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Unravelling the onset causes of Adiabatic Shear Banding in Ti- and W- alloys

   School of Metallurgy & Materials

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  Dr Beñat Gurrutxaga-Lerma  Applications accepted all year round  Funded PhD Project (UK Students Only)

About the Project

This PhD project concerns the study of the fundamental microscopic mechanisms that lead and/or contribute to the formation of shear bands in Ti-based W-based alloys. The project is part of a concerted and innovative larger scale effort at modelling shear band initiation at the mesoscale. Adiabatic Shear Banding (ASB) concerns the formation of extremely narrow bands (<1𝜇m) of highly deformed material. The bands act as weak spots, very deleterious to the material’s mechanical performance of pieces with a propensity to form them. Ti- based and W-based alloys, of use in the aerospace and nuclear industries, are two of the main material systems known to display them. The bands typically form under high strain rate (fast) loading, as may be encountered in aerospace and defence applications (e.g., birdstrikes, armour/anti- armour applications), and, crucially, as may be unwittingly introduced during the manufacturing of metallic pieces (e.g., machining & additive manufacturing, forming, etc).

Despite their reduced dimensions, the bands are so highly deformed that the material’s morphology and microstructure is extremely different to that of the bulk: the band is extremely hardened, cracks and voids are typical, and more often than not part of the material in the band has melted and recrystallised. As a result, adiabatic shear bands act as dramatic weak spots for the material and often leading to failure.

Adiabatic shear band growth is a relatively well understood thermo-plastic instability: an unspecific material instability leads for some reason to the initial localisation of plastic deformation along a narrow region of the material. If this initial localisation is intense enough and the loading under which is formed is fast enough, then the heat generated inside the material during plastic flow may not have sufficient time to diffuse away from the band. This results in thermal-softening, which in turn favours more localised deformation, in a self-feeding instability that causes the material in the band to heat up, often to the point of prompting localised melting and/or dynamic recrystallisation.

But what is the initial instability that leads to all of this? Despite decades of study, the root causes that lead to the formation of adiabatic shear band remain largely unknown. This is because of a number of factors:

1. The bands form suddenly, very quickly and unpredictably, so their formation is difficult to observe experimentally. This is in fact one of the key reasons why shear banding is so undesirable: it seems to happen at random, and if it does, it leads to catastrophic failure.

2. The loading under which they form is fast (usually above 103/s). This, alongside the unpredictability, makes direct observation of band formation very difficult.

3. Once formed, the band wipes out all microstructural features that may have contributed to its formation, so post-mortem characterisation can’t serve but to characterise the band’s own microstructure, but not what caused it.

In this project, a number of proposed and hypothesised microscopic mechanisms leading to the onset of ASB will be explored at the atomistic and mesoscopic scales. Using lattice dynamics and molecular dynamics (MD), the role of the kinematic generation of dislocations (a dislocation core instability that leads to the multiplication of dislocations moving at moderate “resonance” speeds) will be explored in Ti and W alloys. This will concern both lattice dynamics theoretical studies and observational MD studies of the kinematic reactions, and result in a mapping of resonance speeds vs multiplication rates with which to determine whether on its own, kinematic generation suffices to account for the fundamental instability that causes ASB. Alongside this, the effect of microstructure will be studied:

(1)heterogeneous generation at phase boundaries in target precipitates (gamma/gamma’ boundaries, grain boundaries,...) will be studied using targeted molecular dynamics simulations and machine learning approaches;

(2) the role of supersonic dislocations will be characterised;

(3) dislocation mobility laws at high temperatures and under high strain rates will be produced;

(4) the role of homogeneous and Frank-Read source mechanisms will be studied.

All these reactions will be mapped into a purpose-built thermo-elastodynamic code of dislocation dynamics, which will enable the study of the formation of ASBs at larger lengthscales and over extended timescales than those molecular dynamics allow.

Through this, it is hoped that alloying strategies able to hinder the formation of ASBs will be developed and identified, whilst the root causes of ASB are characterised and studied for the first time.

The project will be held at Dr. Beñat Gurrutxaga-Lerma in the School of Metallurgy and Materials at the University of Birmingham. The candidate will have at least a 2:1 class degree in Materials Science, Physics, Applied Mathematics, Engineering, or other relevant discipline. A background or interest in computer programming would be advantageous. For further information, please contact Dr. Beñat Gurrutxaga-Lerma at [Email Address Removed]

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Funding Notes

A fully funded (stipend+fees) PhD studentship is available for UK home students of this project.

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