How Does Etchant Bring Out Grain Boundaries?

Thanks Redfred, I understood the concept, but not the details.

Drew K

Precisely my point.

So, is the bond between the grains weaker than within? I know when I heat treat the knives I make in the forge I want a tight grain pattern, but not why.

Drew

Don't know off the top of my head. But I do know who to ask at work. That is if I can find her.

When material solidify it happens all over but not all together causing little centres in the liquid to start forming solids. A specific crystal structure form emerges depending on the mix and the rate of cooling. These grains grow towards each other but as the liquid crystalises the remaining liquid changes in mixture and eventually when the grains reach each other a boundary layer is formed between these grains that are of a different mixture than the grain.

These different mixtures have different grain structures and as they solidify the shrinkage of the different parts differ. This causes strains to form at the grain lines. When an acid is placed on this surface the area where the strain is higher the acid will attack it more vigorously.

Another factor may be that the material in the boundary has a higher affinity for the acid and is therefore removed faster.

As for the knife making, remember that a chain breaks at its weakest link. If it has no weak link theoretically the chain will never break. It is for this reason that swords are forged and not cut from billet. The forging process eases out the strain so that it is the same all over.

Yes, I understand how the forging eases strain, but the metal is not liquid in the forge. Simple annealing will release the stresses formed by bending or hammering metals.

According to some knife making articles I have read, it is the quenching and heat treating that sets the grain pattern. I will have to read more to understand it better though. The articles I read talk about how to create martensite within the knife blade.

Drew

When metals are forged the material is plasticised and the grains 'flow' so to speak. If the correct forging process is applied the grains are elongated and can be similated by the glass in glass fibre moulding where the strands are orientated. These long grains are then 'woven' into the metal to form a mat like structure. This structure is much more difficult to break than the normal round grains.

During the hardening process of steel the material is heated till the martinsitic state and then cooled down quickly freezing in this state. The martensite structure is a fine structure with small grains due to the fast cooling process but it induces big stresses. Although the steel is very hard now it is also very brittle and will crack if not tempered quickly enough. During the tempering process the material is heated into the area where the molecules can re-orientate themselves without changing the basic structure of the steel. This reduces the stresses in the steel and a little of the hardness is lost but the material is now much less brittle.

In knife making the hardness of the blade is more a function of the alloy being used rather than the heat treatment. For a blade to be able to hold its sharp edge the molecules should be very little mobile. When the blade is sharpened the sharpest you can make it is when the very edge consist of a single molecule. Molecules are like people they prefer being in the crowd and therefore with time a single molecule will move to fit in with the rest making the knife blunt. For a good edge an alloy that has little molecular mobility is then used.

I hope I have shed a bit of light on this very technical question.

Regards,

Johan

Grain boundaries are usually highly stressed part of a microstucture hence are prone to be attcked more when subjected to the etchants. Etchants (acid or alkali) can thus readily bring them out.

Attack of the grain boundary itself isn't the goal if you want to identify grain structure - the goal is essentially to remove a thin layer of material without mechanically damaging the surface and leave a highly cleaned and polished surface -this couldn't readily be achieved by mechanical means. The boundaries are seen as lines between contrasting grains and the contrast is derived from the way that light is reflected off the crystal lattice in different orientations. These reflections are said to have "reflectivity" (a silly sounding thing to say, come to think of it, but that is the term we use), as the individual grains will appear to flip or switch from light to dark and back again as the surface or light source is moved through a specific angle.

I work with single crystal and directionally solidified industrial gas turbine components and we blast and etch thousands of castings a year for inspection to make sure that grain structure meets customer requirements. With good surface prep (blast) and a good etch and rinse, parts are routinely read to identify individual grain structure defects as small as 15 thousands of an inch without magnification, as well as clusters of even finer defects. In general, this inspection consists of holding the parts under bright lighting and rotating them to look for grains that don't belong, don't run the right direction, aren't the right size, ... In the alloys we use, it is possible to see contrast between adjacent grains whose lattices are misoriented from one another by just a couple of degrees. Crystalline orientation and degrees of misorientation at grain boundaries can be determined using x-ray backscatter Laue.

Etching is also a useful tool for us in the lab in conjunction with microscopes to understand dendritic structure, segregation and solutioning characteristics, etc.

Grain or phase boundaries are simply an optical contrast that occurs when the light that is incident upon the material is reflected differently from the grain boundary than it is from the interior of the grain; or in the case of an as-cast material, the primary solidification cell.

The image from an SEM is produced by a similar mechanism, except that you do not rely on incident light.

Etching is a corrosion process that is as effective as the process control. Etching occurs when there is an electrolytic activity at the interface of the etching solution and material of interest. The boundaries between phases, grains, etc. may have different electromotive potentials with respect to the etchant, although this difference ranges in magnitude, and is dependent upon the combinations of materials and etching environment.

Grain boundaries have a lower density than the grain interiors and increased surface area.Grain boundaries consequently have a higher material removal rate than the grain interior during etching.

One more comment: the terms "stressed", and "highly stressed" are often inappropriately used. The terms "cold-worked", "deformed", and "strained" are usually more appropriate. Semi-conductor materials are often described as highly-stressed when they are actually just shaped differently than desired, i.e., scrap.