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Subject: Re: mechanical properties of Al-Sc alloys

Date: 04/22/02 at 9:32 AM
Posted by: Jostein R°yset
E-mail: Jostein.Royset@hydro.com
Message Posted:

In Reply to: Re: mechanical properties of Al-Sc alloys posted by Roberta Lazzeri on 04/22/02 at 7:20 AM:

Dear Roberta,

The effect of Sc on the mechanical properties of an Al alloy will be dependent on the alloy to which you add it, and on the thermomechanical processing of the alloy.

In general, the effect on the strength is more pronounced when Sc is added to alloys that are regarded as "non heat-treatable" (i.e. pure Al, Al-Mn and Al-Mg alloys). In these alloys, an increase in tensile yield strength and ultimate tensile strength of 100 MPa or more should be easy to achieve by adding typically 0.2 wt.% Sc, solutionize the alloy (if feasible) and then do precipitation hardening at 250 - 320░C for somewhere between 3 and 30 hours. (Whether the alloy still should be classified as non heat-treatable is another issue ... )

For heat treatable alloys such as Al-Cu, Al-Mg-Cu,
and Al-Zn-Mg, Sc is not necessarily added for the purpose of particle strengthening, but maybe more for the purpose of microstructure control, for instance by preventing recrystallisation. In this case dispersoids of Al3Sc may be formed at temperatures in the range 400 - 550░C, and these dispersoids do per se give only a minor increase in the yield strength and ultimate tensile strength. However, there may be a significant contribution to the strength from the non-recrystallised structure. I would assume that when Sc is added to heat treatable alloys, the achievable increase in yield strength and ultimate tensile strength is in the order of 10%
(say, anywhere between 3% and 20 %).

When it comes to crack growth rate, I must admit that I know close to nothing. I recall that some fatigue results for Al-Mg alloys with and without Sc are reported in the literature:

H.-J. Gudladt, P. Nagy: The Cyclic Crack Propagation Behaviour of a Hardenable Polycrystalline Al-Mg-Sc Alloy, Mater. Sci. Forum 13/14 (1987) 407-412

A.R. Krause, R.A. Chernenkoff and R.G. Davies: The Effect of Transition Metals on the Fatigue Response of a 5754 Type Alloy, Proc. Light Metals 1999, Eds. M. Bouchard and A. Faucher, August 22-26, 1999, Quebec City, Quebec, Canada 429-438

O. Roder et al.: Correlation Between Microstructure and Mechanical Properties of Al-Mg Alloys Without and With Scandium, Mater. Sci. Forum 217-222 (1996) 1835- 1840

O. Roder et al.: Fatigue Properties of Al-Mg Alloys With and Without Scandium, Mater. Sci. Eng. A234-236 (1997) 181-184

Gudladt and Nagy foud that there is little, if any, effect of Sc on the crack propagation rate in the alloys that they studied.

Krause et al. found that Sc significantly enhanced the fatigue performance of an Al-Mg-Mn alloy.

Roder er al. found that adding Sc to an Al-Mg alloy increased the resistance to microcrack growth but slightly decreased the resistance to macrocrack growth.

***

If you could be a little bit more specific with regards to which alloys that you are going to add Sc to, and to what kind of thermomechanical processing that you want to apply to them, I'd be happy to participate in a discussion about what kind of properties one would expect.


Best Regards,
Jostein R°yset


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