Decreasing grain size with element addition

[scott_alexsk]

Hello,

I have found that in some processing cases that additional elements are added to metals which result in increased potential for smaller grain size. Does anyone know why this is so? Am I signiificantly misunderstanding the issue?

Thanks,
-scott

 

[Astronuc]

That's possible if an element forms an intermetallic compound with a melting temperature higher than the predominant solute, e.g. silicides, and I think oxides as well. The compounds with higher melting temperatures precipitate first and form nucleation sites for grains.

Or is there an effect on solidification kinetics? Cooling/quench rate is also a factor.

Hot and cold working with annealing can produce smaller grains, but that is because dislocation bands form new grain boundaries.

 

[Adrian4English]

Have you never heard of the marbles effect? This concerns itself with the most efficient packing efficiencies and the lowest energy states associated with them. I'm a Chem Eng and this comes into that in a major way in gas behaviour.
Basically most things on an atomic scale "want" (to anathropamise) to achieve the lowest energy state. In the case of metals this is a pure crystal. But the way in which metals cool and solidify of course introduces defects and gives strength to the multiple crystals formed (the annealing process, I was never that good at metalurgy).
Adding atoms of a different "diameter" can prevent or enhance this crystalisation process making metals more malleable, brittle or ductile.
I hope this helps.

 

[yamini]

actually the thing is like when you add alloying elements to the metals the structure becomes more finer due to which there will be more potential.this is what i have studied in physical metallurgy,as i am presently pursuing my b.tech in metallurgy and material technology.
if you have an idea about TTT diagram and Iron-carbon diagram,i think you will get your answer.for reference: Physical metallurgy textbook by Sidney.H.Avner.

 

[Adrian4English]

Well as I think I was trying to say my backgrond is in gas but I think the basic thermdynamic principles can apply to metallurgy as well. Take for example 2,2 dimethyl propane. The molecule is basiclly a tetrahedron and it packs more efficiently than methyl butane or normal pentane itself; this is shown by the density of such gases at almost all pressures and temperatures.

 

[Astronuc]

Alloying affects the metal chemistry certainly, but it is the presence of different phases with different solubilities, or more precisely different precipitation temperatures, which affects the nucleation of grains during the solidification process.

Quench rates affect the kinetics. Slow quenching allows for larger grains.

Subsequent thermo-mechanical processing, e.g. hot work vs cold work, and annealing temperatures affect grain size.

Metals seldom exist in their lowest energy state, which is one factor that contributes to their corrosion susceptibility. Chemically, metals prefer to form oxides or other compounds with a variety of anions.