Is a eutectic alloy always stronger than a hypo or hyper eutectic one?

[Femme_physics]

Question at title. Out of curiosity.

 

[Studiot]

At what temperature?

The eutectic will be liquid at some temperature, which the noneutectic will still have some solid strength left.

 

[Femme_physics]

Sorry-- I mean all when they're all solids. I understand that eutectic alloys coalesce and form grain bounderies, and hyper-hypo ones have lamellar solid phase. I wonder if the grain bounderies always stronger than the lamellar.

 

[Studiot]

Alloys and phase diagrams can be very simple or very complex. Not all form eutectics either.
For example the cupro-nickel system is fully miscible in all proportions, but forms a very pronounced strength maximum at about 58% nickel.
Further the actual strength of alloys depend strongly on their thermal history. Part of the manufacturing process includes standard heat treatment/quenching to make them harder/stronger or softer and more ductile.

Given all that I would say it is a brave person that makes the above claim.

What alloy are you thinking of in particular?

 

[Femme_physics]

Alloys and phase diagrams can be very simple or very complex. Not all form eutectics either.

Yes, I mean "among" the eutectic ones

What alloy are you thinking of in particular?

None particular, all eutectic ones...let's say alimony-lead for the sake of argument. If you solidify it at the eutectic point it would be stronger than hypo-hyper eutectic points, right? Can we argue that all eutectic metals will be stronger solids when they form in the eutectic point, and that's due to the fact they solidify with grain bounderies and not with a lamellar phase?

 

[Mapes]

Can we argue that all eutectic metals will be stronger solids when they form in the eutectic point, and that's due to the fact they solidify with grain bounderies and not with a lamellar phase?

I'm not sure where you heard that eutectic compositions don't freeze to form lamellae? And hypo- and hyper-eutectic alloys compositions certainly contain grain boundaries; alloys in general do. Lamellar boundaries separate two different phases; grain boundaries separate regions of different crystal orientation. They are not mutually exclusive.

Nevertheless, I could be convinced that the eutectic composition is the strongest composition, given that the α and β phases have similar strength. The eutectic point would be optimal because, besides the solid-solution strengthening of the two phases, one obtains the many interfaces of a fine lamellar structure (or, in the extreme, an amorphous structure). If one of the phases is much stronger than the other, however, then I would hypothesize that the addition of the second phase would only decrease the strength.