Hello,
I am curious about grain size and metal strength, one would think that the larger the grains the more distance on continuity of atoms exists. and therefore you would have higher strength. However, I read the opposite is so that with short continuity of crystal arrangement, and hence smaller grains the strength is higher, can anyone shed some light on this?
Regards Digby Millikan BE
Sure. At relatively low temperatures compared to the melting point the strength is controlled by the motion of crystal defects called dislocations (analogous to moving a rug by putting a bump in it locally).
For steels:
"The refinement of the grain size of ferrite provides one of the most important strengthening routes in the heal treatment of steels. The grain size effect on the yield stress can therefore be explained by assuming that a dislocation source operates within a crystal causing dislocations to move and eventually to pile up at the grain boundary. The pile-up causes a stress to be generated in the adjacent grain, which, when it reaches a critical value, operates a new source in that grain.
In this way, the yielding process is propagated from grain to grain. The grain size determines the distance dislocations have to move to form grain boundary pile-ups, and thus the number of dislocations involved. With large grain sizes, the pile-ups will contain larger numbers of dislocations, which will in turn cause higher stress concentrations in neighboring grains.
In practical terms, the finer the grain size, the higher the resulting yield stress and, as a result, in modern steel working much attention is paid to the final ferrite grain size. While a coarse grain size of d-1/2 = 2, i.e. d = 0.25 mm, gives a yield stress in mild steels of around 100 MPa, grain refinement to d-1/2 = 20, i.e. d = 0.0025 mm, raises the yield stress to over 500 MPa, so that achieving grain sizes in the range 2-10 μm is extremely worthwhile."
See entire article at:
Pittsburgh Pete
Dear Digby Millikan:
A good response by "metalengr".
I would only add that the grain size is like the opening in a truss structure. The smaller the opening, the stiffer the structure. (Neglecting a host of piddling engineering details ;>) ). Note also that the largest recorded strengths are for things like carbon nanotubes, single crystal iron... *very* small indeed. It is enfolding these into a bulk material that the strength is lost, and entropy obeyed.
When you stress some structural materials, the large grain size is broken into small grains sizes in the stage called "plastic deformation". Also called "work hardening". Along with the increased surface area of small grains, comes a propensity for corrosion.
David A. Smith
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