Indentation-based micromechanical investigation of B2 intermetallics and bcc metals

Gagneur, Vincent ORCID: 0009-0006-8486-3023 (2025). Indentation-based micromechanical investigation of B2 intermetallics and bcc metals. University of Birmingham. Ph.D.

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Abstract

Whether it is for aerospace or nuclear applications, materials’ high temperature performances are often a limiting characteristic for improving the efficiency of next generation equipment. Few classes of materials have the properties to withstand the high temperature and high mechanical stress of aerospace gas turbines, or the irradiation environment of nuclear power plants, which often comes at the expense of their room temperature properties, limiting their processability and applications. One of the top class of materials for aerospace gas turbine application are nickel-based superalloys. Their success is in part due to their microstructure, consisting of a face-centred-cubic (fcc) y nickel matrix, reinforced with ordered-fcc y’ NiAl precipitates, offering astonishing high temperature performance in terms of yield strength, creep strength and melting point, whilst retaining a decent amount of room temperature ductility for processing. However, recent developments of nickel-based superalloys seem to reach a limit, and no major improvement of their high temperature performance have been seen over recent years, which opens the question of exploring other classes of materials to push mechanical properties even further. Body-centred-cubic (bcc) metals usually display higher melting points than fcc metals, as well as being lower cost, and could potentially outcompete them for high temperature applications. Recently, a new class of alloys using bcc metals has been proposed, termed “bcc-superalloys”, taking inspiration of nickel-based alloys in their microstructure template. These exploit a combination of a bcc disordered matrix, reinforced with ordered-bcc (B2) intermetallics. Many potential alloy systems display such phase equilibria that could realise the desired high temperature performance e.g. HEAs namely (Zr,Ti)(Ta,Nb,V)Alx type, beta titanium + TiFe, or Fe + NiAl. However, their application is currently limited by their lack of room temperature ductility. Much has yet to be discovered as to bcc-superalloy mechanical behaviour, and the bcc crystal structure offers its share of unique behaviour, controversies, and challenges. Designing bcc+B2 alloys with increased room temperature ductility will necessarily involve a study of bcc, B2 and bcc+B2 mechanical properties, and in particularity their micromechanics, which dictate the larger scale deformation behaviour. A question then is to combine high throughput alloy development, in response to modern economic and environmental challenges, and refined micromechanical alloy characterisation. Micromechanical testing techniques, whether they be transmission electron microscopy (TEM), micro tensile or micro-pillar compression, and/or single grain testing, are usually complex and challenging experimental procedures, which can be poorly adapted to the context of rapidly surveying a wide range of different alloy systems. Micromechanical characterisation is especially challenging when performed on bcc and B2 materials, as their multiple possible active slip planes, namely {100}, {110}, {112} and {123}, often have very similar geometry, leading to common issues of ambiguity in slip system identification. An alternative technique for slip activity characterisation is indentation-based slip trace analysis. The technique consists in indenting a polished sample using either a microhardness or a nanoindentation testing equipment, imaging the slip traces generated around the indents using scanning electron microscopy (SEM), and measuring the local crystal orientation using electron back-scattered microscopy (EBSD). From there, theoretical slip trace orientations at the surface can be calculated, and compared to the observed orientations, informing us on the likely active slip planes of the material. Since the experimental procedure is significantly less demanding than TEM or single crystal testing, the large increase in statistics obtainable could by-pass the limitations surrounding ambiguity in bcc slip behaviour, and be powerful in the context of novel alloy development where there is a need for quick property assessments. In this work, we will explore the possibility of using automated indentation based slip trace analysis to efficiently characterise the slip activity in bcc and B2 compounds, and assist the development of novel bcc+B2 alloys with improved room temperature ductility. In Chapter 1, the prior literature is reviewed, setting the background motivation of aerospace, current & future materials, then discussing in detail bcc, B2, and bcc+B2 alloys and their micromechanics, as well as the specific challenges posed by the bcc lattice when it comes to slip activity characterisation. In Chapter 2, the methodology for slip activity assessment developed throughout this thesis experimental chapters (3 to 5) is presented, giving recommendations for the associated experimental work and data analysis to replicate the experiments made throughout this thesis, or use the technique on different materials. In Chapter 3, an automated slip trace analysis algorithm is presented, designed to fully exploit the potential of indentation-based slip trace analysis approaches. The methodology gives access to larger statistics than alternative TEM, micromechanical and/or single crystal testing approaches, with increased speed and repeatability over manually slip trace identification, in a highly simplified experimental procedure. It is first demonstrated in the simpler fcc Nickel system, and then used to characterise the slip activity of bcc Fe, B2 TiFe, B2 Ti2AlMo, and B2 Ti2AlNb. It is shown how unambiguous <111> {112} and/or {123} type slip is likely to be observed under many experimental constraints, and used to distinguish between key bcc and B2 slip activities, whilst the global statistics of plane orientations matched by the slip traces can be used to assess the likely dominant slip planes. In Chapter 4, the new slip trace analysis technique is applied on a compositionally graded sample, B2 FeAl to B2 NiAl, with the aim to characterise the evolution of the slip activity as a function of composition. It is shown how, using the technique developed, we can accurately predict the B2 composition at which the transition in active slip system between <100> dominated B2 NiAl and <111> dominated B2 FeAl occurs. This approach can be used to rapidly study the impact of alloying additions on B2 slip behaviour in different ternary systems, and then to determine which B2 compositions are more likely to favour ductile behaviour within bcc+B2 systems. Also, the quasi continuous information on slip activity as a function composition obtained offers a new kind of data to refine crystal plasticity models. Finally, in Chapter 5, we dive deep into the statistics obtained via the technique. Through sample slip activity simulation, hypothesis testing, and confidence level assessment, it is shown how reliable conclusions on the samples’ bulk slip activity can be made, whilst resorting to only minimal assumptions on the samples slip behaviours. We also show that the ambiguity in slip plane identification can be bypassed in almost any context, provided that sufficient data is gathered, which may now be acquired thanks to the automation and simplicity of the experimental procedure. All experimental results obtained throughout these chapters, using our newly developed technique, give consistent results with literature, and the technique is currently being used to assist the development of bcc+B2 systems of particular interest, notably titanium-based bcc-superalloys. Ongoing work attempts to apply the technique to dual phase alloys, giving new insights as to bcc + B2 slip transfer mechanisms and the influence of B2 slip activity on the overall alloy slip activity.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):