Key Challenge 2
Effect of Irradiation on Deformation and Micromechanics
Leads: Michael Preuss (Experimental) and Daniel Balint (Modelling)
Metals generally exhibit strong hardening effects during irradiation and Zr-alloys are no exception. Typically, the yield stress of Zr-alloys doubles during the early stage of irradiation while a dramatic loss of strain-to-failure is observed. Irradiation hardening is currently qualitatively described by <a> loops being barriers for dislocations during mechanical loading. Irradiation hardening has been reported to saturate below 1025 nm-2 although irradiation damage might still continue to develop in respect of <a> loop arrangement and <c> loop formation. In addition, the role of irradiation-induced precipitates on irradiation hardening in Zr-alloys is unknown. Interestingly, despite the drastic increase in hardness, the failure mode of irradiated Zr-alloys remains ductile. The dramatic change of mechanical behaviour compared to as-fabricated fuel cladding has very significant impact on potential cladding failure modes investigated in Key Challenge 4. Hence, providing a mechanistic understanding of the micromechanics that underpin the performance of irradiated Zr-alloys, in combination with effects such as hydride formation, is critical in order to enable the development of more physically informed safety codes. This will be achieved by addressing the following scientific objectives:
To develop new small- to meso-scale mechanical evaluation methodologies that minimise the interrogated volume of active material but ensures the generation of engineering relevant data;
To identify the role of the various defect structures generated throughout the lifetime of fuel cladding/channel box material on the slip-mode distribution and irradiation-hardening;
To quantify the effect of the defect structures on strain localisation (dislocation channelling), meso-scale strain patterns and stress hot spots associated with potential material failure;
To develop a new discrete dislocation and defect cluster modelling approach that captures the irradiation-induced increasing strain localisation, and informs on the critical stress hot spots;
To utilise DDP (discrete-dislocation plasticity) to establish an irradiation-sensitive crystal plasticity slip rule that provides quantitative prediction of prism and basal strain softening, and effects of dislocation channelling.
Work Packages
WP 2.1: Development of Meso-Scale Testing
There is an urgent need, particularly for the nuclear fission and fusion community, to develop mechanical test procedures to reliably produce engineering design data from small samples. A primary challenge is the manufacture of samples that have the smallest possible volume to still represent the polycrystalline material. MIDAS will focus on understanding the minimum critical testing volume thorough crystal plasticity modelling (predict constitutive laws based on typical texture but different number of grains) and identify new, cost-effective sample manufacturing methodologies, such as laser-ablation, to manufacture small-scale uniaxial tensile test samples while also developing intelligent sample gripping when employing small-scale mechanical testers already available. A second tranche will focus on small-punch creep testing currently developed by UKAEA for fusion applications. This technique development will be carried out initially on non-irradiated material enabling comparison with more standard mechanical testing. Subsequent work will demonstrate the new capability on neutron-irradiated samples utilising, new set-ups developed at the MRF at Culham and will support WP 2.2.
WP 2.1 Key Deliverables
Development of a methodology for small-scale testing to provide engineering design data.
Test applicability of small-punch creep testing for evaluating cladding material.
WP 2.2: Dislocation & defect cluster plasticity modelling
DDP is ideal for studying the effects of irradiation damage on hardening and ductility because it captures strain localization naturally as a result of dislocation nucleation and structure evolution, and accommodates point defect diffusion coupled to dislocation climb, coupling to stress-driven diffusion, elastic/plastic second phases and precipitates. A model based on will be developed to incorporate populations of <a> (prism) and <c> (basal) loops of vacancy and/or interstitial character represented by climb dipoles coupled to point defect diffusion that may expand/contract/coalesce, and are parameterized in the model for size, density, distribution and strength. 2D planar DDP modelling can provide deep physical insights but does not allow for the capture of cross-slip, or dislocation loop interactions at the very local level so that, in our work, we will also utilise fully 3D DDP to provide further insight. Experimental observations by TEM and XRD analysis (KC-1) will be correlated with <a> loop density and distribution in model simulations of cladding load conditions to understand and quantify how <a> loops harden the material by impeding dislocations in the early stages of loading, and the role they play in reduced ductility by the opening of dislocation channels resulting from dislocations clearing away defect clusters. A model benchmark will be replicating the hardening combined with low strain-to-failure observed in experiments for irradiated materials (WP 1.2, and from literature); a model will be developed to capture the interaction of glide dislocations with <a> and <c> loops, as well as the potential nucleation of glide dislocations from sufficiently large loops. Irradiation-induced nano-precipitates or He bubble formation (fusion) will be modelled either as point obstacles or as shearable regions in the crystal, to investigate their effect on hardening and channeling, and in particular to investigate their role in the observation that slip bands are smaller in number but greater in intensity in irradiated material. A recently developed methodology for embedding hydride precipitation (interstitial loops modelled as expanding climb dipoles) will be utilised to couple stress caused by hydride volume misfit with dislocation activity and the diffusion of hydrogen in solution. A recent model of plasticity in hydrides will also be used to allow transmission of slip across hydrides of a sufficiently small size. Strain localisation patterns and stress hot spots will be predicted and correlated against loop, defect, and dislocation structures.
WP 2.2 Key Deliverable
Development of a dislocation and defect cluster plasticity model that allows interpretation of observed behaviour in irradiated material as a result of loop distributions and their interaction with glide dislocations, precipitates, He bubbles (fusion) and hydrides.
