• norsk
    • English
  • English 
    • norsk
    • English
  • Login
View Item 
  •   Home
  • Fakultet for naturvitenskap (NV)
  • Institutt for fysikk
  • View Item
  •   Home
  • Fakultet for naturvitenskap (NV)
  • Institutt for fysikk
  • View Item
JavaScript is disabled for your browser. Some features of this site may not work without it.

(Scanning) Transmission Electron Microscopy Studies of Grain Boundary Segregation relevant to Intergranular Corrosion in Al-Mg-Si-Cu Alloys

Holmestad, Jon
Doctoral thesis
View/Open
Fulltext not available (Locked)
URI
http://hdl.handle.net/11250/283573
Date
2015
Metadata
Show full item record
Collections
  • Institutt for fysikk [2567]
Abstract
The effect of the alloy composition on the susceptibility to intergranular corrosion

in Al-Mg-Si-Cu alloys has been investigated by Transmission Electron

Microscopy. The main focus of this thesis has been the effect of Mg:Si-ratio

and the effect of Mn-content on the intergranular corrosion, since Mg and

Si are the main alloying elements of the 6xxx alloys and Mn is a common

alloying element.

The addition of 0.40 wt% Cu to a 6082 alloy increases the hardness of the

alloy by approximately 10 HV in all ageing conditions. The susceptibility

to intergranular corrosion is also increased in a slightly underaged condition

due to the formation of an enriched layer of Cu at the grain boundary.

By adjusting the Mg:Si-ratio from 0.87 in the 6082 alloy to 2.00, the susceptibility

to intergranular corrosion is reduced. In this Mg-rich alloy, an

enriched layer of both Mg and Cu was observed at the grain boundary.

The presence of Mg reduces the cathodic potential created by the Cu and

reduces the potential difference between the grain boundary layer and the

adjacent depleted zone.

In leaner alloys, a reduced susceptibility to intergranular corrosion is observed

when the Mg:Si-ratio is increased from 0.80 to 2.90. The alloy with

a high Mg:Si-ratio has the same amount of excess Mg as the 6082-type alloy

with a ratio of 2.00. The Mg-rich lean alloy shows an enriched layer of both

Mg and Cu at the grain boundary, while the Si-rich only shows an enriched

layer of Cu. Once again, the Mg+Cu has a reduced electrochemical potential

compared to the pure Cu layer in the Si-rich alloy and the resistance to

intergranular corrosion is increased.

By removing the Mn from the 6082-type alloy with excess Mg, a fully recrystallized

structure with small grains and an aspect ratio close to 1 is

achieved. This alloy show significant susceptibility to intergranular corrosion, even though the enriched grain boundary layer contain both Mg

and Cu. It is likely that the grain structure makes it easier for the corrosion

attack to penetrate more deeply into the sample since most of the

grain boundaries are approximately perpendicular to the surface. In the

Mn-containing alloy, most of the interior grains are very large and have an

aspect ratio >> 1. The intergranular corrosion attacks can only propagate

parallel to the surface and the penetration depth is low. This underlines

the geometrical aspects of intergranular corrosion in alloys where the grain

boundary chemistry is otherwise similar.

In a very Si-rich version of the 6082-type alloy, there is still only observed

an enriched layer of Cu. The absence of an enriched layer of Si is not yet

understood, but more quantitative work needs to be done. The Si might

all disappear to the grain boundary precipitates and dispersoids, but that

investigation has been beyond the scope of this work.

It has not been found any periodicity or structure to the enriched layer at the

grain boundary, so the main theory at the moment is that it is not periodic.

The equipment needed to investigate this have only recently become available

and there are some issues with orienting the sample perfectly, meaning

that so far it is too early to conclude whether there is any periodicity in the

enriched boundary layer or not.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2015:23

Contact Us | Send Feedback

Privacy policy
DSpace software copyright © 2002-2019  DuraSpace

Service from  Unit
 

 

Browse

ArchiveCommunities & CollectionsBy Issue DateAuthorsTitlesSubjectsDocument TypesJournalsThis CollectionBy Issue DateAuthorsTitlesSubjectsDocument TypesJournals

My Account

Login

Statistics

View Usage Statistics

Contact Us | Send Feedback

Privacy policy
DSpace software copyright © 2002-2019  DuraSpace

Service from  Unit