[b]Is Aluminum more brittle than Nickel?[/b]

[coatingsguy]

I'm trying to prove metallurgically which metal is more brittle during impact - Nickel or Aluminum. I've located some mechanical properties data, and it shows Al has a Elongation % of 50-70 while Ni is only 30%. It lists the Young's Mod of Al at 67 GPa and Ni at 207, which would make Ni a stiffer material. But does that necessarily make it more brittle? I've read that Rockwell F is used to test the hardness of softer materials such as Aluminum. However, I've also read that materials may behave differently when subjected to a sudden impact than they do when subjected to a tensile test. I haven't been able to locate stress-strain curves or impact data for these metals. Any help?

 

[metengr]

Nickel and aluminum are considered ductile metals because of their crystal structure. Elongation is a direct measure of ductility for materials, and to some degree an indirect measure of toughness. Toughness would be best described as the area under a stress-strain curve for each metal. If you were to subject Al and Ni to Charpy impact testing, the impact energy for Al would be higher than Ni because of the differences in elongation. Typically, the higher the elongation, the larger the area under the stress-strain curve.

Therefore, a more appropriate response is that Ni is less ductile in comparison to Aluminum. I would not use brittle to characterize either metal.

 

[Goahead]

May I suggest that the problem has not been presented in the usual perspective of brittle failures?
Generally one prefer to speak of testing conditions that cause brittle fracture, and not of brittle materials.
As an example a certain steel, ductile at room temperature, may fracture in brittle mode if tested at a temperature lower than that of transition from ductile to brittle (DBTT). Other examples concern triaxial vs. uniaxial loading modes, speed of testing and presence of stress concentrators.
If you are interested in the results of impact testing of Charpy specimens you will obtain a numeric value.
But are values obtained on aluminum comparable to values for nickel? I would be surprised.

http://www.welding-advisers.com/

 

[metengr]

Goahead;
Yes, the Charpy impact values would be comparable for both metals. Charpy impact testing was originally developed to provide a quick and rather in-expensive method of evaluating the effect of temperature on the fracture behavior of metals - nothing more and nothing less. Ni and Al will not exhibit ductile to brittle transition fracture behavior. Therefore, impact testing would not yield any more meaningful results for toughness as the area under their respective stress-strain curves.

If you really want to compare fracture toughness between these ductile metals, J integral testing is the correct approach.