Rachel C. Thomson

Low temperature copper solubilities in Fe-Cu-Ni

An atom probe characterization of the copper concentration in the matrix of a model Fe–1.1at.%Cu–1.4% Ni alloy has been performed after isothermal ageing for various extended times at low temperatures. The matrix copper concentrations from material annealed at 500, 550 and 600°C are approximately 10% lower than the equilibrium values predicted from the Thermocalc™ program with the Kaufman database. Isothermal annealing for 4000 h at either 400 or 300°C was not sufficient to attain the equilibrium copper concentration. The kinetics of copper precipitation was investigated with the Starink-Zahra model derived for nucleation and growth reactions and a good fit with the atom probe data was found for an activation energy of 250 kJ mol?1.

Carbide precipitation in 12Cr1MoV power plant steel

The chemistry and other characteristics of carbide precipitates in 12CrlMoV steel of the type used in the power generation industry were studied using energy dispersive X-ray spectroscopy and electron and X-ray diffraction techniques, and the results have been compared against thermodynamic calculations. As a result of the much larger substitutional solute concentrations present in the alloy, unlike the carbides that occur in steels containing smaller concentrations of chromium and molybdenum, it is found that the equilibrium M23C6 carbide precipitates very rapidly during heat treatments of the kind used routinely for stress-relief purposes. The chemical compositions of carbides therefore do not change much during subsequent service at elevated temperatures.

Characterization of Carbides in Steels Using Atom Probe Field-Ion Microscopy

Atom probe field-ion microscopy has played an important role in the characterization of carbide precipitates in steels, and has contributed to an understanding of the transformation characteristics of complex alloy carbides within the range of possible microstructures in engineering steels. This paper initially discusses cementite formation from pearlitic, bainitic, and martensitic microstructures. Alloy carbides precipitating in secondary hardening, creep resisting, and high speed steels are subsequently discussed. It is demonstrated that atom probe field ion microscopy is a valuable technique in the development of these technologically important alloys.

The partitioning of substitutional solute elements during the tempering of martensite in Cr and Mo containing steels

Abstract The partitioning of substitutional solute elements between cementite and ferrite during the tempering of martensite both in a commercial 2.2Cr1Mo0.15C wt% steel and in a novel 2.2Cr1Mo0.4C wt% steel has been studied using atom probe field ion microscopy. No partitioning of Cr, Mo and Mn was observed between the cementite and ferrite in the early stages of tempering. No concentration spikes of substitutional solute elements were found at the transformation interface on an atomic scale. The enrichment of Cr, Mn and Mo in the cementite during prolonged ageing, before the onset of soft impingement, was found to be the same in both the low and high carbon alloys. The results provide further support for the theory that cementite precipitates from supersaturated ferrite via a paraequilibrium displacive transformation mechanism.

A multicomponent diffusion model for prediction of microstructural evolution in coated Ni based superalloy systems

Abstract A multicomponent model which can simulate the microstructural evolution of a coated Ni based superalloy system has been developed. The model consists of a one-dimensional finite difference diffusion solver to calculate the component distribution, a power law based model for predicting surface oxidation and a thermodynamic calculation routine for determining the phase evolution. Apart from forecasting concentration and phase profiles after a given thermal history, the model can estimate the losses due to oxidation and the remaining life of a coating based on a concentration and/or phase fraction dependent failure criteria. The phase constitution and concentration profiles predicted by the model have been compared with an experimental NiCoCrAlY coated CMSX-4 system, aged for times up to 10 000 h between 850 and 1050°C, and many experimental features can be predicted successfully by the model. The model is expected to be useful for assessing microstructural evolution of coated turbine blade systems.

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.

Cementite precipitation during tempering of martensite under the influence of an externally applied stress

The precipitation of cementite under the influence of an externally applied stress, during the tempering of martensite in steels, is investigated using transmission electron microscopy. The stress appears to favour the development of particular crystallographic variants of cementite in any given plate of martensite. Hence, a Widmanstätten array of cementite particles in a normally tempered sample changes to an array consisting of just one variant in stress-tempered samples. The results are discussed in the context of the mechanism of carbide precipitation during the lower bainite reaction.

Prediction of multiwire submerged arc weld bead shape using neural network modelling

Abstract A significant problem faced by todays welding engineers is the need to relate welding parameters to the quality of the finished weld. This is usually achieved by experience, and necessitates many experimental trials, eventually leading to optimal welding parameters. Important characteristics in the evaluation of linepipe seam weld quality are the weld bead shape and size, which can have a significant effect on the microstructure and mechanical properties of the weldment through heat flow effects. The present paper describes the application of neural network techniques to the prediction of the outer diameter weld bead shape for three wire, single pass per side, submerged arc, linepipe seam welds, using the weld process parameters as inputs. Novel methods of digitisation of weld macrostructures produced under different experimental conditions, used in the training of the neural network, are discussed. The contribution of a particular welding process parameter (input relevance) to the variation in the final weld bead shape is also considered. It is shown that it is possible to develop a neural network model that will predict the shape of an entire weld bead, without the necessity to assume it to be symmetric, with a relatively high degree of confidence.

Modelling microstructural evolution and mechanical properties of austempered ductile iron

Abstract Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminium alloys. The aim of the present research is to develop a generic model that will enable the producers of ADI to optimise their product in terms of microstructure and mechanical properties, hence minimising the need for expensive and exhaustive experimental trials and reducing alloy development lead times.

MC carbides in the Hf containing Ni based superalloy MarM002

Unexposed MarM002 alloy turbine blades and blades exposed at 700-1000oC for up to 250 days were studied by TEM, SEM, EDX and XRD. The shapes, distribution and compositions of the MC carbides do not vary significantly during exposure at 700 and 800oC. All MC carbides contain significant amounts of Hf, Ta, Ti and W, and they are present with a very broad range of compositions. The latter is caused by a miscibility gap in the MC carbide phase which causes the formation of MC carbides to shift from Hf lean in the initial stages of solidification to Hf rich in the later stages. In the unexposed samples the Hf lean (Ti,Ta)C carbide is the most numerous one. The shape of the complex MC carbide diffraction peaks are explained semi-quantitatively on the basis of the compositional range of the MC carbides.