G.D. West

Combined EBSD/EDS tomography in a dual-beam FIB/FEG–SEM

An automated method for collecting combined three‐dimensional (3D) electron backscatter diffraction (EBSD)/energy dispersive spectroscopy (EDS) data sets on a dual‐beam focused ion beam (FIB)/field emission gun scanning electron microscope (FEG–SEM) microscope is described. The method uses simple scripting files on the dual beam to move between the EBSD collection and the FIB slicing positions, which are linked to a commercial EBSD data collection programme. The EDS data are collected simultaneously with the EBSD patterns analogous to combined two‐dimensional (2D) EBSD/EDS. The technique has been successfully applied to study both the interdiffusion zone between a coating and a substrate and a complex multi‐phase coating on a nickel‐based superalloy sample. This analysis is shown to enable the complex grain shapes, location of precipitates and phase interconnectivity within these samples to be determined without the ambiguities associated with 2D stereographic analysis.

Characterisation of Intermetallic Phases in Multicomponent Al-Si Casting Alloys for Engineering Applications

Multicomponent Al-Si based casting alloys are used for a variety of engineering applications. The presence of additional elements in the Al-Si alloy system allows many complex intermetallic phases to form, which make characterisation non-trivial due to the fact that some of the phases have either similar crystal structures or only subtle changes in their chemistries. A combination of electron backscatter diffraction (EBSD) and energy dispersive X-ray analysis (EDX) have therefore been used for discrimination between the various phases. It is shown that this is a powerful technique for microstructure characterisation and provides detailed information which can be related to microstructure evolution during initial casting and subsequent heat treatment. The mechanical properties of different intermetallic phases have been investigated as a function of temperature using the nanoindentation technique. In particular, the hardness and modulus of a number of phases have been established for a range of alloy compositions. Physical properties of some of the intermetallic phases are also discussed. Phase identity, composition, physical and mechanical properties are set in context to inform alloy design strategies.

On the Roles of Oxidation and Vaporization in Surface Micro-structural Instability during Solution Heat Treatment of Ni-base Superalloys

Micro-structural instability at the surface that develops during solution heat treatment of a typical third generation Ni-base superalloy, CMSX10N has been reported. It is shown that elemental Ni vaporizes from the surface during solutioning leading to de-stabilization of γ phase. With increasing extent of vaporization, a phase mixture of β, γ′, and the refractory (W and Re-rich) precipitates occur within the surface layers resulting in the complete breakdown of the cuboidal γ/γ′ phase morphology that is usually observed. It is demonstrated that the conditions at the surface have a marked effect on the vaporization kinetics and subsequent evolution of surface phases—the presence of a continuous dense oxide such as Al2O3 or the presence of sacrificial Ni-foils interspersed in the furnace significantly suppresses elemental vaporization from the sample surface.

Role of Elemental Sublimation during Solution Heat Treatment of Ni-Based Superalloys

The role of elemental evaporation on the microstructural stability of blade surfaces has been investigated on solutioned and aged samples of Ni-based single-crystal superalloys. Evaporation of Ni and Cr at the casting surface during solution heat treatment leads to the formation of a Ni- and Cr-depleted layer at the surface. Nucleation and growth of γ′ phase occur within this layer through subsequent long-range diffusion of Re, Ta, and W between the γ′ layer and the substrate. Beyond a critical Ni and Cr loss, incipient melting initiates at the surface and principally γ′ and TCP phases are stabilized with de-stabilization of γ phase. Nucleation of TCP phases occurs at grain boundaries arising from cellular recrystallization during the ramp-up cycle. Therefore, on quenching, a range of microstructures are observed at the casting surface.

Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment

Discontinuous precipitation in single-crystal Ni-base superalloys during solution heat treatment has been studied. It is found that discontinuous precipitation occurs at temperatures approaching the solvus, where volume diffusion is dominant. Diffusion of Al ahead of the boundary leads to gamma prime precipitation and is accompanied by a loss in the driving force available for advancement of the grain boundary. The rate of gamma prime precipitation was tracked using in situ neutron diffraction during isothermal hold. Gamma prime precipitation is accompanied by super-saturation of Cr and W within the channels ahead of the interface. The driving force calculated for the initial stages of DP was [10-5 to 10-4] N/[μm2 of the grain boundary]. The results provide an insight into discontinuous precipitation during solution heat treatment of Ni-base single-crystal alloys and are useful in optimizing the heat treatment process to avoid surface defect formation.

Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds

The microstructure in the heat-affected zone (HAZ) of multipass welds typical of those used in power plants and made from 9 wt pct chromium martensitic Grade 92 steel is complex. Therefore, there is a need for systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds manufactured using the traditional arc welding processes in order to understand possible failure mechanisms after long-term service. In this study, the microstructure in the HAZ of an as-fabricated two-pass bead-on-plate weld on a parent metal of Grade 92 steel has been systematically investigated and compared to a complex, multipass thick section weldment using an extensive range of electron and ion-microscopy-based techniques. A dilatometer has been used to apply controlled thermal cycles to simulate the microstructures in distinctly different regions in a multipass HAZ using sequential thermal cycles. A wide range of microstructural properties in the simulated materials were characterized and compared with the experimental observations from the weld HAZ. It has been found that the microstructure in the HAZ can be categorized by a combination of sequential thermal cycles experienced by the different zones within the complex weld metal, using the terminology developed for these regions based on a simpler, single-pass bead-on-plate weld, categorized as complete transformation, partial transformation, and overtempered.

Microstructural characterisation of oxide formation from MCrAlY coatings on nickel-based superalloys

Abstract Nickel-based superalloys are commonly used in industrial gas turbine engines for power generation due to their ability to withstand the arduous environments. Coating systems are employed to increase the service life of components by increasing the oxidation\corrosion resistance and high temperature capabilities. MCrAlY coatings are a common coating system used, not only to protect the substrate from oxidation and corrosion but also to act as a bond coat for thermally insulating coatings. The ‘M’ in the MCrAlY coatings is generally Ni, Co or a mixture of the two, with Co being added to improve the hot corrosion resistance (in conjunction with Cr) and to make the coatings more ductile. These coatings work by forming oxides that act as protective layers against further oxidation (known as the thermally grown oxide, TGO). Alumina is most commonly formed oxide from the MCrAlY bond coat, which acts as a reservoir of elements for the oxide formation. It is necessary to understand the formation and nature of the TGO as a function of composition variation in the coating. This mechanistic understanding can be used to inform modelling methodologies for both service life prediction and the development of new coatings. This research is therefore concerned with the effect of compositional changes in the MCrAlY coating and their effect on the TGO formation. Various ageing temperatures and times have been studied, representative of service conditions. The alumina layer has been studied, however, the formation of other oxides, including isolated spinels, has been characterised in detail. Microstructural observations were carried out using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and electron backscattered diffraction. Higher resolution characterisation was carried out using a dual beam focused ion beam scanning electron microscope for site specific TEM preparation and STEM/TEM for microstructural evaluation. The results are then discussed in the light of both the bond coat composition and ageing conditions.

The Characterisation of Oxide Scales Grown on Nickel Containing Steel Substrates Using Electron Backscatter Diffraction

The oxidation of steels is critically influenced by its constituent alloying elements. These alter the classical three-phase model of the external oxide scales and in addition can introduce internal oxidised regions. This paper considers the oxidation of a number of nickel containing Fe based alloys of varying compositions, including stainless steel. These have been oxidised under different conditions to produce a number of scale morphologies, which have been characterised using two SEM based techniques; Electron Backscatter Diffraction (EBSD) and Energy Dispersive x-ray Spectroscopy (EDS). Results have shown that nickel promotes the formation of a fibrous internal scale, consisting of iron oxide particles (or iron/chromium oxide in the case of stainless steel) along grain boundary regions. Nickel is rejected from these oxide particles and consequently nickel content is enriched in neighbouring metallic areas.

Characterisation of high temperature oxidation using electron backscatter diffraction

Abstract Electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) has been used to characterise the oxide scales formed on a low alloy steel. The technique provides a powerful combination of local phase information and orientational relationships both within and between phase layers. It has revealed that hematite grain growth occurs almost exclusively along the 0001 direction for the entire range of samples examined. Wüstite and magnetite grains were also found to grow preferentially along orientations close to the 001 direction. EBSD is also well suited to characterising more complex scales such as those formed during hot working (e.g. millscale), and those formed on Fe–Ni alloys. In the latter complementary chemical information from energy dispersive spectroscopy (EDS), which was acquired simultaneously with the EBSD, enables the identity of crystallographically similar phases to be distinguished. EDS also shows that no nickel exists in the external scale and that it instead accumulates at the interface with the scale and at adjacent grain boundaries.

Modelling of Nb influence on phase transformation behaviours from austenite to ferrite in low carbon steels

In this paper, a new model has been developed to predict the phase transformation behaviours from austenite to ferrite in Nb-containing low carbon steels. The new model is based on some previous work and incorporates the effects of Nb on phase transformation behaviours, in order to make it applicable for Nb-containing steels. Dissolved Nb atoms segregated at prior austenite grain boundaries increase the critical energy for ferrite nucleation, and thus the ferrite nucleation rate is decreased. Dissolved Nb atoms also apply a solute drag effect to the moving transformation interface, and the ferrite grain growth rate is also decreased. The overall transformation kinetics is then calculated according to the classic Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The new model predictions are quite consistent with experimental results for various steels during isothermal transformations or continuous cooling.