Modification of Properties of Aluminium Alloys by Surface Segregation of

Nanoscale Trace Element Particles

http://www.nt.ntnu.no/users/yingday/NanoNFR/

NFR Nanomat “forskerprosjekt”

Will support

• 1 postdoc 2 years (Yingda)

• 1 PhD student 3 years (Brit)

Background

• Results on surface activation by segregation of trace element Pb

• Not possible to continue Øystein’s work within LMSS

• Continuing interest in exploring the mechanism of activation

ObjectivesTo investigate

• the mechanisms of segregation of trace elements in Group IIIA - VA, especially elements, such as Pb, which at present have practical significance, and elements such as Bi, In, Sn, Ga and Na, which may become important in the near future, but not adequately investigated, e.g., as a result of increased recycling of aluminium alloys,

• the mechanism by which the segregated elements activate the aluminium surface electrochemically with consequences in thermomechanical and chemical treatment and corrosion protection, in particular the melt-down hypothesis,

-1

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

0.0001 0.001 0.01 0.1 1 10 100

Current density [mA/cm2]

Pot

enti

al [

VS

CE]

1

2

3

4

1: As extruded2: 500°C3: 550°C4: 600°C

[Sheng et al. (1998)]

Melting point depression as function of segregated particle size

Pb segregation mechanism [Gundersen (2002)]

Al

Stable oxide Stable oxide

Unstable oxide &corrosion products

Al3+

Solution containing Cl¯

PbAl alloy (liquid?)

• the effect of second trace, impurity and alloying element, such as Mn, Cr, Zn, Fe, Si, Cu, Mg etc. on reducing the activation effect,

• the role of oxygen partial pressure during annealing,

• solubilities of activating and activation-hindering elements in aluminium and in one another through a literature search, and

• theoretically high temperature oxidation of aluminium in the presence of activating and activation-hindering elements by use of thermodynamic data available in the literature,

• to develop models which describe the activation phenomenon and its reduction/elimination by a second element and which can be developed into practical processes to engineer surfaces of aluminium alloys.

Progress• Brit’s training

– literature studies– elementary metallography & electrochemistry (Øystein)– thermodynamics– FEGSEM (Øystein)– high resolution TEM (Yingda)

• Completion of work on 8006 (Yingda)– cause of activation at lower temperatures than model alloys – surface grain structure

Plans• Obtain model alloys

– binaries & ternaries

– variation in grain size

• Characterization– as-received

– after annealing

– in situ hot stage

– post-mortem

• Publications