WP 2.3: Fundamentals of Irradiation Hardening
Bend testing of micro-scale cantilevers cut by focused ion beam (FIB) milling into suitably oriented grains of polycrystalline samples has been developed at Oxford over the last decade. The methods have been successfully deployed to various materials systems, including the determination of CRSS values for the main slip systems in hcp metals.
Within MIDAS we will determine the strength of hardening on major slip systems, firstly in proton-irradiated material, irradiated at the Dalton Cumbrian Facility (DCF), followed by investigating our neutron-irradiated material. Nano-indentation testing (spherical, and Berkovic tips) correlated with indented grain orientations from EBSD will be used for initial rapid screening, followed by the more quantitative micro-cantilever work on a subset of samples parameterized by alloy chemistry, irradiating particle, dose, etc. An additional aspect we aim to explore is the impact of He bubble formation in Zr (generated by a dual- beam set up at DCF) in order to understand irradiation hardening of Zr alloys related to a fusion environment. Tests will be carried out to relatively high strains, enabling CP-FEM parameter extraction to capture both the initial increase in the yield strength and the subsequent reduction in flow strength (softening). A recent development allows the cantilevers to be pulled and bent upwards, making the high-strain region of the triangular section tensile, which enables local rupture strength to be established. Nano-indentation facilities at the MRF will be used for work on active samples, with facilities at Oxford also used for proton-irradiated samples. This will enable the work to be extended to service temperatures, allowing activation energies to be established for different slip systems, and channel formation processes. In addition to testing at fixed strain rates, deformation during periods of fixed loading will be used to explore creep response of different slip systems at temperature. We will use micro-cantilever testing to explore oxygen ingress during thermal exposure and link to KC-3. Mechanical testing will be augmented by TEM inspection of foils extracted from bent cantilevers, to provide additional information on dislocation structures.
Complementary 3D DDP simulations of interactions between dislocation loops and glissile extended dislocations on targeted slip systems will be undertaken using the EasyDD code being developed by Tarleton at Oxford. The aim will be to understand how strongly loops impede dislocation motion, the processes by which they are removed to form channels, the effects of IIP, and small hydrides. Statistical representations of the channel formation and consequent strain softening will be formulated and passed on to higher length-scale CP-FEM models.
WP 2.3 Key Deliverable
Mechanistic understanding of irradiation hardening and subsequent strain softening on different slip systems, and influence by alloy chemistry, irradiation damage level, and irradiating species. The embodiment of this understanding in to DDP and CPFEM methods.
WP 2.4: Strain Localisation and Stress Hot-Spots
We will employ fully automated loading experiments inside an SEM, currently developed at Manchester, to capture the displacement of gold nanoparticles on the surface of the gauge section during mechanical loading in order to obtain strain and rotation maps with a spatial resolution, revealing individual slip traces while covering areas of about 1000 grains (Figure 2.1a and b). Combined with EBSD analysis and utilising Python scripts to correct EBSD data and combining data sets, we will analyse slip activity, slip trace spacing, Burgers direction1 and slip trace interaction with SPPs, hydrides etc. of large areas with unprecedented spatial resolution. Additional FIB-lift out and (S)TEM analysis will be carried out where more detailed dislocation analysis is required. Both statistically representative volume elements (SRVEs) and explicit, microstructurally-faithful CP model reconstructions will be established in order to investigate and quantify the observed slip activations, spatial slip distributions, localisations and stress hot-spots providing unique model validation (Fig. 2.1c). CP modelling (Fig. 2.1d) may then be exploited to investigate the SPP role in slip activation, the localisation of slip at hydrides, and the local states giving rise to hydride shear vs persistent (basal) slip parallel to hydrides; this will provide insight for subsequent work in delayed hydride cracking (KC-4). The models will also be utilised to investigate the consequences of irradiation-induced dislocation channeling on stress hot-spots. Experimental work will start on proton-irradiated samples (early life condition) before quickly moving to neutron-irradiated samples that will enable us study mid/end-of-life material conditions. As the activity of our neutron-irradiated material is relatively low, it will be possible to prepare mechanical test samples at MRF and undertake the in-situ loading experiments at Royce@Manchester. HR-DIC analysis will be carried out at Manchester, HR-EBSD analysis at Oxford and/or MRF, and the accompanying modelling activity will be mainly driven by Imperial. Of particular interest here is how the change of relative CRSS value for the main slip modes (WP KC-2.3) impacts actual slip activity along the two principal directions of a sheet or tube. Further, strain patterns will be analysed in terms of slip transfer across grains and neighbourhood effects, and HR-EBSD will be utilised to correlate slip trace patterns to stress hot spots. New DDP methods have recently been established to incorporate GB slip transfer2, such that better quantitative assessments of stresses developed locally are achievable. These models will be developed for Zr-alloys, taking full quantitative account of the fluence and its spatial variation, for the purpose of identifying mechanisms that could further enhance early failure of Zr-alloys when irradiated to high fluences.
WP 2.4 Key Deliverables
Understanding the effect of microstructure evolution during on slip activity, slip patterns and relationship to stress hot spots at subgrain level.
Identifying any possible mechanisms that might lead to further mechanical degradation during high burnup.