Self-healing Metals

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“Self-healing metal” sounds cooler than it really is.  But, I’ll like it a lot more if it gets my daughter a Ph.D.   So, let’s make some sense of it.  The term itself expresses the goal, which is to find alloys that can heal from the nano-cracks that are caused over time and thereby avoid failures or extend the useful life of the object, with possible additional benefits for safety or resource conservation. 

On to the physics:  When a stress (force) is applied to a material, a strain in some spatial dimension will occur (length, angle, volume), and when the force exceeds the elastic limit of the material, it causes permanent dislocations in the microstructure after.  If additional stress is added, the material will reach its ultimate strength at which it breaks.  That’s easy, like bending and then snapping a branch.

So, let’s apply that to steel.  Austenitic steels, which include high levels of chromium and nickel and low levels of carbon, usually have high corrosion resistance and are highly formable, and thus they are the most widely used grade of stainless steel.  But even these get stressed.

Rather than the permanent dislocation in the microstructure, imagine a “phase change,” in this case a martensitic transformation, where atoms shift very slightly in a simultaneous, cooperative movement or flow.  How much is very slightly?  A distance of less than the atomic diameter. 

That martensitic phase can be tricky.  TRIP steel, (a trade term for TRansformation Induced Plasticity) is a high strength alloy that has only one shot at being formable.  Once that part is manufactured and formed, it is mostly martensitic and can be brittle (but also strong, taking significant force before it breaks suddenly – I.e. no bending). If it’s a part that has to sustain high loads repeatedly, it’s bad news. The martensitic phase will crack, and those cracks propagate and lead to failure.

SEM

Above: A scanning electron microscope (SEM)

As a general note on fatigue: usually, the loads are not particularly high.  It isn’t such a danger to have that brittle phase there temporarily.  Micro-cracks form for a lot of reasons, and it’s the growth of those cracks that matters more.  In a brittle material, it’s harder to form dislocations and easier to grow cracks.

The first goal, then, is to find an alloy that would remain formable but undergo the martensitic transformation under stress but also back-transform when the stress is removed (super elasticity).   So, stress a metastable austenitic material (alloy), and rather than causing permanent dislocation of atoms, the metal changes to the martensitic phase where the atoms flow slightly until the stress is removed, at which point they return to their original positions.  Basically, you don’t worsen cracks wherever they are.  That dislocation of atoms also causes a slight volume change, which helps compress the tips of those microcracks which helps in stopping them from propagating. For an application, imagine a metal part subject to vibration and the cyclical states of stress and no stress.

The second goal is to further find identify an alloy that, aside from it’s elasticity, is also strong, as super elastic materials usually are not that strong and have limited application. 

That’s the science, and then there’s the laborious process of figuring out the properties of various alloys at a scale where even a scanning electron microscope (SEM) can only provide clues regarding topography and diffraction changes.  Thus she’ll also use a transmission electron microscope (TEM) which has atom-by-atom resolution for better visualization…  Harvard may be thought of as more of a liberal arts school, but they she’s trying to borrow theirs as it is newer.  How does that happen?

Also in play are atomic force microscopy (AFM), which allows a clearer view of topography changes, nano-indentation (pictured above)  which requires exacting aim, and other spiffy gizmos which allow the devoted to study a material surface area of 50 nm2 while attempting to identify alloys with optimal behaviors in regard to temperature, stress, strain recovery and residual stress.

With no guarantee.  Such is scientific investigation.  Useful side discoveries welcomed.


Picoindenter

The indenter above is positioned within the SEM to study the material.

  B1

This is a tiny diamond indenter tip that, when viewed at 18 millionths of a meter, looks a bit unwieldy, if not oppressive.  Jackie is aiming for the very small lighter gray dots  (superelastic phase) in the larger darker gray areas.  The manufacturer’s website has more info if interested.

shiny

The above is one the samples she’s processed, helpfully described as “It’s shiny.”  Some alloys are very expensive and purchased from others; some she makes herself I think.

Now that you have the “textbook” case, let’s turn to her advisor’s YouTube video, where my daughter attempts to put on her “No, really, I’m a scientist” face and otherwise demonstrates good humor and an optimal office chair roll.

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