Why alloys are considered to be harder

[aek]

Hey all,

i'm trying to figure out why alloys are considered to be harder, stronger and tougher than their original metal...

what i can up is that maybe two metals can share electrons and be stronger? I'm not really sure as you see... if anyone can help me with the question i'd really appreciate it...

thanks in advance
aek

 

[brewnog]

Alloys are essentially impurities in metals.

If you imagine a pure metal as a load of marbles, packed together, all the same size, you can see how planes of marbles can easily slide over one another, and you can see how cracks can quite easily propagate. If you then put in an 'alloy', - try putting in a large marble for every 50 normal sized marbles, you will be able to see how these dislocations are 'pinned'. In a real metal, these larger particles can make the metal harder.

Wait for the resident experts to get here and explain properly, or do some googles! Depending on what level you're trying to learn at, http://www.schoolscience.co.uk/content/5/chemistry/steel/steelch2pg5.html may be useful.

 

[Astronuc]

Basically, Brewnog gave the answer.

The alloy can have substitutional elements, e.g. Sn in Zr, in which the atoms of the added element (Sn) actually displace/replace the atoms of the main element (Zr), while retaining the same crystal struction. This is also known as solid solution.

Or, the alloy can have interstitial elements, e.g. C in Fe, in which the atoms of the added element fit in between the lattice positions of the main element. In addition, these alloying elements may form areas of a different crystal structure, i.e. a different phase.

An extension of this latter phenomenon is the formation of intermetallic phases. In Zr, the elements Cr, Fe and Ni form intermetallic compounds, Zr(Fe,Cr)₂ (Laves phase) and Zr₂(Fe, Ni) (Zintl phase). In Ni-based superalloys, one will find Ti and Ni aluminides which have important properties affecting the strength and toughness of the superalloys.

One interesting paper to check out - PHASE DIAGRAM CALCULATIONS FOR NI-BASED SUPERALLOYS (1 page sample) - http://doc.tms.org/ezMerchant/prodt...1-352X-101/$FILE/01-352X-101F.pdf?OpenElement

which addresses the highly alloyed multi-component system Ni-Al-Co-Cr-Hf-Mo-Nb-Ta-Ti-W-Zr-B-C.

Anyway, as Brewnog pointed out, the different atoms (in substitution or interstitially) interfere with the slipping of atoms (and gliding of dislocations) when a metal is deformed.

BTW, strength and hardness go hand in hand, more or less.