carried out under the financial support provided by Rolls-Royce plc and Engineering and Physical Sciences Research Councils, UK under the Rolls-Royce/ESPRC Strategic Partnership (EP/M005607/1 & EP/H022309/1). This study was also supported by Nanotechnology Platform Project (NIMS Nanofabrication Platform) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Financial support was also received from the Seventh Framework Programme of the European Commission: ESTEEM2, contract number 312483. Requests for access to the underlying research data should be directed to the corresponding author and will be considered against commercial interests and data protection.This is the author accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S1359645415002281